Tough polymers crack opening displacement

The presented results and the additional information taken from various references indicate the direct relevance of the size of the network strands for the crack opening displacement and consequently for the toughness of the polymer. In polymers under load, the molecular chains at the tip of the crack break after the deformation zone ahead of the crack has grown to a critical width 5C, that is the crack opening displacement. This value 5C is proportional to the length of the molecular strands of the network and is linked in this way to the molecular structure of the polymer. However, the molecular mechanism for chain breakage in the deformation zone is not known at present. 

A craze at the crack tip in glassy polymer has a crack opening displacement of 4 /rm. Given that the fracture toughness is 1 MNm / and the plane strain modulus is 1.5 GPa, calculate the length of the craze and the craze stress. 

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