Crack-wave interaction is common in blast, producing mixed-mode cracks when stress waves are oblique to cracks. How to measure mixed-mode crack-tip stress under blast stress waves is difficult. To this end, optical caustics method was employed to study a running crack in a PMMA plate subjected to obliquely incident blast stress waves, by which stress problem was transformed into optical problem. Incident P and S waves, as well as reflected waves, were visualized as fringe patterns, and mixed-mode crack-tip stress was represented by a caustics pattern (shadow spot). However, mixed-mode caustics patterns under P waves were not consistent with the classical interpretation, hence a modified mixed-mode caustics interpretation was proposed and then verified by P-wave compression and tension loading cases. In modified interpretation, an iteration procedure was proposed to obtain modified stress intensity factors, i.e. K_I and K_II, and crack-tip positions which were more precise than those by the classical interpretation. P-wave compression phase decreased K_I and crack velocity sharply but produced the largest K_II, while the following P-wave tension phase had an opposite effect. S waves and reflected waves produced higher K_I and crack velocity with lower K_II. K_II and crack velocity histories explained curved crack path and microscopic fracture surface. Finally, the reason for the success of the modified interpretation was discussed, and it is indicated that transient stress superposition and delayed redistribution in the crack tip are the mechanism underlying the modified interpretation.

裂纹波相互作用在爆破中很常见，当应力波倾斜于裂纹时会产生混合模式裂纹。爆炸应力波下如何测量混合模式裂纹尖端应力是困难的。为此，采用光学腐蚀法研究了在倾斜入射爆炸应力波作用下的PMMA板中的运行裂纹，从而将应力问题转化为光学问题。入射的P波和S波以及反射波均显示为条纹图案，并且混合模式的裂纹尖端应力由焦散图案（阴影点）表示。然而，P波下的混合模式焦散模式与经典解释不符，因此提出了一种改进的混合模式焦散解释，然后通过P波压缩和拉力情况进行了验证。在修改的解释中，K _I和K _II，以及裂纹尖端的位置，比经典解释的精度更高。P波压缩相使K _I和裂纹速度急剧降低，但产生了最大的K _II，而随后的P波拉伸相却产生相反的作用。S波和反射波产生较高的K _I和较低的K _II裂纹速度。ķ _II裂纹速度历史解释了弯曲的裂纹路径和微观断裂面。最后，讨论了改进解释的成功原因，并指出裂纹尖端的瞬态应力叠加和延迟再分布是改进解释的基础机制。