Issue 33

V. Anes et alii, Frattura ed Integrità Strutturale, 33 (2015) 309-318; DOI: 10.3221/IGF-ESIS.33.35 315 unitary damage is computed for one loading cycle and then is multiplied by the number of loaded cycles. On the other hand, the load depicted in Fig. 9 b), can be considered as a loading block and the inherent damage unity is related to all load cycles performed during the loading period. None of these loading cycles have the same fatigue damage because they have different amplitudes and load sequence. Thus in order to quantify a unitary damage from a general loading it is necessary to account with all the loading cycles. (a) (b) Figure 8 : Variable amplitude loadings, a) Time history, b) Equivalent stress spectrum. (a) (b) Figure 9: Two different loading paths with same maximum stress. Unitary block damage concept The main idea that supports the unitary block damage concept is based in the relation between the maximum equivalent stress fatigue life estimates (for a loading block) and the experimental results. This relation is given in Eq. (4). exp Re _ estimated erimental f Block f N lative Damage N  (4) The unitary damage concept aims to establish the loading block damage, which under cyclic loading conditions can be linearly added as if it was a simple sinusoidal loading cycle, like the ones used to obtain the uniaxial SN curves. It is expected that the experimental fatigue life be less than the estimated one; thus, the block unitary damage can be estimated as follows in Eq. (5). exp 1 Re _ erimental estimated f f Block N N lative Damage  (5) Where 1 Re _ Block lative Damage is the block unitary damage. In order to estimate the relative damage without falling back on experimental tests, it was developed the virtual cycle counting method based in the SSF equivalent shear stress time histories.