过载作用下聚乙烯疲劳裂纹扩展行为研究

论文针对中密度聚乙烯材料(MDPE)，采用平板试样进行了I型疲劳裂纹扩展和单次过载下裂纹扩展试验．发现与金属材料类似，单次拉伸过载对聚乙烯(PE)的疲劳裂纹扩展有明显的迟滞作用，降低了裂纹扩展速率．试验还通过变载荷刻线法获取疲劳裂纹扩展前缘的实际形貌和变化规律，对常规变载荷刻线方法进行了调整和验证，其修正方法对高分子材料的疲劳裂纹扩展前缘刻线具有较好的效果．通过观察发现含楔形塑性区的裂尖钝化是裂纹迟滞的主要原因．过载引入的塑性区内残余应力对裂纹迟滞也起了重要作用．论文利用Dugdale模型计算了塑性区尺寸，使用基于残余应力的Wheeler模型对过载迟滞进行了很好的拟合．

Effect of Single Overload on Fatigue Crack Growth in Polyethylene

The fatigue crack growth behavior of Medium Density Polyethylene(MDPE) under constant amplitude load was experimentally investigated based on plate specimens. The effect of additional imposed single overload was also studied. The results showed that the retardation of fatigue crack growth was obvious when the tensile overload was applied, which was similar to metallic materials. An optimized line scribed method by changing the load was developed to obtain the real shape and the change law of the crack tip during the crack growth. The method was successfully applied to describe the shape of crack tip of polymer material during the fatigue crack growth. It was found that the crack blunting due to the wedge-shaped plastic zone is a primary cause to crack retardation. The residual stress introduced by the overload was another reason for the fatigue crack growth retardation. The plastic zone size at the crack tip was calculated by Dugdale theory. Finally, Wheeler model based on the residual stress was successfully used to fit the decreased crack growth rate.