Issue 33

D. G. Hattingh et alii, Frattura ed Integrità Strutturale, 33 (2015) 382-389; DOI: 10.3221/IGF-ESIS.33.42 386   eff eff k         (2)   Re f eff eff a b       (3) In relationships (2) and (3), k  (  eff ) is the negative inverse slope, while  Ref (  eff ) is the reference shear stress amplitude extrapolated at N A cycles to failure (see Fig. 5). Constants  ,  , a and b are material parameters to be determined experimentally. In particular, by recalling that  eff is equal to unity under fully-reversed uniaxial fatigue loading and to zero under torsional cyclic loading [10], the constants in the MWCM’s calibration functions can directly be calculated as follows [10, 14]:         1 0 0 eff eff eff eff eff k k k k                  (4)   ,Re 2 A A f eff A eff A                 (5) Figure 5 : Modified Wöhler diagram. In Eq. (4) k(  eff =1) and k(  eff =0) are the negative inverse slope of the uniaxial and torsional fatigue curve, respectively; in Eq. (5)  A and  A are instead the endurance limits extrapolated at N A cycles to failure under fully-reversed uniaxial and torsional fatigue loading, respectively. It is worth pointing out here that the reference shear stress,  A,Ref (  eff ), and the negative inverse slope, k  (  eff ), to be used to estimate lifetime under multiaxial fatigue loading are assumed to be constant and equal to  A,Ref (  lim ) and to k  (  lim ), respectively, for  eff values larger than an intrinsic threshold denoted as  lim [10, 11]. This correction, which plays a role of primary importance in determining the overall accuracy of the MWCM, was introduced to take into account the fact that, under large values of ratio  eff , the use of the MWCM is seen to results in conservative estimates [15, 16]. According to the experimental results due to Kaufman and Topper [17], such a high level of conservatism can be ascribed to the fact that, when micro/meso cracks are fully open, an increase of the normal mean stress does not result in a further increase of the associated fatigue damage. This important finding is taken into account by the MWCM via  lim that represents the upper bound for stress ratio  eff [10, 16]. According to the theoretical framework briefly summarised above, the MWCM can be used to estimate fatigue lifetime by following the simple procedure described in what follows. Initially, the maximum shear stress amplitude,  a , and the effective critical plane stress ratio,  eff , have to be determined at the assumed critical location [18, 19]. Subsequently, according to the calculated value for  eff , the corresponding modified Wöhler curve can directly be estimated from Eqs (4) and (5). Finally, the number of cycles to failure under the investigated loading path is predicted via the following trivial relationship [10, 14]: log N f log  a 1 1 k  (  eff =0) k  (  eff =1) N A  A,Ref (  eff =0)=  A  A,Ref (  eff =1)=  A /2 1 k  (0<  eff <1)  A,Ref (0<  eff <1) 1 k  (  eff >1)  A,Ref (  eff >1) Increasing  eff