PSI - Issue 31

Alok Gupta et al. / Procedia Structural Integrity 31 (2021) 15–21 Alok Gupta et al. / Structural Integrity Procedia 00 (2019) 000–000

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However, in the subsequent test run at ‘80g’ level, a sequential failure of struts/connectors of the SLM bracket was observed, as listed in Table 2. The first failure was in the middle diagonal strut which fractured after 2315 cycles at ‘80g’ level, see Fig. 6. It can be safely assumed that the first crack in the middle diagonal struts may have initiated during the test run at ‘60g’ level after 635,866 cycles when a drop in modal frequency (of 3 Hz) was noticed which later resulted in fracture at ‘80g’ level, see Location 1, Fig. 6. To understand the failure mechanisms, the fracture surface of failed location 1 was analyzed using a Quanta FE600 SEM machine to characterise the fracture surface. Fig. 7 shows the Scanning Electron Microscope (SEM) micrograph depicting the details of fractured strut at location 1. The crack initiated from the surface at the geometric discontinuity feature characterized by a Lack of Fusion (LOF) void, as seen in Fig. 7a. A combination of high stress and the presence of stress raising feature was the reason for location 1 to become the prime location of first failure under High Cycle Fatigue (HCF). Fig. 7b shows some distinct beach marks on the fracture surface which typically suggest that there has been a variable crack front advancement with every loading cycle i.e. interruptions. From this it is inferred that the vibration ‘g’ loading was severe enough to leave the beach marks on the fracture surface during the final propagation stage and before a complete fracture of the strut happened. The ‘struts & connectors’ SLM bracket was designed to have a minimum vibration load capability of ‘20g’ in its assembled state. The fundamental vibration mode at 84Hz had its modal deflection response predominantly in the axial direction (Fig. 4), i.e. the weakest direction to cause fatigue damage, should an excitation happen at this frequency. The fundamental vibration mode fell far away from the 1 st LP shaft speed (48Hz) of the targeted aero engine application, thus met the first design requirement, which ensured that the SLM bracket would not experience resonance due to 1 st LP shaft forcing frequency during the engine operation. From the dwell tests, although no integrity issues were observed during the test run at ‘60g’ level, a slight drop in modal frequency of the bracket assembly at 635,866 cycles at ‘60g’ level indicated that the crack may have initiated in the middle diagonal strut (location 1, Fig. 6) which later resulted in complete fracture of the diagonal strut during the next test run at ‘80g’ level after 2315 cycles. Therefore, it can safely be inferred that the SLM bracket has at least ‘40g’ capability for 10 6 cycles. Also, a sequential fracture of the bracket struts or connectors (Fig. 6, listed in Table 2) with a good number of cycles between each of the failure locations proved that the ‘struts & connectors’ shape SLM bracket has the required redundancy in the load transfer path.

Table 2. Shaker table dwell test observations and results summary.

Dwell Test Sequence

Dwell Test Blocks

Number of Cycles

Observation

‘g’ Level

Frequency (Hz)

1 2

30g 40g

84 84

172,239

A preliminary dwell test to ensure system performs as expected.

1,000,000

No integrity issues were reported.

A slight drop in fundamental frequency (to 81Hz), but no cracks/integrity issues were reported. Test was continued to complete 1x106 cycles at ‘60g’ dwell. Total cycles at ‘60g’ dwell = 1,000,000, No struts were observed to be cracked or fractured. Middle diagonal strut found fractured (Location 1, Fig. 6). Middle main strut found fractured (Location 2, Fig. 6). Cross connector was found fractured (Location 3, Fig. 6). Right main strut was found fractured (Location 4, Fig. 6). Total cycles at ‘80g’ dwell = 50,919 cycles Left main strut was found fractured (Location 5, Fig. 6).

3

60g

84

635,866

4

60g

81

364,134

5 6 7

80g 80g 80g

81 79 77

2,315

16,225 24,110

8

80g

68

8,269

9

20g

60

17258

In summary, the fatigue performance of the SLM bracket exceeded the target value of ‘20g’. Many more component tests of such nature should be performed in future to prove the maturity and establish scatter in the fatigue performance of SLM components.