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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used in tests stress level for constructing Eq. (3 ) with factors “ C ae ” and “ b ” has to be lower in comparison with in service acting average stress level on in-flight operated units. The number of cycles for the average value of N̅ ae can be considered in dependence on test frequency for a unit. Usually, test duration in t =2 min is enough for determination of factors “ C ae ” and “ b ” with a high correlation factor for Eq. (3).

3. Durability diagnostics by AE signals appeared in the second semi-cycle of bending loading

The discussed problem can be resolved if unified material plastic deformation, ε 01 , in the semi-cycle of loading dependence on durability, N f , exists. Based on the results of the investigation presented in the preceding part of the paper one can be concluded that relation between the summarized value of N ae in the second semi-cycle of loading and ε 01 takes place in the form:   p ae N C 00 01   (10) 3.1. Tests procedure and results This consideration was attested on the group of turbine blades of superalloy EI437B with spherical shape of γ’ phase. Each blade placed in the special tests-bed (see Fig. 2) pressed by the root, and on the flat surface, piezoreducer was installed. Then all blades were bent up to a stress level not exceeded 1% of the average value of in-service stress for the tested blade. This procedure was directed to the uniformly prepared state of material blades for AE-signal analyses because different blades experienced an in-service difference in bending stress-state. Then, again all blades were bent up to stress level with 1% more than preceding stress-level of bending. During this second bending summarized value of AE-signals was registered. After the described procedure blade fatigue tests were performed by the first bending form on the resonance regime. Statistical analysis of received tests data has shown that between ( N ae ) 01 in the second semi-cycle of loading and durability N f exists good correlation with correlation factor near to 0.99 in the next form:   01 7.22 1.432lg lg ae f N N   (11) Based on the discussed above test results, the methodology of lifetime diagnostics for α -criterion appearing because of crack origination was considered. At the beginning of tests, a constant time interval (for example, for 3 minutes) was considered in which summarized AE-signals value, ( N ae ) t , was registered. The discovered value of ( N ae ) t was compared with a registered in-tests number of cycles, ( N f ) α , when α -criterion appeared in accordance with Shanyavskiy and Banov (2014) following well-known rule using the next relation:     m ae t f N C N    (12) The discussed methodology was attested applicable to turbine blades of superalloy EI437B with spherical shape of γ’ phase. The same test-bed was applied to a blade bending that shown in Fig. 1. Piezoreducer for AE-signal registration was installed on the same flat root area of a blade that was discussed above. Two stress levels of 260 and 280 MPa were chosen for this series of tests. 4. Diagnostics of α -criterion Consequently, durability has to determined based on the same form of Eqs. (2), (3) but with another factor.