PSI - Issue 39

Mario Álvarez-Blanco et al. / Procedia Structural Integrity 39 (2022) 379–386 Author name / Structural Integrity Procedia 00 (2021) 000–000

380

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1. Introduction In the last few years, the 3D printing technology or fused deposition modelling (FDM) has experienced an exponential growth of interest in several research fields, such as aerospace, industry and medicine (e.g., see Redwood et al. (2017)). The possibilities of complex manufacturing, large variety of compatible materials and, thus, a wide range of properties, generate many questions to be explored, see e.g. Gorelik (2017) and Khosravani et al. (2021). Additive manufacturing (AM) parts show anisotropic mechanical properties (influenced by manufacturing process), complex heterogeneous geometries for mechanical analysis (due to printing defects or nozzle trajectory, for example) and high strain rate dependence. All these features make the study of crack morphology in AM components a very interesting field of research, see Wiberg et al. (2019). Three-dimensional printed specimens often consist of two distinct parts: inner grid pattern and outer surface. They share similarities with sandwich composites, such as an internal honeycomb pattern with exterior shells, and the interest in bending behaviour and fracture mechanisms. Many works about crack analysis for sandwich composites have been published, as the work of Carlsson et al. (2005), but literature about fracture in printed materials is limited, e.g. Ameri et al. (2021). Therefore, the analysis of fracture patterns and mechanical behaviour at mesoscale and the study of defects and their influence in printed specimens are two topics of interest in this work. Hence, the main objective of this work is the analysis of the fracture path in polylactic acid (PLA) specimens under three-point bending test. The Digital Image Correlation (DIC) technique was applied during the experimental tests to record the strain field before and after the crack appeared. 2. Materials and methods 2.1. Test samples The printing material used in this work to manufacture the test samples is PLA (9051-89-2) using a 2.85 mm diameter filament, via FDM in the 3D printer Epsilon W50 . Table 1 shows some of the printing parameters set according to the material supplier’s recommendations.

Table 1. 3D-printing parameters. Parameter

Value

Unit

Density

1.24

g/cm 3

Layer height

0.4

mm

Hot end temperature Bed temperature Deposition speed Nozzle diameter

200

°C °C

45 40

mm/min

1

mm

Tensile samples were manufactured with a dog-bone shape and 100% infill core density for the mechanical characterization of the PLA under uniaxial tensile tests. The middle cross-section of the specimen is 10x4 mm 2 and 80 mm length, according to UNE-EN ISO 527-2 (2012). However, these standards are not specifically designed for AM materials and the radius at section transition of the specimen was increased to avoid printing defects in those critical areas. The manufacturing process optimization for AM tensile specimens is described in detail by Özen et al. (2021). Samples were created for the three-point bending tests, based on UNE-EN ISO 178 (2011). These specimens are rectangular prisms with a cross-section of 20x10 mm 2 and 100 mm length. They are composed of a grid pattern, determined by the infill core density percentage (under 100%), and an outer shell surrounding it. The lattice core is a squared grid orientated 45° with respect to the X-axis (see Figs. 1 and 2). The shell consists of vertical surfaces (walls) and horizontal surfaces (top and bottom layers), defined separately during the printing design. Fig. 1 represents the one-quarter cut of a 50% infill core density sample with one shell.