Sample preparation and test method for interface bonding performance of fiber reinforced composite material

Abstract

The invention provides a sample preparation and test method for interface bonding performance of a fiber reinforced composite material. The sample preparation method comprises the following steps: sampling a to-be-detected fiber reinforced composite material to obtain an initial sample piece; in the initial sample piece, a to-be-detected fiber extends along the thickness direction of the initial sample piece; and carrying out thinning processing on the initial sample piece in a manner of grinding and polishing the two processed surfaces of the initial sample piece along the thickness aspect, so as to obtain a test sample piece. According to the sample preparation method for the interface bonding performance of the fiber reinforced composite material, the thickness of the test sample piece is controlled within a proper range and the roughness of the test surface is controlled within a proper range in a grinding and polishing manner, so that a continuous annular separation belt is formed on the contact periphery of the ejected fiber-matrix, and the interface bonding performance of the fiber reinforced composite material is improved. The probability of fiber crushing or fragmentation is reduced, so that the test success rate is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of composite material testing, in particular to a sample preparation and testing method for the interface bonding performance of a fiber reinforced composite material. Background Art

[0002] Fiber reinforced polymer (FRP) is a composite material formed by winding, molding or pultrusion of reinforcing fiber materials (such as glass fiber, carbon fiber, aramid fiber, etc.) and matrix materials (such as synthetic resin).

[0003] The overall mechanical properties of FRP materials depend not only on the intrinsic properties of the fibers and resin matrix but also on the interfacial bonding between them. Single-fiber ejection testing is a relatively comprehensive method for measuring interfacial bonding. However, current single-fiber ejection tests are prone to fiber crushing or breakage, resulting in inaccurate test results and a high test failure rate. Summary of the Invention

[0004] The main purpose of the present invention is to provide a sample preparation and testing method for the interfacial bonding properties of fiber-reinforced composite materials, so as to solve the technical problem that the current single fiber ejection test is very prone to fiber crushing or fragmentation, which makes the test results lose accuracy and leads to a high test failure rate.

[0005] To achieve the above object, the present invention provides a method for preparing a sample of the interface bonding performance of a fiber reinforced composite material, comprising the following steps: A sample is taken from the fiber-reinforced composite material to be tested to obtain an initial sample sheet. In the initial sample sheet, the fibers to be tested extend along the thickness direction of the initial sample sheet.

[0006] The initial sample sheet is thinned by grinding and polishing the two processed surfaces along the thickness to obtain a test sample sheet. The thickness of the test sample sheet is 50-100 μm, and the roughness Ra of the two test surfaces along the thickness of the test sample sheet is ≤15 nm.

[0007] According to an embodiment of the present application, in the step of thinning the initial sample sheet by grinding and polishing the two processed surfaces along the thickness of the initial sample sheet, the step of grinding and polishing one processed surface after completing the grinding and polishing includes: After the initial sample sheet was bonded to the sample loading platform of a precision grinder, one treated surface was sequentially ground and polished using the grinding tools of the precision grinder. The grinding process was as follows: in the first stage, coarse grinding was performed using sandpaper with a grit of 600-1000 to a thickness of 1.50 mm. In the second stage, fine grinding was performed using sandpaper with a grit of 2000 to a thickness of 1.25 mm.

[0008] Remove the initial sample from the sample loading platform, flip it over, and secure it to the loading platform. Grind and polish the other treated surface sequentially. The grinding process involves using 600-800 grit sandpaper, then 1000-1500 grit sandpaper to thin the sample to 500 μm and 250 μm, respectively. Finally, use 2000 grit sandpaper to thin the initial sample to 40-70 μm.

[0009] The polishing process includes: performing precision polishing on a velvet polishing cloth for ≥30 min.

[0010] According to an embodiment of the present application, during the polishing process, the polishing liquid is sprayed and replenished every 2 minutes.

[0011] The present application also discloses a method for testing the interfacial bonding performance of fiber-reinforced composite materials, comprising the following steps: The test sample sheet prepared by the above sample preparation method is fixed on the support platform, and the central area of the test sample sheet is located above the gap of the support platform.

[0012] The indenter on the nanoindenter is used to push out the fiber to be tested in the central area, and the force-displacement relationship curve between the indenter and the sample is recorded by a displacement sensor. The slope of the force-displacement curve is analyzed to determine the maximum load to determine the interfacial bonding performance of the fiber-reinforced composite material.

[0013] According to the embodiment of the present application, the fiber to be tested pushed out by the pressure head is the target fiber. The position relationship of the target fiber satisfies: The target fiber is located at the center of the hexagonal area surrounded by the other six fibers to be tested, wherein the fiber spacing is , r is the fiber radius, and the diameter of the target fiber differs from the diameter of the surrounding fibers to be tested by less than 20%.

[0014] Or, the target fiber exists independently, and the distance between the target fiber and the nearest fiber to be measured is , r is the fiber radius.

[0015] According to an embodiment of the present application, the central area of the test sample piece is a range surrounded by 1 / 2 of the long side and 1 / 2 of the short side of the fiber distribution area of the test sample piece.

[0016] According to an embodiment of the present application, the width of the gap W = (5-10) d, the depth of the gap H = 150 ± 50 μm, and d is the diameter of the fiber to be measured.

[0017] According to an embodiment of the present application, 0.8 μm<Rb of the supporting platform<1.6 μm.

[0018] According to an embodiment of the present application, 0.5d≤the diameter of the indenter≤0.75d, where d is the diameter of the fiber to be tested.

[0019] According to an embodiment of the present application, in the process of using the indenter on the nanoindenter to push out the fiber to be tested in the central area, when the indenter pushes out the fiber, the loading rate is set to 10 mN / s, and the maximum load is set using an iterative method.

[0020] In the above-mentioned sample preparation method for the interface bonding performance of fiber-reinforced composite materials, the thickness of the test sample sheet is controlled within an appropriate range and the roughness of the test surface is controlled within an appropriate range by grinding and polishing, which helps to form a continuous annular detachment zone at the periphery of the fiber-matrix contact, reduce the probability of fiber crushing or fragmentation, and thus improve the test success rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0022] Figure 1 Flow chart for single fiber ejection sample preparation and testing; Figure 2 The overall schematic diagram of the experimental device; Figure 3 It is a structural diagram of the pressure head; Figure 4 This is a structural diagram of the support platform; Figure 5 This is a schematic diagram of the structure of a manual precision grinder; Figure 6 It is an enlarged schematic diagram of the ejection area; Figure 7 A partial schematic diagram of the selected single fiber ejection area; Figure 8 Schematic diagram of the positional relationship between the target fiber and the surrounding fibers; Figure 9 Schematic diagram of the preparation process of the MPCF / EP tow composite material proposed for implementation case 1; Figure 10 The SEM images of the fibers prepared in Example 1 after ejection; the left image is an SEM image of the holes formed on the front of the fibers after ejection; the right image is an SEM image of the ejected fibers; Figure 11 This is the IFSS test result of the MPCF prepared in Example 1 before and after air plasma treatment for 20 minutes; Figure 12 Schematic diagram of the silicon wafer support plane in Comparative Example 1; Figure 13 The left figure is an SEM image of the front side of the fiber after ejection prepared in Comparative Example 1; wherein, the left figure is an SEM image of the front side of the fiber after ejection at one magnification; the left figure is an SEM image of the front side of the fiber after ejection at another magnification; Figure 14 This is a SEM image of the side of the test sample prepared in Example 1; Figure 15 This is a roughness test image of the side of the test sample prepared in Example 1; Figure 16 This is a typical force-displacement curve obtained from the test in Example 2; Figure 17 This is an SEM image of the side of the test sample prepared in Example 2; Figure 18 This is a graph showing the average IFSS value of samples tested in Example 2; Figure 19 These are SEM images of the front and back surfaces of the fiber prepared in Example 2 after ejection; the left image is the SEM image of the front surface; the left image is the SEM image of the back surface; Figure 20 This is an SEM image of the side of the test sample prepared in Comparative Example 2; Figure 21 These are SEM images of the front and back sides of the fiber prepared in Example 2 after ejection; the left image is the SEM image of the front side; the left image is the SEM image of the back side.

[0023] The realization of the objectives, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0024] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0025] It should be noted that all directional indications (such as up, down, etc.) in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0026] In addition, the terms "first," "second," and so on, used in this disclosure are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, features specified as "first" or "second" may explicitly or implicitly include at least one of these features.

[0027] Moreover, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that ordinary technicians in this field can implement them. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0028] Existing single-fiber ejection tests are mostly applied to high-modulus and high-strength fibers. Conventional single-fiber ejection tests, when applied to typical FRP systems (e.g., glass fiber / polyester systems with a modulus <90 GPa and aramid fiber / epoxy systems with a compressive strength <1.5 GPa), expose systemic flaws. First, microcracks generated by conventional mechanical thinning processes severely weaken the fiber-matrix interface, and sample thickness errors cause load transfer path deviations. Second, the gap width of the universal support platform is fixed at 3d (d is the diameter of the fiber to be tested), which causes excessive stress concentration factors in low-modulus fiber testing and fiber breakage rates as high as 65% or more. Third, traditional visual interpretation methods rely on operator experience and are unable to accurately identify the maximum load (Fc) at the critical point of fiber debonding. Experimental data show that existing techniques have a failure rate exceeding 80% for testing low- and medium-modulus FRPs. This can easily lead to fiber crushing or breakage, resulting in inaccurate test results and severely limiting the universality of single-fiber ejection technology.

[0029] The present invention provides a method for preparing a sample of the interface bonding performance of a fiber reinforced composite material, comprising the following steps: Step S100: sampling the fiber-reinforced composite material to be tested to obtain an initial sample sheet, wherein the fibers to be tested extend along the thickness direction of the initial sample sheet.

[0030] Cut samples from the fiber-reinforced composite material (e.g., FRP laminate) to be tested. Initial sample sheets should be as uniform and flat as possible. Cut perpendicular to the direction of extension of the FRP laminate, ensuring that the fibers to be tested are perpendicular to both surfaces of the initial sample sheet.

[0031] For example, a precision cutting device is used to perform initial cutting and sampling on the fiber reinforced composite material to be tested (such as an FRP laminate).

[0032] The specific operations are as follows: First, install the FRP laminate in the positioning fixture of the cutting machine, and ensure that its long side is parallel to the cutting direction through a mechanical limit device; then, use a precision right-angle calibrator to fit its reference surface to the axis of the cutting head and the vertical edge of the sample respectively, and use the rotating fine-tuning mechanism to make the two form a precise 90° orthogonal relationship to ensure the flatness and verticality of the cutting edge; finally, set the displacement of the instrument after each cut to a certain distance, which corresponds to the thickness of the sample, to ensure that the thickness of each sample is uniform, providing high-quality initial sample pieces for subsequent steps.

[0033] In some embodiments, the thickness of the initial sample sheet is 2 mm. The initial fiber-reinforced composite material to be tested is generally thicker, such as about 8 cm. When the thickness of the initial sample sheet obtained by cutting and sampling is 2 mm, it is convenient to further reduce the thickness.

[0034] Step S200: Thinning the initial sample sheet by grinding and polishing the two treated surfaces along the thickness direction to obtain a test sample sheet. The thickness of the test sample sheet is 40-70 μm, and the roughness Ra of the two test surfaces along the thickness direction of the test sample sheet is ≤15 nm.

[0035] The initial sample is ground and polished surface by surface, either mechanically or manually. During the grinding process, the treated surface of the initial sample is rubbed against sandpaper to reduce the thickness. A series of sandpaper grits can be used for the grinding process, with the grit increasing gradually. Begin with the lower grit sandpaper and continue with the higher grit sandpaper. After grinding, the surface is polished, which is then referred to as the test surface, with a roughness Ra ≤ 15 nm.

[0036] After polishing one treated surface, the other treated surface is polished. After polishing, the test sample sheet has a thickness of 40-70 μm, and the roughness Ra of the two test surfaces along the thickness of the test sample sheet is ≤ 15 nm. In some embodiments, the test sample sheet has a thickness of 40-70 μm.

[0037] If the thickness of the test sample is too small, the matrix of the test sample may be too thin and unable to provide sufficient radial constraint, resulting in premature failure. If the thickness of the test sample is too large, it may lead to incomplete fiber debonding, and the axial load required for fiber ejection needs to overcome a larger matrix constraint force, which may lead to mixed interface shear failure modes due to plastic deformation of the matrix.

[0038] For example, the thickness of the test sample sheet is 50 μm. Under this condition, it is suitable to conduct a single fiber ejection test on a fiber-reinforced composite material with a resin matrix.

[0039] In the above-mentioned sample preparation method for the interface bonding performance of fiber-reinforced composite materials, the thickness of the test sample sheet is controlled within an appropriate range and the roughness of the test surface is controlled within an appropriate range by grinding and polishing, which helps to form a continuous annular detachment zone at the periphery of the fiber-matrix contact, reduce the probability of fiber crushing or fragmentation, and thus improve the test success rate.

[0040] In some embodiments, in the step of thinning the initial sample sheet by grinding and polishing the two processed surfaces along the thickness of the initial sample sheet, the step of grinding and polishing one processed surface after completing the grinding and polishing comprises: Step S210: After the initial sample sheet is bonded to the sample loading platform of a precision grinder, the processed surface is sequentially ground and polished using the grinder of the precision grinder. The grinding process is as follows: the first stage is rough grinding to a thickness of 1.50 mm using sandpaper with a mesh size between 600-1000. The second stage is fine grinding to a thickness of 1.25 mm using sandpaper with a mesh size between 2000. The polishing process includes precision polishing on a velvet polishing cloth for ≥30 minutes.

[0041] In this step, the present invention adopts a manual precision grinder (see Figure 5 ) Perform double-sided thinning on the initial sample. Precision grinders offer higher precision and are easier to operate.

[0042] The specific operation is as follows: Position the initial sample piece in the center area of the sample loading platform, apply 502 adhesive, and then use a blade to scrape it at a 30° angle to ensure that the thickness of the adhesive layer is evenly distributed on the platform surface, so that the initial sample piece is fixed on the sample loading platform.

[0043] Place the sample loading platform into the grinder and precisely control the amount of thinning using the height adjustment knob. Sandpaper or polishing cloth can be installed on the grinder as needed. Based on the type of sample to be tested, in the first stage, sandpaper with a mesh size between 600-1000 is used for coarse grinding to a thickness of 1.50 mm. In the second stage, sandpaper with a mesh size between 2000 is used for fine grinding to 1.25 mm. After thinning, precision polishing is performed on a velvet polishing cloth for ≥30 min to complete the first surface processing. In some embodiments, during the polishing process, the polishing liquid is sprayed on every 2 minutes.

[0044] Step S220: Remove the initial sample from the sample loading platform, flip it over, and secure it to the loading platform. The other processed surface is then ground and polished. The grinding process involves using 600-800 grit sandpaper and then 1000-1500 grit sandpaper to thin the sample to 500 μm and 250 μm, respectively. Finally, use 2000 grit sandpaper to thin the initial sample to 40-70 μm.

[0045] In this step, the other treated surface is ground and polished. For example, after heat treatment at 100-120°C (below the thermal aging temperature of the matrix and fibers) to soften the adhesive (e.g., 502 adhesive), highly flexible stainless steel elbow tweezers are used to non-destructively separate the upper and lower surfaces of the initial sample. After flipping and securing the initial sample, the sample is thinned to 500 μm and 250 μm using 600-800 grit and 1000-1500 grit sandpaper, respectively. In the final grinding stage, 2000 grit sandpaper is used to precisely thin the initial sample to 40-70 μm, depending on the fiber type.

[0046] The subsequent polishing process can refer to the polishing in step S210. For example, perform precision polishing on a velvet polishing cloth for 30 minutes or longer, reapplying polishing fluid every 2 minutes. After thinning, immerse the sample carrier in an acetone solution. When the sample has shifted 0.1-0.3 mm, immediately use elbow tweezers to transfer it. Test the surface roughness of the thinned sample, which must meet Ra ≤ 10 nm. This indicates that the polishing effect meets the requirements. Finally, transfer the sample to a constant temperature and humidity environment for storage.

[0047] It should be noted that when removing 502 adhesive, the usual practice is to immerse it in acetone for 5-10 minutes. However, long-term acetone immersion may cause swelling of the composite interface. In order to minimize the impact of acetone immersion on the interface performance of the composite material, high temperature is used to quickly soften the 502 adhesive to debond it after the first grinding is completed. After grinding and polishing, the ejection sample (i.e., the test sample) is removed by soaking it in acetone for 10 minutes.

[0048] In the sample preparation process, a double-sided gradient thinning process was adopted, and 600-2000 grit sandpaper was used for staged coarse / fine grinding (1.75 mm→40-70 μm). Combined with the sample flipping and fixing strategy, precise thickness control across scales was achieved; then, the adhesive was softened by heat treatment at 100-120°C, and highly elastic stainless steel elbow tweezers were used to perform synchronous separation of the upper and lower surfaces to avoid mechanical damage.

[0049] The present application also discloses a method for testing the interfacial bonding performance of fiber-reinforced composite materials, comprising the following steps: Step S10: Fixing the test sample sheet prepared by the above sample preparation method on the support platform, with the central area of the test sample sheet located above the gap of the support platform.

[0050] In this step, a nanoindenter is used to perform a ejection test on the test sample. The polished test sample is placed vertically above the gap (such as a slit) of the support platform, and the fiber-containing area is placed at the center of the support platform. The positional relationship between the indenter, sample, and support platform can be found in Figure 6 .

[0051] The test sample piece is fixed to the support platform by bonding or other means. During the subsequent ejection process, the pressure head is located above the gap to eject a single fiber. When fixing, it is necessary to ensure that the central area of the test sample piece is located above the gap of the support platform. In this way, during the ejection process, the fibers to be tested in the central area can be selected for ejection, thereby systematically eliminating interference from the edge area. The fibers close to the edge of the sample may be affected by factors such as stress concentration at the sample boundary and processing defects, causing the test results to deviate from the actual situation. The fibers in the central area are in a relatively uniform stress field, which can more accurately reflect the interface properties between the fiber and the matrix.

[0052] Specifically, the area at the edge is very prone to interface damage, because during the sample grinding process, the edge fibers are more likely to contact the fixture / abrasive, and their surface morphology (such as grooves and microcracks) may be irreversibly damaged due to mechanical wear, resulting in a reduction in the actual fiber-resin contact area and a low IFSS measurement value. In addition, the grinding process may cause a certain degree of damage to the resin in the edge area, resulting in the formation of a local weak interface, and the interface debonding occurs earlier than the true strength during the ejection process. Finally, the edge of the sample is subjected to non-uniform stress during grinding and testing, which causes stress concentration in the micro-area of the fiber-resin interface. These reasons may lead to inaccurate IFSS measurement results. Selecting fibers in the central area for single-fiber ejection experiments can reduce the impact of the above factors on the accuracy of IFSS measurement results.

[0053] Step S20: Use the indenter on the nanoindenter to push out the fiber to be tested in the central area, and record the force-displacement relationship curve between the indenter and the sample through the displacement sensor, analyze the slope of the force-displacement curve to determine the maximum load to determine the interfacial bonding performance of the fiber-reinforced composite material.

[0054] In this step, the maximum displacement and the maximum load are set as the end conditions of the experiment. In some embodiments, the indenter is controlled at a load of 10 mN / s.

[0055] Two conditions are required for successful fiber ejection: first, the force-displacement curve must conform to the standard curve, indicating a clear peak (brittle interface) or load plateau (ductile interface). Second, electron microscopy indicates the formation of a continuous annular detachment zone at the fiber-matrix contact boundary and a certain degree of fiber descent (fiber axial displacement Δz ≥ 0.3d, and the displacement direction deviates from the loading axis by an angle θ ≤ 2°). In ejection experiments, the maximum load (Fc) before debonding is extracted by analyzing the force-displacement curve. This is based on the fact that the slope dF / dx shows an inflection point or a sudden drop when the interface begins to debond. For brittle interfaces, the slope drops abruptly (dF / dx → 0), corresponding to the peak load Fc. For ductile interfaces, the slope of the force-displacement curve decreases gradually, forming a plateau. In other words, the maximum load (Fc) for interfacial debonding can be determined by the sudden change in the first-order derivative dF / dx. Fc corresponds to the point at which dF / dx drops to 50% of the slope in the elastic phase. Substituting the determined Fc into formula (1) can calculate the IFSS of the composite material.

[0056] (1) Where: —Interface shear strength, in megapascals (MPa); —Interface debonding failure load, in micronewtons (μN); is the fiber radius, in micrometers (μm); is the sample thickness in micrometers (μm).

[0057] This application effectively determines the ejection load based on a dual-modal verification mechanism. Based on the mechanical response criterion, the debonding initiation point is identified in real time using the force-displacement curve characteristics (peak point at the brittle interface / plateau at the ductile interface). This is combined with the first-order derivative dynamic threshold (dF / dx drops to 50% of the elastic slope) to precisely locate the peak load Fc.

[0058] Furthermore, the morphological verification criteria require that ejected fibers form a continuous annular detachment zone (interface debonding integrity) under an electron microscope, with displacement Δz ≥ 0.3d and deflection angle θ ≤ 2° (load axis alignment) to mitigate errors caused by lateral slip. This new strategy overcomes the limitations of traditional single mechanical criteria, achieving precise cross-scale matching of IFSS calculations and enabling highly accurate determination of ejection loads for fiber-reinforced polymer composites.

[0059] In the above-mentioned test method for the interfacial bonding performance of fiber-reinforced composite materials, since the thickness of the test sample sheet is controlled within an appropriate range and the roughness of the test surface is controlled within an appropriate range by grinding and polishing, it is helpful to form a continuous annular detachment zone at the periphery of the fiber-matrix contact, reduce the probability of fiber crushing or fragmentation, and thus improve the test success rate.

[0060] In some embodiments, the fiber to be tested ejected by the pressure head is a target fiber. The positional relationship of the target fiber satisfies: The target fiber is located at the center of the hexagonal area surrounded by the other six fibers to be tested, wherein the fiber spacing is , r is the fiber radius, and the diameter of the target fiber differs from the diameter of the surrounding fibers to be tested by less than 20%.

[0061] Or, the target fiber exists independently, and the distance between the target fiber and the nearest fiber to be measured is , r is the fiber radius.

[0062] The selected fibers must have one of the following two positional relationships in order to be selected as target fibers. The first positional relationship is that the target fibers satisfy the hexagonal central symmetric arrangement. This arrangement requires that the insertion position of the target fibers must satisfy symmetry to maintain a uniform stress field distribution. The surrounding fibers arranged in a hexagonal symmetric pattern form a uniform matrix support network, which makes the matrix constraint conditions of the central fibers close to idealization (isotropy) and avoids asymmetric stress concentration. Moreover, the fibers in actual composite materials are mostly regularly / randomly distributed, and the test results of the central fibers are closer to the interface behavior under actual service conditions. This method can simulate the actual composite material structure. The specific parameter requirements are fiber spacing. (r is the fiber radius) to avoid insufficient matrix wetting or stress overlap due to too small a spacing; and the difference between the target fiber diameter and the surrounding fiber diameter should be less than 20%.

[0063] The second positional relationship is that the target fiber exists independently, and the fiber spacing (r is the fiber radius). Figure 8 This method can reduce the influence of other fibers on the stress field around the target fiber, and the experimental results only reflect the intrinsic interface strength between the fiber and the matrix.

[0064] In some embodiments, the central area of the test sample piece is a range surrounded by 1 / 2 of the long side and 1 / 2 of the short side of the fiber distribution area of the test sample piece.

[0065] In this embodiment, when the fibers are ejected, the fibers located in the range of 1 / 2 of the long side (L1) and 1 / 2 of the short side (L2) of the fiber distribution area are selected for ejection by observation under a light microscope. Figure 7 This area is located in the center of the test sample, which further improves the accuracy of the measurement results.

[0066] In some embodiments, the width of the gap W = (5-10) d, the depth of the gap H = 150 ± 50 μm, and d is the diameter of the fiber to be measured.

[0067] In related technologies, a universal support platform is used with a fixed gap width of 3d (d is the diameter of the fiber to be tested). This causes the stress concentration factor to exceed the standard in low-modulus fiber testing, and the fiber breakage rate is as high as over 65%.

[0068] The gap width and gap depth of this application have been optimized. When the gap width is insufficient, the fiber is likely to contact the edge of the support structure during ejection, and the contact stress will cause plastic deformation of the fiber surface. If the gap width is too large, the effective support length of the fiber will be insufficient, resulting in asymmetric bending under axial load, which will increase the experimental error. For a schematic diagram of the support platform structure, please refer to Figure 4 .

[0069] In some embodiments, 0.8 μm<Rb of the support platform<1.6 μm.

[0070] For example, the support platform is precision-machined from austenitic stainless steel (06Cr19Ni10) and combined with mirror-grade polishing (0.8μm<Ra<1.6μm), which significantly reduces fiber contact friction damage and improves the platform's corrosion resistance and long-term stability.

[0071] For example, the support platform utilizes femtosecond laser micromachining technology, creating a high-precision rectangular gap on the support platform's working surface via a femtosecond laser, achieving micron-level structural control (width W = 5-10d, depth H = 150±50 μm), thus avoiding the edge burrs and heat-affected zones associated with traditional machining. Through a triple innovation in materials, processes, and structure, the platform overcomes the fiber damage and data deviation issues associated with rough machining and slit width mismatch on traditional support platforms, providing a highly reliable hardware foundation for microscale fiber interface performance testing.

[0072] In some embodiments, 0.5d≤the diameter of the indenter≤0.75d, where d is the diameter of the fiber to be tested.

[0073] The size of the indenter is determined by the diameter of the fiber to be tested. The diameter D of the indenter should be between 1 / 2 and 3 / 4 of the diameter of the fiber to be tested. If the indenter is too small, the pressure will be concentrated in the center of the fiber, causing the fiber to be "punctured" instead of being completely ejected. If the indenter is too large, the edge of the indenter will come into contact with the resin matrix, pulling off some resin during ejection, resulting in a combination of interface debonding and matrix damage, resulting in an inflated interface strength value. Figure 3 .

[0074] In some specific embodiments, the shape of the pressure head is a flat pressure head, and the side wall of the pressure head is configured with a curved transition structure to achieve precise force application in the fiber axial direction.

[0075] The indenter size (D=1 / 2-3 / 4d) is dynamically matched based on the fiber diameter to be tested. The lower limit of the indenter diameter (≥0.5d) avoids excessive stress concentration, ensures uniform transmission of ejection force, and suppresses brittle fracture of fibers. The upper limit of the indenter diameter (≤0.75d) avoids contact between the indenter edge and the resin matrix, eliminates the coupling effect of interface debonding and matrix destruction, and ensures universal matching of the pure shear mode of interface strength testing: through parametric design (D / d=0.5-0.75), it adapts to fibers of different diameters (5-50μm), solving the data distortion problem caused by size mismatch of traditional fixed-size indenters.

[0076] In some embodiments, during the ejection of the fiber under test within the central region using the indenter on the nanoindenter, the loading rate is set at 10 mN / s. The maximum load is determined using an iterative method: The critical load (Fc) of the sample is determined through preliminary testing as an initial reference value, and the ejection test is performed based on Fc. If ejection is successful, the maximum load threshold is adjusted to 50–100 mN above Fc. If unsuccessful, the maximum load is increased in steps of 50 mN until ejection is complete.

[0077] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific implementation.

[0078] Example 1 See also Figures 9-11 Example 1 provides a preparation and testing method for an MPCFRP single fiber ejection experiment, comprising the following steps: Step 1: Sample preparation For sample preparation, please refer to Figure 9After removing the surface sizing, mesophase pitch-based carbon fiber (MPCF) was placed in a plasma cleaning chamber using air as the treatment gas, a power of 150 W, and a variable treatment time of 20 min. The treatment pressure was controlled at 20 Pa. In a vacuum environment, atoms and their combinations (atomic clusters) in the air undergo ionization and are converted into a plasma state. This plasma then directly contacts the carbon fiber surface, where its high energy properties cause physical and chemical reactions such as etching and oxidation. The MPCF fiber bundles were then cut into 150 mm long and 2 mm wide samples and stored securely until further use. The mold was then cleaned with anhydrous ethanol to ensure a clean surface. Release agent was repeatedly applied to the mold surface to ensure smooth demolding. 502 adhesive was used to securely adhere one end of the carbon fiber to a groove at one end of the mold. A 50 g iron block was placed at the other end. Constant tension was applied to the fiber bundle to ensure it remained parallel to the mold. 502 adhesive was then used again to secure the other end, and excess fiber strands were trimmed off at both ends. According to the standard epoxy resin curing procedure, 50.00 g of epoxy resin and 10.85 g of IPDA were accurately weighed in a ratio of 100.0:21.7. During this process, IPDA was slowly dripped into the E51 mixture. After initial stirring with a glass rod, the mixture was placed in a homogenizer and vacuumed to remove air bubbles and achieve a homogenous state. The mold containing the fiber bundle was preheated in an 80°C oven for 30 minutes. Subsequently, using the glass rod as a guide, the well-mixed E51 was slowly and continuously poured into the mold grooves, ensuring that the resin completely covered and filled the grooves. The mold was then placed in an oven for curing. The curing schedule was set as follows: first, maintain the temperature at 80°C for 60 minutes, then increase the temperature to 120°C for 60 minutes, and finally increase the temperature to 150°C for 120 minutes to complete the curing process.

[0079] Step 2: Sample thinning and polishing During the initial cutting, an automatic precision cutting machine was used to cut the cured carbon fiber composite material into small pieces with a thickness of 2 mm. The sample was double-sided polished using a manual precision grinder, and the sample was polished to 1.5 mm using 800 mesh, 1200 mesh, and 2000 mesh sandpaper respectively. It was then manually polished on a microscope polishing machine with a speed of 350 r / min, applying polishing liquid every minute for 15 minutes. The sample was heated in an oven to soften the 502 adhesive, and then the polishing process was repeated on the other side. Finally, the sample was thinned to 50 μm. The thickness of the sample was photographed by electron microscopy, as shown in Figure 2. Figure 14As shown. It can be seen that the thickness of the sample is 58.77 μm, which is accurately controlled between 40-70 μm, and the thickness is uniform, and the upper and lower surfaces are flat and smooth, indicating that the thinning process proposed in the present invention can achieve the expected effect. The roughness of the sample is as follows Figure 15 As shown, it can be seen that the Ra of the sample is ≤ 10 nm.

[0080] Step 3: Single fiber ejection test A single fiber ejection experiment was conducted using a nanoindenter. The polished sample was secured to a support platform with pressure-sensitive tape. The sample was positioned so that the slit was located at the sample's center of symmetry and the fiber axis was perpendicular to the platform. The indenter was a flat indenter with a diameter of 5 μm. The support platform had a diameter of 3 mm and contained a slit in the center with a width of W = 100 μm and a gap depth of H = 150 ± 50 μm. (Fibers in fiber-reinforced composites typically have a diameter of 10-20 μm, so a slit of this width satisfies the requirement of W = (5-10)d for most fibers.) During fiber ejection, the loading rate was set to 10 mN / s. The maximum load was set using an iterative method. The force-displacement curve between the indenter and the sample was recorded using a displacement sensor. The maximum load (Fc) was determined by analyzing the slope of the force-displacement curve.

[0081] Experimental results and analysis Use scanning electron microscopy (SEM) to observe the fiber state after ejection. Figure 10 MPCF can be successfully debonded, with no obvious damage to the matrix and fiber, and no obvious crack propagation at the interface. MPCF air plasma treatment for 20 minutes before and after desizing. IFSS test results can be found in Figure 11Each sample underwent five effective ejection cycles, and the effective IFSS was the average of these five results. The IFSS values for the untreated, 20-minute desizing, untreated, and 20-minute desizing samples were 18.19 MPa, 18.60 MPa, 16.20 MPa, and 20.46 MPa, respectively. The IFSS of the unsizing MPCF composite did not significantly improve after plasma treatment. This is because the presence of the commercial sizing agent on the fiber surface allows the air plasma to preferentially contact the sizing agent, severely impacting the plasma's modification effect. Consequently, the IFSS improvement was minimal. After removing the commercial sizing agent from the MPCF surface, the IFSS of the composite decreased. This is because the removal of the sizing agent reduces the roughness of the fiber surface, impairing its wettability with the resin and weakening the interfacial bonding between the fiber and the resin. The IFSS of the desizing MPCF significantly increased after plasma treatment, reaching 20.46 MPa, a 26.2% increase compared to the pre-treatment value of 16.2 MPa. Air plasma treatment effectively improves the interfacial bonding strength between fiber and resin by increasing the surface roughness and chemical activity of the fiber.

[0082] Summary of experimental results Through single-fiber ejection experiments, the present invention successfully demonstrated that plasma treatment altered the surface roughness and chemical activity of the MPCF through physical etching and chemical reactions, thereby enhancing the interfacial bonding strength between the fiber and the resin. During the experiment, the fiber was successfully debonded and ejected from the matrix without significant damage, verifying the reliability of the experimental method and the high quality of the sample preparation.

[0083] Comparative Example 1 Compared with Example 1, steps 1 and 2 are exactly the same. In step 3, except for the support platform being different from the previous one, the rest of the operations and experimental equipment are exactly the same. In Comparative Example 1, the support platform is designed as a silicon wafer. During the experiment, the sample is bonded to two silicon wafers by crystal glue, leaving a gap between the silicon wafers. By moving the relative position of the silicon wafers, the size of the gap can be adjusted to facilitate fiber ejection. For a schematic diagram of the sample placed on the support platform, please refer to Figure 12 The platform made using this method is simple to operate and easy to use. However, the gap width W and gap depth H between the silicon wafers cannot be precisely controlled, which introduces significant uncertainty into the experiment and can easily lead to overall plastic failure of the sample during ejection.

[0084] Please refer to the SEM image of the sample after ejection in Comparative Example 1. Figure 13 As can be seen from the figure, after ejection, the fiber is crushed by the press head, the fiber is squeezed and deformed, and cannot be ejected successfully. At this time, it is impossible to determine the debonding time of the fiber and the resin.

[0085] From Example 1 and Comparative Example 1, it can be seen that the support platform proposed in the present invention can significantly improve the effect of fiber ejection and avoid the occurrence of fiber fragmentation to a great extent.

[0086] Example 2 Step 1: Sample preparation A laminate was prepared by combining quartz fiber (QF) with polyimide (PI) using compression molding. The laminate was then subjected to thermo-oxidative aging at 350°C for 40 h and 80 h, respectively.

[0087] Step 2: Sample thinning and polishing The QF / PI laminate composite material prepared in the previous step was initially cut using a precision cutter, ensuring that each small piece of composite material was 2 mm thick. The sample was then thinned by double-sided sanding using an MDG1 manual precision sander. The sample was affixed to the sample stage using 502 adhesive. The first side was sanded to 1.25 mm using 800-grit and 2000-grit sandpaper, respectively. The polishing process lasted for 30 minutes or longer before the sample was removed. The sample was then sanded to 500 μm using 800-grit sandpaper, 250 μm using 1500-grit sandpaper, and finally to approximately 50 μm using 2000-grit sandpaper. The polishing process lasted for 30 minutes or longer.

[0088] Step 3: Single fiber ejection test A single fiber ejection experiment was performed using the nanoindenter (with the same indenter) and support platform in Example 1. The loading rate was set to 10 mN / s, and the maximum load was determined to be 240 mN by the iterative method.

[0089] Typical force-displacement curves obtained are shown in Figure 2. Figure 16 The thickness of the sample was photographed by electron microscope, as shown in Figure 17 As shown. It can be seen that the thickness of the sample is 60.23 μm, which is accurately controlled between 40-70 μm, and the thickness is uniform, and the upper and lower surfaces are flat and smooth, indicating that the thinning process proposed in the present invention can achieve the expected effect. Subsequently, the average IFSS values of the samples without treatment, after 40h heat aging, and after 80h heat aging were measured, as shown in Figure 18 As shown in Figure 3, this result is consistent with the change law of thermal aging of QF / PI composite materials.

[0090] Finally, the debonding morphology of the front and back sides of the sample was judged, such as Figure 19Microscopic observations show that the sample achieved uniform interfacial debonding in the matrix, with a clear debonding interface morphology and a moderate separation gap. The fiber axial orientation remained vertical, and the ejection process did not induce fiber buckling or matrix plastic deformation, indicating that the interfacial debonding behavior conforms to the brittle fracture mechanism and the ejection effect is good.

[0091] Specifically, Figure 16 This is a typical force-displacement curve of a QF / PI laminate composite sample after 40 h of service. Figure 17 is a SEM image of the side of the QF / PI laminate composite sample of the original non-heat-aged sample. Figure 18 19 is the interface shear strength diagram of the original, 40h serviced and 80h serviced laminate composite material samples, and 19 is the ejection morphology diagram of the QF / PI laminate composite material sample of the original non-heat-aged sample.

[0092] Comparative Example 2: Step 1: Sample preparation: The same as in Example 2 except that aging is not performed.

[0093] Same as Example 2.

[0094] Step 2: Sample thinning and polishing.

[0095] The experimental process is as follows: The QF / PI laminate composite prepared in the previous step is initially cut using a precision cutter, ensuring that each cut piece of composite material is 2 mm thick. The sample is then thinned by double-sided grinding using an MDG1 manual precision grinder. The sample is affixed to a sample table using 502 adhesive. The first side is then polished to 1.25 mm using 800-grit and 2000-grit sandpaper, respectively. The polishing process lasts for 30 minutes or longer before the sample is removed. The sample is then polished to 500 μm using 800-grit sandpaper, 250 μm using 1500-grit sandpaper, and finally to approximately 100 μm using 2000-grit sandpaper. The polishing process lasts for 30 minutes or longer.

[0096] Step 3: Single fiber ejection test A single fiber ejection test was conducted with a loading rate of 10 mN / s. Due to the maximum load protection limit of the instrument, the load was determined to 400 mN using an iterative method, but no debonding point appeared.

[0097] The thickness of the sample was measured by electron microscopy. Figure 20 As shown in the figure, the thickness of the sample is 96.41 μm, which exceeds the reasonable ejection thickness. The debonding morphology of the sample is judged, as shown in the figure. Figure 21 shown.

[0098] Microscopic observations show that the sample exhibits multi-stage fragmentation within the matrix, accompanied by the expansion of the matrix plastic deformation zone. The fractured interface induces localized matrix buckling deformation, leading to delamination failure at the fiber / matrix interface and fiber fragmentation within the matrix.

[0099] Failures caused by overly thick samples are primarily due to two factors. First, an imbalance in the fiber aspect ratio. Excessive fiber embedment can easily lead to initial fiber bending due to residual stress or processing defects. This can cause bending stress concentration during ejection, leading to fiber bending and subsequent breakage in the matrix. Second, long fibers can lead to a surge in debonding loads. Interfacial friction resistance is often positively correlated with fiber embedment length (i.e., sample thickness). When the sample thickness is too high, the required debonding force often exceeds the limits of the nanoindenter, resulting in experiment termination or fiber collapse.

[0100] The above technical solutions of the present invention are only preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention's description and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for preparing samples of the interface bonding properties of fiber reinforced composite materials, characterized in that: The following steps are involved: Sampling the fiber-reinforced composite material to be tested to obtain an initial sample sheet; within the initial sample sheet, the fibers to be tested extend along the thickness direction of the initial sample sheet; The initial sample sheet is thinned by grinding and polishing the two processed surfaces along the thickness to obtain a test sample sheet; the thickness of the test sample sheet is 40~70μm, and the roughness Ra of the two test surfaces along the thickness of the test sample sheet is ≤15nm.

2. The method for preparing samples of the interface bonding performance of fiber reinforced composite materials according to claim 1, characterized in that: In the step of thinning the initial sample sheet by grinding and polishing the two processed surfaces along the thickness of the initial sample sheet, the step of grinding and polishing one processed surface after completing the grinding and polishing comprises: After the initial sample sheet is bonded to the sample loading platform of the precision grinder, a processed surface is sequentially ground and polished by the grinder of the precision grinder; the grinding process is as follows: in the first stage, sandpaper with a mesh size of 600-1000 is used for coarse grinding to a thickness of 1.50 mm; in the second stage, sandpaper with a mesh size of 2000 is used for fine grinding to a thickness of 1.25 mm; The initial sample piece is removed from the sample loading platform, flipped over and fixed on the sample loading platform, and the other processed surface is sequentially ground and polished; the grinding process is as follows: 600-1000 grit sandpaper and 1000-1500 grit sandpaper are used to thin the sample piece to 500 μm and 250 μm respectively, and 2000 grit sandpaper is used in the final grinding stage to thin the initial sample piece to 40-70 μm; The polishing process includes: performing precision polishing on a velvet polishing cloth for ≥30 min.

3. The method for preparing a sample of the interface bonding performance of a fiber reinforced composite material according to claim 1 or 2, characterized in that: During the polishing process, the polishing liquid was sprayed again every 2 minutes.

4. A method for testing the interfacial bonding performance of fiber-reinforced composite materials, characterized in that: The following steps are involved: Fixing the test sample sheet prepared by the sample preparation method according to any one of claims 1 to 3 on a support platform, with the central area of the test sample sheet located above the gap of the support platform; The indenter on the nanoindenter is used to push out the fiber to be tested in the central area, and the force-displacement relationship curve between the indenter and the sample is recorded by a displacement sensor. The slope of the force-displacement curve is analyzed to determine the maximum load to determine the interfacial bonding performance of the fiber-reinforced composite material.

5. The method for testing the interfacial bonding performance of fiber-reinforced composite materials according to claim 4, characterized in that: The fiber to be tested pushed out by the pressure head is the target fiber; the positional relationship of the target fiber satisfies: The target fiber is located at the center of the hexagonal area surrounded by the other six fibers to be tested, wherein the fiber spacing is , r is the fiber radius, the diameter of the target fiber differs from the diameter of the surrounding fibers to be tested by less than 20%; Or, the target fiber exists independently, and the distance between the target fiber and the nearest fiber to be measured is , r is the fiber radius.

6. The method for testing the interfacial bonding performance of fiber-reinforced composite materials according to claim 4, characterized in that: The central area of the test sample piece is a range surrounded by 1 / 2 of the long side and 1 / 2 of the short side of the fiber distribution area of the test sample piece.

7. The method for testing the interfacial bonding performance of fiber-reinforced composite materials according to claim 4, characterized in that: The width of the gap is W = (5-10) d; the depth of the gap is H = 150 ± 50 μm; d is the diameter of the fiber to be measured.

8. The method for testing the interfacial bonding performance of fiber-reinforced composite materials according to claim 4, characterized in that: 0.8 μm<Ra of the supporting platform<1.6 μm.

9. The method for testing the interfacial bonding performance of fiber-reinforced composite materials according to claim 4, characterized in that: 0.5d≤the diameter of the indenter≤0.75d, where d is the diameter of the fiber to be measured.

10. The method for testing the interfacial bonding performance of fiber reinforced composite materials according to any one of claims 4 to 9, characterized in that: In the process of using the indenter on the nanoindenter to push out the fiber to be tested in the central area, when the indenter pushes out the fiber, the loading rate is set to 10 mN / s, and the maximum load is set using an iterative method.

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