PSI - Issue 66

Nur Mohamed Dhansay et al. / Procedia Structural Integrity 66 (2024) 87–101 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Tensile tests were conducted according to the ASTM E8 standard. Specimens had gauge diameter of 5 mm and gauge length of 25 mm. tests were carried out on a Criterion Model 44 machine (MTS, Minnesota, USA) with a displacement rate of 0.13 mm/min

Fig. 1. Compact tension specimen orientations(a) ZX or Edge orientation; (b) XZ or Vertical orientation and (c) XY or Flat orientation.

2.3. ∆ Kth testing Fatigue crack growth rate testing was conducted at ambient laboratory conditions on an Electropuls E3000 dynamic tester utilising the da/dN software package (Instron, Norwood, United States). The crack mouth opening displacement (CMOD) compliance technique was used to capture crack length. This was periodically checked by visual measurements. Sinusoidal type loading was selected and frequencies ranged between 60-80Hz. Investigations conducted by Wanhill (Wanhill, (1974)) and Boyce and Ritchie (Boyce and Ritchie, (2001)) observed negligible change in the Δ K for a given crack growth rate at frequencies of 0.1-50 Hz and 1000Hz. These were also conducted at various stress ratios and Kmax values. Concluding that there are no adverse effects on results due to frequency change. Therefore, it was assumed that the frequency range used in this study did not have an adverse effect on Δ Kth. Δ Kth testing utilised the load shedding technique in accordance to the ASTM E647 (ASTM International, (2013)). The load shedding gradient was set at – 0.08mm -1 . The near-threshold FCGRs are evaluated using the two-parameter approach outlined by Sadananda and Vasudevan and Vasudevan et al (Sadananda and Vasudevan, (2005); Vasudeven et al., (1994)). It is understood that without these two critical parameters being achieved, a crack will not propagate (Vasudeven et al., (1994)). 3. Results 3.1. Microstructure The LPBF AF, SR and DA bi-modal microstructures are shown in Fig. 2. With regards to the AF and SR microstructure, we find the typical needle-like fine martensitic ( α ’) microstructure, along with columnar PGB in the build direction. The martensite has a hierarchical nature as discussed by (Ter Haar and Becker, (2018); Yang et al.,