PSI - Issue 28

James Allum et al. / Procedia Structural Integrity 28 (2020) 591–601 J.Allum et al. / Structural Integrity Procedia 00 (2019) 000–000

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2. Methods and materials This section details the methods and materials utilised in this study, including details of the manufacturing process, specimen preparation, measurement and mechanical characterisation. 2.1. MEAM process Specimens were manufactured using natural PLA (3DXTECH® branded Nature Works® polylactide 4043D, Sigma Aldrich) with a density of 1.25 g/mm using a RepRap X400 3D-printing system. Specimens were produced as hollow four-sided boxes with walls formed by individual extruded filaments. Geometry was defined by direct custom GCODE created with scripts and in-house software (FullControl GCODE Designer – contact the corresponding author for details). This enabled precise and accurate nozzle positioning throughout the deposition process as well as explicit control of print speed, extrusion volume and manufacturing sequence (unidirectional deposition) as shown in Fig. 1. dogbone specimens for tensile testing were developed at the scale of individual extruded filaments by controlling the volume of extrusion at different stages of the toolpath sequence (Fig. 2), made possible by the direct GCODE development. Two four-sided box types were generated, differentiated by the orientation of their dogbone form relative to the build orientation (Fig. 2). This enabled generation of specimens loaded in the F direction (longitudinally) (Fig. 2 (a)) and Z direction (transverse - normal to filaments) (Fig. 2 (b)). All boxes were produced with wall dimensions of 45 mm (H) x 45 mm (W). The wall thickness was 0.5 mm in the gauge region, incrementally widening to a shoulder with thickness of approximately 2x the gauge thickness (controlled by extrusion volume). The boxes were deposited from a heated nozzle at 210 °C and platform temperature of 60 °C. Printhead speed was constant at 1000 mm -1 . Custom GCODE ensured the nozzle position and deposition sequence was precisely controlled at all stages. Thus, any direct thermal variation across the geometry was prevented by maintaining a symmetrical toolpath (as shown in Fig. 1) and a constant print speed. Every region of the part geometry was subjected to the same cooling times and shared the same thermal history.

Fig. 1. Symmetrical printing strategy utilised to manufacture four-sided hollow boxes. (The arrows indicate the print nozzle movement along the toolpath).

2.2. Specimen preparation Specimens were prepared by the following sequence: (i) The four-sided hollow boxes were cut at each of the corners with a razor blade mounted in a specially made tool, resulting in four equal walls. (ii) Using a second specially designed tool, the walls were cut using an array of seven razor blades spaced 5 mm apart from each another. A hydraulic press employed to compress the blades into the walls ensured even and consistent pressure and a clean cut. This yielded six specimens from each wall (each with a width of 5 mm) with a total of