To trigger breaking chains, collapsing bridges etc. there is a new getAppliedImpulse to measure 'stress' on the constraints. A bridge can be build using 'hinge' constraint, or a bunch of point-to-point constraints.
The stress/appliedImpulse can be read out in the very latest RC3 preview43 build, see the included breaking_chain.blend.
I realize this is an old post, but where could you detect the constraint's id? I am just looking for a place that this could be utilized becasue as far as I can tell the forces applied to the joints are not exposed to use something like this.
As you can tell I added a flag and a float to use for detecting if the joint is breakable and what the break threshold would be. Can someone tell me how to access the world so I can call the function?