Tensile testing of Poly(methyl methacrylate)(PMMA) films has been conducted to clarify the effects of humidity history upon their tensile properties. Prior to testing, PMMA film specimens were kept under three different humidity conditions for 3 days to adjust the water content to each condition. The conditions used in this study were 11%, 54% and 98% relative humidity (RH) respectively. The tensile strength of PMMA films at these storage humidities tends to decrease with increasing humidity. The tensile deformation behaviour is ductile at 11% and 54% RH although brittle at 98% RH. When tensile tests were performed for different storage terms at 98% RH using specimens which had been equilibrated to 11% RH before testing, the tensile strength decreased within a short storage term. The tensile deformation behaviour, however, remains ductile. The tensile deformation behaviour drastically changes to a brittle mode after a 3 day storage term at 98% RH. This suggests that the tensile strength is affected by moisture around the specimen surfaces. This is attributed to a reduction in the crazing stress as a result of surface moisture. Brittle failure, however, occurs after a long storage term at the high humidity levels tested. This suggests that the ductile manner is caused by both absorbed water and the moisture around the specimen surfaces.
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