Issue 35

R. Citarella, Frattura ed Integrità Strutturale, 35 (2015) 523-533; DOI: 10.3221/IGF-ESIS.35.58 530 Figure 8 : R-G stability diagram with the G -curve corresponding to a load of 143 MPa. The R -curve (Fig. 9) used has the following equation [7]:    0.52 18.08 0.51 21.49 R K da da (7) with K R in MPa*m 1/2 and da in mm; in this R -curve the Irwin plastic correction is included and, analogously, SIFs calculated by BEM are obtained with allowance for Irwin correction; Figure 9 : Al2024-T3 R-curve. The G-curve, superimposed to the R-curve, is obtained in the following way:  crack tip N. 1 has been automatically propagated for a certain number of increments in order to get the SIFs for a variable crack length;  each SIF is corrected (with the Irwin criterion) by artificially modifying the correspondence between SIFs and related crack increment, in particular by backward shifting the crack length for each step of a quantity rp. From Figs. 10a-b it is evident that with a load of 143 MPa, before instability of crack tip N.1, Von Mises stresses are less than 385 MPa in most part of the ligament, in such a way that a failure based on plastic collapse is still premature. Consequently the real mechanism of lap joint failure is primarily related to fracture instability, responsible for the residual ligament reduction up to a condition in which the plastic collapse becomes effective. The numerical result of fracture instability at 143 MPa is close to the experimental collapse load equal to 139 MPa (specimen N. 5 in [7]) with consequent validation of the proposed procedure.