Issue 47

D. Benasciutti et alii, Frattura ed Integrità Strutturale, 47 (2019) 348-366; DOI: 10.3221/IGF-ESIS.47.26 351 where C = R (  ) is the covariance matrix of x ( t ), see Eq. (2) . Similarly to x ( t ), the PSD matrix S' ( f ) of vector s ( t ) is the Fourier transform of the correlation matrix R' (  ) in Eq. (4):           T T ) ( ) ( ) ( ' A SA A R A A RA R' S           f f T    (6) The previous result yields by the fact that the Fourier transform    is a linear operator and A is a matrix of constants. By following the same procedure, it is straightforward to find the PSD expression of the hydrostatic stress σ H ( t ):     ) ( Re2 ) ( ) ( 9 1 ) ( , H f S f S f S f S yy xx yy xx    (7) Its zero-order spectral moment (i.e. area of S H ( f )) gives the variance:     yy xx yy xx H H C V V t Var V , 2 9 1 )(      (8) Expressions (6)-(7) characterize the deviatoric and hydrostatic stress in the frequency-domain completely. A NALYSIS STEPS OF THE P B P CRITERION his Section summarizes the main analysis steps to be followed when implementing the PbP criterion (see Fig. 1). If the PbP method is applied to the output of a FE analysis, the steps have to be repeated for each nodal result in the model. The input data required by the analysis are:  PSD matrix S ( f ) of the biaxial stress (σ x ( t ), σ y ( t ), τ xy ( t )), along with the covariance matrix C in Eq. (2) calculated from S ( f ). The stress may refer to a physical point in a structure, or to a node in a FE model (in which case matrix S ( f ) is output directly by a FE analysis). If S ( f ) is not known, it may be estimated from measured time-histories;  parameters of tension and torsion S-N curves: strength amplitudes σ A , τ A at N A =2  10 6 cycles and inverse slopes k σ , k τ . The S-N lines may represent design curves at prescribed survival probability (97.7%). Figure 1 : Analysis steps and quantities involved by the PbP spectral method in frequency-domain. Step 1 – From stress vector to deviatoric/hydrostatic stress The first step is to characterize the stress vector s ( t ) by its PSD matrix S' ( f ) in Eq. (6), and by its covariance matrix C' , see Eq. (5) or equivalently compute C' from S' ( f ). Then, characterize the hydrostatic stress σ H ( t ) by its power spectrum S H ( f ) in Eq. (7), and by its variance V H , see Eq. (8). INPUT stress PSD covariance matrix S (f), C Material properties σ A , τ A (at N A cycles) k σ ,  k τ ANALYSIS STEPS OUTPUT Deviatoric/Hydrostatic 1 Principal system i‐th damage d 1 , d 2 , d 3 Reference S‐N line ρ ref , J Aref , k ref Total damage d tot 3 3 2 4 5 T f =D cr /d tot Fatigue life 4 PSD (deviatoric) PSD (projections) S' (f) S' p (f) C' , V H C' p11 , C' p22 , C' p33 U 2 2