PSI - Issue 38

Alok Gupta et al. / Procedia Structural Integrity 38 (2022) 40–49 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

44

5

respectively). The discrete stress – strain loops, taken at regular cyclic interval, for each cyclic test runs are shown in Fig. 5(c) and Fig. 5(d) for LT1 and LT2 tests, respectively. The LT2 test caused a higher level of plasticity meaning that the loops have become wider compared to those for the LT1 test. Note that the cyclic softening effect is also evident in Fig. 5(c) and Fig. 5(d), with a faster rate of softening for the LT2 test where fracture is caused in fewer cycles compared to the LT2 test.

(c)

(a)

Stage 1

Stage 1

Stage 2

Stage 2

(d)

(b)

Fig. 5. LCF test results: (a)  max and  min variation for LT1 test, (b) shape of cyclic loops at discrete cycles for LT1 test (c)  max and  min variation for LT2 test, and (d) shape of cyclic loops at discrete cycles for LT2 test

3.3. Bracket LCF tests The displacement controlled LCF tests on the bracket were also carried out in two separate runs. In the first run, the cyclic test (BT1) was performed on the bracket with a displacement range of ± 1 mm for a target number of 3000 cycles (minimum). This displacement range was chosen so that the bracket is loaded at or close to the elastic limit, i.e., the cyclic test loop comes with a small width, which was verified by checking the first test loop before continuing with the full BT1 test up to 3280 cycles. The cyclic test loops for the 1 st and last cycles of the BT1 test are shown in Fig. 6. As seen in Fig. 6, the maximum loading force in the last cycle is slightly higher than the maximum force in the first cycle which indicates a small level of cyclic hardening at this level of cyclic loading. The BT1 test was stopped after 3280 cycles at ±1 mm displacement range with no failure observed in any of the struts or connectors of the bracket. In the second run (BT2), the displacement range was increased to ±4 mm to increase the amount of plasticity. This was to simulate the loading under an extreme design event of an aero-engine and study the performance of the bracket at these displacement levels. The bracket performance under BT2 test was then studied against the cyclic data from the LCF coupon tests. In the BT2 test, the bracket experienced failure of a strut at 211 th cyclic when a drop in loading was observed. For the BT2 test, the cyclic test loops for the 1 st cycle, the 210 th cycle (the cycle just before