## 2 Material Testing
### Tensile Testing  
The nominal values of the minimum breadth \(b_{\text{t,1}}\), thickness \(b_{\text{t,2}}\) and overall length \(L\) of the tensile specimens are in accordance with EN ISO 527-2 [33] and are shown in Figure 2a. Geometric models designed in accordance with these standard specifications have been developed in standard technical modelling software and then transferred to PrusaSlicer 2.4.0 [34] software to prepare Gcode toolpathing instructions for the printer. To assess the orthotropic response of the printed polymers, the specimens have been printed in three different orthogonal layering orientations, with three specimens printed in each orientation for statistical confidence. The co-ordinate system shown in Figure 2b has been defined to facilitate reference to these orientations, with the global \(y\)-axis aligning with the layering direction during printing, and the \(x\)- and \(z\)-axes defining the plane of the printing bed. The printing orientations have been labelled to reflect the global axis that aligns with the major cross-sectional axis of the specimen during printing. Of particular interest is the behaviour of the specimens printed in the z-orientation where the axial stresses are applied across successive printed layers during testing. A total of 18 tensile specimens have been printed in the present study, with properties including dimensions \(b_{1}\) and \(b_{2}\) and the coefficient of variation (COV) of these dimensions shown in Table 2. The specimens have been printed with 100% infill in order to assess the material behaviour directly, as opposed to any metamaterial response arising from a geometric infill. The tensile specimens are labelled thus: T[test identification number]-[material]-[printing orientation according to Figure 2b]-[specimen index]. The operational parameters recommended by the printer manufacturers for PLA and ABS have been applied during printing, which are a filament deposition diameter of 0.2-0.4 mm, a deposition rate of 10 g/h, nozzle application temperatures of 210 \({}^{\circ}\)C and 240 \({}^{\circ}\)C for PLA and ABS, respectively, and a chamber temperature of 80 \({}^{\circ}\)C; heat treatment was not applied to the specimens post-printing. The measured lengths of the specimens are all within 170 \(\pm\) 0.3 mm, equating to a maximum deviation of 0.17% from the nominal length. The values of the cross-sectional dimensions \(b_{\text{t,1}}\) and \(b_{\text{t,2}}\) shown in Table 2 are the average of three measurements taken along the narrow central segment of the tensile specimens; the coefficients of variation (COV) of the deviations of the three cross-sectional measurements with respect to the nominal dimensions are also shown in Table 2. Printing accuracy tends to be highest in the direction of layering (the \(y\)-axis), and so deviations are smallest for the dimension aligned with that axis, i.e., \(b_{\text{t,1}}\) for the X specimens and \(b_{\text{t,2}}\) for the Y specimens. Prior to testing, a clip-gauge extensometer is attached across a 50 mm reference length to measure extension and thus longitudinal strains (see Figure 3). The test specimen is clamped into the jaws of a Tinius Olsen HK25 Universal Testing Machine and, in accordance with ISO 527-1 [35], is loaded in tension at a rate of 1% of the reference gauge length per minute, i.e., 0.5 mm/min, until failure, equating to an average strain rate of 1.67 \(\times\) 10\({}^{-4}\) s\({}^{-1}\).