IN SITU CARACTERIZATION OF SLOW CRACK GROWTH IN ETHYLENE-BUTENE COPOLYMERS UNDER FATIGUE

摘要

An original method, utilising a digitised video system, has been implemented in order to monitor and record the crack propagation in axisymmetrical notched samples during fatigue testing. This technique is capable of controlling a fatigue test with a constant stress intensity. Different types of crack propagation were observed according to the level of K~(max): ⅰ) continuous growth at low stress intensity with progressive rupture of microfibrils in the crazed zone at crack tip and, ⅱ) discontinuous growth at high stress intensity with heavy damage ahead of the crack tip and extraction of coarse fibrils. Quantitative measurements show that propagation kinetics is controlled by stress intensity following the Paris law with an exponent close to 4.