Plane strain and plane stress analysis of fatigue crack propagation in medium density polyethylene pipe materials

Abstract

AbstractUsing the Crack Layer Theory, differences in damage formation under different stress states during fatigue crack propagation in an ethylene‐butene copolymer were quantified and compared. Despite having vastly different stress states and crack propagation behaviors, arc specimens (28 mm thick) and single edge notched (SEN) specimens (2 mm thick) were shown to have the same specific enthalpy of damage, ∼300 J/g, a parameter in the Crack Layer Theory that is a measure of the material's intrinsic toughness. Damage in the SEN specimen consisted of crazing the significant material yielding; the latter damage type is associated with plane stress conditions. In the predominantly plane strain arc specimen, material yielding was minimal compared to crazing, the dominant damage form. After measuring these damage forms and applying the Crack Layer Theory, the constancy of the specific enthalpy of damage was established. Also the dissipation coefficient, β, a second parameter of the Crack Layer Theory, was shown to be a process‐dependent parameter, which was inversely proportional to the lifetime of the specimen: βSEN = 4.6 × 10−5, βarc = 1.1 × 10−4, which corresponds to lifetimes of 140,000 and 30,000 cycles to failure, respectively.