Fracture analysis of a single polyacrylonitrile-based carbon fiber in rigid-fixing and flexible-fixing chopping processes

Abstract

Chopping is an efficient way to produce short carbon fiber (CF). Generally, there are two types of fixing constraints available in the chopping process: rigid-fixing and flexible-fixing. Simplified experiments were performed using glass and rubber as the fixing constraints in cutting a single polyacrylonitrile-based CF to reveal the influence of the fixing constraints in CF chopping. The cutting forces and the bending angles with different fixing constraints were analyzed. Furthermore, the failure surface of the CF was observed. Due to an additional bending effect in the flexible-fixing cutting, the failure surface of the CF was rough, and the cutting-off force was approximately 5% of the force in rigid-fixing cutting. Therefore, flexible-fixing cutting is a suitable way to decrease the cutting-off force in CF chopping. Moreover, it was concluded that the fiber fracture in rigid-fixing cutting is caused by compression, whereas in flexible-fixing cutting, it results from bending. We hope our work is beneficial to the design of the chopping procedure for short CF.

Keywords

fracture fiber rigid flexible carbon fiber cutting

Due to its high specific strength and modulus,^1–4 polyacrylonitrile (PAN)-based carbon fiber (CF) is widely used as reinforcement in composite materials, such as CF-reinforced plastics, CF-reinforced ceramics, and CF-reinforced metals. According to the fiber length, CF can be classified into continuous and short types. Short CF is highly attractive because of its good process compatibility with traditional manufacturing processes, for example, injection molding and extrusion.

To date, the mechanical properties of short CF-reinforced polymers have been studied widely.^5–8 However, few papers discuss the production technology of short CFs. The chopping process of short CF is similar to sheet metal cutting, which involves feeding a tool that applies transverse pressure to the fiber. However, the CF exhibits a totally different machining behavior compared to typical metals because of its anisotropic and abrasive nature.⁹ Typically, in industrial production, short CFs are produced by a fiber cutter.¹⁰ There are two primary types of manufacturing technologies available for chopping CFs: squeeze roller technology (SRT)¹¹ and pressing roller technology (PRT).^12,13 The cutting force of a filament in SRT and PRT is different due to the different material properties of the fixing constraints. In SRT, the material of the squeeze roll is metal, which is difficult to partially deform; as a result, this cutting process can be regarded as rigid-fixing cutting. In PRT, the material of the active roll is rubber, which is easy to pit on the surface when it is cut; thus, this cutting process can be regarded as flexible-fixing cutting. Hence, the fracture process of CF in these two CF manufacturing technologies is different. To reveal the influence of different constraints in CF chopping, it is important to study the fracture process of these two cutting methods.

Normally, it is difficult to directly observe the fracture process of a single CF because of its micro-dimension in diameter. Hence, several researchers used indirect parameters (e.g. cutting force or failure surface) to analyze the fiber fracture process with different constraints. Shin et al.¹⁴ conducted experiments on Zylon yarns under tension-shear loading conditions. The cutting force was measured by pressing a blade transversely at a constant rate against a yarn gripped at its end. Mayo and Wetzel¹⁵ tested the cutting force of organic and inorganic single fibers. They used a custom-designed fixture that forced a cutting blade into a single fiber at varying blade and fiber angles. The details of the progression of failure were inferred from post-failure imaging. Garcia-Leiva et al.¹⁶ studied the fracture mechanics of Sigma SM1140+ fiber. The fiber was tested with nano-indentation, tension, or diametral compression (which is called rigid-fixing cutting). Hudspeth et al.¹⁷ tested single filaments with different indenter shapes. The method of fiber failure was analyzed via post-mortem fracture surfaces.

This paper outlines two experiments that were designed to analyze the cutting process with different fixing constraints corresponding to two different CF manufacturing technologies. One material of the fixing constraint was glass; this cutting process was defined as rigid-fixing cutting. The loading condition in this process was the same as in SRT. The other material of the fixing constraint was rubber; this cutting process was defined as flexible-fixing cutting. The loading condition in this cutting process was the same as in PRT.

The results of the experiments, especially the cutting forces, can provide guidance for selection of the material of the roll. Moreover, the cutting force is also related to the tool wear; thus, the conclusions are also useful to the optimization of cutting blades for CFs.¹⁸ Based on the experimental results, we determined how different constraint materials influence the fracture process in the CF cutting process.

Experimental details

Materials

In this study, PAN-based CFs (T700SC from Toray) were tested. All CFs were in the sizes of 12 K (i.e., comprising 12,000 filaments). Tows in lengths of 40–60 mm were teased apart, and single fibers were carefully extracted. The diameter of a single CF is 7 µm. Fibers were tested as received, with no additional heat or drying treatment, at 22 ± 2℃ and 65% relative humidity (RH). Some of the important mechanical properties of the material used are listed in Table 1. The hardness of the CF was estimated to be 648 HV.¹⁸

Table 1.

Mechanical properties of the carbon fiber used in this study

Name	Standard (K)	Linear density (g/km)	Density (g/cm³)	Tensile strength (Gpa)	Tensile modulus (Gpa)	Strain to failure (%)
T700SC	12	800	1.8	4.9	230	2.1

The material of the cutting blades was carbon steel (Japan SK2, JIS G4401) with no extra surface treatment. Table 2 shows the amount of each component in the material. The dimensions of the blade were 0.075 mm × 19 mm × 0.9 mm (the thickness was 0.075 mm). The tool wedge angle was 22° and the rake angle was 79°. The hardness of the tool surface was 658–693 HV. The edge radius of the cutting blades was 4.4 µm, which was measured with a microscope at 1000 × magnification.

Table 2.

Amount of the chemical components of the Japan SK2 carbon tool steel¹⁰

Name	Chemical components (%)
Name	C	Si	Mn	P	S
SK2	1.15–1.25	0.10–0.35	0.10–0.50	Lower than 0.030	Lower than 0.030

Two materials of the fixing constraints were used in the experiments: glass and rubber. The dimensions of these two fixing constraints were 60 mm × 60 mm. The thickness of the glass was 20 mm, and the thickness of the rubber was 40 mm. The rubber was made of polyurethane (PU). The Shore Hardness of different rubbers was 57A, 75A, 84A, and 92A, based on tests performed according to ASTM D 2240.¹⁹

Experimental setup

Both the cutting-off force and the fiber deformation tests were conducted on a designed system, as shown in Figure 1. To avoid the influence of environmental factors on the cutting force, a vibration isolation table ZDT15-09, manufactured by the Jiangxi Liansheng Technology Company, was used. A micro-displacement platform (Newport M-VP-25XA1) with an on-axis accuracy of ±1 µm was situated on the table to control the tool feed. Then, the micro-displacement platform was connected to a tri-axial motion control system (Newport ESP301-3 N). A pressure sensor (Kistler 9256C1), whose sensitivity in the vertical Z-axis is −26 pC/N, was used to measure the cutting force. The pressure sensor, which was fixed on the moving component of the micro-displacement platform by screws, was connected to a charge amplifier (Kistler 5080A), which was connected to a data acquisition system (Kistler 5697A). Finally, the data acquisition system was connected to a personal computer, and the load–stroke curve was recorded in real time.

Figure 1.

(a) Schematic and (b) image of the experimental setup used for cutting a single carbon fiber.

Experimental methods

Firstly, the base surface of the micro-displacement platform was cleaned using alcohol, and then the fixing constraint was set up in the middle of the base. Next, the surface of the base was adjusted, with the use of a gradienter, to be horizontal. It is important to record the time that the cutting blade comes into contact with the fixing constraint. After setting the fixing constraint, the blade was fed along the direction of −Z at a feeding rate of 1 × 10⁻³mm/s, and a load–stroke curve was obtained via the computer. The stroke for which the load increased rapidly was marked as the initial point (specifically, that stroke was marked as 0).

After setting the initial point, the blade was returned to the position of +100 µm. Next, a single CF was carefully placed to have its axis aligned in the direction of the Y-axis on the fixing constraint. The blade was fed from +20 to −10 µm along the Z-axis in the rigid-fixing cutting process and from +20 to −80 µm along the Z-axis in the flexible-fixing cutting process. The load and stroke were recorded at intervals of 0.01 s in the rigid – fixing cutting process, and 0.1 s in the flexible – fixing cutting process. Moreover, to factor out the influence of rubber deformation on the cutting force of CFs, an auxiliary experiment was conducted to determine the cutting force of rubber under similar conditions. In this case, the rubber was tested without the introduction of any CF in the flexible-fixing cutting process.

Images of the CF were recorded by a microscope camera (with magnification of 200×). The bending angle (θ) of a single fiber was recorded at the cutting depth of every 5 µm. In this work, θ was measured using a computer-aided design software (Autodesk CAD) based on the enlarged images of the single fiber in the flexible-fixing cutting process. Firstly, the images were imported into the CAD software. Then, the included angles were drawn and measured according to the fiber outline. To minimize experimental error, each experiment was repeated 10 times.

Scanning electron microscope (SEM) imaging of the failure fiber surface was performed using a Zeiss-Merlin field emission SEM. An accelerating voltage of 5 kV was used. The working distance was between 9 and 11 mm. Furthermore, the testing samples were not subjected to additional treatment due to the electrical conductivity of the CF.

Results and discussion

Force behavior

The schematic of the transverse compression of a single CF is shown in Figure 2. The compression load F in the cutting blade and stroke δ were measured.

Figure 2.

Schematic of a single carbon fiber contact with the fixing constraint: (a) rigid-fixing cutting; (b) flexible-fixing cutting.

In the rigid-fixing cutting process, the cutting force of the single fiber is F_c, and F_c = F. In the flexible-fixing cutting process, the thickness of the blade (0.075 mm) is greater than the diameter of a single fiber (0.007 mm), and the cutting depth is greater than the diameter of the fiber; therefore, the cutting blade comes into contact with the rubber. To obtain the precise value, the cutting force of the CF in the flexible-fixing cutting experiment should be corrected by subtracting the compression force F_r₁ + F_r₂, which occurs from the rubber deformation. Because the diameter of a filament only contacts one 10th of the blade thickness, the force F_r₁ + F_r₂ was simplified as F_r, which was determined by the auxiliary experiment. Thus, the cutting force of a single fiber in the flexible-fixing cutting is calculated by F_c = F – F_r.

The cutting force–stroke curves of the rigid-fixing cutting and flexible-fixing cutting processes are plotted in Figures 3 and 4, respectively. The cutting force–stroke curves have two turning points in both the rigid-fixing cutting process and the flexible-fixing cutting process: (a) the instant the blade contacts the single fiber and (b) the breakage of the single fiber, which is the point of peak transverse force. The cutting-off force is defined as the force at which the single fiber is broken. The cutting-off depth is defined as the stroke from point (a) to point (b).

Figure 3.

Cutting force–stroke curve in the rigid-fixing cutting process.

Figure 4.

Cutting force–stroke curves in the flexible-fixing cutting process.

In the experiment, the cutting force increases with the increase of the stroke. When a single fiber is severed, the load suddenly decreases to zero. In rigid-fixing cutting, the growth rate of the cutting force is 43.73 mN/µm, and when the stroke reaches 4.86 µm, the single fiber is broken. In flexible-fixing cutting, the cutting force with rubber hardness of 92 A shows the highest growth rate, which is 0.74 mN/µm. As the hardness of the rubber decreases, the growth rate of the cutting force decreases; this occurs because the cut resistance of the rubber decreases as the hardness decreases.

Figure 5 illustrates the cutting-off forces in the cutting of a single fiber with different fixing constraints. The results show that the cutting-off force in flexible-fixing cutting is significantly less than the cutting-off force in rigid-fixing cutting. The cutting-off force in flexible-fixing cutting is approximately 10 mN, which is only 5% of the cutting-off force in rigid-fixing cutting (244 ± 77 mN). Moreover, the cutting-off force decreases as the hardness of the fixing constraint decreases. Therefore, compared with rigid-fixing cutting, flexible-fixing cutting is a suitable way to reduce the cutting force during the cutting process. In addition, the use of the rubber in the flexible-fixing cutting process has the effect of a buffer action, which can decrease the impact of the cutting blade when the fiber is cut off.

Figure 5.

Cutting-off forces in the rigid-fixing cutting and flexible-fixing cutting process.

Microscopy of the failed fibers

Figure 6(a) presents a schematic diagram of a single fiber cutting. Figure 6(b) shows the SEM image of a CF failure surface in the rigid-fixing cutting process. The fiber appears to have undergone rapid crack progression, and the cutting mark is clearly observed along the cutting direction. The whole failure surface is smooth. This feature is similar to that reported by Pelton,²⁰ in which fiber failure was described as being caused by compression in the transverse direction.

Figure 6.

Scanning electron micrograph of the fracture surfaces of carbon fibers: (a) schematic image of cutting; (b) failure surface in rigid-fixing cutting; (c) failure surface in flexible-fixing cutting.

During the feeding of the cutting blade, the blade develops a transverse compressive stress between the fiber and the cutting edge. This transverse compression induces local flaws in CF, such as cracks and kink bands,²¹ which leads to a gradual local failure.

Figure 6(c) presents microscope images of the CF failure surface in the flexible-fixing cutting process. Compared with the rigid-fixing cutting process, the fracture surface in flexible-fixing cutting appears rough and rugged. This feature is similar to the flexural fracture surface reported by Naito et al.²² When the single CF is in bending deformation, it is inevitable that many more crystallites will become misoriented. The fiber begins to buckle with the formation of kink bands. At this point, with an additional load, a tensile crack forms on the tension side of buckled fiber, and then the kink bands propagate inwards. Finally, the tensile crack and the kink bands meet, resulting in failure.²³ Due to the misoriented crystallites, a tremendous amount of new surface is created and the failure surface exhibits a rugged texture. The same appearances were observed in other literature.¹⁵

Based on the failure surfaces observed, fiber fracture in the flexible-fixing cutting process can be regarded as the effect of bending, while fiber fracture in the rigid-fixing cutting process is attributed to transverse compression.

Bending angle variation of a single CF in the cutting process

Figure 7 depicts enlarged failure surface images of CFs in the rigid-fixing and flexible-fixing cutting processes. The images, taken by a microscope camera, record the bending angle (θ) of the single CF in the cutting process.

Figure 7.

Deformation of single carbon fibers in (a) the rigid-fixing cutting process and (b) the flexible-fixing cutting process.

In the rigid-fixing cutting process, due to the high hardness of the glass, the CF sustains the compression caused by the blade, as shown in Figure 7(a). During the entire cutting process, the single CF clings to the glass surface. After fracture, the CF returns back in the longitudinal direction. In summary, the deformation of a single CF in rigid-fixing cutting can be divided into two steps. Step 1: the cutting blade comes into contact with the single fiber, and then the load begins to increase from zero. Step 2: the single fiber is compressed to fracture, and then the fiber returns along the longitudinal direction.

In the flexible-fixing cutting process, because the Young’s modulus of the rubber is lower than the Young’s modulus of the CF, the rubber deforms first.²⁴ As a result, the single fiber is pressed into the rubber by the cutting blade. With the deformation of the rubber, the rubber and the cutting blade cause the single CF to bend, as shown in Figure 7(b). The bending angle of fiber, θ, decreases with the feed of the cutting blade. Finally, the CF is bent to failure. The entire cutting process in flexible-fixing cutting can be divided into three steps. Step 1: the cutting blade contacts with the single fiber. Step 2: with the feeding of the cutting blade, the single fiber is pressed into the rubber. At this step, θ decreases gradually with the increase of the stroke. Step 3: the single fiber breaks, and the end of the single fiber lifts up suddenly, resulting in a dramatic decrease in θ.

The bending angle distribution curve illustrates that a point of inflection exists, as shown in Figure 8. The bending angle of the single fiber decreases from 180° to approximately 140° as the stroke increases at different rates for different values of the rubber hardness. The decline rate of the bending angle with 92 A is the highest, at 1.92°/µm, whereas the decline rate of the bending angle with 57 A is only 1.1°/µm. When the CF is fractured, θ decreases dramatically from approximately 140° to approximately 70°. The inflection of the bending angle distribution curve can be used to identify the fracture moment of the CF during the cutting process.

Figure 8.

Bending angle distribution curve of a single fiber with respect to the stroke in the flexible-fixing cutting process.

In the flexible-fixing cutting process, the fiber breakage occurs at varying failure angles, θ_fail, as shown in Figure 9. The failure angle is related to the cutting-off depth. As the rubber hardness increases, the θ_fail increases, because the deformation resistance of the rubber increases.

Figure 9.

Failure angle and cutting-off depth in the flexible-fixing cutting process.

Fracture process analysis

The force results show that the CF demonstrates higher cutting-off forces in the rigid-fixing cutting process than in the flexible-fixing cutting process. This result is likely due to different fracture processes for the CFs in cutting processes with different fixing constraints. CFs show an anisotropic strength in different directions, with the strength along the longitudinal direction being higher than in the transverse direction. Failure mode analysis of CF is complicated. Moreland²⁵ believed that transverse compression is always required for fiber failure and bending. Our understanding of CF failure is based on the bending angle and the failure surface microscopy analyzed above.

The fracture process of the PAN-based CF is different for different fixing constraints. In the rigid-fixing cutting process, there is no obvious warping of the fiber. The fiber is only sustained by transverse compression, and the pressure stress in the CF caused by the cutting blade can be calculated with Hertz’s theory. With the feeding of the cutting blade, the pressure stress of the CF increases gradually, and finally the CF is fractured when the stress reaches the material limitation.¹¹ In the flexible-fixing cutting process, the warping of the single fiber is obvious, and the angle of the single fiber changes with the feeding of the blades. The fiber is sustained with longitudinal compression and tension, which is also called the bending stress. The cutting process analysis is shown in Figure 10(a).

Figure 10.

(a) Schematic of the cutting analysis in the rigid-fixing cutting process and in the flexible-fixing cutting process. (b) Single carbon fiber fracture process analysis in the rigid-fixing cutting process and in the flexible-fixing cutting process.

In the rigid-fixing cutting process, the load in the transverse direction is 244 mN. This load will produce sharp kinks or local stress concentration and then lead to fiber failure. The failure surface of the fiber in this cutting process is smooth. This result is evidence that the fiber failure is caused by the instantaneous local failure, and distinct tool marks are observed on the failure surface. The detail fracture process of single CF can be seen in Figure 10(b).

In the flexible-fixing cutting process, the load in the transverse direction is approximately 10 mN. This load is too small to produce local stress concentration in CFs. Because the elastic transverse modulus of CF is significantly higher than that of PU rubber, the fiber is pressed into the rubber and bent by the cutting blade in a small area. This cutting process can be regarded as a three-point bending test, in which the breakage of CF is caused by bending.²² The fiber typically buckles on compression and forms kink bands at the innermost surface of the fiber. With the deformation of CF, a tensile crack is initiated at the tension side that propagates transversely across the fiber. As the blade moves, a tensile crack is initiated inwards, meeting the kink bands in the same plane, as shown in Figure 10(b). This single CF failure analysis is similar to the results given by Dobb et al.²⁶ The appearance of the rough failure surface provides evidence that the failure of the CF in the flexible-fixing cutting process is caused by bending.

Conclusion

This study established two cutting processes using different fixing constraints: rigid-fixing and flexible-fixing. The cutting forces for the two different cutting processes were obtained. Certain conclusions can be drawn from this study as follows.

The cutting-off force in the flexible-fixing cutting process is significantly lower than the cutting-off force in the rigid-fixing cutting process. Therefore, the flexible-fixing cutting process is a suitable approach to decrease the cutting-off force in CF chopping.

A bending angle exists when a single fiber is cut in the flexible-fixing cutting process. Therefore, the inflection of the bending angle distribution curve can be used to identify the fracture of a CF during the cutting process.

In PAN-based CFs, the fiber breaks under transverse compression stress in rigid-fixing cutting and via bending in flexible-fixing cutting.

Footnotes

Declaration of conflicting interests

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

Funding

The authors 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 number 51375175), the Natural Science Foundation of Guangdong Province, China (grant numbers 2015A030313201, 2014A030312017), the Science and Technology Planning Project of Guangdong Province, China(grant number 2015A010105007), and the Fundamental Research Funds for the Central Universities.

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