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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5. Results

5.1. FEM-DBEM

Using the LC approach, accurate SIFs can be computed even with coarse meshes as shown in Giannella et al. (2017b) or Citarella et al. (2016b). Moreover, no modelling of the blades is required when using such an approach. The DBEM model containing the initial crack comprises 2341 linear and quadratic elements. SIFs vs. normalized abscissa, calculated along the along initial crack front (Fig. 4c), are shown in Fig. 9. The step-by-step crack propagation simulation was set up with 25 steps, each comprehensive of the following steps:  crack adder (the crack was inserted into the DBEM uncracked model);  crack loading (the crack faces were loaded with FEM stress results);  solve (the DBEM model was solved and SIFs were computed);  crack life (the da/dN was computed from Eq. 6);  crack extend (the new crack front was predicted). For each step of crack advance, loads on the new crack surface generated during the propagation were automatically added, as provided by the global FEM analysis of the uncracked model. The average crack advance per step has been set equal to 0.15 mm for the initial 5 steps and gradually increased up to 1.5 mm for the last 5 steps. Figure 10 shows the DBEM model containing the propagated crack; such model comprises 4101 linear and quadratic elements.

5.2. FEM

The FEM propagated crack in Figure 11 clearly shows a crack path almost overlapped to the one calculated with DBEM code. Nevertheless, both crack paths calculated numerically do not replicate the experimental crack paths shown in Fig. 3. Such differences could be attributed to non modelled effects like blade vibrations, fluid pressure, etc. but the modelling is under development and in the near future will also add such loading conditions

Figure 10. (a) DBEM submodel with the propagated crack; (b) close-up on the propagated crack; (c) through-the-thickness view of the crack; loads applied on the crack face elements in orange; J-path to calculate J-integral along crack front in purple.