Short- to long-term deformation behavior, failure, and service life of amorphous polymers under cyclic torsional and multiaxial loadings

When investigating materials to be utilized, attention inevitably focuses on their resistance over the service life. Despite the popularity of amorphous polymers, ranging from their applications in structural components to their ability to increase the toughness of biocomposites, the investigation of their short- to long-term resistance has been considerably limited to date. Here, an improved testing equipment and model are proposed to describe the resistance of amorphous polymers under cyclic loadings. Two failure mechanisms are considered for the low- to high-cycle regimes: plastically induced and fatigue, and a history dependent fatigue damage model and a plastic evolution law with a relaxed shear resistance are proposed. Against state-of-the-art models, the proposed model is able to simulate the experimentally observed ultralow- to highcycle failure and service life under torsional and multiaxial loads. The experimental and model results are similar, suggesting that the model is a capable tool for simulating costly and timeconsuming tests. Interestingly, the predicted progress of material failure with plastic deformation was found to resemble the observed development of accumulated void volume. The failure (void volume) development strongly influenced the onset and growth of tertiary cyclic creep and thus, the entire service life. The amorphous structure also appeared to effectively resist failure under torsion.