PSI - Issue 68

Shanyavskiy A. et al. / Procedia Structural Integrity 68 (2025) 453–457

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A. Shanyavskiy et al. / Structural Integrity Procedia 00 (2025) 000–000

Shanysvskiy (2021) and Shlyannikov (2019) and growth toward the hub part. The fatigue strength and durability of this part is estimated and designed based on low cycle fatigue (LCF) criterion by Shanyavskiy (2003) and Williams (2003). However, the SEM investigation on the morphology of compressor disk fracture pattern shows the subsurface fatigue crack initiation that is typical for fracture in the very high cycle fatigue (VHCF) regime by Bathias (2004), Sakai (2004), Shanyavskiy (2022), Pan (2020), Sakai (2010), Ranc (2022) and Yoshinaka (2023). The discovered discrepancy between designed loading conditions and observed features of a real in-service fracture pattern for I-stage compressor disk stimulated the study on regularities of fatigue crack growth in two-phase titanium alloy used for compressor disks under high-frequency loading. Fatigue cracks in aircraft structures grow usually during the ‘flight cycles’ producing the clear beach marks on fracture surface (Fig. 1) by Shanyavskiy (2003). To simulate the in-service loading condition for the compressor turbo-jet engine disk and determine crack path, the mathematical modeling was performed. Considering the same peak loading regimes for a regular civil aviation flight it can be noted that SIF of existing crack will stepwise increasing from flight to flight due to corresponding crack length increment. The experimental procedure was designed to simulate such stepwise change in SIF values with pauses between cyclic loadings. In this case the fracture pattern of cracked specimens has beach marks that are similar to the features of compressor disk fracture surface.

Fig. 1. Fracture surface of compressor disk with beach marks (dotted lines) and crack origin (indicated by arrow).

2. Material and experimental procedure The FCG specimens were machined from the turbo-jet engine compressor disk made of two-phase titanium alloy Ti-6Al-4Mo. The compressor disk was in-service for a regular designed lifetime of 8000 hours and did not show any cyclic degradations of mechanical characteristics. The FCG specimen has a rectangle cross section with reduced gage section. The artificial edge crack was machined in the plane of the maximum stress. The shape and dimensions of the specimens are shown in Fig. 2(a). The mechanical characteristics of the material were determined according to standard ASTM E-8 by using the tensile specimens machined from the compressor disk. The obtained data on the material state are following: yield stress σ Y = 960 MPa, ultimate tensile strength σ U = 989 MPa, density ρ = 4500 kg/m 3 . The FCG tests were performed by using ultrasonic piezoelectric fatigue testing system working at 20 kHz by Bathias (2004). The specimen was subjected to fully reversed cyclic loading ( R = –1). The piezoelectric system allows the control the displacement amplitude applied to the specimen. The displacement amplitude was re-calculated into stress intensity factor range by using numerical simulation. The loading was realized in the stepwise way at different SIF levels. Within one test run the SIF value was assumed to be constant i.e. the deviation of SIF is less than 7% of the initial value as in standard ASTM E-647. The specimen was cooled by dry compressed air during the test. The calibrated digital camera with an optical system for image magnification was engaged to record and control the crack size at the lateral surface of the specimen. The one specimen was subjected to the series of cyclic loading runs at different SIFs. The criterion of a single run stop was reaching a given number of cycles ( N run = 10 7 cycles) or a given crack increment leading to SIF deviation more than 7 %. The first specimen was loaded in the way of SIF decreasing