PARAM, NLFAT

Bulk Data Entry Forces OptiStruct to run models in which fatigue solutions reference small displacement nonlinear analysis (NLSTAT) subcases.

Parameter Values Description
NLFAT <YES, NO>
Defaults:
  1. If this parameter does not exist in the deck, the default behavior is NO.
  2. If this parameter exists in the deck; however, no value (YES/NO) is specified, then the default behavior is NO.
YES
Forces OptiStruct to run models in which fatigue solutions (using the FATLOAD entry) reference Small Displacement Nonlinear Analysis (NLSTAT) subcases.
NO (Default)
Does not allow NLSTAT subcases to be referenced by the FATLOAD data entry and the solution errors out if such models are run.

Comments

  1. PARAM, NLFAT, YES can be introduced to force OptiStruct to run fatigue analysis based on a nonlinear subcase (ANALYSIS=NLSTAT). Nonlinear analysis results are not recommended for use in a fatigue analysis because nonlinear stresses and strains cannot be scaled and superimposed, however, acceptable results may be generated in the following cases:
    • During a nonlinear run (ANALYSIS=NLSTAT), acceptable results may be generated if a model uses elasto-plastic material and NEUBER correction is turned OFF (Plasticity field on FATPARM Bulk Data Entry). This statement is based on the assumption that nonlinear analysis is run until shakedown is achieved. If strain results from the nonlinear analysis are used prior to shakedown, the fatigue results will most likely be incorrect since the magnitude of plastic strain may not be close to the steady state strain value.
    • Additionally, acceptable results may be generated if contact based nonlinearity exists in the model (ANALYSIS=NLSTAT) and the contact area is not expected to change appreciably as the load increases (for example, surfaces that match from the beginning).

    In both cases, the Scale and Offset fields on the FATLOAD entry should be left blank as the stresses and strains cannot be scaled and superimposed.

    Regardless of the model type, fatigue results based on nonlinear results using PARAM, NLFAT, YES should always be interpreted with caution.