Analysis and resolution of irregular breaks in tensile testing specimens fabricated via material extrusion

Purpose: A review of the literature on the tensile testing of parts produced by material extrusion reveals that specimens frequently exhibit non-regular fracture outside the gage length. This study aims to examine the influence of specimen design on the outcomes of tests used to ascertain the material tensile properties along the principal directions. Design/methodology/approach: The ASTM D638, ISO 527 and ASTM D3039 standards for tensile tests are compared. Finite element analyses are used to investigate the stress concentration that occurs on the fillet of specimens based on the infill strategy. Digital image correlation is used to monitor the local stress concentrations. Findings: The experimental tensile tests and digital image correlation results confirm that non-regular failures occur, particularly when the raster lines are parallel to the load direction. A novel strategy is then proposed and applied to the ISO 527 standard. The experimental results demonstrate that this infill strategy allows the material resistance to be fully exploited, showing the ductile behaviour of the material and preventing local peaks of stress due to discontinuities. Practical implications: The method proposed in this study offers a practical approach for accurately testing the inherent properties of materials produced through material extrusion, minimising errors associated with conventional testing methodologies. This has significant implications for both academic and industrial research focused on the characterisation of additively manufactured materials. Originality/value: This paper investigates the reasons for irregular fracture when determining the tensile anisotropy in material extrusion through the most common specimens. It introduces a novel method to address existing challenges in this field.