Numerical and experimental analysis of strain-induced crystallisation and failure in elastomers under multiaxial loading

• Applicants: Dr.-Ing. Eric Euchler; Professor Dr.-Ing. Markus Kästner
• Subject Area: Mechanics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 517438497

Natural rubber (NR) is relevant for a broad range of technical applications reaching from vehicle tyres to health care products. Among the rubbery polymers, NR is characterised by its outstanding mechanical properties. In particular, it is known for its high tear resistance. This is attributed to a reinforcement that comes along with the formation of crystallites in highly-stretched regions, which is referred to as strain-induced crystallisation (SIC). However, large stretches can also lead to the growth of intrinsic defects, that is cavitation. Accordingly, the mechanical behaviour of NR involves a complex interaction of different physical mechanisms. As a consequence, complicated crack patterns can arise even in case of simple settings. For instance, crack branching and deviation have been observed in single-notched specimens of initially homogeneous material under uniaxial tension. Deformation-induced micro-mechanical mechanisms are subject of current research and important influences have not been experimentally investigated yet. In particular, even though multi-axiality of deformation and hydrostatic stress contributions, respectively, are known to significantly restrain SIC and to promote cavitation, most studies are restricted to homogeneous specimens under uniaxial tension. Mechanical models are limited to either SIC or cavitation, so far. Furthermore, it is not possible to adequately predict the possibly complex crack patterns that can arise in crystallising elastomers, yet. This project aims at developing a coupled model of SIC and crack growth in NR, with the latter possibly promoted by cavitation. With this model, complex crack phenomena shall be analysed, for which it is essential to adequately account for the interaction between the different micro-mechanical phenomena as well as multiaxiality of loading. For this purpose, the research group Kästner will extend existing continuum-mechanical models for SIC and develop a fracture phase-field model, which takes into account that cavitation can promote crack growth and crystallinity can enhance the resistance against fracture. As a basis for the model development, the research group Euchler will analyse deformation-induced morphological changes by means of a comprehensive experimental programme. For instance, biaxial experiments will be performed considering different stretch ratios in order to study the influence of multiaxiality of deformation. Micro-structural evolution will be investigated by means of X-ray scattering methods and microtomography, respectively. Furthermore, full-field measurement of deformation will be used as well as recently proposed energetical approaches for determining crystallinity, which are based on evaluating the temperature field. For the first time, a quantitative validation of these energetical methods against results from X-ray scattering will be performed.

• DFG Programme: Research Grants