Effects of liquid environments on the mechanical fatigue behavior of polyethylene

Environmental stress cracking (ESC) of polyethylene (PE) in the presence of active liquid environments results in fracture under applied stress conditions much lower than for failure in the absence of the ESC agent. However, recent studies have shown that the fatigue lifetime of high density polyethylene (HDPE) shows a marked increase in pure ESC liquids and their aqueous solutions. The aim of this present study is to investigate in more detail the effects of ESC agents on the fatigue behavior of HDPE.; The existence of craze-like voids inside the halo region in the main craze damage zone occurs in both air and ESC environments. Secondary crazes outside of the main damage zone are observed only in the presence of liquid ESC environments. These secondary crazes enhance the fatigue lifetime, since their formation serves as an additional mechanism to dissipate the applied cyclic load. Concurrent plasticization results in a blunted crack tip which decreases fatigue crack propagation (FCP) rates.; SEM analysis offers evidence that Igepal plasticizes the base of fibrillated structure as well as crack tips and broken fibrillated structures, resulting in "dimple-like" structures. The base region contains highly disordered crystalline/amorphous material which is more easily penetrated by the ESC agents. A stress-enhanced diffusion process apparently is operative at higher stress/strain rates resulting in facilitated formation of secondary crazes and blunted crack tip.; In air environment, HDPE exhibits a strong dependence on experimental variables, such as test frequency, waveforms, and stress/strain rates. In contrast, when the samples were tested in Igepal, the same experimental variables did not affect the fatigue testing results at all.; Methyl cellosolve is a more efficient cracking agent than Igepal. Secondary crazes and a blunted crack tip also were observed in the presence of methyl cellosolve. This behavior, which is the same as in the case of Igepal, should result in a slower FCP rate. However, methyl cellosolve plasticizes the bulk sample more uniformly and quickly, which decreases the fatigue lifetime.; Model fitting results indicate that all empirical models do not represent the data satisfactorily, although it is proposed that the crack layer model might be used with sufficient additional data to elucidate the fatigue mechanism.