求高人：多损伤起始演化准则对应的总损伤量如何定义衡量？

hopeflydream

以ductile damage 和shear damage为例，如果同一种材料同时使用了这两种损伤起始和演化准则：1）根据Abaqus Analysis User's Manual中20.2.1 Damage and failure for ductile metals小结中说到“The overall damage variable, D, captures the combined effect of all active mechanisms and is computed in terms of the individual damage variables, di, according to a user-specified rule”，不知如何指定“user-specified rule”？2）如果没有指定，系统默认的SDEG是如何计算的？
谢谢各位不吝赐教！