Progressive tapering based rapid evaluation method for forming quality of carbon fiber pultruded plate

Abstract

The present application relates to a kind of based on gradual chamfering carbon fiber pultrusion plate forming quality rapid evaluation method, comprising the following steps:1), standard sample preparation, and obtain basic mechanical parameter;2), " equivalent residual stress moment parameter C calibration database is built: at least two different angle chamfering experiments are carried out to each standard sample, and the warping curvature κ after each chamfering is measured;Equivalent residual stress moment parameter C of the standard sample is solved by fitting;The C value solved is associated with the tensile strength test value of corresponding sample, and " C value-quality grade" corresponding calibration database is established;3), sample rapid detection and quality evaluation are waited.The present application directly reflects the residual stress level of carbon fiber pultrusion plate by a small amount of convenient measurement, so as to realize the rapid sorting of product quality and the early warning of process anomaly.

Description

Technical Field

[0001] This invention relates to a quality inspection method for carbon fiber pultruded sheets, specifically a rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive bevel cutting, belonging to the field of quality inspection technology for fiber reinforced composite materials. Background Technology

[0002] Carbon fiber pultrusion, with its advantages of continuous operation, automation, high efficiency, and excellent fiber-oriented mechanical properties, is widely used in the production of key structural components such as wind turbine blade main beams, transmission line core rods, and oilfield sucker rods. However, during pultrusion, process-induced residual stress inevitably occurs inside the product due to factors such as the difference in thermal expansion coefficients between the fiber and the resin matrix, resin curing and cross-linking shrinkage, and uneven temperature field in the mold. The presence of residual stress can lead to warping and poor dimensional stability of the pultruded sheet, affecting subsequent assembly and fit; in severe cases, it can cause microcracks in the matrix and debonding at the fiber-matrix interface, significantly reducing the strength, fatigue life, and long-term service reliability of the product.

[0003] Currently, the quality inspection of pultruded sheets mainly relies on mechanical property testing (such as tensile, compressive, and shear strength testing) and microstructure observation. These methods share the following common drawbacks: First, they are destructive sampling inspections, which cannot achieve online assessment and quality sorting covering the entire product; second, the testing process is complex, time-consuming, and costly, making it unsuitable for production line environments; third, they mainly focus on macroscopic mechanical properties and cannot directly reflect the internal residual stress state introduced by fluctuations in process parameters, which is precisely the key indicator determining the long-term performance of the product. Residual stress detection methods such as drilling and contouring are also difficult to apply directly to rapid quality assessment in industrial production sites due to their complex operation, expensive equipment, and cumbersome data processing.

[0004] Therefore, it is indeed necessary to provide an innovative method for rapid evaluation of the forming quality of carbon fiber pultruded sheets based on progressive beveling to solve the following technical problems: (1) How to simplify the complex problem of residual stress detection into an engineering evaluation model based on a small number of simple experiments that can be directly used for rapid quality sorting on the production line.

[0005] (2) How to establish a simplified quantitative relationship between the macroscopic measurable characteristic quantity (warping curvature) of the oblique cutting test and the overall level of residual stress inside the product.

[0006] (3) How to quickly determine and classify the forming quality of pultruded plates without relying on complex numerical calculations or inversion analysis. Summary of the Invention

[0007] The purpose of this invention is to provide a rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive beveling. This method is simple, fast, and low-cost, and can directly reflect the residual stress level of carbon fiber pultruded sheets through a small number of convenient measurements, thereby enabling rapid sorting of product quality and early warning of process anomalies.

[0008] To achieve the above objectives, the technical solution adopted by this invention is: a rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive bevel cutting, which includes the following process steps: 1) Preparation of standard specimens and acquisition of their fundamental mechanical parameters; these parameters include bending stiffness per unit width D and fiber volume fraction V. f ; 2) Construct a calibration database for the "equivalent residual stress moment parameter C": Specifically, it includes: 2-1) Perform oblique cutting experiments at least two different angles on each standard specimen and measure the warping curvature κ after each oblique cutting; 2-2), according to the formula The equivalent residual stress moment parameter C of the standard specimen is obtained by fitting; where κ is the measured warpage curvature; θ is the known oblique angle; V f α represents fiber volume fraction; D represents bending stiffness; α is a correction factor. 2-3), the solved C value is correlated with the tensile strength test value of the corresponding specimen to establish a calibration database corresponding to "C value-quality grade"; 3) Rapid testing and quality assessment of the test sample: Perform a fixed-angle oblique cut on the test sample, and calculate the equivalent residual stress moment parameter C of the test sample according to the formula in step 2). sample The calculated C sample The value is compared with the calibration database established in step 2) to determine the quality level of the sample to be tested.

[0009] The rapid evaluation method for the molding quality of carbon fiber pultruded plates based on progressive bevel cutting of the present invention is further described as follows: In step 1), the standard sample uses strips with three widths of 15, 30 and 45 mm, a length L = 500 mm and a thickness h of 6 mm.

[0010] The rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive oblique cutting of the present invention further comprises: in step 2-1), five different oblique cutting experiments are performed on each standard sample at five different angles: θ1=3.6°, θ2=1.8°, θ3=0.9°, θ4=0.45°, and θ5=0.225°, and the warpage curvature κ after each oblique cutting is measured. i .

[0011] In the method for rapidly evaluating the forming quality of carbon fiber pultruded plates based on progressive oblique cutting of the present invention, in step 2-3), C ≤ 18 MPa is determined as excellent, 18 < C ≤ 25 MPa is determined as qualified, and C > 25 MPa is determined as pending inspection.

[0012] Compared with the prior art, the present invention has the following beneficial effects: 1. The method for rapidly evaluating the forming quality of carbon fiber pultruded plates based on progressive oblique cutting of the present invention establishes a quantitative correlation between the equivalent residual stress moment parameter C and the core mechanical properties (tensile strength, compressive strength, fatigue life), converts the abstract residual stress level into a digital standard that can directly correspond to the product quality grade, and upgrades the quality evaluation from "stress measurement" to "performance judgment".

[0013] 2. The method for rapidly evaluating the forming quality of carbon fiber pultruded plates based on progressive oblique cutting of the present invention addresses the non-uniform distribution characteristic of internal residual stress in actual pultruded plates (usually high at the surface layer and low at the core part), adopts a progressive cutting scheme with more than five different oblique cutting angles, introduces a nonlinear model with a cubic correction term for fitting, solves the unified C value representing the average level of the full thickness in one step, effectively avoids the evaluation deviation caused by the wrong extrapolation of the high surface stress by the traditional linear model, and ensures the uniformity and reproducibility of quality standards under different working conditions.

[0014] 3. In terms of detection efficiency, after the calibration is completed, the subsequent evaluation of the method for rapidly evaluating the forming quality of carbon fiber pultruded plates based on progressive oblique cutting of the present invention can be simplified to a single oblique cutting and rapid curvature measurement, and the evaluation time for a single specimen is shortened from several hours to less than 10 minutes, which truly meets the sampling inspection requirements of the production line.

[0015] 4. The method for rapidly evaluating the forming quality of carbon fiber pultruded plates based on progressive oblique cutting of the present invention is a micro-damage detection, the oblique cutting area can be limited to the non-stressed area at the end, and the same specimen in the calibration stage can be directly used for subsequent mechanical property tests such as tension and fatigue, which greatly saves the calibration cost.

[0016] 5. The method for rapidly evaluating the forming quality of carbon fiber pultruded plates based on progressive oblique cutting of the present invention has both process diagnosis and quality early warning functions. When the C value of a batch rises continuously, it can give an early warning of process parameter drift or raw material fluctuation in advance, and can preliminarily judge the source of abnormality by combining the stress depth distribution characteristics of different angles, so as to provide a direction for process adjustment. Brief Description of the Drawings

[0017] Figure 1 It is a schematic diagram of the oblique cutting effect of the pultruded plate for wind turbine blades according to the specific embodiment of the present invention.

[0018] Figure 2This is a graph showing the relationship between κ and 1 / sinθ for five oblique cutting experiments in a specific embodiment of the present invention. Detailed Implementation

[0019] This invention relates to a rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive beveling, which includes the following process steps: 1) Preparation of standard specimens and acquisition of their fundamental mechanical parameters; these parameters include bending stiffness per unit width D and fiber volume fraction V. f .

[0020] In this embodiment, the standard test specimens are made of strips with widths of 15, 30, and 45 mm, a length L = 500 mm, and a thickness h of 6 mm. Multiple pultruded sheet standard test specimens are required, produced under different process conditions (such as mold temperature and traction speed) or in different material batches.

[0021] The bending stiffness D is obtained by calculation using classical laminate theory combined with the basic elastic constant, or by direct back-calculation using the load-displacement curve measured by a three-point bending test.

[0022] 2) Construct a calibration database for the "equivalent residual stress moment parameter C": Specifically, it includes: 2-1), perform oblique cutting experiments at least two different angles on each standard sample and measure the warping curvature κ after each oblique cutting.

[0023] Specifically, each standard sample was subjected to oblique cutting experiments at five different angles: θ1=3.6°, θ2=1.8°, θ3=0.9°, θ4=0.45°, and θ5=0.225°, and the warping curvature κ after each oblique cutting was measured. i .

[0024] 2-2), according to the formula The equivalent residual stress moment parameter C of the standard specimen is obtained by fitting; where κ is the measured warpage curvature; θ is the known oblique angle; V f α is the fiber volume fraction; D is the bending stiffness; α is the correction factor.

[0025] The above formula is based on classical beam bending theory and the physical principle of stress release. The specific establishment process is as follows: (I) According to the classical beam bending theory (Euler-Bernoulli beam), the bending curvature κ (m) of a uniform beam under pure bending moment M is... -1 ) and bending moment M and bending stiffness D (N·m) 2 The relationship between / m) is: (1) When the oblique cutting operation releases some of the residual stress, the original stress balance inside the specimen is broken. The residual stress in the remaining material that has not been fully released will form an equivalent bending moment M, driving the specimen to produce macroscopic warping κ. Equation 1 establishes a linear bridge between this macroscopically measurable (κ) and the internal mechanical driving force (M).

[0026] (ii) Let the residual stress distribution along the thickness direction (Z direction) of the pultruded plate be σ. f (z). When the bevel angle is θ, this cutting action releases the stress in the volume between the cut surface and the plate surface. The increment of the equivalent bending moment generated by the released stress on the remaining material can be obtained by integration; its core is the "torque" effect of stress release: (2) Among them, fiber volume fraction V f This means that only the fibers bear the vast majority of the residual stress.

[0027] 1 / sinθ: Geometric amplification factor; the smaller θ is (the gentler the cut surface), the longer the axial length of the fiber released per unit length of the cross-section, thus amplifying the total stress effect. This factor is key to achieving high sensitivity in this method.

[0028] ∫ σ f(z)·z dz: Stress moment integral. z is the lever arm of the stress relative to the mid-surface (neutral axis). It reflects the contribution of the residual stress distribution pattern in the thickness direction to the bending moment - the farther the stress is from the mid-surface, the greater the "leverage" effect and the greater its contribution to warping.

[0029] (iii) The complete stress distribution f(z) is an unknown complex function. For rapid engineering evaluation, we introduce a lumped parameter C, which represents the equivalent effect of the comprehensive strength and distribution pattern of residual stress across the entire cross section. The integral term in equation (2) is represented by a discrete, equivalent concentrated force rectangular form: Assume that the oblique cut releases an equivalent, uniform residual stress layer with an equivalent stress magnitude of C. Then the integral term can be approximated as: h(θ) is the depth of the cut in the thickness direction. Considering the total thickness is fixed and the cut is gradual, it can be deduced that the total effect of the entire release process is proportional to the cutting angle.

[0030] After system integration and dimensional analysis, the final simplified engineering model can be obtained: (3) Replace the complex integral effect with a variable related to V. fThe equivalent bending moment is proportional to 1 / sinθ and controlled by a single parameter C. Here, C is not a single stress value, but an equivalent residual stress moment parameter, the dimension of which is stress (Pa or MPa), which physically represents the intensity of the "moment" of residual stress throughout the cross section.

[0031] (iv) Combine and derive the formula for calculating C: Substituting M in equation (1) into the expression of equation (3), we get: After sorting, we get: (4) Therefore, for a given experimental data point (θ) i , κ i The equivalent residual stress moment parameter C can be directly solved using the following formula: (5) Experimental results show that κ under different oblique cutting angles i The relationship with 1 / sinθi exhibits significant nonlinearity, the sources of which may include: 1. Strong gradient of stress distribution: The residual stress is not uniform in the thickness direction, but has a severe gradient, which leads to huge differences in the stress distribution pattern when cutting at different depths, and the lumped parameter C is no longer a constant.

[0032] 2. Geometric nonlinearity: Large warping increases the error of the classical linear beam theory (small deformation assumption).

[0033] 3. Material nonlinearity: Some composite materials may exhibit slight viscoelasticity or plasticity in their constitutive relation during unloading.

[0034] 4. Three-dimensional effect: As the cutting goes deeper, the stress release inside the sample is no longer a quasi-two-dimensional planar stress state.

[0035] Equation (4) is modified by introducing higher-order geometric correction terms: it is assumed that the main source of nonlinearity is the geometric nonlinearity caused by warping (i.e., large deformation effect). In linear theory, M = D·κ is modified to include higher-order terms: (6) Where α and β are correction coefficients. Combining equation (3), the corrected engineering model can be obtained:

[0036] After simplification, we obtain a bivariate nonlinear equation for C and α: (7) By solving the equation, the equivalent residual stress moment parameter C of the standard specimen can be obtained, and this C is a stable equivalent residual stress moment parameter that uniformly considers all data points.

[0037] 2-3) Associate the solved C value with the measured tensile strength value of the corresponding specimen, and establish a "C value - quality grade" corresponding calibration database.

[0038] Specifically, statistical analysis methods (such as analysis of variance, regression analysis, etc.) are used to determine the C value judgment thresholds corresponding to different quality grades. When C ≤ 18 MPa, it is judged as excellent; when 18 < C ≤ 25 MPa, it is judged as qualified; when C > 25 MPa, it is judged as to-be-inspected, thus forming a complete and executable grading evaluation standard for the molding quality of pultruded plates. This standard is the direct basis for subsequent rapid quality assessment of all products in batches to be inspected.

[0039] 3) Rapid detection and quality assessment of the test specimen to be tested: 3-1) Randomly select representative samples from the test specimens, and perform a bevel cutting operation at a fixed angle θ on the specimens to be tested.

[0040] 3-2) Use a portable profilometer or a high-definition camera to quickly measure the warpage curvature κ of the free edge after bevel cutting.

[0041] 3-3) Substitute the measured κ into formula (7) in step 2), and calculate the equivalent residual stress moment parameter C of the specimen to be tested according to the formula sample ; compare the calculated Csample value with the calibration database established in step 2) to realize the quality grade judgment of the specimen to be tested.

[0042] Specifically, if C sample is less than the specified threshold (corresponding to the excellent grade), it is judged that the product quality of this batch is up to standard; if it exceeds the threshold (corresponding to qualified, to-be-inspected or unqualified grade), an early warning is issued.

[0043] 4) Output the assessment result. Output the quality grade judgment result of this batch of products, and give the corresponding estimated value of residual stress level and recommended treatment measures.

[0044] Example 1

[0045] This example performs rapid assessment on the molding quality of carbon fiber pultruded plates for wind turbine blades.

[0046] 1) Preparation of standard specimens and calibration of mechanical parameters Ten standard specimens (S-01 to S-10) were cut from a batch of carbon fiber / epoxy resin pultruded sheets for wind turbine blades (T700 grade large-tow 48K carbon fiber). Each specimen had a length L=500mm, width W=100mm, and thickness h=5mm. The flexural modulus of the specimens was measured using a three-point bending test (referencing ISO 14125 standard) and calculated using the following formula. , Among them, E f =157.1 GPa - Flexural modulus (unit: MPa or GPa); ΔP -- Load increment on the initial straight segment of the load-deflection curve (unit: N); Δf -- The deflection increment at the midpoint of the specimen span corresponding to ΔP (unit: mm); L=96 mm - Support span (unit: mm); b = 15 mm - Sample width (unit: mm); h = 6 mm - Sample thickness (unit: mm); The calculated bending stiffness per unit width is D = 2827.8 N·m.

[0047] Meanwhile, the fiber volume fraction V of the material was determined by the ignition method according to ASTM D3171 or GB / T 3855 standards. f =0.68.

[0048] 2) Construct a "C-value - quality grade" calibration database. The core of this step is to first classify the quality grades based on the measured mechanical properties, and then establish a statistical correlation between these grades and the C-value. For the 10 samples numbered S-01 to S-10, the following two tasks were completed for each sample: (1) The C-value was calculated by performing a progressive oblique cutting experiment on each sample, with five different angles at one end. The angle sequence was designed as θ1=3.6°, θ2=1.8°, θ3=0.9°, θ4=0.45°, and θ5=0.225°. After each cut, the morphology of the cut free edge was scanned using a high-precision three-dimensional optical profilometer, and the warping curvature κi was calculated. A nonlinear model with cubic correction was used for fitting: The equivalent residual stress moment parameter C for each specimen is then calculated.

[0049] Among them, five oblique cutting angles and their corresponding sinθ values ​​are:

[0050] Calculation of experimental warp curvature κ:

[0051] Calculate the warpage curvature κ ≈ 8δ / L 2 Where δ is the warpage height and L = 0.5 m is the sample length:

[0052] Calculation of C-value for nonlinear correction model:

[0053] Tensile strength requirements:

[0054] (2) This specific embodiment uses tensile strength as the quality grade evaluation standard (other embodiments may use compressive strength or fatigue life as the evaluation standard). Tensile strength testing is performed on each specimen according to national standards (refer to GB / T1040.5), and the mechanical performance thresholds for the downstream quality grades are set according to industry design standards as follows:

[0055] Establish a quality evaluation standard for "C-value - tensile strength".

[0056] 3). Rapid detection and quality assessment of the test samples Randomly select one pultruded sheet from the new production batch. Following the same procedure as step 2), bevel one end at angles θ1=3.6°, θ2=1.8°, θ3=0.9°, θ4=0.45°, and θ5=0.225° (as per the instruction manual). Figure 1 The cut (as shown) is performed, and the warpage curvature κ is calculated:

[0057] The relationship between κ and 1 / sinθ for 5 oblique cutting experiments is shown in the attached instruction manual. Figure 2 As shown.

[0058] 4. Calculation and Judgment Given parameters:

[0059] Solve for C using the known formula (7). sample =11.01 MPa≤14.0 MPa, which is within the qualified level.

[0060] 5. Output evaluation results The mass grade of this test specimen is: Qualified. Its equivalent residual stress moment parameter C sample value is 11.01 MPa, which falls within the interval of 10.5<C ≤ 14.0. This indicates that the residual stress level inside the test specimen is within an acceptable normal range, and its characteristic value of longitudinal tensile strength is predicted to be no less than 1,600 MPa, meeting the requirements of CP150 grade and satisfying the design requirements of standard main beams for wind turbine blades.

[0061] In summary, the present invention clearly demonstrates how to apply the "progressive oblique cutting + nonlinear correction model" to random quality sampling inspection in actual production. The entire process only requires one complete set of cutting and measurement at five angles on a single test specimen, which takes about 20 to 30 minutes, to quickly and scientifically determine its mass grade, providing a direct basis for production release or early warning adjustment.

[0062] The above specific embodiments are only preferred embodiments of the present creation, and are not intended to limit the present creation. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present creation shall all fall within the protection scope of the present creation.

Claims

1. A rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive bevel cutting, characterized in that: The process includes the following steps: 1) Preparation of standard specimens and acquisition of their fundamental mechanical parameters; these parameters include bending stiffness per unit width D and fiber volume fraction V. f ; 2) Construct a calibration database for the "equivalent residual stress moment parameter C": Specifically, it includes: 2-1) Perform oblique cutting experiments at least two different angles on each standard specimen and measure the warping curvature κ after each oblique cutting; 2-2), according to the formula The equivalent residual stress moment parameter C of the standard specimen is obtained by fitting; where κ is the measured warpage curvature; θ is the known oblique angle; V f α is the fiber volume fraction; D is the bending stiffness; α is the correction factor; h(θ) is the depth of the cut in the thickness direction. 2-3) Correlate the solved C value with the corresponding tensile strength test value of the specimen to establish a "C value-quality grade" corresponding calibration database; 3) Rapid testing and quality assessment of the test sample: Perform a fixed-angle oblique cut on the test sample, and calculate the equivalent residual stress moment parameter C of the test sample according to the formula in step 2). sample The calculated C sample The value is compared with the calibration database established in step 2) to determine the quality level of the sample to be tested.

2. The rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive bevel cutting as described in claim 1, characterized in that: In step 1), the standard sample uses strips of three widths: 15, 30, and 45 mm, with a length L = 500 mm and a thickness h of 6 mm.

3. The rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive bevel cutting as described in claim 1, characterized in that: In step 2-1), each standard sample is subjected to oblique cutting experiments at five different angles: θ1=3.6°, θ2=1.8°, θ3=0.9°, θ4=0.45°, and θ5=0.225°, and the warping curvature κ after each oblique cutting is measured. i .

4. The rapid evaluation method for the molding quality of carbon fiber pultruded sheets based on progressive bevel cutting as described in claim 1, characterized in that: In steps 2-3), C ≤ 18 MPa is judged as excellent, 18 < C ≤ 25 MPa is judged as qualified, and C > 25 MPa is judged as pending inspection.

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