## 1 Introduction
While design formulae have been proposed on the basis of numerical modelling, experimental validation of these models could only be conducted in the low slenderness range owing to commercially available metallic elliptical tubes satisfying width-to-thickness limits imposed by design standards. Such restrictions on the maximum slenderness of experimental specimens are circumvented in the present study by using PLA and ABS, which exhibit considerably larger elastic ranges than carbon steel, stainless steel and aluminum. Research into additively manufactured PLA and ABS is extensive across several fields, such that a full review of related literature is outside the scope of the present study. When discussing material properties, the macroscale performance of any 3D-printed part is not only a function of the feedstock material but also the layering orientation, infill pattern, deposition speed and overall component geometry. The asymmetry of the responses of 3D-printed PLA and ABS specimens in either tension or compression has been well-attested to previously [3; 4; 5; 24; 25], with compressive strengths up to 30% higher than those in tension observed in some cases, while elastic moduli in tension and compression have also been shown to differ significantly [24; 25]. It has been shown [26] that quicker deposition rates can lead to higher tensile strengths since the decrease in cooling times between passes allows for more effective interlayer bonding to develop, although the same study also found that a combination of lower printing speed and higher application temperature can lead to a reduction in geometric deviations. Investigations into the influence of printing layering orientation on mechanical behaviour [27; 28] suggest noticeable anisotropy in the behaviour of 3D-printed PLA depending on the direction of load transfer relative to the layering orientation. As is also confirmed in Section 2, greater ductility tends to be observed in ABS specimens when compared to those printed using PLA. The greater deformation capacity of ABS has proved useful in exploring large-deformation elastic buckling phenomena such as the buckling of lattice domes [29] and the manifestation of snap-through buckling in arches [30]. Studies of the mechanical behaviour of 3D-printed ABS have determined mechanical properties in tension [1; 4; 24; 25; 31] and compression [24; 25; 31]. Finer deposition layer thicknesses have been shown to lead to greater overall strength [31], while the effect of print layer orientation on mechanical properties has also been attested to previously [32]. In summary, the aims of the present study are to (i) determine material properties appropriate for calculating design compressive resistances of 3D-printed elliptical tubes, and (ii) assess the suitability of existing guidance for designing elliptical 3D-printed tubes in compression. The feedstock materials employed in the present study are Ultimaker PLA [6] and MCPP ABSX [7]. Mesoscale material specimens of these polymers are printed in three orthogonal layering orientations and then tested in either tension or compression to determine orthotropic material properties including elastic modulus, strength, failure strains and Poisson's ratio. Next, elliptical tubes printed from the feedstock polymers of various cross-sectional aspect ratios and tube wall thicknesses are loaded in compression, with the results for the load-deflection behaviour, ultimate loads and failure modes discussed. The compressive resistances of the elliptical tubes determined from testing are then compared to the predictions of a design method proposed for slender steel elliptical tubes in compression [22], which has been appropriately rescaled based on the mechanical properties determined from mesoscale material testing. It is envisaged that the successful use of these slender steel analogues in the design of 3D-printed polymer elliptical sections provides a basis to extend the use of such approaches to other thin-walled 3D-printed polymer profiles.