PSI - Issue 34

Zoé Jardon et al. / Procedia Structural Integrity 34 (2021) 32–38 Zoé Jardon/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction 1.1. Additive manufacturing of Metals

Additively manufactured components are questioned for their fatigue performance and therefore not adopted for safety critical applications so far. Due to high temperature gradients during the additive manufacturing process, material imperfections and residual stresses that are known to be detrimental for the fatigue performance of the components might be introduced. As stated by Mercelis and Kruth (2006), the residual stresses alter the component’s structural integrity and are one of the main sources of component deformation and cracking. Hence, the feasibility of integrating Structural Health Monitoring systems in “smart” metallic structures has been investigated (Stranza et al.,2015, De Baere et al.,2014). 1.2. Effective Structural Health Monitoring The effective Structural Health Monitoring system (eSHM), developed and patented by the Vrije Universiteit Brussel, fully exploits the flexibility offered by the 3D printing process by integrating a smart continuous monitoring technology inside additively manufactured parts. The system is based on the detection of pressure changes in 3D curved internal channels embedded in the fatigue critical regions of the component. The pressure is continuously monitored at the channel extremities by externally mounted pressure sensors. Based on the measurement of pressure variations, the system is capable of detecting the presence (Stranza et al.,2015) and finding the location of a fatigue crack (Hinderdael et al.,2016). The capillaries, represented in red on Fig. 1, are filled with air and set initially at a low pressure 1 . After the structure has been fatigue loaded, cracks can be initiated and will then propagate through the structure. If the crack encounters a capillary on its way, a contact arises between the outer pressure 2 and the low pressure volume inside the capillary 1 . Due to the pressure difference between both volumes 2 > 1 , the sudden crack-capillary contact induces the propagation of pressure waves through the capillary towards both capillary extremities and the sensors detect the presence of the crack. 1.3. Objectives This work proposes an assessment of the production of a parallelepiped fatigue sample (25x15x115mm) with integrated eSHM capillary using a hybrid approach combining both additive and subtractive drilling/milling operations. Different printing strategies are presented and compared in terms of final geometry and internal channel roughness, known to be crucial to avoid undesired fatigue initiation (Hinderdael, 2017) and to obtain an accurate detection and localization of the fatigue crack (Hinderdael, 2016, Jardon, 2019). The encountered difficulties during the production process are identified and solutions are proposed. 1.4. Hybrid DED-machine The production of the fatigue sample with integrated eSHM-system is performed using the MiCLAD in-house hybrid Directed Energy Deposition machine developed by the Additive Manufacturing Research Group of the Vrije Universiteit Brussel, presented in the work of Ertveldt et al. (2020). It is equipped with a 5-axis CLC control and has the particularity to allow the combination of both DED additive and subtractive high-speed drilling/milling operations for the production of a part. Fig. 2 shows the experimental set-up and identifies every component of the machine. Directed Energy Deposition (DED) is one of the most known and widespread technologies of 3D metal printing characterized by the simultaneous focused thermal laser energy and powder delivery (ASTM F2792-12a). During the Fig. 1 : effective Structural Health Monitoring system (eSHM) : working principle