You are viewing the documentation for an older COMSOL version. The latest version is available here.
Random Vibration
Use the Random Vibration node to define the random vibration model, the solution field, the fatigue stress measure, and fatigue evaluation parameters for random vibration fatigue evaluation. The Random Vibration feature can be applied to domains, boundaries, edges, or points.
Model Input
See Common Physics Interface and Feature Settings and Nodes for information about the Model Input.
Moment Computation
From the Random vibration model list, select the random vibration model containing the reduced-order model vibration results. From the Stress method list, select the evaluation method for stresses. For Equivalent von Mises stress (EVMS), select the physics interface that was used to compute stresses, from the Physics interface for stresses list. If you select a Shell interface, also select a Through-thickness location — From physics interface, Top, or Bottom. This determines from which position in the shell stresses or strains are taken for fatigue evaluation. When you select From physics interface, the local z-coordinate given in the Default Through-thickness Result Location section in the settings for the Shell interface is used.
Note that if From physics interface is selected from the Through-thickness location list, and the z-coordinate is changed, the Model Reduction study step must be recomputed.
If User defined is selected from the Stress method list, a user-defined stress expression must be entered. From the Shape function integration order list, select the Gauss point order matching the user-defined stress expression.
The user-defined stress expression must be linear with respect to displacements. Therefore, expressions involving principal stresses, and so on, are not allowed.
Select the Lower frequency fL, the Upper frequency fU, and the Number of frequencies nf to define the integral limits and discretization in the moment computations.
Cycle Counting Model
Select a cycle counting model — Dirlik or Bendat. From the Upper integration limit list, select Automatic or User defined. For User defined, enter a finite value for the upper integration limit Su of the integral for the fatigue usage factor. For Duration, enter the time td that represents the duration of the random vibration.
Fatigue Model Selection
From the Criterion list, select S–N Curve, Basquin, or Approximate S–N Curve.
Fatigue Model Parameters
Depending on the selected fatigue model, you have to enter the corresponding fatigue model parameters. For S–N Curve, select an S–N curve fSN(N) from the S–N curve list, which contains all functions defined under Global Definitions. Enter a Cycle cutoff Ncut. Select the Spatially varying S–N data check box if the S–N curve varies with position in the structure. Specify the Life cutoff parameter, Lcut
In many situations, it is reasonable to model the S–N curve data as constant with respect to position in the structure. This will significantly reduce the computation time of the random vibration fatigue evaluation.
For Basquin, enter the Fatigue strength coefficient σ'f, the Fatigue strength exponent b, the Cycle cutoff Ncut, and the Life cutoff parameter, Lcut. For Approximate S–N curve, enter the Transition stress St, the Transition life Nt, the Endurance limit Se, the Endurance life Ne, and the Life cutoff parameter, Lcut.