Effects of high humidity and high temperature exposure on polycarbonate (PC) and cellulose-acetate (CA) were investigated. After the specimens were exposed under 85°C 40% or 85°C 85% for 8 days, tension tests and fracture toughness tests were carried out. The fracture behavior of the specimens was studied by fractography using a video microscope. The results were summarized as the tensile strength-failure strain diagram and the fracture toughness-crack extension resistance diagram. The failure strain decreased because a mechanical scratch at the corner of the specimen had grown during the exposure. Specimens which were finshed smoothly after exposure showed an increase in failure strain. The appearance of fracture surface in CA specimens was influenced by the location of initial defects which causes them fracture. The change in tensile strength of PC was small while that of CA decreased with the increase of exposure humidity. For the case of PC, the exposure increased the fracture toughness but diminished the crack extension resistance. For the case of CA, the exposure did not so much influence the fracture toughness while it diminished the crack extension resistance significantly. The difference in fracture behavior between two materials was discussed by use of the crack extended model in an ideally plastic solid.
