Hysteretic Damping Form

Access the Hysteretic Damping form as follows:

Click the Define menu > Analysis Cases command to display the Analysis Cases form.
Click the Add New Case button or highlight a previously defined analysis case and click the Add Copy of Case or the Modify/Show Case button to display the Analysis Case Data form.
Select Steady State or Power Spectral Density from the Analysis Case Type drop-down list to display the Analysis Cases Data - Steady State or Analysis Cases Data - Power Spectral Density form, respectively.
Click the Hysteretic Damping Modify/Show button to display the Hysteretic Damping form.

Hysteretic damping is used for steady-state and power-spectral-density analysis. The damping matrix is calculated as a linear combination of the stiffness matrix scaled by a coefficient that you specify, and the mass matrix scaled by a second coefficient that you specify. The damping matrix becomes the imaginary part of the complex stiffness (impedance) matrix that is solved at each frequency. Specify the damping in one of the following ways:

Constant Hysteretic Damping for all Frequencies option. The mass and stiffness coefficients are constant for all frequencies. Use the defaults for Mass Proportional Coefficient and Stiffness Proportional Coefficient or use the edit boxes to revise the coefficients.
Interpolated Hysteretic Damping by Frequencies option. The mass and stiffness coefficients are linearly interpolated by frequency. Specify the two constant coefficients (Mass Proportional Coefficient, Stiffness Proportional Coefficient) at a series of frequency points. Between specified points, the coefficients are linearly interpolated. Outside the specified range, the coefficients are constant at the value given for the closest specified frequency point.

Note: Although the mass coefficient is provided for completeness, only the stiffness coefficient is normally used.

When constant hysteretic coefficients are used, hysteretic damping depends only on displacement, not on velocity. This is often a better model for solids than is viscous damping, which is more suited to fluids. To model viscous damping, specify a stiffness coefficient that increases linearly with frequency.

Tip: You can approximate modal damping using a hysteretic stiffness coefficient that is twice the modal damping ratio. For example, to approximate a constant modal damping ratio of 5%, use a constant hysteretic stiffness coefficient of 0.10. This approximation is only accurate at the modal frequency, but that is usually where the most significant response is.