Effect of Fibre Orientation and Hostile Solutions on Stress Relaxation of Glass/Polyamide Composites

Abstract

Polyamide creates high-performance composite materials, which are replacing the traditional epoxy composites in several applications. In this context, exposure to hostile environments is expected. On the other hand, due to the viscoelastic nature of the matrix, these composite materials are prone to stress relaxation. Therefore, the stress relaxation behaviour of glass/polyamide 6 composites was studied considering different fibre directions, as well as exposure to NaOH and HCl solutions. Stress relaxation tests on the bending mode were carried out, and the stress recorded during the loading time (7200 s). All tests were characterized by a stress decrease over time, but laminates with higher fibre angles were more prone to stress relaxation. However, exposure to hostile solutions promoted more significant decreases, where the highest stress relaxation was achieved for alkaline environments with values that were three times higher for laminates with fibres at 0 • and around one and half times higher for 45 • fibre alignments when compared with the control samples. Finally, the Kohlrausch-Williams-Watts (KWW) model showed that it can be used to predict stress relaxation time, due to the accuracy that was obtained between the experimental and theoretical results. Polymers 2020, 12, 20 2 of 12On the other hand, in terms of corrosive environments (alkaline and acidic solutions), the literature is much more abundant with respect to thermoset composites than thermoplastics. For instance, studies developed by Stamenovic et al. [11] in glass-polyester composites demonstrated that alkaline solutions are responsible for the decrease of tensile strength and elastic modulus, because they are highly corrosive, whereas the opposite tendency occurs when these composites are exposed to acid solutions. However, for both solutions, the changes observed are very dependent on the pH value and exposure time. Feng et al. [12] studied the effect of different solutions (H 2 SO 4 , NaCl and NaOH) on glass fibre-reinforced polymer (GFRP)/epoxy composites and observed a decrease in hardness, flexural strength and elastic modulus, and this trend is even higher for higher concentrations. Moreover, Amaro et al. [13] analysed the effect of alkaline (NaOH) and acid (HCl) solutions on the flexural properties of these composites, and the worst bending properties were obtained when the glass/epoxy composites were exposed to NaOH solutions. The effect of hydrochloric acid (HCl) and sulphuric acid (H 2 SO 4 ) was also compared by these authors, and the HCl solution was responsible for the poorest results [14]. Similar conclusions were also obtained by Kamal and Kadhim [15] on nano-silica-reinforced (glass/Kevlar) fabric polyester hybrid composites, due to the higher corrosive effect of alkaline solutions. Finally, the effects of strong acids (HCl, H 2 SO 4 , HNO 3 and H 3 PO 4 ) on mechanical properties of glass/polyester glass-reinforced pipes (GRP) at normal and high temperatures were evaluated by Mahmoud and Tantawi [16]. They observed significan...

Tensile stress relaxation behavior of a novel wide‐angle fabric/EPDM composite at room temperature

“…Figure 6 shows the time stress curves for the fabric/EPDM composite with the different fabric structures at the same initial stress. All curves showed the typical stress relaxation behavior (the time‐dependent stress attenuation), 14–18,38 but the degree of relaxation was different, indicating that the introduction of different fabric structures did not change the stress relaxation mechanism of the composite. In the following, the W‐60 composite will be used as an example to deeply explore its stress relaxation mechanism.…”

“…15 In addition, many researchers have proposed a mechanistic explanation, namely that the fibers affect the structure of the polymer matrix near the interface, thereby altering its stress relaxation behavior. [16][17][18] Meanwhile, the stress relaxation behavior of the fabric has been extensively studied. Hezavehi et al studied the stress relaxation behavior of the worsted fabric under constant torsional strain in the folded state.…”

“…Femandes et al study showed that the stress relaxation of the EPDM composite filled with carbon black was highly dependent on strain level, deformation history, and filler content 15 . In addition, many researchers have proposed a mechanistic explanation, namely that the fibers affect the structure of the polymer matrix near the interface, thereby altering its stress relaxation behavior 16–18 . Meanwhile, the stress relaxation behavior of the fabric has been extensively studied.…”