Issue 31

A.R. Maligno et alii, Frattura ed Integrità Strutturale, 31 (2015) 97-119; DOI: 10.3221/IGF-ESIS.31.08 105 The estimated residual life (i.e. the number of cycles for the crack to grow to the specific length) evaluated by Zencrack was 1.72×10 6 cycles whereas the analytical calculations gave a residual life of 2.03×10 6 cycles, with a difference of nearly 15%. This discrepancy between the analytical solution and the FE-based solution (with Zencrack) was likely due to the following factors:  r i /B = 11 is slightly out of the valid range of 10 (as given in the reference);  the analytical solution assumes that the shape of the crack remains constant, i.e the ratio a/c = 1, whereas in the FE simulation the crack shape would take a more natural shape, based on the energy release rate at each crack-front node. In order to get the residual life closer to the target value of 2.03×10 6 cycles it was decided to assess a more suitable initial crack size using Zencrack. The analytical study suggested an increase of about 17% in the crack dimensions to achieve the fatigue life of 1.72×10 6 cycles. Therefore, the size of the initial crack was modified to 4.74mm. Fig. 10 shows the results of the analytical calculations with initial crack size equal to 4.74 mm: with such an assumption, the analytical assessment matches exactly the Zencrack results calculated with an initial crack size equal to 3.92 mm. It was therefore decided to assume an initial crack size a = 3.1 mm (i.e.: 3.92 - 0.82) in the FE analyses. Figure 10 : Determination of new crack size. The initial a =3.92 mm was increased to a = 4.74 mm in the analytical calculation to match Zencrack results. Results of the numerical analysis for the crack size a = 3.1 mm (stress range 80 MPa) presented in Fig. 11 show that the residual life achieved for this crack size is very close to the target value of 2.03×10 6 cycles. This figure also presents a plot of the results of a Zencrack analysis, in which the crack shape was forced to maintain the rato a/c = 1 as the crack propagated through the wall thickness. This plot demonstrates that there is a tangible effect of the crack-front shape evolution. Figure 11 : Comparison of analytical and numerical residual life (target=2.0x10 6 cycles)