起裂处缺陷形状对PMMA试件动态断裂影响效应研究

Study on the effect of crack initiation shape on dynamic fracture of PMMA specimen

为研究冲击荷载下起裂处不同形状缺陷对脆性材料动态断裂的影响效应,选用PMMA(聚甲基丙烯酸甲酯)作为实验对象,落锤为加载源,将预制缺陷形状设定为唯一变量,同时保持缺陷在落锤加载方向的长度不变,使用新型数字激光动态焦散线实验系统,对PMMA试件进行冲击三点弯实验,同时对试件的破坏形态、时程特征、动态应力强度因子以及裂纹扩展速度进行分析。实验结果表明:除正方形缺陷(两应力集中点)试件外,其他缺陷形状试件均以I型断裂为主;一应力集中点缺陷试件最早发生破坏,两应力集中点缺陷试件次之,无应力集中点试件最晚发生破坏;一应力集中点对照组中,三角形缺陷试件发生破坏所需能量汇聚时间短于竖线型缺陷试件;试件起裂速度与能量汇聚时间呈正相关,起裂时间越早的试件起裂速度越小。由此得到结论:起裂端缺陷形状对板件的破坏形态及非缺陷周边的动态力学性能无明显影响,但会通过影响板件的起裂时动态强度因子来改变板件的起裂时间和峰值速度。

In order to study the effect of different shape defects on the dynamic fracture of brittle materials under impact load,choose PMMA(polymethyl methacrylate)as the experimental object,drop hammer as the loading source,at the same time,the length of the defect in the loading direction of falling weight is kept unchanged,using a new type of digital laser dynamic caustics experimental system,carry out the impact three-point bending test on PMMA specimens.At the same time,analyses the failure mode,time history characteristics,dynamic stress intensity factor and crack growth rate of the specimens.The experimental results show that except for the square defect(two stress concentration points)specimens,the fracture of other defect shape specimens is mainly mode I.The specimen with one stress concentration point is the first to be damaged,the second is the specimen with two stress concentration points,and the last one is the specimen without stress concentration point.In the control group of specimen with one stress concentration point,the energy convergence time of triangular defect specimen is shorter than that of vertical defect specimen,the energy convergence time has a positive correlation on the crack initiation speed.It can be concluded that:the shape of the incision at the initiation end has no significant effect on the failure form of the plate and the dynamic mechanical properties of the non-incision periphery,but the initiation time and peak velocity of the plate will change by affecting the dynamic strength factor at crack initiation of the plate.