Multilevel Strain Controlled Fatigue on a Type 304 Stainless Steel

A series of cumulative damage strain-controlled fatigue tests at 20°C has been carried out on a Type 304 stainless steel with two, three, and five strain levels, both in an increasing and decreasing order. Experimental results show that if the strains are applied in an increasing order, the summation of cycle ratios is greater than unity, whatever the number of applied levels. For a decreasing order, this summation is less than one. However, for the same difference between high and low levels, this summation is closer to unity when the number of applied levels increases. The cumulative damage effect is evaluated using an approach which takes into account the sequence effect of loading. The procedure is based on the modification of the damage evolution with respect to that corresponding to constant amplitude loading. This is explained by an interaction effect due to a previous loading. With the interaction effect parameter suggested, the procedure is generalized to any discrete strain pattern. An application of the method is carried out to estimate the sums of life fractions required for failure for the material investigated. The correlation between predictions and experimental results is then discussed.