Micromechanisms of slow crack growth in polyethylene

Notched specimens of polyethylene (PE) have been subjected to constant and cyclic loading at 80 Cdegrees, and their microde formation behaviour investigated by OM, SEM and TEM. Under constant loading, a transition from full ligament yielding to slow crack growth (SCG) was observed as the stress intensity factor, K, decreased, reflected by a macroscopic ductile-brittle transition with decreasing applied load. SCG was characterized by formation of a wedge-shaped crack tip deformation zone, whose internal structure became progressively finer as K decreased further. The behaviour of relatively SCG resistant third and second generation grades of PE in the low K limit was inferred from TEM of specimens subjected to accelerated testing in Igepal(TM) to be breakdown of diffuse zones of interlamellar voiding rather than development of a mature fibrillar structure. This latter failure mode gave smooth fracture surfaces, similar to those observed under conditions, which have been linked to a transition from discontinuous (stick-slip) crack growth to continuous crack growth with decreasing peak K. TEM again indicated the smooth fracture surfaces obtained under low level cyclic loading conditions to be associated with breakdown of regions of interlamellar voiding, suggesting the micromechanisms of failure to be similar in both types of accelerated test. On the other hand, at higher K, crack advance under dynamic loading led to more extensive fibrillar retraction than for static loading.