PSI - Issue 46

Alok Gupta et al. / Procedia Structural Integrity 46 (2023) 35–41 Alok Gupta et al. / Structural Integrity Procedia 00 (2021) 000–000

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3. Results and Discussion 3.1. Bracket LCF responses

The displacement controlled LCF tests on the bracket were performed in two separate runs. In the first run, the cyclic test (BT1) was performed on the bracket with a displacement range of ± 1 mm which was chosen so that the plastic strains in the bracket elements are not significant i.e. total strain is close to the elastic limit. This was confirmed by reviewing the width of the first few cyclic loops before continuing to proceed to the full BT1 test. The cyclic test loops for the 1 st and the last cycles of the BT1 test are shown in Fig. 3(a), and the cyclic softening effects on the max, mean and min stresses are shown in Fig. 3(b).

Fig. 3. Bracket LCF test (BT1) results: (a) shape of the second and the last loops, and (b) Cyclic softening effects on max, mean and min stresses.

The BT1 test was performed for a total of 3439 cycles with no failure observed in any of the struts or connectors of the bracket. As seen in Fig. 3(a), the maximum loading force in the last cycle is lower than the maximum force in the first cycle which indicates a small level of cyclic softening at this level of cyclic loading. From Fig. 3(b) the cyclic softening is more evident only in the tensile part of the loading with no softening in the compressive part of the loading. In the second test run (BT2 test), the displacement range was increased to ±2 mm to increase the amount of plasticity. The BT2 test was to load the bracket such that it simulates the elasto-plastic strain loading level which could happen under an extreme design event of an aero-engine and to study the performance of the bracket at these displacement levels. In the BT2 test, the bracket experienced the 1 st local failure at the top of the right most main strut at the 15 th cyclic when a sudden drop in loading was observed, as shown in Figs. 4(a) and 4(b). The BT2 test was continued after the 1 st failure, and subsequently the 2 nd failure was noticed at the top of the left most main strut at the 286 th cycle, presented in Figs. 4(c) and 4(d). The history of the bracket during the BT2 test is shown in Fig. 4(e) which indicates that:  The bracket has significant residual strength (up to 65% of tensile loading) even after first two failures in the bracket.  The bracket has asymmetric stiffness where the loading in tension and compression are different. This asymmetric behavior is due to the thin struts and connectors of the bracket having subtle difference in their behavior under compression and tension loading.  Cyclic softening was observed during the bracket LCF test.