Increase in Mass

The use of mass scaling (/DT/NODA/CST) may lead to a mass instability. As a node penetrates, its global stiffness increases (the instantaneous interface stiffness, Kt is added); therefore, its nodal time step decreases. In order to match the minimum time step, Radioss adds the needed mass to the node. Unfortunately, this added mass increases the kinetic energy and the penetration gets bigger.

Figure 1. Effect of Mass Scaling on Interface TYPE7
Unless the interface is able to stop the penetration, the added mass (due to mass scaling) will keep on getting bigger and bigger. Therefore, the computation is likely to stop, since the mass variation may get huge very quickly (few cycles). If this is the case, the interface should be modified:
  • Gap should be increased
  • Initial stiffness can be increased
  • Mesh should be modified to be refined and uniform in the contact zone