The effects of diffusion on environmental stress crack initiation in PMMA

Slow strain rate tensile tests were conducted on un-notched PMMA samples in a range of liquid environments in order to assess the effects of diffusion on craze formation. The environments were selected to have a spread of dif fusion rates and consisted of water, methanol, propan-2-ol, 355 trimethylhe xanol and ethylene glycol. Absorption tests in these environments showed th at methanol had the fastest diffusion rate and greatest degree of swelling, followed by water. Ethylene glycol and trimethylhexanol showed no weight g ain, and propanol caused a steady dissolution of the PMMA. Tensile tests we re conducted after the samples had been pre-immersed in the liquid environm ents for times ranging from 1 minute to 14 days. These were compared with e quivalent tests in air, with the point at which the environment curve depar ted from the air curve being used to establish where crazing commenced. Tes ts were also conducted in methanol and ethylene glycol at a range of strain rates. It was found that with methanol and to a lesser degree with water a nd trimethylhexanol, crazing was delayed in tests with longer pre-immersion . This is due to mechanisms of flaw blunting and/or the introduction of sur face compressive stresses. Similar effects were seen with the methanol test s at slower testing rates. It was found that these effects can occur very l ocally to the sample surface, where no weight gain could be measured and in the very early stages of craze development. (C) 1998 Kluwer Academic Publi shers.