PSI - Issue 37

P.N.B. Reis et al. / Procedia Structural Integrity 37 (2022) 934–940 P.N.B. Reis et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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reinforcement reduces the strain amount. The effect of the printing parameters on short-term creep behaviour was analysed by Tezel et al. (2019), and it was noticed that the 3D-printing orientation significantly affects the dimensional stability. However, the effect of the printing orientation and layer thickness is more pronounced with increasing tension rates. On the other hand, as the layer thickness increases, the creep strength of PLA decreases. Waseem et al. (2020) used the Response Surface Methodology (RSM) to predict the creep rate and rupture time by undertaking the layer height, infill percentage, and infill pattern type (linear, hexagonal, and diamond) as input process parameters. The analysis of variance (ANOVA) results showed that the most influencing factors for creep rate were layer height, infill percentage, and infill patterns, while for rupture time the infill pattern was found significant. Finally, the optimized levels obtained for both responses for hexagonal pattern were 0.1 mm layer height and 100% infill percentage. Creep tests at different stress level and temperatures were performed by Seifans et al. (2021) and the results showed that the stiffer composite configuration exhibits a linear creep behaviour with very slow creep strain rate and minimal creep strain, while the off axis less stiff configuration of 45º exhibited non-linear creep with significantly larger secondary creep strain rate and noticeable creep strain even at room temperature. Rashid and Koҫ (2021) found that the introduction of continuous fibre reinforcements into Onyx (a nylon-based material) significantly improved the creep strength, howeve r, different fibres contribute differently to the composite material’s creep and recovery behaviour. For example, the maximum creep strain observed for all composites at 120ºC is even lower than the maximum creep observed for neat Onyx at 30ºC. Compared to the composite reinforced with continues Kevlar fibres, the minimum creep strain was observed in composites with fibreglass. In terms of stress relaxation, Seifans et al. (2021) observed stress relaxation, even at room temperature, for additively manufactured composites based on continuous carbon fibres. The increase in the initial stress level and temperature led to larger stress relaxation. Stiffer composites showed less stress relaxation, while the weaker fibre/matrix interface for the off-axis samples did not impede the matrix’s viscoplastic behaviour l of the matrix. In this case, this leads to significant stress relaxation compared to stiffer composites configurations and with better fibre/matrix interfaces. The mechanical and thermomechanical properties of vitrimer (material derived from the thermosetting polymers) were studied by Kuang et al. (2020) and observed that heat and light exposure induces stress relaxation and network rearrangements. For example, the normalized stress relaxation modulus gradually decreased with time for different temperatures (from 30ºC to 70ºC), because the catalyst can activate disulfide BER upon increasing temperature. On the other hand, the stress relaxation process was accelerated by the increase in light intensity, which can be attributed to a different reaction mechanism compared to heat-induced disulfide exchange reactions. It is possible to conclude that there are some studies on creep and stress relaxation of 3D printed composites, but they are not sufficient for a full knowledge. Therefore, the main goal of the present study is to analyse the time dependent behaviour of hybrid nanocomposites, and, for this purpose, experimental tests were carried out in compression, tensile and bending modes in which the load (for stress relaxation tests) and the displacement (for creep tests) are recorded during the loading time. G6- Impact™ was the filament used that is a compos ite consisting of a HiPS matrix with carbon fibre and graphene (20%). With this architecture of matrix reinforced, it is possible to achieve a strong composite with a certain degree of flexibility and an exceptional impact performance. 2. Materials and Methods The specimens used in this study were produced on a CreatBot F430 3D Printer using a commercial G6- Impact™ filament, which consists of a HiPS matrix reinforced with 20 wt.% of carbon fibre and graphene. The filament had a 1.75 mm diameter and was extruded through a 0.4 mm diameter nozzle onto a platform heated by a print head in a user-defined pattern to achieve the desired shape. The nozzle is made of hardened steel. When a particular layer is finished, the print head is raised and continues to deposit the next layer. The printing parameters are summarized in Table 1. The deposition angle direction is [0/45/-45/90/90/-45/45/0], which means that the first layer is printed in a unidirectional pattern at an angle of 0° from the horizontal. The second layer is rotated 45° away from the horizontal, the third rotated − 45° and the fourth rotated 90° and this sequence continues as mirror until the code is completed. This is a quasi – isotropic sequence and was selected because the extensional stiffness of the laminate is practically the same in each in-plane direction.