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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1. Introduction The SLM process, a type of Additive Manufacturing (AM), offers advantages such as high material usage efficiency, freedom to manufacture complex shape parts and avoidance of tooling costs [Gupta et al. (2021)]. Ti-6Al 4V offers a high strength to weight ratio and has been widely studied and considered to make the weight sensitive parts in the aerospace industry [Baragetti et al. (2019)]. Over 50% of component failures in the aviation industry are attributed to fatigue [Gorelik (2017)]. On an aero engine, brackets are used to support the externals accessories such as the sensor/controls unit, electrical harness or pipes and are designed to withstand engine driven accelerations to meet the CS-E 650 certification requirement set out by the European Aviation Safety Agency [European Union Aviation Safety Agency (2018)]. Additionally, as per CS-E 810, it needs to be demonstrated that no complete detachment of the bracket, leading to a hazardous situation, will occur in an extreme event such as a Fan Blade Off (FBO) event [European Union Aviation Safety Agency (2018)]. Due to the stringent certification requirements and complex engine operating environment, conventionally made bracket designs are often non-optimised. Brusa et al. (2017) studied the fatigue performance of AM parts built using two different AM processes whereas Leuders et al. (2017) compared the fatigue performance of a topologically optimised component made using the SLM and Investment Casting (IC) processes through an experimental study, but both these studies provided limited relevance to the real applications. Mardaras et al. (2017) suggested a simplistic approach to perform fatigue assessment of AM parts, but they did not present an actual case study. Although these studies look promising, the information published so far is limited and does not cover the wide enough scope to successfully implement AM parts in load bearing applications [Gupta et al. (2021)]. In this paper, the fatigue performance of a weight optimized SLM Ti-6Al-4V bracket has been presented. The bracket design was aimed to support an electronic sensor on an aero-engine. The modal and fatigue performances of the bracket were evaluated through shaker table testing and the minimum fatigue ‘g’ load capability was established. 2. Experimental Campaign 2.1. Test set-up The SLM bracket studied in this paper was named as the ‘struts & connectors’ shape bracket and is shown in Fig. 1. The bracket was designed with struts running in the longitudinal and transverse directions as load carrying features to support a sensor unit on the engine. The main load carrying struts run in the longitudinal direction whereas the transverse running struts, termed ‘connectors’, were to provide lateral stiffness to the load carrying struts. The SLM bracket was optimized for its weight and was designed to meet the three main requirements: (i) the fundamental frequency of the bracket assembly shall be outside of 1 st Low Pressure (LP) excitation frequency of 48 Hz, (ii) the bracket assembly shall have a minimum ‘g’ load capability of ‘20g’ and (iii) the bracket shall have some level redundancy in the load path. The minimum ‘20g’ capability was required for the SLM bracket to cope with the vibrations from other prime sources during engine normal operation and wind-milling vibrations post an in-flight shutdown of the engine, may be due to an ultimate failure event. The optimized AM bracket mass was 91.7 grams which was significantly less (> 50%) than the mass of an equivalent sheet metal Ti-6Al-4V bracket (200 grams). The configuration and mounting of the bracket assembly for the shaker test are shown in Fig. 2. A dummy aluminium block was machined to represent the sensor unit and was integrated with the bracket using two 0.25”  bolts. The total mass of the bracket assembly was 306.7 grams (Bracket mass = 91.7 grams, dummy block mass = 179 grams and fasteners mass (2 off) = 36 grams). An additional aluminium fixture was machined as an interface between the SLM bracket and the shaker table. The bracket was attached to the interface fixture via three Inconel 718 0.25”  bolts tightened to a torque of 15.2 N.m. The interface fixture was fastened onto the shaker armature using four bolts (of M10 size) in high strength steel. The overall fixture assembly was designed as a stiff assembly so that it did not alter the dynamic characteristics of the bracket assembly. To support the test planning, a preliminary Finite Element (FE) modal analysis was performed in Ansys Mechanical v19.1 to identify the fundamental mode of the bracket assembly. The FE mesh consisted of the quadratic tetrahedral (Tet10) elements with 6.44 x 10 5 nodes and 4.10 x 10 5 elements. The boundary conditions were defined to simulate