DIC-based actual measurement method of crack tip strain energy density factor

Abstract

The invention provides a DIC-based actual measurement method of a crack tip strain energy density factor, and relates to the field of fracture mechanics. The method comprises the following steps that a DIC system is established and debugged; speckles are sprayed on the surface of a CT sample, the CT sample is placed on a testing machine, the initial length of the CT sample is measured, and the DIC scale ratio is calculated; the method comprises the following steps: performing a tensile test on a CT sample by using a testing machine, acquiring an image during the tensile test by using a DIC system, analyzing the change of speckles on the surface of the CT sample before and after deformation, and positioning the position of a crack tip to obtain strain information of a pixel point of the crack tip; and analyzing the strain information of the pixel points at the tip of the crack, and solving a strain energy density factor. According to the method, starting from the concept of the strain energy density factor, the strain of the crack tip can be directly measured, the transition by calculating the stress intensity factor is not needed, and the calculation efficiency is improved. In addition, the method belongs to non-contact optical measurement, and instrument sensitivity errors caused by contact measurement can be avoided.

Description

Technical Field

[0001] The present invention relates to the field of fracture mechanics, and particularly to a method for measuring the strain energy density factor at the crack tip based on DIC. Background Art

[0002] The crack tip of a cracked object has stress singularity, that is, the stress infinitely close to the crack tip tends to infinity. To solve this problem, researchers have proposed several fracture mechanics indexes that can reflect the stress level at the crack tip. Among them, the most widely used is the stress intensity factor K. However, the stress intensity factor can only reflect the magnitude of the stress level and can be used to predict whether the crack will expand, but it cannot predict the expansion direction of the crack. Therefore, the strain energy density factor S is proposed in the industry. This index can not only reflect whether the crack will expand, but also judge the expansion direction of the crack by calculating the minimum value of the strain energy density factor at the crack tip in each direction.

[0003] However, in the prior art, the calculation of the strain energy density factor S must simplify the actual problem into a plane strain state or a plane stress state in order to obtain the coefficient κ. This is relatively simple for engineering problems that can be simplified into a two-dimensional plane state, but it is sometimes difficult to achieve for complex actual three-dimensional engineering components. If forced to simplify, errors may occur in the results.

[0004] In addition, the strain energy density factor must be calculated through the stress intensity factor, which results in the lack of an effective measurement method for the strain energy density factor in the tests or actual measurements of actual engineering components. It can only measure the stress intensity factor at the crack tip first and then calculate through the relationship between the two, and the process is relatively cumbersome, and the advantage of the strain energy density factor as an index based on the energy state is not utilized.

[0005] Due to the above series of disadvantages, the use of the strain energy density factor as a fracture mechanics index has been greatly restricted. Summary of the Invention

[0006] Object of the Invention: To propose a method for measuring the strain energy density factor at the crack tip based on DIC, overcome the above disadvantages of the strain energy density factor, measure the strain at the crack tip through digital image correlation technology (DIC), provide a method for directly measuring the strain energy density factor at the crack tip, without the need to measure the stress intensity factor first and then calculate the strain energy density factor, nor the need to simplify the actual engineering component into a plane strain state or a plane stress state, thereby effectively solving the above problems existing in the prior art.

[0007] A method for measuring the strain energy density factor at the crack tip based on DIC proposed by the present invention includes the following steps:

[0008] Set up and debug the DIC system, where the DIC system includes at least one high-definition camera and a light source;

[0009] Spray speckles on the surface of the CT specimen, place it on the testing machine, measure its initial length, and calculate the DIC scale ratio;

[0010] Use the testing machine to conduct a tensile test on the CT specimen, use the DIC system to collect images during the tensile test, analyze the changes in the speckles on the surface of the CT specimen before and after deformation, locate the crack tip position, and obtain the strain information of the crack tip pixel points;

[0011] Analyze the strain information of the crack tip pixel points and solve the strain energy density factor.

[0012] In a further embodiment, the surface of the CT specimen does not contain an initial crack. Place the CT specimen without an initial crack on the testing machine for a tensile test, and a crack will occur after reaching the first predetermined load.

[0013] In a further embodiment, the surface of the CT specimen contains an initial crack. Place the CT specimen with an initial crack on the testing machine for a tensile test, and the initial crack will propagate after reaching the second predetermined load.

[0014] In a further embodiment, the DIC scale ratio is the actual object length corresponding to the length of a single pixel in the image obtained by the DIC system.

[0015] In a further embodiment, use the testing machine to conduct a tensile test on the CT specimen. Under the triaxial stress state, the strain energy density ω at the center position of the pixel point directly opposite the crack tip satisfies the following relationship:

[0016]

[0017] In the formula, σ1, σ2, and σ3 are the three principal stresses at the center position of the pixel point; ε1, ε2, and ε3 are the three principal strains; E is the elastic modulus of the material; υ is the Poisson's ratio of the material.

[0018] In a further embodiment, use the testing machine to conduct a tensile test on the CT specimen. Under the uniaxial stress state, the strain energy density ω at the center position of the pixel point directly opposite the crack tip satisfies the following relationship:

[0019]

[0020] In the formula, σ1 is the first principal stress; ε1 is the first principal strain; E is the elastic modulus of the material.

[0021] In a further embodiment, analyze the strain information of the crack tip pixel points and solve the strain energy density factor S:

[0022] S = ω·r = ωl / 2

[0023] Where ω is the strain energy density at the center position of the pixel directly opposite to the crack tip; r is taken as the distance from the center point of this pixel to the crack tip, and r = l / 2, where l is the DIC scale ratio.

[0024] In a further embodiment, the present method is applicable to the measurement of the strain energy density factor at the crack tip of a cracked object composed of isotropic materials.

[0025] In addition, the present invention also provides an electronic device, which includes a processor and a memory storing computer program instructions; when the processor executes the computer program instructions, the above-mentioned actual measurement method of the strain energy density factor at the crack tip based on DIC is implemented.

[0026] In addition, the present invention also provides a computer-readable storage medium, in which at least one executable instruction is stored. When the executable instruction runs on an electronic device, the electronic device is caused to execute the above-mentioned actual measurement method of the strain energy density factor at the crack tip based on DIC.

[0027] Compared with the prior art, the present invention has at least the following beneficial effects:

[0028] (1) The existing strain energy density factor must be calculated from the stress intensity factor and cannot be solved independently. It still depends on the stress intensity factor, and there is currently no direct measurement method; while this invention patent starts from the concept of the strain energy density factor and can be directly obtained by measuring the strain at the crack tip, without the need to transition through calculating the stress intensity factor, thus improving the calculation efficiency.

[0029] (2) For three-dimensional complex components, the existing strain energy density factor method must simplify them to a plane stress state or a plane strain state, which is difficult to simplify for some actual engineering components; in this invention patent, the strain energy density factor can be directly obtained from the true strain value at the crack tip of a three-dimensional component, without the need to simplify the engineering component two-dimensionally.

[0030] (3) The present invention belongs to a non-contact optical measurement method, which can avoid the instrument sensitivity error brought by the contact measurement method. Description of the Drawings

[0031] Figure 1 It is a polar coordinate definition diagram of the crack tip.

[0032] Figure 2 It is a flowchart of the method of this patent.

[0033] Figure 3 It is the establishment and schematic diagram of the DIC system.

[0034] Figure 4 This is the schematic diagram for analyzing the pixel points at the crack tip of the patented method.

[0035] Figure 5 This is a schematic diagram of the CT specimen in the embodiment after spraying speckles and being installed on the testing machine.

[0036] Figure 6 This is the analysis diagram of the pixel points at the crack tip of the CT specimen in the embodiment.

[0037] Figure 7 This is the comparison diagram of the strain energy density factor measured by the patented method at the crack tip of the CT specimen in the embodiment under different crack lengths and the strain energy density factor calculated by the stress intensity factor based on the empirical formula. Detailed implementation manners

[0038] In the following description, a large number of specific details are given to provide a more thorough understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present invention, some technical features well known in the art are not described.

[0039] Before elaborating on the embodiments, first define or explain some proprietary terms involved in this application:

[0040] DIC: Digital Image Correlation, digital image correlation technology;

[0041] CT specimen: Compact Tension specimen, compact tension specimen.

[0042] The strain energy density factor S can not only reflect whether the crack propagates, but also judge the crack propagation direction by calculating the minimum value of the strain energy density factor at the crack tip in each direction.

[0043] Define the polar coordinates of the crack tip as shown in Figure 1 Let the strain energy density at a point at a distance r from the crack tip be ω, θ be the polar coordinate angle value of this point, and r be the polar coordinate radius value of this point. The traditional strain energy density factor S at the crack tip needs to be calculated by Equation (1):

[0044]

[0045] In the formula, K I 、K II and K III are the stress intensity factors at the crack tip in the states of mode I crack, mode II crack and mode III crack respectively; the coefficients a 11 、a 12, a 22 and a 33 can be calculated respectively by formulas (2)-(5):

[0046]

[0047] In the formulas, G is the shear modulus, which can be calculated by formula (6), and the coefficient κ can be calculated by formula (7).

[0048] G = E / 2(1 + ν) (6)

[0049]

[0050] In the formulas, E is the elastic modulus of the material; υ is the Poisson's ratio of the material.

[0051] It can be seen from formula (7) that in order to obtain the coefficient κ, the actual problem must be simplified to a plane strain state or a plane stress state in the calculation of the strain energy density factor S. This is relatively simple for engineering problems that can be simplified to a two-dimensional plane state, but it is sometimes difficult to achieve for complex actual three-dimensional engineering components. If forced simplification is carried out, errors may occur in the results. In addition, it can be seen from formula (1) that the strain energy density factor must be calculated through the stress intensity factor. This results in the fact that in the tests or actual measurements of actual engineering components, there is no effective measurement method for the strain energy density factor. Only the stress intensity factor at the crack tip can be measured first, and then calculated through the relationship formula (1) between the two, and the process is relatively cumbersome, and the advantage of the strain energy density factor as an energy state-based index has not been utilized. Due to the above series of disadvantages, the use of the strain energy density factor, a fracture mechanics index, has been greatly restricted.

[0052] To solve the above technical problems, this embodiment adopts a method for actually measuring the strain energy density factor at the crack tip based on DIC. This method is applicable to the measurement of the strain energy density factor at the crack tip of a cracked object composed of isotropic materials. The flow chart is shown in Figure 2 :[[]]END]]

[0053] S1. Establish a DIC system using a high-definition camera. The establishment and basic principle of the DIC system are as Figure 3 shown. Debug the DIC system to make it meet the accuracy requirements.

[0054] S2. Spray speckles on the surface of the cracked object, install the object on the test equipment, and through length measurement, obtain the DIC scale ratio l (the actual object length corresponding to the length of a single pixel in the image obtained by the DIC system is l, that is, the side length of a single pixel is l)

[0055] S3. Conduct tests or actual measurements on cracked objects, use the DIC system to collect images during the tests or actual measurements, analyze the changes in the speckles on the object surface before and after deformation, locate the crack tip, and obtain the strain information at the crack tip.

[0056] S4. Solve the strain energy density factor based on the strain information of the pixel points at the crack tip obtained through DIC analysis.

[0057] A typical pixel map is as Figure 4 shown. The grid in the right figure represents each pixel point after image magnification. Through image analysis, the position of the crack tip is located, and the pixel point directly opposite the crack tip is as Figure 4 shown. Let the side length of the pixel point be l. Taking the triaxial stress state as an example, according to the definition of strain energy density, the strain energy density ω at the center position of the pixel point is solved. For isotropic materials, it can be expressed as Equation (8):

[0058]

[0059] where σ1, σ2, and σ3 are the three principal stresses at this point; ε1, ε2, and ε3 are the three principal strains; E is the elastic modulus of the material; and υ is the Poisson's ratio of the material. ε1, ε2, and ε3 in the above formula can all be obtained through DIC analysis. If the component is in a uniaxial stress state, then Equation (8) can be further simplified to Equation (9):

[0060]

[0061] In the formula, σ1 is the first principal stress; ε1 is the first principal strain; and E is the elastic modulus of the material.

[0062] Through Equation (8) or Equation (9), the strain energy density ω at the center of this pixel point can be calculated, and thus the strain energy density factor S at the crack tip can be calculated through Equation (10):

[0063] S = ω·r = ωl / 2 (10)

[0064] For this pixel point, its polar coordinate angle θ = 0, and r is taken as the distance from the pixel center point to the crack tip, that is, r = l / 2.

[0065] Taking the fatigue crack growth test of a steel compact tension (CT) specimen as an example, the specific implementation process of the present invention is described:

[0066] The dimensions of a certain CT specimen are designed as Figure 5 shown. The crack length is defined as the distance from the center of the screw loading force to the crack tip, denoted as a. Random speckles are sprayed on the specimen surface. The specimen is installed on the testing machine, and the DIC system is adjusted so that its accuracy meets the requirements, as Figure 5 shown. The maximum force F is applied at the positions of the two screw holes.max A cyclic load with a load amplitude of 37 kN and a stress ratio R = 0.1 was applied, and the CT specimen was in a mode-I crack state at this time. As the crack propagated, the change in the speckle was analyzed by DIC to obtain the strain information near the crack tip at different crack lengths. Taking the crack length a = 19.574 mm and the applied load reaching F max = 37 kN as an example, the vertex of the crack tip was located by image, as Figure 6 shown. The pixel point directly opposite the vertex of the crack tip ( Figure 5 the left side in the figure) was analyzed. Through DIC analysis, the principal strain at the center position of this pixel point was obtained. At this time, the principal strain was ε1 = 2.252% = 2.252×10 -2 , as Figure 6 shown. Since the CT specimen was in a mode-I crack state, the strain energy ω = 53.3 J / m 3 of this pixel point could be calculated by substituting it into Equation (9). If it was a multi-axial stress state, Equation (8) needed to be substituted. Further, substituting the calculated strain energy density ω into Equation (10), according to the scale ratio of this DIC system, the actual measured value of the strain energy density factor at the crack tip was obtained as S = 1.99×10 -3 MN / m.

[0067] The Chinese standard "GBT 6398-2017: Metallic materials - Fatigue testing - Fatigue crack growth method" gives the empirical formula for calculating the stress intensity factor when the CT specimen is under tension, as shown in Equations (11)-(13):

[0068]

[0069] In the formula, K I is the stress intensity factor under mode-I crack, F is the axial force, T is the plate thickness, which is 15 mm in this example, W is the distance from the axis of the axial force to the far edge position, which is 60 mm in this example, and g is the shape factor, which can be obtained from Equation (12).

[0070] Since the CT specimen was in a mode-I crack state, Equation (1) could be simplified to Equation (14):

[0071]

[0072] Thus, the theoretical calculated value of the strain energy density factor at the crack tip was calculated from Equation (14) as S = 1.94×10 -3 MN / m. The calculation and comparison at different crack lengths were similar. Thus, the comparison between the two at different crack lengths was as Figure 7As shown, it can be seen that the result matching degree is good. Both increase with the increase of the crack length, and the error is within 20%, which can meet the requirements of engineering calculations. This shows that the method for measuring the strain energy density factor at the crack tip based on DIC proposed in this invention patent has good accuracy, and the testing method is relatively simple and convenient. It can meet the measurement of the strain energy density factor at the crack tip of cracked components made of various engineering single isotropic materials, and provide support for the fracture mechanics analysis of cracked components and structures in various high-end equipment manufacturing industries such as aerospace and ocean engineering.

[0073] The technical process of the method for measuring the strain energy density factor at the crack tip based on DIC disclosed in the above embodiment can be implemented in whole or in part by software, hardware, firmware or any other combination.

[0074] When implemented by hardware, in the above embodiment, all or part of the working logic and calculation process can be run on an electronic device after being compiled by software. The electronic device includes a processor, a memory, a communication interface and a communication bus. The processor, the memory and the communication interface complete the communication with each other through the communication bus. The memory is used to store at least one executable instruction, and the executable instruction causes the processor to execute the technical process of the method for measuring the strain energy density factor at the crack tip based on DIC disclosed in the above embodiment.

[0075] When implemented by software, in the above embodiment, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions or computer programs. If the above method is implemented in the form of a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of the embodiments of the present application, in essence, or the part that contributes to the related technology, can be embodied in the form of a software product. The software product is stored in a storage medium and includes several instructions to cause an electronic device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the methods described in the embodiments of the present application. The foregoing storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memories (ROMs), magnetic disks or optical discs that can store program codes. In this way, the embodiments of the present application are not limited to any specific hardware, software or firmware, or any combination of hardware, software and firmware.

[0076] As described above, although the present invention has been shown and described with reference to specific preferred embodiments, it should not be construed as a limitation of the present invention itself. Various changes can be made to it in form and detail without departing from the spirit and scope of the present invention defined by the appended claims.

Claims

1. An experimental measurement method for strain energy density factor at crack tip based on DIC, characterized in that, The steps include the following: Establish a DIC system and debug it. The DIC system includes at least one high-definition camera and a light source; Spray speckles on the surface of the CT specimen, place it on the testing machine, measure its initial length, and calculate the DIC scale ratio; Use the testing machine to conduct a tensile test on the CT specimen, use the DIC system to collect images during the tensile test, analyze the changes in the speckles on the surface of the CT specimen before and after deformation, locate the crack tip position, and obtain the strain information of the crack tip pixel points; Analyze the strain information of the crack tip pixel points and solve the strain energy density factor.

2. The measured method of the strain energy density factor at the crack tip based on DIC according to claim 1, characterized in that, The surface of the CT specimen does not contain an initial crack. Place the CT specimen without an initial crack on the testing machine for a tensile test, and a crack will be generated after reaching the first predetermined load.

3. The measured method of the strain energy density factor at the crack tip based on DIC according to claim 1, characterized in that The surface of the CT specimen contains an initial crack. Place the CT specimen with an initial crack on the testing machine for a tensile test, and the initial crack will propagate after reaching the second predetermined load.

4. The measured method of crack tip strain energy density factor based on DIC according to claim 1, characterized in that The DIC scale ratio is the actual object length corresponding to the length of a single pixel in the image obtained by the DIC system.

5. The measured method of crack tip strain energy density factor based on DIC according to claim 1, characterized in that Use the testing machine to conduct a tensile test on the CT specimen. Under the triaxial stress state, the strain energy density ω at the center position of the pixel directly opposite the crack tip satisfies the following relational expression: In the formula, σ1, σ2, and σ3 are the three principal stresses at the center position of the pixel; ε1, ε2, and ε3 are the three principal strains; E is the elastic modulus of the material; υ is the Poisson's ratio of the material.

6. The measured method of the strain energy density factor at the crack tip based on DIC according to claim 1, characterized in that, Use the testing machine to conduct a tensile test on the CT specimen. Under the uniaxial stress state, the strain energy density ω at the center position of the pixel directly opposite the crack tip satisfies the following relational expression: In the formula, σ1 is the first principal stress; ε1 is the first principal strain; E is the elastic modulus of the material.

7. The measured method of crack tip strain energy density factor based on DIC according to claim 5 or 6, characterized in that, Analyze the strain information of the crack tip pixel points and solve the strain energy density factor S: S = ω·r = ωl / 2 In the formula, ω is the strain energy density at the center position of the pixel directly opposite the crack tip; r is taken as the distance from the center point of this pixel to the crack tip, and r = l / 2, where l is the DIC scale ratio.

8. The measured method of the strain energy density factor at the crack tip based on DIC according to claim 1, characterized in that, It is applicable to the measurement of the strain energy density factor at the crack tip of a cracked object composed of isotropic materials.

9. An electronic device, characterized in that, The device includes: a processor and a memory storing computer program instructions; when the processor executes the computer program instructions, it implements the method for measuring the strain energy density factor at the crack tip based on DIC as described in any one of claims 1 to 8.

10. A computer-readable storage medium, characterized in that, At least one executable instruction is stored in the storage medium. When the executable instruction runs on an electronic device, the electronic device executes the method for measuring the strain energy density factor at the crack tip based on DIC as described in any one of claims 1 to 8.

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