Interpreting instrumented three point bend fracture tests on craze forming polymers

Abstract

Tests on three point bend loaded specimens, containing a sharp initial notch of measured length, are the basis of a standard method (ISO/CD 17 281) for measuring the toughness of a plastic under either quasi-static or impact loading. In some polymers the fracture surface reveals that a long, stable, coplanar craze has extended from the notch tip during loading. Doubts about how to treat this notch extension have sometimes confused interpretation of the test. Using a quasi-static Dugdale–Barenblatt cohesive zone model, this paper presents a simple correction to the standard linear elastic analysis for tests in which a craze length can be measured. The corrected toughness results are higher and linearity restrictions on their validity can be significantly relaxed. Results are presented from Charpy type impact fracture tests on polyethylene. The computed craze stress reveals a craze size dependence which is thought to reflect a two stage process of craze fibril extension.