PSI - Issue 49

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Teiba Ahmed et al. / Procedia Structural Integrity 49 (2023) 37–42 Teiba Ahmed et al. / Structural Integrity Procedia 00 (2023) 000–000

Sine wave designs were created using custom G-Code generation software (FullControl GCode Designer) (Gleadall, 2021). Standard CAD model slicers are only capable of producing walls with a minimum of two lines because a CAD model for a thin wall has two sides (i.e. external CAD surfaces) and slicer algorithms are designed to print all external surfaces in the model. In contrast, a single printed line down the medial axis of the wall is possible with custom print paths. Single-wall designs were chosen for increased printing speed and because two wall designs contain internal pores between the walls, which lead to a reduction in mechanical strength (Allum et al., 2020). The specimens were printed in a box formation to increased wall stability during the print and increase the rate at which specimens could be manufactured (Fig 2. (b)). Non-printed travel lines on the corners of the box allowed for ease of sample retrieval (Fig 2. (a)). The box design allowed four replicates of each specimen to be manufactured. Three were tested and averaged for characterisation, whilst one was kept as a spare. The sinusoidal wave design is 30 mm in length. A 10 mm straight extrusion (tail) was used to stabilise the print after every corner and to provide area for clamping in the mechanical testing machine (Fig 2. (c)). Sinusoidal designs had a fixed amplitude of 3 mm, as any larger was expected to be obtrusive or undesirable for the wearer. Wavelengths ranged from 5 – 15 mm and infinity (control straight sample). The exact wavelength was dictated by the number of waves in the specimen to ensure there was always an integer number of waves to allow consistent testing. Three thicknesses of each design (1.52 mm, 2.00 mm and 2.40 mm) were printed and denoted as thin, medium, and thick. The layer height was 0.48 mm, 0.48mm and 0.8mm respectively. These parameters were found experimentally to achieve good print quality that was expected to meet aesthetics-expectations of end-users. All specimens were 3D printed in Ultimaker branded PLA on an Ultimaker 2+ using a 0.8 mm diameter nozzle.

Force

Top View

(c)

(a)

(a)

print path

printhead

sinusoidal design 30mm

tail 10mm

print path

pla+orm

cut and retrieve sample

non-printed travel corners

Fig. 2. (a) Top View showing non-printed travel corners; (b) print head deposition on platform; (c) final specimen retrieved showing end for force application.

2.2. Cantilever Testing Parameters Cantilever testing was deemed most appropriate as this is representative of the in-situ stress for wearable orthotic devices. A jig was created to clamp the specimens to one side of standard 3-point bend testing rig. Mechanical testing was performed on an Instron 3345 machine with a 1 kN load cell, applying force lateral to the length of the specimens, ensuring that the loading pin was directly above the apex of the final sine wave. To prevent the final sinewave from contacting the rig during specimen deflection in the test, a 3 mm protrusion of the tail was required to ensure sufficient clearance (Fig. 3. (a)). Tests were taken to a maximum of 14 mm displacement which was the upper limit of the samples. The speed of the loading pin was 5 mm.min -1 to ensure quasistatic loading of the samples in a reasonable time (< 3 minutes). One specimen from each group was cut between the clamped portion to the point