PSI - Issue 38

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

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leaving the gauge portion of the specimens as built. The as built SLM Ti-6Al-4V bracket geometry in built orientation is shown in Fig. 2. The bracket design is a ‘struts & connectors’ shape where the main struts are to carry the operating load and auxiliary struts and connectors are to provide stiffness and strength to the main struts [Gupta et al. (2021b)].

(a)

(b)

Build Direction

Fig. 1. Dimensions of SLM Ti-6Al-4V test specimens: (a) tensile and (b) LCF test specimens (dims in mm): (not to the scale)

Fig. 2. SLM Ti-6Al- 4V ‘struts & connectors’ bracket geometry (dims in mm): (not to the scale)

2.2. Experimental Set-up The tensile coupon tests were performed using an electric Mayes 250 test rig which was fitted with a Mayes 250 kN load cell. A pair of LVDTs were used to measure the test strain rate (SR). Two room temperature (RT) tensile tests were done, each at two different SR: 0.0002 s -1 (TT1) and 0.0265 s -1 (TT2). The load application was parallel to the build direction, i.e. perpendicular to the layer deposition direction. The LCF material tests were conducted on a Tinius Olsen H25KS electromechanical testing system fitted with a Tinius Olsen DSCCTOL 25 kN load cell. The strain was measured using a high temperature side-contact extensometer. Fully reversed strain controlled LCF coupon tests (load ratio = −1) at RT were performed with a constant SR of 0.002 s -1 until fracture of the specimens. The LCF tests were performed under symmetric push-pull conditions using the saw-tooth type (SWT) waveform at two different strain (total) values: ±1.0% (LT1) and ±1.2% (LT2), and the objective was to understand the cyclic response of SLM Ti-6Al-4V material under these strain levels which are much in excess of the strains expected in the brackets under normal operating conditions of an aero-engine. The LCF test on the bracket was conducted on the same Tinius Olsen H25KS electromechanical testing machine. The set-up for the LCF test on the bracket is shown in Fig. 3 where the bracket is installed on the test machine via the

bolted top and bottom fixtures. In this set-up, the bottom fixture remained stationary whereas the top fixture travelled by a pre set distance. A LVDT was used to control and measure the movement of the bracket. The bracket LCF test was again a displacement controlled (load ratio = −1) test using the SWT waveform. Firstly, the bracket was tested at a displacement value of ±1 mm (BT1) for a target 3000 LCF cycles without failure. In the second test run, the same bracket was tested at ±4 mm displacement value (BT2). The BT1 and BT2 tests were to prove the performance of the bracket under the maximum normal operating loading and the extreme loading event respectively. To optimize the overall testing time, the BT1 test was at a Displacement Rate (DR) of 0.002 mm/s and the BT2 test was at a DR of 0.008 mm/s. Furthermore, it was also required to show that the bracket has redundancy in the load path i.e., the bracket does not show a significant drop in its load carrying capability in the case of a failure of a strut or connector.

Top grip (moving)

Top fixture

Direction of travel

Bottom grip (fixed)

Bottom fixture

Fig. 3. Bracket LCF test set-up