Interpreting instrumented three point bend fracture tests on craze formingpolymers

Tests on three point bend loaded specimens, containing a sharp initial notc h of measured length, are the basis of a standard method (ISO/CD 17 281) fo r 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 abou t how to treat this notch extension have sometimes confused interpretation of the test. Using a quasi-static Dugdale-Barenblatt cohesive zone model, t his paper presents a simple correction to the standard linear elastic analy sis for tests in which a craze length can be measured. The corrected toughn ess results are higher and linearity restrictions on their validity can be significantly relaxed. Results are presented from Charpy type impact fractu re tests on polyethylene. The computed craze stress reveals a craze size de pendence which is thought to reflect a two stage process of craze fibril ex tension.