Daniel Shaw
Ansys Employee

You have 3 options, but options 2 and 3 are not “good” options.  They are require substantial user effort, because you are stretching the capabilities of the entry level FT.

1.       Using Ansys nCode DesignLife is the best option if you have access to an Ansys nCode DesignLife license.  It has advanced fatigue capabilities which make this type of simulation straightforward.

2.       Adjusting the S-N curve is a conservative option that is difficult to implement.  Goodman is a mean stress correction theory (Gerber is another one) that calculates an adjusted alternating stress (Salt) based on the mean stress and the ultimate tensile stress (UTS) of the material.  If you are not familiar with Goodman (or Gerber), you can find descriptions of them on-line.  If you are using the S-N method, then mean stress from the preload should be less than the yield stress (YS) of the material.  So, a conservative approach is to just assume that the mean stress caused by the preload is equal to the YS throughout the entire model and then use Goodman along with the mean stress (assume to be the YS), the UTS, and the S-N curve to develop an “adjusted” S-N curve that includes the mean stress effect.  Then use that adjusted S-N curve along with just the alternating stress from the FE to estimate the fatigue life.  This approach requires substantial effort and may be difficult to implement if you need the preload to accurately calculate the alternating stress in the FE.

3.       You can use the FT with a constant amplitude, non-proportional loading to evaluate your type of loading, but you need to adjust the loading in the FE model.  To use this approach, you  need to create two Mechanical environments and use a Solution Combination to combine their effects.

With 2 Environments and constant amplitude, non-proportional loading , the FT calculates the alternating stress to be one-half of the difference between the two load cases (Salt = 0.5*(E1 - E2)), and the mean stress to be the average of the  two load cases (Smean = 0.5*(E1 + E2)).  So, you need to set up the environments to ensure that the combination produces a varying load that alternates about the static load.  This type of loading should work.

When those load cases are combined within the FT, they will produce the following mean and alternating stresses.

Smean = 0.5 * (E1 + E2) = 0.5 * [ (vl/2 + sl) + (-vl/2 + sl) ] = static load = preload

Salt = 0.5 * (E1 - E2) = 0.5 * [ (vl/2 + sl) - (-vl/2 + sl) ] = 0.5*varying load = 0.5*external load

To use this approach, you need to:

·         apply these two loadings in the FE to create the two result sets,

·         combine the result sets in a Solution Combination (just add them together),

·         insert a FT under the Solution Combination, and