PSI - Issue 77

Sergio Cicero et al. / Procedia Structural Integrity 77 (2026) 56–63 Sergio Cicero et al./ Structural Integrity Procedia 00 (2026) 000 – 000

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deposition modeling (FDM), first proposed and trademarked by Stratasys (Crump and Muir (1992)). This technology, also known as FFF (Fused Filament Fabrication), involves extruding a filament of molten material through a nozzle. The extruded material is deposited layer by layer to build the final component following a predefined digital model. FFF technology requires materials that can be printed while maintaining their functionality, which in practice implies a low coefficient of thermal expansion, adequate fluidity, and sufficiently high mechanical properties once the material has solidified (e.g., Moreno Nieto et al. (2018)). Its main limitations include the risk of warping during printing, surface roughness, the resulting low strength in many cases (especially in the Z direction), and the final anisotropy. FFF allows for the printing of distinct types of materials, such as polymers, metals, and composites. When it comes to polymers and polymer-matrix composites, acrylonitrile-butadiene-styrene (ABS) and polylactic acid (PLA) are the most commonly used materials. The authors have previously analyzed the mechanical behavior of these two materials, including the notch effect under fracture conditions (Cicero et al. (2020), Cicero et al. (2021)), as well as that of acrylonitrile-styrene acrylate (ASA) (Cicero et al. (2024)), which constitutes an alternative to ABS in aggressive environmental conditions. Additionally, the authors have also analyzed polymer-matrix composites, such as graphene reinforced PLA (PLA-Gr) (Cicero et al. (2021)) and carbon fiber reinforced ASA (Cicero et al. (2025)). Using FFF technology, the quality of printed parts, production efficiency, and mechanical properties are affected by a large number of printing parameters, such as nozzle temperature, layer thickness, bed temperature, printing speed, infill level, raster orientation, infill structure type, etc. (e.g., El Magri et al. (2020), El Magri et al. (2021), Hsueh et al. (2021)). In this context, numerous studies have analyzed how tensile properties change with printing parameters (e.g., Ziemian et al. (2015), El Magri et al. (2020), El Magri et al. (2021), Hsueh et al. (2021)). In general, for (strictly) polymeric FFF materials, tensile properties are highest when the raster orientation coincides with the loading direction, when the infill level is 100%, the printing speed is moderate, and the layer thickness is small. On the other hand, studies analyzing the effect of printing parameters on final fracture behavior are more limited, especially in relation to the notch effect. In this sense, the notch effect refers to the ability of a given material to increase its resistance to fracture (or other cracking processes) in the presence of stress concentrators with a finite radius at their tip. In other words, if fracture resistance in the presence of cracks (infinitely sharp defects with a zero radius at their tip) is quantified by fracture toughness, when the defect is a notch with a finite radius at its tip, the fracture resistance is quantified by the apparent fracture toughness. The greater the notch effect in the material, and for a given notch radius, the greater the apparent

fracture toughness. The need to study this effect arises from three fundamental issues: 1) Many failure conditions in structural components are caused by notch-type defects.

2) Treating notches as if they were cracks leads to results that may be overly conservative. The development of specific methodologies for notch assessments reduces this conservatism and can also be used to enhance structural

integrity by introducing a sufficiently high notch effect at structurally critical points. 3) The magnitude of the notch effect is highly variable and depends on each material.

The most widely used theoretical framework to analyze the notch effect is, probably, the Theory of Critical Distances (TCD), a well-known body of knowledge that allows the analysis of fracture, fatigue, and stress corrosion cracking processes in notched materials to be performed. Although it was first proposed in the middle of the last century (Peterson (1938), Neuber (1958)), it has been extensively developed in the last two decades ( Taylor (2007), Cicero et al. (2012), Cicero et al. (2014), Cicero et al. (2015), Ibañez-Gutiérrez et al. (2019), Ng and Susmel (2020)). With all this, this work analyzes the notch effect in three polymers and two composites which are widely used in FFF. The polymers are ABS, PLA, and ASA, whereas the composites are PLA-Gr (1 wt.%) and ASA-CF (10 wt.%). In all of them fracture tests have been previously performed on three-point bending specimens (with three raster orientations) containing U-shaped notches with notch radii ranging from 0 mm (crack-type defects) up to 2 mm. Section 2 includes a description of the materials and methods used in the analysis; Section 3 presents the results and the corresponding discussion and analysis; and Section 4 summarizes the main conclusions.