Issue 35

R. Citarella, Frattura ed Integrità Strutturale, 35 (2015) 523-533; DOI: 10.3221/IGF-ESIS.35.58 529 covering the ligament between the hole N. 5 and the crack tip N.1 (left side of hole N. 6 in Fig. 3); K eq is the equivalent SIF, calculated as in [9] but in this problem nearly coincident with the mode I SIF. When a propagating crack reaches a hole we have supposed the nucleation of a small, not detectable crack of 1 mm length on the opposite hole side (Fig. 7), as a consequence of local high stress gradients on an aged lap-joint (399620 fatigue load cycles). Starting from the 8 th iteration, in order to improve accuracy of K values, the Irwin correction has been adopted. The second link-up (Table 1) between crack tip N. 2 (right side of hole N.10) and hole N. 11 (Fig. 3) is obtained with a load of 143 MPa, as suggested by Von Mises stresses exceeding 385 MPa in a large part of ligament (Figs. 7b-c); again we have supposed the nucleation of a small, not detectable crack of 1 mm length on the right side of the hole N.11 (Fig. 7d). Then, with the same load of 143 MPa and considering the main crack configuration obtained after the second link-up, the R -curve analysis provides an unstable crack growth (Fig. 8 and Table 2) of crack tip N. 1 up to hole N. 4 (Fig. 7d) and subsequent lap joint failure due to plastic collapse (the residual ligament cannot stand anymore the applied load and undergoes extensive yielding). Namely, with reference to the crack tip N. 1, the G -curve becomes tangent to the R -curve, reaching higher values and higher gradients (Fig. 8) and causing the aforementioned unstable growth. x y a b c x y d 1 mm nucleated crack 1 mm nucleated crack Crack Tip N.1 Hole N.4 Hole N.5 Hole N.10 Hole N.11 Figure 7a-d : Close up of the main crack after first link-up on the left side of main crack (a) ; internal point position, in the configuration preceding the second link-up (b) and related Von Mises stresses on the residual ligament of the main crack right side (c) ; close up of the main crack after second link-up on the right side (d) .