Fatigue assessment with variable amplitude loading

The S-N diagram plots the life time of a material at constant stress amplitude and R ratio. It is, however, not possible to assert the life time, using the diagram, if the load amplitude changes. The most obvious way to determine the life time in this case is to simulate the service load history in the laboratory. Unfortunately, this is a rather involved procedure that is not feasible in most cases. It would be helpful if it were possible to estimate the life time directly from the S-N curves. One way to do this is to use Miner s rule (also known as Palmgren-Miner rule) [99] that will be explained now. The rule is rather simple and thus easy to employ, but it has some disadvantages, discussed at the end of this section. 

To use Miner s rule, a partial damage of the component is calculated for each loading step. Assume that the component is loaded with k different stress amplitudes aa,j j = 1, , A and that the number of cycles for amplitude j is rij. We can then use the S-N curve to determine the number of cycles to failure for each of the stress amplitudes, Miner s rule now assumes that 

To summarise, it can be stated that Miner s rule is useful to provide a first approximation of the life time, but it has to be used with great care, for the calculated life time may not be a conservative estimate. 

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Fatigue assessment with variable amplitude loading... 

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