The experimental analysis of creep and corrosion properties of polymeric tube reinforced by glass, carbon and Kevlar fibers

Polymeric tubes, including epoxy and reinforcing fibers, are widely used in the petroleum and aerospace industries due to their high strength and corrosion resistance. In this study, corrosion and creep properties of resin-based tubes reinforced by Glass fibers (GFR), Carbon fibers (CFR), and Kevlar fibers (KFR) were investigated using tubes made by using a 45-degree unilateral winding method. The highest creep strain was obtained for the CFR equal to 0.7445 and the lowest was obtained for KFR with the Kevlar fibers being severely damaged. The lowest corrosion rate per year was for the CFR sample, equal to 113in/year×1000. The corroded samples were subjected to a tensile test and a 2% improvement in ultimate tensile strength was achieved for GFR. To evaluate the results and the quality of adhesions between fibers and resins, SEM images were taken of the samples.


Introduction
There has been significant research concerning polymeric composite materials in which fibers are used to reinforce resins which are thermoset or thermoplastic polymers [1,2]. These materials have a high strength to weight ratio and their significant properties include high tensile strength, flexural strength, fatigue life, and corrosion resistance [3]. The properties of polymeric composites depend on the polymer properties as well as on the type, direction, and length of the fibers and the quality of resin and fiber bonding [4]. The fibers transfer stress from the polymer matrix to themselves, reduce the stress concentration and improve the sample's toughness [5]. Also, composite tubes, due to their durable, corrosion-resistant, and lightweight structure are a great alternative to metal and concrete pipes [6]. In addition, the inner surface of the composite tube is very smooth which reduces energy consumption, pressure drop, sedimentation, and the friction coefficient that allows it to not require a pipe protection cover against corrosive fluids and environmental factors [7]. Apart from the type and arrangement of the fibers, the properties of composite tubes depend on the compatibility and interleaving between fibers and resin matrix [8]. An incompatible resin, lack of bonding, and weak adhesion strength lead to delamination and failure of fiber-reinforced materials [9]. Experimental and laboratory trialand-error is the most commonly adopted strategy to validate the performance and determine the compatibility of fiber reinforcement materials [10]. Many kinds of research have been done on the energy absorption or creep and corrosion behavior of fiber-reinforced composite samples [11,12].
Liu et al [13] produced carbon fiber reinforced polymer tubes and the transverse impact test was done on the samples with their failure mechanisms investigated. The results show that higher impact energy led to more circumferential cracks. Also, by increasing the impact energy, the tubes absorbed energy by generating circumferential and longitudinal cracks. Wang et al [14] investigated the corrosion behavior and durability of concrete-filled fiber-reinforced polymer tubes subjected to the various conditions with additional sustained axial load and continuous water immersion. Test results showed that more degradation occurred for wet Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
conditions compared to the dry environment. Ren et al [15] did a compression test at various temperatures and pressures on shape memory polymer composite tube reinforced by carbon fibers wound with ±45°layers. The results showed that increased compression resulted in less buckling load. Yang et al [16] did an experimental analysis on the creep behavior of composite tubes under flexural loading. The flexural creep test was done at various stress, 45% to 75% of ultimate flexural strength at 100°C for 500 h, and the tubes' deformation, mechanical performance, and service reliability were evaluated by the superposition principle after a 12-year design life.
Acquiring knowledge of the corrosion and creep properties of fiber-reinforced composite tubes according to compatibility role can help extend their application in various industries, in which little research has been done so far. In this paper, composite tubes with polymer base material were reinforced by glass, carbon, and Kevlar fibers with a unilateral winding at an angle of 45 degrees. Another main aim and the novelty of this study is that it considers the compatibility of fibers and resins. In this study, a corrosion test in an acidic environment and a creep test were done on tubes. Finally, SEM images were taken on the samples to validate the results.

Materials and producing tubes
To obtain high-quality samples with reinforcing fibers, a suitable resin material was used. The resins used for manufacturing with GFR, CFR, and KFR are Swancor-901, Epiran-10, and Epiran-06FL types, respectively, which were achieved by trial-and-error in our previous research [1]. Two additives such as cobalt actuate and peroxide acid solutions were added to the resin at 1.5% and 15% by weight, respectively, which facilitates the curing process [17]. After preparing the resin, the fibers were immersed in the resin solution for 20 min (Gel time). The detailed specifications according to the producers' datasheet of the fibers can be seen in table 1. To produce composite tubes, a shaft with a 26 mm diameter, called a mandrel, was used and placed in the winding machine. To ensure proper adhesion of the fibers to the mandrel and smoothness of the surface, the mandrel was polished first, and a very thin layer of resin called gel coat was sprayed with a thickness of 0.7 mm. Table 2 presents the specifications and layering fibers of the tubes.
After wrapping impregnated fibers around the mandrel, the samples were placed for 2 h at ambient temperature for initial curing and then were placed in an oven for 2 h at 70°C for final curing. Figure 1 shows the produced tube and, their average characteristics are given in table 3.

Determining mechanical properties
The creep test was performed on samples at 50 MPa stress and 200°C, according to the ASTM-D7337 standard [18] and the increase in length and strain of the samples were measured until rupture point. The creep samples were cut from the produced tube and a schematic image of them is presented in figure 2.
For the corrosion test, the samples were first polished with a grade 3000 emery and dried for 120 min at 50°C. The test was carried out according to ASTM-C582 standard in 37% concentration acidic HCl solution [19]. The samples were exposed to acid for 24 h at 40°C and the corrosion rate was calculated according to     [20]. The images of the corrosion samples and the tensile test device are shown in figure 3.

Specification of samples
To ensure the quality of adhesion and compatibility between resins the fibers, SEM imaging was done at the cross-sectional area of the samples. Figure 4, shows these SEM images and the fibers between resins have been  identified, and the bonding between them is plotted, with no discontinuities, porosities, or cavities, which are common defects in fiber reinforcement samples. Also, the TGA test was done on samples to study the thermal stability and weight loss of the samples by increasing temperature [21]. TGA analysis can provide valuable information regarding the composition and thermal stability of polymeric materials. This can be done to compare different materials or through accelerated means for lifetime predictions [22]. The TGA curve of produced samples can be seen in figure 5. The degradation in the samples can be accomplished by processes such as cross-linked breakages due to the increasing temperature. For all produced samples, the degradation occurred at about 330°C. By using carbon fibers, 20%, and 8% higher degradation have been achieved compared to the samples in which glass and Kevlar fibers were used, respectively. These results were obtained due to the better thermal stability of carbon fibers compared to other tested fibers, and appropriate compatibility and bonding between reinforcement fibers and resins. Mass changes at sample temperatures between 250°C and 300°C can be correlated to changes in molecular structure and is believed to reflect the extent of matrix depolymerization which may have occurred [22].

The creep test
The tensile creep test was performed on the samples and the strain data was extracted from the device until the complete rupture of samples. The results of the creep test of the composite tube can be seen in table 4. Creep in polymer composites may occur at any temperature, even at low temperatures, due to the viscoelastic deformation of the base material, although normally the fibers do not creep at this temperature and the creep behavior of the composites is influenced by the geometry of the segment and the properties of their constituents.
To investigate the creep behavior of fiber-reinforced polymers, creep compliance (D(t)) can be used [23], which is defined by equation (2). In this equation, ε(t) is the instantaneous strain, and σ 0 is the constant stress applied to the samples. The strain changes over time and the D(t) values for produced samples with different     composite layers were delaminated. After the mentioned time (133 min), the abrupt separation between the carbon fibers and the resin occurred, followed by the creep stress only carried by the carbon fibers, which is illustrated by the slope of the diagram. In the case of GFR, the separation of the layers was first observed at times of 60 min and 104 min, respectively. Then at 128 min, the fibers started to separate from the base material, and finally, the fibers were ruptured so that, the creep compliance diagram for GFR also shows the slope change, which confirms this. In the case of KFR, the separation of the layers occurred slower, but eventually, the rupture of the sample occurred earlier than the CFR and GFR. The difference in the stress transfer mechanism in the samples is due to the difference in the creep rupture behavior of the samples. Also, to investigate the behavior of the fibers against the applied stress and heat, the images of samples are visible in figure 8. Examination of the surface of the samples revealed that the outer layers of the fibers broke down earlier, and then the inner layers in the middle of the composite were fractured. At the beginning of loading, the stress was tolerated by the fibers in all layers. After exposure to heat, the temperature affects the outer layers and causes submission in these layers. This causes more stress to be applied to the inner layers, and then, after the outer layers failed, the inner layers were exposed to higher temperatures that eventually led to their failures. This indicates that each layer was ruptured at different times.
As can be seen in figure 8, there is good agreement with the results. The GFR and CFR had a similar process until destruction, and all layers failed at approximately the same time. In these samples, the fibers were less damaged due to their higher temperature tolerance; and in the GFR, more severe destruction occurred compared to CFR. In the KFR, considering that the temperature tolerance of these fibers is much lower than the other two fibers, the fibers of the outer Kevlar fibers have been severely damaged, resulting in early damage to the sample at lower strain times and rates.

The tensile strength of corroded samples
To evaluate the produced samples' resistance to an acidic solution, the corrosion test was performed. The corrosion test results for composite tubes are visible in table 5. The lowest corrosion rate per year was obtained for CFR, equal to 113 in/1000 × year, which is 134% lower than KFR. In general, the base material, resin, has a higher corrosion resistance than metal and steel samples due to its lower reactivity and absorption of corrosive materials [24]. These materials also prevent osmotic blistering and are used as insulating material against corrosive materials; they have higher thermal and chemical resistance and less water absorption [25]. To determine the effect of the acidic environment on produced composite samples, the uniaxial tensile test was performed on samples before and after the corrosion test. The maximum tensile strength of the samples and their results are shown in table 6. The results show that the tensile strength of the produced samples increased after the corrosion test, and their fracture strain decreased, which were similar to obtained results in the previous literature [26,27]. The tensile strength of the samples depends on the reinforcing fibers, which prevent growth in the rupture.
The acidic environment creates a better bond and interaction between the base material and the reinforcing fibers by treatment aging under acidic conditions [27]. The highest change in the ultimate tensile strength after corrosion was obtained for sample GFR, equal to 111.7 MPa, with a 2% improvement. On the other hand, acidic corrosion had the least effect on CFR, which is consistent with the high resistance of carbon fibers to acidic environments [28]. After corrosion testing, the fracture strain of all samples decreased; The highest change in the amount of reduction was obtained for GFR, equal to 2.5%. To better understand the results of the tensile test, the stress-strain graphs of the samples are shown in figure 9.