Effects of Precracking Methods and Specimen Width on Fracture Toughness of Plastics

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The method of introduction of the precrack in fracture toughness testing of plastics has been shown to have a significant effect on the measured value of fracture toughness. In this paper, fracture toughness tests of PMMA (Polymethylmethacrylate) and PC (Polycarbonate) have been carried out by CT specimens with two kinds of precracks, fatigue precracking and razor blade slit recommended in ASTM standard D 5045. It was found that the average value of the plane strain fracture toughness, KIc of the razor blade slit specimens was 45% higher than that of the fatigue precracking specimens in the case of PMMA and the former approached to the latter when SR heat treatment was conducted after the razor blade slit. Furthermore, the effect of uncracked ligament, W-a (W : specimen width, a : crack length) of 3 point bend specimens on KIc for PC has been investigated experimentally and analytically. 3-dimensional elastic-plastic finite element analysis of the specimens with various uncracked ligaments has been performed by using a stress-strain curve of PC, in which nonlinear stress-strain behavior yields before the yield stress and strain softening yields after that. It was found that KIc could be obtained even for PC if the size criterion of ASTM D 5045, W-a > 2.5(KQ/σY)2 was satisfied. However, yielding zone at the crack tip of plastics is larger than that of metals when the value of (KQ/σY)2 of both materials is same. Therefore, it is recommended that a larger specimen of 2≤W/B≤4 recognized in ASTM D 5045 is used when the value of 2.5(KQ/σY)2 is nearly equal to W-a.

Effects of Precracking Methods and Specimen Width on Fracture Toughness of Plastics

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