Fatigue Damage Evaluation under Random Stress Sequences in Terms of Effective Stress

A cumulative fatigue damage rule has been proposed for random stress sequences from a view point of internal stress and effective stress. Applied stress was devided into two parts through practical measurements and simulation; internal stress as a backward stress for the deformation and effective stress as a drag due to plastic flow. For the fatigue damage evaluation, a cumulative damage rule like Miner's was operated with the help of ES-N diagram and effective stress measurements during fatigue process. The former was obtained by fatigue tests where effective stresses were kept constant during the tests and the latter was made by frequent monitoring of effective stress during the tests, which resulted in an experimental expression of formulae. It was found that the internal stress at a given cycle can be expressed in a regressive form in terms of the stress level and the internal stress of the latest stress cycle. That is, σi(n)=F(σa(n-1), σi(n-1), σa(n)) where subscripts (n-1) and (n) are the latest number of cycle and the current number of cycle, respectively. The damage rule and experimental expression of effective stress were examined on two materials with or without strain-ageing hardenability, namely S35C steel and Al-1.1 mass% Mn alloy. Agreement between the experimental results and the simmulations was found to be fairly good.