Issue 41

M. A. Meggiolaro et alii, Frattura ed Integrità Strutturale, 41 (2017) 98-105; DOI: 10.3221/IGF-ESIS.41.14 101 Figure 2 : The effective strain range of the hysteresis loop,  eff     op , is the loop portion where the microcrack is completely open, thus can suffer fatigue damage. The opening stress  op , which depends on  m and affects  eff , would be the physical cause for mean stress effects on fatigue crack initiation. Topper’s tests showed that the ratio  op /  max can be negative, meaning microcracks can be entirely open even under a compressive load  op < 0 , in particular under high stresses and negative stress ratios R  min /  max . Therefore, it is interesting to rewrite Eq. (3) as a function of R as:                 op Yc S R 2 max max 1 (4) If fatigue damage is caused by  eff    (  op   min )/E , to calculate it using  N curves available in the literature, which associate  (not  eff ) to the fatigue crack initiation life N , they should be properly adapted. To do so, Topper and co- workers proposed (and validated for some materials) an equation  eff  N that intrinsically includes the mean load effects:         c eff c L N 2 (2 ) (5) This  eff  N curve is illustrated in Fig. 3, where   c and c' are material properties, usually different from the equivalent Coffin-Manson’s parameters, and  L is the strain fatigue limit, defined as the largest effective strain amplitude  eff /2 that does not cause fatigue damage in  N specimens. Figure 3 : Universal  eff  N curve, which describes fatigue lives under any mean load.