General Mechanical

General Mechanical

Non-constant amplitude fatigue analysis

    • Jared McFadden


      I am trying to do a fatigue analysis on two models but am getting results that seem incorrect. Both models use the same part with same S-N data but one has a larger load applied, about 2x more than the other. I created an ASCII file for both that put the load history as a ratio of the maximum load. The issue is that the results of the model with the smaller load are saying that the fatigue life is less, which I know is incorrect. I have attached the load history plot and the result for both loads. I tried changing settings in the fatigue tool and noticed that changing the "infinite life" setting is the only one that makes a difference. Would the fact that the load history for the smaller load having a higher frequency than the other be the reason for these seemingly incorrect results? The images are in order of: small load result, small load history, large load result, large load history.

      Thank you for any help!

    • Vinayak Vipradas
      Ansys Employee

      Hi Jared, 

      Results for multiple frequency cases will use the number of cycles last defined S-N data point for undefined, low-stress values. Extrapolation of the S-N data curve outside your data points is not done. So at high frequencies, there will be many cycles per second so a shortened life could be predicted if your S-N data does not cover appropriate stress range. 

      The infinite life setting is used to reduce the effect that small stress cycles have on the calculated fatigue damage. For non-constant amplitude loading, cycles with very small alternating stresses can over predict the fatigue damage. For example, with non-constant amplitude proportional loading, the history data may produce many cycles with near zero alternating stress. These near zero stress cycles should produce negligible fatigue damage. It is extremely conservative to assume that the life associated with these stress cycles is 1e6 cycles.

      For more information about fatigue life results, please refer to the following resources:
      Fatigue Results ( 
      Frequency-Based Fatigue using Harmonic Analysis in Ansys Mechanical — Lesson 4 - YouTube

      Vinayak Vipradas


      • Jared McFadden

        Hi Vinayak,

        Thank you for your response! I noticed that my S-N curve did not cover all stress values experienced by the part at the lower load level so I added more data points to encapsulate all stress values. Even with doing this I was given the same result. I increased the infinite life value but all that does is increase the fatigue life by whatever factor the infinite life is increased by. Also, the fatigue life for the lower load case is showing the entire part as one single fatigue life value. I am really confused as to why my results are the way they are so any explanation would be great!

        Thank you.

    • taylor godiva

      a problem that is really hard to find an answer to. I asked for help from a @Geometry Dash Scratch teacher I know. I believe there will be the answer that we all want! 

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