Integration of Damage Differentials (IDD): General presentation and verification results

Integration of Damage Differentials (IDD) is proposed for fatigue life assessment, and such a method is completely different from any other life prediction approach. IDD can be applied under arbitrary loading including the general case of plane stress condition, that is, the stress–time functions σx(t), σy(t) and τxy(t) can be deterministic or random and are not restricted to be cyclic, neither with specified variations, nor the three of them proportional. Then, no reversals and cycles could be distinguished along the loading path in the σx − σy − τxy coordinate space. Hence, any trial to apply again the well-known notions of loading cycles (amplitudes) and damage per cycle would conceptually fail. That is why damage differentials dD are introduced in order to be summed (integrated), where еach damage differential dD is per loading (stressing) differential ds. Two empirical factors of (material’s sensitivity to) multiaxial loading non-proportionality, fc and fτ, are involved in the definition of dD. The IDD method has been implemented in software. Results of IDD verifications under multiaxial non-proportional loadings are shown. Based on them, an empirical data bank developed so far is given for IDD parameters.