Issue34

R. Citarella et alii, Frattura ed Integrità Strutturale, 34 (2015) 514-523; DOI: 10.3221/IGF-ESIS.34.57 514 Focussed on Crack Paths DBEM crack propagation for nonlinear fracture problems R. Citarella, V. Giannella, M. Lepore Dept. of Industrial Engineering, University of Salerno, via G. Paolo II, 132, Fisciano (SA), Italy rcitarella@unisa.it A BSTRACT . A three-dimensional crack propagation simulation is performed by the Dual Boundary Element Method (DBEM). The Stress Intensity Factors (SIFs) along the front of a semi elliptical crack, initiated from the external surface of a hollow axle, are calculated for bending and press fit loading separately and for a combination of them. In correspondence of the latter loading condition, a crack propagation is also simulated, with the crack growth rates calculated using the NASGRO3 formula, calibrated for the material under analysis (steel ASTM A469) . The J-integral and COD approaches are selected for SIFs calculation in DBEM environment, where the crack path is assessed by the minimum strain energy density criterion (MSED). In correspondence of the initial crack scenario, SIFs along the crack front are also calculated by the Finite Element (FE) code ZENCRACK, using COD, in order to provide, by a cross comparison with DBEM, an assessment on the level of accuracy obtained. Due to the symmetry of the bending problem a pure mode I crack propagation is realised with no kinking of the propagating crack whereas for press fit loading the crack propagation becomes mixed mode. The crack growth analysis is nonlinear because of normal gap elements used to model the press fit condition with added friction, and is developed in an iterative-incremental procedure. From the analysis of the SIFs results related to the initial cracked configuration, it is possible to assess the impact of the press fit condition when superimposed to the bending load case. K EYWORDS . DBEM; Crack Propagation; Nonlinear Fracture Mechanics. I NTRODUCTION he crack growth analysis of flaws is one of the most important parts for structural integrity prediction of metallic components in the presence of initial and accumulated in-service damages. The problem of residual fatigue life prediction with non-linear loading conditions in structural elements is complex and analytical solutions are generally not available. In the present study, the Dual Boundary Element Method (DBEM) [1], as implemented in the commercial software BEASY [2], is used to simulate the stress scenario on a hollow cylinder, loaded with non-linear boundary conditions. The Stress Intensity Factors (SIFs) are calculated by both the COD ( Crack Opening Displacement) [3-5] and the J-integral [6- 11] approaches for different load cases: pure bending, press-fit (involving nonlinear contact conditions at the interface between hub and axle) [12] and a combination of them, to understand the impact on the crack of each load case (bending and press fit). Then, the growth of the semi-elliptical crack, initiated from the axle surface, is simulated considering the simultaneous application of bending and press-fit and using the Minimum Strain Energy Density (MSED) criterion for crack path assessment [13]. When considering the crack propagation the SIFs from J-integral approach are adopted. T