Issue 37

R. Sepe et alii, Frattura ed Integrità Strutturale, 37 (2016) 369-381; DOI: 10.3221/IGF-ESIS.37.48 377 Several variations of the basic procedures are available; for example, it is possible to define the target by means of a function which implies an equality or an inequality, too; in the latter case the distance is to be considered null if the inequality is satisfied. Once the project variables have been redefined a new run is performed and the process restarts up to the completion of the assigned number of shots. It is possible to plan a criterion of arrest in such way as to make the analysis stop when the distance from the target reaches a given value. In the most cases, it is desirable to control the state of the analysis with a real-time monitoring, with the purpose to realize if a satisfactory condition has been obtained. S ENSITIVITY ANALYSIS n this paragraph a sensitivity analysis of the nucleation parameters and of the main mechanical properties of the material on the material toughness of a thin plate made of aluminium alloy 2024 T3, with a through crack in the middle transversal section, was performed. The values of the material properties used are:  = 324 MPa,  = 0.0045, n = 9.8; for what concerns the Tvergaard parameters they were set equal to q 1 = 1.255, q 2 = 1.20, q 3 = 1.58, as obtained from the calibration of the model performed in [26]; the other necessary physical parameters values were derived from literature and put equal to f 0 = 0.09, D 0 = 0.15 mm, f c = 0.2 and    0.1; the first attempt value for the nucleation parameters were set equal to   N = 0.3, S N = 0.5, f N = 0.05, which represents the default values in the algorithms of the WARP 3D code. The dimensions of the plate were 800 mm in the longitudinal direction, 600 mm in the transversal direction and 1.6 mm in the thickness. The static crack propagation test, from both a numerical and an experimental point of view, were developed by considering a uniform applied longitudinal displacement at both the ends of the plate. The experimental results are available in [18]. At first, the experimental R-curve of the plate was compared with a numerical one (Fig. 4), obtained by using the WARP 3D code; the used FE model consisted of 33216 nodes and 23680 8-noded solid elements. The elements linked to the nodes belonging to the crack plane were modelled by considering a material model based on the GT criterion, as implemented in the used code. As it is shown in the plot of the Fig. 4, in the range of the considered crack elongation, numerical results are in good agreement with the experimental ones: the two R-curves are quite parallel. Further developed numerical investigations show that the distance between the numerical and the experimental R-curves depends substantially on the values considered for the nucleation parameters in the GT model. 0 20 40 60 80 100 120 140 160 0 2 4 6 8 10 crack elongation (mm) remote stress (N/mm 2 ) Numerical experimental Figure 4 : R-curve: numerical-experimental correlation. I