Issue 33

V. Anes et alii, Frattura ed Integrità Strutturale, 33 (2015) 309-318; DOI: 10.3221/IGF-ESIS.33.35 311 This synergistic way to account with each pyramid level is an important feature because under random multi-axial loadings all of these pyramid levels may be activated in a sequential or simultaneous way. Fig. 2 summarizes the synergistic process of evaluating random multi-axial fatigue. Figure 2 : Fatigue estimates process under random loading conditions. In this paper is presented by the present authors a contribution to levels 2, 3, and 4. For level 2, it was developed the SSF equivalent shear stress, and for level 3 it was developed the virtual cycle counting method, which is deeply dependent on the SSF equivalents stress time histories. Regarding level 4, it is presented the block extraction methodology, where loading blocks are extracted from loading spectra in order to be evaluated with the virtual cycle counting (vcc) technique and damage accumulation rules. SSF EQUIVALENT STRESS ( LEVEL 2) SSF physical foundations t is well-know that normal and shear stresses have different damage scales [11,12]. This is so because in experiments it is obtained different fatigue lives regarding the same stress amplitude in both uniaxial loading conditions (shear and axial). Usually, for structural materials, the uniaxial axial SN curve stands above the shear one. Thus, to obtain the same fatigue life in both loading conditions it is required higher normal stress amplitude comparatively to the shear stress amplitude. (a) (b) (c) (d) Figure 3 : Illustration of different deformation patterns from different loading types. a) Without applied loads, b) axial loading, c) shear loading, and d) biaxial loading. Under multiaxial loading conditions, fatigue damage results from the combination of both axial/normal and shear loading components of a multiaxial loading where it is necessary to account the axial and shear damage contribution to the overall damage. To do that, it is necessary to have both axial and shear damages in the same damage scale. For example, the von I