Issue 9

An. Carpinteri et alii, Frattura ed Integrità Strutturale, 9 (2009) 46 – 54 ; DOI: 10.3221/IGF-ESIS.09.05 49 axis. The direction cosines of u -, v - and w -axis can be computed with respect to the PXYZ frame, as a function of the two angles  and  . Figura 2 : PXYZ and Puvw coordinate systems, with the w -axis normal to the critical plane  . The stress vector w S acting at point P , the normal stress vector N and the shear stress vector C acting on the critical plane are given by: wσ S w     w SwN w   N SC w   (4) For multiaxial constant amplitude cyclic loading, the direction of the normal stress vector   t N is fixed with respect to time and consequently, the mean value m N and the amplitude a N of the vector modulus   tN can readily be calculated. On the other hand, the definitions of the mean value m C and amplitude a C are not unique owing to the generally time- varying direction of the shear stress vector )( t C . The procedure proposed by Papadopoulos [23] to determine m C and a C is adopted [14]. Fatigue strength estimation The multiaxial fatigue limit condition presented in Ref .[15] corresponds to a nonlinear combination of the maximum normal stress ( a m N N N   max ) and the shear stress amplitude ( a C ) acting on the critical plane: 1 2 1, 2 1, max                      af a af C N (5) As is well-known, the effect of a tensile mean normal stress superimposed upon an alternating normal stress strongly reduces the fatigue resistance of metals, while a mean shear stress superimposed upon an alternating shear stress does not affect the fatigue life [24]. Therefore, the following multiaxial fatigue limit condition is here adopted [18]: 1 2 1, 2 1, ,                      af a af eqa C N (6) where:           u m af a eqa N N N 1, , (7)