Abstract

Strain gages and dynamic caustics were used in this experiment to measure the V-notch crack initiation toughness and propagation toughness of polymethyl methacrylate (PMMA) specimens. Furthermore, a method of pasting strain gages on V-notch PMMA specimens for three-point bending test was proposed. The specimens were divided into five groups according to the opening angle (ω), and each group corresponded to two pasting methods: acute-angle pasting method and obtuse-angle pasting method. The results of notch stress intensity factors (NSIFs) obtained by strain gage measurement were consistent with the results calculated using the dynamic caustics method. The concept of the dimensionless notch stress intensity factor (DNSIF) was introduced for a better analysis of the effect of opening angles on cracking. The experimental results revealed that the initiation of V-notch crack in the specimens were inhibited more when ω was larger than 90° than for small opening angles. Under this condition, the specimens displayed higher cracking initiation toughness. Furthermore, the time required for crack initiation was longer, and the cracking was instantaneous. An analysis of the vicinity of the V-notch crack tip showed that with the increase in ω, the stress component decreased, and the rate of decrease became higher. This also explains the phenomenon wherein the specimens transformed from being easy to crack to not being so as ω increased. The experimental results showed that the crack propagation velocity and propagation toughness were also affected by the opening angle of the V-notch crack.

摘要

本实验使用应变计和动态焦散测量聚甲基丙烯酸甲酯 (PMMA) 试样的 V 型缺口裂纹萌生韧性和传播韧性。此外，还提出了一种在 V 型缺口 PMMA 试件上粘贴应变片进行三点弯曲试验的方法。将试件按张角(ω)分为五组，每组对应两种粘贴方式：锐角粘贴法和钝角粘贴法。通过应变计测量获得的缺口应力强度因子 (NSIF) 结果与使用动态焦散法计算的结果一致。引入无量纲缺口应力强度因子 (DNSIF) 的概念是为了更好地分析开口角度对开裂的影响。实验结果表明，当 ω 大于 90° 时，与小张角相比，试样中 V 型缺口裂纹的萌生受到更多抑制。在此条件下，试样表现出较高的开裂起始韧性。此外，裂纹萌生所需的时间较长，开裂是瞬时的。对V型缺口裂纹尖端附近的分析表明，随着ω的增大，应力分量减小，且减小率变大。这也解释了随着 ω 的增大，试件由易开裂转变为不易开裂的现象。实验结果表明，裂纹扩展速度和扩展韧性也受V型缺口裂纹张开角度的影响。