## 5 Conclusions  
Testing has been conducted on specimens fabricated from polylactic acid and acrylonitrile butadiene styrene thermoplastic polymers using the fused filament fabrication  
Figure 17: Comparison of experimental results and design curve [22] for resistance reduction factor \(\rho\) for all EHS specimens.  
Figure 18: Ratios of experimental ultimate load to design load resistances for all EHS specimens.  
additive manufacturing technique. Tensile and compressive material testing specimens have been printed in one of three orthogonal layering orientations and tested in order to determine orthotropic material properties. It has been found that the elastic moduli of the PLA and ABS specimens can be considered isotropic in both tension and compression. Noticeable anisotropy is observed in the results for tensile strength, with the materials exhibiting their weakest response in tension when the loading direction is across successive layers (printing orientation Z). Conversely, this loading mode is in fact the strongest in compression. Overall, the PLA material is stronger and stiffer than the ABS material, although the ABS material is noticeably more ductile. The results of the present study confirm that 3D-printed PLA can achieve compressive strengths in excess of 70 N/mm\({}^{2}\), while 3D-printed ABS can achieve compressive strengths in excess of 35 N/mm\({}^{2}\).  
A series of 24 elliptical hollow section specimens have been printed in PLA and ABS. The specimens have been tested in compression whereupon it is seen that the brittle failure mode of the PLA specimens initiates at the point of minimum curvature in the elliptical cross-section; this effect is more readily observable in the more ductile ABS specimens. Comparison has been made between the experimental results and an existing method for the design of slender steel elliptical hollow sections. The study has provided experimental validation of the design rule in the high slenderness range, which was previously only conducted via numerical simulations. This has been made possible by the increased elastic range of PLA and ABS when compared to metallic materials. Moreover, it is shown that the design method generally provides safe-sided results for PLA and ABS elliptical tubes upon appropriate rescaling of the design parameters, thus demonstrating the suitability of applying design methods originally calibrated for slender steel cross-sections to 3D-printed polymer profiles.  
Conceptualization, F.M.; methodology, all authors; software, F.R.; validation, F.M.; formal analysis, F.M.; investigation, F.M. and S.D.S.; resources, F.M. and F.R.; data curation, F.M. and S.D.S.; writing--original draft preparation, F.M.; visualization, F.M. and F.R.; supervision, F.M. and F.R.; project administration, F.R.; funding acquisition, F.M. and F.R. All authors have read and agreed to the published version of the manuscript.  
This work has been supported by the UK Manufacturing Symbiosis Network Plus (UKMSN+) which is funded by the UK Engineering and Physical Sciences Research Council (EPSRC), grant EP/S036091/1. Additional financial support was provided by the Digital Architecture and Robotics Laboratory and the Division of Civil and Building Services Engineering within the School of the Built Environment and Architecture at London South Bank University. The APC has been supported by Loughborough University.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors wish to extend their thanks in particular to [PERSON] of the Strengths of Materials laboratory at London South Bank University for conducting the experiments and to [PERSON] of the Digital Architecture and Robotics Laboratory for his assistance in printing the test specimens. The authors are also grateful to students [PERSON] and [PERSON] for their assistance in developing geometric models, printing the specimens, and preparing experiments.  
The authors declare no conflicts of interest.