Experimental Determination of Circumferential Mechanical Properties of GFRP Pipes Using the Split-Disk Method: Evaluating the Impact of Aggressive Environments

Abstract

This study investigates the mechanical properties of Glass Fiber-Reinforced Plastic (GFRP) pipes in the circumferential direction using the split-disk method, with a focus on understanding the influence of aggressive environmental conditions. The split-disk method was used to determine the key mechanical properties, including the hoop tensile strength and modulus, and also the Poisson’s ratio, which are critical for the performance of GFRP pipes under internal pressure. The experiment was conducted under controlled laboratory conditions, and the results were analyzed to assess the effects of exposure to aggressive environments (saltwater and alkaline solutions at 20 °C and 50 °C). The correlations between the UTS, elastic modulus, and Poisson’s ratio highlight how GFRP pipes degrade under environmental exposure. As the UTS decreases, so do the stiffness and lateral deformability, with the most significant reductions occurring in chemically aggressive environments at high temperatures. Exposure to an alkaline solution weakens the GFRP pipes, with the strength dropping more sharply at higher temperatures, with the UTS decreasing by 21%. Saltwater exposure reduces the elastic modulus, especially at higher temperatures, with a 14% decrease, accelerating material degradation and reducing deformation resistance. An alkaline solution further lowers the modulus, with a 21% decrease at 50 °C, showing the lowest stiffness. Air exposure, in contrast, has a less severe effect, with the pipes retaining much of their mechanical integrity. These findings collectively suggest that environmental degradation affects the overall mechanical behavior of GFRP pipes, providing valuable insights for the design and maintenance of GFRP piping systems, particularly in industries where exposure to aggressive environments is common. This study underscores the importance of considering environmental factors in the material selection and design processes to ensure the long-term reliability of GRP pipes.

1. Introduction

Glass Fiber-Reinforced Plastic (GFRP) pipes are widely used in various industries due to their exceptional mechanical properties, corrosion resistance, and durability [1]. These materials are particularly favored in environments where conventional materials like steel may suffer from rapid degradation due to aggressive environmental factors [2,3,4]. GFRP pipes support sustainable development by reducing maintenance needs and extending service life, particularly in challenging conditions, which conserves resources and minimizes environmental impact. However, to ensure the long-term reliability of GFRP pipes, it is very important to accurately characterize their mechanical properties, particularly under the influence of environmental factors such as temperature, humidity, and chemical exposure.

The mechanical properties of GFRP pipes, particularly their circumferential (hoop) strength, are essential for their application in various industries, including water supply, chemical processing, and oil and gas transportation. Several methods exist for evaluating the circumferential tensile properties of composite products, including the hydrostatic pressure test, flat crush test, segment-type ring burst test, ring burst test, and split-disk test [5]. Unlike the hydrostatic burst test, which requires testing the entire product, the split-disk test offers distinct advantages, such as simplicity, cost-effectiveness, and higher efficiency [5]. The split-disk method has emerged as a standard technique for evaluating these properties. This method involves subjecting a ring or disk cut from the pipe to a diametrical compression, providing valuable insights into the material’s hoop strength and failure characteristics. Numerous studies [5,6,7,8,9,10,11,12,13] have explored different aspects of this testing method, each contributing to a more nuanced understanding of composite materials’ behavior under various conditions, including the influence of aggressive environments [2,14,15,16,17,18].

The exploration of mechanical testing methods is a central theme in the study of composite materials. Zhao et al. [5] used the split-disk test for measuring filament-wound composites’ mechanical properties using 3D Digital Image Correlation and finite element modeling, finding that friction between the ring and disk significantly affects the strain measurements and identifying a low strain zone at the ring split due to local bending. In a similar work, Kaynak et al. [6] investigated the split polymer test method, focusing on the behavior of polymer composites under stress, exploring how different processing parameters affect continuous fiber-reinforced epoxy composite tubes produced by filament winding. Their findings revealed that the type of epoxy resin had little effect on the performance, while carbon fibers and winding angles greater than 60° significantly improved the tubes’ strength. Both studies underscore the importance of split-disk techniques in advancing the understanding of material behavior under conditions that mimic real-world scenarios.

Furthermore, these findings align with the work of Sapozhnikoy et al. [7] and Dick and Korkolis [8], who concentrated on the mechanical measurements in polymer and tubular materials, respectively.

The study presents [19] a method for characterizing filament-wound composite pipes using glass/vinylester and carbon/epoxy materials. Tubular samples were tested through a split-disk and a newly designed biaxial test, which minimized the stress concentration and ensured alignment. An optical technique assessed the void content, and failure envelopes in the σ2 − τ12 plane were obtained and compared with common failure theories. The Puck criterion accurately predicted failure, especially under combined torsional and compressive loads. Another study [10] uses finite element analysis to explain why split-disk tests yield lower FRP rupture strains than flat coupon tests. It was found that geometric discontinuities and bending at the FRP ring gap increased local strains, leading to earlier rupture. Additionally, the effects of adhesive properties, FRP stiffness, overlap geometry, and friction were examined, further elucidating the behavior of these materials.

Further examining composite structures, Benyahia et al. [11] investigated thick, ±55° filament-wound glass/epoxy tubes for offshore applications using quasi-static tests. They developed a redesigned fixture system that reduced the edge stress concentration during split-disk tensile tests on pipes with an 86 mm diameter and a thickness of 6.2 mm. Testing both notched and unnotched specimens revealed that increased notch size and number reduced the yield stress and increased the yield strain, negatively impacting the structural integrity.

Another paper [12] focused on stress and strain distribution in the longitudinal and circumferential directions of glass–polyester composite pipes under tensile testing, widely used in chemical and process industries. Tension tests on flat samples and rings from ±55° filament-wound pipes determined the tensile strengths in both directions.

A modified split-disk test (ASTM D 2290 [20]) was considered in [13] to assess the stress–strain behavior of two GFRP rings using a simple specimen preparation method. GFRP-confined glass fiber rings of 1.75 mm, 2.50 mm, and 3.25 mm thicknesses, wrapped with E-glass and S-glass fabrics, underwent uni-axial tensile testing. The results for the E-glass composites were compared to those of the S-glass composites. Finite element simulations of selected specimens were also conducted by the authors, yielding strain efficiency factors that align well with the experimental data for design applications.

A significant focus of research in the specialized literature is the effect of environmental exposure on the degradation mechanisms of FRP materials. Silva et al. [21] investigated the degradation of GFRP laminates under accelerated aging, highlighting the challenges in predicting long-term degradation, particularly under salt fog exposure. The authors of the paper [17] explored the impact of accelerated thermal and hydrothermal aging on E-glass fiber-reinforced epoxy resin composite pipes, emphasizing how hydrothermal conditions severely affect the mechanical properties. Mahmoud and Tantawi [15] studied the effects of strong acids on glass–polyester GRP pipes, demonstrating the detrimental effects of acidic environments on mechanical performance. Additionally, research on the tensile properties of glass–polyester pipes exposed to various acidic and alkaline solutions [16] found that alkaline exposure led to a significant reduction in the tensile properties, particularly with higher alkalinity, while acid treatments improved properties such as the tensile strength and elasticity. The study also observed substantial changes in weight, impact resistance, flexural strength, and hardness in glass–polyester GRP pipes exposed to strong acids, with sulfuric acid causing the most significant loss in strength, as revealed by X-ray diffraction analysis. Similarly, Bi et al. [22] used acoustic emission techniques to investigate aging damage in GFRP pipes subjected to acidic conditions and external mechanical loads, identifying matrix cracking and delamination as primary degradation mechanisms. Zhang and Deng [23] applied molecular dynamics simulations to study GFRP degradation in seawater, showing reductions in the Young’s modulus and interlaminar strength with increasing seawater content and temperature.

The present study presents a significant advancement in understanding the mechanical properties of GFRP pipes by investigating their behavior in the circumferential direction using the split-disk method, particularly under aggressive environmental conditions. Unlike previous studies, this research emphasizes the effects of exposure to saltwater and alkaline solutions at elevated temperatures, which are critical factors in real-world applications of GFRP pipes. By determining the key mechanical properties such as the hoop tensile strength, elastic modulus, and Poisson’s ratio, this study highlights how these properties are affected by environmental degradation. Importantly, the research incorporates rigorous statistical analysis and finite element analysis to evaluate the correlations between the ultimate tensile strength, elastic modulus, and Poisson’s ratio, providing a comprehensive understanding of the mechanical behavior of GFRP pipes under different conditions.

By examining how these environmental conditions degrade critical properties such as the hoop tensile strength, elastic modulus, and Poisson’s ratio, this study provides essential data for designing GFRP systems with extended service life in harsh conditions. This research not only identifies degradation patterns but also supports sustainable infrastructure by informing maintenance strategies and material choices that reduce environmental impact through longer-lasting installations. Therefore, this study contributes to sustainable development by supporting the design and selection of materials that enhance the resilience and longevity of industrial infrastructure.

2. Materials and Methods

2.1. Samples and Immersion Conditions

The tested samples consisted of a Glass Fiber-Reinforced Epoxy (GRE) pipe with an internal diameter of 81 mm and a wall thickness of 4 mm. The specimens were cut to have the geometrical characteristics presented in Figure 1, according to standard [20]. For each experimental condition, three replicate specimens were tested to account for variability and ensure the reproducibility of results.

The immersing solutions were selected to reflect significant differences in the pH as seen in

Table 1. Immersing solutions’ pH.

Immersing Solution	pH
	at 20 °C	at 50 °C
saltwater	7.06	8.12
alkaline solution	13.05	12.38

. Their pH values were measured using the MULTI 9630 pH meter. The alkaline solution was prepared following CSA S806 [24], containing 118.5 g of Ca(OH)₂, 0.9 g of NaOH, and 4.2 g of KOH per liter of water, while the other environment consisted of a 3.5% sodium chloride (NaCl) solution.

In order to analyze the effect of the temperature on the mechanical behavior of GFRP pipes, some specimens were inserted in an oven at a constant temperature of 50 °C, as shown in Figure 2. To ensure accurate and consistent experimental conditions, both the pH and temperature were carefully controlled throughout the study. The pH of the immersion solutions, including the saltwater and alkaline solutions, was monitored and the pH level was kept within a narrow range (±10%) to ensure that chemical reactions affecting the degradation of the GFRP pipes occurred consistently across all specimens. Fluctuations outside this range were minimized by frequent adjustments to the solutions.

2.2. Design of Experiments

The Design of Experiment (DoE) and Analysis of Variance (ANOVA) techniques utilize arrays to systematically organize the factors influencing the behavior of the GFRP material and the corresponding levels at which they should be configured. A full factorial design was implemented, comprising six experimental conditions to examine the effects of the temperature (20 °C and 50 °C) and environment (air, saltwater, and alkaline solution). The testing conditions are detailed in

Table 2. Testing conditions.

Test no.	Temperature, °C	Environment
1	50	saltwater
2	50	alkaline solution
3	20	saltwater
4	20	alkaline solution
5	50	air
6	20	air

.

2.3. Mechanical Testing

The test fixture, illustrated in Figure 3 and produced using a lathe machine, consists of two semicircular metal plates (D-blocks) connected to the upper and lower arms of the fixture with pins. During testing, the bending moment generated at the junction of the split-disk test fixtures primarily affects the apparent tensile strength rather than the true tensile strength of each test [13]. Consequently, the test fixture was specifically designed to minimize the influence of this bending moment.

The experimental tests were carried out using the Walter Bai LF300 Walter+Bai AG, (Löhningen, Switzerland) universal testing machine with the capacity of 300 kN, as shown in Figure 4, at a constant strain rate of 3 mm/min. All tests were performed at room temperature. The data were continuously monitored by the computer and data acquisition system until the specimen attained its ultimate tensile strength (UTS) and subsequently fractured.

The ultimate tensile strength was determined with the formula (1) [20]:

$$\sigma ={\displaystyle \frac{F}{2·A}}={\displaystyle \frac{F}{2·b·t}}$$

(1)

where F—measured force, N; A—section area in the calibrated area, mm²; b—width in the calibrated area, mm; t—pipe wall thickness, mm.

The tensile testing machine measured the force, F, necessary for the calculation of the tensile strength in the circumferential direction.

The procedure followed during the split-disk tests involved several key steps to ensure accurate measurements and results. Firstly, the reduced section dimensions of the specimens were measured using digital calipers. Thickness measurements were taken at four points on each specimen, two of which were located in the gauge sections, while the width of the reduced sections was also recorded. These measurements allowed for the calculation of the reduced section areas, using the minimum thickness and width values.

Next, the specimens were mounted onto the split-disk test fixture, ensuring that the reduced sections were aligned with the split in the fixture. Once mounted, the testing machine was set to a constant loading rate, and the test was initiated. During the test, load and strain data were continuously recorded until the specimen failed. These data were then used to generate stress–strain curves for each specimen. After the tests, the ultimate hoop tensile strength and hoop tensile modulus values were calculated by averaging the measurements from all specimens in each test group. Standard deviations were also computed and reported to reflect the variability within the test groups, providing a comprehensive understanding of the material’s performance under the tested conditions.

To determine the Poisson’s ratio for the investigated samples, strain gauges were attached to the surface of each specimen in two directions to measure both longitudinal (ε_x) and transverse strains (ε_y), as illustrated in Figure 5a. The strain gauges used had a gauge factor of k = 2.19 and a resistance of 350 Ω, with data recorded using the MGCplus acquisition system (Figure 5b).

The uncertainty range for each measured property was calculated using the standard deviation of the measured values. For each experimental condition, three measurements were taken, and the standard deviation was determined to quantify variability. This approach provides a statistical representation of the uncertainty associated with the measurements.

Hooke’s law for an orthotropic material in a system of rectangular Cartesian coordinates with axes perpendicular to the planes of symmetry of elastic characteristics can be conveniently represented in matrix form [25]:

$$\left[\begin{array}{l}〈{\epsilon }_x〉\\ 〈{\epsilon }_y〉\\ 〈{\epsilon }_z〉\\ 〈{\gamma }_{xy}〉\\ 〈{\gamma }_{yz}〉\\ 〈{\gamma }_{zx}〉\end{array}\right]=\left(\begin{array}{cccccc}{a}_{11}& a_{12}& a_{13}& 0& 0& 0\\ a_{21}& a_{22}& a_{23}& 0& 0& 0\\ a_{31}& a_{32}& a_{33}& 0& 0& 0\\ 0& 0& 0& a_{44}& 0& 0\\ 0& 0& 0& 0& a_{55}& 0\\ 0& 0& 0& 0& 0& a_{66}\end{array}\right)\times \left[\begin{array}{l}〈{\sigma }_x〉\\ 〈{\sigma }_y〉\\ 〈{\sigma }_z〉\\ 〈{\tau }_{xy}〉\\ 〈{\tau }_{yz}〉\\ 〈{\tau }_{zx}〉\end{array}\right],$$

(2)

a_ij—components of the flexibility matrix.

The components of the tensor can be expressed in terms of technical constants:

$$a_{11}={\displaystyle \frac{1}{E_x}};$$

(3)

$$a_{22}={\displaystyle \frac{1}{E_y}};$$

(4)

$$a_{33}={\displaystyle \frac{1}{E_z}};$$

(5)

$$a_{12}=-{\displaystyle \frac{{\nu }_{xy}}{E_x}}=-{\displaystyle \frac{{\nu }_{yx}}{E_y}};$$

(6)

$$a_{13}=-{\displaystyle \frac{{\nu }_{xz}}{E_x}}=-{\displaystyle \frac{{\nu }_{zx}}{E_z}};$$

(7)

$$a_{23}=-{\displaystyle \frac{{\nu }_{zy}}{E_z}}=-{\displaystyle \frac{{\nu }_{yz}}{E_y}};$$

(8)

$$a_{44}={\displaystyle \frac{1}{G_{xy}}};$$

(9)

$$a_{55}={\displaystyle \frac{1}{G_{yz}}};$$

(10)

$$a_{66}={\displaystyle \frac{1}{G_{zx}}},$$

(11)

where E_x, E_y, E_z—elastic moduli;

ν_xy, ν_yx, ν_zx, ν_xz, ν_zy, ν_yz—Poisson’s ratios;

G_xy, G_yz, G_zx—shear moduli.

In the performed experiment, there is only circumferential stress (denoted ${\sigma }_x$).

Circumferential strain (denoted ${\epsilon }_x$) and axial strain (denoted ${\epsilon }_y$) were measured.

Then:

$$\begin{array}{l}{\epsilon }_x=a_{11}{\sigma }_x\\ {\epsilon }_y=a_{12}{\sigma }_x=a_{21}{\sigma }_x\end{array}$$

(12)

The ratio of measured strains gives one Poisson’s ratio:

$${\epsilon }_y/{\epsilon }_x=-{\nu }_{xy}$$

(13)

But there is the following relation:

$${\displaystyle \frac{{\nu }_{xy}}{E_x}}={\displaystyle \frac{{\nu }_{yx}}{E_y}}$$

(14)

2.4. Theoretical Analysis of the Stress State in the Zone of Circular Notches

The presence of circular notches in the tested specimens leads to distortion of the homogeneous stress state and the occurrence of stress concentration. This phenomenon is important to consider when establishing the strength limits of composite pipe material. In addition, theoretical analysis allows us to establish a complete picture of stress and strain distribution in the stress concentration zone. Comparison with the results of the experimental measurements of strains in the zone of installation of strain gauges is important to verify the results of the study. When conducting experiments on the strain measurement, strain gauges with dimensions of 8 × 8 mm were used. At significant values of strain gradients, the sensor readings correspond to some average strain values. The theoretical study makes it possible to determine a detailed picture of strain distribution within the area of sensor installation. In addition, theoretical analysis makes it possible to predict the value of stresses at the points of their maximum concentration. Taking into account the stress concentration caused by the presence of semicircular notches is necessary for the reasonable application of the results of the study to analyze the strength of pipes without holes.

For theoretical analysis of the stress concentration zone, the section of the specimen in the vicinity of the hole is considered. Due to the symmetry with respect to the axes oh and ou, it is sufficient to consider the fourth part shown in Figure 6.

When analyzing this problem, the pipe material is considered as a homogeneous orthotropic elastic body. The mathematical formulation of the problem is reduced to the following complete system of equations [26]:

$${\displaystyle \frac{\partial {\sigma }_x}{\partial x}}+{\displaystyle \frac{\partial {\tau }_{xy}}{\partial y}}=0$$

(15)

$${\displaystyle \frac{\partial {\tau }_{yx}}{\partial x}}+{\displaystyle \frac{\partial {\sigma }_y}{\partial y}}=0$$

(16)

$${\epsilon }_x={\displaystyle \frac{\partial u}{\partial x}},$$

(17)

$${\epsilon }_y={\displaystyle \frac{\partial v}{\partial y}},$$

(18)

$${\gamma }_{xy}={\displaystyle \frac{\partial u}{\partial y}}+{\displaystyle \frac{\partial v}{\partial x}},$$

(19)

$${\epsilon }_x={\displaystyle \frac{{\sigma }_x}{E_x}}-{\nu }_{yx}{\displaystyle \frac{{\sigma }_y}{E_y}}$$

(20)

$${\epsilon }_y={\displaystyle \frac{{\sigma }_y}{E_y}}-{\nu }_{xy}{\displaystyle \frac{{\sigma }_x}{E_x}}$$

(21)

$${\gamma }_{xy}={\displaystyle \frac{{\tau }_{xy}}{G_{xy}}}$$

(22)

In physical relations (22): E_x, E_y—modulus of elasticity in axial and circumferential directions, respectively; ν_xy, ν_yx—Poisson’s ratios; G_xy—shear module.

On two lines, x = 0, y = 0, symmetry conditions are set on the boundary y = h and a uniform tensile stress corresponding to the applied load during tests is set. The other boundaries are free from external loads.

The complete system of Equations (15) and (16) can be reduced to the equivalent two solving equations with respect to displacements. The physical relations (20…22) in the inverse form are as follows:

$${\sigma }_x={\displaystyle \frac{E_x}{1-{\nu }_{xy}{\nu }_{yx}}}\left({\epsilon }_x+{\nu }_{xy}{\epsilon }_y\right);\hspace{0.17em}$$

(23)

$$\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}{\sigma }_y={\displaystyle \frac{E_y}{1-{\nu }_{xy}{\nu }_{yx}}}\left({\epsilon }_y+{\nu }_{yx}{\epsilon }_x\right);\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}$$

(24)

$${\tau }_{xy}=G_{xy}{\gamma }_{xy}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}$$

(25)

After replacing deformations in physical relations (23) by displacements using (17) and (18), the equations of equilibrium (15) and (16) are reduced to the following solving equations with respect to displacements:

$${\displaystyle \frac{E_x}{1-{\nu }_{xy}{\nu }_{yx}}}\left({\displaystyle \frac{{\partial }^{2}u}{\partial x^2}}+{\nu }_{xy}{\displaystyle \frac{{\partial }^{2}v}{\partial x\partial y}}\right)+G_{xy}\left({\displaystyle \frac{{\partial }^{2}u}{\partial x\partial y}}+{\displaystyle \frac{{\partial }^{2}v}{\partial x^2}}\right)=0$$

(26)

$${\displaystyle \frac{E_y}{1-{\nu }_{xy}{\nu }_{yx}}}\left({\displaystyle \frac{{\partial }^{2}v}{\partial y^2}}+{\nu }_{yx}{\displaystyle \frac{{\partial }^{2}u}{\partial x\partial y}}\right)+G_{xy}\left({\displaystyle \frac{{\partial }^{2}v}{\partial x\partial y}}+{\displaystyle \frac{{\partial }^{2}u}{\partial y^2}}\right)=0$$

(27)

The solution of boundary value problems for areas of complex shape is possible using approximate methods. In this paper, the finite element method is used, which allows us to find numerical solutions with accuracy sufficient for practical applications. The calculations were conducted in the ANSYS v.2020 R1 program complex in the framework of static analysis. For discretization of the problem, a plane 6-node element PLANE183 with the option “plane stress state” was used. The approximation of displacements u and v within the finite element is given by quadratic functions. The finite element model shown in Figure 7, which utilizes model symmetry to optimize computation, consists of 2717 nodes and 1306 triangular elements. This symmetry allowed for a more efficient setup, minimizing the node and element count while preserving accuracy in stress distribution and deformation predictions.

The elastic material properties of the orthotropic material were specified according to the experimental results obtained for the specimen that was kept in air at 20 °C. The calculations were performed for a nominal tensile stress of 100 MPa on line y = h.

3. Results and Discussion

3.1. Tensile Properties

The final aspects of the specimens tested are presented in Figure 8 and the tensile properties obtained in the experimental analysis are shown in Figure 9. The values represented in Figure 9 represent the mean of the measurements for the three tested specimens.

The unexposed pipe, evaluated in air at room temperature, serves as the baseline and shows the highest strength. The graphs from Figure 9a show that pipes exposed to saltwater experience a noticeable reduction in strength (18%), especially at higher temperatures. While the reduction is more pronounced at 50 °C, the effect is slightly less at room temperature, suggesting that the damaging effects of saltwater increase with heat. Similarly, exposure to an alkaline solution weakens the GFRP pipes, with a more severe drop in strength at elevated temperatures (UTS decreases with 21%). However, under cooler conditions, the alkaline solution has a milder impact. This indicates that chemical degradation is accelerated by heat. Interestingly, the pipes exposed to air at 50 °C retain most of their strength, showing only a moderate reduction. This suggests that while high temperatures alone do cause some weakening, the absence of a corrosive medium such as saltwater or alkaline solution helps maintain a higher level of structural integrity.

The second graph from Figure 9 shows that exposure to different environmental conditions causes a noticeable reduction in stiffness. Samples exposed to saltwater show a reduction in the elastic modulus, particularly at elevated temperatures (E decreases with 14%), suggesting that saltwater accelerates material degradation, which affects the pipe’s ability to withstand deformation. The alkaline solution further reduces the modulus, especially at 50 °C (E decreases with 21%), where the lowest stiffness is observed.

In the case of the Poisson’s ratio (Figure 9c), it can be observed that exposure to saltwater, particularly at 50 °C, lowers the Poisson’s ratio, implying that the material becomes less prone to lateral expansion under stress. The alkaline solution has a similar effect, with the lowest values observed at higher temperatures. However, under 20 °C conditions, the Poisson’s ratio values in the saltwater and alkaline solution are closer to the unexposed specimen, indicating less material degradation at room temperature.

Similarly, in the scientific paper [2], it was found that at higher temperatures (50 °C), the UTS tends to decrease compared to lower temperatures (20 °C). Additionally, the tensile modulus in the axial direction was higher at 20 °C, indicating a stiffer material response under cooler conditions. Also, these findings are consistent with those reported in [27], which show that environmental shifts, such as transitioning from tap water to seawater or alkaline solutions, have a significant impact on the tensile strength. These studies notably highlight that alkaline environments cause the greatest reduction in the tensile strength.

The previous study of the authors [2] highlighted that the temperature played an essential role in affecting the mechanical properties, while the type of solution—especially alkaline—had a significant effect on the flexural and tensile moduli. In saltwater environments, the flexural modulus was higher compared to other conditions, possibly due to the formation of a corrosive precipitate, as confirmed by XRD and FTIR analyses.

Using the Formula (14), the minor Poisson’s ratio can be calculated as follows:

$${\nu }_{xy}={\displaystyle \frac{{\nu }_{yx}{E}_x}{E_y}}$$

(28)

The values of the Young’s modulus (

Table 3. Experimental data regarding Young’s modulus and Poisson’s ratio.

Exp. no.	Immersing Conditions	E Axial (Ex), GPa	E Circ. (Ey), GPa	Major Poisson’s Ratio ${\mathit{\nu }}_{\mathit{y}\mathit{x}}$	Minor Poisson’s Ratio (Formula (28) ${\mathit{\nu }}_{\mathit{x}\mathit{y}}$
1	50°—saltwater	11.17	20.47	0.53	0.29
2	50°—alkaline solution	12.83	18.28	0.47	0.33
3	20°—saltwater	11.64	19.84	0.67	0.39
4	20°—alkaline solution	13.32	21.72	0.61	0.37
5	50°—air	13.38	18.99	0.58	0.41
6	20°—air	13.89	23.5	0.62	0.37

) for the axial direction of the pipe were determined in the previous work of the authors [2].

3.2. Numerical Results

The distribution of the circumferential and axial strains is shown in Figure 10, and the results for the circumferential and axial stresses are presented in Figure 11. Pictures expanded from ¼ part to the whole area are shown for clarity.

Analysis of the numerical results shows the presence of a significant non-uniformity of strain distribution in the vicinity of circular notches. However, the most significant strain gradients appear in the vicinity of the edges of the circular notches. Within the area occupied by strain gauges, the strains vary only slightly. Thus, the value of circumferential deformation in the entire study area varied in the range from −0.00026 to 0.011. Within the sensor installation area, the circumferential deformation varied from 0.0036 to 0.0049.

This allows us to use the average values of strains determined from the changes in the resistance of the sensors to find the elasticity characteristics of the pipe material. The analysis of stress distribution in the cross section with minimum area reveals a significant stress concentration. Thus, the average stress on the line y = 0 amounted to 180 MPa, and the maximum stress on the contour of the hole 306 MPa. This fact should be taken into account when processing the results of fracture experiments on specimens with circular notches.

3.3. Statistical Analysis

In order to highlight the impact of environmental conditions (A—temperature and B—solution type) on the mechanical properties, the Pareto charts’ and main effect plots’ graphical representations are used.

The Pareto charts presented in Figure 12 indicate that the environmental conditions (temperature and solution type) have a marked effect on the mechanical properties of the GFRP pipe, with temperature being the more significant factor in influencing the tensile strength, modulus, and Poisson’s ratio. The solution type also impacts the strength and modulus, but it seems to have little effect on the Poisson’s ratio. The findings from [2,28] also demonstrate that the temperature has a pronounced effect on both the compressive and tensile strength of GFRP material.

The main effects plots from Figure 13a show that there is a clear decrease in the tensile strength as the temperature increases from 20 °C to 50 °C. The reduction is quite significant, indicating that elevated temperatures weaken the material considerably. The tensile strength decreases significantly in alkaline and saltwater environments compared to air, but the reduction is less pronounced than that caused by the temperature. Like the tensile strength, the tensile modulus decreases notably with an increase in the temperature from 20 °C to 50 °C, indicating that the stiffness of the GFRP pipe decreases at higher temperatures. Alkaline solutions also negatively affect the modulus but to a lesser extent. Regarding the Poisson’s ratio, it can be observed from Figure 13c that it also decreases with an increase in the temperature, indicating that the material becomes less ductile as the temperature rises. The Poisson’s ratio is lowest in alkaline solutions, showing a distinct V-shaped trend.

The interaction plots (Figure 14) demonstrate how the temperature and solution type together affect the mechanical properties of GFRP pipes under aggressive environments. For the tensile strength (UTS), at the reference condition (20 °C and air), the material has its highest strength. However, as the temperature rises to 50 °C, the tensile strength decreases across all solution types, with the most severe reduction occurring in saltwater. This suggests that exposure to higher temperatures significantly weakens the material, and the aggressive nature of saltwater exacerbates this effect. While the alkaline solution also reduces the tensile strength, its impact is less severe compared to saltwater at elevated temperatures. A similar pattern is observed for the tensile modulus (E). As the temperature increases to 50 °C, the tensile modulus drops noticeably, especially in the alkaline solution, which has the most detrimental impact. For the Poisson’s ratio, it can also be seen that, as the temperature rises to 50 °C, the Poisson’s ratio decreases in both the air and alkaline solutions, with alkaline conditions showing the most pronounced drop, indicating a loss of ductility.

4. Conclusions

The present study provides important insights into the degradation of GFRP pipes under varying environmental conditions, with a particular focus on the impact of the temperature and corrosive agents. The findings demonstrate that GFRP pipes are highly susceptible to mechanical degradation when exposed to elevated temperatures and aggressive environments like saltwater and alkaline solutions. Exposure to saltwater causes a significant decrease in the ultimate tensile strength (UTS) and elastic modulus as the temperature rises, with a notable reduction at 50 °C. This leads to diminished resistance to deformation, reduced stiffness, and lower ductility, making the material more brittle. Similarly, this study shows that alkaline environments, particularly at high temperatures, result in even more severe degradation of GFRP, significantly reducing both the strength and stiffness.

In contrast, exposure to air at 50 °C results in only moderate reductions in the UTS and elastic modulus, with the Poisson’s ratio remaining relatively stable. This suggests that, in the absence of corrosive agents, GFRP pipes retain much of their stiffness and ductility, even at elevated temperatures. These results highlight the importance of considering environmental factors such as the pH and the composition of the environment when assessing the long-term durability of GFRP pipes.

This study also emphasizes that combined environmental factors, especially the temperature and solution type, have a synergistic effect on the mechanical degradation of GFRP pipes. Exposure to both saltwater and alkaline environments at elevated temperatures proves to be more detrimental than exposure to individual factors alone.

While this study provides valuable insights, there are limitations. The experimental setup focused primarily on saltwater, alkaline solutions, and air, excluding other potentially aggressive environments, such as acidic solutions. Additionally, only short-term exposure was considered, and the effects of prolonged exposure remain unclear. Variations in manufacturing processes and material formulations could further influence GFRP pipe performance, meaning the results may not be universally applicable to all types of GFRP materials.

Future research should expand on these findings by investigating a broader range of environmental conditions, including acidic environments and varying concentrations of saltwater and alkaline solutions. Long-term exposure studies will be essential to understanding the degradation mechanisms in real-world applications. Moreover, incorporating other loadings, such as cyclic loading, could offer a more comprehensive view of GFRP performance under practical conditions. Further exploration of GFRP formulations, resin types, and fiber orientations will deepen the understanding of how material composition influences degradation under environmental and thermal loads.