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Limitations of Penalty Joint Treatment in LS-DYNA


LS-DYNA supports various joint definition types such as spherical, cylindrical, etc (please refer to the LS-DYNA User’s manual for a complete list). Irrespective of the joint definition type and the elements associated, translational constraints are applied to the joint nodes to model appropriate behavior. The constraints are applied using the default penalty formulation whose stiffness depends on the maximum frequency of all joints in the model, timestep and the timestep-scale-factor (TSSFAC). The default penalty formulation computes the necessary forces in an attempt to model the joint behavior based on the joint type but in some instances the forces generated by the penalty treatment may be insufficient to maintain the constraint. As a good practice, it is always recommended to monitor the relative displacements between the joint nodes to see if there is any separation which would indicate a insufficient forces being generated by the penalty treatment. There are two ways to fix if there is some relative displacement between the nodes. The first approach would be to increase the penalty scalefactor RPS in *CONTRAINED_JOINT keyword whose value is based on trial-and-error appraoch. The second approach is to switch the penalty formulation to an implicit Lagrange-Multiplier formulation which computes the exact force to enforce the joint constraint. Beginning 971 and onwards, this option is now available for both SMP and MPP versions of LS-DYNA and can be invoked using the *CONTROL_RIGID keyword using the parameter LMF.

  • V S N Ganeswara Rao T says:

    if we change the formulation from penalty to implicit Lagrange-multiplier through LMF card in *CONTROL_RIGID, it will apply this approach to all the joints in the model. if we want this to be changed to a particular joint, what should be the approach?

  • Suri Bala says:

    Ganeswara Rao,

    Currently this option works for all joints or none.

    Suri

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