Method for testing fatigue propagation law of three-dimensional surface crack reinforced by CFRP

Abstract

The present application relates to the testing method of three-dimensional surface crack fatigue propagation law, in particular to a kind of CFRP reinforced three-dimensional surface crack fatigue propagation law testing method. With Q235 as steel plate substrate, precast surface crack is processed on the surface of steel plate by milling machine or micro electric spark, and then the propagation rate and law of surface crack are observed under the pull-pull load of fatigue testing machine. The present application contains specimen design, which prevents the damage of specimen at the pinhole and chamfer by design method, ensures that the damage of specimen only occurs in the surface crack area during the experiment, and ensures that the experiment achieves the expected effect. The experimental loading test method for the specimen measures and characterizes the slip of CFRP and the propagation of surface crack by combining various test methods. The present application not only observes and analyzes the macroscopic crack propagation and fracture form, but also the microscopic fracture mode and crack source area inside the steel plate, better reveals the mechanism problem of steel plate fatigue crack propagation.

Description

Technical Field

[0001] Test method for crack fatigue propagation law of CFRP-reinforced three-dimensional surface. Background Technology

[0002] Steel structures are widely used in civil engineering and construction. Surface cracks are a common defect in steel structures. These cracks can propagate under environmental and fatigue conditions, leading to fracture and damage, causing unnecessary losses or accidents. Studies show that 90% of steel structure failures are fatigue failures. Whether in bridge structures, pipelines, or aerospace structures, surface cracks are the most frequent defect. Most engineering structural failures are caused by the gradual propagation of surface cracks, severely jeopardizing the safety and service life of engineering structures. Therefore, studying the propagation patterns of surface cracks in different materials is a prerequisite for evaluating their structural fatigue performance. CFRP (Cemented Solid Reinforced Polymer) reinforcement technology, as a novel technology, features high strength, high efficiency, and good adaptability, effectively reducing the occurrence of such safety hazards.

[0003] Because fatigue testing and static load testing use different methods and loading approaches, the failure modes of specimens differ under these two loading conditions. Therefore, specimen design for fatigue testing is a crucial step in ensuring accurate experimental results, and the design varies depending on the loading method. Currently, most experimental research in fracture mechanics focuses on through-cracks. Fatigue life, crack propagation rate, driving force models, and stress intensity factor calculations are rarely studied for three-dimensional surface cracks. The specimen preparation methods and crack parameter measurement techniques for through-cracks are not suitable for surface crack specimens. Therefore, a new experimental testing method suitable for surface cracks is needed.

[0004] Common crack monitoring methods include the CTOD method and the strain gauge method. However, for CFRP-reinforced structures, the cracks are covered by CFRP, and these two direct-contact measurement methods cannot obtain parameters of the crack surface, nor can they measure the displacement field at the crack tip and the crack propagation length. Non-contact or indirect measurement methods are needed to measure crack propagation, such as using a three-dimensional displacement deformation (DIC) system to measure the three-dimensional deformation of the specimen, thereby reflecting the deformation and propagation of the crack in the length and depth directions; or using the beach marking method to leave marks on the material cross-section corresponding to the number of cycles, so that measurements can be taken and observed after the experiment.

[0005] CFRP bonding reinforcement has become a popular method in engineering due to its speed and efficiency. However, the reinforcement effect is mainly determined by the weakest bonding layer. During CFRP bonding, a resting period of 3-7 days is required to ensure proper bonding. The effective bond length is calculated using the shear stress formula for the bonding layer. Commonly used bonding layer models include solid models, spring models, and cohesion models. The shear modulus of the bonding layer is measured experimentally, while Poisson's ratio is obtained based on the adhesive parameters.

[0006] The stress intensity factor of unreinforced surface cracks has been calculated using a formula fitted by Newman-Raju through extensive numerical calculations. However, the stress intensity factor at the crack tip decreases after CFRP reinforcement, and there is no guiding formula for calculation. Furthermore, the selection of specimen shape and design / manufacturing standards for existing mechanical pin-clamp fatigue testing machines are currently lacking, making it impossible to guarantee that the specimen will fail only in the surface crack area during testing. Therefore, traditional testing methods cannot ensure that the experiment achieves the expected results. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by providing a testing method for the fatigue propagation law of cracks on CFRP-reinforced three-dimensional surfaces. It solves the problem of difficulty in selecting and designing the shape of test specimens for fatigue testing machines with mechanical pin clamps. The designed specimens avoid failure at the pin holes, thickness changes, and chamfers, thus ensuring that the central crack is in the weakest area during the experiment, allowing for observation of crack propagation.

[0008] The technical solution of this invention: A method for testing the fatigue propagation law of cracks on CFRP-reinforced three-dimensional surfaces, characterized by comprising the following steps:

[0009] A. A modified "dumbbell-shaped" tensile MT specimen is fabricated using Q235, Q345, or X80 high-strength steel as the base material. Pre-existing surface cracks are machined onto the steel plate surface using a milling machine or micro-electrical discharge machining. The propagation rate and pattern of these surface cracks are then observed under tensile-tensile loads in a fatigue testing machine. This shape design fills a gap in existing standards regarding the design and fabrication of surface crack specimens. Specifically, it includes the pin hole dimensions, pin end thickness, tensile section thickness, transition zone chamfer, surface crack shape, and crack size. This design method prevents specimen failure at the pin hole and chamfer areas, ensuring that the specimen fails in the surface crack region during the experiment, thus guaranteeing the expected experimental results.

[0010] B. Based on step A, process the pre-fabricated surface cracks into a semi-elliptical or rhomboid shape;

[0011] C. Use the beach marking method to measure crack propagation length and depth. Mark approximately 6 to 8 times during the entire loading process, mainly depending on the specimen life and size. Select 40,000 or 20,000 cycles for marking based on the specimen crack size and life.

[0012] D. Use DIC technology to observe the direction and path of surface crack propagation after CFRP reinforcement. The three-dimensional DIC is set up on both sides of the specimen to obtain deformation in two directions. The DIC system takes pictures corresponding to the marking stages of the loading cycle. 20 sets of pictures are taken in each stage (loading frequency 10Hz). In the analysis stage, the maximum strain in each shooting stage is taken as the crack propagation peak value corresponding to the maximum fatigue load in the cycle, which also corresponds to the maximum stress intensity factor.

[0013] In step D, when the bonding length increases to a certain value, the shear stress of the bonding layer no longer increases. The calculation formulas for the shear stress of the bonding layer and the strain distribution of CFRP are shown in (1) to (4):

[0014]

[0015]

[0016]

[0017]

[0018] In the formula This indicates the elastic modulus of the steel plate. This represents the elastic modulus of CFRP. Indicates the shear modulus of the adhesive layer. Indicates the thickness of the steel plate. Indicates CFRP thickness. Indicates the thickness of the adhesive layer. Indicates the tensile force at the loading end. Indicates the bonding length.

[0019] E. Based on the beach markings left in step C, organize the crack propagation observation data and calculate the crack propagation rate based on the stress intensity factor results calculated by the finite element method. The length of the marked lines is measured using a vernier caliper and a magnifying glass. For unreinforced specimens, the average length of the front and back sides is used as the crack length. For reinforced specimens, the measured beach marking lengths are used for both the reinforced and unreinforced surfaces. A crack propagation driving force model is also established.

[0020] F. The driving model in step E is adopted as a two-degree-of-freedom model. This model can more accurately describe the propagation rate of surface cracks in both length and depth directions. It is more accurate than the single-degree-of-freedom model. The two-degree-of-freedom model can be specifically expressed as formulas (8) to (9):

[0021] The crack length propagation rate can be expressed by equation (8):

[0022]

[0023] The crack propagation rate in the depth direction can be expressed by equation (9):

[0024]

[0025] The effective stress intensity factor amplitude in the driving force model is calculated using formula (10). The effective stress intensity factor is represented by the difference between the maximum stress intensity factor and the stress intensity factor threshold value.

[0026]

[0027] The reinforced form in formula (10) There is no applicable analytical formula; it is necessary to solve it using the finite element method, or to use the modified stress intensity factor formula, (11), for calculation:

[0028]

[0029] The fundamental term in the above formula is the stress intensity factor of the unreinforced semi-elliptical surface crack, and the correction term function is... This is the stress intensity factor attenuation correction term after CFRP reinforcement. After extensive finite element calculations, the value of the correction term is between 0.8 and 0.4.

[0030] At this point, the crack driving force model can be expressed as equation (12):

[0031]

[0032] After the experiment, the fracture surface of the damaged specimen was observed by scanning electron microscopy (SEM). Since the size of the fracture specimen is limited by the scanning electron microscope, the fracture surface needs to be cut with a medium-speed wire cutter before observation. The size of the fracture specimen is controlled within 10×20mm. Then, gold sputtering is performed on the surface of the specimen before SEM fracture scanning is performed.

[0033] SEM fracture surface scanning helps to provide the microscopic morphology of the fracture surface, identify crack propagation initiation regions, microcrack propagation patterns, initial oxidation areas of the material, and the fracture mode of the specimen, as well as other microscopic mechanisms.

[0034] The specimen designed in this invention can prevent failure at the pin hole and chamfer, ensuring that the specimen fails in the surface crack area during the experiment, thus ensuring the experiment achieves the expected results. Compared with the traditional fatigue loading mode, this invention uses the beach fatigue marking method to mark the fatigue crack propagation inside the material, allowing for observation and measurement of the crack propagation rate after the experiment. This invention not only focuses on macroscopic crack propagation and fracture modes but also conducts relevant observation and analysis on the microscopic fracture mode and crack initiation region inside the steel plate, better revealing the mechanism of fatigue crack propagation in steel plates. It has the following beneficial effects:

[0035] 1. The experimental testing method for the fatigue propagation law of CFRP-reinforced three-dimensional surface cracks in this invention effectively solves the problem of difficult specimen shape selection and design in existing mechanical pin-clamp fatigue testing machines. The provided "dumbbell-shaped" specimen can be designed according to the pin dimensions of the testing machine, resulting in a small specimen size that facilitates design, processing, and experimentation. Under the guidance of the design method, it can be effectively guaranteed that the specimen will fail at the designated location during the experiment, avoiding experimental failure due to undesirable failure locations. The surface processing shape of the crack in this invention can improve the stress concentration factor at the crack tip, allowing the crack to reach the crack initiation stage more quickly compared to the original semi-circular initial crack, making the crack propagation process more obvious and facilitating subsequent crack observation and measurement.

[0036] 2. In the preliminary experimental stage, this invention mainly determines the crack propagation threshold value by gradually increasing the load. First, 0.4 times the specimen yield load is selected as the benchmark, and the maximum load is gradually increased by 10% until crack propagation occurs or the specimen shows obvious axial plastic deformation at 50,000-100,000 cycles. This load is defined as the fatigue crack propagation threshold value load, and it is substituted into the calculation when solving the effective stress intensity factor amplitude as the stress intensity factor threshold value.

[0037] 3. During the fatigue test, the "beach marking method" was used to mark the crack propagation paths corresponding to different cycle numbers during fatigue crack propagation. Comparison of experimental displacement results showed that this method does not cause crack closure or additionally increase the crack propagation rate, which is helpful for subsequent measurement of crack parameters and calculation of crack propagation rate. This method can measure the crack propagation rate before and after reinforcement, as well as the propagation rate in the length and depth directions. Furthermore, it is supplemented by a DIC system to observe the crack propagation pattern and initial crack propagation direction, making the crack propagation monitoring results more accurate and specific.

[0038] 4. Since the propagation law of the crack propagation source region is quite complex, SEM micro fracture scanning also provides microscopic image support for the crack propagation direction, propagation origin, crystal fracture mode and fracture mechanism of the surface crack source region. It can help to better understand the crack propagation mechanism and law of the surface crack source region, and can observe the propagation rate of microcracks in the crack source region, and explain the crack initiation phenomenon. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a structural schematic diagram of the improved "dumbbell-shaped" MT specimen;

[0041] Figure 2 This is a flowchart illustrating the experimental testing method for three-dimensional surface cracks.

[0042] Figure 3 A schematic diagram of the surface crack shape processing;

[0043] Figure 4 Schematic diagram of shear stress in the CFRP-cracked steel plate bonding layer;

[0044] Figure 5 A schematic diagram of the load spectrum for the beach crack marking method;

[0045] Figure 6 A scatter plot of the expansion rate;

[0046] Figure 7 This is a schematic diagram of microcrack propagation on the fracture surface obtained by SEM.

[0047] Figure 8 This is a schematic diagram of the microscopic lattice fracture mode. Detailed Implementation Example

[0048] like Figures 1 to 8 As shown, this is a test method for the fatigue propagation law of cracks on a three-dimensional surface reinforced with CFRP.

[0049] This embodiment tests the fatigue propagation law of three-dimensional surface cracks in CFRP-reinforced composite materials. The method is used to measure the crack propagation law of three-dimensional surface cracks in composite materials. The specimen used in this method is an MT specimen, and this method is applicable to specimens of different sizes. The specimen shape is as follows. Figure 1 As shown.

[0050] The specific execution process of this test method is as follows:

[0051] A. According to Figure 1 The dumbbell-shaped MT tensile specimen was prepared as shown, and the specimen dimensions were measured and recorded.

[0052] B. First, determine the yield load and ultimate failure load of the specimen through quasi-static loading, and observe and record the failure mode and location of the specimen.

[0053] C. Based on the static load data, the fatigue load value of the specimen is initially estimated, and the fatigue crack propagation threshold load is found by using the step-by-step loading method. Then, this load is used to calculate the fatigue crack propagation threshold stress intensity factor.

[0054] D. Record the pre-crack size data, including the initial pre-crack size and the pre-crack fatigue crack size. For example... Figure 6 As shown.

[0055] E. In the formal experiment, the beach marking method is used, with marking performed every 40,000 or 20,000 cycles. The number of marking cycles and the axial displacement are recorded, such as... Figure 5 As shown.

[0056] F. DIC imaging is performed during the marked load cycle. The fatigue load loading frequency is set to 10Hz, and the DIC imaging frame rate is 20 images per second. This allows the acquisition of crack images corresponding to the load peak.

[0057] Because the crack propagation rate differs by several orders of magnitude from that of the stable propagation stage in the later rapid propagation stage, when the crack is found to have propagated to the edge of the specimen, or when the axial displacement of the specimen increases rapidly, the loading program should be terminated and the fracture surface of the specimen should be protected.

[0058] When marking the fracture surface for reading, select the 6 to 8 clearest lines on the fracture surface for measurement and record the data, taking the average of multiple readings.

[0059] The driving force factor in the crack propagation driving force model is the stress intensity factor. The stress intensity factor of the reinforced three-dimensional surface is calculated using finite element method or a fitted calculation formula.

[0060]

[0061] In the above formula The semi-elliptical shape function can be referenced from the Newman-Raju formula, and the attenuation factor after reinforcement is also relevant. It ranges from 0.4 to 0.8 and is related to the crack size and the parameters of the reinforced CFRP.

[0062] The Newman-Raju formula is expressed as:

[0063]

[0064]

[0065] In the formula For the semi-elliptical shape parameters, This is the function for fitting the crack shape.

[0066] During the experiment, the marked load did not cause crack tip closure, so the entire loading process can be described by the Paris formula for crack propagation rate. The crack propagation amount corresponding to each cycle is:

[0067] Corresponding to the crack propagation length at each stage, plot the crack propagation amount and the effective stress intensity factor amplitude in both constant and logarithmic coordinate systems. The function graphs are analyzed in terms of length and depth, respectively, and the fitting driving force model is obtained.

[0068] Extended parameters .

[0069] The fracture surface was cut at a medium speed to control the size of the scanned specimen within 10×20mm. After surface gold spraying to increase conductivity, SEM scanning electron microscopy analysis was performed.

[0070] like Figure 7 As shown, during the scanning process, the overall cross-section was first observed at a low magnification of 200x, and then a high magnification was used to observe the areas with clear crack propagation lines. The magnification ranged from 2000x to 16000x to analyze the propagation law and propagation rate of the microcracks.

[0071] like Figure 8 As shown, high magnification (approximately 8000–20000x) is used to observe the fracture mode of the lattice in areas where the crystal is clear, thereby inferring the fracture mode of the fracture surface.

[0072] By combining microscopic crack initiation fracture analysis and macroscopic fatigue crack propagation analysis, a complete fracture law study was conducted on the specimen.

[0073] This test method meets the requirements of ASTM E647-15e1 fatigue crack propagation rate test, and the experimental loading method meets the requirements of GB / T 4164-2007 plane strain fracture toughness of metallic materials. Experimental methods.

[0074] This test method is based on the surface crack propagation rate after CFRP reinforcement. It can measure the crack propagation of the covered surface in multiple ways, improving test accuracy and enriching the test methods.

[0075] The specimen designed in this invention prevents failure at the pin holes and chamfers, ensuring that failure occurs in the surface crack area during the experiment, thus guaranteeing the expected results. Compared to traditional fatigue loading methods, this invention employs the beach fatigue marking method to mark the crack propagation pattern within the material, allowing for observation and measurement of the crack propagation rate after the experiment. This invention not only focuses on macroscopic crack propagation and fracture modes but also conducts relevant observation and analysis on the microscopic fracture modes and crack initiation regions within the steel plate, better revealing the mechanism of fatigue crack propagation in steel plates.

[0076] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A test method for the fatigue propagation law of cracks on CFRP-reinforced three-dimensional surfaces, characterized in that, The fatigue surface crack propagation specimen shape design method using pin loading resulted in a "dumbbell" shaped overall specimen. Using Q235, Q345, or X80 high-strength steel as the base material, pre-fabricated surface cracks are machined on the steel plate surface using a milling machine or micro-electrical discharge machining. The propagation rate and propagation pattern of the surface cracks are then observed under tensile-tensile loads in a fatigue testing machine using a specific fatigue loading method. The process includes the following steps: A. Prepare a "dumbbell-shaped" center crack tensile MT specimen and measure its actual dimensions; B. Test the yield strength, ultimate strength, failure mode, and failure displacement of the specimen material under static load; C. Perform fatigue crack pre-cracking loading. By gradually increasing the fatigue tensile load, the load required for crack propagation is finally determined as the pre-cracking load, with each increment being 10% of the maximum load. D. Calculate the effective bonding length of CFRP according to the formula, and then bond the effective length of CFRP to the surface of the steel plate; E. During fatigue loading, the "beach crack marking method" is used to mark crack propagation. A fixed cycle number marking method is used, and for specimens with different crack sizes, marking is performed once every 40,000 or 20,000 cycles, and the number of marking cycles is half the number of loading cycles. At the same time, DIC digital imaging technology is used to monitor the strain field of the crack covered by CFRP. F. Measure the fracture surface cracks, measuring the length and depth of the cracks under each marked load level, and substituting them into the driving force model to analyze the fatigue crack propagation rate; the driving force model adopts a two-degree-of-freedom model to describe the propagation rate of the surface crack in both length and depth directions, specifically expressed as: The crack propagation rate along the length direction can be expressed by the following formula: The crack propagation rate in the depth direction can be expressed by the following formula: The fracture surface was scanned using a scanning electron microscope (SEM) to analyze its microscopic fracture mode and fracture mechanism.

2. The test method for the fatigue propagation law of CFRP-reinforced three-dimensional surface cracks according to claim 1, characterized in that: In step D, when the bonding length increases to a certain value, the shear stress of the bonding layer no longer increases. The calculation formulas for the shear stress of the bonding layer and the strain distribution of CFRP are shown in (1) to (4): ; ; ; In the formula, This indicates the elastic modulus of the steel plate. This represents the elastic modulus of CFRP. Indicates the shear modulus of the adhesive layer. Indicates the thickness of the steel plate. Indicates CFRP thickness. Indicates the thickness of the adhesive layer. Indicates the tensile force at the loading end. Indicates the bonding length.

3. The test method for the fatigue propagation law of CFRP-reinforced three-dimensional surface cracks according to claim 1, characterized in that: Crack initiation and propagation are influenced by stress intensity factor. For the control of three-dimensional semi-elliptical surface cracks, the stress intensity factor is expressed by the Newman-Raju formula.

4. The test method for the fatigue propagation law of CFRP-reinforced three-dimensional surface cracks according to claim 1, characterized in that: In step E, the "beach marking method" is used during fatigue loading, and 0.5 times the maximum load is selected.

5. The test method for the fatigue propagation law of CFRP-reinforced three-dimensional surface cracks according to claim 1, characterized in that: Step F also includes using a scanning electron microscope (SEM) to observe the crack initiation region, microcrack propagation direction, lattice fracture mode, and fracture mechanism when performing microscopic analysis of the fracture surface.

6. The test method for the fatigue propagation law of CFRP-reinforced three-dimensional surface cracks according to claim 1, characterized in that: In step A, the measured dimensions include the pin hole size, pin end thickness, tensile section thickness, transition zone chamfer, surface crack shape and crack size, and then the surface crack propagation rate and propagation law are observed under tensile load in a fatigue testing machine through a specific fatigue load loading method.

Images