Modelling crack tip failure mechanisms in polymers

The concept of representing K _c as a critical stress at a critical distance is described. Fractures stemming from crazes are then considered and the stresses and distances associated with this type of failure are given. The determination of critical distances, and hence stresses, using blunt notches is described and it is found that in some cases crazes are formed, but in others they are not. In these fractures, the stresses are approximately 500 MN m⁻². A model of fatigue cracking which incorporates a stress and distance concept is used to show that at high K _c values craze-type failures result from initially much higher stresses, again about 500 MN m⁻². At low K values, damage appears to occur for stresses around 2000 MN m⁻². It is suggested that primary bond rupture may be involved for the very high values and that the 500 MN m⁻² stress may represent the breaking of van der Waals’ forces to form crazes.