Due to the unique features such as high stretchability, chemical toleration, wear resistance, anti- slipping, and durability; industrial applications of cross-linked polymers have been growing through past decades. Their practical function can cover a broad range of applications from insensitive to sensitive industries such as automotive, medical, mining, and petroleum. In most such cases, the polymeric components are simultaneously exposed to mechanical loads as well as environmental elements like humidity and oxygen. This affair can considerably reduce the nominal service life of the polymers and cause irreversible damage to their functionality. In the literature, this chem- ical/physical phenomenon is generally regarded as the aging mechanism, which is classified into different subcategories based on the source of aging. Thermo-oxidation, hydrolysis, and hygrother- mal are some of the main types of chemical aging in which oxygen, temperature, and water play major roles in the deterioration of polymer matrix.Such classes can be either result of individual aging elements or due to combined synergized ef- fects of the above-mentioned environmental factors. Any type of aging can induce irreversible damages to the polymer matrix, which results in the reduction of materials’ nominal service life. While predicting any of these types of aging is a great interest of researchers in the last few years, there is still a long way to find a comprehensive model which can consider the synergized effects of chemical aging. Here, multi-physics models are devised to describe the solo and combined long-term effects of different environmental elements on the constitutive behavior of cross-linked polymers. These models are relevant for all decay mechanisms formed by the occurrence of two simultaneous micro- mechanisms; (i) formation/reduction of the cross-links, and (ii) chain scission. We modeled each sub-mechanism through a linear kinetic equation which is then coupled into the modular network concept to allow the calculation of synergies and parallel damage accumulation through multiple sources. In this respect, hydrolytic aging is considered to be induced by chain scission and re- duction of the cross-link, while thermal aging is considered as a superposition of two competing effects of cross-link formation and chain scission. Besides, hygrothermal damage can be regarded as a race among three micro-structural phenomena; i) chain scission due to the presence of temperature, ii) reduction of cross-links attributed to the attendance of water, and iii) formation of cross-links as a consequence of oxygen reactions. These proposed models are micro-mechanically based and are mainly relevant to thin samples due to our underlying assumption of homogeneous diffusion of oxygen and water throughout the matrix. These models have been validated against extensive data sets obtained from experiments we specifically designed for concept validation. In view of their interpretation, precision, and deep insight, it provides insight into the nature of damage accumulation. The model is a good choice for advanced implementation in FE applications.
Read
