抄録
Many studies on the problem of fracture strength due to the Griffith crack in glassy amorphous polymers have hitherto been presented. The specific value of energy on the fracture surface can be calculated upon the relation between the fracture stress and the size of the Griffith crack. The measurement of the specific value of energy on the fracture surface is shown as extremely high over the theoretical value. This discrepancy is attributed to the energy consumed in a viscous flow in a thin layer of material at the fracture, resulting in molecular orientation such as craze.
In this paper is reported the effect of the craze at the tip of the Griffith crack on the tensile strength of polycarbonate and polystyrene at various strain rates that has been studied. Craze was artificially induced by bringing the material into contact with kerosene, n-pentane, n-hexane and n-heptane as craze accelerators. The results show that the craze at the crack tip has reduced the tensile strength both of polycarbonate and polystyrene at high strain rates where the Griffith theory could be applied. However, the mode of fracture in polycarbonate at low strain rate has changed from brittle to ductile fracture and no effect of craze on the tensile strength has been noticed. The tensile strength of polycarbonate has not been affected by mere absorption of environmental reageants without craze. Conversely, an increase has been observed in polystyrene. It is indicated, in consequence, that almost 50% of the apparent specific value of energy on the fracture surface have been consumed for craze formation at the crack tip prior to fracture both in polycarbonate and in polystyrene.
