PSI - Issue 12

Venanzio Giannella et al. / Procedia Structural Integrity 12 (2018) 404–415 V. Giannella/ Structural Integrity Procedia 00 (2018) 000 – 000

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Figure 3. (a) Failure induced by the fatigue crack-growth; (b) typical disk failure scenarios (Shlyannikov et al., 2001).

3. DBEM analyses

3.1. Introduction

A DBEM submodel (Fig. 4), enclosing the domain in which the crack is likely to propagate, is extracted by the FEM submodel (Fig. 2b). It must be sufficiently larger than the volume affected by the experimental crack-growth (Fig. 3a) and, when still uncracked, comprises 3099 linear elements. A quarter-circular part-trough crack, with radius equal to 0.15 mm, has been inserted in the DBEM submodel (Fig. 4), in the most critical point, as pointed out by the FEM global-local stress analyses and consistently with the experimental outcomes. The initial crack inclination (nearly 45°) has been defined consistently with the experimental inclination. The area surrounding the crack insertion point as well as the crack faces have been step-by-step remeshed along the propagation, using quadratic (9 nodes) quadrilateral elements or quadratic triangular (6 nodes) elements. During the remeshing process, several rings of internal points were positioned along the crack front in order to build the J paths needed to allow the J -integral evaluation. One element far from the crack has been constrained in the three directions in order to remove the rigid body motion (the load applied on the crack faces is self-equilibrated). SIFs are calculated with the J - integral approach (Rigby et al. 1993, 1998; Dell’Erba et al. 2000) whereas the crack path assessment is based on the Minimum Strain Energy Density (MSED) criterion (Sih, 1074). Different approaches have been proposed in the past to perform fracture assessments by using in combination FEM and DBEM: Fixed Displacement (FD), Fixed Load (FL) and Loaded Crack (LC). A complete benchmark among the three approaches has been presented by Giannella et al. (2017a) on a complex industrial application in which also load spectrum effects have been considered. A further benchmark between FD and LC approaches on a crack propagating in aeroengine turbine stage has been presented by Citarella et al. (2016b). In summary, the LC approach turned out to exhibit both computational and accuracy advantages vs. FD and FL and, therefore it was selected to tackle the fracture problem presented in this paper.