Impact toughness of the nanofiber embedded glass fiber-reinforced composites under water absorption condition

Composite manufacturing

This study utilized three different materials: glass fiber-reinforced polymers (GFRP), as a control sample, ABS (acrylonitrile butadiene styrene), and PC (polycarbonate). The primary material being tested was GFRP, while the other materials were employed to improve its properties through the application of an electrospinning process. Apart from the control sample, there were two additional variations of each specimen, distinguished by their electrospinning process durations of 10 min and 30 min, respectively (as shown in Table 1). Polycarbonate (PC) and acrylonitrile–butadiene–Styrene (ABS) pellets were provided by Tisan Ltd, Türkiye. The solvents tetrahydrofuran (THF) and N, N-dimethylformamide (DMF) were provided by Inovenso Ltd, Türkiye. X300 Biaxial Glass Fabrics (300 g m⁻²) from Metyx Ltd, Türkiye, and an ERA 4000 epoxy system, comprising resin (A) and hardener (B) in a 2:1 weight ratio, provided by Tekno Marin Ltd, Türkiye, were utilized for composite fabrication. The transparent ERA 4000 epoxy possesses a density of 1.1 g cm⁻³ and contains no solvents. The first curing duration is 1 h, while the complete curing period for the epoxy is 6 h. The glass fiber features a 0°/90° woven angle, both with and without chopped strand mats. The tensile strength ranges from 1200 to 1500 MPa, whereas the elastic modulus varies between 70 and 75 GPa.

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Table 1.

Compositions of each material.

Designation	Composition
GFRP	7-layer Glass fabric + Resin
P10	7-layer glass fabrics with Polycarbonate nanofiber electrospun for 10 min. + Epoxy Resin
P30	7-layer glass fabrics with Polycarbonate nanofiber electrospun for 30 min. + Epoxy Resin
A10	7-layer glass fabrics with ABS nanofiber electrospun for 10 min. + Epoxy Resin
A30	7-layer glass fabrics with ABS nanofiber electrospun for 30 min. + Epoxy Resin

Electrospinning combines features from both electro-spraying and traditional dry-spinning methods used for fiber production. ABS and PC were separately dissolved in various solvents with varying concentrations and then combined at room temperature. Subsequently, the prepared solution was loaded into a syringe and sprayed onto the glass fabric surface. ⁷ Figure 1 provides a schematic representation of the electrospinning and vacuum bagging process concisely. Tetrahydrofuran (THF) and N-dimethylformamide (DMF) were utilized as solvents with ABS and PC to fabricate electrospun nanofibers. ABS was dissolved in DMF at varying concentrations of 22, 30, and 37 wt%. Whereas PC was dissolved in a THF: DMF mixture of (60:40) with concentrations of 15, 20, and 25 wt%. The two solutions were combined at ambient temperature for approximately 5–6 h to achieve uniformity. The NE300 Multi Nozzle Electrospinning Machine from Inovenso Ltd was employed for the electrospinning procedure. The solution with differing concentrations was introduced into a syringe and connected to a syringe pump. Using high electrical energy, both ABS and PC polymeric solutions were transformed into nanofibers through syringe pumping and sprayed onto glass fabric. Subsequently, nanofiber-embedded glass textiles were utilized in the fabrication of composite plates.

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Figure 1.

Schematic illustration of preparing steps of the samples.

The process of producing GFRP-embedded nanofibers involves the use of a vacuum infusion. Prior to the experiment, the area within the vacuum chamber was thoroughly cleaned, and polyvinyl alcohol (PVA) Liquid Mold Release was poured and solidified. The sample was then cut to fit inside the chamber, along with a flux net that aids in the even distribution of epoxy resin, and a separator. Next, the specimen was placed within the vacuum chamber, and the epoxy resin was applied on top, ensuring it was spread as evenly as possible. This process continued until the epoxy resin was fully utilized, resulting in a material consisting of seven layers. The vacuum bag was carefully positioned over everything to ensure there are no holes. Once the vacuum was activated, the pressure was monitored to detect any issues, such as unintended holes. The production of GFRPs continued for a duration of approximately 1 to 1.5 h. At the end of this timeframe, the system was inspected, and any excess resin that had been vacuumed could be observed. Specimens were cut from each material for the water physical and mechanical tests, in accordance with the established standards using a water jet device.

Microstructural observation

Microscopic investigations were performed to examine the internal structures and interfacial connections of the composite fracture surface. Electron microscopy was utilised to investigate nanoscale ABS and PC solutions within the glass fibers. The electrospun mats were subjected to vacuum drying at 60 °C for 2 h prior to scanning electron microscopy (SEM) analysis. The entire electrospun fabrics were gold-coated using a sputtering apparatus (Q 150 Netherlands) to prevent electrostatic charging and to achieve crisp pictures. Images from the scanning electron microscope were acquired using a Phenom-World XL instrument at an energy of 15 kV. Before and after the water absorption test, in order to determine microstructural changes in composite structure light microscopy was used as well.

Differential scanning calorimetry

Differential scanning calorimetry (DSC) analysis was conducted on composites to ascertain critical transition temperatures, including melting (Tm), glass transition (Tg), and cold crystallization temperatures, utilizing a heating profile ranging from 25 to 225 °C at a rate of 10 min⁻¹, alongside heating and cooling profiles. Sample weights ranged from 10 to 15 mg, and the equipment operated for approximately 50 min for each assay.

Contact angle test

The experiment took place according to the ASTM 5725-99 standard. The objective was to assess the water immersion characteristics of our materials, determining their hydrophobic or hydrophilic tendencies. The contact angle was measured, serving as an indicator of their interaction, in order to gauge the wettability of the solid materials by a liquid. The contact angle test operates through a straightforward mechanism. The test device, depicted in Figure 2 consists of a video camera, a syringe, and a light source. To conduct the test, the sample is placed in a designated area, and water from the syringe is gradually dispensed until it makes contact with the sample. At the moment the water droplet touches the sample, the device's software promptly captures an image of the sample and records the corresponding data. This process is repeated a minimum of three times, capturing images of three distinct locations on the sample, as illustrated in Figure 2(a)-(b). The contact angle (θ) is a numerical measurement used to quantify the interaction between a liquid and a solid surface. It represents the angle formed between the normal line drawn on the solid surface and the tangent line of the liquid surface at the specific point of interest along the interface where the three phases (solid, liquid, and gas) meet.

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Figure 2.

(a) Polycarbonate sample material with the droplets of water after testing, (b) contact angle measurement test device.

Ozone chamber test method

The ozone impact is the most prominent factor, particularly in high-voltage electrical systems and the aviation sector. Ozone can degrade polymer matrices, resulting in surface fissures on glass fiber composites. Ozone-resistant polymers or protective coatings are employed for this reason. The application areas of glass fiber reinforced epoxy composites include aircraft external surface panels, radar domes, electric vehicle body panels, high voltage electrical insulators, boat hulls and superstructures, and ventilation and chimney systems. This test enables the evaluation of the material's ability to withstand weather conditions in an ozone environment. The ozone chamber is designed to simulate ozone-rich environments. The chamber is equipped with an ozone generator, which produces controlled levels of ozone gas. The equipment consists of a tube that flows oxygen, where a constant electrical discharge (corona effect), is generated through a transformer, which transforms oxygen molecules ( O 2 ) adding an oxygen atom. Then the ozone gas is sucked by a Venturi system and mixed with water to be treated in a mixer that feeds the ozone chamber. ²² The test was conducted by the esteemed Dorukmak Limited Company and was based on the ASTM D1149-2018 standard. From each material, three specimens were exposed to ozone ( O 3 ) at 50 ppm for 72 h, and 75 ppm for 72 h to 144 h.

Water immersion test method

The water immersion test follows the guidelines set by the ASTM D570 standard. Prior to the test, the materials were dried in an oven at 50°C for 24 h to remove any excess moisture. Once the drying process was completed, the specimens’ initial weights were measured using a precision balance with a precision of 1/10,000 at room temperature and recorded for future comparison. The specimens were immersed in distilled water at temperatures of 25°C and 65°C for 1000 h. 25°C was picked as it signifies room temperature and is one of the most commonly utilized reference values in the literature. Figure 3 shows the water tanks where the samples were kept at room temperature and 65 °C, and the high precision balance next to them. Thermal aging samples were kept in a water tank insulated with styrofoam and containing a controllable heater. Thermocouple and control were used to keep the water temperature between 65–68 C. The samples were then placed individually in a container filled with a distilled water tank, and their weights were periodically checked at specific intervals ranging from 24, 48, 72, 120, 240, 500, and 1000 h. After each soaking interval, the sample surfaces were cleaned with cloth and promptly reweighed. After weighing, the samples were promptly returned to the tank. The water absorption was calculated using the formula1:

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Figure 3.

Water absorption test apparatus.

Izod-impact test method

The method employed in this process was derived from the ASTM D256 standard. Izod impact test was applied to each group to determine impact resistance before and after the water retention test and Ozone test. In the case of water immersion, the impact test was carried out on specimens that had been immersed for 240 h and 500 h, as these intervals allow for regular observations of any visible differences in the material's behavior. The impact strength (IS) of specimens was calculated using the following formula 2;

Where, Ec (joules)is the absorbed energy by breaking, b(mm) is the width, and h(mm) is the thickness of the sample.

Performance analyses of the composites

Figure 4 presents SEM pictures of the cracked surface, illustrating the microstructures of the composite materials prior to the water absorption test. SEM images were acquired from the fractured surface to analyse the fiber-matrix interfacial adhesion of the composites and the influence of nanofibers. The micrograph of the GFRP composite in Figure 4(a) demonstrates a strong connection between the fiber and matrix contact. Impact testing revealed that the fibers were detached from the matrix due to either fracture or pull-out failure. Figure 4(b) illustrates the micro-image of the A10 composite sample. It was seen that following the fracture, the fibers displaced and gaps emerged. Upon detailed observation of the damaged regions, it is evident that while the matrix has effectively wrapped up the fibers, fractures develop at the fiber peripheries post-fracture. This indicates that the manufacture was successful and an effective matrix-fiber interaction was established. The fiber diameter was measured to be roughly 14 µm. Figure 4(c) displays the microscopic image of the P10 composite sample. The micro image revealed that the matrix closely wrapped the fiber. Furthermore, it was noted that the fibers fractured at an angle, indicating ductility post-fracture. Figure 4(d) presents the microscopic image of the A30 composite material. The observation indicated that the matrix fiber interface exhibited strong characteristics, with the matrix closely enveloping the fiber. The observation indicated that the fibers exhibited breakage at an angle akin to that shown in Figure 4(c). The micrograph of the P30 composite sample is presented in Figure 4(e). The micro-image revealed a homogeneous distribution of fibers and matrix. The detailed observation revealed a strong fiber-matrix interface, with the matrix effectively enveloping the fibers. A strong fiber-matrix interface is essential for effectively transferring the load from the matrix to the fiber, thereby enhancing overall strength. Furthermore, effective interface bonding serves as evidence of successful production.

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Figure 4.

SEM images of un-condition composite samples; (a) GFRP, (b) A10, (c) P10, (d) A30, (e) P30.

Figure 5 shows the DSC results of the produced composites. The glass transition temperatures (Tg) results of the composites vary between 69.9 °C and 72.2 °C. The incorporation of additional materials into the manufactured composites results in alterations to the Tg temperature of the composite by ±1–2 degrees. The cold crystallization temperature of GFRP was approximately 118 °C, while the incorporation of nanofibers marginally delayed this temperature. As the doping duration extended from 10 min to 30 min, the Tcc temperature surpassed 119.3 °C. ABS and PC have nothing that appreciably impacts the molecular structure of epoxy. The quantity of ABS/PC may be insufficient to affect the thermal properties. No substantial interaction (chemical or physical) takes place between ABS/PC and the epoxy-GFRP matrix.

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Figure 5.

DSC results of composites.

Generally, the use of additional components typically elevates the Tcc temperature of the composite, facilitating crystallization at reduced temperatures. ² Nonetheless, it is expected that the complete attachment of the nanofiber additive to the fiber, along with its diminutive size, results in an adverse consequence.

The effect of the addition of nanofiber treatment on the GFRP surfaces was observed using water contact angle measurements. PC and ABS nanofibers showed a relatively hydrophobic behavior, different from glass fibers, as expected from the literature^29,30 according to the contact angle test results (as shown in Figure 6). PC was measured with approximately 80 ° of contact angle, similar to ABS, which had approximately 78 °. The introduction of nanofibers between the glass fibers in the composite led to a decrease in water uptake in the glass fiber-reinforced nanofiber-embedded polymer composites. This is because the nanofibers acted as barriers and hindered the penetration of water into the composite structure. Furthermore, since the nanofibers were hydrophobic, they repelled water and prevented further immersion, thereby enhancing the material's resistance to water uptake. ¹³ Consequently, the presence of nanofibers not only reduced water absorption but also improved the overall properties of the composite material in terms of water resistance. Consequently, the hydrophilic nature of glass fiber was enhanced with the hydrophobic nature of PC, and ABS nanofibers for humid environments.

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Figure 6.

Contact angles of PC material were taken from three different points on the sample.

Figure 7(a) shows the results of the nanofiber-reinforced composites after water absorption at various soaking times. The results indicate that all samples rapidly absorbed water within the initial about 120 h. After the initial 120 h, the samples retained water at rates ranging from 3% to 10%. During the initial 120 h, GFRP and P10 samples retained around 9.6% and 8% water, respectively, but A10, A30, and P30 samples retained 3%, 6.6%, and 6.5% water, respectively. The water retention rate of GFRP composite is seen to be at least 20% more than that of composites using nanofiber over the initial 120 h. Following 240 h of water retention, an increase in the water retention rate was noted in samples GFRP and A10, but a decline was detected in the remaining samples. The samples typically attained their peak water retention capacity between 240 and 500 h of water retention. This phenomenon can be attributed to water gradually occupying all spaces inside the composite and permeating its internal structure, compromising the contact. In a similar study, Oguz et al. ³¹ elucidated that the primary cause of water retention in GFRP composites is the rapid formation of hydrogen bonds between the hydroxyl group of the epoxy resin and water molecules. After 480 h, the water retention rates of GFRP and A10 samples rose, whereas the water retention rates of A30 and P30 samples began to decline. Following 500 h of water retention testing, the water retention rates of nearly all samples began to decline. Following 500 h of water retention, the water retention rates of all samples, except the GFRP composite, began to decline. Following 1000 h of water retention, the retention rates of all samples diminished markedly. The control sample displayed the highest water absorption rate, with an expected uptake of approximately 11 percent. However, by incorporating nanofibers into the glass fiber, the material's susceptibility to water immersion can be significantly reduced. When comparing the effects of ABS and PC materials on reducing water absorption, ABS proved to be more effective. Among the samples, A10 exhibited the lowest water uptake percentage, reaching only 4% after 500 h. Furthermore, the remaining samples demonstrated their highest water absorption percentages at the 120-h and 240-h marks. P10 experienced an 8% water uptake after 120 h of immersion, and this value remained consistent at the 240-h mark. P30 and A30 exhibited a similar water uptake of approximately 7% after 120 h. The glass fiber-reinforced polymer composite with 10-min electrospun ABS nanofiber embedding proved to be the most suitable material for humid environments at the conclusion of the experiment.

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Figure 7.

Water gain ratios of the composites (a) At room temperature, (b) At 65°C condition.

Figure 7(b) shows the water gain ratios of the samples as a function of the time. 65 °C was chosen as temperature in order to enhance the water absorption process known as “accelerated aging” and to increase water absorption. Also to examine the impact of hydrothermal aging solely on fabrics without reaching the glass transition temperature (Tg) of the resin. ³¹ In the water retention test conducted at 65°C, the GPRP, A10, and P10 samples achieved the maximum water retention rate after 500 h. Following 500 h of water retention, the water retention rates of GFRP and P10 samples significantly diminished. This condition demonstrates that the fiber-matrix interface is compromised and the composite structure is impaired. The A30 and P30 composites achieved the maximum water retention rate in roughly 360 h. GFRP was identified as the composite that holds the most amount of water at both 65 °C and room temperature. All composites were found to retain more water at 65°C than at room temperature.

Water absorption composites by many mechanisms, including the free volume within the polymer and capillary wicking along the fibers. The capillary effect enables uncoated GFRP fiber to absorb water molecules into the composite structure upon exposure to water. ³² The SEM image illustrates that this event caused surface fiber separation, fracture cavities, and uncoated glass fiber pull-out from the matrix surface. However, it was much less diffused in nanofiber-reinforced composites. Water molecules initially induce the swelling of glass fibers, resulting in the formation of microcracks in the matrix around the enlarged fibers. The injected water molecules impair the interfacial adhesion between the fiber and matrix. Ultimately, water-deformed fiber and matrix constituents commence detachment from the fiber, diminishing fiber-matrix interface adhesion and influencing the mechanical and physical characteristics of the composites. ³³

Impact test results after water absorption

Figure 8(a) shows the impact strength results of the composites as a function of absorption time. In contrast to the control sample, a noticeable improvement in the impact outcomes when nanofibers were incorporated was observed. This improvement was particularly evident after the samples were submerged in water. The absorption of water resulted in the degradation of the interface between the fiber and matrix, leading to the exposure of the fibers. Following 240 h of water retention testing, the impact strength of the GFRP composite diminished from 48.95 ± 4.2 kJ/m² to 36.2 ± 1.4 kJ/m². Following 500 h of water retention testing, an anticipated drop in the impact strength of the GFRP composite was noted; nonetheless, an increase of approximately 10% was recorded. Xian et al. ³⁴ explained this decrease after the water retention test as follows: cracks and stress concentrations readily form under complicated loads, resulting in the premature breakdown of composites. Moreover, the molecular architecture of epoxy resin comprises a significant quantity of hydroxyl groups, which are susceptible to moisture absorption, resulting in resin hydrolysis, plasticization, and fiber/resin debonding, hence expediting composite failure in the operational environment.^35,36

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Figure 8.

Composites’ impact test results based on water soaking time; (a) Impact strength depending on the time, (b) Water soaking time range.

A comparable scenario was noted in samples P10 and P30 following 240 water retention assessments. Karmarker et al. ³⁷ observed that the filling of gaps in the structure, following short-term water absorption, enhanced the shear strength of jute fiber-reinforced composites. Dhakal et al. ³⁸ discovered that a short-time water absorption test enhanced the mechanical characteristics of a hemp-reinforced polyester matrix composite. Nonetheless, prolonged water absorption resulted in the development of microcracks surrounding the fiber, and the permeation of water via these fissures compromised the fiber-matrix interface and diminished the mechanical characteristics. Following 500 h of water retention, the A10 sample exhibited the maximum impact strength at 58 ± 11.2 kJ/m². These are increasing, as explained by Liao et al. ³⁹ as the change in characteristics during thermal ageing results from the conflicting influences of post-curing induced thermo-oxidative degradation and matrix hardening.

Table 2 shows impact strength with the standard deviation of the samples under different conditions. A10 exhibited the lowest water absorption level at 500 h. Following 500 h of water retention, the maximum impact strength was recorded in the A10, P30, P10, A30, and GFRP samples, respectively. Following 1000 h of testing, the maximum impact resistance recorded was 51.3 ± 9.9 kJ/m² for the P30 samples, 45.8 ± 10.3 kJ/m² for the A30 samples, 44.5 ± 5.6 kJ/m² for the A10 samples, and 43.4 ± 5.1 kJ/m² for the P10 samples. Upon comparing the impact strength of the composite reinforced with PC nanofiber for 30 min to that reinforced for 10 min after 1000 h of water retention, an enhancement of approximately 18% was noted (as seen in Figure 8(b)). Similarly, a comparison of the impact strength of the composite reinforced with ABS nanofiber for 30 min and that reinforced for 10 min after 1000 h revealed an increase of roughly 2%. Overall, during 1000 h of water retention, nanofiber-reinforced composites’ impact resistance markedly improved compared to GFRP. Before water retention, the impact strength of the GFRP composite was measured at 48.95 ± 4.2 kJ/m², whereas the impact strengths of the P10, P30, A30, and A10 samples were recorded as 58.3 ± 7.1, 65.3 ± 10.6, 76.9 ± 16, and 78.7 ± 11.3 kJ/m², respectively. When compared to nano ABS reinforced composite, GFRP increased by around 57% in the dry condition after 30 min. As a result of water absorption after 1000 h, a decrease of approximately 43% in impact strength is observed in glass fiber-reinforced composites without nanofibers, while a decrease of approximately 4% is observed in P30 samples, which is seen as the lowest decrease amount among all samples. When we examine the change in impact strength, we can list the samples that exhibit the best performance as P30, P10, A30, A10, and GFRP. In addition to all these points, it is understood that the use of high amounts in both nanofiber types shows the best properties after 1000 h of water absorption. A similar study Xian et al. ¹¹ reported that the surface cracks of tensile and bending of PA6CFEP before and after 120 days of hygrothermal aging were further analyzed to elucidate the deterioration process during service.

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Table 2.

Impact strength results of samples under different conditions.

	Uncond.	Underwater at Room Temperature			Thermal Aging	Ozane Condition
	0 h	240 h	500 h	1000 h	1000 h	a	b	c
GFRP	48.9 ± 4.2	36.2 ± 1.4	38.6 ± 6.3	27.9 ± 2.4	26.1 ± 8	44 ± 6.1	43 ± 4	42 ± 7
A10	78.8 ± 11.3	58.0 ± 11.2	54 ± 13	44.5 ± 5.6	42.5 ± 2	76 ± 5.1	77 ± 2	71 ± 11
A30	76.9 ± 16	61.9 ± 16.3	48.4 ± 6.6	45.8 ± 10	36.4 ± 11	58 ± 10	60 ± 12	49 ± 6
P10	58.3 ± 7.1	63.3 ± 17	52.6 ± 11.5	43.4 ± 5.1	34.2 ± 12	49 ± 1	43 ± 4	46 ± 7
P30	65.3 ± 10.6	74.8 ± 14	56 ± 9	51.3 ± 9.9	38.04 ± 10	70 ± 10	68 ± 3	63 ± 7

All results in the table are expressed in megapascals (MPa). a:50ppm_72 h, b: 75ppm_72 h, c:75ppm_144 h.

The composites’ light microscope images following water retention are displayed in Figure 9. Delamination and pull-out degradations were noted in the microstructure image of the A10 sample during a 1000-h water absorption test (Figure 9(a)). Gaps between the fiber and the matrix are also observed to occur during this period. Fiber breakdown was seen in Figure 9(b) following the breakage in the A30 sample's microscope picture following the 1000-h water retention test. Fiber fractures and deformations brought on by water and gaps between the plates are seen in the fracture images of the P10 sample in Figure 9(c) following a 1000-h water retention test. Following a 1000-h water retention test, the P30 sample's fracture picture showed indications of toughness fracture and microfibril degradation (Figure 9(d)). Toughness breakage is supported by the outcomes thus far. Figure 9(e) distinctly illustrates that the 1000-h water retention fracture image of the GFRP sample exhibited a greater creation of void structures following water deformation compared to the other images. This circumstance demonstrates the significant reduction in impact resistance. In a comparable investigation, Oguz et al. ³¹ observed that, following impact resistance testing, the control samples exhibited no complete breakage; however, delamination occurred on the tension side of the glass/epoxy group samples.

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Figure 9.

Microscopic images of the composites; (a) A10, (b) A30, (c) P10, (d) P30, (e) GFRP.

Figure 10 shows SEM images taken from fractured surfaces after the 1000 h water retention test. Figures 10 a-e present SEM images of the water sorption composite, highlighting degradation, pull-out, and gaps.The matrix was fragmented into microscopic parts. Figure 10(b) illustrates that the fibers of the A10 sample scattered and detached from the matrix after 1000 h of water retention. The matrix was seen to break down, resulting in a diminished connection with the fiber. Upon examination of the microstructure of A10 and A30 composite materials post-impact testing, following 1000 h of water immersion, Figures 10(b) and 10(c) revealed the existence of pull-outs and delaminations. Simultaneously, the SEM images in Figures 10(a) and (b) indicated that it was detached from the fiber matrix during water retention. Both microscopy analyses resulted in a reduction of the impact strength of the A10 sample. Bridge building was noted during fracture as a result of increased nano reinforcement in the A30 sample (Figure 10(b)). The introduction of water resulted in the swelling of the glass fiber within the composite structure, allowing it to effectively fill the gap between the fiber and the matrix. Figure 10(a) illustrates the separation of the fiber and matrix following water retention. Matrix deterioration markedly diminished the impact strength. ³⁹ Figure 10(c) illustrates the decomposition of the nanofibers, which detached from the atrium during water retention. The breaking down of the fiber matrix, as shown in the SEM pictures, led to the degradation of the fiber-matrix interface. The impact strength diminished due to the matrix's ability to transfer the stress to the fiber. Furthermore, the detailed image revealed that the fibers were detached from the matrix. Upon examination of the micrograph of the A30 sample in Figure 10(d), it was seen that the matrix exhibited a spongy texture. The researchers similarly discovered that the fiber matrix contact was compromised due to the rounded structure, resulting in fiber degradation. The breakdown of the fiber and the spongy contraction of the matrix compromised the composite structure, resulting in a reduction of its strength. The detailed image in Figure 10(d) illustrates that the nanofibers were detached from the matrix following water retention. Upon examining the microstructure of the GFRP composite material in Figure 10(e), it was observed that the fibers had detached from the matrix following water retention. A failure to effectively transfer load from the matrix to the fiber following the degradation of the fiber-matrix contact resulted in a reduction of strength during water absorption.

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Figure 10.

SEM images of composite samples after 1000 h water absorption test; (a) GFRP, (b) A10, (c) P10, (d) A30, (e) P30.

Impact test results under 65°C water absorption

The impact strength results obtained after the long-term water retention test at room temperature and 65 °C are shown in Figure 11. Looking at the results in general, the impact strength of the composites decreased after 1000 h both at room temperature and at 65 °C. This situation can be explained as follows; this phenomenon is attributed to hydrothermal aging, which leads to the physical dissolution of the resin on the GFRP surface, resulting in corrosion pits that expose the fibers to hydrolysis and create substantial flaws. ³⁹ Furthermore, water molecules may induce matrix expansion, leading to the development of cavities and fissures within the composite structure. ⁴⁰ After thermal aging, impact test results were calculated as 42.5 ± 2, 38.04 ± 10, 36.49 ± 11, 34.2 ± 12 and 26.2 ± 7.6 kJ/m² for A10, P30, A30, P10, and GFRP composite samples, respectively. After thermal aging, it was understood that the Nano ABS additive showed better impact resistance than the nano PC additive.

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Figure 11.

Impact test results of composites after 1000 h water absorption test.

Microscopic and SEM image analyses are impeded by the extensive damage to the fracture surfaces, resulting from the interaction of fragments produced during the test with other components of the specimen. Furthermore, the preparation of these samples for examination in an SEM is compromised by the delicacy of the composite and their current state as sticks. This type of occurrence, loss of the fracture surface due to impacts or contamination, is common in real cases and prevents the analysis of failures of many fractured parts in service or during aircraft maintenance. Comparable structures resulting from water retention at ambient temperature were noted in both microscopic and SEM pictures (Figure 12(a)-(f)). Nonetheless, it can be asserted that the deterioration of nanofibers is more pronounced in the SEM images. The matrix was additionally seen to be progressively deteriorated.

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Figure 12.

Microscopic and SEM images of the composites after 1000 h thermal aging at 65°C; (a)A10, (b)A30, (c)P10, (d)P30, (e)GFRP, (f)A30, (g) P30.

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Figure 13.

Impact test results under different ozone environments.

Impact test results after ozone exposure

Figure 13 shows ımpact test results of the samples under dıfferent ozone environment. Since the ozone exposure occurred on the sample surface, no significant changes occurred between timeframes. Following 72 h of testing at 50 ppm, sample A10 had a maximum impact strength of 76 kJ/m². Under identical conditions, the P30 sample exhibited an impact strength of 70 kJ/m², the A30 sample 58 kJ/m², and the P10 sample 49 kJ/m². Although the impact strength of certain samples improved after 72 h at 75 ppm, a marginal decline was noted in others. This issue of short-term water absorption was elucidated in the aforementioned section. The moisture absorbed by the fibers under brief wet conditions enhances fiber strength temporarily.^32,33,41 Upon doing the 75 ppm 144-h ozone test, the impact strength diminished in a manner analogous to the water retention test. Similarly, to the water immersion experiment, A10 showed the most impact strength with approximately 77 kJ/ m² and P30 was the second one with approximately 68 kJ/m² impact strength.

Relative to the original condition of the composites, the impact strength of the GFRP composite diminished by roughly 9% in a 50 ppm ozone environment. Upon increasing the ozone level to 75 ppm, a reduction of 11% was observed. The impact strength of the A10 composite material at 50 ppm and 75 ppm after 72 h diminished from 78.8 MPa to 76 MPa and 77 MPa, respectively. It is recognised that an increase in ABS density correlates with a more significant reduction in impact resistance. The impact strength of the A30 sample under 50 ppm and 75 ppm for 72 h diminished from 76.9 MPa to 58 MPa and 60 MPa, respectively. Likewise, the strength values of the P10 sample diminished under ozone conditions relative to the usual air environment. When the duration was extended to 144 h in Ozane medium at 75 ppm, the impact strength of all samples diminished. The factors contributing to these declines are as follows: Ozone degrades polymer chains, resulting in embrittlement and diminished energy absorption capability upon impact. Degradation of the fiber-matrix contact, resulting in early failure under impact stresses. The composite thus absorbs diminished energy before to failure, resulting in a reduction in impact strength. PC or ABS nanofibers may initially improve toughness; nevertheless, they might deteriorate in an ozone-rich environment. The contact between nanofiber and epoxy deteriorates, resulting in less impact energy absorption.^42–44 In the P30 sample, exposure to 755 ppm ozone for 144 h resulted in a reduction in impact strength from 65.3 MPa to 63 MPa.

It was anticipated that the drop in properties would be more significant during water immersion conditioning compared to ozone exposure. ² This expectation arises from the fact that ozone attack predominantly affects the external surface of the laminate, while water immersion allows for diffusion to penetrate inside the bulk epoxy matrix. As a result, water immersion conditioning is likely to have a more pronounced effect on the overall properties of the material, whereas ozone exposure primarily impacts the surface.