Yasushi Ikai

Fatigue. A Fatigue Damage Rule of a Steel Based on Plastic Strain.

A fatigue damage rule of a structural steel based on plastic strain ranging from high-cycle to low-cycle fatigue regime is proposed, especially paying attention to load sequence effect. Four types of fatigue tests including i) constant strain amplitudes, ii) mean strain amplitudes, iii) surplus and iv) reversible strains against the foregoing half cycle were conducted. The fatigue damage was evaluated at each half cycle, so that a loading sequence may be evaluated in a unique manner. The fatigue damage rule was optimized for these fatigue tests and also compared to rules by other researchers.The load sequence effect (Hi-Lo or Lo-Hi) was found to exist but small in strain criterion compared to that in stress criterion. A step-wise strain increasing test was compared to a step-wise stress increasing test and resulted that coaxing effect was not big under strain controlled condition.

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 Miners 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.