An interaction effect between stresses is considered in cumulative creep damage tests with two levels to explain the deviation of the summation of time ratios from unity. With the cumulative damage concept previously suggested for the creep process, empirical relations have been established in order to take this effect into account. The parameters considered are the difference between stresses and the damaging time ratio. Essentially, this approach gives a theoretical value of the sum of time ratios greater than unity for decreasing stresses, whereas this value is smaller than unity for increasing stresses. The correlation between predictions and some published experimental results is discussed.
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