Applying dead loads to fatigue

I was recently asked how to apply a dead load in a fatigue study (a none varying load that is applied to a structure all of the time, as opposed to a live or dynamic load that generates an alternating stress).

The diagram below shows the definition of alternating stress (or stress amplitude) and mean stress, both of which are important in influencing the lifetime of a material.

The normal inputs to a fatigue study are the stress amplitudes, but as you can see in the next picture, mean stresses also have an impact on the lifetime of a material. The higher the mean stress, the lower the predicted lifetime.

Dead loads are not alternating but do contribute to the mean stress, so it is important that we can consider them in a fatigue study. But how do we do this?

Alternating load only case

In a normal fatigue study, the method takes the max stress, S_max from a static study loadcase and we use the loading ratio (R = S_min/S_max) to define S_min directly.

For example,

• Picking ‘zero based’ means we define S_max but S_min = 0.
• Picking ‘fully reversed’ means we define S_max and S_min = – S_max.
• Picking ‘loading ratio’ means we can define any arbitrary S_min from the S_max value.

The fatigue setup simply allows us to pick the relevant static study and along with the loading ratio this provides our alternating stress values. These are then compared to the S-N curve to get the lifetime or damage plots.

Dead load case

Dead loads however are constant and therefore by definition not alternating. This means they only apply a non-zero mean stress, which must be considered for the reasons argued above, but cannot be applied in SolidWorks Simulation in the normal way.

The method we use therefore is the ‘Find cycle peaks’ method. This method takes the peaks of multiple loads to find the damage so we must build our static studies with the ‘peaks’ in mind.

Therefore if we had one alternating stress and one dead load, we would need two studies:

• Static study 1 including S_max and the dead load.
• Static study 2 including S_min and the dead load.

You can see here that we are not using one study for S_max and implying S_min from the loading ratio, rather we are doing two separate studies with S_max in one and S_min in the other. However both include the dead load.

Finally, in the fatigue study you reference both of these studies using the ‘Find cycle peaks’ method. This then takes both S_max + dead load and S_min + dead load extremes and combines them to find the worst case for fatigue.