Learning Forum

[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