PSI - Issue 37

S. Valvez et al. / Procedia Structural Integrity 37 (2022) 738–745

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S. Valvez et al. / Structural Integrity Procedia 00 (2019) 000 – 000

and production time than with traditional techniques, Mohamed et al. (2015). Several applications for this technologies have emerged in the defence and aeronautics industries, Singamneni et al. (2019), automotive sector, improving supply chain and logistics, biomedical applications, Chen et al. (2016), and production of customized parts. AM does not require specific tools and material waste is minimal, Ngo et al. (2018). Nowadays, the fused filament fabrication (FFF), also known as fused deposition modelling (FDM), is one of the most widely used additive manufacturing (AM) techniques. Created by Stratasys co-founder Scott Crump in 1988, this technology was commercially available in the late 1990s, Sood et al. (2010). A continuous filament of a thermoplastic polymer is used to print the desired structure, which is heated at the nozzle to achieve a semi-liquid state and then extruded onto the platform or on top of the pre-printed layers. For this printing method, the thermo-plasticity of the raw material is an essential property because it allows the filaments to fuse during printing and then to solidify at room temperature after printing, Sheoran and Kumar (2020), Mohamed et al. (2015). The simplicity of the process, printing at high-speed and low cost are the main benefits of this technique (FFF). However, some disadvantages are also reported in the literature, such as mechanical properties as a function of process parameters, poor surface finish, appearance of laminated parts and limited raw materials, Mohamed et al. (2015), Stansbury and Idacavage (2016). According to Mohamed et al. (2015), for example, the main processing parameters that affect the mechanical properties of printed parts are layer thickness, filament width and orientation and air gap (in the same layer or between layers). Nevertheless, Sood et al. (2010) reported that the main cause of mechanical weakness is the inter-layer distortion. Despite the potential for large-scale printing, FFF still remains limited because the part quality and mechanical properties of additively manufactured parts depend on proper selection of process parameters, Ngo et al. (2018). The anisotropic behaviour and sensitivity to the choice of process parameters underscore the importance of selecting process parameters for 3D printed structures, Raney et al. (2017), Letcher et al. (2015), Soury et al. (2013), Nidagundi et al. (2015), Durgun and Ertan (2014). On the other hand, literature also emphasizes the influence of post process annealing on certain mechanical properties, Torres et al. (2015). In this context, annealing is a post-processing technique used to increase the strength and surface quality of FFF print parts. It was identified that this post-processing heat treatment increases the interlaminar toughness of polymers, making their performance better than injection moulding samples, Hart et al. (2018). Furthermore, better mechanical properties are expected after annealing, Wach et al. (2018). Hong et al. (2019), for example, reported that the flexural and compressive strengths increased with the annealing treatment of PLA parts. The samples after heat treatment at 130 ºC for 300 s reached a flexural strength 58.3% higher than that of neat PLA, while the compressive strength after treatment at 140 ºC for 600 s promoted an increase of 39.8%. Bhandari et al. (2019) found that the tensile strength of PLA and PETG-based composites, both reinforced with short carbon fibers (CF), increased, respectively, two and three times after annealing treatment. In a similar study, Rangisetty and Peel (2017) studied the effect of annealing treatment on composites of PLA, ABS and PETG reinforced with CF. Three annealing temperatures (65 ºC, 110 ºC and 85 ºC, respectively) were used due to the different glass transition temperatures of the matrices. Over a 60-min period, authors achieved improvements of 16.8%, 3.34% and 12.4%, respectively. Kumar et al. (2021) studied Polyethylene Terephthalate Glycol (PETG) and Carbon Fiber reinforced Polyethylene Terephthalate Glycol (CFPETG) composites printed with different infill densities (25%, 50%, 75% and 100%). Hardness, tensile, impact and flexural strength were compared before and after annealing for both materials. Regardless of the material, the highest mechanical properties were obtained for the specimens with 100% infill density submitted to the annealing treatment. When comparing the effect of the annealing treatment, authors found improvements of 21%, 25%, 23% and 18% increase in hardness, tensile strength, impact strength and bending strength, respectively, for the CFPETG samples compared to the PETG samples. Therefore, from the literature reported above, it is possible to conclude that significant benefits in terms of mechanical properties are obtained with the annealing treatment. In this context, the main objective of this work is to study the effect of thermal annealing on the flexural strength and hardness of PETG, carbon fibre reinforced PETG (CFPETG) and Kevlar fibre reinforced PETG (KFPETG). For this purpose, temperatures of 90 ºC, 110 ºC and 130 ºC as well as exposure times of 30 min, 240 min and 480 min will be considered. Finally, the mechanical characterization obtained by the experimental tests will be discussed considering the volumetric changes and the inverse of sample