PSI - Issue 38

W. Radlof et al. / Procedia Structural Integrity 38 (2022) 50–59 W. Radlof et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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Keywords: lattice structures, electron beam melting, fatigue life evaluation, damage, digital image correlation, thermography, potential drop method 1. Introduction Additively manufactured (AM) structures have a great potential to be applied in load-bearing porous implant structures. Due to the flexibility of the manufacturing process, e.g. the electron beam melting (EBM) process, the porous structure design can be adjusted to achieve desired mechanical properties, for example to match the elastic modulus of human bone or to obtain individual ultimate and fatigue strengths. Besides the appropriate design and the choice of the material, the understanding of the fracture and fatigue behavior is fundamental for the reliable operation of AM load-bearing components, which are often fatigue-loaded. In the recent years, various studies have been focused on the investigation and further development of approaches for a damage and fatigue tolerant design of AM components and lattice structures as has been shown in the review papers of Zerbst et al., 2021 and Benedetti et al., 2021. Therefore, the application of basic concepts for the determination of the lifetime, the fracture and fatigue behavior as well as the influence of manufacturing related imperfections on the mechanical performance on AM parts were investigated. But, the authors have emphasized that the damage mechanism and damage evolution of AM structures under cyclic loading are not yet well understood. In addition, fatigue in AM structures is a highly local damage phenomenon due to the presence of manufacturing related imperfections. In this regard, it is necessary to determine local parameters such as stresses and strains for an accurate fatigue prediction. (Zerbst et al., 2021; Benedetti et al., 2021) Most research studies on AM lattice structures have been limited to uniaxial fatigue loading, with the majority to compression-compression (Benedetti et al., 2021; Radlof et al., 2021). However, if lattice structures are used in biomedical implants, they are exposed to complicated physiological stress-states, which necessitate further investigations for other load cases such as bending, shear or multiaxial loading (Hedayati et al., 2017; Pérez-Sánchez et al., 2018). The main problem in studying the mechanical behavior of lattice structures is that no standards are available for either the monotonic bending and torsion tests or for their fatigue testing. Therefore, this study is divided into two parts: Firstly, the design and the experimental testing of AM lattice structures, subjected to cyclic bending and torsion loading, is shown. Secondly, in the main part, the implementation of several image-based and in-situ measurement techniques into the experimental setups, for determining local phenomena, like strain hotspots, strut damage and crack growth, are presented. Thereby, a better understanding of the fracture and fatigue behavior will be provided by using digital image correlation (DIC), temperature field measurement and potential drop method.

2. Materials and experimental 2.1. Specimen design

The samples were manufactured by means of EBM using an A1 EBM machine (Arcam SE/GE Additive, Mölndal, Sweden) with Ti6Al4V ELI powder with a particle size in the range of 15 – 45 μm and a layer thickness of 50 μm. After manufacturing, residual powder was removed with the powder recovery system from Arcam. But, no other post processing was applied. Three different porous-designs (50%, 60% and 70%) with designed cubic unit cells were manufactured with varying strut thicknesses from 0.78 – 1.08 mm and pore sizes from 1.24 – 1.59 mm. The types of porosity-designs are listed in Table 1 and named as 50% porosity, 60% porosity and 70% porosity. Two types of samples were manufactured as shown in Figure 1. The cross section of the solid grip zones had a rectangular contour for bending testing (Fig. 1a) and a cylindrical contour for torsion testing (Fig. 1b). A solid transition zone connected the porous gauge sections with the respective grip zones to avoid stress concentrations.

Table 1. Porous structure designs.