Ductile Failure and Delayed Necking in Polyethylene

Abstract

This chapter investigates the time dependent (delayed) necking under displacement (ramp) and load (creep) control conditions. The results obtained from these experiments are employed to characterize a similar phenomenon, the ductile failure of PE pipes. Ductile failure of a PE pipe manifests itself in appearance of a bulge on the pipe wall (ballooning). The bulge is extended in the longitudinal direction of the pipe and accompanied by significant thinning of the wall. The study of PE ductile failure under sustained hydrostatic pressure tests is a very involving and time consuming process. The estimation of the time to ductile failure on the basis of the test for delayed necking is in a good agreement with the direct observations of PE pipes ductile failure in the sustained hydrostatic pressure test performed in accordance with ISO/TR 9080. The neck formation and propagation constitutes the yielding process in a PE tensile drawing experiment. It is generally accepted that all the modes of fracture in polyethylene are intimately associated with the development of cavitation, drawing, and crazing, or material as a precursor to fracture.