PSI - Issue 51

J. Majko et al. / Procedia Structural Integrity 51 (2023) 160–165

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J. Majko et al. / Structural Integrity Procedia 00 (2022) 000–000

production of variously shaped objects (Chua et al. (2010), Marsalek et al. (2021), Krawiec et al. (2021), Rojek et al. (2020)). However, in recent years, the 3D printing development has led to a significant expansion of the application possibilities of other materials. Some of them are composites: materials composed of a matrix and fibres. Each of these components has a specific function. For example, the matrix protects the fibres from the external environment and ensures the geometric stability of the object. Fibres significantly increase the strength of structures, thanks to which composites can be competitive materials compared to conventional construction materials (Goh et al. (2019)). The significant advantages of composites are low weight, resistance to corrosion, strength-to-weight ratio, etc. (Jones, (1999)). The disadvantages are high production costs, directional dependence of mechanical properties, difficulty repair of damage and complexity of mechanical properties (Šofer et al. (2021), Gljušćić et al. (2022), Kastratović et al. (2021)). The reinforcement in composites can take on different forms, such as fibres, dispersed particles, or flakes (Khalid, (2021)). In the case of fibres, composites can be reinforced with either long or short fibres. Short fibres are often incorporated into thermoplastic filaments, which in the case of FFF technology, eliminates the need for significant modifications of the extrusion system. Application of long fibres require modification of 3D printers, such as the addition of a second nozzle, that can deposit fibres in specific locations. The most commonly used materials in 3D printing are polyamides. One of the main disadvantages of nylon is its hygroscopicity, which denotes the tendency of the material to absorb moisture. This affects the properties of the nylon (Humeau et al., (2018)) and negatively impacts the 3D printing process. The material can flow uncontrollably through the nozzle when heated to its melting temperature, leading to the deposition of material in inappropriate places and an increase in the proportion of voids in the structure. This not only reduces the print quality but also significantly affects the mechanical properties of the printed structures. As a result, users of 3D printers should take care to properly store the nylon material. In the past, the authors have published an article focusing on the tensile properties of nylon reinforced with chopped carbon fibre, which showed the effect of orientation on the obtained results (Majko et al., (2022)). During specimen preparation process, the authors encountered issues with improper storage in the workplace, which led to limited usability of the material and also required the purchase of new material. This comprises additional investment costs and significant delays in experimental measurements due to the need to carry out public procurement. During the public procurement process, the idea of how to use the improperly stored material emerged. One of the options chosen by the authors of the article was to adjust the printing parameters, which will be described in further detail in the next section of the article. The primary goal was to print specimens from the improperly stored material and to perform an initial assessment of used material on the tensile strength. 2. Specimen preparation and experimental procedure 2.1. Manufacturing process The specimens were printed using two printers that operate on the principle of the Fused Filament Fabrication method - the Markforged MarkTwo and the Prusa i3. The reason for this is that the former printer does not allow adjustment of the nozzle temperature. Therefore, the authors used the latter printer, which they have available at their workplace, in order to investigate the effect of the nozzle temperature on print quality of wet nylon. 2.2. Material A series of tensile tests were performed on specimens printed from nylon reinforced with chopped carbon fibre. Table 1 comprises tensile properties of observed material specified by the printer manufacturer.

Table 1. Mechanical properties of nylon reinforced with chopped carbon fiber specified by manufacturer (Markforged, (2022)). Parameter Nylon Reinforced with Chopped Carbon Fibre (Onyx) Tensile modulus of elasticity [GPa] 1.4 Ultimate strength [MPa] 30 Tensile strain at brake [%] 58