采用动态焦散线实验系统，对有机玻璃（PMMA）在冲击载荷下的Ⅰ型和Ⅰ－Ⅱ混合型裂纹在起裂和扩展时的动态断裂特性进行了研究。结果表明：随着 PMMA 由Ⅰ型断裂转变为Ⅰ－Ⅱ混合型断裂，从落锤作用在试件上到裂纹起裂所需时间不断增加，说明裂纹起裂需要的能量有所增加，同时从裂纹起裂到最终贯通所需时间不断减少，说明裂纹平均扩展速度也不断增大；在Ⅰ型断裂中，PMMA 的断裂韧度 KⅠC为2．04 MN ／m3／2，而在Ⅰ－Ⅱ混合型断裂中，PMMA 的断裂韧度 KⅠC低于Ⅰ型断裂时的断裂韧度 KⅠC，但是 KⅡC有所增大；对于Ⅰ－Ⅱ混合型断裂，PMMA 极限扩展速度约为366 m／s，当达到极限扩展速度后，裂纹尖端出现微裂纹增韧现象，使裂纹的表面能迅速增大，随后裂纹的扩展速度迅速减小。%The initiation and propagation of mode Ⅰ and mixed mode Ⅰ -Ⅱ cracks in Polymethylmethacrylate (PMMA)subjected to impact loading was studied by using a dynamic caustics test system.It was shown that the initiation from pure mode Ⅰ to mixed mode Ⅰ -Ⅱ needs more energy for PMMA,because the time needed for initiation increases. The time from crack initiation to crack arrest decreases,so the crack average propagation velocity increases;the fracture toughness KⅠC of PMMA is 2.04 MN /m3 /2 for mode Ⅰ fracture;for mixed mode Ⅰ -Ⅱ fracture,the fracture toughness KⅠC of PMMA is lower than that of mode Ⅰ fracture,but the fracture toughness KⅡC increases;for mixed mode Ⅰ -Ⅱfracture,the maximum propagation velocity of PMMA is about 366 m /s,then the micro-cracks toughening phenomenon appears at the crack tip,the surface energy of crack increases,and its propagation velocity decreases rapidly.
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