BIAXIAL YIELDING BEHAVIOR OF HIGHLY ORIENTED POLYPROPYLENE TUBE

Polypropylene tubes with highly oriented molecular structures induced by die-drawing have been tested at various axial/hoop stress ratios to study the first quadrant (tensile-tensile) failure surface with particular regard to the effect of draw ratio. The experimental tests have been conducted on a flexible test rig designed for testing flat sheet and tube under different loading conditions whilst controlling the applied strain rate to within 120-200 s(-1). A microcomputer-based system has been developed to control the test and automatically log the data. The experimental results are compared with the predictions of the available anisotropic failure theories. The first-quadrant failure surfaces obtained indicate that the yield behaviour of polypropylene in both isotropic and anisotropic states is hydrostatic-pressure dependent with the degree of pressure dependency increasing with draw ratio. The anisotropic tubes are very weak in the hoop direction compared to the axial direction even when biaxially drawn, and yield by rapid circumferential expansion except when the applied axial stress is very much greater than the hoop stress.