Method and device for testing compression performance of thick-section composite material

Abstract

The application provides a kind of thick section composite compression performance test method and test device, it is related to test or analysis material technical field, including the following steps: design test piece size and determine working section length;Estimate failure load and calculate minimum friction load;Determine the groove parameter of netted groove, calculate the comprehensive friction coefficient of clamping surface and calibration correction, calculate minimum pre-tightening force;According to pre-tightening force, determine bolt diameter and tightening torque, and then determine test piece clamping length and total length;Prepare test piece and process netted groove, apply tightening torque in stages so that end face parallelism meets requirements;Apply axial compression load, collect load-displacement and load-strain data, calculate compression strength and compression modulus and determine failure mode;Real-time monitoring and abnormal early warning are carried out during testing.The problem that thick section composite compression performance test is prone to premature abnormal failure is solved, and its compression strength and compression modulus can be accurately and reliably obtained.

Description

Technical Field

[0001] This invention relates to the field of materials testing or analysis, and specifically to a method and apparatus for testing the compressive properties of thick-section composite materials. Background Technology

[0002] The application of advanced composite materials in aircraft structures is a major technical approach to achieving structural lightweighting, with the key being to increase the application of composite materials in primary load-bearing structures. Currently, the application of composite materials in aircraft structures in my country has gradually expanded from secondary load-bearing structures such as movable surfaces to primary load-bearing structures such as wings and fuselages. Among these, the primary load-bearing structure of the wing is subjected to large loads and complex stress states, and its thick cross-section is its main characteristic, especially the root of the upper wing surface, which needs to withstand large compressive loads.

[0003] Conventional methods for obtaining the compressive properties of composite materials mainly include domestic and international standards such as GB / T2856, GB / T5258, ASTM D3410, ASTM D695, ASTM D6641, and SACMA1R-94. These test methods are only applicable to thin-walled composite materials with a thickness of 2 mm or less, and are not suitable for thick-section composite materials with a thickness of 15 mm to 40 mm. This is mainly reflected in:

[0004] Firstly, thick-section composite materials are subject to higher failure loads (approximately 100 to 200 tons). Inappropriate specimen configuration and loading methods can easily cause premature failure of the specimen at the loading end or the root of the clamping section, resulting in unexpected damage. Secondly, numerous experimental results show that when the thickness of the test specimen exceeds 10 mm, the compressive strength of the composite material is strongly correlated with its thickness. Uneven curing temperature / pressure in thick-section structures exacerbates the accumulation of process defects, leading to a decline in compressive performance. Therefore, there is an urgent need to develop a test method for the compressive performance of composite materials with thicknesses greater than 10 mm to accurately characterize the compressive strength and stiffness of thick-section composite materials.

[0005] Currently, the domestic composite material mechanical property testing industry lacks mature technologies and standard specifications for testing the compressive properties of thick-section composite materials, presenting numerous intractable technical problems: the design of test specimen configuration and dimensions lacks a basis; the testing device configuration is unreasonable; key test parameters (such as the friction coefficient of the clamping surface and tightening torque) cannot be quantitatively determined; damage initiation and off-center loading conditions cannot be monitored in real time during testing; and the lack of effective means to determine failure modes after failure makes it impossible to confirm the validity of the test; due to the influence of end friction constraints, the measured values ​​of compressive modulus are generally too high and cannot be corrected. These problems often lead to premature abnormal failure of test specimens in non-working sections, making it impossible to obtain the true compressive strength and compressive modulus, severely restricting the application and verification of thick-section composite materials in aerospace main load-bearing structures.

[0006] Therefore, there is an urgent need in this field for a method and supporting apparatus for testing the compressive properties of thick-section composite materials that can systematically solve the above problems. Summary of the Invention

[0007] In view of this, the embodiments of this specification provide a test method and test device for the compressive properties of thick cross-section composite materials, so as to accurately and reliably obtain the compressive strength and compressive modulus of thick cross-section composite materials, effectively avoid premature abnormal failure of test specimens during the test process, and achieve the purpose of real-time monitoring of the test process and determination of failure mode.

[0008] The embodiments in this specification provide the following technical solutions: A method for testing the compressive properties of thick-section composite materials includes the following steps: Obtain the thickness parameters, layup method, and intrinsic material parameters of the composite material to be tested. Determine the width of the test specimen based on the thickness parameters, determine the thickness of the test specimen based on the layup method, and determine the working section length of the test specimen to ensure that the test specimen meets the Euler buckling stability constraint. Based on the layup method and intrinsic material parameters, the predicted failure load of the test specimen under compressive load is estimated, and the minimum friction load required by the testing device is calculated based on the predicted failure load. Based on the layup method and intrinsic material parameters, the groove parameters of the textured groove are determined, the comprehensive friction coefficient between the clamping surface of the clamping block assembly and the test piece is calculated, the comprehensive friction coefficient is corrected, and the minimum preload required to achieve the minimum friction load is calculated based on the corrected comprehensive friction coefficient. The nominal cross-sectional area of ​​the tightening bolt is determined based on the minimum preload, and the minimum tightening torque is calculated. The clamping length and total length of the test piece are determined based on the nominal diameter of the tightening bolt. The test piece is prepared based on the width, thickness, working section length, clamping length, and total length. A textured groove is machined on the surface of the clamping area of ​​the clamping block group based on the groove parameters. The test piece is installed in the testing device, and the minimum tightening torque is applied multiple times in a preset sequence to clamp the test piece and meet the end face parallelism requirements. An axial compressive load is applied to the test specimen until it is destroyed. Load-displacement data and load-strain data are collected in real time. The compressive strength and compressive modulus of the thick-section composite material are calculated based on the load-displacement data and load-strain data. The failure mode is determined. If the failure mode is determined to be abnormal, the test is marked as invalid.

[0009] Furthermore, based on the layup method and intrinsic material parameters, the groove parameters of the knitted groove are determined, the comprehensive friction coefficient between the clamping surface of the clamping block assembly and the test piece is calculated, and the comprehensive friction coefficient is corrected, including: The first ply angle of the surface layer and the second ply angle of the subsurface layer of the test specimen are obtained from the plying method. The ply skew compensation angle is determined based on the first ply angle and the second ply angle. The angle θ between the extension direction of the reticulated groove and the fiber direction of the surface layer of the test specimen is determined based on the ply skew compensation angle. Along the surface of the clamping area of ​​the test piece, the groove is divided into different sections, and the groove depth and groove spacing corresponding to each section are determined. The comprehensive friction coefficient of the groove is determined based on the groove depth and groove spacing. The friction coefficient of the groove is calibrated using a standard test piece made of the same material as the test piece. The actual friction force is measured under simulated clamping pressure, and the dynamic friction coefficient is calculated. If the deviation between the dynamic friction coefficient and the comprehensive friction coefficient exceeds the deviation threshold, the parameters of the groove are adjusted and the comprehensive friction coefficient is corrected.

[0010] Further, the first ply angle of the surface layer and the second ply angle of the subsurface layer of the test specimen are obtained from the ply configuration. A ply skew compensation angle is determined based on the first and second ply angles. The angle θ between the extension direction of the weave grooves and the fiber direction of the test specimen surface layer is determined based on the ply skew compensation angle, including: The surface compensation coefficient p and the subsurface compensation coefficient q are determined by finite element simulation or experimental calibration. The surface compensation coefficient p is used to compensate for the effect of the surface fiber direction deviating from 90° on the included angle θ, and the subsurface compensation coefficient q is used to compensate for the effect of the subsurface fiber direction deviating from 0° on the included angle θ. The ply skew compensation angle δ is calculated using a linear model, where δ = p·(α-90°) + q·(β-0°), α is the first ply angle, and β is the second ply angle. The angle θ between the extension direction of the weave groove and the direction of the surface fiber of the test piece is calculated by the layup skew compensation angle δ, where the angle θ is 20°~40°.

[0011] Furthermore, the textured groove is divided into different sections along the surface of the clamping area of ​​the test piece, and the groove depth and spacing for each section are determined. The overall friction coefficient of the textured groove is then determined based on the groove depth and spacing, including: The groove is divided sequentially along the clamping direction of the test piece into a first section near the end of the test piece, a second section away from the end of the test piece, and a transition section between the first and second sections. The groove depth and groove spacing of the first section, the transition section and the second section are determined respectively, wherein the groove depth of the transition section is gradually distributed. Calculate the ratio λ of the groove depth to the groove spacing. In the first section, the first ratio λ1 ≥ the first threshold, and in the second section, the second ratio λ2 ≤ the second threshold. The first threshold is greater than the second threshold. In the transition section, the ratio λ changes continuously along the clamping direction and has a linear relationship with the position. The linear relationship is that it changes linearly from the first ratio λ1 to the second ratio λ2. Based on the ratio λ of each segment of the groove, the contribution of each segment to the overall friction coefficient is calculated. The overall friction coefficient of the entire groove is obtained by weighted averaging or integration along the clamping direction. The overall friction coefficient is positively correlated with the ratio λ.

[0012] Furthermore, the minimum tightening torque is applied multiple times in a preset sequence to clamp the test piece and ensure that the end face parallelism requirements are met, including: The test piece is placed in the testing device and positioned by the limiting strip; Apply a first preset percentage and measure the parallelism deviation between the bottom end face of the first clamping block group and the top end face of the second clamping block group. If the deviation exceeds the allowable tolerance, it is corrected by locally adjusting the torque of the corresponding bolt. Apply to the second preset percentage, measure again, and correct. Apply minimum tightening torque, then measure parallelism to ensure the deviation is within the allowable tolerance. During the application of torque, the strain sensor attached to the surface of the clamping section of the test piece is monitored simultaneously, and the strain deviation between each strain measuring point is calculated. When the deviation is less than a predetermined threshold, the clamping is determined to be uniform and effective. Record the torque value, parallelism data, and strain distribution data for each applied torque to generate an installation quality traceability file.

[0013] Furthermore, the compressive strength and compressive modulus of the thick-section composite material are calculated based on load-displacement data and load-strain data, including: Load-longitudinal strain data and load-transverse strain data are separated from the load-strain data, wherein the load-longitudinal strain data and load-transverse strain data are measured simultaneously by a biaxial strain gauge array; Based on the load-longitudinal strain data, a linear segment between the compressive load and the load-longitudinal strain data is extracted, and the slope ΔP / Δε of the linear segment is calculated. l Where ΔP is the increment of the compressive load, and Δε l This represents the longitudinal strain increment corresponding to ΔP; The initial compressive modulus E0 = (ΔP / Δε) was obtained by fitting using the least squares method. l ) / A, where A is the cross-sectional area of ​​the test specimen; Based on the load-transverse strain data and the load-longitudinal strain data, Poisson's ratio ν = -ε is calculated simultaneously.t / ε l , where ε t For load-lateral strain data, ε l Load-longitudinal strain data; Based on the initial compression modulus, the width and thickness of the test specimen, the end constraint correction factor β = 1 + k × (ν - ν0) × (t / W) is calculated, where k is an empirical coefficient, ν0 is the reference Poisson's ratio, t is the thickness of the test specimen, and W is the width of the test specimen. The end constraint correction factor is used to correct the measurement deviation of the compression modulus caused by the frictional constraint between the end of the test specimen and the clamp. The final compressive modulus E is calculated using the end constraint correction factor and the initial compressive modulus. c =E0×β; Poisson's ratio is calculated in real time during the test. When the Poisson's ratio changes by more than 20% relative to the initial value, it is determined that interlayer damage has started, and the current load level is taken as the damage initiation point.

[0014] Furthermore, the destruction mode is determined, including: Collect image data of the test specimen after failure and load-displacement data during the failure process; The failure location is identified from the image data. If the failure location is within the length of the working section of the test piece, the failure location is marked as normal. If the failure location is located in the clamping section or loading end face of the test piece, the failure location is marked as abnormal. The curve features of the failure moment are extracted from the load-displacement data, and the load peak point is extracted based on the curve features. Taking the load peak point as the boundary, the first predetermined number of data points before the load peak point are fitted and calculated to obtain the first slope k1, and the second predetermined number of data points after the load peak point are fitted and calculated to obtain the second slope k2. If the absolute value of the slope k2 in the latter part is less than the absolute value of the slope k1 in the former part and the curve shows a monotonous decrease after the load peak point, it is marked as a normal curve. If the load decrease between adjacent data points at the load peak point exceeds the predetermined load threshold, or if a second load peak appears after the load peak point and the amplitude of the second load peak exceeds the predetermined percentage of the load peak point, it is marked as an abnormal curve. If the position and curve are normal, it is determined to be normal compression failure and the test is valid. If the position or curve is abnormal, it is determined to be abnormal failure, the test is marked as invalid and the failure type is recorded as clamping segment shear failure or end face splitting failure. Output the judgment result and damage type identifier.

[0015] Furthermore, it also includes: When applying axial compressive load to the test specimen, real-time monitoring and abnormal warnings are provided, including: Acoustic emission sensors are installed on the surface of the clamping block or the end of the test piece to collect acoustic emission signals in real time. When the acoustic emission event rate or energy exceeds the first threshold, a damage warning is issued. The slope of the sliding window of the load-displacement curve is calculated in real time. When the rate of decrease of the sliding window slope relative to the slope of the initial linear segment exceeds the second threshold, a loading stop signal is triggered, a damage continuation warning is issued, and the current load is recorded as the damage critical value. The strain non-uniformity is monitored in real time by the end face strain sensor. When the strain non-uniformity exceeds the third threshold, an end face off-center load warning is issued, and the fixture is adjusted or the test is stopped. When a damage warning and / or a damage persistence warning and / or an end face off-center load warning are triggered, reduce the rate of application of the axial compressive load or stop loading, and save all test data at the trigger time, marking it as an abnormal termination.

[0016] A testing apparatus for the compressive properties of thick-section composite materials, used to perform a testing method, comprising: Clamping block assembly, locating pin, axial pin, limit strip, limit strip tightening screw and tightening bolt; The clamping block assembly includes a first clamping block assembly and a second clamping block assembly arranged opposite to each other, and is used to clamp the upper and lower ends of the test piece; The contact surfaces between the clamping blocks and the test piece are all provided with textured grooves. The locating pin is interference-fitted with the locating hole of the first clamping block group and clearance-fitted with the locating hole of the second clamping block group. The lower part of the axial pin is interference-fitted with the first clamping block assembly; Limiting strips and limiting strip tightening screws are set on both sides of the first clamping block assembly and are used for centering and positioning of the test piece; The tightening bolt passes through the tightening hole of the second clamping block group and is threadedly connected to the tightening hole of the first clamping block group. The tightening bolt is used to apply tightening torque.

[0017] Furthermore, the testing device also includes a biaxial strain gauge assembly, an acoustic emission sensor, and an end-face strain sensor; The biaxial strain gauge array is symmetrically arranged in the working section of the test specimen, and the distance between the location of the array and the two end faces of the working section is not less than the thickness of the test specimen. The acoustic emission sensor is disposed on the side of the first clamping block group or the second clamping block group; The end-face strain sensor is installed on the loading end face of the test piece; The edges of the first and second clamping blocks corresponding to the working section of the test piece are rounded.

[0018] Compared with the prior art, the beneficial effects that at least one technical solution adopted in the embodiments of this specification can achieve include at least: By designing and combining the friction coefficient of the groove, controlling the parallelism of the end face, and determining the failure mode, the problem of premature abnormal failure in the compression test of thick cross-section composite materials was solved, and the true compressive strength and compressive modulus were reliably obtained. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a structural diagram of a testing device for the compressive properties of thick-section composite materials according to an embodiment of the present invention; Figure 2 This is a first disassembled diagram of the testing device for the compressive properties of thick cross-section composite materials according to an embodiment of the present invention; Figure 3 This is a second disassembled diagram of the testing device for the compressive properties of thick cross-section composite materials according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the installation of a testing device for the compressive properties of thick-section composite materials according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the torque applied by the testing device for the compressive properties of thick-section composite materials according to an embodiment of the present invention; Figure 6 This is a flowchart of a method for testing the compressive properties of thick-section composite materials according to an embodiment of the present invention; Figure 7 This is a schematic diagram of a method for testing the compressive properties of thick-section composite materials according to an embodiment of the present invention; Figure 8 This is a schematic diagram of the geometric dimensions of a thick plate compression test specimen according to an embodiment of the present invention; Figure 9 This is a schematic diagram of the textured groove according to an embodiment of the present invention.

[0021] The attached figures are labeled as follows: 1. First clamping block group; 1-1. First clamping block; 1-2. Second clamping block; 1-3. Tightening hole; 1-4. Positioning hole; 1-5. Corrugated groove; 2. Second clamping block group; 2-1. Third clamping block; 2-2. Fourth clamping block; 3. Axial pin; 4. Limiting strip; 5. Limiting strip tightening screw; 6. Tightening bolt; 7. Test piece; 8. Mounting plate; 9. Baffle; 10. Fastening bolt; 11. Positioning pin. Detailed Implementation

[0022] The embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] Example 1: Test apparatus for compressive properties of thick cross-section composite materials.

[0025] like Figures 1 to 5 As shown, this embodiment provides a testing device for the compressive properties of thick-section composite materials, used to perform the testing method described in this invention. The testing device includes: a first clamping block assembly 1, a second clamping block assembly 2, an axial pin 3, a limiting strip 4, a limiting strip tightening screw 5, a tightening bolt 6, a positioning pin 11, as well as a biaxial strain gauge assembly, an acoustic emission sensor, and an end-face strain sensor.

[0026] The first clamping block group 1 (located above) and the second clamping block group 2 (located below) are arranged opposite to each other to clamp the upper and lower ends of the test piece 7. The first clamping block group 1 consists of the first clamping block 1-1 and the first clamping block 1-2, and the second clamping block group 2 consists of the third clamping block 2-1 and the fourth clamping block 2-2.

[0027] The surfaces of the first clamping block group 1 and the second clamping block group 2 that contact the test specimen 7 are both provided with textured grooves 1-5. The textured grooves are formed by machining or electrical discharge machining, and the angle θ between their extension direction and the fiber direction of the surface layer of the test specimen 7 is 30° (adjustable within the range of 20°~40° depending on the layup method). The groove spacing is 2mm, and the groove depth is 0.5mm. Testing shows that the coefficient of friction μ of this textured groove can reach 0.23. The 30° angle of the textured grooves avoids fiber directions such as 0°, ±45°, and 90°, preventing damage to the fibers.

[0028] The edges of the first clamping block group 1 and the second clamping block group 2 corresponding to the working section of the test piece 7 are provided with rounded chamfers with a radius of 1mm to 3mm to prevent the test piece from being cut and damaged by the right-angled edge of the tooling when it expands outward during compression.

[0029] A total of eight locating pins 11 (four on the top and four on the bottom) are installed in the locating holes 1-4 of the first clamping block group 1 and the second clamping block group 2, respectively. The locating holes of the first clamping block group 1 and the locating pins 11 are interference-fitted; the locating holes of the second clamping block group 2 and the locating pins 11 are clearance-fitted, with a clearance tolerance of ±0.01mm. This design ensures high parallelism between the upper and lower end faces of the testing device, meeting the requirements of combined loading.

[0030] The lower part of the axial pin 3 is interference-fitted with the first clamping block assembly 1. The upper part of the axial pin 3 has a diameter 0.02mm to 0.03mm smaller than that of the lower part and is provided with a groove. This design can reduce axial friction and provide a more stable guiding effect than linear motion bearings under heavy loads (100 tons to 200 tons).

[0031] The limiting strip 4 and the limiting strip tightening screw 5 are located on both sides of the first clamping block group 1. When installing the test piece 7, adjust the position of the limiting strip 4 according to the width of the test piece, tighten the limiting strip tightening screw 5, so that the test piece 7 rests against the upper and lower limiting strips, and the centering and positioning can be completed.

[0032] The tightening holes 1-3 include a through hole portion provided in the second clamping block assembly 2 and a threaded hole portion provided in the first clamping block assembly 1. The tightening bolt 6 passes through the through hole portion of the second clamping block assembly 2 and is threadedly connected to the threaded hole portion of the first clamping block assembly 1 to apply tightening torque. The preload generated by tightening the bolt 6 causes the clamping block assembly to clamp the test piece 7.

[0033] The biaxial strain gauge assembly is symmetrically attached to the working section of test specimen 7, with the attachment positions located at a distance from both ends of the working section that is not less than the thickness of test specimen 7. The biaxial strain gauge assembly is used to simultaneously measure longitudinal and transverse strain.

[0034] Acoustic emission sensors are installed on the side of the first clamping block group 1 or the second clamping block group 2 to monitor acoustic emission signals of internal damage initiation in the material in real time.

[0035] End-face strain sensors are installed at the four corners of the loaded end face of the test piece 7 to monitor the stress uniformity of the end face in real time. Finally, the first clamping block group 1 and the second clamping block group 2 are fixed to the mounting plate 8 by the baffle 9 and the fastening bolts 10.

[0036] Example 2: Test method for compressive properties of thick cross-section composite materials.

[0037] This embodiment provides a method for testing the compressive properties of thick-section composite materials, using the aforementioned testing apparatus. Figure 6 and Figure 7 As shown, it includes the following steps: Step 1: Obtain input parameters and make preliminary design of test piece dimensions.

[0038] The dimensional parameters of test piece 7 are as follows: Figure 8 As shown, the dimensional parameters include width W and working section length L. work Clamping length h, total length H, thickness t.

[0039] Recommended layup method: If measuring the 0° compressive strength of the composite material, it is recommended to use [90°, 0°]. ns Orthogonal layups (where 90° layups are placed on the outermost layer to provide support for the outer 0° layer fibers, preventing premature sublayer buckling failure) can also be used, along with [90°, 45°, 0°, -45°]. ns Isotropic layup, or other layup methods used in design.

[0040] The width W of test specimen 7 is determined based on the thickness t, satisfying 1.5×t ≤ W ≤ 10×t. This width-to-thickness ratio range effectively reduces testing errors caused by free edge effects (a larger width leads to increased specimen preparation costs and increased testing machine tonnage). The working section length L of test specimen 7 is then determined. work To satisfy the Euler buckling stability constraint: the estimated failure load is taken as the critical buckling load P. cr Substitute into the formula Where K is the equivalent bending stiffness per unit width, and β is the boundary correction coefficient, which can be 1 to 2. The maximum allowable working section length L can then be calculated by reverse calculation. work-max Taking into account both the loading boundary effect and the space required for strain gauge mounting (at least 15mm), the minimum working section length L is determined. work-min Finally in L work-min and work-max Select an appropriate working section length L work .

[0041] Step 2: Estimate the failure load and calculate the minimum friction load.

[0042] Based on the ply configuration and intrinsic material parameters, the predicted failure load P of specimen 7 under compressive load is estimated. pred To ensure that the test specimen does not fail at the ends, the fixture must provide a frictional force of at least 10% of the maximum load, i.e., the minimum frictional load F. fric_min =0.1×P pred (This ratio is based on engineering experience.)

[0043] Step 3: Determine the parameters of the groove, the overall friction coefficient, and their calibration.

[0044] (1) Determine the angle θ between the extension direction of the reticulated groove and the direction of the surface fiber.

[0045] The first ply angle α of the surface layer and the second ply angle β of the subsurface layer of specimen 7 were obtained from the ply configuration. The surface layer compensation coefficient p and the subsurface layer compensation coefficient q were determined through finite element simulation or experimental calibration. p compensates for the effect of a 90° deviation of the surface layer fiber direction on the included angle θ, and q compensates for the effect of a 0° deviation of the subsurface layer fiber direction on the included angle θ. The ply skew compensation angle δ was calculated using the following formula: δ=p·(α-90°)+q·(β-0°).

[0046] The angle between the extension direction of the groove and the fiber direction of the surface layer of the test piece is θ = 30° + δ, and the value of θ ranges from 20° to 40°. When the surface layer is 90° and the subsurface layer is 0°, δ = 0° and θ = 30°, and the above values ​​are preferred.

[0047] (2) Segmented design of the groove.

[0048] The positions of grooves 1-5 are as follows: Figure 9 As shown.

[0049] Along the clamping direction of test piece 7, the grooves 1-5 are sequentially divided into a first section near the end of the test piece, a second section away from the end of the test piece, and a transition section between the two. The groove depth d and the groove spacing p of each section are determined. Specifically, the ratio λ1 = d1 / p1 of the first section is greater than or equal to a first threshold (e.g., 0.25), and the ratio λ2 = d2 / p2 of the second section is less than or equal to a second threshold (e.g., 0.15), with the first threshold being greater than the second threshold. The groove depth of the transition section is gradually distributed, such that the ratio λ of the transition section changes linearly from λ1 to λ2 along the clamping direction (λ is a linear function of position x).

[0050] (3) Calculation model of comprehensive friction coefficient.

[0051] The overall friction coefficient μ is calculated using the following formula: μ=μ0×(1+2d / p)×(1+0.1×Ra)×f m, Where μ0 is the reference friction coefficient (taken as 0.20~0.25), d / p is the ratio of depth to spacing; Ra is the secondary micro-roughness (unit μm) formed after the anilox groove is machined, and f m The material pairing factor (determined based on the pairing relationship between the fiber type of the test specimen and the clamping block material, with a value ranging from 0.9 to 1.1) is used. The overall friction coefficient of the entire groove is obtained by weighted averaging or integration along the clamping direction, and the overall friction coefficient is positively correlated with the ratio λ.

[0052] (4) Calibration of friction coefficient.

[0053] Before testing, a standard specimen with the same material, ply, and surface treatment as test piece 7 was used to conduct a friction coefficient calibration test under simulated clamping pressure (equal to the average compressive stress generated on the clamping surface by the minimum preload). The critical tensile force required for the standard specimen to slide after being clamped was measured, and the dynamic friction coefficient μ was calculated. cal If |μ cal If -μ| / μ>10%, the calibration is considered unqualified, and the machining parameters of the anilox groove (such as groove depth, groove spacing, or secondary roughness) need to be adjusted or the clamping block assembly needs to be replaced before recalibration. If multiple calibrations still fail, the μ obtained from the calibration should be used. cal Replace the design value μ and recalculate the subsequent tightening torque.

[0054] Step 4: Calculate the preload, select the bolt, and determine the clamping length.

[0055] Based on the minimum friction load F fric_minBased on the combined friction coefficient μ, calculate the minimum required preload F. pre_min =F fric_min / μ.

[0056] Based on the minimum preload F pre_min The nominal diameter D of bolt 6 is initially selected (the nominal cross-sectional area is first determined by the minimum preload, and then the nominal diameter is determined by the nominal cross-sectional area). To ensure that the bolt does not yield during use, the ultimate available preload of the bolt also needs to be checked. The ultimate preload of the bolt is calculated using the following formula: F max = (0.6~0.7) × σ s ×A， Where, σ s Where A is the yield strength of the bolt material (MPa), and A is the nominal cross-sectional area of ​​the bolt (mm²). 2 The selected preload F pre_min It should not be greater than F max If F pre_min >F max If the nominal diameter D of the bolt is not met, the bolt needs to be increased or a bolt of higher strength grade should be selected, and the calculation should be recalculated until the requirements are met.

[0057] Finally, the nominal diameter D of bolt 6 was determined, and the minimum tightening torque T was calculated. min = K×D×F pre_min , where K is the torque coefficient (0.23 was tested and determined in this device).

[0058] The clamping length h of the test piece 7 is determined based on the nominal diameter D of the tightening bolt 6 (e.g., h = (3~5) × D). The clamping length h and the working section length L are then used to determine the clamping length h. work The total length of test piece 7 is determined to be H = 2h + L. work .

[0059] Step 5: Prepare the test specimen and machine the textured groove.

[0060] Based on the design dimensions (width W, thickness t, clamping length h, working section length L) work Test piece 7 is prepared with a total length H. At the same time, based on the groove parameters determined in step 3, textured grooves 1-5 are machined on the surface of the clamping area of ​​the first clamping block group 1 and the second clamping block group 2 (by electrical discharge machining or mechanical milling).

[0061] Step 6: Install the test piece and apply tightening torque in stages.

[0062] The test piece 7 is positioned using the limiting strip 4. Tightening torque is applied in stages and progressively. Phase 1: Applying the target torque T min30%. Use a feeler gauge or dial indicator to measure the parallelism deviation between the bottom end face of the first clamping block group 1 and the top end face of the second clamping block group 2. If the deviation exceeds the allowable tolerance (e.g., 0.02 mm), it is corrected by locally adjusting the torque of the corresponding bolt.

[0063] Second stage: Apply 60% of the target torque, measure parallelism again and make corrections.

[0064] The third stage: Apply 100% of the target torque, and finally measure the parallelism to ensure that the deviation is within the allowable tolerance range.

[0065] During the application of torque, the strain sensors attached to the surface of the clamping section 7 of the test piece are monitored simultaneously, and the strain deviation between each strain measurement point is calculated. When the strain deviation is less than 10% of the average value, the clamping is considered uniform and effective. The torque value, parallelism deviation, and strain distribution data for each applied torque are recorded to generate an installation quality traceability file.

[0066] Step 7: Perform a compression test and collect data in real time.

[0067] The installed testing device is placed between the upper and lower pressure plates of the testing machine, and an axial compressive load is applied to the test specimen 7 until the specimen fails. Load-displacement data and load-strain data are acquired in real time. Among them, the load-strain data are obtained simultaneously through a biaxial strain gauge group, acquiring both load-longitudinal strain and load-transverse strain.

[0068] Step 8: Calculate the compressive strength and compressive modulus (including end constraint correction). Separate the load-longitudinal strain data (ε) from the load-strain data. l ) and load-lateral strain data (ε t Extracting the compressive load P and longitudinal strain ε l The initial compressive modulus E0 = (ΔP / Δε) is obtained by fitting the linear segment between them using the least squares method. l ) / A, where ΔP is the load increment, Δε l The longitudinal strain increment is represented by A, where A is the cross-sectional area of ​​the specimen.

[0069] Simultaneous calculation of Poisson's ratio ν=-ε t / ε l .

[0070] An end constraint correction factor β = 1 + k × (ν - ν0) × (t / W) is introduced, where k is an empirical coefficient (taken as 1.5 to 2.5), ν0 is a reference Poisson's ratio (taken as 0.30 to 0.35), t is the specimen thickness, and W is the specimen width. This correction factor is used to correct for the measurement deviation of the compressive modulus caused by the frictional constraint between the specimen end and the clamp.

[0071] Calculate the final compressive modulus E c =E0×β. The compressive strength is obtained by dividing the failure load by the initial cross-sectional area.

[0072] Poisson's ratio is calculated in real time during the test. When the Poisson's ratio changes by more than 20% relative to the initial value, it is determined that interlaminar damage has started, and the load level is recorded as the damage initiation point.

[0073] Step 9: Determine the destruction mode.

[0074] Image data of test specimen 7 after failure and load-displacement data during the failure process were collected.

[0075] Location identification: Identify the location of damage from the image data. If the location of damage is within the working section length of test piece 7, the location is marked as "normal". If the location of damage is located in the clamping section or the loading end face, it is marked as "abnormal".

[0076] Curve Feature Extraction: Extract curve features at the failure moment from the load-displacement data to determine the load peak point. Using this point as a boundary, fit a predetermined number of data points before the peak point and calculate the slope k1 of the first segment; fit a predetermined number of data points after the peak point and calculate the slope k2 of the second segment. If the absolute value of the slope k2 of the second segment is less than the absolute value of the slope k1 of the first segment and the curve shows a monotonically decreasing trend after the peak point, the curve is marked as "normal". If the load decrease between adjacent data points at the peak point exceeds a predetermined threshold (e.g., 10% of the estimated failure load, with a reasonable range of 5% to 15%), or if a second load peak appears after the peak point and its amplitude exceeds a predetermined percentage of the first load peak, the curve is marked as "abnormal".

[0077] Comprehensive judgment: If the position and curve are normal, it is judged as normal compression failure and the test is valid; if the position or curve is abnormal, it is judged as abnormal failure (clamping segment shear failure or end face splitting failure), the test is marked as invalid and the failure type is recorded.

[0078] Output the judgment result and damage type identifier.

[0079] Step 10: Real-time monitoring and anomaly warning.

[0080] During the application of axial compressive load, the following monitoring is performed simultaneously: 1. Acoustic emission monitoring. Acoustic emission sensors are installed on the surface of the clamping block or the end of the test piece to collect acoustic emission signals in real time. When the acoustic emission event rate or energy exceeds a first threshold, a damage warning is issued.

[0081] 2. Stiffness monitoring. The slope of the sliding window of the load-displacement curve is calculated in real time. When the rate of decrease of this slope relative to the slope of the initial linear segment exceeds a second threshold, a loading stop signal is triggered, a damage continuation warning is issued, and the current load is recorded as the damage critical value.

[0082] 3. Off-center load monitoring. The strain non-uniformity at four measuring points is monitored in real time by end face strain sensors. When the non-uniformity exceeds the third threshold (10%~20%), an end face off-center load warning is issued, prompting adjustment of the fixture or termination of the test.

[0083] When any of the above warnings is triggered, the rate of application of axial compressive load is automatically reduced or loading is completely stopped, and all test data at the trigger time is saved and marked as abnormal termination.

[0084] Example 3: Taking a certain type of carbon fiber composite material as an example, with a thickness t=20mm and a layup of [90°, 0°], ns Using the above method, with p=0.1 and q=0, we obtain θ=30°; the groove spacing is 2mm and the depth is 0.5mm; the estimated failure load is 500kN, and the calculated preload is approximately 217kN. M16 bolts are selected, with a tightening torque of approximately 800N·m. In actual testing, the test piece underwent normal compression failure in the working section. The measured compressive strength met expectations, and the corrected compressive modulus deviated from the theoretical value by less than 5%.

[0085] Beneficial effects of the embodiments of the present invention: Existing standards for testing the compressive properties of composite materials (such as GB / T 2856 and ASTM D695) are only applicable to thin-walled composite materials with a thickness of ≤2mm. There are currently no effective testing methods for thick-section composite materials with a thickness of 10mm to 40mm. This invention proposes for the first time a complete compressive property testing scheme for thick-section composite materials, including a test specimen size design method, a combined loading test device, a textured groove friction surface treatment, and quantitative calculation of tightening torque. This solves the industry problem of being unable to obtain the true compressive properties of thick-section composite materials due to their large loads and tendency to premature failure.

[0086] The friction coefficient of the clamping surface is increased to 0.23 by using a textured groove (30° included angle, 2mm spacing, 0.5mm depth), achieving a combined loading of end-face compression and friction loading. High parallelism of the end faces is ensured by using an interference / clearance fit (tolerance ±0.01mm) with locating pins. Axial friction is reduced by using a diameter-reducing groove design on the axial pin. Stress distribution is optimized by using segmented, gradually changing textured grooves. These techniques effectively prevent premature abnormal failure of the test specimen in the clamping section or on the loading end face, ensuring that failure occurs in the working section, thereby obtaining the true compressive strength and compressive modulus. Experiments show that using the method of this invention to test 20mm thick composite materials increases the test success rate from less than 30% with existing methods to over 95%.

[0087] Existing methods typically ignore the influence of end friction constraints on the compressive modulus, leading to inflated modulus measurements. This invention addresses this by attaching biaxial strain gauges to the working section of the test specimen, simultaneously measuring longitudinal and transverse strain, and introducing an end constraint correction factor to precisely correct the initial compressive modulus. This eliminates measurement bias caused by end friction, resulting in more accurate compressive modulus data with deviations from theoretical values ​​controlled within 5%. Furthermore, by monitoring Poisson's ratio abrupt changes in real time, the initiation point of interlaminar damage can be accurately determined, providing more comprehensive mechanical property parameters for composite material structural design.

[0088] To address the anisotropic characteristics of composite materials, this invention proposes a linear model for ply skew compensation angles. It establishes a quantitative relationship between the ply angles of the surface and subsurface layers and the optimal extension direction of the grooves, thus preventing fiber damage caused by the groove direction being parallel to the fiber direction. This model can be calibrated through finite element simulation or a small number of experiments, is applicable to any ply configuration, and possesses strong universality and engineering application value.

[0089] This invention provides a formula for calculating the minimum tightening torque, and combines the measured torque coefficient K=0.23 of the textured groove with engineering experience of 10% friction force, making the clamping force design based on sound principles. Simultaneously, by employing steps such as applying torque in stages, checking parallelism, monitoring strain uniformity, and recording installation parameters, a complete installation quality traceability archive is formed, ensuring the consistency and reproducibility of clamping conditions for each test.

[0090] By setting acoustic emission sensors on the side of the clamping block, setting strain sensors at the four corners of the end face, and analyzing the slope of the load-displacement curve in real time, this invention can detect abnormal situations such as damage initiation, stiffness reduction, or off-center loading in a timely manner during the test, and automatically reduce the loading rate or stop loading, avoiding invalid testing and tooling damage, thus improving the safety and intelligence level of the test.

[0091] In summary, this invention provides a systematic, reliable, and high-precision method and apparatus for testing the compressive properties of thick-section composite materials. It has significant creative and practical value and can be widely used in the characterization of the mechanical properties of thick-section composite material structures in aerospace, automotive, wind power and other fields.

[0092] The above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any substitution of equivalent components or equivalent changes and modifications made within the scope of protection of this patent should still fall within the scope of this patent. Furthermore, the technical features, technical features and technical solutions, and technical solutions in this invention can be freely combined and used.

Claims

1. A method for testing the compressive properties of thick-section composite materials, characterized in that, Includes the following steps: Obtain the thickness parameters, layup method and intrinsic material parameters of the composite material to be tested. Determine the width of the test piece (7) based on the thickness parameters. Determine the thickness of the test piece (7) based on the layup method. Determine the working section length of the test piece (7) so that the test piece (7) satisfies the Euler buckling stability constraint. Based on the layup method and intrinsic material parameters, the predicted failure load of the test specimen under compressive load is estimated, and based on the predicted failure load, the minimum friction load required by the testing device is calculated. Based on the layup method and the intrinsic parameters of the material, the groove parameters of the textured groove (1-5) are determined, the comprehensive friction coefficient between the clamping surface of the clamping block group and the test piece (7) is calculated, the comprehensive friction coefficient is corrected, and the minimum preload required to achieve the minimum friction load is calculated based on the corrected comprehensive friction coefficient. Along the surface of the clamping area of ​​the test piece (7), the textured groove (1-5) is divided into different sections, and the groove depth and groove spacing corresponding to each section are determined. The comprehensive friction coefficient of the textured groove (1-5) is determined according to the groove depth and the groove spacing. The friction coefficient of the textured groove (1-5) is calibrated. The actual friction force is measured under simulated clamping pressure using a standard test piece of the same material as the test piece (7), and the dynamic friction coefficient is calculated. If the deviation between the dynamic friction coefficient and the comprehensive friction coefficient exceeds the deviation threshold, the parameters of the textured groove (1-5) are adjusted and the comprehensive friction coefficient is corrected. The grooves (1-5) are sequentially divided along the clamping direction of the test piece (7) into a first section near the end of the test piece (7), a second section away from the end of the test piece (7), and a transition section between the first and second sections. The groove depth and groove spacing of the first section, the transition section, and the second section are determined respectively, wherein the groove depth of the transition section is gradually distributed. The ratio λ of the groove depth to the groove spacing is calculated respectively, wherein the first ratio λ1 of the first section is ≥ the first threshold, and the second section... The second ratio λ2 of the segment is less than or equal to the second threshold, and the first threshold is greater than the second threshold. The ratio λ of the transition segment changes continuously along the clamping direction and is linearly related to the position. The linear relationship is a linear change from the first ratio λ1 to the second ratio λ2. Based on the ratio λ of each segment of the textured groove (1-5), the contribution of each segment to the comprehensive friction coefficient is calculated. The comprehensive friction coefficient of the entire textured groove (1-5) is obtained by weighted average method or integration along the clamping direction. The comprehensive friction coefficient is positively correlated with the ratio λ. The nominal cross-sectional area of ​​the tightening bolt (6) is determined according to the minimum preload, and the minimum tightening torque is calculated. The clamping length and total length of the test piece (7) are determined according to the nominal diameter of the tightening bolt (6). The test piece (7) is prepared according to the width, the thickness, the working section length, the clamping length and the total length. The textured groove (1-5) is machined on the surface of the clamping area of ​​the clamping block group based on the groove parameters. The test piece (7) is installed in the test device, and the minimum tightening torque is applied multiple times in a preset order so that the test piece (7) is clamped and meets the end face parallelism requirements. An axial compressive load is applied to the test piece (7) until the test piece (7) is destroyed. Load-displacement data and load-strain data are collected in real time. The compressive strength and compressive modulus of the thick cross-section composite material are calculated based on the load-displacement data and load-strain data. The failure mode is determined. If the failure is determined to be abnormal, the test is marked as invalid.

2. The test method according to claim 1, characterized in that, Based on the layup method and the intrinsic parameters of the material, the groove parameters of the textured grooves (1-5) are determined, the comprehensive friction coefficient between the clamping surface of the clamping block assembly and the test piece (7) is calculated, and the comprehensive friction coefficient is corrected, including: The first layup angle of the surface layer and the second layup angle of the subsurface layer of the test specimen (7) are obtained from the layup method. The layup skew compensation angle is determined based on the first layup angle and the second layup angle. The angle θ between the extension direction of the reticulated groove (1-5) and the fiber direction of the surface layer of the test specimen (7) is determined based on the layup skew compensation angle.

3. The test method according to claim 2, characterized in that, The first ply angle of the surface layer and the second ply angle of the subsurface layer of the test specimen (7) are obtained from the plying method. A ply skew compensation angle is determined based on the first ply angle and the second ply angle. The angle θ between the extension direction of the weave groove (1-5) and the fiber direction of the surface layer of the test specimen (7) is determined based on the ply skew compensation angle, including: The surface compensation coefficient p and the subsurface compensation coefficient q are determined by finite element simulation or experimental calibration. The surface compensation coefficient p is used to compensate for the effect of the surface fiber direction deviating from 90° on the included angle θ, and the subsurface compensation coefficient q is used to compensate for the effect of the subsurface fiber direction deviating from 0° on the included angle θ. The ply skew compensation angle δ is calculated using a linear model, where δ = p·(α-90°) + q·(β-0°), α is the first ply angle, and β is the second ply angle. The angle θ between the extension direction of the textured groove (1-5) and the surface fiber direction of the test piece (7) is calculated by the ply skew compensation angle δ, wherein the angle θ is 20°~40°.

4. The test method according to claim 1, characterized in that, The minimum tightening torque is applied multiple times in a preset sequence to clamp the test piece (7) and ensure that the end face parallelism requirement is met, including: The test piece (7) is placed in the testing device and positioned by the limiting strip (4); Apply to the first preset percentage, measure the parallelism deviation between the bottom end face of the first clamping block group (1) and the top end face of the second clamping block group (2), and if the deviation exceeds the allowable tolerance, correct it by locally adjusting the torque of the corresponding bolt; Apply to the second preset percentage, measure again, and correct. Apply the minimum tightening torque and finally measure the parallelism to ensure that the deviation is within the allowable tolerance. During the application of torque, the strain sensor attached to the surface of the clamping section of the test piece is monitored simultaneously, and the strain deviation between each strain measuring point is calculated. When the deviation is less than a predetermined threshold, the clamping is determined to be uniform and effective. Record the torque value, parallelism data, and strain distribution data for each applied torque to generate an installation quality traceability file.

5. The test method according to claim 1, characterized in that, The compressive strength and compressive modulus of the thick-section composite material are calculated based on the load-displacement data and load-strain data, including: Load-longitudinal strain data and load-lateral strain data are separated from the load-strain data, wherein the load-longitudinal strain data and the load-lateral strain data are measured simultaneously by a biaxial strain gauge array; Based on the load-longitudinal strain data, a linear segment between the compressive load and the load-longitudinal strain data is extracted, and the slope ΔP / Δε of the linear segment is calculated. l Where ΔP is the increment of the compressive load, and Δε l This represents the longitudinal strain increment corresponding to ΔP; The initial compressive modulus E0 = (ΔP / Δε) was obtained by fitting using the least squares method. l ) / A, where A is the cross-sectional area of ​​the test piece (7); Based on the load-lateral strain data and the load-longitudinal strain data, Poisson's ratio ν = -ε is calculated simultaneously. t / ε l , where ε t For load-lateral strain data, ε l Load-longitudinal strain data; Based on the width and thickness of the test piece (7), the end constraint correction factor β = 1 + k × (ν - ν0) × (t / W) is calculated, where k is an empirical coefficient, ν0 is a reference Poisson's ratio, t is the thickness of the test piece (7), and W is the width of the test piece (7). The end constraint correction factor is used to correct the compression modulus measurement deviation caused by the frictional constraint between the end of the test piece and the clamp. The final compressive modulus E is calculated using the end constraint correction factor and the initial compressive modulus. c =E0×β; Poisson's ratio is calculated in real time during the test. When the Poisson's ratio changes by more than 20% relative to the initial value, it is determined that interlayer damage has started, and the current load level is taken as the damage initiation point.

6. The test method according to claim 1, characterized in that, The destruction mode is determined, including: Collect image data of the test specimen (7) after it is damaged and the load-displacement data during the damage process; The damage location is identified from the image data. If the damage location is within the length range of the working section of the test piece (7), the damage location is marked as normal. If the damage location is located in the clamping section or loading end face of the test piece (7), the damage location is marked as abnormal. The curve features of the failure moment are extracted from the load-displacement data, and the load peak point is extracted based on the curve features. Taking the load peak point as the boundary, the first predetermined number of data points before the load peak point are fitted and calculated to obtain the first slope k1, and the second predetermined number of data points after the load peak point are fitted and calculated to obtain the second slope k2. If the absolute value of the slope k2 in the later segment is less than the absolute value of the slope k1 in the earlier segment and the curve after the load peak point shows a monotonically decreasing trend, then the curve is marked as normal. If the load decrease between adjacent data points at the load peak point exceeds a predetermined load threshold, or if a second load peak appears after the load peak point and the amplitude of the second load peak exceeds a predetermined percentage of the load peak point, then the curve is marked as abnormal. If the position and curve are normal, it is determined to be normal compression failure and the test is valid. If the position or curve is abnormal, it is determined to be abnormal failure, the test is marked as invalid and the failure type is recorded as clamping segment shear failure or end face splitting failure. Output the judgment result and damage type identifier.

7. The test method according to any one of claims 1 to 6, characterized in that, Also includes: When applying an axial compressive load to the test piece (7), real-time monitoring and abnormal warnings are provided, including: Acoustic emission sensors are installed on the surface of the clamping block or the end of the test piece to collect acoustic emission signals in real time. When the acoustic emission event rate or energy exceeds the first threshold, a damage warning is issued. The slope of the sliding window of the load-displacement curve is calculated in real time. When the rate of decrease of the sliding window slope relative to the slope of the initial linear segment exceeds the second threshold, a loading stop signal is triggered, a damage continuation warning is issued, and the current load is recorded as the damage critical value. The strain non-uniformity is monitored in real time by the end face strain sensor. When the strain non-uniformity exceeds the third threshold, an end face off-center load warning is issued, and the fixture is adjusted or the test is stopped. When the damage warning and / or the damage persistence warning and / or the end face off-center load warning are triggered, reduce the rate of application of axial compressive load or stop loading, save all test data at the trigger time, and mark it as abnormal termination.

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