A crack layer approach to fatigue crack propagation in high density polyethylene

Fatigue crack propagation (FCP) in high density polyethylene (HDPE) is observed to occur with an accompanying layer of damage ahead of the crack tip. The crack layer theory, which accounts for the presence of both the damage and the main crack, is applied to the problem. It is observed that the kinetic behavior of HDPE under fatigue consists of three regions: initial acceleration, constant crack speed (“deceleration”), and reacceleration to failure. Within the first two regions, crack propagation appears “brittle,” while in the third region “ductile” behavior is manifested. Ultimate failure occurs via massive yielding of the unbroken ligament. Two damage mechanisms are found to be responsible for HDPE failure: formation of fibrillated voids and yielding. Both mechanisms are present throughout the entire lifetime of the crack, but the former dominates the “brittle” crack propagation region, while the latter is more prominent in the “ductile.” Throughout the analysis the resistance moment R_t is approximated as the total volume of transformed material associated with crack advance. Crack layer analysis produces a satisfactory fit of the experimental data and yields a specific enthalpy of damage, γ^*, value in the 1–2 cal/g range.