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I am trying to work out fatigue life of a part using Miner's rule, however I am having some trouble working out which stress values to use. I have seen some examples but there seems no rhyme or reason to the values they have used in the accumulative Miner's rule, where fatigue failure occurs when :

$$\sum_{j=1}^{j=k}\frac{n_j}{N_j}=1$$

Where $n_1, n_2...n_k$ represents the number of cycles at specific overstress levels and $N_1, N_2...N_k$ represent the life (in cycles) at these overstress levels.

The example below just used the maximum positive values, with no regard for negative values, even if they were nearly the same as that maximum positive value. Surely this negative stress would have some impact on fatigue, more so than a zero based loading?

enter image description here

So if I have a stress vs time plot (shown below) that varies from positive to negative quite randomly, where do I get these overstress levels from? Are they the alternating stresses, mean stresses, or max/min stresses that are over the endurance limit of the material?

enter image description here

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I used Miners rule to summarized the fatigue life of different stress cycles for various times . As : stress A for B cycles + stress C for D cycles + stress E for F cycles. I don't think it applies to single stress applications.

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  • $\begingroup$ Yes it is accumulative damage, but my textbook doesn't say what stress to use. $\endgroup$ – david_10001 Jun 16 '18 at 12:28
  • $\begingroup$ I used maximum stress for each cycle, $\endgroup$ – blacksmith37 Jun 17 '18 at 14:56
  • $\begingroup$ I think I have worked out the answer. My textbook seems to plot the mean stress and alternating stress on an AM diagram and draw a line from the ultimate tensile strength on the mean axis through that point. Then use some trig to find the y-intercept on the alternating axis. They use this value as their equivalent stress value for the Miner's rule $\endgroup$ – david_10001 Jun 18 '18 at 9:41

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