PSI - Issue 37

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

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

this context, it is not surprising that we have witnessed in recent years the emergence of polymeric nanocomposites (PNCs) with the aim of improving the properties of host matrices through small amounts of fillers. For example, carbon nanotubes, nanofibers or other types of nanoparticles are commonly used to reinforce polymeric matrices and, consequently, improve the mechanical performance of the resulting components (Ivanova et al., 2013; Campbell and Ivanova 2013). Nevertheless, as consequence of the inherent viscoelasticity of the matrix phase, polymer composites are prone to creep and stress relaxation, making it a great challenge when they are used in long-term applications. Therefore, a better understanding of the stress-relaxation and creep behaviour of composites enables us to predict the dimensional stability of load-bearing structures. In terms of creep behaviour, for example, Dul et al. (2016) studied the benefits obtained when graphene nanoplatelets (xGnP) were incorporated in an acrylonitrile – butadiene – styrene (ABS) matrix. Daver et al. (2016) characterized the creep response of polyolefin-rubber nanocomposites produced by additive manufacturing and they observed an instantaneous increase in strain due to elastic response of the material, which was followed by a viscoelastic response due to molecular rearrangement. However, composites with carbon nanotubes showed higher creep deformation compared with those without the nano-reinforcements. The reason for this decrease was explained by the authors due to the fact that carbon nanotubes are incorporated into the composites via a low viscosity thermoplastic concentrate (Plasticyl LDPE2001). Furthermore, the low properties when compared to those obtained with compression moulded composites were a consequence of the porosity promoted by the gaps occurred between extruded filaments. Mohamed et al. (2016) studied the relationships between the manufacturing conditions and the bending creep stiffness for PC – ABS produced by FDM and found the optimum parameters to improve the flexural creep stiffness. In a similar study, these authors found that the highly effective parameters for creep rate are layer thickness, number of contours, raster angle and build orientation, while road width and air gap have low impact on the creep rate of FDM processed part (Mohamed et al., 2017). Niaza et al. (2017) studied the long-term creep behaviour of Polylactide (PLA) and polylactide/hydroxyapatite (PLA/HA) composites and observed that the nanoparticles promoted an increase in hardness and creep resistance. A systematic characterization of 3D-printed ABS components was developed by Zhang et al. (2018) and the printing orientation was found to have a significant effect on the creep properties. For example, while 90º was the most creep resistant orientation, 0º had the highest tensile properties. It should be noted that 0º refers to the specimen printed along the x-axis, while 90º refers to the specimen printed along the y-axis. This was explained by the authors due to the fact that the creep test specimens are thin with only two or three layers, as suggested by the creep tester manufacturer, while the tensile specimens are much thicker (ten layers) according to the ASTM standard. Therefore, the creep specimens are more sensitive to the printing orientation. Mohammadizadeh et al. (2018) studied the creep behaviour of nylon and nylon reinforced with carbon, Kevlar and glass fibres. They observed that increasing the temperature increases the strain, but the inclusion of reinforcing fibres decreases the creep deformation. Neat nylon showed the highest strain both at room temperature and at 100 °C, which was, for example, at 100 °C, almost 25 times greater than the strain observed in composites. According to the authors, when the fibres were added to the nylon matrix the creep resistance was improved almost 40 times compared to the neat nylon. On the other hand, both creep compliance and creep recovery were higher for the fiberglass/nylon composite and lower for the carbon/nylon composite. However, the increase in temperature decreases the creep resistance due to the weakening of the bonding strength and, consequently, a decrease in the stress transfer at the interface. In fact, the viscoelastic properties of a polymer determine the thermomechanical behaviour of the composites under an applied load at different temperatures. Salazar-Martín et al. (2018) analysed the creep behaviour of polycarbonate fused deposition modelling parts and concluded that the printing orientation significantly affects the elastic and viscoelastic components of the creep deformation. When the specimens are printed in the YZ direction, the creep resistance increased compared to other orientations, because on YZ orientation most of the deposited filaments are contours arranged in the same direction in which the specimen is pulled, while the specimens printed in the XZ direction showed the highest creep strain because they are pulled perpendicular to their deposited filaments. Regarding the creep displacement, it increases with increasing the air gap value because the density of the material decreases. In fact, the air gap is the process parameter that most affect the investigated samples, in which the elastic and viscoelastic components are prevailing over the plastic one. Continuous fibre reinforced additively manufactured (CFRAM) components were studied by Mohammadizadeh et al. (2019) and, in terms of creep behaviour, they found a viscoelastic behaviour with large deformation in the first stage, following a sharp decline in strain, and, later, a gradual decrease in strain. Furthermore, increasing the temperature increases the strain percentage, and fibre