Abstract
The double torsion geometry provides a simple method to determine the stress intensity factor, K c , as a function of the crack velocity, v. The log K c vs log v relationship is a useful quantitative description for investigating the effect of molecular weight (MW) and molecular weight distribution (MWD) on the fracture toughness or the crack opening displacement u c . A theory which explains the sigmoidal shape of the fracture energy, Y c vs MW is formulated on the assumptions that (a) plastic deformation at the crack tip is induced by polymer molecules having MW larger than the critical MW, M c, and (b) craze fibrils are stabilized by molecular entanglements. The MWD has little effect on Y c provided that Y c is expressed in terms of number-average MW, ~ M n . A rapid increase in Y c in the region of MW ⋍ M c is attributed to increase in the entanglement density.
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