PSI - Issue 31

Sanjin Braut et al. / Procedia Structural Integrity 31 (2021) 33–37 Sanjin Braut et al. / Structural Integrity Procedia 00 (2019) 000–000

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engineering , Reid (2020). It is based on the data of blade fatigue strength given in Table 2. It can be seen that the estimated mean is µ = 151.63, MPa while the standard deviation is equal to σ = 6.934.

Table 2. Fatigue strength of the blade at 2·10 7 cycles obtained via Locati method

Fatigue strength at 2 · 10 7 cycles, MPa

No.

Excitation frequency, Hz

S-N curve One-slope model

S-N curve Two-slope model

1 2 3 4 5

382 384 384 382 385

156.5 143.6 144.4 149.9 160.9

157.9 144.7 145.3 150.4 162.7

Fig. 2. Compressor blade test sample

Fig. 3. Cumulative distribution function of fatigue strength at 2 · 10 7 cycles

4. Conclusion In this paper, a procedure for accelerated fatigue testing of a gas turbine compressor blade based on the modified Locati method is presented. The blades were made of the aluminum alloy 2618 (T851). The tests were performed on a vibration shaker with the frequency that corresponds to the blade’s first natural frequency. First, S–N curves were estimated according to two approximate representation namely one-slope and two-slope models. Then, the blades were tested at different stress levels starting at a level below foreseen fatigue strength at the specified fatigue life. In each loading block, a blade was subjected to 2ˑ10 7 cycles. After the first block was completed, the stress level in the second loading block was increased by 20 MPa and a new 2ˑ10 7 cycles were applied. A procedure was repeated until the blade failure occurred. It was assumed that blade failure occurs when the natural frequency drops by more than 2%. After the failure, the fatigue strength was calculated according to Palmgren -Miner rule. Finally, the mean fatigue strength of the compressor blade was determined as well as its standard deviation.