Issue 33

M. Cova et alii, Frattura ed Integrità Strutturale, 33 (2015) 390-396; DOI: 10.3221/IGF-ESIS.33.43 393   2 2 2 , 1 1 1 2 2 1 a eq b T b S b S                                 (10) S UPERPOSITION OF STATIC AND MULTIPLE INDEPENDENT PULSATING LOADINGS he procedure above described can be extended to multiple time variable components. It is relevant if such components are independent and separated, i.e. if each one is different from zero only when other components are null. Let us consider, for instance, another time variable function [σ] V2 , compared to the single one of Eq. 1. With two separated time variable components, the resulting stress condition is:               1 2 1 2 S V V t f t f t        (11) An example of two separated time-functions is given in Fig. 2. If f 1 is different from 0 between t 1 and t 3 , then f 2 will be different from 0 elsewhere, for instance from t 3 to t 5 . Figure 2 : definition of two “separated” time functions. Similarly to previous equations, the normal stress on the plane defined by tensor n is:             1 2 1 2 T T T n S V V t n n n n f t n n f t        (12) In this case relative maxima and minima will occur at t 1 (or t 3 or t 5 ), t 2 and t 4 . At these instants, necessarily an extreme occurs; generally, they could be local maxima or minima of σ n . For fatigue strength assessment, the maximum amplitude shall be computed; such a maximum amplitude is defined by the largest difference between the values obtained at extremes. For instance, by investigating the extremes at t 2 and t 4 , the mean value and relative amplitude shall be computed slightly differently from previous case:             , ,2 4 2 4 1 2 1 1 1 2 2 2 T T T n m n n S V V t t n n n n n n                               , ,2 4 2 4 1 2 1 2 1 2 1 1 2 2 1 1 max , 2 2 T T n a n n V V T T V V V V t t n n n n n n n n                            (13) According to previous considerations, the equivalent amplitude at a generic direction n, turns out: T