Issue 39

M. Muñiz Calvente et alii, Frattura ed Integrità Strutturale, 39 (2017) 160-165; DOI: 10.3221/IGF-ESIS.39.16 162                               n j j ij ref j n i S fail i C N B GP A A P P ij ...1 ...1 , int, ) log( ) log( exp 1 ) 1( 1    (3) where j A  is the angle interval assigned to each value of ij GP , which is the damage value for the plane j of the specimen i . To start the iteration process, an initial estimation of i eq A , , close to 40º, must be assumed because it depends on the values of B , C and the three Weibull parameters, which are still unknown. Figure 1 : a) Iterative process applied to fit the PFCDF; b) Material plane selected for the projection of the normal and shear stresses [10]; c) Difference between MCC and MCE multiaxial fatigue criteria [10]. Step 4: Estimation of B and C: The estimation of B and C must be obtained by minimizing the least square equation proposed in [1] with respect to B, C and 1  , 2  , … t  for different sizes: 2 1 log log            i i i i C GP BN Q  (4) where  is the median value for each of the different equivalent angle intervals obtained in the previous step, n is the sample size and i GP and i N are the maximum value of the critical parameter and the number of cycles to failure of the i-th specimen, respectively. Step 5: Estimation of Weibull parameters: The probability of failure for each of the specimens is obtained using a plotting point position rule [4]: 4.0 3.0    N i P (5) a) b) c)