HIGH-SPEED DOUBLE TORSION TESTS ON TOUGH POLYMERS .2. NONLINEAR ELASTIC DYNAMIC ANALYSIS

Although the High Speed Double Torsion (HSDT) test readily generates steady rapid crack propagation in tough structural polymers, computation of the dynamic fracture resistance from displacement data using a transient dynamic torsional beam-on-elastic-foundation model leaves considerable scatter in the results. Much of this scatter is here traced to the use of a single high-rate, small strain clastic modulus to model these highly nonlinear materials for generation mode analysis. The use of nonlinear, large-strain dynamic shear stress-strain data, from a test which measures the impedance to HSDT displacement loading of a previously cracked specimen, increases the consistency of fracture resistance data for pipe-grade polyethylenes. For crack velocities approaching the torsional wave speed, consistency is further improved by accounting for energy transmitted to the crack front by axial stresses set up by differential warping. The corrected data show fracture resistance to be almost constant for crack velocities exceeding some critical minimum.