A Damage Cumulation Law under Non Proportional Cyclic Loading with Overloads for the Prediction of Crack Initiation

Abstract

For a sequence of cyclic loadings of constant amplitude and containing overloads, we propose a method for damage cumulation in non proportional loading. This method uses as data cyclic stabilized states at non proportional loading and initiation or fatigue curve in uniaxial case. For that, we take into account the mean cell size and we define a stabilized uniaxial state cyclicaly equivalent to a non proportional stabilized state through a family of cyclic strain stress curves (C-S-S-C). Although simple assumptions like linear damage function and linear cumulation are used, we obtain a sequence effect for difficult cross slip materials as 316 stainless steel, but the Miner rule for easy cross-slip materials. We show that for the second case, in a load controled test, the non proportional loading is less damaging than the uniaxial one for the same equivalent stress, while the result is opposite in a strain controlled test.