Issue 31
R. Citarella et alii, Frattura ed Integrità Strutturale, 31 (2015) 138-147; DOI: 10.3221/IGF-ESIS.31.11 146 In Fig. 14 the growth angle from the initial cracked configuration is shown with its variation along the crack front and against the analyzed crack growth criteria: it is possible to see that the all criteria provide similar predictions. At the beginning of crack growth, the growth rate of the crack front midpoint is faster than that at the intersection with the surface (break through points): the reason is that the maximum stress-intensity factor (Fig. 15) is attained at the deepest point of the initial crack front. Consequently, a straight-fronted notch evolves towards a curved front and the flaw aspect ratio b/c increases. In Fig. 16 the crack depth as calculated by DBEM [1] and by FEM (ZENCRACK) is shown along the crack propagation, proving a satisfactory level of consistency between the aforementioned computational approaches. Figure 13 : Superficial crack length (mm) vs. cycles for the analyzed specimens (considering the outcomes of the DBEM code Beasy and of the FEM code ZENCRACK). Figure 14 : Growth angle, from the four different criteria, as varying along the initial crack front (considering the outcomes of the DBEM code Beasy and of the FEM code ZENCRACK). Figure 15 : SIF’s (MPa mm) along the crack front related to the first step (considering the outcomes of the DBEM code Beasy and of the FEM code ZENCRACK). Figure 16 : Crack advance (mm) of crack front center point point vs. number of cycles. C ONCLUSIONS he computed FEM crack propagation results are found to be in good qualitative (the crack path) and quantitative (the crack growth rates) agreement with experimental findings and numerical outcomes available from literature. A rather complex 3D crack growth behavior is present in case of superimposed tension and in phase torsion and the fatigue life is decreased if compared to a pure tension fatigue load. This is related to the increase of the mode mixity effect. Moreover, it can be emphasized the reduced calculation times of the FEM approach, in comparison with the DBEM approach shown in [1], the latter providing some advantages in the preprocessing phase (e.g. remeshing during crack propagation). The crack insertion and the whole crack propagation is fully automatic, with repeated remeshing realized at T
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