PSI - Issue 66

538 4

Slobodanka Boljanović et al. / Procedia Structural Integrity 66 (2024) 535– 542 S. Boljanovi ć and A. Carpinteri/ Structural Integrity Procedia 00 (2025) 000–000

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Surface elliptical flaw occurrence is still an important issue in large system reliability, causing a reduction in failure strength performances, aerodynamic loads imbalance, and ultimately, additional maintenance costs; if left untreated, it can suddenly lead to fatigue-induced hazard events. Therefore, the present research study proposes a novel computational design strategy for evaluating the fatigue resistance of elliptical crack-like flaw in order to achieve long-term safe-integrity performances of damaged systems either in design phase or during exploitations. 4. Safety-relevant performance design of surface elliptical flaws 4.1. Fatigue behavior of a surface elliptical crack-like flaw The failure strength of plate ( w = 100 mm, t = 9.6 mm, Fig1a), made of 7075T6 alloy can be explored in terms of the crack growth path. Such fatigue-relevant evaluations are performed in the case of maximum stress equal to S max = 150 MPa with the stress ratio R = 0.1, assuming that the semi-elliptical flaw is characterized by the following crack lengths: a 0 =1.92 mm, b 0 =2.4 mm in de p th and surface direction, respectively. Driving mode progression due to semi-elliptical flaw is herein quantified via the crack growth rate in depth and surface direction using Eq.10 coupled with Eq.1 to 9. Relevant disturbance stress state analysis through the crack growth paths is shown in Fig.2a and b for four different crack lengths in depth direction, where the vertical axis is normal to the centreline of the plate and the horizontal axis correlates to the plate front face, respectively. Further, experimentally explored crack paths by Putra and Schijve (1992) are employed to verify the predictive capability of the developed computational design strategy, as is shown in Fig.2a and b. From different comparisons it can be inferred that generated crack paths adequately correlate with those tested through relevant experiments.

Fig. 2. Crack path evaluations: (a) 1 – a = 4.89 mm, 2 – a = 5.74 mm, (b) 1– a = 6.30 mm, 2 – a = 6.95 mm, experiment (PCA14) discussed by Putra and Schijve (1992) and calculated curves are the present research results.

4.2. Fatigue strength estimation Now the failure resistance of the plate with a semi-elliptical flaw (Fig.1a) is analyzed through the residual life evaluation. The plate, made of 9Ni-4Co-0.2C steel, is subjected to cyclic loading, characterized by two different maximum stresses ( S max = 207 MPa and 310.5 MPa). Relevant initial crack growth sizes and corresponding maximum stresses are shown in Table 1, whereas the width and the plate thickness are equal to w = 152.5 mm and t = 7.4 mm, respectively.