The coupling effects of seawater aging and fatigue micro-crack on mechanical degradation of carbon fiber/epoxy plain woven composites

Abstract

Carbon fiber reinforced plastics applied in the marine industry are often exposed to conditions of seawater and alternating loads for extended periods, which can cause the complex degradation process of the composites. In this work, the coupling effects of micro-cracks and the seawater aging on the carbon fiber/epoxy resin-based plain woven composites were experimentally investigated. The micro-cracks were introduced into the composites through the three-point bending fatigue load. The results indicate that the residual stress generated by different expansions between carbon fiber and epoxy resin that reduces the interfacial properties of the composite is the primary reason for seawater aging. Besides, seawater aging causes the bending strength of the specimens with fatigue damage to decrease more seriously than that of the undamaged specimens. This is because micro-cracks increase the specific surface area of the composites in contact with seawater to provide channels for seawater diffusion. The coupling effects suggest the maintenance cycle of the composite parts should be gradually reduced if micro-cracks occur in some composites.

Keywords

Plain woven composite pre-fatigue damage seawater aging coupling effect specific surface area

Carbon fiber reinforced plastics (CFRPs) have been increasingly applied in the marine field, such as the shipbuilding industry, because of their excellent anticorrosion behavior under seawater conditions.^1–5 When the ship is continuously affected by harsh environments such as waves and gravel during its service, some fatigue cracks will appear on the surface of the composites.^6–8 This kind of fatigue effect is very common in the real service life of the composites. The micro-crack caused by the fatigue effect will thus induce the seawater to diffuse into the interior of the composites, which may threaten the service life of the CFRP.⁹ In practical application, the prediction of aging life for composites without considering fatigue damage may be too ideal, which will eventually lead to premature failure of structural members, resulting in devastating disasters. Therefore, it is essential to study the performance changes of CFRPs under the interaction of seawater aging and fatigue damage for their promotion and application in the shipbuilding industry.¹⁰

Many researchers have investigated the effect of seawater aging under different conditions on the performance degradation of fiber reinforced composites.^11–13 Li et al. explored the effects of seawater aging at different temperatures and concentrations on the static/dynamic mechanical properties of CFRPs.^14,15 It was found that the higher temperature caused a more significant change in the moisture absorption rate with the aging time. However, the moisture absorption rate was not sensitive to the change in NaCl concentration.¹⁶ Koshima et al. studied the effect of long-term seawater immersion on the mechanical properties and fatigue life of carbon fiber plain woven composites (PWCs).¹⁷ The results showed that the fatigue strength degradation depended on the stress ratio and the fiber/matrix interface strength degradation. Stamenović et al. analyzed the effect of pH on the tensile properties of glass fiber reinforced plastics.¹⁸ Furthermore, some researchers have studied the mechanism of seawater aging on different fibers^19–22 and resin matrix^23–27 based composites. In summary, different scholars have explored the influence mechanism of different temperatures, pH, and aging time on the mechanical properties and hygroscopic diffusion of composites.^28,29 The composites are subjected to various alternating loads in the application process, resulting in micro fatigue damage, which will further accelerate the seawater aging process.³⁰ However, studying the effects of seawater aging may be too optimistic for composite life prediction without considering the additional effects of fatigue cracks. Therefore, it is of great significance for the life prediction of composite materials comprehensively to study the two coupled boundary conditions of fatigue crack and seawater aging on the degradation of mechanical properties of composite materials.

This study aims to explore the performance degradation mechanism of composites in the actual marine environment. First, the details of prefabricating the fatigue crack and the seawater aging experiments are introduced in the Experiments section. Then the three-point bending properties of different samples that with no crack or with crack and unaged, aged and the dried sample after seawater aging are compared. To reveal the mechanism of the seawater aging, the experiments of interlaminar properties and the surface properties of the composites were designed. Besides, the moisture absorption law of epoxy resin with different sizes was characterized to reveal the accelerating aging mechanism of the composites under the coupling effect of the seawater and fatigue. Finally, the final section concludes the significant findings of this work.

Experiments

Materials

T700S-12 k plain woven carbon fiber fabrics provided by Toray Inc. (Japan) were used as the reinforcement. The matrix was based on JC-02A (diglycidyl ether of bisphenol A) epoxy resin and JC-02B (improved methyl tetrahydro phthalic anhydride) hardener. The plain woven fabric layers were laid on the mould and cured by the vacuum assisted resin transfer moulding process. In this work, the curing process parameters of all samples were heated at 90°C for 2 h, 110°C for 1 h and 135°C for 6 h. The carbon fiber volume fraction was about 56%.

Prefabricated fatigue crack

Figure 1 shows the method to predefine the cracks in the PWC. The fatigue test was performed by a hydraulic servo fatigue test machine at 90% stress level until the micro-cracks are prefabricated. Figure 1(a) shows the detail of the three-point bending fatigue test. The fatigue test selected a sine wave force loading method, and the stress ratio (R = minimum stress σ_min/maximum stress σ_max) was set to 0.1. The load frequency was set to 4 Hz. The whole fatigue loading process was monitored in real time by a high-speed camera. Figure 1(b) shows the dimension of the specimen for the fatigue test to introduce the fatigue cracks (PWC-FC). Figure 1(c) gives the strain–cycle curve of the composites under the 90% stress level of the fatigue load. The strain–cycle curve indicates three stages of PWC under the three-point bending fatigue load. Stage I and stage II are the crack initiation and propagation stages, respectively. Besides, stage II is the primary stage of the composites under the fatigue load. Therefore, the number of cycles for prefabrication fatigue damage was selected from stage II, since obvious fatigue damage was observed on the compression surface of the specimen and the damage morphology was always in a stable state in stage II. The observed fatigue damage morphology is shown in Figure 1(d) and (e).

Figure 1.

Prefabricated fatigue crack procedure. (a) The fatigue loading test; (b) specimen specifications; (c) the strain–cycle curves of specimens at 90% stress levels; (d) real-time observation of fatigue cracks and (e) microscopic topography of prefabricated crack.

Seawater aging experiment

Table 1 lists the composition and proportion of artificial seawater referring to ASTM D1141. In this work, the temperature of the seawater was chosen to be 70°C to accelerate the seawater aging effect. The dried samples were weighed on an electronic balance with accuracy to 0.1 mg. During the seawater aging process, specimens were periodically weighed and removed for mechanical testing at various time intervals. In this work, PWC, PWC-FC, and specimens for the mode I interlaminar fracture test were aged for 126 days and the pure epoxy resin was aged up to 30 days. The moisture absorption rate M _t of the sample was calculated by equation (1):

M_{t} (%) = \frac{m_{t}^{} - m_{0}^{}}{m_{0}^{}} \times 100 %

(1) where m₀ is the weight of the dry specimen and m_t is the weight of the wet specimen at time t.

Table 1.

Chemical composition of substitute ocean water

Compound	Concentration g/L
NaCl	24.530
MgCl₂	5.200
Na₂SO₄	4.090
CaCl₂	1.160
KCl	0.695
NaHCO₃	0.201
KBr	0.101
H₃BO₃	0.027
SrCl₂	0.025
NaF	0.003

Characterization method

Flexural properties of the PWC and PWC-FC were determined using a three-point bending configuration according to GB/T 1449-2005. The three-point bending test was conducted with a universal testing machine (Suns, UTM5205). The maximum load of the sensor was 5 kN. The span of the test was 64 mm, and the loading speed was 2 mm/min. Compressive strength and modulus of neat epoxy resins were evaluated according to GB/T 1448-2005. The size of the specimen for the mode I interlaminar fracture test was 125 × 25 × 4 mm³, which referred to the ASTM D5528 standard. The loading speed was 1 mm/min. Each mechanical property test was configured with five samples.

The elemental and chemical functional group analyses were performed for unaged and aged samples using energy dispersive X-ray spectroscopy (EDS, Quanta-450-FEG; FEI Co., Hillsborough, CA, USA) and Fourier transform infrared (FTIR) spectroscopy (Spotlight 400 and Frontier; PerkinElmer) to confirm the effect of seawater aging on the matrix as well as the interface. The scanning electron microscope (SEM, Quanta-450-FEG; FEI Co., Hillsborough, CA, USA) and VHX-5000 Ultra-field microscopy system were used to characterize the interface and failure morphology.

Results and discussion

Effects of seawater aging on PWC

Figure 2 shows some test characterizations of composites and epoxy resin before and after seawater aging. Table 2 shows the flexural test results of specimens undried after seawater aging and samples dried after seawater aging. Figure 2(a) shows the flexural stress–strain curves of the samples that unaged, undried after seawater aging and dried after seawater aging. The results in Figure 2(b) show that the average flexural strength of samples undried after seawater aging and dried after seawater aging decreased by 4.6% and 0.8%, respectively. Compared with the samples that unaged and dried after seawater aging, the discreteness of flexural strength and flexural modulus of the sample undried after seawater aging are larger. Therefore, it is proved that seawater aging has a negative effect on the flexural properties of PWC. It can be found that the flexural modulus of the samples undried after seawater aging and dried after seawater aging has no apparent change. In CFRPs, the flexural modulus mainly depends on the mechanical properties of the carbon fibers along with the span direction and the carbon fiber volume fraction.¹¹ Due to the stability of the carbon fibers in the seawater environment, the flexural modulus of PWC-FC after seawater aging is almost not affected.¹² As shown in Figure 2(c), the compressive strength of the epoxy resin decreased by 6% after seawater aging. The compressive properties recovered after drying the internal moisture in the resin. This phenomenon was similar to the PWC. The initial peak load in the mode I interlaminar fracture test shown in Figure 2(d) indicates the seawater aging gave rise to the reduction of interlaminar properties of the PWC.

Figure 2.

Mechanical test results of seawater aging on the plain woven composites (PWCs). (a) Flexural stress–strain curves of different composites; (b) comparisons of flexural strength and modulus of different composites; (c) compressive stress–strain curves of resin under different conditions and (d) load–displacement curves of mode I interlaminar fracture test for the PWCs.

Table 2.

The statistical results of flexural properties of plain woven composites undried after seawater aging and dried after seawater aging

Flexural property (MPa)	Sample	Observed value (MPa)	Average (MPa)	Coefficient of variation	Variance value (σ²)
Flexural strength	Undried after aging	839.5846.9836.1802.8797.7	824.6	2.7%	20.3
Flexural strength	Dried after aging	876.5872.6836.9847.5858.5	858.4	1.9%	14.9
Flexural modulus	Undried after aging	50.1 × 10³51.5 × 10³51.4 × 10³53.2 × 10³53.0 × 10³	51.8 × 10³	2.5%	1.1 × 10³
Flexural modulus	Dried after aging	51.8 × 10³51.6 × 10³51.0 × 10³52.5 × 10³52.1 × 10³	51.8 × 10³	1.1%	0.5 × 10³

To illustrate the effect of the seawater aging on the interfacial performance of the composites, SEM images of the unaged and aged samples after the mode I interlaminar fracture test are shown in Figure 3. It can be found that the carbon fiber of the unaged sample is covered by a large number of resin fragments, indicating the good interfacial bonding property. The resin peaks around the carbon fiber also indicate good interfacial properties between the carbon fiber and the resin. When the composite is aged by the seawater, it is obvious that the fiber surface is very smooth and covered by less resin. Therefore, seawater aging caused a reduction of the interfacial property of the composites. The degradation of the interfacial performance may be due to the fact that the artificial seawater environment caused residual stress inside the composites because the expansion of the resin and carbon fiber was inconsistent. In this work, the seawater aging effect on the PWC is a physical hygroscopic expansion process.

Figure 3.

Scanning electron microscope (SEM) images of the unaged and aged samples after the mode I interlaminar fracture test. (a) Unaged sample and (b) aged sample.

Figure 4 shows the FTIR spectroscopy of unaged and aged epoxy resin.³¹ It indicates that no new functional groups were formed after the seawater aging. The changes of the flexural strength and the functional groups before and after seawater aging proved that epoxy resin did not undergo chemical changes during the aging process in this work. Besides, the internal elements distribution of the aged epoxy resin and PWC were analyzed using an EDS as shown in Figure 5(a) and (b). It was found that elements such as sodium (Na), chloride (Cl), magnesium (Mg), calcium (Ca), and other elements in artificial seawater could penetrate epoxy resin and PWC. The mass of these elements in epoxy resin was only 1.09% and it was much less in PWC. According to the research reports of Starkova et al.,³² the free volume of composites during seawater aging affects the diffusion of a small number of elements, which may be related to the free volume of composites. Freedom volume refers to the volume that the molecular itself does not actually occupy the total mass. It means the difference between the measurement volume and the actual volume is the same as the pore rate of the composites. When the seawater sample diffuses, the seawater will stay in the free volume, which will increase the diffusion rate of the sample. Therefore, the freedom volume will cause a small amount of artificial seawater to exist in this type of form. Overall, the chemical effects of seawater aging on epoxy resins and PWCs may not be significant in this work.

Figure 4.

Fourier transform infrared (FTIR) spectroscopy of unaged and seawater aged epoxy resin.

Figure 5.

Elements results of epoxy resin and plain woven composite (PWC) after seawater aging. (a) Epoxy resin and (b) PWC.

To sum up, the diffusion of seawater in PWC is mainly carried out in epoxy resin matrix and the fiber/matrix interface. First, the moisture absorption of the resin is mainly due to the diffusion of the water in the resin network and the gap or micro-crack. In addition, the high crosslink density area of the resin will hinder the diffusion of seawater, and seawater is easier to diffuse in the low crosslink density area, which can further give rise to some defects such as fiber debonding and cracking.³³ The cracks caused by interfacial damage provide a channel for water diffusion, which makes water diffuse through the interface through capillarity. Tsenoglou et al. studied the water penetration of the composites under different interface strength levels.³⁴ The authors explained it as the result of the two processes, diffusion through the polymer matrix and through a network of micro-channels formed along the imperfectly bonded polymer–fiber interface.

Effects of seawater aging on PWC-FC

Figure 6 shows the flexural test results of PWC-FCs after the seawater aging process. As shown in Figure 6(a) and (b), the flexural strength of PWC-FC after seawater aging decreased by 8.2% but recovered to 99.7% of the original sample when the PWC-FC was dried again after seawater aging. Table 3 shows the flexural properties of PWC-FC that undried after seawater aging and dried after seawater aging. Compared with PWC, the decrease in flexural strength of PWC-FC after seawater aging was more prominent, indicating that cracks accelerated the seawater aging process and negatively affected the mechanical properties of the composites. It can be seen that the flexural strength of PWC-FC shows a larger standard deviation than that of PWC. This is because the distribution of cracks during the prefabricated fatigue damage is random. The crack is not necessarily in the bending failure area of the sample, thus affecting the discreteness of the PWC-FC flexural strength. From the view of stability, the overall standard deviation of flexural strength of PWC is smaller than that of PWC-FC. Therefore, this work paid more attention to the average value of bending intensity.

Figure 6.

Flexural properties of plain woven composite with fatigue cracks (PWC-FC) under different conditions. (a) Flexural stress–strain curves and (b) comparisons of flexural strengths and modulus.

Table 3.

Statistical results of flexural properties of plain woven composite-fatigue cracks that undried after seawater aging and dried after seawater aging

Flexural property (MPa)	Sample	Observed value (MPa)	Average (MPa)	Coefficient of variation	Variance value (σ²)
Flexural strength	Undried after aging	740.9770.7717.6777.6814.2	764.2	4.8%	33.0
Flexural strength	Dried after aging	818.4823.7860.2828.7820.0	830.2	2.0%	15.4
Flexural modulus	Undried after aging	52.2 × 10³51.6 × 10³52.7 × 10³52.1 × 10³52.4 × 10³	52.2 × 10³	0.8%	0.4 × 10³
Flexural modulus	Dried after aging	50.4 × 10³51.0 × 10³52.1 × 10³51.0 × 10³50.5 × 10³	51.0 × 10³	1.3%	0.6 × 10³

To show intuitively the effect of fatigue damage on seawater aging, the bending damage morphologies under the same strain before and after seawater aging were compared. Figure 7(a) to (c) and Figure 7(d) to (f), respectively, show the primary damage process of PWC-FC that was unaged and after seawater aging under the three-point bending load. I, II, and III, respectively, represent the 1%, 1.27%, and failure strains of the specimens during the three-point bending test. According to Figure 7(a) and (d), PWC-FC showed much more initial damage after seawater aging. When the strain reached 1.27%, the unaged sample damage only extended along with the thickness direction of the specimen. However, the aged sample had shown apparent delamination and a local shear band appeared on the compression surface. When the composites reached the failure strain, the samples showed apparent delamination failure between the middle layer and the tension layer of two samples. However, the aged sample also showed an obvious shear band area.

Figure 7.

The propagating failure process of plain woven composite with fatigue cracks (PWC-FC) under different conditions. (a)∼(c) Unaged sample under different strains and (d)∼(f) aged sample under different strains.

According to the above, water enters the resin and reinforcement after PWC-FC aging. As the moisture absorption of the resin is far greater than that of the reinforcement, the volume expansion between the two phases will then mismatch, resulting in the local stress and deformation of the composites. In addition, water enters the fiber/matrix interface, which will reduce the interface bonding strength. These two conditions lead to interfacial debonding and matrix cracking of the composites, and the declination of mechanical properties. Therefore, PWC-FC after seawater aging often showed more serious damage morphology.

Mechanism of coupling effects on the PWC

Figure 8 shows the coupling effect mechanism of the seawater aging and preset crack on the PWC. The square root of aging time versus absorption percentage of PWC and PWC-FC with time is shown in Figure 8(a). According to equation (1), the average saturated water absorption rate was 0.39%. Figure 8(a) shows that the whole moisture absorption process was subjected to partial Fick diffusion law.³⁵ Besides, it takes about 33 days for PWC-FC and 70 days for PWC to reach their maximum moisture absorption rates. PWC-FC obtained the maximum equilibrium water absorption earlier probably because the existence of cracks increased the specific surface area in contact with seawater. At the same time, fatigue cracks provided more channels for seawater diffusion in the composites. To verify this conjecture, epoxy resins of different sizes were designed to represent different specific surface areas, and the corresponding seawater aging tests were carried out.

Figure 8.

The coupling effect mechanism of seawater aging and cracks on the plain woven composite (PWC). (a) Moisture absorption curves of PWC and of plain woven composite with fatigue cracks (PWC-FC); (b) moisture absorption curves of epoxy resin with different sizes and (c) schematic diagram of seawater diffusion of PWC and PWC-FC.

The experimental results of epoxy resins with different dimensions are shown in Figure 8(b). Figure 8(b) indicates that the equilibrium water absorption rate of the sample with a smaller volume was higher than that of the sample with a larger volume. From Figure 8(b) (i), it can be seen that the equilibrium water absorption was proportional to the specific surface area of the epoxy resin. This suggests that the hygroscopicity of epoxy resin had an apparent size effect. Thus, the specific surface area is one factor affecting the resin matrix's moisture absorption rate.

Figure 8(c) shows the schematic diagram of seawater diffusion of PWC and PWC-FC to reveal the mechanism of two coupling effects of seawater aging and micro-cracks on PWCs. Figure 8(c) (i) shows that the seawater passes smoothly through the resin-rich area when the seawater diffuses in PWC. However, the seawater will be hindered when it diffuses to the carbon fiber, resulting in slow diffusion through the area among numerous carbon fiber filaments. However, as shown in Fig. 8(c) (ii), the seawater diffusion channels increase when there are cracks on the surface of PWC. This is because the specific surface area in the composites increases, accelerating the seawater diffusion and reducing the water saturation time. Accelerated seawater aging will eventually lead to accelerated degradation of composites with micro-cracks and decrease their mechanical properties.

Conclusions

This work experimentally compared the flexural properties of PWC and PWC-FC after the seawater aging process to reveal the coupling effects of seawater aging and micro-cracks on the PWC. Combined with some additional experiments such as EDS, FTIR spectroscopy, mode I interlaminar crack, and the moisture absorption of the composites and resin, some significant conclusions were obtained. First, the bending properties of PWC-FC after seawater aging decreased by 8.2%, while that of PWC only decreased by 4.6%. However, the bending and compressive properties recovered when the samples were dried after seawater aging. There were no chemical changes from the EDS and FTIR spectroscopy of the composite and epoxy after seawater aging, suggesting that seawater aging was mainly a reversible hygroscopic expansion process. Second, the physical hygroscopic expansion process caused by seawater aging was due to different expansions of resin and carbon fiber. This kind of expansion caused residual stress inside the composite that further reduced the mechanical properties of the composites. Finally, the micro-cracks increased the specific surface area of the composite causing the PWC-FC to show worse flexural properties than the PWC after seawater aging. This work provides a new idea for the application of composites in complex and harsh marine environments. When the composite is damaged because of some tiny loads, seawater will aggravate its corrosion effect. In this case, the engineers should reduce the maintenance life to slow down the seawater aging effects.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation, China (Grant No. 52073224, 52173080, 12002248), Key Research and Development Program of Xianyang Science and Technology Bureau, China (Grant No. 2021ZDYF-GY-0035), Local Transformation Program of Major Scientific and Technological Achievements of Xi'an Science and Technology Bureau, China (Grant No. 2021SFGX0003), Technology Innovation Guidance Special Program of Shaanxi Province, China (Grant No. 2022CGBX-10), Young Talent fund of University Association for Science and Technology in Shaanxi, China (Grant No. 20210509). Scientific Research Project of Shaanxi Provincial Education Department, China (Grant No. 22JC035), The Youth Innovation Team of Shaanxi Universities.

ORCID iDs

Tao Liu

Xingzhong Gao

Wei Fan

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