I am trying to model a device used for absorbing the energy of a falling object until the object stops. The system requires that the device hold steady under a tensile load up until a critical load is reached (say 100 lbs). Once this critical load is reached, the plates begin to slip past each other with a resistance of 100lbs.

As an example of the devices operation, if a rope was tied to each end of the device and  two people were to pull on the ropes with the device in the middle, the system would remain motionless until the 100lb limit is reached and then the device would begin to extend so that the tension in the ropes would never exceed 100lbs.

I have tried modeling this using dampers, springs but was unsuccessful. The dampers and springs cannot be set to release at a constant value, they are  functions of velocity and distance, respectively. I have attempted to use force functions and friction between the plates but this seems to be very unstable.

I would appreciate any ideas to model this problem.