Issue 35

R. Citarella, Frattura ed Integrità Strutturale, 35 (2015) 523-533; DOI: 10.3221/IGF-ESIS.35.58 523 Residual strength evaluation by DBEM for a cracked lap joint R. Citarella Department of Industrial Engineering, via Giovanni Paolo II, 132, Fisciano (SA), University of Salerno, Italy rcitarella@unisa.it A BSTRACT . The present work summarizes a numerical procedure aimed at the evaluation of the residual strength of a cracked lap joint, based on the competing failure mechanisms regulated by the R-curve analysis and plastic collapse. The model adopted for Stress Intensity Factors (SIFs) evaluation is based on the use of the Dual Boundary Element Method (DBEM) within the theoretical frame of Linear Elastic Fracture Mechanics (LEFM). The value of failure load was available from experiments, allowing a comparison with numerical results and consequent validation of the described procedure. K EYWORDS . Residual strength; DBEM; Lap joint. I NTRODUCTION he residual strength of an aircraft structure degrades during the life due to fatigue cracks, especially in the presence Multiple Site Damage (MSD), eventually evolving towards Widespread Fatigue Damage (WFD). Broek [1] published a report on the residual strength behaviour of 2024-T3 Alclad sheet panels. Small MSD cracks in combination with a lead crack can significantly reduce the load level driving to unstable crack propagation. Test data on residual strength of various types of airframes with multiple-site fatigue cracks are presented in [2], where it is showed how residual strength is affected by structural design features, bending stresses, material plasticity, arrangement of multiple-site cracks and stable growth of cracks under static loading. Structures with MSD fail when Stress Intensity Factors (SIFs) are within a range from the plane-strain fracture toughness (K Ic ) to plane-stress fracture toughness (K c ) and when net stresses are 30 to 90% of the yield stress. Full-scale tests show that the presence of MSD adjacent to a lead crack reduces the residual strength by 15% [3]. SIFs can attain critical values in a way similar to strain energy release rates. The criterion for failure due to unstable crack growth can therefore be written as  I Ic K K or  I c K K (1) where K Ic and K c are the fracture toughness of the material under plane strain and plane stress conditions respectively. It is experimentally found that K Ic is constant for thick sections of a given material. K c is found to vary with crack length and component geometry and is applicable to thinner sections where stable crack growth can occur. From Eq.1, the failure criterion can be written as:    c c c Y a K (2) T