PSI - Issue 42

Numan Berat Yondu et al. / Procedia Structural Integrity 42 (2022) 1567–1575 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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After observing the fully cycle-dependent failure behavior of the material, dwell periods are added to the test matrix. Dwell fatigue tests are planned to simulate the engine’s operating stress levels by holding the specimen at max load for the duration of dwell. Instead of applying the static hold in each cycle, to observe the cyclic and static response of the material separately “Block” test method is used which is given in the Figure2 . Dwell durations are chosen with a proportion of the estimated test duration. The aim of increasing the dwell periods is to increase the time-dependent mechanisms. For most of the tests, the ratio between total dwell duration and total cyclic duration aimed to be maintained b etween “0,8 - 1,2” in average “1”, total duration of dwell application differs form 50 minutes to 420 minutes.

Figure2. Schematically representation of block test loading wave type Unlike from standard axial fatigue testing in which dwell application can be either tension or compression, in fully reversed rotating bending fatigue test during the holding periods specimens are subjected to both compression and tension stresses simultaneously. Due to the fact that fully reversed movement of the test setup surfaces subjected to tension and compression stresses are disturbed randomly. In Figure3 it can be clearly observed that at higher stresses with applying relatively short dwell periods, fracture mechanisms can be accepted to be cycle dependent since there is no significant change in failure cycles unlike the results in lower stresses.

TASNİF DIŞI

Figure3. High temperature dwell effect comparison At room temperature, effect of applied dwell duration showed more obvious decrease in life unlikely from high temperatures tests. Normalized Cycles Ratio (NCR) is used to understand materials dwell fatigue sensitivity which is basically ratio between dwell fatigue life results and non-dwell fatigue results . In Figure4 at room temperature life, the reduction is clear. Compressive and tensile balanced dwell loadings seem to be more effective at room temperature