PSI - Issue 72

Nenad Vidanović et al. / Procedia Structural Integrity 72 (2025) 499– 506

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cut HPT casing had been already in service for 12 678 engine hours (1 773 ground-air-ground cycles). When the HPT casing was returned to MRO workshop after another 2 807 cycles, cracks were detected at all EDM marks during regular fluorescent penetrant inspection (FPI). The affected area is not repairable, and testing of samples or component itself was not possible due to geometry restrictions, and replacement might be time-consuming and/or too expensive. According to the Clean, inspect and repair (CIR) manual for this type of engine, the part should have been replaced but since the HPT casing is costly, the MRO requested the evaluation of the residual life of the casing based on available but also limited data for the measured crack length of 0.41 mm after 2 807 cycles at notch of 2 mm in depth (Fig. 1.).

Fig. 1. EDM cut (notch) with crack.

The main problem, converted into question, which the authors faced with was: How to determine Paris’ coefficients for the material of the damaged component which has been in operation for a long period at elevated temperatures, since testing in this case was not applicable due to geometry restrictions? Although the experimental methods have been used for a long time and are still the most relevant methods for fatigue life predictions, it is costly approach and often there are no resources for experiments or in some cases it is impossible to carry out experiments when structures are of complex geometry (such as HPT casing). On the other hand, the approach based on numerical simulations, verified by the experiments, shows notable advantages regarding costs and efficiency. The remaining service life can be evaluated knowing fatigue crack growth rate and the number of loading cycles to fai lure, as defined by Paris’ law: = ∙ ∆ (1) where: – the range of stress intensity factor (SIF), and – experimentally determined material coefficients which depend on fatigue parameters (such as stress ratio and temperature) where knowing them the design service lifetime can be considered reliable, and exploitation and maintenance schedule of the component can be determined. The extensive research of the Inconel 718 coefficients presented in the literature showed a wide range of and values (effects on the fatigue crack growth behaviour) and they are presented in the following Table 1 (Grbović at al. (2025)):