The fracture behaviour of polyethylene terephthalate
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Tensile, notched tensile and cleavage tests have been carried out on amorphous and crystalline polyethylene terephthalate of different molecular weights.
At −160°C where the materials are brittle, the brittle strengths of the amorphous materials are slightly molecular-weight-dependent at high and medium molecular weights: the high molecular weight leads to higher strength than the medium. However, the low-molecular-weight material has a very much lower brittle strength. Surface studies suggest that this sudden change is the change from failure from crazes in the high- and medium molecular-weight material to failure from inherent flaws in the low-molecular-weight material. The crystalline material is weaker than the amorphous, failure arising from inherent flaws in every instance.
It was found that the notched tensile and cleavage results represent crack propagation from very blunt cracks with large surface energies. The cleavage results indicate very little difference in the materials at −160°C but substantial differences at +20° C. Crystalline samples have generally consistently higher fracture surface energies than the amorphous The low-temperature results substantiate the role of flaws and crazes indicated in the tensile tests. The 20°C results have been interpreted using tensile data to evaluate the work done to failure at a blunt crack tip.
KeywordsTensile Test Fracture Behaviour Cleavage Result Polyethylene Terephthalate Crystalline Sample
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