PSI - Issue 39

Andrey Shanyavskiy et al. / Procedia Structural Integrity 39 (2022) 327–332 Author name / Structural Integrity Procedia 00 (2019) 000–000

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It is evident that established durability exceeds the number of flight cycles. This discrepancy is related to the fact that for each flight, the engine disk is damaged for several cycles at the transient in-flight modes. Previous analysis for the D-18T engine had shown that the change in loading modes is 3-4 cycles per flight as described in Shanyavskiy and Stepanov (1995), Shanyavskiy and Losev (1999). Since the total number of flight cycles up to the moment of the incident with the engine is equal to 3732, the maximum possible operating durability of the engine is 14,928 unit loading cycles. Based on the estimated durability of the engine and using the data of fatigue tests for smooth specimens from the VT3-1 titanium alloy presented in Handbook (1975) and shown in Fig. 6 the calculated stress level calc max σ is evaluated to be approximately 820 MPa.

Fig. 6. S-N curve for the low cycle fatigue behavior of the titanium alloy VT3-1.

The stress state of the fan disk was also estimated for various values of crack length in a depth direction. The conception of the equivalent stress σ eq and the methodology for its determination described in detail by Shanyavskiy (2018) was applied. Since the unified kinetic curve for the VT3-1 titanium alloy which was discovered by Shanyavskiy and Losev (1999) associates the striation spacing δ with the equivalent stress intensity factor K eq , the value of σ eq can be calculated by the equation of the stress intensity factor for a semi-elliptical crack presented in Murakami (1987). Based on the data of measured striation spacing δ depending on the crack length a in a depth direction obtained for section 1 and shown in Fig. 5a the results of equivalent stress level evaluation are presented in Table 1. It is obvious that the value of σ eq decreased with the increase in the crack length a in a depth direction.

Table 1. The data on the values of the equivalent stress σ eq estimated for several crack lengths a in a depth direction. a (mm) δ (μm) K eq (MPa∙(m) 1/2 ) σ eq (MPa) 5 1.9 60 550 10 2.5 62 390 15 5.3 65 335 20 6.2 70 315 48 – 80 230

Based on the analysis of established peculiarities and regularities for the fatigue crack propagation in the fan disk combined with the stress levels and the stress concentrator in the disk, the recommendations regarding the inspection interval in terms of the number of flight cycles were provided. Moreover, it is worth noting that without a cut in the analyzed location of the fan disk the operating time of the engine would exceed 15,000 flights.