Fatigue Acceleration of Slow Crack Growth in Ethylene/Hexene Copolymer Pipes. 1988/1989 Annual Report

摘要

A fatigue accelerated pipe testing method which accurately ranks the crack propagation resistance of medium density polyethylene pipes is described. The crack propagation mechanism within this test is determined by optical and electron microscopy of fracture surfaces and partially cracked specimens. The mechanism gradually progresses from brittle failure, which is characterized by a zone of crazes at the crack tip and a macroscopically flat fracture surface, to a more ductile failure, where large scale material yielding becomes very pronounced. Even though a similar mechanism was observed in both the ethylene/hexene and ethylene/butene copolymer pipes, the Crack Layer Theory could be used to extract a material parameter which differentiated the pipes according to their crack propagation resistance. This parameter, the specific enthalpy of damage, was shown to remain constant under different loading conditions. With the exception of the most ductile case, the dissipation coefficient decreased with increasing load, which is indicative of the higher crack speed at increased loads.