Mechanical Degradation and Fatigue Life of Amorphous Polymers

Due to favorable properties (cheap price, easy processing, preeminent combination of toughness and strength, clearness, recyclability etc.), amorphous polymers are widely used in windows, sporting goods, vehicles, aeronautic equipment, electronics, and health technology. However, their applications may suffer from fatigue, when material fails at significantly lower stress levels than under monotonic loading conditions; fatigue loads result in polymer degradation which can affect horrific accidents (e.g., the air disaster of China Airlines Flight 611) and tremendous financial losses. Despite this motivation, fatigue behavior of amorphous polymers has been scarcely investigated so far. In this study, micro-mechanical characteristics of amorphous structure and their influence on macroscopic deformation behavior (ratcheting) and fatigue life are investigated. It was found (SEM results) that polymer degradation is the process of failure (shear banding affecting micro-cracking and fracture) causing finally breakdown of polymer network. The degradation process was very rate sensitive, and the crack initiation phase before rapid rupture of the material encompassed the majority (even 95 %) of the total fatigue life. Certain fracture surfaces showed sharpened protrusions indicating that the separation of the fracture surfaces from each other occurred precisely on those protrusions. The vein-like, cellular, and rippled patterns of shear bands on fracture surfaces increased fracture toughness and thus, fatigue resistance and life.