Issue 33

V. Anes et alii, Frattura ed Integrità Strutturale, 33 (2015) 309-318; DOI: 10.3221/IGF-ESIS.33.35 316 Virtual cycle counting concept The virtual cycle counting (vcc) [16] is a non-Rainflow approach to evaluate loading block damage and it is based on the SSF equivalent shear stress previously presented. In the present authors´ opinion, equivalent stress approaches are much more suitable to account block damages than other ones, because they condensed in one damage parameter the instantaneous values of axial and shear damages, and also it allows in an easy way to implement cycle counting techniques in finite element packages. The concept and use of the vcc approach is much easier than other methods usually adopted to quantify block damage accumulation. Basically, the vcc method estimates the relative damage described in the previous section and is presented in Eq. (6). In the numerator of Eq. (6), it is added all SSF equivalent stress absolute values at each peak and valley found between two consecutive zero stress points along the SSF equivalent shear stress time history of a loading block. In denominator hold two times the maximum SSF equivalent shear stress found in the entire SSF time history. Fig. 10 illustrates the vcc method and their variables.   , ssf,max, 2 ssf peak valley Block abs vcc      (6) (a) (b) (c) Figure 10 : Virtual cycle counting variables identification. The block fatigue life is estimated as follows in Eq. (7), 1 ,max, _ b ssf block f block A N vcc         (7) where A, and b are the power law regression variables obtained from pure shear fatigue life results from the considered material. ,max, ssf block  it the maximum SSF equivalent stress within a loading block, and vcc is the virtual cycle counting. R ANDOM LOADINGS ( LEVEL 4) Damage assessment of loading spectra ne way to deal with random loadings is to extract loading blocks from the random loading time history in order to estimate multiaxial random accumulative damage. In order to do that, the SSF equivalent stress criterion in association with the virtual cycle counting method can be used. The paradigm that supports the block extraction methodology presented here is based in the SSF equivalent stress time histories and is described in the following. Fig. 11, shows a multiaxial loading represented with the SSF equivalent shear stress time evolution. Here the block extraction is based on the SSF maximum peak, the first maximum peak is settled as the reference damage on the SSF virtual cycle procedures, see Block 1 on Fig. 11, until that peak be exceeded, see Block 2 on Fig. 11, at this point the block 1 ends and starts the second block region with a new damage scale given by the SSF time evolution new peak, Block 3, in Fig. 11. The same methodology is used to extract the other blocks. O