Open challenges in tensile testing of additively manufactured polymers: A literature survey and a case study in fused filament fabrication

Additive manufacturing (AM, also commonly termed 3D printing) is progressing from being a rapid prototyping tool to serving as pillar of the Industry 4.0 revolution. Thanks to their low density and ease of printing, polymers are receiving increasing interest for the fabrication of structural and lightweight parts. Nonetheless, the lack of appropriate standards, specifically conceived to consistently verify the tensile properties of polymer parts and benchmark them against conventional products, is a major obstacle to the wider uptake of polymer AM in industry. After reviewing the standardisation needs in AM with a focus on mechanical testing, the paper closely examines the hurdles that are encountered when existing standards are applied to measure the tensile properties of polymer parts fabricated by fused filament fabrication (FFF, aka fused deposition modeling, FDM), which is presently the most popular material extrusion AM technique. Existing standards are unable to account for the numerous printing parameters that govern the mechanical response of FFF parts. Moreover, the literature suggests that the raster- and layer-induced anisotropic behaviour and the complicated interplay between structural features at different length scales (micro/meso/macro-structure) undermine pre-existing concepts regarding the specimen geometry and classical theories regarding the size effect, and ultimately jeopardise the transferability of conventional tensile test standards to FFF parts. Finally, the statistical analysis of the tensile properties of poly(lactic acid) (PLA) FFF specimens printed according to different standards (ASTM D638 type I and ASTM D3039) and in different sizes provides experimental evidence to confirm the literature-based argumentation. Ultimately, the literature survey, supported by the experimental results, demonstrates that, until dedicated standards become available, existing standards for tensile testing should be applied to FFF with prudence. Whilst not specified in conventional standards, set-up and printing parameters should be fully reported to ensure the repeatability of the results, rectangular geometries should be preferred to dumbbell-like ones in order to avoid premature failure at the fillets, and the size of the specimens should not be changed arbitrarily.