The influence of hygrothermal aging on the hoop tensile strength of glass fiber wound polymer composites fabricated via filament winding technique

The study investigates the impact of moisture environment treatment, on the hoop tensile strength (HTS) of glass fiber-reinforced polymer (GFRP) composites, through hygrothermal aging. GFRP cylinders were fabricated with varied parameters—volume fraction, winding angle, and stacking sequences using a filament winding machine. The fabricated samples are subjected to hygrothermal aging using seawater and tap water with oil at 80 °C for 1080 h (45 days). The HTS tests were performed on unaged and aged samples. There was a reduction in HTS for aged samples which is attributed to heat, seawater contamination, and oil. The highest and lowest HTS values recorded are 402.9 MPa and 118.3 MPa for unaged and tap water with oil-aged samples respectively. HTS in aged samples is compared with unaged samples. The study opens up avenues in identifying the best-suitable combination for retaining HTS under various aging conditions.

have been used for decades in domestic and commercial applications, are one of the critical components in these industries.However, traditional metallic cylinders have several limitations-corrosion susceptibility, weight, and the risk of sudden failure.These limitations are especially concerning for applications that require the storage of highly flammable gases, such as oxygen cylinders for mountaineers, fuel tanks for automobiles, and LPG storage cylinders, where safety is a critical factor [3]. Furthermore, there is a limited availability of pressure cylinders in the market that are suitable for various weather conditions, further limiting the operational capabilities of pressure cylinders [4].In light of these challenges, there is a growing demand for alternatives to traditional metallic-based cylinders.Composite materials, particularly polymer-based composites reinforced with fibers like graphene, carbon, glass, Kevlar, and aramid, have emerged as promising candidates due to their higher strength-to-weight ratio in manufacturing light-weight pressure vessels [5,6].E-glass fiber or carbon fiber are commonly used as reinforcements in pressure cylinder applications.Matrix materials such as epoxy, vinyl ester, and polyester resins, are essential for bonding the fibers and effectively distributing the load [7,8].
Despite the benefits of polymer-based composites, their processing remains challenging.Several fabrication methods have been investigated for developing pressure cylinders-compression molding, hand lay-up, and spray techniques etc [9,10].However, filament winding technique has emerged as the preferred method for producing polymer composite pressure cylinders.This technique ensures fiber flow continuity while reducing stress concentration zones and the risk of breaks/cracks in the fabricated cylinder [11,12].Recent studies have investigated the effects of stacking sequences in composite pressure vessels, focusing on laminate quality and structural deformation using fiber-reinforced polymer composites [13][14][15].Furthermore, vessels made from composite materials such as carbon and glass fibers, are found to be better performing.Liners used in pressure cylinders/vessels play another important role in creating leak-proof barrier between the cylinder's contents and the outer material.Liners are made up of variety of matrix materials such as aluminum, polyethylene, HDPE, and vinyl ester.Therefore, selecting suitable liner and fiber-based composites necessitates meticulous scrutiny of vessel strength, particularly through assessments of HTS.Furthermore, the selection of materials is influenced by various environmental factors including moisture, exposure to reactive chemicals, and temperature fluctuations.Therefore, it is essential to assess how these diverse environmental conditions affect the efficiency and performance of composite materials made products.When natural fibers are used as reinforcement, the influence of environmental factors on the mechanical properties of the vessel is significant.Natural fibers are more susceptible to moisture than synthetic fibers like carbon and glass [16,17].Researchers have explained the environmental effect of the combination of synthetic and natural fiber on mechanical properties [18].These environmental factors also have an impact on matrix materials.However, moisture's effect on the mechanical properties of resin is usually less insignificant.Materials naturally degrade over time as they age.These aging effects can be influenced by a variety of environmental factors common to the oil and gas industries.Hygrothermal aging is the most common aging process and poses significant material property risks, particularly for natural fiber-reinforced polymer composites [19].
Among all existing aging processes, the hygrothermal aging process is predominant and dangerous from a material property's point of view, especially in the case of natural fibers reinforced polymer composites [20].Hygrothermal aging mediums include tap/distilled water, alkali water, and other commonly used oils (such as corrosion-resistant oil and automobile lubricant).Recent research has looked into carbon fiber and ramie fiber composites under various hygrothermal conditions, such as immersion in distilled water and exposure to different temperatures for testing the mechanical properties [21].Furthermore, CFRP composites are frequently exposed to hygrothermal environments during service, where temperature and moisture can impair material properties [22].Also, recent research has revealed a discrepancy between the long-term degradation performance of GFRP composites and the retained strength of glass fibers during underwater attack [23,24].Stacking sequences of composite laminates emerge as important parameters to elucidate the moisture diffusion behavior and property degradation of composites.Working temperature, exposure duration, and loading conditions influence the aging process [25][26][27].A research group investigated the mechanical and thermal properties of carbon/glass fiber-reinforced epoxy hybrid plates subjected to hygrothermal aging, exposed to varying temperature ranges, and distilled water immersion for extended periods [28].Hygrothermal aging experiments were conducted on the glass epoxy fiber composite using tap water at controlled temperatures [29].Additionally, the effects of water and alkali-treated hemp fiber blended with Polypropylene composite were studied [30].The author also developed fiber-reinforced phenolic composites and examined the impact of water absorption on their mechanical properties [31].Furthermore, elastomer-based composites underwent oil aging, and thermoplastic vulcanizate composite materials were subjected to hot oil aging [32,33].Moreover, glass epoxy composites were subjected to hygrothermal aging in various operating environments, including process water, seawater, and machine oil, with different durations and temperatures, and their mechanical performance was analyzed using different regression models [34].
The combination of tap water and oil aging is a method used to assess the durability and performance of fiber composites under different environmental conditions.Tap water exposure evaluates the material's response to moisture absorption and hygrothermal aging, simulating real-world conditions such as outdoor exposure or immersion in aqueous environments.On the other hand, oil aging examines the composite's resistance to degradation when exposed to petroleum-based fluids, mimicking potential industrial or automotive applications where contact with oils or lubricants may occur.By subjecting fiber composites to both tap water and oil aging, researchers can assess their suitability for various practical environments and applications, providing valuable insights into their long-term performance and durability [35][36][37].In harsh environments, the mechanical properties of FRP composites deteriorate as the working temperature rises between 70-80 °C.The aging period, chemical treatment (such as exposure to the sea and distilled water), and working temperature have all been shown to influence the overall strength of FRP composites [38][39][40][41][42]. Recent research has shown that moisture influences matrix osmotic cracking due to the expansion of the gap between fiber and matrix, as well as the debonding of fiber and matrix.Longer periods of hygrothermal aging result in a proportional reduction in the strength of FRP composites [27,43].Although several studies have been conducted on the hygrothermal aging of FRP composites, the findings are limited to a few working environments and loading conditions.There has been less research on the hygrothermal aging effect of filament-wound GFRP composite pipes in various moisture mediums/environments.More research is needed, particularly on the effects of aging on HTS, that is an important property in filament-wound composites.Thus, there is scope for studying the effects of hygrothermal aging on the HTS of fiber-wound GFRP composites.
The current study is an experimental investigation of the impact of the moisture environment, on the HTS of GFRP composites, using hygrothermal aging techniques.The filament wound GFRP composite samples were fabricated at varied conditions-volume fraction, winding angle, and stacking sequence.The fabricated samples were subjected to hygrothermal aging in two conditions-seawater and tap water in an oil bath.The effect of such hygrothermal treatments on the HTS of GFRP composites is measured using a split disk test.The performance of GFRP composites was investigated under varied conditions-aging parameters, stacking sequence, and fiber volume fraction.The study explores the use of developed GFRP composites as a replacement for existing metallic materials used in pressure vessels.

Fabrication of GFRP composites
The GFRP composites developed in the current study were made of E-glass fibers reinforced in a polymer matrix.The Epoxy resin and PVC liner were procured from the local market.The E-glass 1200 tex fiberglass direct roving shown in figure 1 was reinforced in Lapox L-12 resin with K-6 as a hardener to fabricate the GFRP composite.The PVC liner of 192 mm diameter, 360 mm length, and 3 mm wall thickness was used to line the filament material.The physical properties of the raw materials are given in table 1.
GFRP composites fabricated via the filament winding process are found to possess many advantages in terms of strength, durability, and design flexibility.This manufacturing method involves winding the continuous strands of glass fibers onto a rotating mandrel in a predetermined pattern.The resin matrix is applied simultaneously or sequentially to impregnate and bond the fibers, resulting in a strong composite structure.The filament winding process allows for precise control over fiber orientation, volume fraction, and stacking sequence, resulting in tailored composite components with high mechanical performance.Cylindrical structures, such as pressure vessels made up of GFRP composites produced via the filament winding process Initially, the PVC liner/mandrel was housed between the movable and fixed jaws as shown in figure 2(a).The E-glass fibers were initially wound on the PVC mandrel in the dry condition-as shown in figure 2(b)-to ensure proper winding and to avoid slippage during the actual winding process.Later wet winding was employed.The fibers were pulled through the resin tank as shown in figure 2(c).The fabricated sample is shown in figure 2(d), that are cooled down overnight at room temperature.The GFRP samples were fabricated with varied winding parameters-fiber volume fraction, winding angle, and stacking sequence.In this study, ten different composition GFRP cylinders with varied winding parameters are fabricated using the filament winding technique.Further, ten compositions are divided into two parts (6-normal case and 4-special case cylinders) based on specific winding angles and stacking sequences.The normal and special conditions followed for the fabrication of GFRP samples are tabulated in tables 2 and 3.The normal/regular GFRP samples are identified as P-samples and the special case samples are identified as SC-samples.The special case includes the combination of different winding angles (3 different winding angles on the single cylinder), stacking sequence, and volume fraction-based winding machine conditions and available parameters.The samples for hygrothermal aging treatment were prepared as per ASTM D2290 [46] and samples were cut using CNC milling at Super Enterprises, Mangalore, India.

Hygrothermal treatment of GFRP samples
The hygrothermal treatment involves subjecting materials to varied conditions-moisture and temperatures conditions that mimic real-world environmental exposure.The hygrothermal conditions for this process were determined using literature data [47,48].Hygrothermal treatment uses a variety of moisture mediums, such as drinking water, wastewater, thermal water, oil, and natural gas.In the present research, GFRP samples were hygrothermally treated with two moisture mediums: seawater (SW) and tap water with oil (TWWO).Figure 3 illustrates the hygrothermal treatment for GFRP composites.As shown in figure 3(a) the sea water for aging was collected from the Arabian Seashore in the water container.The seawater was transferred to the container which was later placed in the controlled chamber figure 3(b).The SAE30 standard oil for TWWO aging was procured from the local market.The oil was mixed with tap water in a fixed ratio of 1:15.The mixture was placed in the controlled oil bath setup shown in figure 3(c).The various fabricated GFRP samples were subjected to hygrothermal treatment in the containers maintained at 80 °C.The hygrothermal aging was conducted for 45 days.

Mechanical testing of treated samples
As HTS is an important property for determining the structural integrity of cylindrical components under internal pressures, a cylindrical-shaped composite ring was used for mechanical testing.HTS determines the material's ability to withstand deformation and failure caused by tensile stresses acting circumferentially around the structure.HTS has a direct impact on the performance and reliability of cylindrical structures subjected to internal pressure.Therefore, materials with higher HTS can withstand higher operating pressures, resulting in more efficient and durable designs.Then, the HTS is critical for selecting materials that can withstand the expected operating conditions, such as pressure, temperature, and environmental factors.Hence in the current work, the effect of hygrothermal aging on the HTS of filament wound GFRP samples is estimated.To determine the HTS, GFRP composite samples in aged and unaged conditions, were subjected to a split disk test.The HTS test was conducted using the circumferential or ring tensile test method in which the samples were subjected to tension radially.The ring-type model of the HTS test specimen including dimensions is shown in figure 4(a).The ring samples were notched at diametrically opposite points of the ring.The inner and outer diameters of the ring are clamped by a specialized split disk holding mechanism that provides uniform pressure distribution.The detailed gripping mechanism (fixture) used for holding the notched samples is shown in figure 4(b).The grips were holding the ring securely without slipping.The ring specimen was placed vertically between the grips of the split disk holding sample.The inner and outer surfaces of the ring were kept parallel to the platens of the sample.It was crucial to ensure that the ring specimen was aligned perfectly with the loading axis of the testing machine to avoid any additional bending stresses during the test.
The hoop tensile test setup for testing the GFRP cylinder samples requires specialized fixtures that apply circumferential tensile loads onto cylindrical specimens.The hoop tensile test using the UTM machine and fixture used for the holding sample are shown in figures 5(a) and (b).The GFRP cylinder specimens were cut as per the test requirement, ensuring proper gripping in the UTM.The clamps are used to fasten the ends of the GFRP cylinder samples securely.These grips can assist with evenly distributing the tensile load along the circumference of the cylinder.A 400 kN UTM is used to apply tensile loads onto the GFRP cylinder specimen.The radial load was applied to the inner diameter of the ring while maintaining the outer diameter was fixed.Loading continued until the samples fractured.Durign testing, parameters such as the load at failure, maximum displacement at fracture, and HTS were continuously recorded.
Finally, the study investigated the failure behavior of GFRP samples exposed to hygrothermal treatment.Images of fractured samples were visually analyzed to identify fracture modes.The fractured surfaces were also examined using HR-FESEM(Carl Zeiss GEMINI 300).

Effect of hygrothermal treatment on HTS
The behavior of the various GFRP ring samples under radial loading is analyzed for both unaged and aged GFRP ring samples.The influence of hygrothermal aging on the HTS of the filament wound GFRP samples is shown in figure 6.It is observed from the scatter plot shown in figure 6(a) that the aged samples exhibit lower HTS as compared to the unaged samples.The maximum HTS achieved in the case of unaged samples is approximately 403 MPa, while in the case of SW-aged and TWWO-aged samples, it is approximately 314 MPa and 279 MPa, respectively.Thus, the hygrothermal aging in the SW medium and TWWO medium has a detrimental effect on HTS.A decrease of about 32% and 43% is observed in SW-aged and TWWO-aged samples, respectively.Similar observations were reported by Krauklis A E et al [49], where the GFRP composite strength was decreased due to hygrothermal aging.Also, Xin H et al [50] conducted flexural tests on GFRP material under various hygrothermal aging conditions, demonstrating a reduction in the flexural strength of the GFRP composite.
Further, as compared to TWWO-aging, the SW-aging yields better in HTS.However, the polynomial fit in figure 6(b) reveals that the hoop strength of the unaged sample surpasses that of both SW-aging and TWWOaging conditions.Therefore, the GFRP materials tend to absorb moisture when exposed to wet environments.This absorption can lead to the weakening of its structural integrity.Hygrothermal aging can also affect the bond between the glass fibers and the polymer matrix.Moisture ingress at the fiber-matrix interface can weaken the bond, reducing the load transfer capability and overall strength of the composite material.
Continuing with this discussion, figure 6(c) illustrates the hoop strain of the aged samples with varying volume fractions and winding angles.When comparing the strain rates of unaged and aged samples prepared under different conditions of GFRP, it is evident that the unaged samples exhibit higher strain rates than the aged ones.Specifically, samples P2 and P3 demonstrate greater strain rates compared to the others, while SC3 with TWWO aging exhibits the least strain value.Additionally, the average strain rate across GFRP samples subjected to different aging conditions indicates that the unaged samples have higher rates than the aged ones, with TWWO aging showing the lowest rate of change in strain (figure 6(d)).Moreover, the stress-strain relationship of the GFRP samples, both aged and unaged, is depicted in figure 6(e).It is observed that as stress increases, the strain rate also tends to increase in unaged GFRP samples.Furthermore, the scattered distribution of points in the aged sample suggests that hygrothermal aging has a significant impact on the strength of the GFRP material.The decrease in hoop tensile strength (HTS) post-aging can be attributed to several factors.Continuous exposure of the glass fiber to moisture and elevated temperatures can lead to the weakening of the resin, reducing its strength [51,52].Additionally, the accumulation of chloride and oil particles in the sample may weaken it internally.Water molecules diffusing across the polymer chains can cause separation of layers and expansion of polymer structures, weakening the intermolecular bonding forces.As a result, the polymer composite becomes more ductile and less stiff, leading to a decrease in HTS [53].

Influence of winding factors on the HTS
The HTS is affected by factors like volume fraction, winding angle, and stacking sequence.Figure 7(a) depicts the effect of fiber volume fraction (Vf) on HTS.The Vf varies between 0.55-0.75,whereas in unaged samples, the increase in Vf increases the HTS and the increment is around 10%.In the case of aged samples, an opposite trend is observed where the HTS decreases with an increase in Vf, and a reduction is approximately 50%.SW-aging has less effect with an increase in Vf as compared to TWWO-aging.However, a higher increase in HTS is observed in the case of TWWO-aging at higher Vf.GFRP composites tend to absorb water when exposed to aqueous environments such as seawater and tap water with oil.This absorption can weaken the interfacial bonding between the glass fibers and the polymer matrix, leading to a reduction in overall strength compared to an unaged sample.Also, the authors have studied the effect of filler concretion on hygrothermal conditions for mechanical properties, which has shown a decrease in the mechanical strength of the composite due to hygrothermal aging [54,55].
Figure 7(b) illustrates the impact of changes in the winding angle on HTS.For unaged samples, the effect of winding angle on HTS is moderate, with a maximum HTS of 350.44 MPa observed for winding angles of 45°/ 55°/65°.The influence of winding angle on the HTS of aged GFRP composite is less pronounced compared to unaged composite.In SW-aged samples, the maximum HTS is 285.34MPa for winding angles of 45°/55°/65°, while TWWO-aged samples reach a maximum HTS of 227.15 MPa for a winding angle of 55°.Previous research on E-glass epoxy composite wounds with an optimal angle of 55°has demonstrated the impact of different environmental conditions on HTS values.The aging process is estimated to have decreased the overall strength of the glass fiber composite [56].Additionally, the author investigated the effects of different winding angles with glass and carbon fiber composites on mechanical strength and water absorption.As the winding angle increases, water absorption in the composite also increases, resulting in reduced strength [57].Therefore, the winding angle significantly influences the strength of composite materials.
The stacking sequence has a significant impact on the hoop tensile strength (HTS) of both aged and unaged samples illustrated in figure 7(c).Among the three stacking sequences, SS2 resulted in the highest HTS, while SS3 showed the lowest strength.However, the influence of the stacking sequence is more pronounced in aged samples compared to unaged ones.Factors such as forced voids, gaps, entrapped air, particles, and the accumulation of chloride and oil particles in aged samples limit the resistance capacity of the fiber and resin.Additionally, the influence of exposure to seawater (SW) and tap water with oil (TWWO) on filament-wound GFRP samples significantly reduces HTS.This reduction can also be attributed to factors like aging temperature (80 °C) and duration (45 days), which facilitate the entrapment of chloride particles and oil molecules between the filaments.In contrast, unaged samples exhibit variation in HTS due to factors such as fiber and matrix composition, interfacial bonding, and fiber fragmentation.Despite these challenges, unaged samples outperform aged ones in terms of HTS.Further research [58] on water absorption with different laminated conditions of glass and flax fiber epoxy composites has shown that the stacking sequence can affect water absorption rates, indicating its importance in determining the strength of fiber-reinforced composites.commonly observed failure in the composites.The delamination increases with an increase in the angle between the adjacent plies.
For a better understanding of the failure modes in the samples, the fractured surfaces were observed under scanning electron microscopy (SEM).The images taken in SEM are shown in figure 10.The microstructure of the unaged fractured sample shown in figure 10(a) depicts the fiber-matrix debonding that has resulted in the rupture of fibers.The SEM image of the SW-aged, fractured sample is shown in figures 10(b), (c).It appears from figure 10(b) that as a result of insufficient matrix material to resist the imposed load, the fibers are scattered.They are not intact with the epoxy resin.This indicates that SW-aging has resulted in adhesive and cohesive failure.From figure 10(c) the resistance provided by the matrix to the applied load is convincing, thus it indicates that the failure of the fiber is due to shearing action (45°angle).Thus, a river-like flow pattern of the matrix is observed.In the case of the TWWO-aged, failed sample shown in figure 10(d) the lack of matrix material between the fibers is observed.As a result, the interfacial bonding has decreased and thus the samples have failed before taking up the actual peak load leaving behind few resin particles.
The findings from the current investigation provide important insights into the behavior of GFRP composites under hygrothermal aging conditions, shedding light on the effects of moisture environments on HTS.These findings have important implications for future research in several areas.Firstly extended research is needed to investigate the long-term effects of hygrothermal aging on GFRP composites, particularly at higher temperatures and in different environmental conditions such as chemical and pollutant exposure.Secondly, there is a need for ongoing research into optimizing the fabrication process of GFRP components to increase their resistance to aging and overall performance.Lastly, identification of the best-suited combination for retaining HTS under various aging conditions opens avenues for developing new materials and customized design strategies.

Conclusions
In the present study, the GFRP composite cylindrical structure was developed with varied parameters-volume fractions, winding angles, and stacking sequence parameters.HTS tests were conducted on both unaged and aged samples exposed to seawater and tap water with oil.The GFRP composite experiences a decrease in HTS when exposed to seawater and tap water with oil.Specifically, there is a 32% decrease in HTS due to seawater aging and a 43% decrease due to tap water with oil aging.Among the samples tested, GFRP composite sample P1, with a fiber volume fraction of 0.55, winding angle of 55°, and a stacking sequence of SS1(± 55°2/90°2/(± 55°2)), exhibits higher HTS and enhanced resistance to aging.The presence of seawater contamination and oil molecules during aging processes contribute to the reduction in HTS by approximately 50%.Further research is needed to identify optimal combinations for retaining HTS under various aging conditions, which could lead to the development of more durable materials.

Figure 1 .
Figure 1.The 1200 tex fiberglass direct roving used for filament wound.

Figure 2 .
Figure 2. Fabrication process of GFRP cylinders-(a) filament winding facility, (b) initial dry winding on mandrel followed by wet winding, (c) glass fibers pulled out of resin bath for wet winding, and (d) fabricated GFRP cylinders.

Figure 3 .
Figure 3. Hygrothermal treatment process-(a) Sea water collection, (b) Sea water aging setup, and (c) Tap water with oil bath aging setup.

Figure 4 .
Figure 4. CAD models of (a) ring-type notched HTS test sample and (b) exploded view of the HTS test fixture assembly.

Figure 5 .
Figure 5. Hoop tensile test setup consisting of (a) universal testing machine (b) Fixture for holding the sample.

Figure 6 .
Figure 6.Effect of hygrothermal aging on GFRP samples-(a) Variation in HTS of the GFRP samples under different aging conditions, (b) Average of HTS for the different aging conditions, (c) Variation in the strain of the GFRP samples with different winding sample, (d) Average strain of samples under different aging conditions, and (e) Hoop stress versus hoop strain of the GFRP aging samples.

Figure 8 .
Figure 8. Images of notched samples before the ring test-(a) Unaged, (b) SW-aged, and (c) TWWO-aged and the images of the samples after the ring test-(d) Unaged, (e) SW-aged, and (f) TWWO-aged.

Figure 7 .
Figure 7. Variation in HTS due to (a) Volume fraction (b) Winding angle (c) Stacking sequence of GFRP composite.

3. 3 .
Damage analysis and fractography study The images of the unaged, SW-aged, and TWWO-aged samples taken before the ring tensile test are shown in figures 8(a)-(c), and the images taken after the test are shown in figures 8(d)-(f).The fractography study of the samples reveals three types of failure modes-(i) a conical mode failure near the notched section as seen in figures 8(d)-(f), (ii) delamination from the PVC liner as seen in figure 9(a), and (iii) random failure at the notch section as seen in figure 9(b).The delamination between the layers of piles with different inclinations is the most

Figure 9 .
Figure 9.The failed modes of the unaged ring sample.

Figure 10 .
Figure 10.Images of fractured surfaces showing-(a) broken fibers, (b) scatted fibers after breaking, (c) matrix flowing in the river pattern, and (d) uneven matrix dispersion post-aging.

Table 2 .
Details of the various fabricated samples.

Table 3 .
Special cases of the fabricated cylinder.