Learning Forum

[Morgan Ramanauskas]

Hi All,

I'm looking for some guidance please. I am trying to run a fatigue analysis using fatigue tool on a dogbone sample subjected to axial tensile and compressive loads, whilst also subjected to a horizontal clamping preload via a contact pad. The contact pad imparts a contact stress which I want to include in the fatigue analysis.

The clamping force is applied as a first time step, prior to appling a compressive axial load in time step 2, which is then reversed in time step 3. The clamping force remains throughout. 

I am trying to run the analysis without reversing the clamping load and only cycling the compressive and tensile loading. I have tried using solution combinations however, this still reverses the clamping load.  The clamping load also seems to be applied at the same time as the axial loading instead of following the application via the time steps. ie clamping load -> axial compressive loading -> axial tensile loading.

Any help would be greatly appreciated.

Kind regards,

Morgan

[danielshaw]

The Mechanical Fatigue Tool (FT) can assess the fatigue behavior of a combined static load (e.g., preload) and alternating load (e.g., cyclic load).  The static load creates a mean stress.  Per the FT, this type of loading is known as constant amplitude, non-proportional loading and can be assessed by inserting a FT under a Solution Combination.

For constant amplitude, non-proportional loading, the FT calculates the alternating stress as one-half of the difference between the two result sets in the Solution Combination (Salt = 0.5*(E1 - E2)), and the mean stress as the average of the two result sets (Smean = 0.5*(E1 + E2)).  To produce accurate fatigue results, you need to define the loading for the two result sets to ensure that the Solution Combination produces the correct alternating and mean stresses.  For example, if your two loading Environments are:

Environment 1 (E1) = static load + alternating load

Environment 2 (E2) = static load - alternating load

The Solution Combination will produce:

Smean = ½* (E1 + E2) = ½* [ (static load + alternating load) + (static load - alternating load) ]

Smean = static load

and

Salt = ½*(E1 - E2) = ½* [ (static load + alternating load) - (static load - alternating load) ]

Salt = alternating load