Low Cycle Fatigue Tests of Polycarbonate under Push-Pull Loading

Abstract

Experiments on fatigue of polycarbonate subjected to a push-pull loading were carried out at the speed of repetition of 3cpm or lower at room temperature. Careful observations of the processes of the appearance of deformation bands and the fracture were made in conjunction with the measurements of the change of load and hysteresis loop. The fatigue behavior and its fracture surface appearance were studied over a range of strains yielding the specimen lives from 10 to 2×103cycles. The fatigue behavior in the low cyclic-life range of polycarbonate was interpreted by taking into account of the behaviors and mechanical properties in the static tension tests.It has been confirmed that the transition of the fatigue fracture mode from the deformation band type to the brittle fracture type exists and the strain of this transition depends more on the maximum strain than on the strain amplitude. Beyond 4% of the maximum strain, the deformation bands appear at 40-50% of the cyclic-life. As the deformation bands grow, the load per cycle decreases and the hysteresis energy per cycle increases. On the fracture surface of the deformation band type, the plastically deformed zone accompanying numerous microscopic deformation bands is observed. Below 4% of the maximum strain, the appearances of the fatigue surface have a number of features in common with fatigue fractures in metals, and the conditions for the onset of catastrophic failure appear to be in accord with the Griffith criterion for brittle fracture. The fracture surface energy of polycarbonate has been evaluated to be 6.77×105erg/cm2 in the cyclic loading.