PSI - Issue 77

Pawel Madejski et al. / Procedia Structural Integrity 77 (2026) 323–330 Author name / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction It is commonly assumed that maximizing the tensile strength of a 3D-printed part is key to improving its mechanical performance. However, this study presents a counterintuitive finding that a PLA specimen with lower tensile strength can actually offer superior energy absorption and delayed failure entirely due to its internal infill geometry. Specifically, while exhibiting lower ultimate tensile strength than the Octet structure, it showed significantly higher ductility and strain tolerance. This result challenges the conventional assumption that strength alone defines mechanical quality in additively manufactured parts, highlighting the important role of internal structure in overall performance. As additive manufacturing technologies, particularly Fused Deposition Modeling (FDM), gain major interest for structural and thermomechanical applications, optimizing the internal architecture of printed parts has become increasingly important. Polylactic acid (PLA) is a biodegradable thermoplastic commonly used in Fused Deposition Modeling (FDM) because of its excellent printability and environmental sustainability. Despite these advantages, PLA naturally exhibits semi-brittle mechanical behavior. Its limited ductility often below 10% elongation at break, makes it sensitive to structural design parameters such as crystallinity, molecular orientation, and infill pattern [1,2]. This study investigates the tensile performance of PLA specimens with five distinct infill topologies such as Cubic, Quarter-Cubic, Lines, Octet, and Triangles. A total of 25 samples were printed using a MakerBot Sketch FDM printer, maintaining consistent process parameters, including infill density, layer height, and line width. The specimens adhered to ISO 257-2-A tensile test geometry. The infill types were chosen to represent a spectrum of mechanical behaviors, ranging from stretch-dominated (e.g., Octet) to bending-dominated (e.g., Lines) configurations. Tensile tests were conducted using an MTS 810 universal testing machine at a constant displacement rate of 1 mm/min. Measured parameters included axial and transverse strain, stress, displacement, and surface temperature distribution via thermal imaging (FLIR camera). The Octet pattern demonstrated the highest ultimate tensile strength of 30.43 MPa due to its efficient axial load transfer. However, the Lines pattern exhibited the highest elongation at break of 0.954% and the greatest strain range c.a. 0.1, with the highest constriction percentage of 8.73%, indicating superior energy dissipation and a more gradual failure mechanism. These findings underscore the critical influence of internal geometry on the mechanical behavior of PLA parts, highlighting the limitations of evaluating structural performance solely based on peak strength. While engineering stress–strain data are typically sufficient for small-strain analysis, their accuracy diminishes beyond the yield point, particularly in cases involving necking or localized deformation. The relevance of true stress–strain models in such regimes has been well established in prior work [3,4]. This study demonstrates that infill topology can be strategically employed to tailor the tensile response of 3D printed PLA, offering valuable design flexibility for applications that demand both strength and resilience, such as thermal sensor housings and energy storage enclosures. 2. Materials and Methods 2.1. Additive manufacturing of biodegradable PLA samples The specimens were fabricated using a Fused Deposition Method (FDM) printer, MakerBot Sketch Large, utilizing biodegradable PLA filament (CadXpert, Poland) as the printing material. The printing process was prepared using Cura slicing software, which enabled precise control over print settings and infill structures. The 3D printing parameters used for printing bio-PLA are as follows: 100% infill density, nozzle diameter of 0.4 mm, layer height of 0.2 mm, line width of 0.4 mm, printing speed of 80 mm/s, and extrusion temperature of 220°C. The geometrical design of the samples conformed to the ISO 257-2-A standard, as shown in Figure 1. Five distinct infill geometries, as shown in Figure 2, were selected for comparative analysis: Cubic, Quarter-cubic, Lines, Octet, and Triangles. Each infill type was applied uniformly throughout the specimen's volume, while maintaining consistent outer shell and layer parameters.