PSI - Issue 38

Alok Gupta et al. / Procedia Structural Integrity 38 (2022) 40–49

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Author name / Structural Integrity Procedia 00 (2021) 000 – 000

failure) and the 212 th cycle (the cycle immediately after failure) are compared in Fig. 7. From Fig. 6 and Fig. 7, the maximum force in the tension appears to be higher as compared to the force in compression which indicates an asymmetric stiffness of the bracket. This was expected because the thin struts and connectors of the bracket will have tendency to buckle with increasing loading, thus lowering down the overall stiffness (hence strength), under compression. As seen in Fig. 7, the BT2 test had wider loops indicating that the bracket is cycling with increased levels of plastic strain. In the BT2 test, some cyclic softening was observed in the bracket performance, similar to the behavior observed in coupon test results in Fig. 5. The location of strut failure is shown in Fig. 8.

Fig. 6. Bracket LCF test loops for BT1 test

Fig. 7. Bracket LCF test loops for BT2 test

4. Property – Performance Relationship 4.1. LCF properties

For both the LCF coupon tests, see Fig. 5(a) and 5(c), the softening in the initial part of the test (in stage 1) is larger which subsequently (in stage 2) gets stabilized with a lower softening behavior. The stage 2 represents a period of crack growth before fracture of the specimens. An SEM examination of fracture surface of the failed LCF specimens from LT1 and LT2 tests was carried out to study the fracture mechanism (see Fig. 9). The fracture surfaces of both the specimens look similar, where a distinct crack initiating feature and a central crack growth region surrounded by shear lip formed during final fracture were witnessed, as seen in Fig. 9(a) and Fig. 9(b). The initiation point for the LT1 coupon is marked as the discontinuity formed due to the combined effect of rough surface and a pore, refer Fig. 9(c). Whereas the initiation point for the LT2 coupon is due to a sub-surface Lack of Fusion (LOF) void, as shown in Fig. 9(d). A rough crack growth region seems to indicate the inter-granular crack growth with some small layer peeling-off effect was also seen. This examination suggests that the LCF strength of SLM Ti-6Al-4V material is dependent on the microstructure, surface roughness and presence of defects (LOF voids and pores) [Y. M Ren et al. (2019)]. The post build Hot Isostatic Pressing (HIP) process is often adopted to reduce the size of, or even close, the inherent defects inside AM parts/specimens to achieve better fatigue performance [Gorelik (2017), Gupta et al (2021a)]. The cyclic softening behavior is linked to the evolution of grain size and dislocation density [Kamaya (2009)], and also to the possible initiation of micro-cracks in the areas of relatively large local misorientations [Kamaya (2012)]. The EBSD scans were carried out on the fractured LT2 test specimen (on longitudinal cut) as shown in Fig. 10. The EBSD scans were taken at 1 mm distance away from the fracture surface (loc. 1, Fig. 10) and also near to the fracture surface itself (loc. 2, Fig. 10) to study the variation in microstructure and any possible accumulation of plastic strain due to pile-up of dislocations. The bulk of the material (at loc.1 in Fig. 10(a)) is dominated by the α’ martensitic grains which are of needle shape with high aspect ratio. T he martensitic α’ needles are finer and smaller in the area close to