EXPERIMENTAL & NUMERICAL STUDY OF DYNAMICALLY LOADED BOLTED &
HYBRID (BOLTED/BONDED) JOINTS
By
Aiswarya Venkadachalam

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Engineering Mechanics - Master of Science
2014

ABSTRACT
EXPERIMENTAL & NUMERICAL STUDY OF DYNAMICALLY LOADED BOLTED &
HYBRID (BOLTED/BONDED) JOINTS
By
Aiswarya Venkadachalam
Joining of composite materials is commonly achieved by bolting, bonding, or a combination of
the two methods (hybrid) in engineering structures such as aircraft, marine and automotive. If not
designed properly, dynamic loading of these joints can be critical to the integrity of the structure,
as failure initiates at fastener-material interfaces. This demands a thorough understanding of the
structural response of joints subjected to various loading conditions. The present study aims at
predicting the strength of single-lap, bolted and hybrid joints with various geometric
configurations under impact loading conditions. Experiments are conducted in a Drop Weight
Impact Testing Machine (DWITM) where tensile loading is applied by a specially designed
fixture. Results show that at room temperature, the strength of the hybrid joints with shorter edge
distances is greater than or comparable to the strength of bolted joints with larger edge distances.
At elevated temperatures (60°C), there is a 25-35% reduction of strength for joints with smaller
edge distances while joint strength for large edge distances is invariant with respect to
environmental conditions. In addition, the effect of clamping force on bolted joint strength is
investigated using a Split Hopkinson Pressure Bar (SHPB). Results confirm that at higher
loading rates, bolting torque plays a dominant role on the bearing strength of the joint structure.
Enhanced 3D finite element (FE) models are developed to substantiate the experimental
predictions and results are found to be in good agreement.

I dedicate this thesis to my mom and my loving husband

iii

ACKNOWLEDGMENTS

I would like to sincerely acknowledge my thesis advisor, Dr. Srinivasan Arjun Tekalur,
for his continuous support throughout the research and writing process. I am always
grateful for his technical assistance, persistent guidance and encouragement which
certainly helped me on both my research as well as on my career. I also thank my
committee members, Dr. Dahsin Liu and Dr. Xinran Xiao for taking time to review my
thesis and making suggestions. In addition, special thanks to Dr. Abhishek Dutta, Dr.
Guojing Li and Dr. Ermias Gebrekidan Koricho for their invaluable advice and insightful
comments in many research aspects which undoubtedly improved the quality of the work.
I sincerely thank our Ph.D. students Wei Zang and Andrew Joe Vander Klok for their
contribution to this project. I am indebted to my colleague, David Gonzalez for helping
me directly and indirectly throughout my research. I thank him for all his help,
discussions, support and fun we had for the last one and a half years. I would like to thank
my team mates Wu Zhou, Steven Ellis Utz and Peter Noel Howes for their assistance in
carrying out experiments. Furthermore, I thank Mike McLean, Adam Klein, Todd Pasch,
Aida Montalvo and the late Gail Berry for their extended help. I gratefully acknowledge
the financial support provided by TARDEC.
Finally, I thank my parents, my better half and my friends, for always being a source of
love, inspiration and encouragement in all my needy times. Without you all this wouldn’t
have been possible.

iv

TABLE OF CONTENTS

LIST OF TABLES

vii

LIST OF FIGURES

viii

KEY TO SYMBOLS

xiii

Chapter 1 Introduction
1.1 Joining methods and strength calculations
1.2 Testing methods
1.3 Bolt preload and joint behavior
REFERENCES

1
1
2
3
6

Chapter 2 Evaluating Bolted Joint Behavior at High Strain Rates
2.1 Abstract
2.2 Introduction
2.2.1 Failure modes on composite joints observed from static testing
2.3 Experimental study
2.3.1 Material and model description
2.3.1.1 Test setup and joint configuration
2.3.2 Results and discussion
2.3.2.1 Effect of bolt-pretension
2.4 Numerical study
2.4.1 Model description
2.4.2 ABAQUS/Standard – Bolt-Preload analysis
2.4.2.1 Contact Properties
2.4.2.2 FE Mesh
2.4.2.3 Bolt preload and boundary conditions
2.4.3 Results and discussion
2.4.3.1 Bolt pre-tension
2.4.3.2 FE model validation
2.4.3.3 Effect of slip from numerical model
2.4.3.4 FE results for tension test
2.5 Limitation
2.6 Conclusion
REFERENCES

7
7
8
9
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11
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15
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27
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34
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39

Chapter 3 Evaluating Hybrid Joint Behavior at Intermediate Strain Rates
3.1 Abstract
3.2 Introduction
3.2.1 Joining methods
3.2.2 Testing methods
3.2.3 Failure modes on composite joints observed from static testing

40
40
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41
42
43

v

3.3 Experimental work
3.3.1 Static Testing Method
3.3.2 Low velocity Impact Testing Method
3.3.3 Specimen configurations and materials
3.4 Experimental Results and Discussion
3.4.1 Experimental observations from static analysis
3.4.1.1 Response of bolted joints
3.4.1.2 Effect of e/d ratio
3.4.1.3 Response of bonded joints
3.4.1.4 Failure modes on bonded joints
3.4.1.5 Response of hybrid joints
3.4.1.6 Effect of e/d ratio
3.4.1.7 Effect of joint type
3.4.2 Experimental observations from impact analysis
3.4.2.1 Force equilibrium
3.4.2.2 Response of bolted joints
3.4.2.3 Effect of e/d ratio
3.4.2.4 Response of bonded joints
3.4.2.5 Failure modes on bonded joints
3.4.2.6 Response of hybrid joints
3.4.2.7 Effect of e/d ratio
3.4.2.8 Effect of joint type
3.4.2.9 Effect of loading rates
3.4.2.10 Effect of hole-clearance
3.4.2.11 Effect of environmental conditions
3.4.2.12 Effect of composite adherend
3.5 Conclusion
REFERENCES

44
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Chapter 4 Conclusions and Recommendations
4.1 High strain rate impact testing
4.2 Static and low velocity impact testing
4.3 Recommendations

80
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vi

LIST OF TABLES
Table 2-1

Metal-metal bolted joint specimen configuration and applied bolt preload.

13

Table 2-2

Bolted joint specimen configurations used for numerical analysis.

20

Table 2-3

Material properties of the specimen, bolt and the loading bars used in
numerical analysis.

21

Table 3-1

Material properties of S2 glass-SC 15 epoxy laminates [16].

49

Table 3-2

Dimensions of pulse shaping materials used to achieve force equilibrium
within the specimen under impact loading.

59

vii

LIST OF FIGURES
Figure 1.1

Strain rates and its corresponding experimental techniques used for
material characterization

2

Figure 1.2

Behavior of joint under static loading

3

Figure 2.1

Typical failure modes observed in single-lap, single-bolt composite joints
loaded at the hole: (a) shear-out (b) cleavage (c) net tension and (d)
bearing.

10

Schematic diagram of Split Hopkinson compression bar used in current
study shown in (a), placement of the load cell to monitor bolt preload
shown in (b) and the effect of preload on bolt and contact surfaces shown
in (c).

12

Bolt joint configuration and key parameters associated with the current
study.

13

Plot of incident pulse, transmission pulse and reflected pulse obtained from
incidence and transmission bar.

15

Figure 2.5

Transmission force of aluminum/aluminum bolted joint clamped at 500lbf.

16

Figure 2.6

Variation of force to slip and maximum bearing
aluminum/aluminum bolted joint for varying preloads.

16

Figure 2.2

Figure 2.3
Figure 2.4

Figure 2.7

force

of

Transmission stresses of aluminum/aluminum bolted joint for varying
preloads: 0lbf, 100lbf, 300lbf, 500lbf and 700lbf.

17

Strain histories from the load cell outfitted on the bolt for varying preloads:
0lbf, 100lbf, 300lbf, 500lbf and 700lbf.

18

Figure 2.9

Loading rate vs. bolt preload of the specimen under high strain rate impact.

18

Figure 2.10

Finite element (FE) model of split Hopkinson pressure bar (top) used for
numerical analysis and schematic represent of applied bolt preload with
equivalent nut pressure on FE model (bottom).

21

Figure 2.11

Plastic properties of the aluminum sample used in numerical analysis.

22

Figure 2.12

Schematic represent of master and slave surfaces assigned to the finite
element model.

23

Figure 2.13

Finite element model showing mesh on aluminum/aluminum bolted joint.

24

Figure 2.14

Steps employed to apply bolt preload on finite element model in static
solver.

25

Figure 2.8

viii

Figure 2.15

Schematic represent of bolt partition where preload is applied on finite
element model.

25

Stresses developed in the bolt region after applying a preload of 500lbf
from finite element modelling.

26

Stresses developed on aluminum-bolt contact surfaces for a applied preload
of 500lbf.

26

Location of strain gauges in experimental method & corresponding nodal
positions in finite element model.

27

Comparison of transmission stresses of aluminum/aluminum bolted joint
from experimental and finite element results for varying preloads: 0lbf,
100lbf, 300lbf, 500lbf and 700lbf.

28

Comparison of bearing strength of aluminum/aluminum bolted joint from
experimental and finite element results for varying preloads: 0lbf, 100lbf,
300lbf, 500lbf and 700lbf.

29

Percentage of error in aluminum/aluminum bolt joint strength from finite
element predictions by comparing against experimental results.

30

Schematic represent of load cell location in experimental method &
corresponding section where nodal values are extracted from finite element
model.

30

Comparison of preloading effect on aluminum/aluminum bolted joint from
experimental and finite element results for varying preloads: 0lbf, 100lbf,
300lbf, 500lbf and 700lbf.

31

Figure 2.24

Schematic represent of contact zone on the metal constituents.

32

Figure 2.25

Different phases on slipping event in aluminum/aluminum bolted joint
preloaded at 500lbf subjected to high loading rates.

33

Representation of bolt-metal contact during the event of slipping (a) at
region I and (b) at region I and III.

33

Correlation of strain-time history and bolt slip on aluminum/aluminum
bolted joint preloaded at 500lbf from finite element predictions.

34

Variation of bearing strength of aluminum/aluminum bolted joint under
tension loading for different e/d ratios from finite element analysis.

35

Failure mode predictions of aluminum/aluminum bolted joint under tension
loading for different e/d ratios from finite element analysis.

35

Figure 2.16
Figure 2.17
Figure 2.18
Figure 2.19

Figure 2.20

Figure 2.21
Figure 2.22

Figure 2.23

Figure 2.26
Figure 2.27
Figure 2.28
Figure 2.29

ix

Figure 2.30

Variation of bearing strength of aluminum/aluminum bolted joint under
tension loading for different w/d ratios from finite element analysis.

36

Failure mode predictions of aluminum/aluminum bolted joint under tension
loading for different w/d ratios from finite element analysis.

36

Typical failure modes observed in single-lap, single-bolt composite joints
loaded at the hole: (a) shear-out (b) cleavage (c) net tension and (d) bearing
(Reproduced from Figure 2.1).

43

Drop tower test setup (left), fixture setup with holding specimen (middle)
and joint specimen (right).

45

Figure 3.3

Fixture used to convert compression to tension load in drop tower machine.

46

Figure 3.4

Hybrid joint specimen configuration used for impact analysis and
geometrical parameters: Front view (shown in left) and side view (shown in
right).

48

Figure 3.5

Bolted joint geometry used for impact analysis.

48

Figure 3.6

Aluminum/S2 glass-SC 15 hybrid samples placed on the specially designed
fixture to maintain alignment and constant pressure.

49

Load-displacement curve of Aluminum/S2 glass-SC 15 bolted joint with
edge distance 1d under static loading.

50

Failure loads of Aluminum/S2 glass-SC 15 bolted joints with different e/d
ratios under static loading.

51

Failure modes of Aluminum/S2 glass-SC 15 bolted joints with different e/d
ratios under static loading.

52

Load-displacement curve of Aluminum/S2 glass-SC 15 bonded joint with
bond area 0.5 in. x 1 in. under static loading (load bottom to top).

53

Typical failure modes definition of an adhesive bonded joint: (a) cohesive
failure, (b) adhesive failure, (c) mixed failure and

53

Failure modes of Aluminum/S2 glass-SC 15 bonded joints with bond area
0.5 in. x 1 in. under static loading.

54

Load-displacement curve of Aluminum/S2 glass-SC 15 hybrid joint with
edge distance 1d under static loading (load bottom to top).

54

Failure load predictions of the Aluminum/S2 glass-SC 15 hybrid joints
with different e/d ratios under static loading.

55

Figure 2.31
Figure 3.1

Figure 3.2

Figure 3.7
Figure 3.8
Figure 3.9
Figure 3.10
Figure 3.11
Figure 3.12
Figure 3.13
Figure 3.14

x

Figure 3.15
Figure 3.16
Figure 3.17
Figure 3.18

Figure 3.19

Figure 3.20
Figure 3.21
Figure 3.22
Figure 3.23
Figure 3.24
Figure 3.25

Figure 3.26
Figure 3.27
Figure 3.28
Figure 3.29

Failure modes of Aluminum/S2 glass-SC 15 hybrid joints with different e/d
ratios under static loading.

56

Failure modes of Aluminum/S2 glass-SC 15 hybrid joints with e/d =1 and
bond area of 0.5 in. x 1 in. under static loading.

56

Failure load comparison of Aluminum/S2 glass-SC 15 bolted, bonded &
hybrid joints under static loading

57

Force vs. displacement plot of Aluminum/S2 glass-SC 15 joints under
static loading (a) bolted joint with e/d = 1, (b) bonded joint with bond area
0.5 in. x 1 in. and (c) hybrid joint with e/d =1.

58

Force histories of dog-bone aluminum specimen from top and bottom load
cell on using different pulse shaping materials: (a) Buna-N-rubber, (b)
polyurethane, (c) polypropylene and (d) latex rubber.

60

Force history of Aluminum/S2 glass-SC 15 bolted joint with edge distance
1d under impact loading from top load cell (load right to left).

61

Bolt transition & rotation of Aluminum/S2 glass-SC 15 bolted joint with
edge distance 1d under impact loading (load right to left).

62

Failure loads of Aluminum/S2 glass-SC 15 bolted joints with different e/d
ratios under intermediate loading rates.

63

Failure modes of Aluminum/S2 glass-SC 15 bolted joints with different e/d
ratios under impact loading.

63

Force history of Aluminum/S2 glass-SC 15 hybrid joint with edge distance
1d under impact loading from top load cell (load right to left).

64

Schematic of joint slip (yellow line) and bending (blue line) of
Aluminum/S2 glass-SC 15 bonded joint with bond area 0.5 in. x 1 in. under
impact loading (load right to left).

65

Failure modes of Aluminum/S2 glass-SC 15 bonded joints with bond area
0.5in. x 1in. under impact loading.

65

Force history of Aluminum/S2 glass-SC 15 hybrid joint with e/d = 1 under
impact loading (load right to left).

66

Failure load predictions of the Aluminum/S2 glass-SC 15 hybrid joints
with different e/d ratios under impact loading.

67

Failure modes of Aluminum/S2 glass-SC 15 hybrid joints with different e/d
ratios under static loading.

68

xi

Figure 3.30
Figure 3.31
Figure 3.32
Figure 3.33
Figure 3.34

Figure 3.35

Figure 3.36

Failure load comparison of Aluminum/S2 glass-SC 15 bolted, bonded &
hybrid joints under impact loading.

69

Failure load comparison of Aluminum/S2 glass-SC 15 bolted, bonded &
hybrid joints under static, intermediate and high loading rates.

70

Behavior of Aluminum/S2 glass-SC 15 hybrid joints with e/d = 1 for
varying hole clearances under intermediate loading rates.

71

Failure load comparison of Aluminum/S2 glass-SC 15 bolted & hybrid
joints with e/d = 1 and 3 between room temperature and 60°C.

72

Bearing strength comparison between Aluminum/S2 glass-SC 15 and
Aluminum/E-glass bolted joints with e/d =1 and 4 under intermediate
loading rates.

73

Bearing strength comparison between Aluminum/S2 glass-SC 15 and
Aluminum/E-glass hybrid joints with e/d =1 and 4 under intermediate
loading rates.

73

Failure mode comparison between Aluminum/S2 glass-SC 15 and
Aluminum/E-glass bolted and hybrid with e/d = 1 and 4 under impact
loading rates.

74

xii

KEY TO SYMBOLS

xiii

Chapter 1 Introduction
1.1 Joining methods and strength calculations
In engineering structures, joining of two or more structural components is often achieved by
mechanical fastening or bonding. Mechanical fastening (uses bolts or rivets) is the common
method of joining structural parts due to their relative ease of installation and resistance to
environmental degradation. Bolted joints are mostly preferred for transferring high loads thereby
widely used in large scale structures. However, high stress concentration regions are developed
around the hole locations in bolted joints where crack can initiate to cause structural failure.
Bonded joint is the ideal choice for thin walled structures where fatigue is of prime concern,
mostly used in aircraft and marine structures. This method of joining offers high efficiency
through smooth load transference, but it has some drawbacks. The strength of the bonded joint
depends heavily on surface treatment of the adherend members and operating conditions of the
joints. Also, the cost of maintenance for bonded joints is much greater than that of bolted
configuration. The strength of the joint depends on the joining method and its maximum load
carrying capacity. Bolted joint strength can be calculated as,

where, bearing area is defined as the thickness of the joint times the diameter of the bolt-hole.
Bonding strength can be determined as,

1

1.2 Testing methods
Strain-rate is defined as the rate at which the deformation occurs within a material with respect to
time. Understanding the behavior of joints under different strain rates is essential to optimize the
design and installation of joints in any structural members.

Figure 1.1 Strain rates and its corresponding experimental techniques used for material
characterization

Figure 1.1 illustrates different types of strain rates and its corresponding experimental
techniques used for characterizing material behavior. Standard testing machines are available to
execute quasi-static analysis to predict the strength of the materials at very low strain rates (1s-1).
These testing methods are used to perform both compression and tension analysis. The
experimental investigation in intermediate strain rate ranges (10 0 s-1 to 102 s-1) employs drop
tower testing machine for compression loading condition [1]. To investigate the behavior of
joints at higher strain rates (102 s-1 to 104 s-1), Split Hopkinson Pressure Bar (SHPB) is generally
used for both compression and tension analysis. Most conventional tools for dynamic
investigations are used to characterize material behavior. However, it is extremely difficult to
study the structural response of joints in both SHPB and drop tower testing system.
2

1.3 Bolt preload and joint behavior
In bolted joints, the applied pre-torque on the bolt and nut causes tension in the fastener and
subsequent compression on the material constituents. This preload helps in holding the joining
members together without slipping and plays a predominant role in defining the frictional force
between the contact surfaces. The amount of torque (T) required to achieve desired preload
(

can be calculated as,

where, K is the nut factor usually considered as 0.2 and D is the diameter of the bolt [2].

Figure 1.2 Behavior of joint under static loading

When a static load is applied on a clamped bolted joint, four distinct regions [3] are evident in
force-displacement response as shown in Figure 1.2. In region 1, as the applied load increases
the entire joint will undergo elastic deformation where there is no relative movement of the
joining members are visible until it reaches a critical load. The magnitude of the region 1 entirely

3

depends on the amount of applied bolt-torque. In region 2, the joint slips until bolt-hole clearance
is eliminated. Relative displacement between the matting surfaces is evident in this region. In
region 3, significant plastic deformation can be seen until it reaches the failure of the joint
(region 4).

4

REFERENCES

5

REFERENCES

[1] K. T. Ramesh, “High Rates and Impact Experiments,” in Springer Handbook of
Experimental Solid Mechanics, W. N. S. J. Prof, Ed. Springer US, 2008, pp. 929–960.
[2] Zhou S, Wang Z, Wu X and Zhou J, "Experimental and numerical investigation on bolted
composite joint made by vacuum assisted resin injection," Compos Part B: Eng, p.
45(1):1620–8, 2013.
[3] G. Kelly, “Quasi-static strength and fatigue life of hybrid (bonded/bolted) composite singlelap joints,” Composite Structures, vol. 72, no. 1, pp. 119–129, Jan. 2006.

6

Chapter 2 Evaluating Bolted Joint Behavior at High Strain Rates
2.1 Abstract
Bolted joints are used extensively in automobile and aerospace applications. Though bolting is
considered to be an easy and non-destructive method of joining, these fasteners are often
considered to be detrimental under impact loading conditions. Therefore, it demands specific
attention to a thorough understanding of the mechanical response of bolted joints at high rates of
loading. In this study, enhanced three dimensional (3D) finite element (FE) models are
developed for single-lap, single-bolt, metal-metal joints in order to observe their structural
behavior when subjected to impact loading in a Split Hopkinson Pressure Bar (SHPB). FE
solutions are compared against experimental results and are found to be in good agreement for
compression loading in SHPB. The validated FE models are further used to investigate the
failure modes of bolted joints under tensile loading condition. The following issues are
thoroughly investigated from the numerical model: (a) the effect of joint slip (b) influence of
geometric parameters on failure mode and (c) fastener preloading effect. Two different
geometrical parameters are investigated: edge distance-to-bolt diameter (e/d) ratio chosen from 1
to 5 and width-to-bolt diameter ratio (w/d) ranging from 1 to 4. The intent of this paper is to
provide a relationship between bolt pre-tension, loading rate and mode of failure by
computational method for high loading rates. These observations from the FE model will provide
ample information to characterize the crashworthiness of bolted structures where impact loading
is of prime concern and can be used as an alternative tool for structural joint test in SHPB.

7

2.2 Introduction
Bolted joints have been significantly used in the automobile and aeronautical industries for many
years where joining two or more structural members together are required. This kind of fastening
technique serves as a leading method for joining structural members by considering its ease of
installation and subsequent disassembly for inspection, maintenance, and repair work when
compared to adhesive or hybrid joints (bolted/bonded). Furthermore, bolts are relatively
inexpensive which makes it easier for replacement. However, due to the structural discontinuities
in their geometry, stresses are concentrated around the bolt-hole locations in the joint assembly,
which makes this region susceptible to failure. These higher stress regions result in a reduction in
structural joint strength which in turn leads to damage and failure of the entire structure.
Therefore, to employ bolted joints more effectively and efficiently, a thorough understanding of
joint mechanisms is highly necessary. The structural response of mechanical joints is
substantially different for different loading conditions. In particular, the dynamic response of
mechanical joints is more complicated because of the development of highly concentrated stress
regions, localized non-linear stiffness, inertial effects, dynamic friction, and damping factors
which are more likely to adversely affect the load carrying capacity of the joint. In the past
several years, exhaustive studies have been performed to understand the mechanical behavior of
bolted joints under quasi-static loading [1], [2], [3] but only a few have addressed the impact
response of mechanical joints [4].
In order to design joints where safety is of paramount concern, detailed knowledge of the stress
distribution in the critical regions is essential especially concerning failure modes of the
assembly. The study of bolted joint behavior is mostly parametric. Its mechanical response
depends on non-dimensional geometric parameters such as w/d, l/d, d/t and e/d where w is the
8

width of the joint, l is the length of joint, t denotes thickness, e indicates the edge distance from
the center of the bolt hole, and d is the diameter of the bolt used for non-dimensionalization.
Previous works on bolted joints are mainly focused on predicting the joint load to failure for
different geometrical configurations and materials. It has been shown that under quasi-static
0 -1

loading, tested at a strain rate of 10 s , the maximum bearing stress before failure (both in
compression and in tension) forms an asymptotic region for varying geometrical parameters.
Mode of failure depends on loading rate and tends to converge after certain values of w/d and e/d
ratios have been exceeded (approximately 2 for composite joints) [1], [2], [3], [4]. Under impact
2 -1

loading conditions, tested at a strain rate of (10 s

4 -1

to 10 s ), the maximum bearing stress of

the joints are significantly higher than the joints subjected to quasi-static loading and
convergence of the asymptotic region of failure mode shifts to e/d ratio 3 (approximately) for
composite joints [4]. Early attempts were made to experimentally study the effects of clamping
force using different sized washers [5]. Also, the effects of bolt pre-load on a composite bolted
joint subjected to compression loading have been investigated [6]. Observations from these
studies clearly indicate that the bolt pre-load varies with applied force within the bolt and can
also be used as a key parameter to determine bearing mode of damage.
2.2.1 Failure modes on composite joints observed from static testing
Figure 2.1 illustrates the typical failure modes of single-lap, single-bolt mechanical joints
frequently observed during experimental investigations: shear-out, cleavage, net tension and
bearing failure [2], [3]. Mixed failure modes (combination of shear-out, cleavage or net tension)
are mostly noticed in the joints subjected to high loading rates where edge distance to diameter
(e/d) ratio is less than 3 [4]. Bearing failure is predominant when e/d ratio is over 3. Since
bearing failure is more gradual and progressive, it is often considered to be the preferred failure
9

as it can withstand loads even after the initiation of failure. All the other failure modes are
catastrophic and detrimental, which has to be avoided in structural member.

Figure 2.1 Typical failure modes observed in single-lap, single-bolt composite joints loaded
at the hole: (a) shear-out (b) cleavage (c) net tension and (d) bearing.

Performing experiments on bolted joints are still an ongoing process, since experimental data are
considered to be the most reliable datum line even today. But, in recent development of
numerical methods, extensive finite element analysis can be possible which is time consuming
but inexpensive and can predict detailed stress-strain distributions within the joint & modes of
failure. But care should be taken that the finite element model and analysis should mimic exact
physical system and realistic loading conditions.
The objective of this paper is to develop an efficient finite element bolted joint model subjected
to impact rates of loading in SHPB and to study its structural response by numerical approach.
10

This paper also discusses the significance of the role of bolt pre-tension and joint slip in bearing
mode of failure for metal-metal bolted joints. Joint slip is the relative movement of the faying
surfaces where the applied axial load surpasses the frictional force produced by the clamping
force of the fastener. The maximum amount of joint slip is predicted as twice the size of holeclearance but an average of one-half size of hole-clearance is observed in practical events. In this
work, a relation between bolt-pretension load, loading rate and maximum force transfer prior to
slipping condition is investigated. Experiments are carried out on split Hopkinson compression
bar and the results are in good agreement with FE solutions which predicts the dependence of
joint strength on loading rate.
2.3 Experimental study
2.3.1 Material and model description
2.3.1.1 Test setup and joint configuration
Experimental set-up for impact testing on SHPB is schematically represented in Figure 2.2. The
SHPB consist of split-bars (incident & transmission) which are made-up of 6061 T6 Aluminum
material with stiffness of 70GPa and has an identical diameter and length of 0.75 in., and 6 feet
respectively. The smaller diameter bar helps in generating high strain rates to characterize the
dynamic behavior of the bolted joint. The incident bar, commonly known as loading bar, is
loaded by a compressive pulse on the impact of the striker launched by a gas gun. The stress
pulse passes through the incidence bar, and at bar-specimen interfacial surface a portion of the
wave gets transmitted through the specimen, and the other portion gets reflected back. Since the
loading rate of incident pulse plays a major role in investigating the material response, it is
important to maintain force equilibrium within the specimen. Therefore, pulse shapers are used
that elongates the pulse width which helps in attaining force equilibrium within the specimen.
11

Both the incident and transmitted pulses are recorded by the strain gages which are bonded on
the split-bars. Electrical-strain gages are installed on the mid-span of each split-bar and are
positioned in diametrically opposite side in order to cancel out bar bending. Strain gages are
connected to a half-bridge circuit configuration. The output response of the circuit is amplified
by an Ectron amplifier (model513-2A) and data acquisition is carried out by using digital
oscilloscope (LeCroy-model354A).

Figure 2.2 Schematic diagram of Split Hopkinson compression bar used in current study
shown in (a), placement of the load cell to monitor bolt preload shown in (b) and the effect
of preload on bolt and contact surfaces shown in (c).

12

Figure 2.3 Bolt joint configuration and key parameters associated with the current study.

In addition, at the end of the transmission bar, a soft material is placed to prevent the
transmission bar from overshooting. A single-lap, single-bolt, metal-metal (Al 6061 T6) bolted
joint is tested for varies preloading conditions. To study the preloading effect, joints are outfitted
with calibrated load cell as shown in Figure. Lubricant is applied at the bar-specimen interfaces.
It helps in reducing the bar-sample interfacial frictional effects on loading. The geometric
description of the tested sample is shown in Figure 2.3. Details on applied preloads and
specimen configurations are presented in Table 2-1. For all the tested specimens the diameter of
the bolt is 6.35 mm.

Table 2-1 Metal-metal bolted joint specimen configuration and applied bolt preload.

13

2.3.2 Results and discussion
In this study, experiments are carried out in split Hopkinson compression bar which has been
developed to obtain loading rates from 200MN/s to 900MN/s. Stress equilibrium within the
sample is verified by comparing force data from the incidence, reflected and the transmission
pulses according to the wave theory [7].The maximum bearing stress of the bolted joint is
calculated as follows:

Equation 1
- Cross-sectional area of the transmission bar
- Young’s Modulus of the transmission bar
- Axial strain of the transmission pulse
- Bolt-hole diameter
- Specimen thickness
Parametric study has been executed on all 5 cases of the specimen series S1to observe the strainrate behavior of the bolted joints. However, to avoid multiple overlaps, experimental data
acquired from the incident and transmitted bar for the specimen clamped at 500lbf are alone
shown in Figure 2.4 It can be seen that the width of the incidence pulse is 150μs and only half
the intensity of the incident pulse is transmitted through the specimen and the remaining is
reflected back to the incident bar.

14

Figure 2.4 Plot of incident pulse, transmission pulse and reflected pulse obtained from
incidence and transmission bar.

2.3.2.1 Effect of bolt-pretension
Torque tightening in the bolted assembly helps the adherend members in contact by creating
tension in the bolt. The main objectives of this experiment are: (a) to investigate the influence of
torque tightening on joint behavior and (b) to examine the change in joint strength subjected to
impact loading conditions. Since the w/d ratio and e/d ratio of the tested samples are already
chosen to be in the asymptotic region, joint failure is confined to bearing mode of failure.
According to the transmitted force data from the experimental observation, behavior of bolted
joints can be classified as four stages. (a) Initial loading, (b) slip, (c) post slip and (d) bearing
damage. Different stages are described in Figure 2.5.
Force to slip (start of slipping stage) and max bearing force (onset of bearing damage stage) from
the experiments are plotted against bolt pretension and it is presented in Figure 2.6. This plot
shows that there is no substantial difference in force to slip between 100lbf and 300lbf. But, it
increases significantly when the value of bolt pretension increases from 300lbf to 500lbf. As we
15

further increase the torque tightening, force to slip drops down. The similar trend is also
observed for max force to failure.

Figure 2.5 Transmission force of aluminum/aluminum bolted joint clamped at 500lbf.

Figure 2.6 Variation of force to slip and maximum bearing force of aluminum/aluminum
bolted joint for varying preloads.

However, the influence of torque can be more phenomenal in slipping and the strain on the bolt
itself. Figure 2.7 and Figure 2.8 shows the transmission stress data for all 5 preloading
condition and the strain history from the load cell outfitted in the bolt. With reference to the
Figure 2.7, it is evident that the slipping can be delayed when we increase the torque tightening
16

of the bolt. For the preloading condition of 100lbf, slip occurs at 40μs. Whereas for higher
preloads (500lbf), slip occurs at 47μs and subsequent increase in preloads doesn’t contribute
much in slipping phenomena. From the overall strain plot (Figure 2.8), it is evident that the
increase in clamping causes smaller deviation in strain except for 100lbf. The strain on the bolt
reduces considerably for higher preloads (700lbf).

Figure 2.7 Transmission stresses of aluminum/aluminum bolted joint for varying preloads:
0lbf, 100lbf, 300lbf, 500lbf and 700lbf.

The loading rate experienced in the joint assembly for varying clamping conditions is presented
in Figure 2.9. The plot shows that the loading rate is higher for hand tightened bolted joints and
it decreases as we further increase the clamping force on the bolt. This is because, when the
clamping force in the bolt increases, it increase the compression on the metal adherends which
in-turn increase the frictional force between the faying surfaces. This helps in preventing the
relative displacement of the adherend members. Provided the bolt-hole clearance is maintained
as constant for all tested samples with the decrease in relative velocity, loading rate is expected
17

to decrease. But for the specific geometrical configuration of tested samples, it is observed that
the specimens pre-loaded at 300lbf are subjected to less loading rate.

Figure 2.8 Strain histories from the load cell outfitted on the bolt for varying preloads:
0lbf, 100lbf, 300lbf, 500lbf and 700lbf.

Figure 2.9 Loading rate vs. bolt preload of the specimen under high strain rate impact.
18

2.4 Numerical study
The mechanical behavior of bolted joint is complex under high strain rate loadings. Therefore, it
is important to substantiate the experimental results by computational results. A three
dimensional finite element model is developed in ABAQUS 6.11.2. Numerical analysis is carried
out for specimen series S1 for different preloading condition subjected to compression loading as
listed in Table 2-1. The model is further employed to study the response of bolted joint for
tensile loading as listed in Table 2-2. In order to observe the effect of slip, the bolt-hole
clearance is specified to the model as 0.1778 mm (0.007 inch), which is considered as the
standard hole-clearance for a bolt diameter of 6.35 mm (0.25 inch). Joint assembly is developed
with the entire SHPB setup in order to replicate the exact loading scenario as shown in
. Option to apply bolt pretension is currently not available in ABAQUS/Explicit solver.
Therefore, ABAQUS/Standard solver (static) is used to apply bolt preload in the assembly.
Clamped bolted joint is then imported to ABAQUS/Explicit solver in order to analyze the joint
behavior using non-linear dynamic formulation by setting-up the nodal positions from static
solution as the initial state for current analysis.
2.4.1 Model description
Geometric characteristics of the assembly and SHPB are imitated as described in the prior
explanations. In this investigation, the main scope is to model Split Hopkinson Pressure Bar
(Figure 2.10) to generate the exact compressive loading state irrespective of the computational
difficulties encountered in finite element analysis. Therefore, some modifications on geometry of
the SHPB model and bolt is incorporated which is not having a significant role in response of the
complete assembly in the event of loading. Changes encompassed in FE model are explained in
detail as follows: On SHPB model, striker is eliminated, since load is applied on surface of
19

incidence bar as a pressure wave. This helps in avoiding wave-splitting occurs at the interfacial
surface between incident bar & striker. Stopper is included as a rigid body at the end of
transmission bar. Nut and Bolt are modeled as an integral part and their threads are also
neglected (this assumption has been used by other researchers also).
Mechanical properties of the specimen (Aluminum) and bolt (Steel) are shown in the Table 2-3.
Sample is prescribed with elastic-plastic formulation (Figure 2.11) whereas bolt and loading bar
(Aluminum) are specified only with elastic property which are not the key part of investigation.
Densities of the materials are defined beforehand in static analysis which will be used in
subsequent dynamic analysis.

Table 2-2 Bolted joint specimen configurations used for numerical analysis.

20

Figure 2.10 Finite element (FE) model of split Hopkinson pressure bar (top) used for
numerical analysis and schematic represent of applied bolt preload with equivalent nut
pressure on FE model (bottom).

Table 2-3 Material properties of the specimen, bolt and the loading bars used in numerical
analysis.

21

Figure 2.11 Plastic properties of the aluminum sample used in numerical analysis.
2.4.2 ABAQUS/Standard – Bolt-Preload analysis
2.4.2.1 Contact Properties
Contact interactions are developed using surface-surface based discretization method by
enforcing linear penalty formulation with finite sliding. Master surface are assigned to more stiff
materials to avoid penetration into the slave surface as shown in Figure 2.12. Four interactions
properties are prescribed to the model: Metal - Metal, Bolt - Metal, Nut – Metal and Bar
(Incidence & Transmission) – Metal interactions. Frictional coefficient for interaction between
bolt-metal & nut-metal is prescribed as 0.7 & 0.3 respectively. Normal contact behavior is
imposed on the model with hard pressure over-closure relationship. Also, tangential contact
behavior is also prescribed by allowing finite sliding formulation. Frictionless contact is defined
for bar-specimen interfacial surface. To avoid complications in convergence of contact solution,
adjustment of slave surfaces are allowed to be exactly in contact with the master surfaces only
for few contact pairs. In others, adjustments of slave surfaces are not permitted. Contact
properties used are mostly based on the study of McCarthy et al. [8].
22

Figure 2.12 Schematic represent of master and slave surfaces assigned to the finite element
model.
2.4.2.2 FE Mesh
Finite element mesh of bolted joint assembly is shown in Figure 2.13. 8-noded linear elements
(C3D8R) are used to mesh the bolted joint assembly along with reduced-integration and
hourglass control. In order to get accurate solutions, a fine mesh is created around the bolt-hole
surfaces and in the regions of loading bars where strain gauges are located. A total of 12,444
elements are created for the metal surfaces and 6764 elements are constructed for the bolt part.
2.4.2.3 Bolt preload and boundary conditions
Bolt pretension is applied to the assembly using ABAQUS/Standard solver. Bold preload
can be calculated as,
Where,

is the bolt-torque applied, K is the nut factor usually considered as 0.2 and D is the

diameter of the bolt. As shown in Figure 2.14 two steps approach is used for applying bolt
pretension on the sample which is influenced from the study of Song Zhou et al. [9].
23

Figure 2.13 Finite element model showing mesh on aluminum/aluminum bolted joint.
In the first step, bolt preload (0lbf – 700lbf) is applied to the assembly by clamping the free ends
of the metals (only in transverse directions). A “Partition surface” is created on the bolt along
bolt-diameter, where bolt pretension is applied normal to its surface. The partition of bolt cross
section where clamping force is applied is shown in Figure 2.15. By pulling the bottom portion
of elements in upward direction & top portion of elements in downward direction, ABAQUS can
automatically generate internal stresses which correspond to the applied load, thereby changing
the length of the bolt. Since bolt & nut are modeled as an integral part, equivalent pressure load
is applied on the nut surface to generate symmetrical stress distribution within the bolt cross
section along the axis (in order to mimic the exact loading condition). In the second step, length
of the bolt is fixed by holding every other constraints from step one. Time-dependent loading is
applied to the FE model using ABAQUS/Explicit solver. Stress pulses are applied at the end of
the incident bar by allowing the bar displacement along axial direction.

24

Figure 2.14 Steps employed to apply bolt preload on finite element model in static solver.

Figure 2.15 Schematic represent of bolt partition where preload is applied on finite element
model.

25

2.4.3 Results and discussion
2.4.3.1 Bolt pre-tension
Figure 2.16 shows the distribution of through-thickness stresses along the bolt shank pre-loaded
at 500lbf. As it can be seen, the shank of the bolt is subjected to a uniform tensile stress and
subsequent compressive stresses are observed in the metal, nut and bolt head region. Also,
significant stress concentrations are noticed on the edge of the bolt-hole region on the metal
surfaces caused by the bearing of the bolt as shown in Figure 2.17. The total compression
produced by the pretension of the bolt ranges from 6.5 to 96 MPa.

Figure 2.16 Stresses developed in the bolt region after applying a preload of 500lbf from
finite element modelling.

Figure 2.17 Stresses developed on aluminum-bolt contact surfaces for a applied preload of
500lbf.
26

2.4.3.2 FE model validation
To the best of authors’ knowledge, this study is the first effort to develop a SHPB model for
structural analysis. It implies the importance of validating the accuracy of the model before
further investigation. In this session, comparison of experimental data (strain gauges reading) is
carried out against the numerical results (axial stress values) for the compression loading in
SHPB. In FE model, stresses are extracted and averaged from two diametrically opposite nodes
on the transmission bar. The location of the strain gauges (experiment) and corresponding nodal
positions (FEA) are shown in the Figure 2.18.

Figure 2.18 Location of strain gauges in experimental method & corresponding nodal
positions in finite element model.

FE analysis is carried out for a total time period of 1000μs with a time step of 1μs. Data are
extracted and averaged for each time step. The respective transmission stresses for specimen
series S1 are plotted against computational results and are shown in Figure 2.19. It can be seen
that the behavior of FE model are comprehensively in good agreement with the experimental
results. The effect of slipping is not much evident from the FE transmission data. Potential
reasons could be: (a) time step in analysis and (b) applied nut-pressure.
In the current analysis, time step is maintained at 1μs. If we further bring down the time
increment, the possibility of visualizing the slipping effect in transmission pulse could be higher.
27

In contrary, it will be an additional increase in computational cost. Also, the slipping phenomena
can be investigated in detail from the specimen itself. Therefore it is not obligatory to refine the
time step of the analysis further.

Figure 2.19 Comparison of transmission stresses of aluminum/aluminum bolted joint from
experimental and finite element results for varying preloads: 0lbf, 100lbf, 300lbf, 500lbf
and 700lbf.
28

In ABAQUS model, an equivalent nut-pressure is applied to the joint assembly for all preloading
condition in order to have a uniform stress distribution along the through thickness direction of
the bolt. To predict the preload strain experienced in the bolt from the numerical model, applying
pressure on the nut surface is unavoidable.
The FE model is hence validated by comparing the maximum bearing strength (Equation 1) of
the bolted joint from the experimental data. The comparison of bearing strengths for all 5
clamping conditions and FE error percentage are shown in Figure 2.20 and Figure 2.21
respectively. With reference to Figure 2.20, it is evident that bearing strength of the bolted joint
increases with increase in preload up to 500lbf and decreases for subsequent preload.

Figure 2.20 Comparison of bearing strength of aluminum/aluminum bolted joint from
experimental and finite element results for varying preloads: 0lbf, 100lbf, 300lbf, 500lbf
and 700lbf.

Figure 2.21 shows that the FE solutions are in very good agreement with experimental results
for higher preloading conditions. The model is also used to verify the behavior of bolt itself.
Hence, the comparison of the bolt strain data is carried out amongst the load cell information and
29

average nodal values (nominal strain) across the bolt cross section as shown in Figure 2.22. The
respective transmission stresses for specimen series S1 are plotted against computational results
and are shown in Figure 2.23. This study is executed only to compare the behavior of the bolt
and but not to compare the exact strain values because of the difference in positions of load cell
and nodal locations where data are extracted. From Figure 2.23, it can be seen that the behavior
of bolt in FE model for higher preloading conditions (300lbf, 500lbf and 700lbf) are in good
compliance with the experimental results.

Figure 2.21 Percentage of error in aluminum/aluminum bolt joint strength from finite
element predictions by comparing against experimental results.

Figure 2.22 Schematic represent of load cell location in experimental method &
corresponding section where nodal values are extracted from finite element model.
30

Figure 2.23 Comparison of preloading effect on aluminum/aluminum bolted joint from
experimental and finite element results for varying preloads: 0lbf, 100lbf, 300lbf, 500lbf
and 700lbf.

2.4.3.3 Effect of slip from numerical model
Though the effect of slipping is not so evident from the transmission bar, it can be studied in
detail from the model itself. All the information presented here in this session is for joint
assembly clamped at 500lbf. Contact region of the bolted joint is shown in Figure 2.24.
31

From the FE model, slipping event can be classified as two phases: (a) Preliminary slip and (b)
Secondary slip (Figure 2.25). Before slipping occurs, there is no relative displacement between
the faying surfaces as the loading pulse just reaches the joint assembly at 315μs which is referred
as initial loading. In the first phase of slip, the pulse reaches halfway through the specimen (at
350μs). It initiates the movement of the contact surfaces between the metals. The load
transference is entirely by the friction between the metal surfaces but still there is no contact
between the bolt and the adherend. This phase extends till the adherend metal displaces to make
contact with the bolt surface.

Figure 2.24 Schematic represent of contact zone on the metal constituents.

In the second phase, the load transference by the friction exceeds and bolt-metal contact is
achieved at the bay region I as shown in Figure 2.26 (a) at 380μs. From this point, bolt slips
further to close the bolt-hole clearance without the requirement of additional loading. At the end
of this phase (at 401μs), bolt-metal contact is attained at bay region I and III as shown in Figure
2.26 (b). From this point, the majority of the load transference is through the bolt. From the
numerical model, the total time period of slipping is calculated to be 51μs within the specimen.
This observation can also be interpreted form the strain history of the bolt as shown in Figure
2.27.

32

Figure 2.25 Different phases on slipping event in aluminum/aluminum bolted joint
preloaded at 500lbf subjected to high loading rates.

Figure 2.26 Representation of bolt-metal contact during the event of slipping (a) at region I
and (b) at region I and III.

33

Figure 2.27 Correlation of strain-time history and bolt slip on aluminum/aluminum bolted
joint preloaded at 500lbf from finite element predictions.

2.4.3.4 FE results for tension test
Numerical analysis is performed for pull-off loading in SHPB for the specimen series S2 and S3
as mention in Table 2-2. The main objective of this study is to determine the stress concentrated
areas by visualizing Von Mises stress distribution and evaluating failure modes for different
bolted joint configurations. Influence of e/d and w/d ratio in the failure mode of the bolted joints
is investigated. Bearing strength and failure modes of joint assembly for varying e/d ratios are
presented in Figure 2.28 and Figure 2.29 respectively.
From Figure 2.28 and Figure 2.29, it can be seen that when we decrease the edge distance, the
failure mode changes from bearing to mixed mode of failures provided the w/d ratio maintained
in asymptotic region. The transition region for this type of joint assembly lie around e/d = 3 for
high strain rate loadings. Shear out failure, which is often considered to be dangerous to the
structural joint is observed for only e/d=1.

34

Figure 2.28 Variation of bearing strength of aluminum/aluminum bolted joint under
tension loading for different e/d ratios from finite element analysis.

Figure 2.29 Failure mode predictions of aluminum/aluminum bolted joint under tension
loading for different e/d ratios from finite element analysis.

Bearing strength and failure modes of joint assembly for varying w/d ratios are presented in
Figure 2.30 and Figure 2.31 respectively. With reference to the aforementioned plots: as the
35

width of the joint decreases (by keeping e/d ratio in asymptotic region), the predominant failure
mode changes from bearing to net tension. This transition point noticeably occurs at w/d = 2.828.

Figure 2.30 Variation of bearing strength of aluminum/aluminum bolted joint under
tension loading for different w/d ratios from finite element analysis.

Figure 2.31 Failure mode predictions of aluminum/aluminum bolted joint under tension
loading for different w/d ratios from finite element analysis.
36

2.5 Limitation
The finite element model is limited for higher preloading conditions. Also, only few parameters
are investigated from the numerical model: strength of the joint, preloading effect, effect of
slipping and the failure modes. The other influential factors such as contact stresses between the
faying surfaces, secondary bending of the joint, etc., are not explored.
2.6 Conclusion
In this study, structural behavior of single-lap metal-metal bolted joint is investigated for high
strain rate loading conditions. Experiments are carried out for compression loading in SHPB.
Refined 3D finite element model is developed using ABAQUS tool with complete experimental
setup which successfully predicts the behavior of joints for varying preloading conditions.
Preloads are applied in static (standard) solver and successfully imported in explicit solver for
analyzing the impact behavior of the joint. The results are in good compliance for higher torque
tightened cases. The study demonstrates that the joint slip, loading rate and bearing strength of
the bolted joints are more influenced by bolt clamping force subjected to impact loadings. The
torque applied to the bolts causes the compression on the metal adherends which increases the
frictional force between the faying surfaces. For time-dependent loading condition, this friction
is directly related to the loading rate and joint slip.
The validated FE model is further employed to investigate the joint failure modes for tension
loading. Bearing strength and failure modes are presented for varying geometrical
configurations. It shows that the failure mode of the bolted joints is entirely relying on joint
geometry. From this study, it is strongly recommended not to use e/d ≤ 3 and w/d ≤ 2.828 for
bolted joints subjected to high strain rate loadings.

37

REFERENCES

38

REFERENCES

[1] E. Godwin and F. Mathews, "A review of the strength of joints in fibre–reinforced," Compos
A Appl Sci Manuf, p. 11(3):155–60, 1980.

[2] G. Kretsis and F. Matthews, "The strength of bolted joints in glass fibre/epoxy laminates,"
Compos A Appl Sci Manuf, p. 16(2):92–102, 1985.
[3] P. Smith, K. Pascoe, C. Polak and D. Stroud, "The behavior of single-lap bolted joints in,"
Compos Struct, p. 6(1–3):41–55, 1986.
[4] A. VanderKlok, A. Dutta and A. Tekalur, "Metal to composite bolted joint behavior
evaluated at impact rates of loading," Composite Structures, vol. 106, p. 446–452, 2013.
[5] Y. Yan, W.-D. Wen, F.-K. Chang and et al, "Experimental study on clamping effects on the
tensile strength of composite plates with a bolt-filled hole," Compos: Part A, p. 1215–
1229, 1999.
[6] H.-S. Wang, C.-L. Hung and F.-K. Chang, "Bearing Failure of Bolted Composite Joints. Part
I: Experimental Characterization," Journal of Composite Materials, 1996, p.1284-1313.
[7] H. Kolsky, "An Investigation of the Mechanical Properties of Materials at very High Rates of
Loading," Proc. Phys. Soc, pp. 676-700, 1949.
[8] C. T. McCarthy, M. A. McCarthy, W. F. Stanley and V. P. Lawlor, "Experiences with
Modeling Friction in Composite Bolted Joints," Journal of Composite Materials, vol. 39,
p. 1881–1908, 2005.
[9] Zhou S, Wang Z, Wu X and Zhou J, "Experimental and numerical investigation on bolted
composite joint made by vacuum assisted resin injection," Compos Part B: Eng, p.
45(1):1620–8, 2013.

39

Chapter 3 Evaluating Hybrid Joint Behavior at Intermediate Strain Rates
3.1 Abstract
The importance of characterizing light-weight composite materials increases with increase in
implementation of composites in vehicle applications. In this study, the structural responses of
metal/composite joints (dissimilar material joints) under low and intermediate loading rates are
investigated in detail. Low velocity impact experiments are carried out on single-lap, single-bolt
Aluminum/S2 glass-SC 15 composite joints subjected to tension loading in instrumented Drop
Weight Impact Testing Machine (DWITM). Drop weight impact testing system is fundamentally
configured to simulate low velocity compressions which are comparable to automobile crashes.
However, tensile loading is induced in DWITM by a special kind of fixture designed and
developed to convert the loading from compression to tension. The effect of geometrical
parameters on bolted assembly is investigated for varying edge-to-bolt diameter ratio (e/d). The
study is further extended to evaluate the strength and failure modes of hybrid (bolted and
bonded) and adhesive joints. Experimental observation shows that the strength of the hybrid joint
is much higher than its bolted counterpart which is dominated by the load carrying capacity of
the bonded member. Also, the effect of hole-clearances, influence of adherend members and
environmental conditions on hybrid joint behaviors are analyzed in detailed.
3.2 Introduction
The use of advanced fiber reinforced composite materials for engineering structures in
aerospace, marine and automobiles has been increasing in recent years because of its excellent
mechanical properties and lighter weight. In structural assembly, joining composite materials to
other components are frequent and are often considered to be critical. A detail understanding of

40

joining techniques is therefore essential in composite assembly to prevent the catastrophic failure
of the structure during services.
3.2.1 Joining methods
Traditionally, joining two or more structural elements are achieved by mechanical fastening or
adhesively bonding technique that has its own advantages and disadvantages. Mechanical
fastening (uses bolts or rivets) method is considered to be the most ideal choice for joining
composite members due to its relative ease of installation and disassembly for inspection.
However, drilling holes in laminates may potentially damage the orientation of fibers in
composite configuration which requires specific attention [1], [2]. In addition to that, composite
materials are considerably brittle when compared to metals and the development of higher stress
regions at the bolt-hole surroundings significantly reduces the overall strength of the structure
[3]. On the other hand, adhesively bonded joints are proved to have exceptional static and fatigue
properties due to the absence of bolt-hole regions [4]. In this type of joining method, continuous
load transference can be achievable along the overlap length and hence considered to be the most
efficient joining method when compared to bolting [4]. On the contrary, it is highly impossible to
install adhesively bonded joints in critical structures due to the difficulty in detecting the onset of
failure within the adhesive layer. Also, the strength of the adhesively bonded joint is hugely
affected by the degradation of adhesive surface [5] within the bonding area.
Therefore, hybrid joints are developed in order to overcome the disadvantages of the
aforementioned joining techniques [6]. In hybrid joints, combination of mechanical bolts and
adhesives are employed together to reinforce the load carrying capability of the structural
assembly. In most practical applications, hybrid joints are used as a fail-safe technique, where if
one joining member fails, load can be taken over by the other member [7]. Also, it has been

41

shown that the inclusion of bolt in hybrid joint helps in enabling additional load transference by
the damaged adhesive bond by keeping any rupture to progress further [6], [7]. Thereby, the
catastrophic failure of the structure is prevented and enhances the long term performances of the
joint [7].
3.2.2 Testing methods
Understanding the behavior of joints under different loading rates is essential to optimize the
design and installation of joints in any structural members. Standard testing machines are
available to execute quasi-static analysis to predict the strength of the joints at very low strain
rates (1 s-1). To investigate the behavior of joints at higher strain rates (10 2 s-1 to 104 s-1), Split
Hopkinson Pressure Bar (SHPB) is generally used for both compression and tension. However, it
is extremely difficult to study the response of joints in intermediate strain rate loadings (10 0 s-1 to
102 s-1). A few experimental investigations have been carried out in drop tower testing machine
to characterize materials for intermediate rates of loading under compression [8], [9]. However,
performing tension test on dissimilar joints using drop tower is very challenging.
Previous works on composite joints are mainly focused on strength predictions on bolted
assembly subjected to static loading conditions. Smith et al. [10] observed the effects of the
geometrical parameters such as width and edge distance on ultimate strength and failure
mechanisms. Experiments are carried out by the authors to predict the strengths and failure
modes of single-lap bolted joints and compared with double lap joint configuration using CFRP
laminates. Based on theoretical approaches, Hart-Smith [3] predicted the substantial benefits of
using hybrid joints on improperly bonded structures. Kelly [11] carried out experimental and
numerical investigation with carbon-fiber reinforced plastic adherends to compare the
mechanical response of bolted, bonded and hybrid joints under static loading conditions. Kelly

42

[12] also studied the distribution of load in hybrid joints and investigated the effect of adhesive
stiffness on load sharing capability of the bolt.
3.2.3 Failure modes on composite joints observed from static testing

Figure 3.1 Typical failure modes observed in single-lap, single-bolt composite joints loaded
at the hole: (a) shear-out (b) cleavage (c) net tension and (d) bearing (Reproduced from
Figure 2.1).

Failure modes are experimentally investigated by number of authors under static loading
conditions [10], [13]. As demonstrated in Figure 3.1, typical failure modes observed in singlelap, single-bolt composite joints under uniaxial tension load are classified as four types: shearout, cleavage, net tension and bearing failure [10],[14]. All failure modes should be taken into
consideration in designing structural joints. Shear-out, cleavage and net-tension failure modes
are catastrophic and detrimental that has to be avoided in engineering structures. However,
bearing failure is the preferred mode in mechanically fastened composite joints as it can
withstand loads beyond the onset of failure.

43

Few research works are published on bolted joints subjected to high loading rates [15]. However,
there is not much information available for joints under intermediate loading rate conditions. In
this study, the main objective is to predict the strength and failure modes of hybrid joint
configurations under low velocity tensile loading, with the focus on optimizing joint design for
vehicle applications. Impact testing is performed at room temperature in standard DWITM to
predict the strength of the joint and the results are compared against its static responses. Also, a
series of bolted and hybrid joints are tested at 60◦C to predict the strength degradation at elevated
temperature.
3.3 Experimental work
3.3.1 Static Testing Method
Static tests are conducted at room temperature using MTS 810 Material Test System. The test
specimens are tabbed in order to maintain axial alignment and clamped with 5 MPa gripping
pressure. Tensile force is applied to the bottom of the specimen with the displacement control
rate of 1 mm/min. To observe the failure mechanism, each joint specimen is loaded to its
ultimate failure load and the load-displacement responses are recorded. Videos and images are
captured using phantom camera which is used to examine the failure process of tested
specimens.
3.3.2 Low velocity Impact Testing Method
Drop weight tension testing is performed using Instron Dynatup 9250HV. This conventional
impact testing machine is mainly used to generate intermediate strain rate loadings in lab
environment which are comparable to actual automobile crashes. Impact on the sample can be
achieved by raising the mass of the system to a specific height and dropping it vertically on to
the testing specimen.

44

Figure 3.2 Drop tower test setup (left), fixture setup with holding specimen (middle) and
joint specimen (right).

Along with variable impact height, the tower is also capable of varying mass, velocity and
impact energy. The movement of the suspended mass is channeled by the columns that are held
in position as shown in Figure 3.2. As the name signifies, DWITM are originally configured to
generate compression loading. In the current study, this instrumented loading device is used to
induce tension load by manufacturing a special type of fixture which can capable of changing the
load from compression to tension. The schematic representation of the tensile fixture is shown in
Figure 3.3.
In the fixture, the joint assembly is clamped with the top of the specimen attached to the
stationary base and the bottom of the specimen attached to the sliding platform which takes the
impact, thereby inducing tensile loading on the tested samples. Soft materials, so called pulse
shapers, are used on top of the tensile fixture in order to achieve force equilibrium across the
specimen length. Load sensors are placed at the top and bottom of the specimen which are used

45

to record the force history of the sample. High speed images are taken at the rate of 15000
frames per second using Phantom V12 high speed camera to examine joint responses. Tests at
elevated temperatures are conducted in environmental conditioning unit equipped with Dynatup
9250HV. The specimens are heated to 60◦C in the heat chamber for 15 min prior to testing to
ensure thermal equilibrium.

Figure 3.3 Fixture used to convert compression to tension load in drop tower machine.

46

3.3.3 Specimen configurations and materials
The schematic representation of the tested specimen and joint parameters are shown in the
Figure 3.4. Single-lap, metal-composite joint configurations are tested to investigate the
mechanical response of structural joints under static and intermediate loading rates. In all the
tested specimens, aluminum of thickness 0.125 in. and e/d ratio of 2 is used as the metal
adherend (Figure 3.5). To study the influence of geometrical parameters, S2 glass/SC 15 epoxy
composite laminates of thickness 0.145 in. are used as the composite adherend for varying e/d
ratios from 1.0 to 4.0. The material properties of the composite laminate are shown in Table 3-1.
The efficiency of three different joining methods (bolted, bonded and hybrid) are examined using
aluminum/S2 glass-SC 15 composite joints. For all bolted and hybrid specimens, clamping force
of 40 in.lb is applied to the bolt of diameter 0.25 in. FM-94K film adhesive is used for bonded
and hybrid joints with the bond area of 0.5 in. x 1 in. The material property of the adhesive
member is provided by the manufacturer. Specimens are heated at 3°C/min and cured at 127°C
for 2 hrs. Special fixtures are employed to preserve the alignment of joint and to maintain the
clamping pressure on the joint assembly (Figure 3.6). The behavior of hybrid joints with e/d =1
is compared for three different hole-clearances: (a) tight fit (b) 0.006 in. and (b) 0.03 in. Also, the
effect of composite adherend is investigated by varying the composite material for bolted and
hybrid configurations with e/d = 1 and 4. E-glass plain weave ply with a matrix of black epoxy
resin of thickness 0.094 in. is used for comparative study. Investigation at elevated temperatures
is conducted on aluminum/S2 glass-SC 15 using bolted and hybrid joints with e/d = 1 and 3.

47

Figure 3.4 Hybrid joint specimen configuration used for impact analysis and geometrical
parameters: Front view (shown in left) and side view (shown in right).

Figure 3.5 Bolted joint geometry used for impact analysis.

48

Table 3-1 Material properties of S2 glass-SC 15 epoxy laminates [16].

Figure 3.6 Aluminum/S2 glass-SC 15 hybrid samples placed on the specially designed
fixture to maintain alignment and constant pressure.

3.4 Experimental Results and Discussion
3.4.1 Experimental observations from static analysis
3.4.1.1 Response of bolted joints
The load-displacement curve of the bolted specimen with edge distance 1d is shown in Figure
3.7. Static response of the bolted joint can be classified as four stages: initial loading, slip, post
slip and bearing failure [17]. In stage 1 (AB), linear behavior of the load-displacement curve is
observed which represents the slip resistance caused by the contact force between the adherend
members. Frictional force between the matting surfaces varies with the bolt preload [18].

49

Figure 3.7 Load-displacement curve of Aluminum/S2 glass-SC 15 bolted joint with edge
distance 1d under static loading.

In stage 2 (BC), a constant steady load is observed as the displacement increases. This stage
indicates the initiation of bolt slipping from exceeding frictional force. The length of slipping
entirely depends on the hole clearance and bolt clamping force. In stage 3(CD), non-linear
response of the load is observed which represents the post slip behavior of the joint. Within this
region, the contact between bolt and specimen is achieved that initiates and develops bearing
failure on further loading. The final stage (DE) represents the sudden reduction in load after
exceeding the maximum capacity of the joint.
3.4.1.2 Effect of e/d ratio
To investigate the effect of edge distance in bolted joint configuration, a total of 16 specimens
are tested with varying edge distance from 1d to 4d. Figure 3.8 illustrates the relationship
between the failure force and e/d ratio of the bolted configuration. The error bars indicate 95%
confidence for 4 samples. Results show that the load carrying capacity of the bolted joints is
directly proportional to the edge distance. Also, it is observed that the specimens with higher e/d
50

ratio, requires more time to reach the peak force which is attributed to the material overlap
length. Failure force of the bolted configuration with edge distance 4d is 172% greater than that
of 1d.

Figure 3.8 Failure loads of Aluminum/S2 glass-SC 15 bolted joints with different e/d ratios
under static loading.

Also, with reference to Figure 3.9, it can be seen that the mode of failure changes from shear
out to bearing as the edge distance increases. Based on the observations it is evident that the
change in failure mode has a clear dependency on the edge distance and there exists a transition
region where the bearing mode of failure starts to dominate. The experiments predict that the
transition region for bolted joint configuration lies around e/d = 3.
Many research works has been carried out to investigate the edge effect on composite joints [10],
[15], [19], and majority of authors seem to agree that a certain minimum edge distance
(transition region) is required to reach the dominating mode of failure. However, in designing
structural joints, the transition e/d ratio has to be decided based on additional geometrical
parameters such as width, thickness, and lay-up sequence [14] that needs to be explored in
future.
51

Figure 3.9 Failure modes of Aluminum/S2 glass-SC 15 bolted joints with different e/d ratios
under static loading.

3.4.1.3 Response of bonded joints
Load-displacement curve for bonded joint with bonding area of 0.5 in. x 1 in. is shown in Figure
3.10. Static response of the bonded joint can be classified as two stages. Initial stage (A”B”)
represents the slip resistance of the joint up to the maximum load carrying capacity of the
adhesive member. In bonded joint, slip resistance is increased by the presence of adhesive
member which helps in smooth load transference when compared to bolted configuration.
However, after exceeding its maximum load carrying capacity, bonded joint fails
catastrophically (B”C”). With reference to Figure 3.10, the strength of the bonded joint is found
to be 7.52 kN and it might depend on the stiffness of the adhesive [11]. Further investigation
needs to be done to verify the load carrying capacity of the joint using adhesives of varying
stiffness.

52

Figure 3.10 Load-displacement curve of Aluminum/S2 glass-SC 15 bonded joint with bond
area 0.5 in. x 1 in. under static loading (load bottom to top).
3.4.1.4 Failure modes on bonded joints
The failure modes on bonded joints can be classified as four types corresponding to the
definition shown in Figure 3.11 [20]. Aluminum/S2 glass-SC 15 bonded joints evaluated for
static loading shows (Figure 3.12) a mixed mode of failure which indicates partial adhesive
failure on both adherend members.

Figure 3.11 Typical failure modes definition of an adhesive bonded joint: (a) cohesive
failure, (b) adhesive failure, (c) mixed failure and
(d) adherend failure (Reproduced from [20]).

53

Figure 3.12 Failure modes of Aluminum/S2 glass-SC 15 bonded joints with bond area 0.5
in. x 1 in. under static loading.

3.4.1.5 Response of hybrid joints

Figure 3.13 Load-displacement curve of Aluminum/S2 glass-SC 15 hybrid joint with edge
distance 1d under static loading (load bottom to top).

The load-displacement curve for hybrid joint with edge distance 1d is shown in Figure 3.13.
Similar to the bolted configuration, behavior of hybrid joints are classified as 4 stages [17]. The
first stage (A’B’) indicates the slip resistance of the joint up to the failure of adhesive member.
In this stage, the complete load transference is established by the bonded member without the
54

contribution of frictional force between the contact surfaces. In the second stage (B’C’), sudden
failure of adhesive member is observed followed by the joint slip. The slipping of bolt is not
much evident in hybrid joints. This might be due to the coupled behavior of joint slip and
adhesive failure. In the third stage (C’D’), non-linear behavior of the load is observed up to the
maximum load carrying capacity of the bolt along with the gradual development of bearing
failure. The final stage (DE) represents the sudden failure of the joint specimen.
3.4.1.6 Effect of e/d ratio

Figure 3.14 Failure load predictions of the Aluminum/S2 glass-SC 15 hybrid joints with
different e/d ratios under static loading.

Total of 16 specimens are tested with varying edge distance from 1d to 4d to examine the effect
of edge distance. Figure 3.14 shows the experimental failure loads of the hybrid joints with
different e/d ratios. As can be seen, maximum load carrying capacity of the hybrid joints are
nearly identical for varying edge distances. This is because; the strength of the hybrid joints
depends entirely on adhesive stiffness.
Also, with reference to Figure 3.15, it can be seen that the failure pattern changes from shear
out to bearing as the edge distance increases and reaches a transition region at e/d = 3. Along
55

with the typical failure modes (Figure 3.1), failure of adhesive member contributes mixed mode
of failure (Figure 3.11) in all hybrid configurations as shown in Figure 3.12.

Figure 3.15 Failure modes of Aluminum/S2 glass-SC 15 hybrid joints with different e/d
ratios under static loading.

Figure 3.16 Failure modes of Aluminum/S2 glass-SC 15 hybrid joints with e/d =1 and bond
area of 0.5 in. x 1 in. under static loading.

3.4.1.7 Effect of joint type
Figure 3.17 illustrates the summary of joint failure loads for bolted, bonded and hybrid
configurations. It is evident that the load carrying capacity of the hybrid joints is much higher
than that of the bolted joints for all edge distances. Failure load of hybrid joints with e/d =1 is

56

nearly 3 times higher than that of the bolted joints with similar configuration and is 1.15 times
higher than that of bolted joints with e/d = 4.

Figure 3.17 Failure load comparison of Aluminum/S2 glass-SC 15 bolted, bonded & hybrid
joints under static loading

Also, it can be seen that the failure loads of the hybrid joints are comparable to the ultimate
failure loads of the adhesive joints. However, hybrid joints are considered to be superior to
bonded joints with respect to failure pattern. This can be interpreted from the load-displacement
curves of the aforementioned joints. As can be seen from Figure 3.18, the load distribution in
hybrid joint is typically similar to that of bonded joint before the onset of adhesive failure. Once
the bonded member fails, the behavior of hybrid joint is similar to that of the bolted joint and the
load transference is achieved by the bolted member alone. Joint load is sustained by the bolt
which results in gradual and progressive failure. On the other hand, failure of bonded joints is
catastrophic which has a detrimental effect on any structure and hence needs to be avoided. This

57

sort of hybrid joint behavior is in disagreement with the observations by Kweon et al [20] where
the bearing mode of failure occurs before the adhesive member fails.

Figure 3.18 Force vs. displacement plot of Aluminum/S2 glass-SC 15 joints under static
loading (a) bolted joint with e/d = 1, (b) bonded joint with bond area 0.5 in. x 1 in. and (c)
hybrid joint with e/d =1.

3.4.2 Experimental observations from impact analysis
3.4.2.1 Force equilibrium
During the event of impact loading, maintaining constant force or at least nearly constant force
throughout the specimen is always a challenging task. Failure to attain force equilibrium may
result in an instantaneous and premature damage in the specimen which may invalidate the test
results. Achieving uniform force distribution within the specimen is therefore essential in order
58

to attain gradual and progressive failure. In the current experiment, the direct impact is made on
top surface of the tensile fixture as shown in Figure 3.3. The resulted impact pulse causes the
movable frame to slide down and induces force at the bottom of the specimen. The induced
tensile force will then propagate along the length of the specimen that requires a short period of
time to achieve uniform distribution of force within the specimen.

Table 3-2 Dimensions of pulse shaping materials used to achieve force equilibrium within
the specimen under impact loading.

In realistic conditions, premature fracture on the specimens is observed before it reaches force
equilibrium. This can be eliminated by adding a softer material, called pulse shapers [14]
between the impact tup and the top surface of the fixture. Determining the suitable type of pulse
shaping material is purely a trial and error method.
In this study, force equilibrium is verified for a myriad of different pulse shaping materials but
the details are presented only for four cases. Investigated pulse shaping materials are categorized
from hard to soft rubber material and are listed in Table 3-2. Conventional dog-bone aluminum
sample of thickness 0.125 in. is used to examine the force equilibrium under tensile loading
conditions. Force data acquired by the top and bottom load cells are presented in Figure 3.19 for
four pulse shaping materials. The differences between the peak force from the top and bottom
load cell measurements are significantly large for hard rubber material as shown in Figure 3.19
59

(a). Continuous development in force equilibrium can be seen from Figure 3.19 (b) and (c)
when medium rubber materials are used. On using soft rubber material, about 80% of force
transmission is achieved which demonstrates the uniform distribution (approximately) of force
throughout the length of the specimen (Figure 3.19 (d)). Therefore Latex rubber of thickness
0.25 in. is used for later analysis.

(a)

(b)

(c)

(d)

Figure 3.19 Force histories of dog-bone aluminum specimen from top and bottom load cell
on using different pulse shaping materials: (a) Buna-N-rubber, (b) polyurethane, (c)
polypropylene and (d) latex rubber.

60

3.4.2.2 Response of bolted joints
Impact behavior of bolted joint configuration is illustrated in Figure 3.20. Similar to static
observation, all four phases on joint behavior under intermediate loading rates are evident with a
substantial bolt rotation of about 5° is observed along with bolt slipping as shown in Figure
3.21. The reason could possibly due to the significant bending on the single-lap joints under
impact rates of loading. This issue can be eliminated by using double-lap configurations [1], but
it is virtually impossible to replace every single-lap joints in practical application.

Figure 3.20 Force history of Aluminum/S2 glass-SC 15 bolted joint with edge distance 1d
under impact loading from top load cell (load right to left).

3.4.2.3 Effect of e/d ratio
A total of 12 specimens are tested for varying edges distances from 1d to 4d and the
experimental failure loads are shown in Figure 3.22. The error bars indicate 95% confidence for
3 samples. Results show that the maximum load carrying capacity of the bolted joint increases

61

with edge distance and plateaus out at higher e/d ratios. The failure load of bolted joint with e/d
= 3 is at least twice as high as the joint failure load of e/d = 1. Furthermore, it can be observed
that the load transfer curve tends to elongate with the increasing overlap area as more time is
required to reach complete failure.

Figure 3.21 Bolt transition & rotation of Aluminum/S2 glass-SC 15 bolted joint with edge
distance 1d under impact loading (load right to left).

Also, with reference to Figure 3.23, it is evident that the bearing failure mode starts to occur for
higher e/d dimensions. A significant delamination on the composite materials is observed when
the loading rate is inflicted. This can be attributed to the rate sensitive properties of S2 glass-SC
15 composite materials at which they are loaded. The potential reason could also be due to the
stacking sequence of the laminate [21] that affects the interlaminar shear stresses around the hole
which demands further investigation.

62

Figure 3.22 Failure loads of Aluminum/S2 glass-SC 15 bolted joints with different e/d
ratios under intermediate loading rates.

Figure 3.23 Failure modes of Aluminum/S2 glass-SC 15 bolted joints with different e/d
ratios under impact loading.

63

3.4.2.4 Response of bonded joints
Impact response of the bonded joint with bonding area of 0.5 in. x 1 in. is shown in Figure 3.24.
Unlike static behavior, crack initiation on adherend member is observed prior to the peak load in
A”B” region. The crack initiation in the adherend member can be attributed to the bending of the
joint as illustrated in Figure 3.25. This crack develops further which causes complete adherend
failure in a simply bonded joint. The peak load (B1”) represents the failure of adherend member
and found to be 5.60kN.

Figure 3.24 Force history of Aluminum/S2 glass-SC 15 hybrid joint with edge distance 1d
under impact loading from top load cell (load right to left).

3.4.2.5 Failure modes on bonded joints
Adhesively bonded joints are tested to observe the failure modes under intermediate loading
rates. Among four, three samples exhibit adherend failure (Figure 3.26) which are used to
calculate average joint failure load. However, one specimen failed due to adhesive failure which
is neglected in data analysis. The potential reason for adhesive failure could be due to inefficient
surface preparation.

64

Figure 3.25 Schematic of joint slip (yellow line) and bending (blue line) of Aluminum/S2
glass-SC 15 bonded joint with bond area 0.5 in. x 1 in. under impact loading (load right to
left).

Figure 3.26 Failure modes of Aluminum/S2 glass-SC 15 bonded joints with bond area 0.5in.
x 1in. under impact loading.

65

Contact between the adhesive and adherend members are mainly defined by the surface
treatement. If the process is not done appropriately, adhesion failure could posibly occur which
significantly reduces the load carrying capacity of the joints. In this investigation, all hybrid and
bonded joints are prepared by abrading the adherend surfaces using fine grit sand paper prior to
bonding. To observe the influence of surface treatment, further investigation needs to be done in
future.
3.4.2.6 Response of hybrid joints
The behavior of hybrid joints is classified as four stages (Figure 3.27). In the initial stage
(A1’B1’), slip resistance of the joint is offered by the combination of bolt and adherend member.
This can be explained by comparing its failure pattern with bonded configuration (Figure 3.24).
Unlike bonded joints, there is no delamination in the composite laminates during initial loading.
Also, the presence of bolt prevents the bending on adherend member.

Figure 3.27 Force history of Aluminum/S2 glass-SC 15 hybrid joint with e/d = 1 under
impact loading (load right to left).
66

Partial debonding on adhesive constituent is spotted at first peak (B1’) which is in contrast to the
static observation where complete failure of adhesive member is observed. The second stage
(B1’C1’) indicates the gradual slip on the joint into the bearing damage. The structural integrity
of the joint sustains partially by adhesive and by bolt member in C1’D1’ region followed by the
complete failure of adhesive at D1’. Once adhesive fails, the joint load is sustained by the bolt
member till it reaches its ultimate failure load.
3.4.2.7 Effect of e/d ratio
Figure 3.28 shows the experimental failure loads of the hybrid joints are almost constant for
varying edge distances. Also, with reference to Figure 3.29, it can be seen that the failure mode
changes from shear out to bearing as the edge distance increases with significant inter-laminar
and mixed mode of failure (Figure 3.11). The failure mode ultimately reaches a transition
region at e/d = 3, where the bearing mode of failure starts to dominate.

Figure 3.28 Failure load predictions of the Aluminum/S2 glass-SC 15 hybrid joints with
different e/d ratios under impact loading.

67

Figure 3.29 Failure modes of Aluminum/S2 glass-SC 15 hybrid joints with different e/d
ratios under static loading.

3.4.2.8 Effect of joint type
Figure 3.30 illustrates the summary of joint failure loads of bolted, bonded and hybrid
configurations under intermediate rates of loading. It can be seen that the failure loads of hybrid
joints are significantly higher for lower e/d ratios when compared to bolted joints of similar
dimensions. Howerver, for higher e/d ratios, there is no significant differences in strength is
observed. Also, it can be seen that the load carrying capacity of the hybrid joints with lower e/d
ratios are comparable to bolted joint loads with higher e/d ratios. Failure patterns of hybrid joints
are similar to that of bolted configuration for all e/d ratios except for the increase in damage area
in where bearing mode of failure dominates. This is due to the combined effect of loading rate
and stiffness of the adhesive member. Adhesively bonded joints fail predominantly by adherent
failure. Hence, it will not be logical to compare the ultimate joint failure load of bonded with
bolted or hybrid configurations.

68

Figure 3.30 Failure load comparison of Aluminum/S2 glass-SC 15 bolted, bonded & hybrid
joints under impact loading.

3.4.2.9 Effect of loading rates
Figure 3.31 illustrates the summary of joint failure loads for bolted, bonded and hybrid
configurations under static, intermediate and high loading rates [22]. The increasing trend in
joint failure force with increasing loading rate indicates that the material properties are a function
of loading rates which are in good agreement with the results by Daimaruya et al [23]. For all
loading conditions, hybrid joints exhibit larger load carrying capacity. Under static and
intermediate loading rates, hybrid joint with e/d = 1 has a comparable strength to bolted joint of
higher e/d dimensions. Also, it can be seen that the hybrid joints with e/d = 1 is the least rate
sensitive configuration.

69

Figure 3.31 Failure load comparison of Aluminum/S2 glass-SC 15 bolted, bonded & hybrid
joints under static, intermediate and high loading rates.

3.4.2.10 Effect of hole-clearance
The influence of hole-clearance in hybrid joints is investigated for three cases: tight fit, 0.006 in.
and 0.03 in. As demonstrated in Figure 3.32, increasing the hole clearance does not play a
dominant role on joint failure load. However, the magnitude of bolt slip increases with increase
in bolt-hole clearance. For an efficient joint design, it is essential to define an optimum range for
bolt-hole clearance under impact loading condition. From the experimental observations, 0.03 in.
clearance is believed to be efficient since tight fit is virtually impossible to achieve in practical
scenarios.

70

Figure 3.32 Behavior of Aluminum/S2 glass-SC 15 hybrid joints with e/d = 1 for varying
hole clearances under intermediate loading rates.

3.4.2.11 Effect of environmental conditions
Experiments are conducted for bolted and hybrid joints with e/d = 1 and 3 at 60°C to investigate
the joint sensitivity to environmental temperature. A total of 32 samples are tested and the results
are illustrated in the Figure 3.33. Reduction in load carrying capacity of joints at elevated
temperature is observed for lower e/d ratios. However, the joint failure load is insensitive to
environmental conditions for larger e/d values under intermediate loading rates. Joints with e/d =
1 exhibits shear out failure mode which initiates from exceeding the compressive strength of the
fiber and matrix along the loading direction. On the other hand, failure in higher e/d ratios is not
dominated by matrix properties. Therefore, it is believed that the reduction in ultimate load for
lower edge distances with increase in temperature can be attributed to the degradation of the
matrix strength in composite laminates at elevated temperatures. The operative temperature is in
the range of glass transition temperature (Tg, 95°C [23]) of SC 15 epoxy which causes gradual
softening of matrix constituent that affects the load carrying capacity of the joints. However,
71

glass transition temperature of S2 glass (Tg, 1056°C [24]) is way beyond the operating range
which is not attributed to the deduction in strength. A fairly small difference in hybrid joint
failure load is observed for higher e/d ratios at 60°C, which may due to the softening of the
adhesive with increase in temperature.

Figure 3.33 Failure load comparison of Aluminum/S2 glass-SC 15 bolted & hybrid joints
with e/d = 1 and 3 between room temperature and 60°C.

3.4.2.12 Effect of composite adherend
To investigate the effect of joining substrates, experiments are conducted on single-lap,
aluminum/E-glass ply epoxy resin composites for bolted and hybrid configurations with e/d = 1
and 4. Four joint specimens are tested for each model to determine the average joint failure force.
The bearing strength of the joints is caluclated as the joint failure load divided by the thickness
of the composite laminates times the diameter of the bolt-hole, regardless of the joining method.
This method of comparison is carried out purely for convenient purposes.

72

Figure 3.34 Bearing strength comparison between Aluminum/S2 glass-SC 15 and
Aluminum/E-glass bolted joints with e/d =1 and 4 under intermediate loading rates.

Figure 3.35 Bearing strength comparison between Aluminum/S2 glass-SC 15 and
Aluminum/E-glass hybrid joints with e/d =1 and 4 under intermediate loading rates.

With reference to Figure 3.34, 35% increase in the bearing strength of the aluminum/E-glass
bolted joint with e/d = 4 is observed. However, there is no considerable difference in bearing
strength for lower e/d ratios. This can be explained based on the failure patterns on the
corresponding adherends Figure 3.36. From experiments, it is observed that S2-glass-SC 15
composite exhibits significant delamination for higher e/d ratios under impact loading rates.
However, in E-glass specimens no substantial delamination can be seen. Observation of increase
73

in bearing strength in E-glass laminates might be attributed to the increase in interlaminar shear
strength of the material when compared to S2-glass-SC 15 composites. For the same reason,
aluminum/E-glass hybrid joints exhibits higher bearing strength when compared to
aluminum/S2-glass-SC 15 composite of similar joint configurations as shown in Figure 3.35.

Figure 3.36 Failure mode comparison between Aluminum/S2 glass-SC 15 and
Aluminum/E-glass bolted and hybrid with e/d = 1 and 4 under impact loading rates.

3.5 Conclusion
Quasi-static and low velocity impact tests are conducted on aluminum/s2glass-SC15 single lap
joints. The strength of the joints is evaluated and the failure modes are characterized for three
types of joint configurations: mechanical fastening, bonding, and hybrid joints. It is shown that
the strength of the hybrid joints with shorter edge distances is stronger than the bolted
configuration with large edge distances. Hybrid joint shows improved load carrying capability by
the presence of adhesive member. And on post-adhesive failure; joint load is sustained by the
bolt thereby prevents catastrophic failure of the joint. Modes of failure for both bolted and hybrid
joints show asymptotic behavior at e/d =3 where bearing mode of failure dominates. Significant
74

delamination on joints under impact rates of loading is observed which is attributed to the rate
sensitivity of the material. At elevated temperatures (60°C), there is a 25-35% reduction of
strength for joints with smaller edge distances while joint strength for large edge distances is
invariant with respect to environmental conditions attributed to the gradual softening of matrix
constituent in composite laminates. The effects of hole clearance, and adherend material are also
identified as critical parameters which should be considered on designing of joints that may
undergo impact rates of loading.

75

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76

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[22] D. Gonzalez, “Mechanical behavior of metal-composite joints under impact rates of
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Chapter 4 Conclusions and Recommendations
Joining of two structural members is commonly achieved by bolting, bonding, or a combination
of the two methods (hybrid) in engineering structures such as aircraft, marine and automotive. If
not designed properly, dynamic loading of these joints can be critical to the integrity of the
structure, as failure initiates at fastener-material interfaces. This demands a thorough
understanding of the structural response of joints subjected to various loading conditions.
4.1 High strain rate impact testing
In this study, enhanced three dimensional (3D) finite element (FE) models are developed for
single-lap, single-bolt, metal-metal joints in order to observe their structural behavior when
subjected to impact loading in a Split Hopkinson Pressure Bar (SHPB). FE solutions are
compared against experimental results and are found to be in good agreement for compression
loading in SHPB. The study demonstrates that the joint slip, loading rate and bearing strength of
the bolted joints are more influenced by bolt clamping force subjected to impact loadings. The
torque applied to the bolts causes the compression on the metal adherends which increases the
frictional force between the faying surfaces. For time-dependent loading condition, this friction
is directly related to the loading rate and joint slip. The validated FE model is further employed
to investigate the joint failure modes for tension loading. Bearing strength and failure modes are
presented for varying geometrical configurations. It shows that the failure mode of the bolted
joints is entirely relying on joint geometry. From this study, it is strongly recommended not to
use e/d ≤ 3 and w/d ≤ 2.828 for bolted joints subjected to high strain rate loadings.
4.2 Static and low velocity impact testing
Quasi-static and low velocity impact tests are conducted on aluminum/s2glass-SC15 single lap
joints. The strength of the joints is evaluated and the failure modes are characterized for three

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types of joint configurations: mechanical fastening, bonding, and hybrid joints with various
geometric configurations. Intermediate loading rate impact testing is conducted on a Drop
Weight Impact Testing Machine (DWITM) where tensile loading is applied by a specially
designed fixture. Results show that at room temperature, the strength of the hybrid joints with
shorter edge distances is greater than or comparable to the strength of bolted joints with larger
edge distances. Modes of failure for both bolted and hybrid joints show asymptotic behavior at
e/d =3 where bearing mode of failure dominates. Significant delamination on joints under impact
loading can be attributed to material rate sensitivity. At elevated temperatures (60°C), there is a
25-35% reduction of strength for joints with smaller edge distances while joint strength for large
edge distances is invariant with respect to environmental conditions.
4.3 Recommendations
The developed finite element model can be further employed to understand other influential
parameters such as contact stresses between the faying surfaces and secondary bending of the
joint under impact rates of loading. Also, material constituent on finite element model can be
modified to study the complex behavior of dissimilar material joints.

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