Sample preparation and testing methods for interfacial bonding properties of fiber-reinforced composite materials

Abstract

This invention provides a method for preparing and testing samples of the interfacial bonding properties of fiber-reinforced composite materials. The sample preparation method includes: taking a sample from the fiber-reinforced composite material to be tested to obtain an initial sample sheet; extending the fiber to be tested along the thickness direction of the initial sample sheet within the initial sample sheet; and thinning the initial sample sheet by grinding and polishing two surfaces along its thickness to obtain a test sample sheet. In the above-described method for preparing samples of the interfacial bonding properties of fiber-reinforced composite materials, the grinding and polishing process controls the thickness and roughness of the test sample sheet within a suitable range, which helps to form a continuous annular detachment zone at the fiber-matrix contact boundary, reducing the probability of fiber crushing or breakage, thereby improving the test success rate.

Description

Technical Field

[0001] This invention relates to the field of composite material testing technology, and in particular to sample preparation and testing methods for the interfacial bonding performance of fiber-reinforced composite materials. 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 highly on the interfacial bonding properties between the fibers and the resin matrix. Single-fiber ejection testing is a relatively complete method for testing interfacial bonding properties. However, current single-fiber ejection tests are prone to fiber crushing or fragmentation, leading to inaccurate test results and a high failure rate. Summary of the Invention

[0004] The main objective of this invention is to provide a sample preparation and testing method for the interfacial bonding performance of fiber-reinforced composite materials, in order to solve the technical problem that current single-fiber ejection experiments are prone to fiber crushing or fragmentation, which leads to inaccurate test results and a high failure rate.

[0005] To achieve the above objectives, the present invention provides a method for preparing samples of fiber-reinforced composite materials to improve their interfacial bonding properties, comprising the following steps:

[0006] An initial sample sheet is obtained by taking a sample from the fiber-reinforced composite material to be tested. Within the initial sample sheet, the fiber to be tested extends along the thickness direction of the initial sample sheet.

[0007] The initial sample sheet is thinned by grinding and polishing its two surfaces along the thickness direction to obtain a test sample sheet. The thickness of the test sample sheet is 50~100μm, and the roughness Ra of the two test surfaces along the thickness direction is ≤15 nm.

[0008] According to an embodiment of this application, in the step of thinning the initial sample sheet by grinding and polishing two processing surfaces along the thickness aspect, the step of grinding and polishing one processing surface includes:

[0009] After the initial sample piece is bonded to the sample platform of the precision grinder, the surface to be processed is sequentially ground and polished using the grinder. The grinding process is as follows: in the first stage, coarse grinding is performed using sandpaper with a grit between 600 and 1000 grit to a thickness of 1.50 mm. In the second stage, fine grinding is performed using sandpaper with a grit between 2000 and 1000 grit to a thickness of 1.25 mm.

[0010] The initial sample sheet was removed from the sample carrier platform, flipped over, and fixed on the platform. The other processing surface was then successively ground and polished. The grinding process was as follows: 600-800 grit sandpaper and 1000-1500 grit sandpaper were used to thin the sample sheet to 500 μm and 250 μm respectively, and in the final grinding stage, 2000 grit sandpaper was used to thin the initial sample sheet to 40-70 μm.

[0011] The polishing process includes precision polishing on a velvet polishing cloth for ≥30 minutes.

[0012] According to an embodiment of this application, during the polishing process, polishing liquid is sprayed every 2 minutes.

[0013] This application also discloses a method for testing the interfacial bonding performance of fiber-reinforced composite materials, comprising the following steps:

[0014] The test sample piece prepared by the above sample preparation method is fixed on the support platform, and the central region of the test sample piece is located above the gap of the support platform.

[0015] The fiber to be tested is extruded from the central region using an indenter on a nanoindenter, 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 and thus the interfacial bonding performance of the fiber-reinforced composite material.

[0016] According to an embodiment of this application, the fiber to be tested ejected by the pressure head is the target fiber. The positional relationship of the target fibers satisfies:

[0017] The target fiber is located at the center of the hexagonal region formed 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%.

[0018] Alternatively, the target fiber exists independently, and the distance between the target fiber and the nearest test fiber is... , where r is the fiber radius.

[0019] According to an embodiment of this application, the central region of the test sample sheet is the area surrounded by half the long side and half the short side of the fiber distribution region of the test sample sheet.

[0020] According to an embodiment of this application, the width of the gap W = (5-10)d. The gap depth H = 150 ± 50 μm. d is the diameter of the fiber to be tested.

[0021] According to the embodiments of this application, 0.8 μm < Rb of the support platform < 1.6 μm.

[0022] According to the embodiments of this application, 0.5d ≤ the diameter of the pressure head ≤ 0.75d, where d is the diameter of the fiber to be tested.

[0023] According to an embodiment of this application, in the process of using the indenter on the nanoindenter to push out the fiber to be tested in the central region, the loading rate is set to 10 mN / s when the indenter pushes out the fiber, and the maximum load is set using an iterative method.

[0024] In the above-mentioned sample preparation method for the interfacial bonding performance of fiber-reinforced composite materials, the thickness of the test sample sheet and the roughness of the test surface are controlled within a suitable range by grinding and polishing. This helps to form a continuous annular detachment zone at the periphery of the fiber-matrix contact, reducing the probability of fiber crushing or breakage, thereby improving the test success rate. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0026] Figure 1 A flowchart for single-fiber ejection sample preparation and testing;

[0027] Figure 2 This is a schematic diagram of the overall experimental setup;

[0028] Figure 3 This is a schematic diagram of the pressure head structure;

[0029] Figure 4 A schematic diagram of the supporting platform;

[0030] Figure 5 This is a schematic diagram of a manual precision grinder.

[0031] Figure 6 This is an enlarged schematic diagram of the ejected region;

[0032] Figure 7 A partial schematic diagram of the selected single-fiber protrusion region;

[0033] Figure 8 A schematic diagram showing the positional relationship between the target fiber and surrounding fibers;

[0034] Figure 9 A schematic diagram illustrating the preparation process of the MPCF / EP tow composite material proposed in Case 1;

[0035] Figure 10 SEM images of the fibers after ejection, prepared for Example 1; the left image is a SEM image of the holes formed on the front side after fiber ejection; the right image is a SEM image of the ejected fibers.

[0036] Figure 11 Figure 1 shows the IFSS test results of MPCF before and after 20 min of air plasma treatment for desizing, as prepared in Case 1.

[0037] Figure 12 This is a schematic diagram of the silicon wafer support plane in Comparative Example 1;

[0038] Figure 13 The images shown are SEM images of the front side of the fiber after ejection prepared in Comparative Example 1; the left image is an SEM image of the front side of the fiber after ejection at one magnification; the right image is an SEM image of the front side of the fiber after ejection at another magnification.

[0039] Figure 14 SEM image of the side of the test sample prepared in Example 1;

[0040] Figure 15 This is a roughness test image of the side surface of the test sample prepared in Example 1;

[0041] Figure 16 The typical force-displacement curves obtained from the test in Example 2 are shown below.

[0042] Figure 17 SEM image of the side of the test sample prepared in Example 2;

[0043] Figure 18 This is a graph showing the average IFSS values ​​of the samples tested in Example 2;

[0044] Figure 19 The images show 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 right image is the SEM image of the back side.

[0045] Figure 20 SEM image of the side of the test sample prepared for Comparative Example 2;

[0046] Figure 21 The images show 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 right image is the SEM image of the back side.

[0047] The realization of the objective, functional characteristics and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0048] 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 a part of the embodiments of the present invention, and not all of them. 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.

[0049] It should be noted that all directional indicators (such as up, down, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicator will also change accordingly.

[0050] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature.

[0051] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0052] 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 (such as glass fiber / polyester systems with a modulus <90 GPa, and aramid fiber / epoxy systems with a compressive strength <1.5 GPa), reveal systemic defects. First, microcracks generated by conventional mechanical thinning processes severely weaken the fiber-matrix interface, and sample thickness errors lead to load transfer path deviations. Second, the fixed gap width of the universal support platform (3d, where d is the diameter of the fiber under test) causes stress concentration factors to exceed limits in low-modulus fiber testing, resulting in fiber breakage rates exceeding 65%. Third, traditional visual interpretation methods rely on operator experience and cannot accurately identify the maximum load (Fc) at the fiber debonding critical point. Experimental data shows that the failure rate of existing technologies for testing medium- and low-modulus FRPs exceeds 80%, easily leading to fiber crushing or breakage and compromising test results, severely limiting the universality of single-fiber ejection technology.

[0053] This invention provides a method for preparing samples to demonstrate the interfacial bonding properties of fiber-reinforced composite materials, comprising the following steps:

[0054] Step S100: Take a sample from the fiber-reinforced composite material to be tested to obtain an initial sample sheet. Within the initial sample sheet, the fiber to be tested extends along the thickness direction of the initial sample sheet.

[0055] Samples are taken from the fiber-reinforced composite material (such as FRP laminate) to be tested, with the initial sample pieces being as uniform in thickness and flat as possible. The cutting direction is perpendicular to the extension direction of the FRP laminate, so that the fibers to be tested are perpendicular to the two surfaces of the initial sample piece.

[0056] For example, a precision cutting device is used to perform initial cutting and sampling of the fiber-reinforced composite material (such as FRP laminate) to be tested.

[0057] The specific operation is 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 the mechanical limiting device; then, use a precision right-angle calibrator to align its reference surface with the axis of the cutting head and the vertical edge of the sample respectively, and use the rotation fine adjustment mechanism to make the two form a precise 90° orthogonal relationship to ensure the flatness and perpendicularity 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 and to provide a high-quality initial sample piece for subsequent steps.

[0058] In some embodiments, the initial sample sheet thickness is 2 mm. The initial thickness of the fiber-reinforced composite material to be tested is generally relatively thick, such as about 8 cm. Obtaining an initial sample sheet thickness of 2 mm by cutting and sampling facilitates subsequent thinning.

[0059] Step S200: The initial sample sheet is thinned by grinding and polishing the two processing 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 is ≤15 nm.

[0060] The initial sample sheet is individually sanded and polished, either mechanically or manually. During sanding, the treated surface of the initial sample sheet is brought into contact with sandpaper for friction to thin it. Specifically, a series of sandpaper grits can be used, with the grit gradually increasing. First, sand with low-grit sandpaper, then with high-grit sandpaper. After sanding, polishing is performed; this surface can be called the test surface, and its roughness Ra ≤ 15 nm.

[0061] After polishing one surface, the other surface is polished. After processing, the thickness of the test sample is 40-70 μm, and the roughness Ra of the two test surfaces along the thickness direction is ≤15 nm. In some embodiments, the thickness of the test sample is 40-70 μm.

[0062] If the thickness of the test sample sheet is too small, the matrix of the test sample sheet may be too thin, which may not provide sufficient radial constraint and lead to premature failure. If the thickness of the test sample sheet is too large, the fiber may not be completely debonded, and the axial load required for the fiber to push out needs to overcome a larger matrix constraint force. This may lead to mixed interface shear failure modes due to matrix plastic deformation.

[0063] For example, the thickness of the test sample sheet is 50 μm, under which conditions a single fiber ejection experiment is performed on a fiber-reinforced composite material with a resin matrix.

[0064] In the above-mentioned sample preparation method for the interfacial bonding performance of fiber-reinforced composite materials, the thickness of the test sample sheet and the roughness of the test surface are controlled within a suitable range by grinding and polishing. This helps to form a continuous annular detachment zone at the periphery of the fiber-matrix contact, reducing the probability of fiber crushing or breakage, thereby improving the test success rate.

[0065] In some embodiments, the step of thinning the initial sample sheet by grinding and polishing two processing surfaces along the thickness aspect includes, in the step of grinding and polishing one processing surface, the following steps:

[0066] Step S210: After the initial sample piece is bonded to the sample platform of the precision grinder, the surface to be processed is sequentially ground and polished using the grinder of the precision grinder. The grinding process is as follows: In the first stage, coarse grinding is performed using sandpaper with a grit between 600 and 1000 grit to a thickness of 1.50 mm. In the second stage, fine grinding is performed using sandpaper with a grit between 2000 and 1.25 mm. The polishing process includes: precision polishing on a velvet polishing cloth for ≥30 minutes.

[0067] In this step, the present invention employs a manual precision grinder (see details for structure). Figure 5 The initial sample sheet undergoes double-sided thinning. Precision grinders offer higher accuracy and are easier to operate.

[0068] The specific operation is as follows: Position the initial sample piece in the center area of ​​the sample carrier platform, apply 502 adhesive, and then use the blade at a 30° angle to scrape the adhesive layer 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 carrier platform.

[0069] The sample platform is placed into the grinder, and the thinning amount is precisely controlled using the height adjustment knob. Sandpaper or polishing cloth can be mounted on the grinder as needed. Depending on the type of sample, the first stage uses 600-1000 grit sandpaper for coarse grinding to a thickness of 1.50 mm, followed by a second stage using 2000 grit sandpaper for fine grinding to 1.25 mm. After thinning, precision polishing is performed on a velvet polishing cloth for ≥30 minutes, completing the first surface treatment. In some embodiments, polishing fluid is sprayed every 2 minutes during the polishing process.

[0070] Step S220: Remove the initial sample sheet from the sample carrier platform, flip it over, and fix it on the platform. Then, grind and polish the other processing surface sequentially. The grinding process is as follows: use 600-800 grit sandpaper and 1000-1500 grit sandpaper to thin the sample sheet to 500 μm and 250 μm respectively, and use 2000 grit sandpaper to thin the initial sample sheet to 40-70 μm in the final grinding stage.

[0071] In this step, another surface is ground and polished. For example, after heat treatment at 100-120℃ (below the thermal aging temperature of the matrix and fibers) to soften the adhesive (such as 502 adhesive), a non-destructive separation is performed from the upper and lower surfaces of the initial sample sheet using high-elasticity stainless steel curved tweezers. After the initial sample sheet is flipped and fixed, it is thinned to 500 μm and 250 μm respectively using 600-800 grit and 1000-1500 grit sandpaper. In the final grinding stage, 2000 grit sandpaper is used to precisely thin the initial sample sheet to 40-70 μm according to the fiber type.

[0072] 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, replenishing the polishing liquid every 2 minutes. After thinning, immerse the sample platform in acetone solution. When the sample shifts by 0.1-0.3 mm, immediately use bent tweezers to hold and transfer it. Test the surface roughness of the thinned sample; it must reach 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.

[0073] It should be noted that the usual practice to remove 502 adhesive is to soak it in acetone for 5-10 minutes. However, prolonged acetone soaking may cause swelling at the interface of the composite material. In order to minimize the impact of acetone soaking on the interface properties of the composite material, the 502 adhesive is softened by high temperature and quickly softened to remove it after the first grinding. After grinding and polishing, the sample (i.e., the test sample piece) is taken out by soaking it in acetone for 10 minutes.

[0074] In the sample preparation process, a double-sided gradient thinning process was adopted, using 600-2000 grit sandpaper for staged coarse / fine grinding (1.75 mm → 40-70 μm), combined with a sample flipping and fixing strategy to achieve precise thickness control across scales; then, the adhesive was softened by heat treatment at 100-120℃, and the upper and lower surfaces were separated synchronously with high-elasticity stainless steel bent tweezers to avoid mechanical damage.

[0075] This application also discloses a method for testing the interfacial bonding performance of fiber-reinforced composite materials, comprising the following steps:

[0076] Step S10: Fix the test sample piece prepared by the above sample preparation method onto the support platform, and the central region of the test sample piece is located above the gap of the support platform.

[0077] In this step, a nanoindenter is used to perform an ejection experiment on the test sample. The polished test sample is placed perpendicularly above the gap (such as a slit) of the support platform, with the fiber-containing area centered on the support platform. Refer to [reference needed for the positional relationship between the indenter, sample, and support platform]. Figure 6 .

[0078] The test sample sheet is fixed to the support platform using methods such as bonding. During the subsequent ejection process, the pressure head is positioned above the gap to eject single fibers. During fixation, it is necessary to ensure that the central region of the test sample sheet is above the gap on the support platform. This allows the test fibers within the central region to be ejected during the ejection process, systematically eliminating interference from the edge regions. Fibers near the sample edges may be affected by stress concentration at the sample boundary, processing defects, and other factors, causing the test results to deviate from the true situation. Fibers in the central region are in a relatively uniform stress field, which can more accurately reflect the interfacial properties between the fiber and the matrix.

[0079] Specifically, areas at the very edges are highly susceptible to interfacial damage because the edge fibers have a higher probability of contact with the fixture / abrasive during sample grinding. Their surface morphology (such as grooves and microcracks) may suffer irreversible damage due to mechanical wear, leading to a reduction in the actual fiber-resin contact area and a lower IFSS measurement. Furthermore, grinding may cause some damage to the resin in the edge areas, resulting in localized weak interfaces. During ejection, interface debonding occurs earlier than the true strength. Finally, the sample edges are subjected to non-uniform stress during grinding and testing, causing micro-stress concentration at the fiber-resin interface. All these factors can contribute to inaccurate IFSS measurements. Selecting fibers in the central region for single-fiber ejection experiments can reduce the impact of these factors on the accuracy of IFSS measurements.

[0080] Step S20: Use the indenter on the nanoindenter to push out the fiber to be tested in the central region, 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 and determine the interfacial bonding performance of the fiber-reinforced composite material.

[0081] In this step, the maximum displacement and maximum load are set as the conditions for ending the experiment. In some embodiments, the indenter is controlled with a load of 10 mN / s.

[0082] Successful fiber ejection requires two conditions: first, the force-displacement curve must conform to a standard curve, meaning the curve exhibits a clear peak point (brittle interface) or a load plateau region (ductile interface); second, electron microscopy reveals a continuous annular debonding zone at the fiber-matrix contact perimeter, with the fiber exhibiting a certain degree of descent (fiber axial displacement Δz ≥ 0.3d, and the displacement direction deviating from the loading axis by an angle θ ≤ 2°). In the ejection experiment, the maximum load (Fc) before debonding is extracted by analyzing the force-displacement curve. This is based on the inflection point or sudden drop in the slope dF / dx when debonding begins. For brittle interfaces, a sudden drop in slope (dF / dx → 0) corresponds to the peak load Fc. For ductile interfaces, the slope of the force-displacement curve decreases gradually, forming a plateau region. Therefore, the maximum load (Fc) can be determined by the abrupt change in the first derivative dF / dx; when dF / dx decreases to 50% of the slope in the elastic stage, Fc is reached. The IFSS of the composite material can be calculated by substituting the determined Fc into formula (1).

[0083] (1)

[0084] In the formula:

[0085] —Interfacial shear strength, in megapascals (MPa); —Interfacial debonding failure load, in micronewtons (μN); The fiber radius is expressed in micrometers (μm). The thickness is measured in micrometers (μm).

[0086] In this application, the ejection load is effectively determined based on a dual-modal verification mechanism. On the one hand, based on the mechanical response criterion, the debonding initiation point is identified in real time through the force-displacement curve characteristics (peak point of brittle interface / plateau region of ductile interface), and the load peak Fc is accurately located by combining the first derivative dynamic threshold (dF / dx drops to 50% of the elastic slope).

[0087] On the other hand, based on the morphological verification criteria, the ejected fibers must form a continuous annular detachment zone (interfacial debonding integrity) under electron microscopy and satisfy the displacement Δz ≥ 0.3d and the deviation angle θ ≤ 2° (load axis alignment) to avoid errors caused by lateral slippage. The new strategy breaks through the limitations of traditional single mechanical criteria, achieves cross-scale accurate matching of IFSS calculations, and realizes high-precision acquisition of ejection loads of fiber-reinforced polymer composites.

[0088] In the above-mentioned test method for the interfacial bonding performance of fiber-reinforced composite materials, by grinding and polishing, the thickness of the test sample sheet and the roughness of the test surface are controlled within an appropriate range, which helps to form a continuous annular detachment zone at the periphery of the ejected fiber-matrix contact, reducing the probability of fiber crushing or breakage, thereby improving the test success rate.

[0089] In some embodiments, the fiber to be tested ejected by the pressure head is the target fiber. The positional relationship of the target fibers satisfies:

[0090] The target fiber is located at the center of the hexagonal region formed 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%.

[0091] Alternatively, the target fiber exists independently, and the distance between the target fiber and the nearest test fiber is... , where r is the fiber radius.

[0092] To be selected as a target fiber, the selected fiber must possess one of the following two positional relationships. First, the target fiber must be arranged in a hexagonal center-symmetric configuration. This arrangement requires the fiber insertion positions to maintain symmetry and a uniform stress field distribution. The surrounding fibers, arranged in a hexagonal symmetry, form a uniform matrix support network, making the matrix constraint conditions of the central fiber nearly ideal (isotropic) and avoiding asymmetric stress concentration. Furthermore, in actual composite materials, fibers are often regularly / randomly distributed, and the test results of the central fiber more closely resemble the interface behavior under real service conditions. This method can simulate the structure of actual composite materials. Specific parameter requirements include fiber spacing. (r is the fiber radius) to avoid insufficient matrix wetting or stress overlap due to excessively small spacing; and the difference between the target fiber diameter and the surrounding fiber diameter should be less than 20%.

[0093] The second positional relationship is that the target fiber exists independently, and the fiber spacing is... (r is the fiber radius). See also the two positional relationships. Figure 8This 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 relatively well.

[0094] In some embodiments, the central region of the test sample sheet is the area enclosed by half the long side and half the short side of the fiber distribution region of the test sample sheet.

[0095] In this embodiment, during fiber ejection, fibers within a range encompassing half of the long side (L1) and half of the short side (L2) of the fiber distribution area are selected and ejected using an optical microscope. The specific regions are illustrated in the diagram. Figure 7 This area is located in the center of the test sample, thus further improving the accuracy of the measurement results.

[0096] In some embodiments, the width of the gap W = (5-10)d. The gap depth H = 150 ± 50 μm. d is the diameter of the fiber to be tested.

[0097] In related technologies, a general-purpose 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 the low modulus fiber test, and the fiber breakage rate is as high as 65% or more.

[0098] The width and depth of the gap in this application have been optimized. When the gap width is insufficient, the fiber is prone to contact with the edge of the support structure during ejection, and the contact stress causes plastic deformation of the fiber surface; an excessively large gap width results in insufficient effective support length of the fiber, causing asymmetric bending under axial load and increasing experimental error. Please refer to the structural schematic diagram of the support platform. Figure 4 .

[0099] In some embodiments, 0.8 μm < Rb of the support platform < 1.6 μm.

[0100] For example, the support platform is made of austenitic stainless steel (06Cr19Ni10) and precision machined, combined with mirror polishing (0.8μm < Ra < 1.6μm), which significantly reduces fiber contact friction damage and improves the platform's corrosion resistance and long-term stability.

[0101] For example, the support platform employs femtosecond laser micromachining technology, using a femtosecond laser to form a high-precision rectangular gap on the working surface of the support platform, achieving micron-level structural control (width W=5-10d, depth H=150±50 μm), avoiding edge burrs and heat-affected zones caused by traditional machining. Through triple innovation in materials, processes, and structure, the problems of fiber damage and data deviation caused by rough machining and mismatched slit widths in traditional support platforms are solved, providing a highly reliable hardware foundation for microscale fiber interface performance testing.

[0102] In some embodiments, 0.5d ≤ the diameter of the indenter ≤ 0.75d, where d is the diameter of the fiber to be tested.

[0103] The size of the indenter is determined by the diameter of the fiber being tested, with the indenter diameter D between 1 / 2 and 3 / 4 of the fiber diameter. If the indenter is smaller than the fiber diameter, the pressure is concentrated in the central area of ​​the fiber, causing the fiber to be "broken" rather than ejected intact. If the indenter diameter is too large, the edge of the indenter will contact the resin matrix, tearing off some resin during ejection, leading to a mixture of interfacial debonding and matrix failure, resulting in an inflated measured interfacial strength value. Please refer to the schematic diagram of the indenter structure. Figure 3 .

[0104] In some specific embodiments, the pressure head is flat and the sidewall of the pressure head is configured with a curved transition structure to achieve precise axial force application to the fiber.

[0105] Based on the dynamic matching of the indenter size (D=1 / 2-3 / 4d) with the diameter of the fiber to be tested, the lower limit of the indenter diameter (≥0.5d) avoids excessive stress concentration, ensures uniform transmission of ejection force, and inhibits brittle fiber fracture; the upper limit of the indenter diameter (≤0.75d) avoids contact between the edge of the indenter and the resin matrix, eliminates the coupling effect of interface debonding and matrix damage, and ensures universal matching of the pure shear mode for interface strength testing. Through parametric design (D / d=0.5-0.75), it adapts to fibers of different diameters (5-50μm) and solves the data distortion problem caused by size mismatch of traditional fixed-size indenters.

[0106] In some embodiments, during the ejection of the fiber under test in the central region using the indenter on a nanoindenter, the loading rate is set to 10 mN / s. The maximum load is determined using an iterative method: a 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 exceed Fc by 50–100 mN; if unsuccessful, the maximum load is increased incrementally in increments of 50 mN until ejection is complete.

[0107] The technical solution of the present invention will be further explained below with reference to the accompanying drawings and specific implementations.

[0108] Example 1

[0109] Please see Figures 9-11 Example 1 provides a method for preparing and testing an MPCFRP single fiber ejection experiment, including the following steps:

[0110] Step 1: Sample Preparation

[0111] Please refer to the sample preparation process. Figure 9Mesophase pitch-based carbon fiber (MPCF), after surface slurry removal, was placed in a plasma cleaner. Air was used as the treatment gas, the treatment power was 150 W, and the treatment time was 20 minutes, controlled at a pressure of 20 Pa. In a vacuum environment, atoms and their combinations (atomic clusters) in the air were ionized into a plasma state. This plasma then directly contacted the surface of the carbon fiber, using its high-energy properties to perform physicochemical reactions such as etching and oxidation. The MPCF fiber bundles were then cut into samples with a length of 150 mm and a width of 2 mm and stored properly for later use. The mold was then cleaned with anhydrous ethanol to ensure its surface was free of impurities. A release agent was repeatedly applied to the mold surface to ensure smooth demolding. One end of the carbon fiber was firmly adhered to a groove at one end of the mold using 502 adhesive, while a 50 g iron block was placed at the other end. A constant tension is applied to the fiber bundle to ensure it remains parallel to the mold. The other end is then secured again with 502 adhesive, and excess fiber strands are trimmed from both ends. Following the standard epoxy resin curing procedure, 50.00 g of epoxy resin and 10.85 g of IPDA are accurately weighed, with a ratio of 100.0:21.7. During the process, IPDA is slowly dripped into E51, and after initial stirring with a glass rod, the mixture is placed in a homogenizer and vacuumed to remove air bubbles and achieve a homogeneous state. The mold containing the fiber bundle is preheated in an oven at 80°C for 30 minutes. Then, using a glass rod as a guide, the uniformly mixed E51 is slowly and continuously injected into the grooves of the mold, ensuring the resin completely covers and fills the grooves. The mold is then placed in the oven for curing. The curing program is set as follows: first, maintain the temperature at 80°C for 60 minutes, then increase the temperature to 120°C and maintain it for 60 minutes, and finally increase the temperature to 150°C and maintain it for 120 minutes to complete the entire curing process.

[0112] Step 2: Sample thinning and polishing

[0113] During initial cutting, an automated precision cutter was used to cut the cured carbon fiber composite material into small pieces with a thickness of 2 mm. The samples were then double-sided polished using a manual precision grinder, applying 800, 1200, and 2000 grit sandpaper to a thickness of 1.5 mm. Following this, manual polishing was performed on a microscope polisher at 350 r / min, applying polishing fluid every minute for 15 minutes. The samples were then heated in an oven to soften the 502 adhesive, and the polishing process was repeated on the other side. The final sample thickness was reduced to 50 μm. The sample thickness was then imaged using an electron microscope. Figure 14As shown in the figure, the sample thickness is 58.77 μm, accurately controlled between 40-70 μm, and the thickness is uniform with smooth upper and lower surfaces, indicating that the thinning process proposed in this invention can achieve the expected results. The sample roughness is as follows. Figure 15 As shown, the Ra of the sample is ≤10 nm.

[0114] Step 3: Single fiber ejection experiment

[0115] Single-fiber ejection experiments were conducted using a nanoindenter. The polished sample was fixed to a support platform using pressure-sensitive tape. The sample position was adjusted so that the slit was located at the center of symmetry, with the fiber axis perpendicular to the platform. A flat indenter with a diameter of 5 μm was used. The support platform had a diameter of 3 mm and a slit with a width W = 100 μm and a gap depth H = 150 ± 50 μm at its center. (In fiber-reinforced composites, the fiber diameter is generally 10-20 μm, therefore, this slit width satisfies the condition W = (5-10)d for most fibers). The loading rate was set to 10 mN / s when the indenter ejected the fiber. 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, and the slope of the force-displacement curve was analyzed to determine the maximum load (Fc).

[0116] Experimental Results and Analysis

[0117] To observe the fiber state after ejection using a scanning electron microscope (SEM), please refer to [link to relevant documentation]. Figure 10 MPCF was successfully debonded, with no significant damage to the matrix or fibers, and no obvious crack propagation at the interface. Please refer to the IFSS test results for MPCF after 20 min of air plasma treatment before and after desizing. Figure 11Five effective ejection tests were performed on each sample, and the effective IFSS was the average of the five results. The IFSS values ​​before desizing (untreated), after desizing (treated for 20 min), after desizing (untreated), and after desizing (treated for 20 min) were 18.19 MPa, 18.60 MPa, 16.20 MPa, and 20.46 MPa, respectively. The IFSS of the untreated MPCF composite did not significantly improve after plasma treatment. This is because the presence of commercial sizing agent on the fiber surface causes the air plasma to preferentially contact the sizing agent, severely affecting the modification effect of the plasma, thus resulting in a minimal increase in IFSS. 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 surface roughness of the fiber, worsens the wettability with the resin, and weakens the interfacial bonding between the fiber and the resin. After desizing, the IFSS of the MPCF significantly improved after plasma treatment, reaching 20.46 MPa, an increase of 26.2% compared to the untreated 16.2 MPa. Air plasma treatment effectively improves the interfacial bonding strength between fibers and resins by increasing fiber surface roughness and chemical activity.

[0118] Summary of Experimental Results

[0119] Through single-fiber ejection experiments, this invention successfully verified that plasma treatment alters the surface roughness and chemical activity of MPCF through physical etching and chemical reactions, thereby improving the interfacial bonding strength between the fiber and the resin. During the experiment, the fiber successfully detached and ejected from the matrix without significant damage, verifying the reliability of the experimental method and the high quality of sample preparation.

[0120] Comparative Example 1

[0121] Compared to Example 1, steps 1 and 2 are exactly the same. In step 3, except for the support platform, the rest of the operation and experimental equipment are identical. In Comparative Example 1, the support platform is designed as silicon wafers. During the experiment, the sample is bonded to two silicon wafers using crystal adhesive. A pore is left between the silicon wafers. The size of the pore can be adjusted by moving the relative positions of the silicon wafers to facilitate fiber ejection. Please refer to the schematic diagram of the sample placed on this support platform. Figure 12 The platform made using this method is simple to operate and easy to use. However, it is impossible to precisely control the gap width W and gap depth H between silicon wafers, which introduces considerable uncertainty into the experiment and can easily lead to overall plastic damage to the sample during ejection.

[0122] Please refer to the SEM image of the sample after ejection in Comparative Example 1. Figure 13 As shown in the figure, after ejection, the fiber is crushed by the pressure head and deformed by compression, making it impossible to eject successfully. At this point, it is impossible to determine when the fiber debonds from the resin.

[0123] Based on Example 1 and Comparative Example 1, it can be seen that the support platform proposed in this invention can significantly improve the fiber ejection effect and greatly avoid the occurrence of fiber breakage.

[0124] Example 2

[0125] Step 1: Sample Preparation

[0126] Based on factory-sized quartz fiber (QF), laminates were prepared by combining it with polyimide (PI) using compression molding. Subsequently, thermo-oxidative aging was carried out at 350 °C for two service times: 40 h and 80 h.

[0127] Step 2: Sample thinning and polishing

[0128] The QF / PI laminate composite material prepared in the previous step was initially cut using a precision cutting machine to ensure that each small piece of composite material was 2 mm thick. Then, an MDG1 manual precision grinder was used to thin the sample by double-sided grinding. During grinding, the sample was adhered to the sample stage using 502 adhesive. The first side was ground to 1.25 mm with 800-grit and 2000-grit sandpaper, and polished for ≥30 minutes. The sample was then removed. The sample was then ground to 500 μm with 800-grit sandpaper, to 250 μm with 1500-grit sandpaper, and finally to approximately 50 μm with 2000-grit sandpaper, followed by polishing for ≥30 minutes.

[0129] Step 3: Single fiber ejection experiment

[0130] A single fiber ejection experiment was conducted using the nanoindenter (with consistent indentation head) and support platform described in Example 1. The loading rate was set to 10 mN / s, and the maximum load was determined to be 240 mN using an iterative method.

[0131] Typical force-displacement curves obtained are as follows Figure 16 As shown. The thickness of the sample was photographed using an electron microscope, as shown. Figure 17 As shown in the figure, the sample thickness is 60.23 μm, accurately controlled between 40-70 μm, and the thickness is uniform with smooth upper and lower surfaces, indicating that the thinning process proposed in this invention can achieve the expected results. Subsequently, the average IFSS values ​​of the untreated, 40-hour heat-aged, and 80-hour heat-aged samples were measured, as shown in the figure. Figure 18 As shown, this result is consistent with the thermal aging behavior of QF / PI composite materials.

[0132] Finally, the debonding morphology of the front and back sides of the sample was judged, such as... Figure 19As shown in the figure. Microscopic observation shows that the sample achieved uniform interfacial debonding in the matrix. The debonding interface morphology was clear and the separation gap was moderate. The fiber axial orientation remained perpendicular. 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.

[0133] Specifically, Figure 16 These are typical force-displacement curves for a QF / PI laminate composite sample that has been in service for 40 hours. Figure 17 This is a SEM image of the side of a QF / PI laminate composite material sample that has not undergone thermal aging. Figure 18 19 shows the interfacial shear strength diagrams of the original, 40h service, and 80h service laminate composite material samples, and 10 shows the ejection morphology diagram of the original unheated QF / PI laminate composite material sample.

[0134] Comparative Example 2:

[0135] Step 1: Sample preparation. Except for not aging, the process is the same as in Example 2.

[0136] Same as Example 2.

[0137] Step 2: Sample thinning and polishing.

[0138] The experimental procedure was as follows: The QF / PI laminate composite material prepared in the previous step was initially cut using a precision cutting machine to ensure that each small piece of composite material was 2 mm thick. Then, an MDG1 manual precision grinder was used to thin the sample by double-sided grinding. During grinding, the sample was adhered to the sample stage using 502 adhesive. The first side was ground to 1.25 mm with 800-grit and 2000-grit sandpaper, and polished for ≥30 minutes. The sample was then removed. The sample was then ground to 500 μm with 800-grit sandpaper, to 250 μm with 1500-grit sandpaper, and finally to approximately 100 μm with 2000-grit sandpaper, followed by polishing for ≥30 minutes.

[0139] Step 3: Single fiber ejection experiment

[0140] A single-fiber ejection experiment was conducted on the needle, with the loading rate set to 10 mN / s. Due to the instrument's maximum load protection limit, the load was determined to be 400 mN using an iterative method, but no debonding point still appeared.

[0141] The thickness of the sample is captured by electron microscopy, such as... Figure 20 As shown, the sample thickness is 96.41 μm, exceeding the reasonable ejection thickness. The debonding morphology of the sample is then assessed, as follows: Figure 21 As shown.

[0142] Microscopic observations revealed that the sample exhibited multi-level fragmentation characteristics within the matrix, accompanied by the expansion of the matrix's plastic deformation zone. The fragmentation interface induced local buckling deformation in the matrix and triggered delamination failure at the fiber / matrix interface, with fiber fragmentation occurring within the matrix.

[0143] The failures caused by excessively thick samples are mainly due to the following two reasons. First, the fiber aspect ratio is unbalanced. If the fiber embedment length is too large, it is prone to initial bending due to residual stress or processing defects. During ejection, bending stress concentration is induced, causing the fiber to bend and easily break in the matrix. Second, long fibers tend to cause a surge in debonding load. The interfacial frictional resistance is often positively correlated with the fiber embedment length (i.e., sample thickness). When the thickness is too large, the required debonding force often exceeds the limit of the nanoindenter equipment, leading to experiment termination or fiber crushing.

[0144] The above technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made under the technical concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A method for testing the interfacial bonding performance of fiber-reinforced composite materials, characterized in that, The following steps are involved: The test sample piece prepared by the sample preparation method is fixed on the support platform, and the central region of the test sample piece is located above the gap of the support platform. The fiber to be tested is extruded from the central region using an indenter on a nanoindenter, 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 and thus the interfacial bonding performance of the fiber-reinforced composite material. The reinforcing fiber material in the fiber-reinforced composite material is glass fiber, carbon fiber, or aramid fiber, and the matrix material is synthetic resin. The fiber to be tested protruding from 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 region formed by the other six fibers to be tested, wherein the fiber spacing is... , where r is the fiber radius, and the diameter of the target fiber differs from the diameter of the surrounding fibers being measured by less than 20%; Alternatively, the target fiber exists independently, and the distance between the target fiber and the nearest test fiber is... r is the fiber radius; The sample preparation method includes the following steps: A sample is taken from the fiber-reinforced composite material to be tested to obtain an initial sample sheet; within the initial sample sheet, the fiber to be tested extends along the thickness direction of the initial sample sheet; The initial sample sheet is thinned by grinding and polishing the two surfaces along the thickness direction to obtain the 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 direction is ≤15 nm. In the step of thinning the initial sample sheet by grinding and polishing two processing surfaces along the thickness direction, the step of grinding and polishing one processing surface includes: After the initial sample piece is bonded to the sample platform of the precision grinder, the surface to be processed is successively ground and polished by the grinder of the precision grinder. The grinding process is as follows: in the first stage, coarse grinding is performed with sandpaper with a grit between 600 and 1000 grits to a thickness of 1.50 mm; in the second stage, fine grinding is performed with sandpaper with a grit between 2000 grits to a thickness of 1.25 mm. The initial sample sheet was removed from the sample carrier platform, flipped over, and fixed on the sample carrier platform. The other processing surface was then sanded and polished in sequence. The sanding process was as follows: the initial sample sheet was thinned to 500 μm and 250 μm respectively using 600-1000 grit sandpaper and 1000-1500 grit sandpaper, and in the final sanding stage, the initial sample sheet was thinned to 40-70 μm using 2000 grit sandpaper. The polishing process includes: performing precision polishing on a velvet polishing cloth for ≥30 minutes, during which polishing liquid is sprayed every 2 minutes.

2. The test method for the interfacial bonding performance of fiber-reinforced composite materials according to claim 1, characterized in that, The central region of the test sample sheet is the area enclosed by half the long side and half the short side of the fiber distribution area of ​​the test sample sheet.

3. The test method for the interfacial bonding performance of fiber-reinforced composite materials according to claim 1, characterized in that, The width of the gap is W = (5-10)d; the gap depth is H = 150 ± 50 μm; and d is the diameter of the fiber to be tested.

4. The test method for the interfacial bonding performance of fiber-reinforced composite materials according to claim 1, characterized in that, 0.8 μm < Ra of the support platform < 1.6 μm.

5. The test method for the interfacial bonding performance of fiber-reinforced composite materials according to claim 1, characterized in that, 0.5d ≤ diameter of the indenter ≤ 0.75d, where d is the diameter of the fiber to be tested.

6. The test method for the interfacial bonding performance of fiber-reinforced composite materials according to any one of claims 1 to 5, characterized in that, In the process of using the indenter on the nanoindenter to push out the fiber to be tested in the central region, the loading rate is set to 10 mN / s when the indenter pushes out the fiber, and the maximum load is set using an iterative method.

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