Cohesive zone modelling of crack growth in polymers - Part 1 - Experimental measurement of cohesive law

The initiation, evolution, and breakdown of the craze ahead of the crack ti p is known to control the onset of slow crack growth in polyethylene. Exten sive craze zones and large specimen deformations in tough polymers render c onventional fracture mechanics approaches incapable of achieving a valid to ughness characterisation for these materials. This paper describes a novel experimental method for measuring the local work of separation in tough pol yethylene, using tensile specimens with deep circumferential notches, which provide a high level of constraint. The traction-separation behaviour of t he material within the constrained ligament is measured directly in situ, y ielding a rate dependent cohesive law, which is believed to provide a physi cally based material description of the damage zone that precedes crack gro wth. Measurements indicate that both the local work of separation and the c ohesive strength are strongly rate dependent, allowing quantification of th e fracture behaviour over a wide range of test speeds and discrimination be tween the stress crack performances of different grades of polyethylene.