Comparing the long term behaviour of tough polyethylenes by craze testing

摘要

The initiation, growth and final failure of a craze at the site of a flaw is known to precede slow crack growth under low constant load in polyethylene pipe. Most established slow crack growth tests rely on being able to generate high constraint at the crack tip in order to promote damage and micro-voiding. However, for recently developed PE80 and PE100 type grades of polyethylene, such methods cannot achieve constrained brittle fracture due to extensive crack tip blunting and the formation of large crack tip craze zones, thus invalidating the use of a conventional fracture mechanics analysis. An experimental method is described here wherein deep notched tensile specimens are used to analyse craze behaviour in tough polyethylenes under plane strain conditions. Under constant load conditions, stress - time characteristics of the craze provide good discrimination between various grades within acceptable times. Under constant speed conditions, traction - separation properties of the craze have been measured directly, yielding a rate and temperature dependent work of separation(γ) which may be thought of as equivalent to G_c. Rate dependent trends in γ distinguish well between the grades allowing assessment of long term properties. The intrinsic physical justification of the method lies in the measurement and analysis of separation properties locally at the craze interface. A cohesive zone modelling technique using the 'Finite Volume' method is introduced indicating how the results may be applied to the prediction of slow crack growth in other geometries.