PSI - Issue 37

Yifan Li et al. / Procedia Structural Integrity 37 (2022) 41–48 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Since the octet-truss lattice was proposed by Fuller (1961), it has received widespread attention from different researchers. Mechanical properties of octet-truss lattices have been studied extensively using theoretical and experimental methods. The analytical models revealing the relationship between relative density and mechanical properties (including modulus, strength and fracture toughness) of octet-truss lattices are proposed and validated (O’Masta et al. 2017; Chen et al. 2018) . The effects of lattice orientations on the mechanical behaviours were also studied by Gu et al. using numerical and experimental methods (Gu et al. 2019a; Gu et al. 2019b). Fatigue failure is one of the most common failure modes of lattice structures because lattice structures in real applications are frequently subjected to cyclic mechanical or thermal loadings. The cell morphology and porosity effects on the compressive fatigue behaviour of lattice structures have been experimentally studied by different researchers (Yavari et al. 2015; Zhao et al. 2016). Different numerical methods were also proposed by different researchers to predict the lattice fatigue life and fatigue crack propagation (Burr et al. 2020; Hedayati et al. 2016; Hedayati et al. 2018). However, no studies have been done about the orientation effects on the fatigue crack growth of octet-truss lattice. As a lattice structure with great potential for use, it is necessary to understand the fatigue response of octet-truss lattice structures in different orientations. An experimental method has been done to study the lattice orientation effects on the fatigue crack growth of octet truss lattice in the present work. Lattice specimens in different orientations were printed using photopolymer with compact tension specimen geometry. The crack paths and propagation rates for different orientations lattices were obtained and compared with each other in this research. 2. Experimental procedures 2.1. Specimen design and manufacture All the lattices in different orientations are 3D printed by using photopolymer into compact tension (CT) specimen geometry according to ASTM E647-15 standard (2015). The definition of lattice orientations and configuration of unit cell structure are shown in Fig. 1. The lattice samples are defined as X, Y and Z orientations. The orientations of octet-truss lattices were defined based on the loading direction and local coordinate of the unit cell. For example, the load direction of orientation-X lattice is parallel to the x-axis in its local coordinate.

Fig. 1. Definition of lattice orientations.

The geometry of the lattice specimen used in this work is shown in Fig. 2. The designed nominal radius of the struts r is 0.5 mm and their nominal length l is 5 mm. The macro dimension of CT specimen designed to be large enough to