Fracture toughness sample thickness effect

The important case of specimens with a fixed total thickness was considered in Ref. [40]. There are certain features of crack bifurcation under these conditions, such as that if the sample with a fixed total thickness has too large a number of layers there will be no bifurcation. Layer thickness and composition are important and efficient parameters to control the bifurcation in laminates. The effect is comparable with a crack bridging phenomenon [21], The bifurcation mechanism increases the laminate fracture toughness by approximately 1.5-2 times. 

In the following sections, we shall discuss the effects of molecular weight, crack speed and molecular orientation on effective fracture surface energy, fracture energy and fracture toughness of glassy polymers. Polymethyl methacrylate will be used for illustration. Other variables, e.g., temperature, sampl thickness, etc., have been well discussed in the literature, we do not include them in this section. 

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