Method for recovering carbon fibers in composite material

Abstract

The invention discloses a method for recovering carbon fibers in a composite material, which comprises the following steps of: carrying out cold and hot circulation treatment on a to-be-recovered part accessory, and fragmenting a matrix material in the to-be-recovered part accessory to obtain a fragmented part accessory; performing ultrasonic vibration treatment on the fragmentation part accessory to further crush the matrix material in the fragmentation part accessory to obtain a plurality of single-layer carbon fiber layers; and performing ablation treatment on the single carbon fiber layer to evaporate the residual block-shaped base material attached to the carbon fiber layer to obtain the recycled carbon fiber tow. According to the technical scheme, by combining the synergistic effect of ultrasonic vibration and hot and cold circulation with the ablation technology, the problems that a traditional recycling mode lacks carbon fiber protection, the size of a recycling object is limited and the like can be solved, and the recycled carbon fiber tows have the advantages of being high in recycling efficiency, low in fiber damage, environmentally friendly and the like.

Description

Technical Field

[0001] This invention belongs to the field of material recycling, and specifically relates to a method for recycling carbon fibers in composite materials. Background Technology

[0002] Carbon fiber reinforced polymer (CFRP), which uses carbon fiber or carbon fiber fabric as reinforcement and resin, ceramics, metal, cement, carbonaceous material or rubber as matrix material, has wide applications in aerospace, automotive and wind turbine blades, but it also generates a large amount of waste materials. The traditional way to deal with these wastes is by landfill or incineration, but this method will lead to the permanent loss of scarce resources, and incineration of thermosetting matrix materials also faces the risk of generating persistent organic pollutants such as dioxins.

[0003] Especially since CFRP is used in aerospace and wind turbine blades, the manufacturing of carbon fiber relies on precisely controlled spinning processes and energy-intensive high-temperature heat treatment. Therefore, more and more technologies are focusing on recycling carbon fiber from waste CFRP components. Patent CN121131019A discloses a continuous crushing system and method for fiber waste. In its method for treating carbon fiber waste, the carbon fiber material is crushed, and the powder is added to cement or paving materials to achieve carbon fiber recycling. However, this method generates a large amount of dust, and the resulting recycled products are of low value, significantly reducing the intrinsic value of the carbon fiber. Patent CN121249010A discloses a method for recycling CFRP carbon fiber. This method involves cutting the CFRP, treating it with an acidic medium containing a phase transfer catalyst to decompose the polymer matrix, and then treating it with an alkaline medium containing a phase transfer catalyst to obtain recycled carbon fiber. Patent CN121244666A also discloses a carbon fiber composite material recycling device, which uses a treatment liquid and microwave radiation to decompose the epoxy resin in CFRP, thereby allowing the carbon fibers in CFRP to maintain their complete structure and mechanical properties. Although the above-mentioned prior art avoids the loss of value in recycling carbon fibers to some extent, it still requires the treatment liquid used in the recycling process after the carbon fibers are recycled, and these acidic or alkaline substances cause greater environmental pollution.

[0004] It is evident that the existing carbon fiber recycling process still pollutes the environment; and because there are strict restrictions on the number and size of CFRP accessories during recycling, it can directly damage the performance of carbon fiber. Therefore, there is an urgent need to develop a carbon fiber recycling method that does not damage the fiber, has no size restrictions, and is environmentally friendly. Summary of the Invention

[0005] To address the problems of value loss, limited size of recyclable materials, and severe environmental pollution associated with existing technologies for carbon fiber recycling, this invention provides the following technical solution:

[0006] A method for recycling carbon fibers in a composite material, the recycling method comprising the following steps:

[0007] The attachments to be recycled are subjected to hot and cold cycling treatment to break up the base material in the attachments to be recycled, thereby obtaining the broken attachments;

[0008] The fractured part attachment is subjected to ultrasonic vibration treatment to further break the matrix material in the fractured part attachment, thereby obtaining multiple single-layer carbon fiber layers.

[0009] The single-layer carbon fiber layer is subjected to ablation treatment to evaporate the remaining blocky matrix material attached to the carbon fiber layer, thereby obtaining recycled carbon fiber bundles.

[0010] Preferably, the process of subjecting the attachment to be recycled to a hot-cold cycle treatment to break up the base material in the attachment and obtain the broken attachment includes the following steps:

[0011] The part to be recycled is placed in the first coolant to cool it down, so that the overall temperature of the part to be recycled does not exceed -180℃, thus obtaining the cooled part;

[0012] The cooling part accessory is placed in a heating furnace and rapidly heated to 150~200°C to obtain the heating part accessory.

[0013] The heating part accessory is placed in the second coolant for cooling, and cracks are generated in the base material to obtain the cracked part accessory.

[0014] Repeat the above steps to further expand and increase the cracks in the cracked component, thereby forming a crack network in the matrix material and obtaining a fragmented component.

[0015] Preferably, the accessory to be recycled is a carbon fiber reinforced matrix composite material.

[0016] Preferably, the first and second coolants are liquid nitrogen; the heating furnace is a high-temperature and high-pressure furnace.

[0017] Preferably, the ablation process is performed using a laser beam.

[0018] Preferably, the matrix material is an epoxy resin.

[0019] Preferably, the ultrasonic vibration treatment is carried out in a warm water bath.

[0020] Preferably, the ultrasonic vibration treatment of the fractured part attachment further breaks down the matrix material in the fractured part attachment to obtain multiple single-layer carbon fiber layers, including the following:

[0021] The fractured part attachment is transferred from the second coolant to the warm water tank for fixed-frequency ultrasonic vibration treatment to obtain the cracked part attachment;

[0022] Based on the equipment conditions of the recycling plant and the type of matrix material of the accessory to be recycled, obtain the ultrasonic recycling formula for the corresponding matrix material of the accessory to be recycled;

[0023] The cracked part is subjected to frequency-conversion ultrasonic vibration treatment according to the ultrasonic recovery formula to obtain a single layer of carbon fiber.

[0024] Preferably, the variable frequency ultrasonic vibration treatment is performed using a variable frequency sawtooth wave. Then, obtaining the ultrasonic recovery formula for the substrate material corresponding to the accessory to be recovered, based on the equipment conditions of the recycling plant and the type of substrate material, includes the following steps:

[0025] The recycling plant was surveyed to obtain its equipment parameters, including the parameters and layout of the ultrasonic vibration equipment, and the depth and area of ​​the warm water tank.

[0026] Determine the type of matrix material in the accessories to be recycled, and establish a reference material database for the corresponding matrix material type based on the equipment parameters of the recycling plant. The database includes the Poisson's ratio of the reference material, the size of the reference material, and the center frequency and frequency variation amplitude of the corresponding reference material.

[0027] Obtain the Poisson's ratio and dimensions of the recycled materials of the accessories to be recycled;

[0028] Using the sawtooth wave frequency output formula, the ultrasonic recovery formula corresponding to the accessory to be recovered is obtained; the sawtooth wave frequency output formula is:

[0029]

[0030] Where f(t) is the oscillation frequency output at time t; A is the center frequency of the reference material; and B is the frequency variation amplitude of the reference material. Poisson's ratio is the reference material. The Poisson's ratio for the accessories to be recycled; Based on the dimensions of the reference material; The dimensions of the accessories to be recycled; For the frequency f(t) itself in A- B and A+ The speed of oscillation between B; mod is the remainder operation, which ensures the loop.

[0031] Preferably, the reference material is subjected to a crushing test, and the center frequency and frequency variation amplitude of the variable frequency sawtooth wave are adjusted using an ergonomic method; and the center frequency and frequency variation amplitude of the reference material are determined based on the crushing test results.

[0032] Compared with existing technologies, this invention has the following advantages: It does not have strict limitations on the size of the parts to be recycled, resulting in more complete carbon fiber bundles. Furthermore, it generates no additional pollutants during the recycling process, exhibiting high recycling efficiency, high economic value of recycled carbon fiber, and a green and environmentally friendly recycling process. Specifically, this invention achieves layering of the carbon fiber parts through thermal cycling hardening and pulverizing of the matrix material; then, water is transformed into ice crystals, increasing their volume and further expanding the interlayer cracks in the matrix material; finally, the ultrasonic vibration frequency is adjusted through sawtooth wave cycles to further pulverize matrix materials of different sizes and shapes, thereby obtaining complete carbon fiber layers. Through physical action, most of the matrix material in the parts to be recycled is peeled off, preserving the integrity of the carbon fiber and minimizing damage to mechanical properties; finally, the remaining small portion of matrix material is treated by ablation, thereby obtaining complete carbon fiber bundles. Compared with traditional processing methods, this invention significantly improves recycling efficiency, fiber protection, and environmental protection, and supports high-quality and high-efficiency recycling of various shapes and functional structures. Meanwhile, by establishing a variable frequency sawtooth wave database in advance, this invention enables intelligent processing when recycling carbon fibers from composite materials with different matrix materials. Attached Figure Description

[0033] Figure 1 This is a process flow diagram of the carbon fiber recycling method described in this invention;

[0034] Figure 2 This is a schematic diagram illustrating the carbon fiber recycling method described in this invention;

[0035] Figure 3 This is a microscopic image of the carbon fiber after laser ablation in the carbon fiber recycling method described in this invention. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Example 1:

[0038] A method for recovering carbon fibers from a composite material involves applying ultrasonic vibration and thermal cycling to the component to be recovered, combined with laser ablation technology, to recover the carbon fibers from the composite material. Figures 1-2 As shown.

[0039] Specifically, it includes the following:

[0040] S1. Perform hot and cold cycling treatment on the attachments to be recycled to break the base material in the attachments to be recycled, and obtain the broken attachments.

[0041] S2. The fractured part attachment is subjected to ultrasonic vibration treatment to further break the matrix material in the fractured part attachment and obtain multiple single-layer carbon fiber layers.

[0042] S3. The single-layer carbon fiber layer is subjected to ablation treatment to evaporate the remaining blocky matrix material attached to the carbon fiber layer, thereby obtaining the desired product. Figure 3 The image shows recycled carbon fiber bundles.

[0043] In step S1, the attachment to be recycled is subjected to a hot-cold cycle treatment of cooling → heating → cooling. By repeatedly creating temperature gradient differences between the layers of the attachment to be recycled, the base material in the attachment to be recycled is broken, thereby obtaining the broken attachment.

[0044] like Figure 2 As shown, step S1 involves subjecting the attachment to be recycled to a hot-cold cycle treatment, causing the base material in the attachment to be recycled to break down, thereby obtaining the broken attachment. Specifically, this includes the following:

[0045] S11. Place the part to be recycled into the first coolant to cool it, so that the overall temperature of the part to be recycled is not higher than -180℃, and obtain the cooled part;

[0046] S12. Place the cooling part accessory into a heating furnace and rapidly heat it to 150~200°C to obtain the heating part accessory.

[0047] S13. The heating part accessory is placed in the second coolant to cool it, and cracks are generated in the base material to obtain the cracked part accessory.

[0048] S14. Repeat steps S12 to S13 to further expand and increase the number of cracks in the cracked component, thereby forming a crack network in the matrix material, and thus obtaining a fragmented component.

[0049] Wherein, the first and second coolants are preferably liquid nitrogen; the heating furnace is preferably a high-temperature and high-pressure furnace; and the accessory to be recycled is CFRP, preferably a resin-based composite material.

[0050] The core of this technical solution lies in utilizing the thermal expansion and contraction properties of the matrix material. By repeating steps S12 to S13, the internal stress at the interface of the attachment to be recycled is continuously aggravated, and the matrix material is made brittle, ultimately achieving microscopic pulverization of the matrix material. During the cooling process in step S11, the matrix material undergoes significant volume shrinkage, and its inherent brittleness increases sharply. In step S12, when the temperature suddenly rises to a high temperature, an interlayer temperature difference appears in the parts to be recycled. The surface matrix material is stretched, and the middle matrix material is compressed, causing micro-cracks to initiate between the layers. With continued heating, moisture infiltrates and fills the cracked areas. In the next step S13, the temperature in the matrix material drops sharply again, and the moisture that has seeped into the interlayer cracks of the matrix material gradually condenses to form ice crystals, which expand in volume, thereby generating significant expansion stress inside the cracks. This stress causes the crack tip to extend, leading to the further development and extension of the original interlayer cracks in the matrix material. After repeating steps S12 to S13 a sufficient number of times, these micro-cracks no longer exist in isolation, but rapidly network and connect along the brittle matrix material to form a crack mesh. Especially at the weakest link, the fiber-matrix interface, large-scale debonding occurs. Ultimately, the matrix material no longer fails merely in the form of macroscopic cracking, but is divided into countless tiny fragments by these interwoven microcrack networks throughout the composite material, especially in areas where the matrix material is concentrated, such as resin-rich areas. This results in "shattering" damage, completely losing its function of transmitting loads and protecting fibers. Consequently, the structural integrity of the matrix material can be completely dismantled, thus obtaining fragmented attachments.

[0051] In step S2, the ultrasonic vibration treatment is used to further break down the matrix material, thereby significantly reducing the matrix material attached to the carbon fiber bundles and causing the carbon fibers in the broken portion to separate into layers, thus achieving the layer-by-layer peeling of the carbon fiber layers. The ultrasonic vibration treatment is carried out in a warm water bath.

[0052] In step S3, the ablation process is performed using a laser beam. By placing the single-layer carbon fiber layer on a laser processing platform, the remaining blocky matrix material attached to the single-layer carbon fiber layer is evaporated by a high-energy laser beam to obtain carbon fiber bundles, thereby completing the recycling of carbon fibers.

[0053] Example 2:

[0054] The difference between this embodiment and Embodiment 1 is that, in step S2, the fractured part attachment is subjected to ultrasonic vibration treatment to further break down the matrix material in the fractured part attachment, obtaining multiple single-layer carbon fiber layers, including the following:

[0055] S21. The fractured part attachment is transferred from the second coolant to the warm water tank for constant frequency ultrasonic vibration treatment to obtain the cracked part attachment; the constant frequency vibration treatment accelerates the cracking of the fractured part attachment;

[0056] S22. Based on the equipment conditions of the recycling plant and the type of base material of the accessory to be recycled, obtain the ultrasonic recycling formula for the corresponding base material of the accessory to be recycled;

[0057] S23. The cracked accessory is subjected to frequency-conversion ultrasonic vibration treatment according to the ultrasonic recovery formula. This frequency-conversion vibration treatment pulverizes the matrix material in the cracked accessory and refines it through continuous frequency-conversion vibration, thereby causing the carbon fibers to peel off layer by layer to obtain a single carbon fiber layer. Further, in step S22, the frequency-conversion vibration treatment uses a frequency-conversion sawtooth wave. Therefore, step S22, based on the equipment conditions of the recycling plant and the type of matrix material of the accessory to be recycled, obtains the ultrasonic recovery formula for the matrix material corresponding to the accessory to be recycled, including the following steps:

[0058] S221. Conduct a survey of the recycling plant to determine the parameters and layout of the ultrasonic vibration equipment, the depth and area of ​​the warm water tank, and other characteristics of the plant.

[0059] S222. Determine the type of matrix material of the accessories to be recycled, and establish a reference material database for the corresponding matrix material type using the equipment of the recycling plant. The database includes the Poisson's ratio, dimensions, center frequency, and frequency variation amplitude of the reference material obtained from laboratory tests.

[0060] Taking epoxy resin as the base material of the accessory to be recycled as an example, the specific steps are as follows: Obtain commonly used epoxy resins on the market, and use these epoxy resins as reference materials. Utilize the ultrasonic equipment of the recycling plant to perform ultrasonic vibration crushing tests on the obtained reference materials to obtain the Poisson's ratio corresponding to each reference material, denoted as... The dimensions of the tested epoxy resin fragments were used as the reference material dimensions, denoted as . ; and the ergonomic method is used, that is, by repeatedly adjusting the center frequency and frequency change amplitude of the variable frequency sawtooth wave, the reference material is crushed and tested, and then the center frequency A and frequency change amplitude B corresponding to the reference epoxy resin at this size are determined based on the crushing test results.

[0061] S223. Obtain the Poisson's ratio and dimensions of the recycled materials of the accessories to be recycled;

[0062] Specifically, when recycling carbon fibers from the same batch of composite materials, the Poisson's ratio of the recycled material is obtained by measuring the elastic modulus and shear modulus of the material to be recycled, denoted as . ; and measure the dimensions of the accessory to be recycled, denoted as . ;

[0063] S224. Obtain the recycling formula corresponding to the accessory to be recycled; using the sawtooth wave frequency output formula, obtain the sawtooth wave frequency output formula, i.e., the ultrasonic recycling formula, for processing the substrate material type corresponding to the accessory to be recycled. The sawtooth wave frequency output formula is as follows:

[0064]

[0065] Where f(t) is the oscillation frequency output at time t; A is the center frequency of the reference material; and B is the frequency variation amplitude of the reference material. Poisson's ratio is the reference material. The Poisson's ratio for the accessories to be recycled; Based on the dimensions of the reference material; Dimensions of the accessories to be recycled; For the frequency f(t) itself in A- B and A+ The speed of oscillation between B; mod is the remainder operation, which ensures the loop.

[0066] When recycling accessories made of a specific epoxy resin, the center frequency A, frequency variation amplitude B, modulation frequency F, and other parameters of the corresponding epoxy resin are obtained from a reference material database. , Substituting the above data into (Formula 1) yields the corresponding sawtooth wave frequency formula for epoxy resin. For example, when A=100kHz, B=80kHz, and F=100Hz are measured... , Substituting the above data into (Formula 1), we obtain the following formula for the ultrasonic recovery of epoxy resin:

[0067] f(t)=100+80х[2хmod(100хt)-1] (Formula 2)

[0068] Where f(t) is the oscillation frequency output at time t, and the frequency unit in the formula is kHz.

[0069] The variation of the ultrasonic output frequency of the sawtooth can be obtained from formula (2), as shown in Table 1.

[0070] Table 1. Variation of ultrasonic output frequency of the sawtooth teeth

[0071] Time (ms) (Fхt)%1 value Output frequency (kHz) Change of state 0 0.00 20 The starting point of the cycle, with the lowest frequency. 2.5 0.25 100 Ascend at a constant speed to the center point 5 0.5 180 Ascending at a constant speed to the highest point 7.5 0.75 100 Descending at a constant speed to the center point 10 1.00 20 The price instantly jumped back to its lowest point, and a new cycle began.

[0072] As shown in Table 1, when the matrix material of the accessory to be recycled is this specific type of epoxy resin, the sawtooth wave needs to be output with the following parameters to better break down the epoxy resin. Specifically, the sawtooth wave needs to cycle extensively between 20kHz and 180kHz, with a total bandwidth of up to 160kHz, a cycle period of T = 0.01 seconds (10 milliseconds), and an average sweep speed of 160 MHz / s. This embodiment, by establishing a reference material database in advance, can quickly determine the frequency conversion parameters of the sawtooth wave to be converted, thereby improving recycling efficiency; at the same time, obtaining specific frequency-converted sawtooth waves according to different material types further improves ultrasonic processing efficiency and avoids energy waste; it also provides conditions for intelligent recycling processes.

[0073] In summary, this invention utilizes the synergistic effect of ultrasonic vibration and thermal cycling to fracture the matrix material. Simultaneously, laser ablation evaporates a few blocky matrix materials within the carbon fiber layer, resulting in complete carbon fiber bundles. This recycling method protects the performance and structure of the carbon fibers in the recycling section, significantly increasing the economic value of the recycled carbon fibers and achieving high recycling efficiency. Furthermore, because no additional chemical substances are added during the recycling process, no additional pollutants are generated, making this method environmentally friendly. Moreover, since this recycling method primarily relies on physical processes, it has fewer size restrictions on the recycled materials, further improving recycling efficiency.

[0074] It should be noted that the technical features in embodiments 1 and 2 above can be combined arbitrarily, and the resulting technical solutions all fall within the protection scope of this application. Furthermore, in this document, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0075] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for recycling carbon fibers in a composite material, characterized in that, The recycling method includes the following steps: The attachments to be recycled are subjected to hot and cold cycling treatment to break up the base material in the attachments to be recycled, thereby obtaining the broken attachments; The fractured part attachment is subjected to ultrasonic vibration treatment to further break the matrix material in the fractured part attachment, thereby obtaining multiple single-layer carbon fiber layers. The single-layer carbon fiber layer is subjected to ablation treatment to evaporate the remaining blocky matrix material attached to the carbon fiber layer, thereby obtaining recycled carbon fiber bundles.

2. The carbon fiber recycling method as described in claim 1, characterized in that, The process of subjecting the attachment to be recycled to a hot-cold cycle treatment to break up the base material in the attachment and obtain the fragmented attachment includes the following steps: The part to be recycled is placed in the first coolant to cool it down, so that the overall temperature of the part to be recycled does not exceed -180℃, thus obtaining the cooled part; The cooling part accessory is placed in a heating furnace and rapidly heated to 150~200°C to obtain the heating part accessory. The heating part accessory is placed in the second coolant for cooling, and cracks are generated in the base material to obtain the cracked part accessory. Repeat the above steps to further expand and increase the cracks in the cracked component, thereby forming a crack network in the matrix material and obtaining a fragmented component.

3. The carbon fiber recycling method as described in claim 2, characterized in that, The accessories to be recycled are made of carbon fiber reinforced composite material.

4. The carbon fiber recycling method as described in claim 2, characterized in that, The first and second coolants are liquid nitrogen; the heating furnace is a high-temperature and high-pressure furnace.

5. The carbon fiber recycling method as described in claim 2, characterized in that, The ablation process is performed using a laser beam.

6. The carbon fiber recycling method as described in claim 3, characterized in that, The matrix material is an epoxy resin.

7. The method for recycling carbon fiber as described in any one of claims 2 to 6, characterized in that, The ultrasonic vibration treatment is carried out in a warm water tank.

8. The carbon fiber recycling method as described in claim 7, characterized in that, The ultrasonic vibration treatment of the fractured part attachment further breaks down the matrix material in the fractured part attachment, obtaining multiple single-layer carbon fiber layers, including the following: The fractured part attachment is transferred from the second coolant to the warm water tank for fixed-frequency ultrasonic vibration treatment to obtain the cracked part attachment; Based on the equipment conditions of the recycling plant and the type of matrix material of the accessory to be recycled, obtain the ultrasonic recycling formula for the corresponding matrix material of the accessory to be recycled; The cracked part is subjected to frequency-conversion ultrasonic vibration treatment according to the ultrasonic recovery formula to obtain a single layer of carbon fiber.

9. The carbon fiber recycling method as described in claim 8, characterized in that, When using variable frequency sawtooth waves for the variable frequency ultrasonic vibration treatment, the process of obtaining the ultrasonic recovery formula for the substrate material corresponding to the accessory to be recovered, based on the equipment conditions of the recycling plant and the type of substrate material, includes the following steps: The recycling plant was surveyed to obtain its equipment parameters, including the parameters and layout of the ultrasonic vibration equipment, and the depth and area of ​​the warm water tank. Determine the type of matrix material in the accessories to be recycled, and establish a reference material database for the corresponding matrix material type based on the equipment parameters of the recycling plant. The database includes the Poisson's ratio of the reference material, the size of the reference material, and the center frequency and frequency variation amplitude of the corresponding reference material. Obtain the Poisson's ratio and dimensions of the recycled materials of the accessories to be recycled; Using the sawtooth wave frequency output formula, the ultrasonic recovery formula corresponding to the accessory to be recovered is obtained; the sawtooth wave frequency output formula is: Where f(t) is the oscillation frequency output at time t; A is the center frequency of the reference material; and B is the frequency variation amplitude of the reference material. Poisson's ratio is the reference material. The Poisson's ratio for the accessories to be recycled; Based on the dimensions of the reference material; The dimensions of the accessories to be recycled; For the frequency f(t) itself in A- B and A+ The speed of oscillation between B; mod is the remainder operation, which ensures the loop.

10. The carbon fiber recycling method as described in claim 9, characterized in that, The reference material is subjected to a crushing test, and the center frequency and frequency variation amplitude of the variable frequency sawtooth wave are adjusted using an ergonomic method; and the center frequency and frequency variation amplitude of the reference material are determined based on the crushing test results.

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