Many thermoplastic polymers, even 'normally' tough ones like pipe-grade polyethylenes, revert to a low-toughness brittle fracture mode at high crack speeds (>100 m/s). This mode has recently been attributed to a micron-scale crack tip melting mechanism, which may also control resistance to impact fracture initiation. A test to study rapid crack propagation must provide a long crack path along which a high but defined speed can be sustained. Two methods are reviewed here. The High Speed Double Torsion test generates a curved crack front, which complicates the analysis but promotes plane strain fracture. A hydrostatically-pressurised pipe test is simpler but does not allow crack speed to be controlled. In both tests, increasing temperature causes a brittle-tough transition associated with the formation of shear lips. The results presented here focus on a modified high density polyethylene material.
展开▼
机译：许多热塑性聚合物，甚至是“通常”硬质的粘性级联聚乙烯，恢复到高裂缝速度（> 100m / s）的低韧性脆性断裂模式。该模式最近归因于微米级裂纹尖端熔化机构，其也可能控制抗冲击骨折引发的抗性。研究快速裂纹传播的测试必须提供长裂纹路径，可以持续高但定义的速度。这里审查了两种方法。高速双扭转试验产生弯曲的裂纹前部，其使分析复杂化，但促进平面应变骨折。静电加压的管道测试更简单，但不允许控制裂缝速度。在这两个测试中，增加温度导致与形成剪切嘴唇相关的脆性转变。这里介绍了重点在改性的高密度聚乙烯材料上。
展开▼
