EFFECT OF STRAIN RATE ON CRAZE YIELDING, SHEAR YIELDING, AND BRITTLE-FRACTURE OF POLYMERS AT 77 DEGREESK

In the reported experiments, the tensile strength of PMMA, polycarbonate (PC), polychlorotrifluoroethylene, and polysulfone was measured in liquid nitrogen. These polymers deformed by crazing which was induced by the liquid nitrogen. The stress versus log strain rate curve was sigmoidal in that its slope increased and the decreased with strain rate. Above a critical strain rate of about 200 min** minus **1, which varied somewhat with the polymer, crazing was not observed with the optical microscope; the behavior became brittle, and the tensile strength became constant. The shear deformation mode of PC was measured under compression and under tension. The compressive strength versus log strain rate was linear throughout the entire range giving a compression shear activation volume of 360 A**3. The shear tensile strength of PC varied only slightly with strain rate when compared to the compressive strength.