Ductile-Brittle Transition in Mechanisms of Slow Crack Growth in Engineering Thermoplastics

Abstract

A transition in the mechanism and kinetics of slow crack growth (SCG) is described in this paper. Itresembles well-knownductile-brittle transitionin dynamic impact resistance and referred to asductile-brittle transition of second kind (DBT2). There is acorresponding transition temperature (TT2). A discontinuous, stepwise crack propagation and continuous slow crack growth (SCG) are recordedabove and below TT2, respectively. Optical and SEM observations suggestthat the change of SCG mechanism is closely associated with material ability to form a stable process zone (PZ) in front of growing crack.The Paris-Erdogan equationexpressingthe average crack growth rate as a power functionof stress intensity factorhas noticeably different powerat the temperatures above and below TT2. In addition, the crack growth rate normalized by a characteristic strain rate is orders of magnitude higherat temperatures below TT2 than that above TT2. Such transition in kinetics of SCG has an apparent consequence in duration of SCG stage prior to ultimate failure.The observed TT2 appears to be significantly higher than conventional ductile-brittle transition temperature in dynamic impact.TT2 alsoimplies certain limitations for extrapolation of elevated temperature SCG data to ambient temperature commonly used in temperature accelerated testing.