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Abstract

For carbon fiber–reinforced polymer composites applied in aerospace industry, the mechanical performance of carbon fiber in extreme temperature environment is of great importance. In this study, carbon fiber produced by dry-jet wet spinning and wet spinning approaches were cryogenically conditioned at different cooling rates. After cryogenically conditioned at sharp cooling rate, both fibers have around 10% decreases in tensile strength due to the huge hoop stress induced by the quenching process. However, after cryogenically conditioned at slow cooling rate, the interfacial shear strength between the two kinds of carbon fibers and epoxy resin was significantly enhanced. Furthermore, scanning electron microscopy, atomic force microscopy, and the Raman spectroscopy were conducted to detect the micro-structures and surface morphologies of the cryogenically conditioned carbon fibers. This study provided fundamental data for the material design and application of the carbon fiber at extreme temperature environments such as aerospace or other industrial fields.

Keywords

Carbon fiber mechanical properties interfacial properties cryogenic

Introduction

Composites applied in aerospace industry are required to have excellent mechanical properties, including high tensile, bending, and compressing strengths. They should be durable over a range of temperatures, especially in ultrahigh and ultralow temperatures. Due to the low density and excellent mechanical and thermal performance, carbon fiber has been the excellent candidate for the structural material applied in aerospace industry.^1,2 In recent decades, carbon fiber–reinforced polymer (CFRP) composites are rapidly developed and widely applied as typical advanced materials in extreme temperature environments.^3,4 When served at ultralow temperature, for example, the lightweight liquid hydrogen fuel tanks for next-generation reusable space-launched vehicles,⁴ composites need to withstand large cyclic temperature variations. Considerable attentions have been paid to CFRP to explore the mechanical property and failure mode at cryogenic temperatures or after cryogenic conditioning.^5–7 By investigating the properties of cryogenically conditioned CFRP, some scholars observed micro-cracked interface between fiber and polymer. The micro-cracking happened as a response to thermal residual stresses generated through cryogenic cycling, which resulted from large differences of the coefficient of thermal expansion between the carbon fiber and the matrix.^8–10 However, other literature works indicated that the surface roughness of carbon fiber could be improved essentially by cryo-treatment (cooled at −196°C environments by liquid nitrogen).^11,12 These micro-cavities and surface crimp generated by the cryogenic liquid nitrogen could work as mechanical interlocking sites to polymer resin, which would enhance the interfacial properties of composites.¹¹ In fact, the mechanical and interfacial properties of carbon fiber are both important for the tensile behavior of CFRP. Therefore, investigation of carbon fiber under cryogenic condition is very critical to reveal the mechanism of tensile behavior of CFRP in extreme low temperature.¹³

Among carbon fibers produced by different precursors, polyacrylonitrile (PAN)-based carbon fibers are the most widely used ones due to their excellent mechanical properties and cost effective quality.^14–16 PAN precursor fiber is mainly manufactured by two relatively mature processes: dry-jet wet spun and wet spun. During the manufacturing process, dry-jet wet-spun PAN fiber is spun out from the spinneret plate into a short distance of air gap before entering into the coagulation bath. The slight solvent evaporation and decrease in the temperature result in a condensed thin outer layer and a smooth surface of the fiber. While, in the wet spun, fiber is spun out from spinneret plate directly into the coagulation bath, results into a thick and rough skin layer with grooves.^17,18 Meanwhile, both dry-jet wet-spun and wet-spun carbon fibers have been used in aerospace industry or fuel tanks. Their mechanical behaviors in cryogenic condition are very critical for the design of composites used in cryogenic environment.

In this study, the dry-jet wet-spun and wet-spun PAN carbon fibers were cryogenically treated in different cooling rates. The tensile test and Weibull distribution analysis were conducted to evaluate the fiber tensile properties. The micro-bond test was carried out to investigate the interfacial shear strength (IFSS) between cryogenically treated fiber and epoxy resin. The Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) were adopted to detect the micro-structures and surface morphologies of the fibers.

Experiment

Materials and cryogenic treatment process

The wet-spun carbon fibers (T300) and dry-jet wet-spun carbon fibers (SYT49S) were manufactured by Toray Industries, Japan, and ZhongFuShenYing Carbon Fiber Co., Ltd, China, independently. Before cryogenic treatment, samples were prepared by immersing the fibers into acetone solvent for 30 min and then washed by distilled water. Based on the practical applications and Timmerman and Colleagues,^7,8 two kinds of cryogenic processes were adopted in this study: (1) temperature-program-controlled method (TPCM): the cooling rate is 2°C per minute when the treated temperature decreased from 20°C to −196°C and then maintained at −196°C for 12 h; (2) quenching method (QM): the samples were directly put into the cryogenic chamber at −196°C and maintained for 12 h. After treatment, both TPCM- and QM-treated carbon fibers were taken out from the cryogenic box (−196°C) to the laboratory (20°C) and allowed to recover to the ambient temperature. The schematic of cryogenic treatment processes under different cooling rates is shown in Figure 1.

Figure 1.

Schematic of cryogenic treatment processes under different cooling rates.

Single fiber tensile test

The fiber tensile test was conducted using a single fiber tensile testing machine (XS(08)XG; Shanghai Xusai Instrument Co., Shanghai, China) at a crosshead speed of 20 mm/min and a gauge length of 20 mm at 20°C ± 1°C and 65% relative humidity. Fiber diameters were measured by a polarized light microscope (Nikon Eclips LV 100 POL, Nikon Corporation, Tokyo, Japan) with a digital image capturing system. More than 50 specimens were tested for each sample and the mean values were calculated.

The strength of the fiber obeys the two-parameter Weibull distribution expressed by the empirical equation (1)¹⁹

F (σ_{f}) = 1 - \exp [- L {(σ_{f} / σ_{0})}^{β}]

(1)

where $σ_{f}$ is the tensile strength, $F (σ_{f})$ is the failure probability of an individual fiber at an applied stress of $σ_{f}$ , L is the length ratio about the reference length, $σ_{0}$ is the scale parameter, and $β$ is the shape or flaw dispersion parameter. The experimental data can be ranked in ascending order of strength values. The cumulative probability can be assigned as equation (2)²⁰

F (σ_{f}) = \frac{n}{(N + 1)}

(2)

where n is the rank of the tested fiber in the ranked strength tabulation and N is the total number of tested fibers (N is 50 for this study). Therefore, equation (1) can be rearranged as equation (3)²⁰

\ln \ln [1 / (1 - F (σ_{f}))] = β \ln σ_{f} + \ln L - \ln σ_{0}^{β}

(3)

A linear regression analysis can be applied to a plot of $\ln \ln [1 / (1 - F (σ_{f}))]$ versus $\ln σ_{f}$ . Therefore, $σ_{0}$ and $β$ can be obtained by the slope and intercept of this line.

Surface morphology and micro-structure analysis

The micro-scale and nano-scale surface morphologies of the untreated and cryo-treated carbon fibers were investigated using SEM (JSM-5600LV, JEOL Ltd., Tokyo, Japan at 15 kV) and AFM. The AFM analysis was conducted on a MultiMode Digital Instrument Nanoscape III setup in the contact mode under ambient conditions. The micro-structure of the carbon fibers was investigated by Raman spectroscopy (Renishaw inVia Raman microscope, Renishaw Plc., Gloucestershire, UK, 633 nm).

Interfacial shear strength

The interfacial properties of carbon fibers were investigated by the fiber pull-out test. The untreated and cryo-treated carbon fibers were selected from a bundle to prepare the micro-bond samples as described in Zhang and Qiu.²¹ EPOLAM 2008 epoxy resin and its curing agent (AXSON Technologies Shanghai Co. Ltd., Shanghai, China) were adopted and mixed in a weight ratio of 100 parts of epoxy resin and 20 parts of curing agent. Samples were prepared by wrapping the resin micro-droplets on the fibers and then curing for 3 h at 45°C.

The embedded lengths and the diameters of the fibers and beads were measured using the polarizing microscope. Then, the micro-bond test was carried out using a single fiber tensile testing machine (XS(08)XG) with a load cell of 3 N capacity at 20°C ± 1°C and 65% relative humidity. The under clamp displacement rate was 1 mm/min. The schematic view of the fiber pull-out and micro-bond techniques is shown in Figure 2.

Figure 2.

Schematic view of (a) droplet sample and (b) fiber pull-out technique.

The IFSS $τ_{i}$ was calculated using the following equation derived from the well-known shear-lag model²²

τ_{i} = \frac{n P_{m a x} \coth (n L / r)}{2 A}

(4)

where P_max is the peak load, A is the cross-sectional area of the fiber, L is the embedded length, r is the fiber radius, and n is defined as

n = \frac{E_{m}}{E_{f} (1 + v_{m}) \ln (R / r)}

(5)

where E_m = 1.80 GPa is Young’s modulus of the epoxy resin according to the datasheet of manufacturing, v_m = 0.35 is Poisson’s ratio of the matrix, R is the radius of the epoxy beads, E_f is the carbon fiber tensile modulus. The E_f values of the dry-jet wet and wet-spun carbon fibers are determined by the fiber tensile tests.

Results and discussion

Fiber surface morphologies

SEM pictures of the two kinds of carbon fiber and their cryogenically treated samples are shown in Figure 3. It was found that the wet-spun carbon fiber had abundant grooves distributed along its longitudinal direction in Figure 3(a), while the dry-jet wet-spun fiber had a much smoother surface. This is due to the different manufacture processing of the PAN precursor. After cryogenic treatment, all the fiber had a deteriorative surface at both TPCM and QM cooling rates. For dry-jet wet-spun carbon fiber, TPCM and QM treatments induced some grooves along its longitudinal direction. For wet-spun carbon fiber, the grooves appeared to be slightly wider and deeper. Meanwhile, evenly distributed micro-cracks were observed on both QM-treated dry-jet wet-spun and wet-spun carbon fibers, which indicated that the surface of carbon fiber can be damaged by the cryogenic quenching process. The micro-cracks may result in the degradation of the tensile properties, but improve the physical interlock with matrix.

Figure 3.

SEM images of the surface morphologies of (a) untreated, (b) TPCM-treated, (c) QM-treated dry-jet wet-spun carbon fiber; and (d) untreated, (e) TPCM-treated, (f) QM-treated wet-spun carbon fiber.

To further explore the surface roughness, AFM analysis was adopted. The AFM images and calculated results are shown in Figure 4 and Table 1, respectively. Micro-cracks along the radial directions were also found on the surfaces of the QM-treated carbon fibers as shown in Figure 4(c) and (f). These results were in accordance with the SEM image in Figure 3. The calculated results in Table 1 shows that root mean square roughness (RMS) of TPCM-treated dry-jet wet-spun fiber and wet-spun fiber increased 20.7% and 62.8%, respectively, compared with the untreated fibers. Moreover, the maximum height of dry-jet wet-spun carbon fiber and wet-spun fiber increased from 47.83 to 100.41 nm and 237.99 to 335.44 nm, respectively. The surface areas also increased significantly. However, fibers treated by QM process only showed slightly increase in the RMS, maximum height, and surface areas. This indicated a significant enhancement of the surface roughness of fibers by TPCM treatment.

Figure 4.

AFM images of (a) untreated, (b) TPCM-treated, (c) QM-treated dry-jet wet-spun carbon fiber; and (d) untreated, (e) TPCM-treated, (f) QM-treated wet-spun carbon fiber.

Table 1.

Roughness parameters of the cryo-treated carbon fibers.

Fiber type	Dry-jet wet-spun carbon fiber			Wet-spun carbon fiber
Fiber type	Untreated	TPCM	QM	Untreated	TPCM	QM
Surface area (μm²)	4.04	4.41	4.21	4.40	6.05	4.41
Root mean square roughness, RMS (nm)	6.37	7.69	6.72	32.27	52.54	39.31
Maximum height, R_max (nm)	47.83	100.41	52.47	237.99	335.44	277.07

TPCM: temperature-program-controlled method; QM: quenching method.

During cryogenic process, carbon fibers suffered the hoop stress along their radial direction. Because, the carbon fibers exhibit coefficient of thermal expansion along axial (–0.1 ~ –0.5 ppm °C⁻¹) and radial (7 ~ 12 ppm °C⁻¹) directions of the fiber,²³ indicating that contraction in radial and extension in axial directions will be aroused by cryogenic temperatures. Compared with QM process, temperature of the TPCM decreased slower and duration is longer (about 2 h). Therefore, the structure of the TPCM-treated carbon fiber obtained more time to be adjusted under the contraction in radial and extension in axial directions.

Single fiber tensile properties

The single fiber tensile test and Weibull statistical analysis were conducted to explore the tensile properties of the carbon fibers after cryogenic treatments. Figure 5 shows the corresponding Weibull plots. Values of Weibull parameters and mean values of fiber strengths are given in Table 2. According to the fitting results, the $σ_{0}$ values of dry-jet wet-spun and wet-spun carbon fibers were 3.81 and 2.10 GPa, respectively, implying higher stresses were needed to cause fiber failure of dry-jet spun fiber. The Weibull modulus, β, for dry-jet wet-spun and wet-spun carbon fibers were calculated to be 6.47 and 4.82. The Weibull modulus, β, can be attributed to the nature and distribution of the flaws which existed in the fibers and can be regarded as a defect frequency distribution factor. The higher β value of the dry-jet wet-spun carbon fiber indicated less defects and more even distribution of the structure than the wet-spun carbon fibers, which coordinated with results of Baojun et al.²⁴

Figure 5.

Weibull distributions of the single fiber testing and tensile properties of carbon fibers: (a) dry-jet wet-spun carbon fiber and (b) wet-spun carbon fiber.

Table 2.

Weibull parameters for the tensile strengths of the cryo-treated carbon fibers.

Fiber type	Dry-jet wet-spun carbon fiber			Wet-spun carbon fiber
Fiber type	Untreated	TPCM	QM	Untreated	TPCM	QM
Intercept	−8.102	−6.885	−7.376	−5.45	−5.18	−4.59
β	6.47	6.47	6.45	4.82	4.67	4.51
Maximum height, R_max (nm)	3.81	3.71	3.43	3.10	3.03	2.77
Average strength (GPa)	3.53	3.45	3.20	2.86	2.73	2.49
SD	0.42	0.29	0.43	0.59	0.71	0.61

TPCM: temperature-program-controlled method; QM: quenching method.

According to Table 2, all carbon fibers showed decreased tensile strength after cryogenic process. The average strengths of the TPCM- and QM-treated dry-jet wet-spun fibers were 3.45 and 3.20 GPa, decreased 2.2% and 9.3% compared to original fibers. As for the wet-spun fiber treated by TPCM and QM processes, the average strengths were 2.73 and 2.49 GPa, decreased 4.5% and 12.9%. According to Figure 3, the QM-treated carbon fiber showed more micro-cracks than TPCM-treated fiber due to the sudden decrease in the temperature. Therefore, the QM-treated carbon fiber exhibited more degradation of tensile strength. The strength degradation of cryo-treated dry-jet wet-spun fiber is lower than that of wet-spun fiber, because the homogeneous-structured dry-jet wet-spun carbon fiber¹⁷ might have suffered more even deformation and less damage than the wet-spun carbon fiber.

In addition to the tensile strength, the fiber moduli and elongations at break of cryo-treated fibers were also investigated as depicted in the embedded pictures of Figure 5. The pictures showed that the moduli of the dry-jet wet and wet-spun carbon fibers were significantly increased after cryogenic processes, while the strengths and elongations at break of both fibers were degraded considerably. This is because the huge compressive stress on the radius direction induced by the cryogenic process may cause tighter micro-structure and higher interaction of the molecules and thus lead to the enhancement of the fiber moduli. Meanwhile, it may also bring severe structural inhomogeneity or extrinsic defects (micro-cracks), resulting in the decline of fiber strength and elongation at break.

To justify the hypothesis of the tensile property changes of the cryo-treated carbon fibers, the test of Raman spectra was conducted. As shown in Figure 6, in the Raman spectra, the ratios of I_D/I_G of the dry-jet wet-spun and wet-spun carbon fibers both demonstrated significant increase after TPCM and QM processes. In addition, the QM-treated carbon fibers showed the highest ratios of I_D/I_G. Ratio of I_D/I_G is adopted to detect the degree of graphite structure in material. If the ratio of I_D/I_G is higher, it means the less graphite structures and more defects. Therefore, the increase in the ratios of I_D/I_G of the cryo-treated fibers indicates that the graphite structures in the carbon fibers were severely damaged by the huge hoop stress induced by cryo-treatment.

Figure 6.

Raman spectra of the cryo-treated dry-jet wet-spun and wet-spun carbon fibers.

IFSS

During the fiber pull-out test, when an external force was imposed and increased to the critical value, interfacial debonding occurred, followed by a progressive sliding of the droplet along the fiber. The IFSS values between carbon fibers and epoxy resin are shown in Figure 7. The IFSS of the original dry-jet wet-spun carbon fiber was 34.75 MPa, much lower than that of the wet-spun carbon fiber (43.73 MPa), due to its smoother fiber surface morphology as shown in Figures 3(a) and (d) and 4(a) and (d). The IFSS values of TPCM-treated dry-jet wet-spun and wet-spun carbon fibers were 41.87 and 56.93 MPa, which were 20.5% and 30.2% higher than the untreated fibers.

Figure 7.

IFSS values between the cryo-treated carbon fibers and epoxy resin.

The analysis of variance (ANOVA) is a statistical approach to analyze and determine whether the mean of a variable is affected by different types and combinations of factors. In this study, the one-way ANOVA and Tukey’s pair-wise multiple comparison were adopted to evaluate the significance of the IFSS values between the untreated and cryo-treated carbon fibers.⁷ A p-value smaller than 0.05 was considered statistically significant. According to one-way ANOVA, the IFSS of the TPCM-treated carbon fibers significantly enhanced, but that of the QM-treated carbon fibers had insignificant increase, because the rougher surfaces of the TPCM-treated carbon fibers provide stronger mechanical interlocking on the interfaces between the fiber and the matrix.

After carbon fibers were pulled out from the epoxy droplets, the surface morphologies of carbon fibers and damages of the droplets were investigated to evaluate the micro-bonding properties. As shown in Figure 8(a) and (d), the smooth fiber surface and complete epoxy droplet indicated that the untreated carbon fibers were pulled out easily and the interfacial bonding was weak. Figure 8(c) and (f) shows that the QM-treated carbon fiber/epoxy micro-composites exhibited similar SEM images with the untreated ones due to the limited changing of the fiber surface roughness. However, as shown in Figure 8(b) and (e), the broken epoxy droplets on the TPCM-treated carbon fibers indicated that the other ends of the droplets still adhered on the fibers surfaces after the fibers were pulled out, demonstrating the improvement of the interfacial bonding.

Figure 8.

SEM images of carbon fiber/epoxy micro-composites after fiber pull-out test: (a) untreated, (b) TPCM-treated, (c) QM-treated dry-jet spun carbon fiber; and (d) untreated, (e) TPCM-treated, (f) QM-treated dry-jet spun carbon fiber.

Conclusion

In this study, the dry-jet wet-spun and wet-spun carbon fibers were cryogenically treated with both sharp and slow cooling rates. The results showed that the tensile strengths and elongations at break of the carbon fibers degraded after the cryogenic condition, especially for QM process. The surfaces became rougher and the interfacial properties of the treated fibers are improved. IFSS between TPCM-treated carbon fibers and epoxy increased 30.2% for wet-spun fiber and 20.5% for dry-jet wet-spun fiber. In addition, the tensile strength of the cryo-treated CFRP increased 10% due to the improved carbon fiber/matrix bonding. This study could provide fundamental data for the material design and application of the carbon fiber at extreme temperature environment such as aerospace or other industrial fields.

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 of China (grant nos 51503120 and 51303025), Shanghai Natural Science Foundation (grant no. 17ZR1400800), Shanghai Science and Technology Committee (grant no. 14YF1409600), and also funded by the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program.

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Tensile and interfacial properties of dry-jet wet-spun and wet-spun polyacrylonitrile-based carbon fibers at cryogenic condition

Abstract

Keywords

Introduction

Experiment

Materials and cryogenic treatment process

Single fiber tensile test

Surface morphology and micro-structure analysis

Interfacial shear strength

Results and discussion

Fiber surface morphologies

Single fiber tensile properties

IFSS

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References