Comprehensive Experimental Study on the Mechanical Properties of FRP Composite Pipes

Comprehensive Experimental Study on the Mechanical Properties of FRP Composite Pipes

Abstract

Fiber-reinforced polymer (FRP) composite pipes consist of unidirectional continuous glass fibers (hoop glass), a thermosetting polymer vinyl ester resin matrix, chopped glass (discontinuous short fibers), and particulate reinforcing materials impregnated within the resin. These composite pipes are categorized based on their nominal diameter, pressure rating, and stiffness class. However, to date, the mechanical properties of such composites have not been comprehensively investigated. Therefore, this paper presents a systematic approach aimed at thoroughly exploring their mechanical characteristics through axial and hoop tensile strength experiments. The FRP composite pipes used in the current study have glass fibers reinforced at an angle of approximately 89° in the hoop direction. To ensure the accuracy and reliability of the experimental data, we selected three batches associated with each pipe category, with slight variations in their compositions. Three specimens were taken from each batch, and each specimen underwent two types of tests. A total of 18 tests were conducted for each pipe category (2 test types × 3 batches × 3 specimens, i.e., 9 hoop tests and 9 axial tests). Consequently, for 36 pipe categories, a total of 648 tests were performed. Axial tensile tests were conducted using Instron 5569A and Instron 8801 universal testing machines, while hoop tensile tests utilized a split-disk hydraulic testing machine. The average tensile and hoop stresses, along with their associated standard deviations, were calculated based on the population standard deviation (PSD) equation and plotted according to material composition. The results indicate that an increase in the particulate reinforcing material content leads to a decrease in both axial and hoop tensile strengths. However, increasing the proportions of resin, chopped glass, and glass fibers contributes to enhancing both axial and hoop tensile strengths.

Keywords: Mechanical properties, FRP composite pipes, Axial and hoop tensile strength, Comprehensive experimental study

1. Specimen Preparation
   The cross-section of the pipe consists of three layers: the surface layer, the structural reinforcement layer, and the liner layer. The surface and liner layers protect the outer and inner surfaces of the pipe from weather, UV radiation, and corrosive and/or abrasive working fluids, while the reinforcement layer provides mechanical strength.

Figure 1 - Cross-section of a typical particulate FRP composite pipe

The axial and hoop tensile test specimens used in the experimental study were prepared strictly in accordance with the relevant standard guidelines of the International Organization for Standardization (ISO) and the American Society for Testing and Materials (ASTM). These specimens were precisely cut from the test materials, as shown in Figure 2. To avoid any damage to the specimens, a commercially available handheld diamond rotary cutting machine was used.

Figure 2. Extracting specimens from an FRP composite pipe

The specimens for axial (longitudinal) tensile tests were prepared from pipes cut along the longitudinal direction into parallel-sided strips according to ISO 8513 and ASTM 5083 standards. These specimens had a width (b) of 25 mm and a length (l) of 300 mm. The minimum straight length between the supporting fixtures for these axial tensile strips was maintained at 100 mm. The specimen thickness was equal to the pipe wall thickness, which varied among different test materials. The specimens for hoop (circumferential) tensile tests had full diameter and a wall thickness equal to that specified in ISO 8521 and ASTM D2290 standards. These hoop tensile ring specimens had a total width of 25 mm and a minimum width of 15 mm at the reduced notch section.

 Figure 3. Axial tensile strip specimen cut from a particulate FRP composite pipe: (a) 3D CAD model, (b) Schematic diagram, (c) Actual experimental specimen

 Figure 4. Hoop tensile strip specimen cut from a particulate FRP composite pipe: (a) 3D CAD model, (b) Schematic diagram, (c) Actual specimen

2. Mechanical Property Characterization
   2.1 Tensile Tests
   The load-extension results from the axial tensile tests were used to obtain the peak axial tensile load values required for evaluating the axial tensile strength (σ). Figure 5 shows the typical failure modes observed in the axial tensile specimens. Matrix cracking and delamination were significant failure modes, which is logical since the fibers are wound at an 89° angle, and the axial tensile load is perpendicular to the winding (hoop) axis.

Figure 5. Typical failure modes observed in the axial tensile test: (a) Fractured specimen, (b) Delamination, (c) Matrix cracking

Figure 6 shows the typical failure modes observed in the hoop tensile specimens. Fiber fracture became a significant failure characteristic, manifested as complete fracture of the fibers at the reduced cross-section of the specimen. Given the hoop tensile load applied along the circumference and perpendicular to the axis, as well as the radius factor of the hoop-wound tensile specimen, the observed fracture mode appears logical.

Figure 6. Typical failure modes observed in the hoop tensile specimen

2.2 Influence of Composition
As mentioned earlier, we selected three batches associated with each pipe category, with slight variations in their compositions, and three samples were picked from each batch to account for test variability. Given the homogeneity of the constituent materials, the matched winding direction, and the precise control of the CNC machine used in the fiber winding process, analyzing the composition of the constituent materials provides deeper insights into the variations in mechanical strength of particulate FRP composite pipes under different pressure and stiffness ratings.

Figure 7. LOI test sample: (a) Image of the actual test sample and (b) 2D CAD model

A muffle furnace and a high-precision balance were used to perform the loss-on-ignition (LOI) test. The LOI test was conducted to assess the content of the constituent materials. The specimens used for the LOI test were taken from the tested axial tensile strips, away from the fracture area. As shown in Figure 7, these specimens had a width (b) of 25 mm and a length (l) of 25 mm, which helped in understanding the composition of the constituent materials associated with each test category pipe (see Figure 8).

Figure 8. LOI testing equipment and charred sample: (a) Muffle furnace, (b) Electronic balance, (c) Remaining charred residue

Table 1 illustrates the calculation details followed for evaluating the composition (%) of different constituent materials. The specimen was first weighed (Figure 13a), then placed in a crucible (porcelain cup), and finally transferred to the muffle furnace (Figure 13b). The temperature inside the furnace was kept constant to ensure uniform and moderate ignition rates. The ignition process required a significant amount of time to ensure complete combustion of the constituent materials. Depending on the sample thickness, this process took up to 2-5 hours. After burning, the charred residue was allowed to cool, and different components in the residue were separated using tweezers and weighed on a high-precision balance.

Table 1. Typical calculation details for the LOI test

The average tensile stress and hoop stress for each batch were calculated using Eq. (1), and their associated standard deviations were calculated based on the population standard deviation (PSD) equation for the two pipe categories.

Where σm,at and σm,ht represent the average strengths corresponding to the axial and hoop strengths obtained from the tensile tests of the specimens, respectively. σn,at and σn,ht represent the axial and hoop strengths corresponding to the specimen numbers, respectively. PSD and PSDHT represent the standard deviations of the number of specimens subjected to axial and hoop tensile tests, respectively. n represents the number of samples per batch, which is set to 3 (n=3) in the current study.

The average axial and average hoop tensile strengths corresponding to different pipe categories, along with their associated standard deviations, are shown in Figures 9, 10, 11, and 12. Table 2 provides the average composition of the constituent materials and the average axial and average hoop tensile strengths corresponding to each pipe category.

[Image: Figure 9. Average axial and average hoop tensile strengths corresponding to pipe category DN350 and their associated standard deviations: (a) PN3, (b) PN6, and (c) PN10.]

[Image: Figure 10. Average axial and average hoop tensile strengths corresponding to pipe category DN500 and their associated standard deviations: (a) PN3, (b) PN6, and (c) PN10.]

[Image: Figure 11. Average axial and average hoop tensile strengths corresponding to pipe category DN700 and their associated standard deviations: (a) PN3, (b) PN6, and (c) PN10.]

[Image: Figure 12. Average axial and average hoop tensile strengths corresponding to pipe category DN800 and their associated standard deviations: (a) PN3, (b) PN6, and (c) PN10.]

Table 2. Average axial and average hoop tensile strengths

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From Figures 9, 10, 11, 12, and Table 2, it can be observed that for a given pressure rating, the axial tensile strength decreases when moving to a higher stiffness rating. For a given stiffness rating, the axial tensile strength increases when moving to a higher pressure rating, which confirms the enhanced strength requirements needed to withstand higher pressures. Notably, the increase in strength concerns at higher stiffness ratings is negligible; in fact, for pressure ratings PN6 and PN10000 corresponding to stiffness rating SN3, the strength concerns are zero. This is mainly due to the minimum axial tensile strength requirements of stiffness rating SN10000 exceeding those of pressure ratings PN3 and PN6.

Similarly, through the visual analysis of Figures 9, 10, 11, 12, and Table 2, for a given pressure rating, the hoop tensile strength generally decreases when moving to a higher stiffness rating, except for pressure rating PN3, where the effect is minimal or non-existent. Therefore, higher stiffness ratings have lower hoop tensile strengths; the exception for PN3 highlights the minimum hoop tensile strength required by the relevant pressure rating, which exceeds the strength requirements of stiffness ratings SN2500, SN5000, and SN10000. For a given stiffness rating, similar to axial tensile strength, the hoop tensile strength also increases when moving to a higher pressure rating, further confirming the enhanced strength requirements needed to withstand higher pressures. Notably, the increase in hoop tensile strength concerns at higher stiffness ratings is negligible; in fact, for pressure ratings PN6 and PN10000 corresponding to stiffness rating SN3, the strength concerns are zero. This is mainly due to the minimum hoop tensile strength requirements of stiffness rating SN10000 exceeding those of pressure ratings PN3 and PN6.

Figures 13 and 14 show the variation in the average axial and average hoop tensile strengths of particulate FRP composite pipes with the variation in the average composition of particulate reinforcing materials (sand), respectively. Despite a certain amount of scatter in the data, a first-order polynomial curve fit indicates that an increase in the average composition of particulate reinforcing materials leads to a decrease in both the average axial and average hoop tensile strengths. Although a higher proportion of particulate reinforcement enhances the stiffness of particulate FRP composite pipes and reduces costs, as demonstrated by the experimental results, caution should be exercised in determining how much particulate reinforcement to use.

[Image: Figure 13. Variation in the average axial tensile strength corresponding to the average composition of particulate reinforcement]

[Image: Figure 14. Variation in the average hoop tensile strength corresponding to the average composition of particulate reinforcement]

Figures 15 and 16 show the variation in the average axial and average hoop tensile strengths of particulate FRP composite pipes with the variation in the average composition of the resin, respectively. Compared to particulate reinforcement, the average variation in resin composition is much smaller, ranging from 25% to 32%. It can be demonstrated from the overall data that when the resin composition is increased, both the axial and hoop tensile strengths increase. In addition to improving the overall strength, the resin helps keep the fibers in place and oriented and protects the composite structure from environmental damage.

[Image: Figure 15. Variation in the average axial tensile strength corresponding to the average resin composition]

[Image: Figure 16. Variation in the average hoop tensile strength corresponding to the average resin composition]

Figures 17 and 18 show the variation in the average axial and average hoop tensile strengths of particulate FRP composite pipes with the variation in the average composition of chopped glass, respectively. Compared to the scattered datasets corresponding to particulate reinforcement and resin, the linear relationship between the average chopped glass composition and the average axial tensile strength is very evident. This is basically logical since, among all the constituent materials, chopped glass is the material that significantly contributes to the axial tensile strength value (winding angle of 89°) of the hoop-wound particulate FRP composite pipe. The variation in the average chopped glass composition is similar to that of the resin, ranging from 8% to 15%. Although there is a certain amount of scatter in the data related to the average hoop tensile strength, it still increases when the chopped glass composition is enhanced according to a first-order polynomial curve fit. Chopped glass helps coordinate the overall strength of the composite by reinforcing its weaker aspects. Therefore, more chopped glass may lead to the reinforcement of weak/weakened areas in particulate FRP composite pipes.

[Image: Figure 17. Variation in the average axial tensile strength corresponding to the average composition of chopped glass]

[Image: Figure 18. Variation in the average hoop tensile strength corresponding to the average composition of chopped glass]

Figures 19 and 20 show the variation in the average axial and average hoop tensile strengths of particulate FRP composite pipes with the variation in the average composition of glass fibers (hoop glass), respectively. The variation in the glass fiber composition is slightly wider than that of chopped glass, ranging from 9% to 25%. It can be seen from the overall data that when the average glass fiber composition is increased, the average axial tensile strength increases. The linear relationship between the average glass fiber composition and the average hoop tensile strength is very sharp. This is again logical since, among all the constituent materials, glass fibers are the only material that directly affects the hoop tensile strength (winding angle of 89°) of the hoop-wound particulate FRP composite pipe.

[Image: Figure 19. Variation in the average axial tensile strength corresponding to the average composition of hoop glass]

[Image: Figure 20. Variation in the average hoop tensile strength corresponding to the average composition of hoop glass]

3. Conclusion
   This study conducted a comprehensive experimental investigation on the axial and hoop tensile strengths of particulate-reinforced fiber composite (FRP) pipes, delving into the influence of constituent materials on these mechanical properties. Through a series of carefully designed experiments, we analyzed the specific effects of different types of fibers, particulates, and their ratios on the performance of composite pipes. The research results reveal the relationship between material composition and the axial and hoop tensile strengths of composite pipes, providing important design guidance and reference for engineers and manufacturing professionals. These findings not only contribute to optimizing the performance of composite materials but also have significant implications for improving the reliability and extending the service life of related products.

1. Matrix cracking and delamination are the primary failure modes in particulate FRP composite pipes under axial tensile load, while fiber fracture is the typical failure mode under hoop tensile load.

2. The design of pipes with excellent axial tensile strength should incorporate a higher proportion of chopped glass. Conversely, the design of pipes with outstanding hoop tensile strength should include a higher proportion of hoop glass.

3. Increasing the proportion of particulate reinforcement will enhance pipe stiffness and reduce costs. However, it has been observed that a higher proportion of particulate reinforcement leads to a decrease in both axial and hoop strengths.

4. Considering the hoop (circumferential) winding configuration, the main failure modes observed under axial and hoop tensile loads are logical, as the axial load always acts perpendicular to the winding axis, while the hoop load acts along the circumference.

5. The proportion of constituent materials significantly affects pipe performance. Therefore, optimizing performance for specific design requirements is feasible and is only limited by the scope of the existing experimental database.

References:

Farrukh Saghir, Scott Gohery, F. Mozafari, N. Moslemi, Colin Burvill, Alan Smith, Stuart Lucas, Mechanical characterization of particulated FRP composite pipes: A comprehensive experimental study, Polymer Testing, Volume 93, 2021, 107001, ISSN 0142-9418, https://doi.org/10.1016/j.polymertesting.2020.107001.