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# Calculating Stiffness for Impact in Motion Simulation

One of the challenges when setting up impact problems in Simulation Motion is to input the impact parameters (Stiffness, Exponent, Damping and Penetration). Since Jordan Tadic, in his first post, came out with some goodies for the SOLIDWORKS user, I figured it was my turn to dig up Santa’s sack a little early and give Simulation users a tool as well.

In particular, we will take a look at what Stiffness means from an impact perspective:

Stiffness is the parameter used to define the compression that occurs when parts contact each other. This is the equivalent of spring stiffness where the force generated is equal to the stiffness times the penetration between the parts. The other aspect of this value is that the stiffness is a function of geometry.

For example, theoretically, for a steel rod, the stiffness k = A*E / L

A is the cross-sectional area
E is the Young’s Modulus
L is the effective length which is free to deform

You can download and use this Excel Sheet for Stiffness Calculation.

Alternative method

An alternative approach would be to use FEA inside SOLIDWORKS Simulation. You can setup a Gap/Contact problem of two blocks with the appropriate materials. Measure the contact force and the displacement at the interacting faces. Then, take an average stiffness = force/displacement and use that in the Impact model definition.

I definitely think it is time to put up the Christmas tree!!