Guidelines for Using Dynamic Analyses

Following are some guidelines that you should consider when running dynamic analyses:

• To produce accurate results from a dynamic response analysis, you must know the necessary modes to be included in your analysis. The number of modes you include depends upon the nature of the input load and the analysis type:

◦ For all dynamic analyses with base excitation, request enough modes to ensure that the industry standard of 80% total effective mass participation is reached.

◦ For dynamic frequency with frequency-dependent loading, ensure that the highest natural frequency specified in the modal analysis is larger than the highest frequency of the applied load. Furthermore, some recommend including all modes from half the lowest operating frequency to twice the maximum operating frequency.

◦ For dynamic random analyses, include enough modes to cover the entire frequency range of the Power Spectral Density (PSD).

◦ For dynamic time analyses with a time-dependent load, you must compare the results from several analyses to determine the dependence on the number of modes. Use 80% total effective mass participation if you are using base excitation as a starting point. Furthermore, the mode shapes you request must be representative of the deflected shape of the part as if the loads were static. For example, in a dynamic analysis of a flat plate subject to bending loads, ensure that you have included out-of-plane mode shapes.

• You need to consider several issues concerning base excitation when you define a dynamic shock analysis. These include:

◦ Dynamic shock uses response spectra as the forcing function. Creo Simulate uses this response spectra as a weight factor to multiply each individual modal shape and then add them together. Creo Simulate uses one of two methods to add the modal shapes—the Absolute Sum method or SRSS method.

◦ If the frequencies of major contributing modes for your model are not very close together, the SRSS provides a better approximation method. In this case, the Absolute Sum method overestimates the maximum response.

◦ Be sure to include enough modes to capture the response spectra frequency range.

◦ You can define your response spectrum either as uniform or as a function. If you define the response spectrum as uniform, Creo Simulate assumes that the X, Y, and Z values you enter in the Direction of Base Excitation area represent both the magnitude and direction of the response spectrum. For example, if an acceleration spectrum response is normalized to G's, the vector you enter should contain not only the direction of the spectrum response, but also the magnitude of gravity.

If you define the response spectrum as a function instead, Creo Simulate uses the product of the X, Y, Z information and the function you define to derive magnitude and direction. Thus, you can include some or all of the magnitude information in the function if you prefer.