Fatigue Analysis

   
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Fatigue Finite Element Analysis:
Perform a fatigue analysis on a large rotating vertical drum.

Special features:
3D model setup with bricks, beams, time dependent alternating loads.

Output:
Maximum Principal stress, Mean Stress, and Alternating Stress for classic fatigue analysis.

Comments:
The finite element model described accurately a structure that consisted of a central core tube, drive and driven solid spindles on the ends, with bar, angle, and channel elements welded to it that served as braces for a chain drive sprocket and spiral belt conveyor.

Applycon and Autodesk Algor developed a custom routine for changing a stress component of choice, in this case the Maximum Principal Stress, into Sm and Sa, Mean Stress and Alternating Stress, per the figure in the first thumbnail above. Finding these Sm and Sa values for every nodal stress allows a useful plot, like the second thumbnail. Sm can be plotted in the same way, where for each cycle, Sm = (Smax+Smin)/2 and Sa = (Smax-Smin)/2. Knowing the Sy, Sut, and Se, the Yield Stress, Ultimate Strength, and the Endurance limit respectively of the materials under study, permits determination of a design's fatigue susceptibility. Of course, Se is an estimated value based on many qualifying factors (short of destructive testing). Suffice it to say, the designer will choose the lowest reasonable value of Se to ensure that if he errs it will be on the conservative side.

The third thumbnail illustrates some typical values of Se, where Phi = Se/Sut. Notice the shape of the curves: the so-called bell curve. This indicates the fatigue analysis per se is not deterministic, but depends on statistical probabilities that the analyst must qualify to serve his purpose, like generating a conservative design rule.

The fourth thumbnail shows a useful plot of Sm and Sa against various commonly accepted fatigue criteria. Notice the calculated Safety Factors. It is possible to plot the Safety Factor directly on the finite element model as was done for Sa and Sm for the most convenient appraisal of the design's fatigue immunity or susceptibility; a picture (or chart) is worth a thousand words.