Craze initiation criteria during environmental stress cracking of amorphous polymers

A wide range of environmental stress cracking tests was performed to assess more fully the stress/strain/time conditions required for craze initiation under tensile loading. Tests were performed under constant strain rate, co nstant stress rate, constant stress, and constant strain conditions. In eac h instance, tests were conducted both in air and in the active environmenta l liquid. The polymers investigated were all commercial grades of polycarbo nate, poly(vinyl chloride), and polystyrene; the environments used included methanol, propanol, ethylene glycol, and dekalin. Some of the tests were c arried out at elevated temperatures. Departure points were observed in all instances, where the behaviour in the environment deviated from that in air. These points were taken as the poin t of craze initiation. The stress, strain, and time to craze initiation wer e determined in this manner for each test and these values were used to ass ess the validity of several craze initiation criteria, including critical s tress, strain, relaxed strain, and strain energy density. It was found that the critical relaxed strain criterion was the most applicable, apart from cases in which appreciable and relatively rapid diffusion occurred, which p roduced effects that masked the underlying behaviour. Some post-immersion tests were also performed. These tests, which involve a pplication of a creep stress followed by immersion in the environment at a later stage, have been used previously to show that the molecular motion th at precedes craze initiation occurs regardless of the surrounding environme nt. The environment only seems to play a role in determining at what point the craze precursors become true crazes. Further post-immersion tests withi n this study are presented to support this view. PRCPA/1478. (C) 1998 The I nstitute of Materials.