Influence of accelerated aging on mechanical properties and aging behavior of glass fiber reinforced pipe

Abstract

In this work, the accelerated aging test of GFRP in two thermal environments was conducted. The accelerated aging test was conducted at a constant temperature of 95°C for periods of 3000h to analyze and study the aging behaviors and mechanism under two environments. Resin defects caused by aging were found in the SEM images, and degradation of the fiber/resin interface was observed. Both the hoop tensile test and uniaxial compression test showed brittle fracture characteristics, and the strength of hydrothermal aging specimens decreased more significantly. Due to matrix degradation, uniaxial compression fracture after hydrothermal aging was observed that the resin exhibited poor adhesion. The decrease in the hardness of the outer resin layer was attributed to the development of the degree of molecular chain breakage. ATR-FTIR results showed that the accelerated aging process is accompanied by changes in the concentration of functional groups. The change in C-H intensity was attributed to the post-curing phenomenon. Accelerated aging resulted in the formation of carboxylic acids or esters and the introduction of hydroxyl groups. Thermogravimetric tests showed that accelerated aging did not change the thermal decomposition temperature of epoxy resins, but caused a decrease in resin content.

Keywords

Aging behavior glass fiber reinforced pipe hydrothermal aging thermal oxygen

Introduction

Compared with pipelines made of traditional materials, fiber reinforced composite pipelines have many potential advantages, such as high specific strength and stiffness, superior corrosion resistance, and thermal insulation.^1–4 As a typical fiber reinforced composite pipe, glass fiber reinforced pipe (GFRP) has the characteristics of high strength, light weight, and long service life, so it is widely used in the transmission pipeline in the industrial fields such as aerospace, oil field and natural gas. GFRP is widely used as an oil field transportation pipeline due to its advantages such as corrosion resistance, low transport resistance, and ease of installation and transportation.^5–7 In service, the performance of GFRP can be affected by moisture, heat, water, alkalis and other harsh environments.⁸ With the increasing use of GFRP pipelines, the reliability and performance of composite materials over a long period of time are issues that need to be addressed. Therefore, it is necessary to further investigate the long-term aging behaviors and mechanisms of GFRP under service conditions to ensure its safety and predict its long-term performance.

The long-term durability test of GFRP is an important test process to study its aging mechanism, and some researchers have used accelerated aging test. Fonseca⁹ conducted accelerated aging tests on GFRP in a deionized water environment and a QUV chamber for up to 3000 h. By comparing and analyzing the effects of the two environments on the performance of GFRP, no significant chemical degradation was observed by Fourier Transform Infrared Spectroscopy (FTIR), which indicates that the resin mainly undergoes physical degradation during humid-heat aging. The durability of GFRP composites was evaluated by accelerated aging through moisture absorption and temperature changes over a reasonable time frame, and changes in glass transition temperature (T_g) due to moisture absorption were found.¹⁰ Yang¹¹ investigated the thermal aging behavior of anhydride-cured epoxy resin matrix. The epoxy resin matrix was thermally aged at 130°C −160°C for 30 days. The results showed that oxidation and molecular rearrangement occurred in the surface layer of the epoxy resin specimens during thermal aging. Warm seawater immersion was also used in the aging experiments to compare the mechanical properties of GFRP at different immersion times. It was found that seawater immersion reduced the average shear modulus and ultimate tensile strain of the specimens, and in another study, after 4 months of accelerated seawater aging, the strength of the laminates decreased by an average of 45% to 50%, and the strength of the thermoset resin decreased by an average of 18%.^12,13 Degradation of GFRP has a significant effect on their tensile properties,¹⁴ with water and alkali significantly affecting not only the degradation of the glass fibers and matrix, but also the interface between the fibers and the matrix.¹⁵ At the same time, some researchers have also conducted failure analyses of GFRP pipes that have been used, which helps to help us focus on some performance failure indicators. Rodríguez¹⁶ analyzed GFRP that had already failed in the oil transportation field, indicating that the failure of GFRP was caused by the continuous thermal degradation of the transported fluid. During the failure process, the diffusion of low molecular weight hydrolysis degradation of the matrix is activated, and the matrix fiber interface is degraded, resulting in a decrease in the mechanical properties of the composite material. Short term isothermal failure tests were also investigated and cracks and pores were observed on the surface of the specimens which expanded with increase in temperature and time and the hygrothermal stresses were calculated for each layer.^17,18 Compared to CFRP composites, GFRP composites are more susceptible to hygrothermal attack. Elevated temperatures and alkaline environments reduce the tensile properties of the specimens, and a decrease in the strength of the glass fibers leads to a decrease in the performance of GFRP composites.^19,20 During environmental aging, significant degradation at the fiber/matrix interface was found after the experiment.²¹ In addition, some researchers conducted accelerated aging experiments on GFRP, comparing the tensile strength and interlaminar shear strength by varying the type of solution and temperature as a measure of GFRP performance.^11,22–24 Meanwhile, the hygroscopic properties of fiber-reinforced materials greatly reduced the mechanical properties of the composites.²⁵ Zeng and colleagues applied constant axial stress to GFRP in an alkaline environment at 60°C, concluding that the effect of constant axial stress on strength was relatively minor.²⁶ The decrease in strength of GFRP in alkaline environments is mainly related to the concentration of OH -, as alkaline ions can disrupt hydrogen bonding networks and accelerate aging.²⁷ Liao²⁸ and his team analyzed GFRP pipelines that had failed in oil fields. They discovered that the pipes had a low resin content, indicating irreversible chemical degradation.

In order to solve the problem of aging failure of GFRP pipelines in oilfield water transmission field, the laboratory designed accelerated aging test to investigate the failure mechanism under actual working conditions. In this paper, the time-temperature equivalent equation is used to design the test temperature of 95°C, and two kinds of thermal environments are selected to simulate the degradation principle of GFRP in the aging environment under the long-time water transmission condition, and the thermal aging mechanism and its effect on the macroscopic performance are studied comparatively from the results.

Experimental

Materials

The GFRP specimens used in this experiment were provided by Shandong Shengli Xinda Industrial Corporation, using bisphenol A-type diglycidyl ether as the matrix material and methylhexahydrophthalic anhydride curing agent, and the specimens were prepared by winding 13 layers of glass fibers at ±45° and adopting a layer-by-layer curing procedure. The internal structure of the GFRP specimens is shown in Figure 1(b), with a pure resin layer on the outside and a glass fiber/epoxy resin composite layer on the inside, winding a total of 13 layers.

Experimental method

The two thermal environments used in the experiment are thermal oxygen environment and hydrothermal environment, with a temperature below the glass transition temperature of epoxy resin (125°C), selected as 95°C. Set three parallel samples per group to ensure the accuracy of experimental data.

In order to solve the failure problem of non-metallic composite pipes represented by GFRP in oilfield environment, we designed accelerated aging experiments of GFRP pipes under thermo-oxygen and simulated extracted water environments based on the principle of time-temperature equivalence(API 15S-2016) in the hope of clarifying the aging behaviors and aging mechanisms of GFRP pipes under the influence of different environmental factors, which would be convenient for taking preventive measures in a timely manner. Based on on-site research, the operating temperature of GFRP is 40°C and the maximum operating temperature is 65°C. According to the high-performance accelerated testing program and the principle of time-performance fairness, conducting an accelerated aging test at an increased temperature serves as a method to discern long-term aging trends, equation (1)

∆ T = \frac{1}{α} \log (\frac{t_{D e s i g n L i f e}}{t_{T e s t}})

(1)

where

$α$ is the time-temperature shift factor

t_DesignLife is the design life time (hours)

t_Test is the test time (hours)

log is log base 10

$∆ T$ is the test temperature minus the product’s temperature

The experimental temperature was elevated by 30°C beyond the actual working temperature. The aim is to understand the aging behavior under prolonged service conditions through a brief high-temperature aging test. In the experiment, the thermal oxygen environment adopts the air circulation furnace with the temperature of 95°C, and the specimens are cut into 100 mm tubes to carry out the accelerated thermal oxygen aging test.

The specimens were cut into 100 mm tubes and put into the constant temperature air circulation box for accelerated thermal oxygen aging test for periods of 500 h, 1000 h, and 3000 h. Hydrothermal aging test using 500 mm specimen, placed in a constant temperature water tank for periods of 500 h, 1200 h, and 3000 h. The accelerated aging equipment is shown in Figure 2. Based on our field research, GFRP pipes are extensively employed for transporting oilfield produced water. To investigate the aging behavior within this environment, we analyzed the constituents of the produced water collected on-site and replicated its ionic composition in the laboratory by preparing a solution. According to GB/T1462-2005 to determine the volume of the solution is 150 L, to ensure that the loading rate of the hydrothermal aging solution is greater than or equal to 8 mL/cm², the ionic content of the details of Table 1.

Table 1.

Ionic composition under simulated hydrothermal environment.

Ionic species	Ca²⁺	Mg²⁺	SO₄²⁻	HCO₃^-	cl^-	Fe²⁺	Na⁺/K⁺	pH
Concentration	6300	290	635	325	85,000	20	5000	5.7

Analytical method

In order to analyze the structure and morphology of aged specimens, the appearance and color of GFRP pipes under different aging conditions were first observed. The micro morphology of specimens under different conditions was observed by scanning electron microscope (ZEISS Gemini 300), and the trend of internal surface defects, micro holes in cross section and damage observation of fiber/matrix interface during aging were analyzed.

Under 400–4000 cm⁻¹ resolution of 4 cm⁻¹ attenuation Fourier-transform spectroscopy (ATR), the changes of chemical structure on the surface of aging specimens were studied using Fourier transform infrared spectroscopy. The thermal stability of the outer layer of GFRP pipes under different aging conditions was analyzed using Mettler Toledo (TGA/DSC2) in Switzerland. For the hoop tensile test, the GFRP pipes were cut into 25 mm wide ring specimens with a symmetrical notch depth of 5 mm according to ASTM D2290-2008 procedure A. The hoop tensile strength of the specimens was tested according to the split disk method at a loading rate of 10 mm/min using a universal testing machine (Instron 5985) and uniaxial compression tests were carried out using the same machine at a compression rate of 5 mm/min. The cross-section of the specimens was polished and then subjected to microhardness testing. The Vickers hardness of the outer resin layer of the specimens was measured with a 50 g indenter using a microhardness tester, HVT-1000A, manufactured by Laihua company.

Results and discussion

Microstructural characterizations

The aging and failure forms of GFRP pipes caused by the environment can be roughly divided into three stages. The first stage involves significant color changes on the surface of the pipeline. In the second stage, there is detachment between the fibers and the resin, and micropores and cracks appear. In the third stage, macroscopic resin detachment, exposure of glass fibers, and even pipeline fracture occur.

Figure 3 shows the macroscopic changes of GFRP specimens during aging in two different thermal environments, (a) is thermal oxygen aging and (b) is hydrothermal aging. As the aging time increases, the surface color of GFRP specimens gradually darkens. The thermal oxygen aging specimen gradually deepens from the initial light green to yellow, while the color change of the hydrothermal aging specimen is more obvious, gradually deepening from light green to brownish yellow.

The change in color indicates a chemical change in the epoxy resin on the surface during the accelerated aging, resulting in the formation of an oxidized resin layer on the surface. Because of the pure resin layer attached to the outer surface of GFRP, the outer layer is more susceptible to oxidizing reactions compared to other areas.²⁹ During accelerated aging, oxygen diffuses more readily through the resin layer at higher temperatures, resulting in a dark oxidized layer on the surface. For the inner glass fiber resin composite layer, the adhesive interface of the composite material acts as a barrier to oxygen diffusion.

As can be seen in Figure 4, there was little resin underfilling on the inner wall of the specimen before aging, no fiber exposure was observed, and no obvious porosity was observed in the cross-sectional SEM images. As the thermal oxygen aging progressed, the cross-sectional porosity and resin defects intensified, and micropores were observed in the epoxy resin after 3000 h. The appearance of micropores proved that the epoxy resin degraded after aging and small molecules were generated. After thermal aging for 3000 h, the resin defects increased, resulting in large areas of resin-coated glass fibers being exposed and the fiber/resin interface detaching, indicating a decrease in the bonding between the glass fibers and the resin.³⁰

After 500 h of accelerated hydrothermal aging test, the resin defects on the inner surface became severe and the fibers on the inner surface were exposed, as shown in Figure 5. However, the epoxy resin between the fibers was still present. After 1200 h of accelerated aging, it was found that the resin between the fibers was detached and the single glass fibers were exposed, which would affect the performance of the composite material, and micropores appeared in the resin matrix. After 3000 h of hydrothermal aging, the number of pores in the cross-section increased. Magnification reveals that the fiber/resin interfaces were degraded. During the aging process, the GFRP specimens are subjected to debonding and cracking, which leads to weakening of the fiber/matrix interface, resulting in poor internal bonding of the composites.^31,32

Mechanical behavior

In the actual oil field transportation process, the GFRP pipeline has high requirements for its pressure bearing performance. The hoop tensile test was carried out on these GFRP specimens under accelerated aging. As shown in Figure 6, the overall decreasing trend of hoop tensile strength was observed after aging for different periods. The GFRP specimens showed brittle fracture characteristics before and after aging without the characteristic of plastic deformation.

As the aging time increases, the decrease in hoop tensile strength of the hydrothermal aging specimens becomes more significant. The hoop tensile strength of the specimen without accelerated aging is 475 MPa, which decreases to 196 MPa after 3000 h of hydrothermal aging, and the hoop tensile strength reduction rate is 58%. After 3000 h of thermal oxygen aging, it decreased to 276 MPa, and the hoop tensile strength decreased by 42%. In contrast, hydrothermal aging has a more significant impact on hoop tensile strength. This is because the hydrothermal environment not only accelerated the degradation of resin matrix, but also significantly affected the interface between fibers and matrix.^15,19

In order to characterize the macroscopic aging law of the pure resin layer, a Vickers hardness tester was selected to measure the microhardness of the outer resin layer of the GFRP specimens using a 50 g indenter. The experimental results are shown in Figure 7. As the aging time increases, the hardness of the outer resin layer decreases gradually. At the later stage of aging, the decrease of hardness tends to be slow, and after accelerated aging for 2000 h, the hardness gradually tends to stabilize. The reason for the decrease in the hardness of the outer resin layer is that during the aging process, the macromolecular chains in the epoxy resin crosslinked network pyrolyze to form small molecular chains, and small molecules such as CO₂ are thermally cracked to form small molecules,³³ which produce water, free hydroxyl and free hydrogen during the aging process, leading to a decrease in the mechanical properties of epoxy resins, which were verified in the SEM images. The micropores observed in SEM images indicated the production of small molecules. In the later stage of aging, the hardness of the resin layer tends to stabilize because the molecular chains of the degradable part are completely broken. In contrast, the degradation of the resin in the hydrothermal aging specimens was more pronounced.

During the service process of GFRP pipelines, they are usually subjected to compressive loads. Therefore, the influence of aging on the compression performance of GFRP is also one of the issues that needs to be studied. Uniaxial compression tests were carried out on the GFRP specimens using a universal tensile testing machine, and the experimental specimens and processes are shown in Figure 8. In order to simplify the test, we only studied the compressive strength, damage mode and structural degradation process of the hydrothermal aged GFRP specimens, comparing the shorter hydrothermal aged specimens with the longer aged specimens.

The compression curves of specimens with different hydrothermal aging times under axial compression load are shown in Figure 9. The compression process exhibited brittle failure characteristics at room temperature,³⁴ with the load gradually increasing to the highest point and then rapidly decreasing. The compressive strength decreased with the prolongation of aging time. The compressive strength of the 0 h specimen was 137.7 kN, which decreased to 136.2 kN after 500 h and 88.21 kN after 7000 h.

The energy absorption capacity (EA) was used as the absorption capacity of thin-walled pipe structures for axial loads,³⁵ equation (2). The results were characterized by calculating the area under the load displacement curve, as shown in Table 2. With the prolongation of aging time, the compressive strength F_max and energy absorption EA both show a decreasing trend. EA decreased from 378.2 J of the unaged specimen to 219.6 J of the 7000 h aged specimen, only 58% of the unaged specimen

E A = \int_{0}^{d} F (x) d x

(2)

EA: Energy absorption capacity

d: Compression stroke

F(x): Instantaneous compressive load

Table 2.

The compressive performance of GFRP under different aging times.

Aging time(hours)	F_max(kN)	EA(J)
0	137.7	378.2
500	136.2	361.6
7000	88.2	219.6

The process of compression crushing is carried out in a progressive failure mode, where failure occurs first at one end face of the specimen, leading to the generation of the main crack. Then, the main crack gradually expands as the axial load increases. After the experiment, it was observed that the direction of the main cracks was consistent with the fiber arrangement direction, with end faces generated and rapidly expanding, as shown in Figure 8(c). For composite materials, micro damage usually occurs before significant macroscopic damage changes. The SEM image shown in Figure 10 further investigates the micro damage of the compression fracture surface. As shown in Figures 10(a) and (b), the interface bonding of the unaged specimen appears to be relatively good, with a resin layer covering the glass fiber. Due to the stronger adhesion of the matrix, this good coating allows the specimen to absorb more destructive energy during layering for composite materials.^35–37 Figures 10(c) and (d) shows the microstructure of the fracture surface after 500 h of hydrothermal aging. The resin layer was not significantly affected, but micropores were observed in the resin layer between the glass fibers, which were uniformly dispersed on the resin bonding layer, proving the formation of small molecules during the aging process. However, the fiber/matrix interface was not damaged, so the compressive strength only slightly decreased. For the 7000 h specimen shown in Figures 10(e) and (f), due to matrix degradation, the fiber/matrix interface bonding significantly deteriorated, and the matrix aging did not produce good encapsulation effect.²⁹ At the same time, due to the prolongation of hydrothermal aging time, the resin matrix plasticized. In this case, the resin did not play a role in conducting loads, resulting in a significant decrease in compressive strength.

Chemical composition

To further determine the aging characteristics of functional groups in the GFRP resin layer after accelerated aging experiments, ATR-FTIR analysis was performed on the aged specimens. The infrared spectrum was calibrated. 2965 cm⁻¹, 2930 cm⁻¹ and 2870 cm⁻¹ were saturated hydrocarbon C-H stretching vibration, 1730 cm⁻¹ was anhydride cured epoxy resin C = O stretching vibration, 1610 cm⁻¹ and 1510 cm⁻¹ were Simple aromatic ring C = C and C-C stretching vibration peaks, 1455 cm⁻¹ was CH₂ bending vibration; 1360 cm⁻¹ represents C-N telescopic vibration; Both 1290 cm⁻¹ and 1234 cm⁻¹ are stretching vibration peaks of the ether oxygen bond C-O in the ether group, while 3600–3390 cm⁻¹ is stretching vibration of -OH. Figure 11 shows the infrared spectrum of the thermal oxygen aging specimens. After a long period of thermal oxygen aging, the infrared spectrum of the specimen did not observe the generation of new peaks or the disappearance of old peaks. Only some peak intensities changed, indicating that there were only changes in the concentration of functional groups during the thermal oxygen aging process and did not damage the macromolecular framework. The peak located near 912 cm⁻¹ is the characteristic peak of epoxy group C-O-C in epoxy resin. The specimen consistently exhibits epoxy group characteristic peaks during the thermal oxygen aging process, indicating that the epoxy group did not disappear during the thermal oxygen aging process, but only changed in concentration. For ease of comparison, shift the spectral lines vertically to overlap their baselines, as shown in Figures 11(b)–(d). The strength of the C-H characteristic bands near 2958 cm⁻¹, 2930 cm⁻¹, and 2870 cm⁻¹ first increased and then decreased, due to the post curing phenomenon that occurred during the aging process, causing the epoxy resin to further solidify and form a cross-linked network structure, resulting in an increase in cross-linking density. However, after 3000 h of aging, the strength of the C-H peak decreased due to the oxidation of the C-H bond between the methyl or methylene groups of the two benzene rings. An increase in characteristic band strength was observed near 1290 cm⁻¹ and 1234 cm⁻¹, possibly due to the formation of carboxylic acids or esters during thermal oxygen aging, leading to an increase in C-O bond strength.³⁸ The characteristic band near 1600 cm⁻¹ is attributed to the vibration of the aromatic skeleton, and this characteristic peak hardly changes during the aging process, which indicates that the simple aromatic ring hardly changes during the accelerated aging process. The characteristic band near 1730 cm⁻¹ is attributed to the presence of C=O. Due to the incomplete curing of the epoxy resin in the early stage of thermal oxygen accelerated aging, a post curing phenomenon occurred, manifested as an rise and then decrease in the strength of the C=O peak.¹¹ A weak -OH peak appeared near 3500 cm⁻¹, indicating that during the thermal oxygen aging process, water in the air diffuses into the outer layer of the epoxy resin and forms hydroxyl groups during the subsequent accelerated aging process, and with the extension of thermal oxygen aging time, the concentration of hydroxyl bands increases.

In the hydrothermal aging specimen, the position of the characteristic peak did not change in Figure 12, indicating that no chemical changes occurred during the hydrothermal aging process. The C-H stretching vibration peak near 2930 cm⁻¹ did not change, only showed a decrease in intensity, indicating oxidation of the C-H on the main chain. The characteristic peaks near 1290 cm⁻¹ and 1234 cm⁻¹ reflect changes in C-O concentration, while the change in C=O characteristic peak intensity at 1730 cm⁻¹ indicates post curing during accelerated aging. Similarly, a - OH stretching vibration peak appeared near 3400 cm⁻¹, indicating that the entry of water molecules during hydrothermal aging leads to the formation of hydroxyl groups. The results of infrared spectroscopy indicate that the properties of the benzene ring in the epoxy resin are stable during the aging process, and characteristic bands of the benzene ring are observed at different aging times. During the aging process, the chemical bond breaks mainly on the ether bond. With the formation of small molecules during the aging process, large molecules will break and form small molecules, and small molecules will continue to break and produce hydroxyl groups.

When GFRP specimens are heated to a certain temperature, the molecular chain undergoes degradation. As the temperature further increases, the degradation rate accelerates, and the temperature corresponding to a weight loss of 5% is used as an indicator of thermal stability Td.³⁹ Figure 13 shows the thermogravimetric curves of specimens aged under thermal oxygen and hydrothermal aging at different periods. The remaining 95% of the weight area was selected for local magnification in the Figure 13. The original specimen without accelerated aging had a thermal decomposition temperature of 331°C, and after 500 h, the thermal decomposition temperature of the specimen reached 338°C, which is speculated to be due to the post curing effect of the epoxy resin.

Overall, accelerated aging did not significantly affect the thermal decomposition temperature of the outer resin layer of GFRP.³⁹ The resin decomposes during the heating process, leaving glass fibers as the remaining material. After heating, the remaining weight of the hydrothermal aging specimens varies greatly, and shows an increasing trend with increasing aging time. It is speculated that during the aging process, the decrease in the resin content of the outer layer of GFRP leads to an increase in the relative content of glass fibers. After thermogravimetric testing, the remaining weight accounts for a relatively large proportion. Moreover, the longer the hydrothermal aging time, the more epoxy resin decomposes, and the outer resin content decreases.

Aging mechanism analysis

Figure 14 shows the accelerated aging mechanism. The internal structure of GFRP is glass fiber reinforced phase and epoxy resin matrix. The microstructure of unaged specimen is shown in Figure 14(b). The bonding between glass fiber and resin matrix is mainly physical bonding and hydrogen bonding. The unaged specimen resin matrix and glass fiber bond well, and no obvious interface damage was found. The resin filling is good, with only a few cases of incomplete resin filling. During the accelerated aging process, external environmental influences such as oxygen, heat, water, and some ions can cause shrinkage and dissolution of the epoxy resin, leading to the expansion of resin defects and the appearance of micropores as seen in SEM images. This will lead to a decrease in the mechanical properties of GFRP. Due to the transfer of heat, the glass fiber/resin interface undergoes varying degrees of shrinkage, and the interface bonded by hydrogen bonds begins to crack. Subsequently, ions in the medium diffuse into the interface, causing further damage and resulting in significant cracking. Under the combined action of temperature, oxygen, water, and medium ions, the epoxy resin undergoes a post curing phenomenon and then undergoes degradation, manifested as a decrease in the C-H concentration in the infrared spectrum. During the degradation process, the internal macromolecular chains of the resin are destroyed, as shown in Figure 14(c). The degradation produces small molecules, which are manifested as the appearance of micropores on the resin matrix. The increase in C-O bond concentration indicates the formation of carboxylic acids or esters during the degradation process. As the aging time prolongs, the diffusing H₂O will combine with the main chain to form -OH, indicating the continuous acceleration of aging. It was observed in SEM images that hydrothermal aging can accelerate the rupture of the glass fiber/matrix interface, and thermogravimetric experiments show that the degradation of epoxy resin is more significant under hydrothermal aging environment. Figures 14(d) and (e) shows the failure process of the internal structure of GFRP.

Conclusions

In this experiment, we conducted two accelerated aging experiments, thermal oxygen aging and hydrothermal aging, using acid anhydride cured GFRP. The aging temperature was both 95°C and the time reached 3000 h. We simulated the operating conditions of GFRP in the oil field, analyzed and studied its aging behavior and mechanism, and reached the following conclusions:

1. Two types of thermal aging environments have an impact on the appearance color of GFRP specimens, with different degrees of color deepening. The color change of the hydrothermal aged specimen is more pronounced, indicating that the surface resin layer has undergone oxidation. As the aging time prolongs, the number and area of resin defects in both aging environments increase, and micropores appear in the resin matrix while the fiber/matrix interface also undergoes degradation. This is because the molecular chains of epoxy resin decompose and break during the aging process, resulting in the production of small molecules such as CO₂ and H₂O.

2. The hoop tensile strength gradually decreases under two aging environments, and the influence of hydrothermal aging is more significant, because hydrothermal environment not only accelerates the degradation of resin matrix and glass fiber but also significantly affects the interface between fibers and matrix, thereby affecting the trend of decreasing hoop tensile strength. The compressive strength of the uniaxial compression test results decreases with increasing aging time. After 7000 h of hydrothermal aging, the energy absorption rate decreases to 58% of the unaged specimen. The hardness of the outermost resin shows a gradually decreasing trend in both thermal environments, and the rate of decrease shows a decreasing trend. This is attributed to the complete breakage of some molecular chains in the later stages of aging, resulting in a gradual stabilization of hardness.

3. During the aging process, there are only some changes in the concentration of functional groups, and the macromolecular framework is not damaged. The strength of the C-H band first increases and then decreases, which is attributed to the post curing phenomenon during the aging process. The increase in C-O peak intensity indicates the formation of carboxylic acids or esters during the aging process. With the increase of aging time, both types of aging specimens showed an increase in -OH concentration, and the concentration of hydroxyl bands increased. Thermogravimetric experiments showed that the degree of resin decomposition increased, leading to an increase in residual mass as aging progressed.

Footnotes

Acknowledgements

The authors also acknowledge the support of China National Petroleum Corporation (CNPC) for the project “Research on Aging Mechanism and Life Prediction of Non-metallic Pipelines,” Project No. Z1H1412.

Author contributions

Jingjie Dou: Investigation, Data curation, Methodology, Formal analysis, Conceptualization, Writing—original draft. Jing Yan: Methodology. Tan Gu: Funding acquisition. Zhiming Yu: Data curation. Dandan Liao: Writing – original draft. Min Hu: Data curation. Fei Zhao: Conceptualization. Jie Liu: Data curation. Siwei Chen: Resources. Jun Wang: Investigation, Supervision.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the China National Petroleum Corporation; No. Z1H1412.

ORCID iD

Jingjie Dou

Data availability statement

The authors do not have permission to share data.*

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