脆性PMMA带板中的动态裂纹传播实验

设计了一种研究裂纹在脆性带板(Strip)中传播行为的实验装置。将矩形无预制裂纹的脆性有机玻璃(PMMA)试件上下边固支,在万能试验机上施加拉伸载荷直到给定幅值,然后在试件侧边中央部位开启一个尖锐裂纹。由于预加的拉伸载荷作用,新开启的裂纹将沿试件中部快速传播,导致整个平板试件断裂。采用印刷在试件表面的导电断裂丝记录裂纹的动态传播历史。设计了一种逻辑电路,可将多达128路电阻丝的断裂信号转换为1路输出信号,使用高速波形记录仪测量该信号,得到裂纹传播距离与时间关系。对不同形状的试件施加不同幅度的预加载荷,完成断裂实验,确定了裂纹的稳定传播速度。通过大量实验,获得了材料中稳定裂纹速度v0与动态裂纹传播能Gc之间的关系。测试结果表明:PMMA的传播能Gc是传播速度v0的递增函数;当裂纹平均的稳定速度超过某一临界值时,出现明显的速度振荡。

Experiment of Dynamic Crack Propagation in Brittle PMMA Strip

In order to study the dynamic crack propagation in brittle materials, an experimental technique was developed to measure the velocity of crack propagation in a preloaded brittle strip. A rectangular PMMA specimen without any precrack is clamped on upper and bottom sides by two heavy steel fixtures attached to a MTS test machine. When the tensile load attains a prescribed level, a sharp-pointed crack is initiated artificially at the middle of one side of specimen. The crack propagates fast across the specimen due to the tensile preload. Its propagation velocity is measured by equally spaced conductive lines that are printed on the specimen surface before experiment. Preloads with different amplitudes are applied to specimens with different shape in experiment, thus steady propagation speed of crack is determined. Experimental results show that for each preloaded strip, cracks arrive at a steady velocity v₀ after a short acceleration stage. The steady velocity of crack propagation is found to be an increasing function of energy G_c stored in the preloaded strip. This represents velocity-toughening effect of PMMA material. Obvious crack speed oscillation occurs when the average steady velocity exceeds certain critical level.