Issue34

Z. Marciniak et alii, Frattura ed Integrità Strutturale, 34 (2015) 1-10; DOI: 10.3221/IGF-ESIS.34.01 4 ( ) ( ) sin(2 ) 2 ( ) cos(2 ) eq x xy t t t          , (8) where:  x (t) - normal stress along the specimen axis,  xy (t) - shear stress in the specimen cross section,  - angle determining the critical plane position. From Eq. (8) it appears that the equivalent stress  eq (t) is linearly dependent on the stress state components  x (t) and  xy (t), so it can be expressed as 1 1 2 2 1 n eq j j j a x a x a x       , (9) where: a 1 = sin (2  ), a 2 = 2cos (2  ), x 1 =  x , x 2 =  xy . From theory of probability [13] it results that the variance of random variable being a linear function of some random variables is expressed by the following formula 1 2 1 2 2 2 2 1 2 1 2 1 2 2 n eq j xj j k xjk x x x x j j k a a a a a a a                 , (10) where:   eq - variance of equivalent stress  eq ,  x1 - variance of normal stress  x ,  x2 - variance of shear stress  xy ,  x1x2 - covariance of normal  x and shear stress  xy stresses. Under biaxial random stationary and ergodic stress state, the variances  x1 ,  x2 and the covariance  x1x2 in Eq. (10) are constant. In the method of variance for determination of the critical plane position the maximum function of Eq. (10) is searched in relation of the angle  occurring in coefficients a 1 and a 2 . After reduction, the variance of equivalent stress   eq versus the angle  can be written as       1 2 1 2 2 2 sin 2 4 cos 2 2sin 4 eq x x x x            . (11) An exemplary assessment of the critical plane position for loading combination K01 [12] obtained using the variance and damage accumulation methods is shown in Fig. 1. a) b) Figure 1 : Dependence of the normalized value of: a) variance, b) damage accumulation on the angle  of critical plane position for loading combination K01 (λ  = 0.189) [14].