PMMA膨胀环动态拉伸碎裂实验研究

在强动载作用下, 脆性材料的碎裂问题是一个重要的研究课题, 而脆性材料在冲击拉伸载荷下的力学行为的实验研究相对较匮乏. 提出了一种动态拉伸断(碎)裂的液压膨胀环实验技术, 可用于准脆性/脆性材料的动态拉伸. 利用该技术对有机玻璃(PMMA)圆环试件进行了不同膨胀速度下的动态碎裂实验研究. 从回收碎片的断口形貌和碎片内部残余裂纹观察可知试件的破碎由环向拉伸应力造成, 碎片断口处发出的稀疏波会将周围的拉伸应力卸载, 从而抑制其他裂纹的进一步发展. 利用超高速相机记录了试件的膨胀碎裂过程, 利用DISAR激光速度干涉仪获得了试件外表面粒子的径向膨胀速度历史, 通过试件上的应变片获得了试件的应变历史和断裂应变. 实验结果表明: 在拉伸应变率150\mathrm{~}500s^{-1}范围, 材料的动态断裂应变低于准静态加载下的断裂应变, 体现出“动脆”现象; 随着加载应变率的提高, PMMA 材料的碎片尺寸减小; 无量纲化的PMMA圆环的平均碎片尺寸介于韧性碎裂模型和脆性碎裂模型的预测数值之间, 反映出材料的准脆性特性. 

EXPERIMENTAL STUDY OF THE HIGH VELOCITY EXPANSION AND FRAGMENTATION OF PMMA RINGS

The dynamic fracture and fragmentation of brittle solids under impact loading are important research subjects. However, the experimental study on the tensile fracture and fragmentation of brittle solids is relatively limited. A technique using liquid-driving expansion ring setup was developed for the dynamic tensile fracture and fragmentation testing of brittle materials. This technique was used to study the fragmentation properties of PMMA rings at different expansion velocities. From the observations of the fracture morphology and the residual internal cracks of the recovered fragments, it is concluded that the fracture of the rings is caused by the circumferential tensile stress. The unloading stress waves from the fracture points of the fragments inhibit the further development of other cracks close to the fracture points by unloading the tensile stress in the tension regions. The PMMA ring expansion process was captured using ultrahigh speed camera. The specimen surface expansion velocity was measured using laser interference device DISAR (displacement interferometer system for any reflector). The strain history and fracture strain of ring were captured using the strain gauge on the specimen. Preliminary experimental results conducted on PMMA rings show that: (1) In the range of tensile strain rate 150\mathrm{~}500s^{-1}, the dynamic failure strain of PMMA is lower than that under the quasi-static tensile loading, which means that PMMA became brittle under higher strain rate loading; (2) Higher loading rates resulted in the more fragments and the smaller size of the PMMA rings; (3) The “non-dimensional fragment size vs. strain rate” data fall between the theoretical fragmentation predictions for ductile material and brittle material.