CRACK LAYER ANALYSIS OF NONMONOTONIC FATIGUE CRACK-PROPAGATION IN HIGH-DENSITY POLYETHYLENE

The rate of fatigue crack propagation (FCP) in high density polyethylene (HDPE) is a nonmonotonic function of the energy release rate, a phenomenon previously noted for other semicrystalline polymers. Microscopic observations reveal a single craze-like active zone preceding the crack during the initial crack acceleration. Subsequently crack deceleration is associated with a circular active zone. Ultimate failure occurs by crack reacceleration preceded by large scale yielding through an elongated damage zone accompanied by large scale deformation. Thus, FCP involves two mechanisms: quasibrittle and ductile. This anomalous crack propagation behavior is well described by the 'crack layer' (CL) theory.