Issue 37

R. Sepe et alii, Frattura ed Integrità Strutturale, 37 (2016) 369-381; DOI: 10.3221/IGF-ESIS.37.48 378 In order to clarify the influence of the nucleation parameters f N ,  N and S N on the material toughness at the beginning of the crack propagation (output variable), that is, on the start point of the R-curve, a sensitivity analysis was developed; results are reported as a pie-chart in Fig 5. As it is possible to observe, the most meaningful influence on the output variable is due to the parameter f N ; for the variables distribution the following realistic characteristics were assumed: f N  N(0.05; 0.01),  N  N(0.3; 0.06); S N   N(0.5; 0.1). Figure 5 : Sensitivity analysis results. The commercial code used to develop the sensitivity analyses is the St-Orm code, linked to the LS-Dyna code for what concerns the deterministic structural results. In this code the GT model can be related only to shell type elements under plane stress conditions. The cumulative density function (CDF), the mean and the standard deviation of the output variable obtained for an increasing number of MC trials, are respectively reported in the plots of Figs. 6-8. The mean and the standard deviation of the crack initiation time show quite constant values after about eighty MC iterations. That reached mean value corresponds to a critical remote stress of 74 N/mm 2 , which differs from the experimental results of a few percent. As it is possible to observe from the plot of the standard deviation versus the MC iteration number, the reached value of the standard deviation of the crack initiation time is about 10.6, which implies a high probability of varying the critical remote stress of about ± 3% of its mean value, given the considered statistics of the nucleation parameters, i.e. the same order of magnitude of the difference between the numerical and the experimental results. From this considerations, it is possible to conclude that the influence of the calibration of the nucleation parameters is quite low in the reference case, even in case their intervals of variation are rather large. After this, an investigation on the influence of the variability of the mechanical material properties on the same output variable was developed. Figure 6 : CFD of the crack init. time [ms]