## 2 Material Testing
### Compression Testing  
A total of 21 compression specimens have been printed in the PLA and ABS materials, the nominal dimensions of which (cross-section width \(b_{c,1}\), cross-section breadth \(b_{c,2}\), height \(h\)) are in accordance with ASTM D695 [36] as shown in Figure 7. Two separate batches of Ultimaker PLA specimens have been printed at different temperatures, with PLA batch 1 printed at 190 \({}^{\circ}\)C and PLA batch 2 printed at 210 \({}^{\circ}\)C, with all other printing parameters being identical to those employed during the printing of the tensile specimens. The specimens have been printed in the layering orientations shown in Figure 7 to assess the orthotropy of the response in compression, with three to four specimens printed in each orientation for statistical confidence. The compression specimens have been labelled thus: C[test identification number]-[material]-[printing orientation according to Figure 7]-[series index], with measured geometric properties shown in Table 4 along with deviations from the nominal dimensions. The specimens are placed between the loading platens of a Tinius Olsen HK25 universal testing machine, with a displacement transducer set in contact with the loading platen to record vertical displacement. During testing, a downwards displacement is applied monotonically at a rate of 0.25 mm/min until 5 mm of displacement is recorded by the transducer; note that this equates to an average strain rate of 1.64 \(\times\) 10\({}^{-4}\) s\({}^{-1}\), which is almost identical to the strain rate applied during tensile testing. Figure 5: Average values of tensile elastic modulus and ultimate tensile strength from testing. Figure 6: Average values of strain at ultimate tensile strength and failure strain from testing. elastic behaviour, the Z specimens exhibit well-defined peak stress in keeping with the PLA specimens, albeit with a post-peak behaviour that in fact restabilizes strongly leading to a resumption of load-carrying capacity. This behaviour is analogous to the densification effect observable in cellular foams [37] whereby the voids at the microscale within the material are compacted, which manifests as an increase in stiffness at the macroscale. In the case of the ABS-X specimens, the compressive response exhibits a more gradual progression towards nonlinear behaviour similar to that observed in many ductile metallic materials [38], with no clearly defined peak stress observable. Figure 8: Stress–strain curves from compressive testing of batches: **(a)** PLA1; **(b)** PLA2; **(c)** ABS. When defining representative compressive strengths, the ultimate strength corresponds with the peak stress observed for the majority of specimens during testing. For the ABS-X specimens, since there is no well-defined maximum stress, it is more appropriate to employ a 1% proof stress approach to characterize the compressive strength of the material; this proof strength has been calculated for all specimens to facilitate comparison across the material batches. The values obtained from testing for compressive elastic modulus \(E_{\text{c}}\), ultimate compressive strength \(\sigma_{\text{u,c}}\), 1% proof stress in compression \(\sigma_{\text{p1\%,c}}\) and strain at ultimate compressive stress \(\epsilon_{\text{u,c}}\) are shown in Table 4.  
As can also be seen in Figure 9, in contrast to the tensile specimens, the Z specimens are in fact the strongest and stiffest, albeit only marginally so for the majority of sets of specimens. In general, upon examination of Figures 9 and 10, it is reasonable to consider the strength, stiffness and ductility of the 3D-printed materials in compression as isotropic; it should be noted, however, that the compressive strength of the PLA1 specimens, which were printed at a lower temperature than the PLA2 set, are on average 23% weaker in the X orientation than in the Z orientation.