Hey Chen,
It may be better to go the route of creating multiple skeletal meshes of different body parts(what used to be called a “gore mesh”) and detaching them based on hit location. (There is a pretty good tutorial series on this)
Can you tell me exactly what you want to happen? I need to know the general idea before moving forward. I can only assume that you want to detach the leg and have them fall over. That is different than my initial thinking on this, so we need to get on the same page.
-Matt W.