射出成形したPPの微細構造と疲労破壊特性

概要

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Effects of ambient temperatures ranging from -50 to 50℃ on microstructure and fatigue failure were examined for using injection-molded PP sheets. A three-layer structure was formed in the specimen by contact with a cooled mold wall during injection. Higher molecular orientation and hardness were observed in the intermediate layer, which was for med by strong shear under rapid cooling, while lower molecular orientation and a high degree of crystallinity were measured in the core region. These differences in structures and properties between the two layers may cause higher stress under loading, and as a result brittle cracks may be easily formed in the intermediate layer, exerting a large influences on the fatigue lifetime. At temperatures above room temperature, an increase in tan δ (a decrease in E') became more evident on increasing the level of the stress amplitude, and a fracture mode changed from brittle to ductile. At a low temperature of 10℃ and under high stress amplitude, an increase in tan δ was also observed. In this case, however, the tan δ increase may be attributed to internal friction due to inner structural changes under cyclic loading. At much lower temperatures, the value of tan δ clearly decreased, indicating more elastic behavior which was quite different from typical cases.
1997-10-25