PSI - Issue 23

A.A. Shanyavskiy et al. / Procedia Structural Integrity 23 (2019) 57–62 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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with dramatic changes in their geometry. On the other scales, metal has macroscopically the same geometry that was seen before activity in cyclic loading. In the localized area on the meso- or microscale, there will be appeared fatigue crack in the future because the process of plastic deformation takes place due to damages accumulation. Nevertheless, one can be concluded that metals volume around appeared in a future origin experienced plastic deformation. This process can be described based on the well-known Coffin-Manson relation:   k f C N      (1) In each cycle of cyclic loads, accumulated damages characterized summarized AE signals, N ae . Consequently, one can be concluded that average N̅ ae -value dependence on Δ ε Σ -value can be considered in the same form that presented in Eq. (1) and for diagnostics of durability by the AE signals we can use the equation in the next form:   b ae ae f N C N  (2) Specimens of steel with Ultimate tensile stress of 1250 MPa and turbine blades of superalloy EI437B having the spherical shape of γ’ phase were used for the approbation of discussed methodology for durability diagnostics. Tests of specimens were performed on the resonance frequency using the first bending form of vibrations. The special test-bed was constructed by Banov et al. (1982) and Troenkin et al. (1982), Fig. 1, with: (1) recorder of AE signals; (2) preliminary multiplier; (3) filter; (4) block of AE-signals transformation; (5) display for AE-signals visual control; (6) device for energy registration; (7) device for registration of summarized AE signals; (8) generator for loading specimens; (9) device for registration of the number of cycles. 2.1. Tests procedure

Fig. 1. Schema of the designed testing complex with test-bed for blades and system of AE-signals registration and their analyses (see comments in the text).

A transducer for AE-signals registration placed on the plane surface units and was fixed by the spring. Acoustic contact was carried out using special oil. Special device numbered AE-103 was used for AE-signals transformation. During tests, values of such summarized AE-signals, N ae , were registered as an average value in the range of 4∙10 4 - 5∙10 4 cycles. In tests were used the next stress levels for the critical section: specimens of steel and blades were cyclically bent at 270, 285, 298 MPa and 300, 330 MPa respectively. On each stress level, for correct statistical analysis of test results, 8 to 15 units were tested.

2.2. Test results

Systematical behavior of N ae dependence on the number of cycles, N f , was revealed, Fig. 2. There can be considered several stages in AE-signals formation on the discussed dependence.