Fixture and device for in-situ testing of carbon fiber properties, and testing method thereof

Abstract

The present invention discloses a fixture for in-situ testing of carbon fiber properties, its device, and testing method, including a clamping mechanism and an adjusting mechanism, provided with a limit assembly, the limit assembly being provided with a threading channel, the threading channel being provided with a limit step portion; and an adjusting mechanism driving the clamping mechanism to move on a plane perpendicular to the center line of the threading channel. When conducting the test, the carbon fiber can be passed through the threading channel so that the droplets on the carbon fiber abut against the limit step portion, and then one end of the carbon fiber is fixed with a clamping portion. The other end of the carbon fiber is fixed by the upper clamping piece of a universal testing machine, and a tensile test is performed using the universal testing machine. This fixture can more conveniently perform a droplet and single fiber separation test, that is, accurately obtain the debonding load of the fiber and resin interface.

Description

Technical Field

[0001] The present invention relates to the technical field of composite materials, and in particular to a fixture and a device thereof, as well as a testing method for in-situ testing of carbon fiber properties. Background Art

[0002] Polymer-based composites are now widely used in various fields such as aerospace, national defense and military, and transportation engineering. However, the study of interfacial strength in composites, that is, the adhesion between reinforcing fibers and the polymer matrix, remains an unresolved issue. People may think that high adhesion strength is ideal, but this is not always the case. If the interfacial bond strength is too high, it may lead to reduced performance of the composite material. A strong interface may cause a large number of fibers to break simultaneously under tensile loads. Even intrinsically strong fibers may break simultaneously due to the rupture of adjacent weak fibers through the strong interface, which will lead to a decrease in the tensile strength of the composite material. Therefore, it is crucial to accurately measure the mechanical properties of the interface.

[0003] The droplet debonding test is an improved version of the single fiber pull-out test, which solves the problem of sample preparation for the single fiber pull-out test. In this test, the resin will automatically form droplets on the surface of the single fiber due to surface tension. The external load acts directly on the end of the droplet and is transmitted to the fiber through the interface. Through symmetrical loading, when the shear force reaches a critical value, debonding will occur at the interface. However, for high-performance fiber and resin composites, traditional micromechanical tests may have a relatively low success rate and are difficult to succeed because the embedded fiber length is too long, which may cause fiber breakage, droplet and single fiber shedding during the test. In addition, due to its sensitivity to micro-geometric parameters and experimental parameters, the discreteness of the interface shear strength data calculated by the maximum pull-out force is relatively large. Summary of the Invention

[0004] The purpose of the present invention is to provide a fixture and its device and testing method for in-situ testing of carbon fiber properties, so as to solve one or more technical problems existing in the prior art and at least provide a beneficial option or create conditions.

[0005] The technical solutions adopted to solve the above technical problems are:

[0006] The present invention provides a fixture for in-situ testing of carbon fiber properties, comprising a clamping mechanism and an adjusting mechanism, provided with a limit assembly, wherein the limit assembly is provided with a threading channel, and the threading channel is provided with a limit step portion; the adjusting mechanism drives the clamping mechanism to move on a plane perpendicular to the center line of the threading channel.

[0007] The beneficial effects of the present invention are:

[0008] During the test, the carbon fiber is passed through the threading channel so that the droplet on the carbon fiber contacts the limiting step. The other end of the carbon fiber is fixed to the upper clamp of the universal testing machine, and the tensile test is performed using the universal testing machine. This fixture more conveniently performs the droplet and single fiber separation test, that is, accurately obtains the debonding load at the fiber-resin interface.

[0009] As a further improvement of the above technical solution, the limit assembly includes two limit blocks arranged at a relative interval, and the two limit blocks form the threading channel. The structure is simple and it is convenient to abut the droplet against the limit blocks.

[0010] As a further improvement of the above technical solution, the limit block is provided with a guide slope to facilitate the installation of carbon fiber.

[0011] As a further improvement of the above technical solution, it also includes a limit drive structure that drives the two limit blocks to move closer to and away from each other, and is adapted to carbon fibers of different diameters.

[0012] As a further improvement of the above technical solution, the adjustment mechanism includes a base and an adjustment screw rotatably arranged on the base, the adjustment screw is threadedly connected to the clamping mechanism, and the clamping mechanism is slidably arranged on the base.

[0013] The present invention also provides a device for in-situ testing of carbon fiber properties, comprising any one of the above-mentioned fixtures, a universal testing machine, and a modulation and demodulation instrument, wherein the universal testing machine is provided with an upper clamping member.

[0014] The present invention also provides a method for in-situ testing of carbon fiber properties, comprising:

[0015] solidifying the droplets on a single carbon fiber;

[0016] Measuring the fiber diameter d of carbon fiber f and the length l of the droplet e ;

[0017] Take two stressed optical fibers A and temperature compensation optical fibers B with different wavelengths of grating lengths, and make the stressed optical fiber A do the wavelength-tension (Δλ B -F) calibration curve;

[0018] Bond the stressed optical fiber A, temperature compensation optical fiber B and carbon fiber in sequence, and connect the temperature compensation optical fiber B to the modem;

[0019] Passing the carbon fiber through the threading channel and making the droplet abut against the limiting step to stretch the optical fiber;

[0020] When the droplet is pulled off, the wavelength of the optical fiber in the stressed light segment changes Δλ according to the recording of the modem. BThe debonding load on the resin droplet is calculated from the wavelength-tension curve, and finally the interface strength between the resin and carbon fiber is calculated.

[0021] Carbon fiber is not easy to break, and it is easier to obtain the debonding load between the droplet solid and the carbon fiber, and the test success rate is higher.

[0022] As a further improvement of the above technical solution, the debonding load at the interface between carbon fiber and resin is calculated using the conversion relationship between grating frequency shift and tension, so that the debonding load result is more accurate.

[0023] As a further improvement to the above technical solution, a universal testing machine is used for stretching, with a fixture on the machine holding the sleeve at the fusion splice of optical fibers A and B. The stretching rate of the universal testing machine is easy to control and record, and universal testing machines are relatively common, eliminating the need for specialized stretching equipment, making testing much simpler.

[0024] As a further improvement to the above technical solution, when securing the droplet, an adjustment mechanism is used to adjust the position of the clamping mechanism so that the stressed optical fiber A is parallel to the tensile direction of the universal testing machine. This makes the carbon fiber less likely to break, making it easier to determine the debonding load between the droplet solid and the carbon fiber, resulting in a higher test success rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0026] Figure 1 This is a schematic diagram of wavelength-tension calibration of the optical fiber of the present invention;

[0027] Figure 2 This is a structural schematic diagram of an embodiment of a device for in-situ testing of carbon fiber properties provided by the present invention;

[0028] Figure 3 This is a structural schematic diagram of an embodiment of a fixture for in-situ testing of carbon fiber properties provided by the present invention;

[0029] Figure 4 This is the frequency shift-tension calibration curve of the embodiment provided by the present invention. DETAILED DESCRIPTION

[0030] This section will describe in detail the specific embodiments of the present invention. The preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the accompanying drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but it should not be understood as a limitation on the scope of protection of the present invention.

[0031] In the description of the present invention, it should be understood that descriptions involving orientations, such as up, down, front, back, left, right, etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limitations on the present invention.

[0032] In the description of the present invention, if there are words such as "several", it means one or more, and "more" means more than two. Greater than, less than, and exceed are understood as not including the number itself, and above, below, and within are understood as including the number itself.

[0033] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, and connecting should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention based on the specific content of the technical solution.

[0034] Polymer-based composites are now widely used in various fields such as aerospace, national defense and military, and transportation engineering. However, the study of interfacial strength in composites, that is, the adhesion between reinforcing fibers and the polymer matrix, remains an unresolved issue. People may think that high adhesion strength is ideal, but this is not always the case. If the interfacial bond strength is too high, it may lead to reduced performance of the composite material. A strong interface may cause a large number of fibers to break simultaneously under tensile loads. Even intrinsically strong fibers may break simultaneously due to the rupture of adjacent weak fibers through the strong interface, which will lead to a decrease in the tensile strength of the composite material. Therefore, it is crucial to accurately measure the mechanical properties of the interface.

[0035] Micromechanical testing of single-fiber composite specimens can effectively assess interfacial strength. Methods for characterizing single-fiber interfacial properties include the single-fiber break test, the single-fiber pullout test, the single-fiber ejection test, and the droplet 900 debonding test. The droplet 900 debonding test is an improved version of the single-fiber pullout test, resolving the sample preparation challenges associated with the single-fiber pullout test. For simplicity, the single fiber will be referred to as single-fiber 700. In this test, resin automatically forms droplets 900 on the surface of single-fiber 700 due to surface tension. The external load acts directly on the ends of droplets 900 and is transferred to the fiber through the interface. Symmetrical loading causes debonding when the shear force reaches a critical value. At this point, the force-displacement curve and the length of the droplet 900 wrapped around the fiber are recorded, and the interfacial shear strength is then calculated using a formula. The procedure and specimen preparation for the droplet 900 debonding test are simpler than those for the pullout test.

[0036] Assuming that the interface load is uniformly distributed, the shear strength of the interface can be obtained using formula (1).

[0037]

[0038] Where: Fd——fiber debonding load; d f ——fiber diameter; l e ——The length of the fiber buried in the matrix

[0039] In recent years, the development of fiber optic sensing technology has made it possible to obtain internal information of materials. Fiber optic sensors are small in size and have high measurement sensitivity, making them easy to bury inside composite materials to achieve continuous collection of internal information. Among them, the Fiber Bragg Grating (FBG) sensor is a fiber optic sensor based on a wavelength demodulation mechanism. Its signal demodulation accuracy is not affected by light loss during the transmission process, making it easy to achieve distributed measurement. The working principle of the FBG sensor is to use the sensing characteristics of the grating's effective refractive index and the Bragg grating period to the external environment (parameters such as temperature and strain) to convert changes in external parameters into changes in the central wavelength. When the Bragg grating is in a strained state, the reflected light takes a longer or shorter time to travel back, and the wavelength of the reflected light will also change, and its refractive index will also change. The external parameters are measured by detecting the change in the central wavelength.

[0040] The central wavelength of the FBG sensor is controlled by both temperature and strain. When subjected to temperature change ΔT and axial strain Δε, the central wavelength changes Δλ. B As follows:

[0041] Δλ B =K ε Δε+K T ΔT (2)

[0042] Where K ε Indicates the axial strain sensitivity coefficient; K T It represents the temperature sensitivity coefficient and is only related to the material's own characteristics. Therefore, FBG sensors can simultaneously obtain temperature and strain information inside the material, and certain decoupling methods can be used to achieve temperature and strain measurement.

[0043] To measure interfacial shear strength using a fiber optic sensor, the fiber must first be calibrated for wavelength-tension. After calibration, the frequency shift of the fiber and the corresponding tension or stress have a clear conversion factor.

[0044] However, for high-performance fiber and resin composite materials, the traditional micromechanical test may cause fiber breakage during the test due to the excessive length of the embedded fiber, resulting in failure of droplet 900 and single fiber 700 to fall off, so the success rate is relatively low and difficult to succeed; in addition, due to the sensitivity to micro-geometric parameters and experimental parameters, the discreteness of the interface shear strength (IFSS) data calculated from the maximum pull-out force is large. Therefore, referring to Figures 1 to 4 The present invention provides a fixture and a device and a testing method for in-situ testing of carbon fiber properties, and makes the following examples:

[0045] In some embodiments, a fixture for in-situ testing of carbon fiber properties, referring to Figure 3 , including a clamping mechanism 100 and an adjusting mechanism 200. The clamping mechanism 100 is provided with a limiting assembly, and the limiting assembly includes two limiting blocks 110 arranged at a relative interval. The two limiting blocks 110 form the threading channel, and the threading channel is for the carbon fiber to pass through. The carbon fiber will also be referred to as a single fiber 700 hereinafter. When the carbon fiber is set, the carbon fiber can be clamped between the two limiting blocks 110 from the side, so that the droplet 900 is clamped accordingly. The structure is simple, and it is convenient to abut the droplet 900 against the limiting block 110. The bottom of the limiting block 110 can also be understood as a limiting step portion. The limiting block 110 is provided with a guide slope 111. The guide slope 111 plays the role of guiding the carbon fiber to pass through, which is convenient for installing the carbon fiber.

[0046] In some other embodiments, the limiting assembly includes a threading block, the threading block is provided with a threading channel, and the threading channel is provided with a limiting step portion near one end of the clamping assembly.

[0047] The adjustment mechanism 200 drives the clamping mechanism 100 to move in a plane perpendicular to the centerline of the threading channel. Specifically, the adjustment mechanism 200 includes a base 210 and an adjustment screw rotatably mounted on the base 210. An adjustment knob 230 is provided at the protruding end of the adjustment screw. The adjustment screw is threadedly connected to the clamping mechanism 100, and the clamping mechanism 100 is slidably mounted on the base 210. In other embodiments, the adjustment mechanism 200 also includes a linear drive element such as a pneumatic cylinder, an oil cylinder, or an electric push rod, which automatically drives the movement of the clamping mechanism 100.

[0048] During the test, a single carbon fiber can be passed through the threading channel so that the droplet 900 on the carbon fiber abuts against the limit step. The other end of the carbon fiber is fixed by the upper clamp 300 of the universal testing machine, and a tensile test is performed using the universal testing machine. This fixture makes it more convenient to perform the separation test of the droplet 900 and the single fiber 700, that is, to accurately obtain the debonding load of the fiber-resin interface.

[0049] Further improvements include a limit drive structure that drives the two limit blocks 110 to move closer to or further away from each other, that is, the interval between the two limit blocks 110 can be adjusted. Adapt to the test of single fibers 700 of different diameters. Specifically, the limit drive structure is provided with a clamping seat, and the bottom ends of the two limit blocks 110 can be slidably provided on the clamping seat. The clamping seat is rotatably provided with an adjustment screw, and the adjustment screw has two threaded sections with opposite thread directions. The bottom ends of the two limit blocks 110 are respectively threadedly connected to the threaded sections at both ends. By rotating the adjustment screw, the two limit blocks 110 move closer to or further away from each other, thereby adjusting the interval between the two limit blocks 110, and thus adapting to different single fibers 700. At this time, the two limit blocks 110 can also be made into a ring shape, and the two limit blocks are combined into a threading channel for the single fiber 700 to pass through. The droplet 900 is placed under the two limit blocks, and the interval between the two limit blocks 110 is widened to allow the droplet 900 to enter, and then the interval between the two limit blocks 110 is narrowed so that the droplet 900 cannot pass through the threading channel, which can better carry out the separation test of the droplet 900 and the single fiber 700.

[0050] Reference Figure 2 The present invention provides an embodiment of a device, including a fixture, a universal testing machine, and a moderator 400 , wherein the universal testing machine is provided with an upper clamping member 300 .

[0051] The present invention provides an embodiment of a testing method, and the specific steps are as follows:

[0052] (1) A small amount of uncured resin is sucked by a syringe and dropped onto the surface of a single fiber 700. The resin droplets will form elliptical droplets 900 under the action of gravity and surface tension. Multiple droplets 900 can be placed on a single fiber.

[0053] (2) placing the single fiber 700 with the droplets 900 in an oven and keeping it warm until the resin droplets 900 are completely solidified;

[0054] (3) While waiting for the resin to solidify, perform wavelength-tension calibration on the optical fiber. Figure 1 , take two stressed optical fibers A810 and temperature compensation optical fibers B820 with different wavelengths and 5mm grating length, and make the stressed optical fiber A810 as wavelength-tension (Δλ B -F) calibration curve. After calibration, the frequency shift of the grating and the corresponding tension or stress have a clear conversion coefficient. The temperature compensation fiber B820 is used for temperature compensation.

[0055] (4) After the resin is cured, the fiber diameter d is measured using an optical microscope. f and the length l of the droplet 900 e , that is, the length of the fiber buried in the matrix in formula (1);

[0056] (5) Use adhesive 600 to bond a stressed optical fiber A810 and a single fiber 700. The strength of the adhesive 600 must be greater than the shear strength of the resin. Another temperature-compensating optical fiber B820 is welded to the stressed optical fiber A810. A heat shrink tubing 500 is installed at the weld to ensure tensile strength and connected to the modem.

[0057] (6)Reference Figure 2 , using a universal testing machine for stretching, the clamping part 300 on the universal testing machine clamps the sleeve 500 at the fusion joint of the stressed optical fiber A810 and the temperature compensation optical fiber B820, and the fixture clamps the upper surface of the single fiber 700 and the resin droplet 900. The clamping width of the fixture is required to be slightly larger than the diameter of the single fiber 700 and smaller than the diameter of the droplet 900. The single fiber 700 can move freely in the fixture. Figure 3 As shown;

[0058] (7) The fixture of the present invention only needs to be fixed under the universal testing machine, and the adjustment knob 230 controls the left and right displacement of the limit block 110. When the limit block 110 is adjusted to the appropriate position, the locking knob 220 is tightened to fix the limit block 110. After the experiment is completed or when the droplet 900 needs to be replaced, the locking knob 220 must be unlocked first, and then the adjustment knob 230 must be turned;

[0059] (8) The universal testing machine is stretched at a speed of 0.5 mm / min, and the modem starts to record the wavelength change of the stressed optical fiber A810. When the droplet 900 is pulled off, the wavelength change (Δλ B ) and the wavelength-tension curve in step (3) to calculate the debonding load on the resin droplet 900, and finally calculate the interface strength between the resin and the fiber according to formula (1), see Table 1 below.

[0060] Table 1

[0061] Sample number Wavelength change (μm) Force (N) Length (mm) <![CDATA[Area (mm 2 )]]> IFSS(MPa) 1-1 1.71771 0.754368 0.91 0.357175 2.112039 1-2 1.80407 0.819338 0.725 0.284563 2.87929 1-3 1.40958 0.522555 0.775 0.304188 1.717872

[0062] Compared with the traditional droplet debonding test, which requires the use of tensile instruments and force sensors, ordinary force sensors have large numerical fluctuations during testing, and the debonding load finally measured has large errors. The present invention uses the conversion relationship between grating frequency shift and tension to accurately obtain the debonding load at the fiber and resin interface. Secondly, the traditional droplet debonding test requires the use of a universal testing machine and a force sensor to obtain data, which requires fixed instruments and sites. The present invention uses an optical fiber sensor and only needs to provide a tensile device for measurement. Finally, the present invention provides an optical fiber for temperature compensation. By calibrating the frequency shift-temperature curve of the optical fiber, the fiber-resin interface shear strength under different temperature conditions can be accurately measured.

[0063] The above specifically describes the preferred embodiments of the present invention, but the invention is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A method for in-situ testing of carbon fiber properties, characterized in that: include: solidifying the droplet (900) on a single carbon fiber; measuring the fiber diameter df of the carbon fiber and the length le of the droplet (900); Take two stressed optical fibers A (810) and temperature compensation optical fibers B (820) with different wavelengths of grating lengths, and make a wavelength-tension (ΔλB-F) calibration curve for the stressed optical fiber A; Adhere the stressed optical fiber A (810), the temperature compensating optical fiber B (820) and the carbon fiber in sequence, and connect the temperature compensating optical fiber B (820) to a modem; Passing the carbon fiber through the threading channel of the fixture and making the droplet (900) abut against the limiting step portion, and stretching the optical fiber using a universal testing machine; After the droplet is pulled off, the debonding load on the resin droplet (900) is calculated according to the wavelength change ΔλB of the optical fiber in the stressed light segment recorded by the modem and the wavelength-tension curve, and finally the interface strength between the resin and the carbon fiber is calculated; The clamp includes a clamping mechanism (100), which is provided with a position limiting component, the position limiting component is provided with a threading channel, and the threading channel is provided with a position limiting step portion; The adjusting mechanism (200) drives the clamping mechanism (100) to move on a plane perpendicular to the center line of the threading channel.

2. The method for in-situ testing of carbon fiber properties according to claim 1, wherein: The limiting assembly comprises two limiting blocks (110) arranged at a relative interval, and the two limiting blocks (110) form the threading channel.

3. The method for in-situ testing of carbon fiber properties according to claim 2, wherein: The position-limiting block (110) is provided with a guiding inclined surface (111).

4. The method for in-situ testing of carbon fiber properties according to claim 2, wherein: It also includes a limit driving structure for driving the two limit blocks (110) to move closer to and away from each other.

5. The method for in-situ testing of carbon fiber properties according to claim 1, wherein: The adjusting mechanism (200) comprises a base (210) and an adjusting screw rotatably arranged on the base (210); the adjusting screw is threadedly connected to the clamping mechanism (100); and the clamping mechanism (100) is slidably arranged on the base (210).

6. The method for in-situ testing of carbon fiber properties according to claim 1, characterized in that: The universal testing machine is provided with an upper clamping member (300).

7. The method for in-situ testing of carbon fiber properties according to claim 1, characterized in that: The debonding load at the carbon fiber and resin interface is calculated using the conversion relationship between grating frequency shift and tension.

8. The method for in-situ testing of carbon fiber properties according to claim 1, characterized in that: A universal testing machine is used for stretching, with a fixture on the testing machine holding the sleeve at the fusion splice of optical fiber A and optical fiber B.

9. The method for in-situ testing of carbon fiber properties according to claim 1, characterized in that: When the droplet (900) is fixed, the position of the clamping mechanism (100) is adjusted by the adjusting mechanism (200) so that the stressed optical fiber A (810) is parallel to the tensile direction of the universal testing machine.

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