Method for estimating plane strain fracture toughness of material based on small test piece and energy principle

Abstract

This invention belongs to the field of material and structural performance testing technology, specifically relating to a method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles. The method includes the following steps: S1, designing small specimens for fracture toughness testing; S2, conducting fracture toughness testing; S3, analyzing the fracture morphology of the small specimens; S4, calculating the energy consumed per unit area in each region; and S5, calculating the plane strain fracture toughness of the material. The method described in this invention for estimating the plane strain fracture toughness of materials based on small specimens and energy principles can significantly improve the efficiency of obtaining plane strain fracture toughness and save on related experimental costs. Furthermore, the entire process is simple, convenient, and fast, with strong engineering practicality and applicability.

Description

Technical Field

[0001] This invention relates to the field of material and structural performance testing technology, specifically to a method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles. Background Technology

[0002] plane strain fracture toughness K of the material IC This reflects a material's ability to resist crack propagation under stress, an inherent property of the material itself. The plane strain fracture toughness K of the material is obtained. IC It is of great significance. First, K IC It can be used for reliability assessment of engineering structures. In the engineering field, this indicator is crucial for predicting and preventing structural failures, such as in buildings, aerospace components, and automotive parts. IC It can serve as an important parameter for crack propagation calculation and probabilistic damage tolerance assessment. Secondly, K... IC It can be used to assist in material selection and design optimization. In the process of designing and manufacturing engineering materials, understanding their plane strain fracture toughness can help engineers select the most suitable material type and properties for specific applications; by analyzing the K-axis of different materials... IC Comparing values ​​can optimize structural design, improve product performance and lifespan, and reduce unnecessary waste. Furthermore, K... IC It can also be used for structural loss assessment and prediction. Utilizing plane strain fracture toughness, it can help determine the potential damage modes and assess the extent of damage during service, thereby enabling the implementation of relevant measures to repair the damage or replace structural materials, thus better protecting the safety of people's property. Furthermore, K... IC It can be used to promote material improvement and research and development. By studying and understanding the plane strain fracture toughness of materials, it can guide the research and development of new materials, which helps to improve existing materials or discover new materials with higher performance, thus promoting the development of materials science and engineering.

[0003] However, to obtain the plane strain fracture toughness K of the material IC Finding the value of K is quite difficult. Typically, the plane strain fracture toughness K of a material is... IC Only when the specimen thickness meets the relevant requirements can the measured fracture toughness be regarded as plane strain fracture toughness.

[0004] Specifically, current methods for determining the plane strain fracture toughness (K) of metallic materials both domestically and internationally... IC The method standards mainly include ISO 12737, GB / T 4161, and ASTM E339. According to the descriptions in these standards, K... IC The structural dimensions of the test specimens must satisfy the following three inequalities:

[0005] a0,W-a0,B≥2.5(K IC / σ y ) 2 ;

[0006] Where a0 is the pre-existing crack length, W is the specimen width, B is the specimen thickness, and K... IC It is the plane strain fracture toughness of the specimen; σ y The yield strength of the material.

[0007] According to the above formula, for high-strength and ultra-high-strength materials, their yield strength σ y Higher, K IC The yield strength σ is relatively low, therefore the test specimen thickness is small, and the testing process is relatively simple. However, for some materials with high toughness, the yield strength σ is relatively high. y Lower, K IC A higher value indicates that the plane strain fracture toughness K of the material is obtained. IC The required specimen thickness is very large; plane strain fracture toughness specimens often require thicknesses of 50 mm or even over 100 mm. When the specimen thickness is too large, it negatively impacts the material's plane strain fracture toughness K. IC The detection has the following problems:

[0008] First, the force required to conduct fracture toughness tests on these large test pieces far exceeds the maximum tensile force of common universal tensile testing machines, which brings great difficulties to the tests; moreover, conducting related tests will result in a considerable energy consumption.

[0009] Second, when the specimen thickness is too large, the amount of material required for testing is also large, which is not conducive to saving materials and reducing costs, especially for expensive or hard-to-obtain materials, such as single crystals currently used in aero-engine blades, the testing cost is too high.

[0010] Therefore, researching a method to obtain the plane strain fracture toughness of materials using small-sized test specimens (hereinafter referred to as "small specimens") can save materials and reduce testing costs, while also reducing the tensile force required for testing and improving detection efficiency.

[0011] Chinese patents with publication (announcement) numbers CN111767664A and CN108844806A provide some methods for determining the plane strain fracture toughness of metallic materials using samples with small thickness dimensions. However, these methods may have drawbacks such as being cumbersome to operate, requiring the construction of data models, or having a limited scope of application. Therefore, they cannot meet the needs of plane strain fracture toughness testing of materials on a large scale.

[0012] Therefore, providing a method for estimating the plane strain fracture toughness of materials that is widely applicable and easy to operate is of great significance to those skilled in the art. Summary of the Invention

[0013] The purpose of this invention is to address the aforementioned technical problems and, in order to meet the application requirements of accurate surface roughness of specimens, convenient specimen processing, and universality of testing methods, provide a method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles. This method is widely applicable and easy to operate, thereby achieving accurate and efficient detection of the plane strain fracture toughness of materials.

[0014] In view of this, the present invention provides a method for estimating the plane strain fracture toughness of materials based on small specimens and the energy principle, characterized by comprising the following steps:

[0015] S1, Design small specimens for fracture toughness testing: Design at least three different thicknesses of small specimens based on the material's mechanical properties and testing standards;

[0016] S2, Conduct fracture toughness tests: Conduct fracture toughness tests on the small specimens obtained in step S1 according to the test standards, obtain the force-displacement curves of the small specimens, and the small specimens after fracture;

[0017] S3, Small specimen fracture morphology analysis: Three-dimensional morphology images of the small specimen fracture surface were taken to obtain three-dimensional morphology point cloud images of the fracture surface. According to the different fracture surface forms, the fracture surface area was divided into plane strain fracture area and plane stress fracture area, and the area of ​​different fracture areas was calculated.

[0018] S4, Calculate the energy consumed per unit area in each region: Based on the energy principle, using the force-displacement curve obtained in step S2, calculate the energy consumed per unit area in the plane strain fracture region, the energy consumed per unit area in the plane stress fracture region, and the dissipation work per unit area in the entire fracture region during the fracture process.

[0019] S5, Calculate the plane strain fracture toughness of the material: Based on the energy consumed per unit area of ​​each region calculated in step S4, and combining the relationship between the energy principle and the plane strain fracture toughness, the plane strain fracture toughness is calculated.

[0020] Furthermore, in step S1, the relevant dimensions of the small specimen, except for the thickness, are designed according to the test standards.

[0021] Furthermore, in step S1, the thickness of each small specimen should be selected such that the load required for the small specimen in the fracture toughness test does not exceed the maximum tensile force of the tensile machine.

[0022] Furthermore, the small specimen is a compact tensile specimen or a three-point bend specimen.

[0023] Furthermore, in step S2, the fracture toughness test process is divided into two stages:

[0024] Pre-crack stage: The amplitude, loading frequency, and pre-crack length on the surface are set according to the size of the specimen;

[0025] Fracture toughness tensile stage: Based on the specimen size, the rate of increase of the stress intensity factor is controlled by controlling the stress loading rate. The loading continues until the specimen fractures. At the same time, the force and notch-opening displacement data of the specimen are collected using the testing machine software to obtain the force-displacement curve.

[0026] Furthermore, in step S3, a three-dimensional morphological image of the fracture surface is captured using a super depth-of-field three-dimensional video microscope.

[0027] Furthermore, in step S3, the position information of each point on the fracture surface is obtained through the three-dimensional topographic point cloud map of the fracture surface, and the area of ​​each fracture region is calculated based on the position information of each point.

[0028] Furthermore, in step S3, the fracture region is divided according to the different fracture surface types as follows:

[0029] The fracture surface near the surface is under plane stress. The fracture surface at this point forms an angle of approximately 45° with the surface, and the fracture is shear. This region is designated as the plane stress fracture region, and its area is denoted as A. s ;

[0030] The fracture regions other than the plane stress fracture region are in a plane strain state. The fracture surface in this region undergoes normal fracture. This region is denoted as the plane strain fracture region, and its area is labeled A. b ;

[0031] In addition, the entire three-dimensional area of ​​the fracture region is labeled as A. t All planar areas are labeled A. p .

[0032] Furthermore, in step S4, the process of calculating the energy per unit area of ​​each region is as follows:

[0033] Based on the energy principle and the force-displacement curve obtained in step S2, the following equation (5) is obtained:

[0034]

[0035] And, γ s ,γ b ,γ d >0;

[0036] Where, γ s γ represents the energy consumed per unit area in the plane stress fracture zone. bγ represents the energy consumed per unit area of ​​the corresponding plane strain fracture zone. d A represents the work dissipated per unit area during the fracture process; s i A b i A t i represent the sum of the areas of the plane stress fracture zone, the plane strain fracture zone, and all fracture regions in each small specimen, respectively; i represents different small specimens; W t This represents the work done by external forces during the fracture toughness test.

[0037] Then, using samples of various thicknesses, at least three different equations are obtained according to equation (5), and three of these equations are selected to form a set of equations.

[0038] Based on the system of equations, γ is obtained by solving the equations. s γ b and γ d The value of .

[0039] Furthermore, in step S5:

[0040] plane strain fracture toughness K of the material IC The following formula (7) is used to calculate:

[0041]

[0042] Where E is the elastic modulus of the material; v is the Poisson's ratio of the material.

[0043] This invention proposes a method for estimating the plane strain fracture toughness of materials based on small specimens and the energy principle, combining the energy principle and small-scale fracture toughness testing. Compared with existing technologies, the method for estimating the plane strain fracture toughness of materials described in this invention has the advantages of wide applicability, simple operation, high efficiency, and good economy, specifically in the following aspects:

[0044] (1) The method for estimating the plane strain fracture toughness of materials described in this invention has a wide range of applications: For structures of commonly used metallic materials, such as turbine disks of aero engines, wheels and rails of airplanes and high-speed trains, hulls of ships, petrochemical pipelines and high-pressure containers, it is necessary to obtain the plane strain fracture toughness of the materials used. Often, the specimen size required to measure the plane strain fracture toughness of these materials is very large. When it is difficult to obtain the plane strain fracture toughness of the materials using conventional inspection methods, this method can be used to estimate the plane strain fracture toughness.

[0045] (2) The method for estimating the plane strain fracture toughness of materials described in this invention is simple to operate: This method adds only one step to the traditional fracture toughness measurement test, namely, taking a three-dimensional topographic point cloud map of the fracture surface, and there are already relevant devices that can intelligently process this step, so the operation process is simple and easy to handle.

[0046] (3) The method for estimating the plane strain fracture toughness of materials described in this invention is highly efficient: After obtaining the test results, this invention only needs to solve a three-element linear equation to obtain the test results, which is fast and efficient.

[0047] (4) The method for estimating the plane strain fracture toughness of materials described in this invention is economical: This invention avoids the use of large-sized specimens to measure plane strain fracture toughness, which saves materials on the one hand and energy consumption during the test on the other hand, thus making it economical.

[0048] In summary, the method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles described in this invention can significantly improve the efficiency of obtaining plane strain fracture toughness and save the cost of related experiments. Moreover, the entire process is simple, convenient, fast, and has strong engineering practicality and applicability. Attached Figure Description

[0049] Figure 1 This is a flowchart of the method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles, as described in this invention.

[0050] Figure 2 This is the force-displacement curve obtained during the fracture toughness test in Embodiment 1 of the present invention;

[0051] Figure 3 This is a three-dimensional point cloud map of the fracture surface obtained during the fracture morphology analysis of the small specimen in Embodiment 1 of the present invention;

[0052] Figure 4 This is a fracture region division diagram obtained according to the fracture form in Embodiment 1 of the present invention. Detailed Implementation

[0053] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0054] In the description of this application, it should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. For ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following drawings denote similar items; therefore, once an item is defined in one drawing, it need not be further discussed in subsequent drawings.

[0055] It should be noted that the term "and / or" in the specification and claims of this application means at least one of the connected objects, and the character " / " generally indicates that the preceding and following related objects are in an "or" relationship.

[0056] It should be noted that in the description of this application, the directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this application. The directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0057] It should be noted that, in this application, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, it should be noted that the scope of the methods and apparatuses in the embodiments of this application is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0058] This invention provides a method for estimating the plane strain fracture toughness of materials based on small specimens and the energy principle. The implementation process is as follows: Figure 1 As shown, the steps include:

[0059] S1, Design small specimens for fracture toughness testing: Design at least three different thicknesses of small specimens based on the material's mechanical properties and testing standards;

[0060] S2, Conduct fracture toughness tests: Conduct fracture toughness tests on the small specimens obtained in step S1 according to the test standards, obtain the force-displacement curves of the small specimens, and the small specimens after fracture;

[0061] S3, Small specimen fracture morphology analysis: Three-dimensional morphology images of the small specimen fracture surface were taken to obtain three-dimensional morphology point cloud images of the fracture surface. According to the different fracture surface forms, the fracture surface area was divided into plane strain fracture area and plane stress fracture area, and the area of ​​different fracture areas was calculated.

[0062] S4, Calculate the energy consumed per unit area in each region: Based on the energy principle, using the force-displacement curve obtained in step S2, calculate the energy consumed per unit area in the plane strain fracture region, the energy consumed per unit area in the plane stress fracture region, and the dissipation work per unit area in the entire fracture region during the fracture process.

[0063] S5, Calculate the plane strain fracture toughness of the material: Based on the energy consumed per unit area of ​​each region calculated in step S4, and combining the relationship between the energy principle and the plane strain fracture toughness, the plane strain fracture toughness is calculated.

[0064] Furthermore, in step S1, at least three small specimens of different thicknesses can be designed based on the relevant mechanical properties of the material, such as elastic modulus, yield strength, etc., and relevant test standards. At the same time, the other relevant dimensions of the small specimens, except for the thickness, are designed according to relevant test standards.

[0065] As some embodiments of the present invention, the other relevant dimensions of the small specimen, except for the thickness, can be determined according to domestic and international methods for measuring the plane strain fracture toughness (K) of metallic materials. IC The design should be carried out in accordance with the methods and standards, such as ISO 12737, GB / T 4161 and ASTM E339.

[0066] As some embodiments of the present invention, in step S1, if it is desired to obtain effective plane strain fracture toughness through experiment, the thickness B of traditional large-size specimens can first be estimated according to existing methods. The thickness B of the specimen should satisfy the following formula (1):

[0067] B≥2.5(K IC / σ y ) 2(1)

[0068] In equation (1), σ y It is the yield strength of the material, the value of which can be obtained through testing or by consulting a material handbook; K IC The plane strain fracture toughness of the specimen, as some examples of the present invention, can be initially estimated by the following formula (2).

[0069]

[0070] In equation (2), n is the strain hardening exponent of the material, E is the elastic modulus, and ε is the elastic modulus. f ε is the true fracture strain under uniaxial tension. f =ln(1+ψ), where ψ is the reduction of area under uniaxial tension.

[0071] It should be noted that, in addition to equation (2), other relevant formulas can also be used to calculate K according to existing technology. IC Make a forecast.

[0072] After estimating the thickness B of the traditional large-size specimen, the maximum tensile force of the tensile testing machine required to conduct fracture toughness tests on these large-size specimens can be estimated, and the thickness of the large-size specimen can be evaluated based on the estimated maximum tensile force. Then, based on the evaluation results, at least three different thicknesses of small specimens should be designed, specifically selecting the thickness of each small specimen so that the load required for the fracture toughness test does not exceed the maximum tensile force of the tensile testing machine.

[0073] Therefore, it can be seen that in existing technologies, when the specimen thickness is too large, the force required to conduct fracture toughness tests on these large specimens far exceeds the maximum tensile force of common universal tensile testing machines, making the tests impossible. However, in this invention, the plane strain fracture toughness of materials can be estimated using small-sized specimens through the method described herein, and the load required for each small specimen in the fracture toughness test does not exceed the maximum tensile force of the tensile testing machine. Therefore, the method for estimating the plane strain fracture toughness of materials described in this invention has a very wide range of applications and can be applied to the estimation of plane strain fracture toughness of almost all materials.

[0074] Furthermore, the small specimen is a compact tensile specimen or a three-point bend specimen.

[0075] Furthermore, in step S2, the fracture toughness test process is divided into two stages:

[0076] Pre-crack stage: The amplitude, loading frequency, and pre-crack length on the surface are set according to the size of the specimen;

[0077] Fracture toughness tensile stage: Based on the specimen size, the rate of increase of the stress intensity factor is controlled by controlling the stress loading rate. The loading continues until the specimen fractures. At the same time, the force and notch-opening displacement data of the specimen are collected using the testing machine software to obtain the force-displacement curve, i.e., the PV curve.

[0078] Furthermore, in step S3, relevant equipment, including but not limited to video equipment such as ultra-depth-of-field three-dimensional video microscopes, can be used to capture three-dimensional morphological images of the fracture surface to obtain a three-dimensional morphological point cloud map of the fracture surface. Based on the different fracture forms, such as normal fracture and shear fracture, the fracture surface region is divided into a plane strain fracture region and a plane stress fracture region, and the area of ​​different fracture regions is calculated.

[0079] Specifically, in step S3, the positional information of each point on the fracture surface, such as the three-dimensional spatial coordinates (x, y, z), can be obtained through the three-dimensional topographic point cloud map of the fracture surface, and the area of ​​each fracture region can be calculated based on the positional information (x, y, z) of each point. The process of calculating the surface area based on the three-dimensional spatial coordinates (x, y, z) of each point within the surface has been disclosed in detail in the prior art and will not be repeated here.

[0080] Furthermore, such as Figure 4 As shown, in step S3, the fracture region is divided according to the different fracture surface types as follows:

[0081] The fracture surface near the surface is under plane stress. The fracture surface at this point forms an angle of approximately 45° with the surface, and the fracture is shear. This region is designated as the plane stress fracture region, and its area is denoted as A. s ;

[0082] The fracture regions other than the plane stress fracture region are in a plane strain state. The fracture surface in this region undergoes normal fracture. This region is denoted as the plane strain fracture region, and its area is labeled A. b ;

[0083] In addition, the entire three-dimensional area of ​​the fracture region is labeled as A. t All planar areas are labeled A. p .

[0084] Furthermore, in step S4, the process of calculating the energy per unit area of ​​each region is as follows:

[0085] First, by integrating the force-displacement curve obtained in step S2, the energy required for fracture can be obtained, as shown in equation (3).

[0086] W t =∫PdV (3)

[0087] Then, according to the energy principle G IC =K2 IC / E' and G IC =W / A0, we can get the energy G. IC The relationship between the stress intensity factor Kct and the stress intensity factor Kct is shown in equation (4).

[0088]

[0089] Then, assuming γ s γ represents the energy consumed per unit area in the plane stress fracture zone. b γ represents the energy consumed per unit area of ​​the corresponding plane strain fracture zone. d Other energy consumed per unit area during the fracture process, such as dissipated work. The sum of these energies multiplied by the area should equal the work done by the external force, as shown in equation (5).

[0090]

[0091] And, γ s ,γ b ,γ d >0;

[0092] in These represent the sum of the areas of the plane stress fracture zone, the plane strain fracture zone, and all fracture regions in each small specimen, respectively; i represents different small specimens; W t This represents the work done by external forces during the fracture toughness test.

[0093] By using small specimens of at least three different thicknesses, at least three different equations are obtained according to equation (5), and three of these equations are selected to form a set of equations. Within this set of equations, any set of three equations can be used to solve for γ. s γ b and γ d The value of is then determined by ignoring solutions that do not satisfy the constraints, ultimately yielding γ. s γ b and γ d The value of .

[0094] Furthermore, in step S5, the energy per unit area of ​​each region, i.e., γ, can be calculated based on step S4. s γ b and γ d The plane strain fracture toughness is calculated by combining the value of and the relationship between the energy principle and the plane strain fracture toughness.

[0095] Specifically, the calculation process for plane strain fracture toughness is as follows:

[0096] First, based on the principle of energy, we can obtain the following equation:

[0097]

[0098] Based on the above equation (6), the plane strain fracture toughness K can be obtained as shown in the following equation (7). IC The calculation formula is as follows:

[0099]

[0100] Where E is the elastic modulus of the material; v is the Poisson's ratio of the material.

[0101] Example 1

[0102] The following example, using a certain type of high-temperature alloy material, illustrates the method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles as described in this invention:

[0103] S1, Design of small specimens for fracture toughness testing: Design at least three small specimens of different thicknesses based on the material's mechanical properties and testing standards. Specifically: The thickness of the specimen required to obtain the plane strain fracture toughness of the material, calculated using the above formula (1), must exceed 72 mm. At this point, the load required for the test exceeds the maximum tensile force of a commonly used tensile testing machine by 10 KN. Therefore, four sets of small specimens with smaller thicknesses are designed, with thicknesses of 15 mm, 20 mm, 22.5 mm, and 25 mm respectively. This example uses a compact tensile specimen; other relevant dimensions can be designed according to standards.

[0104] S2, Conduct fracture toughness tests. According to relevant testing standards, conduct fracture toughness tests on the four groups of small specimens obtained in step S1, and obtain the results as follows: Figure 2 The force-displacement curves of each small specimen are shown, and as follows: Figure 3 The fracture surface of the small specimen after fracture is shown;

[0105] The fracture toughness test process is divided into two stages. The first stage is pre-cracking. In this example, a constant-amplitude triangular wave with a stress ratio R=0 at room temperature is used for fatigue loading test; and the amplitude, loading frequency, and pre-crack length are set according to the specimen size.

[0106] The second stage is the fracture toughness tensile test. The compact tensile specimen is fixed in a fixture with pins, and an extensometer or other displacement measuring device is installed at the notch of the specimen. If a high-temperature test is to be performed, the high-temperature furnace can be heated to the set temperature and held for 30 minutes; if a room-temperature test is to be performed directly, it can be conducted directly. During the tensile test, the rate of increase of the stress intensity factor is controlled by controlling the stress loading rate according to the specimen size. Loading continues until the specimen fractures. The force and notch-opening displacement data of the compact tensile specimen are collected using the testing machine software to obtain the force-displacement curve, i.e., the PV curve.

[0107] S3, Fracture Morphology Analysis of Small Specimens: Three-dimensional morphology images of the fracture surface were captured using a super-depth-of-field 3D video microscope, obtaining a point cloud map of the fracture surface. Based on different fracture types, such as normal fracture and shear fracture, the fracture region was divided into plane strain and plane stress fracture regions. The results are as follows: Figure 4 As shown; then the area of ​​different fracture regions was obtained through calculation;

[0108] Among them, capturing a 3D point cloud map can obtain the location information (x, y, z) of each point on the fracture surface, and the area of ​​any region can be calculated based on this information.

[0109] Observations show that the fracture surface near the surface is under plane stress, while the fracture surface forms an angle of approximately 45° with the surface, indicating shear fracture. This area is denoted as A. s Apart from this, the area is in a plane strain state, and the fracture occurs normally. This area is designated as A. b Simultaneously, the total area of ​​the entire fracture region is denoted as At, and the planar area is denoted as A. p .

[0110] S4, Calculate the energy per unit area of ​​each region: Based on the energy principle and combined with the force-displacement curve obtained in step S2, calculate the energy consumed per unit area in the plane strain fracture region, the energy consumed per unit area in the plane stress fracture region, and the dissipated work per unit area of ​​the entire fracture region during the fracture process; in this embodiment, γ is calculated. s =0.2497, γ b =0.0376 and γ d =0.0026. During the analysis, if the specimen meets the conditions for plane strain fracture toughness, the plane stress fracture region can be ignored.

[0111] S5, Calculate the plane strain fracture toughness of the material: Based on the energy consumed per unit area of ​​each region calculated in step S4, and combining the relationship between the energy principle and the plane strain fracture toughness, the plane strain fracture toughness of the material can be finally obtained.

[0112] It should be noted that this invention focuses on explaining and illustrating the method and principle for estimating the plane strain fracture toughness of materials. Other related details, such as the fracture toughness testing process and the calculation of the area of ​​each region based on the three-dimensional morphology point cloud map of the fracture surface, have been disclosed and described in detail in the prior art and are well-known technologies in this field, and will not be repeated here.

[0113] In plane strain fracture toughness testing of materials, the reason why obtaining plane strain fracture toughness requires the specimen to meet stringent thickness conditions is that the fracture toughness specimen needs to meet approximate plane strain conditions. The fracture surface of the fracture toughness specimen can be divided into two regions based on the fracture mode: the normal plane strain region and the shear plane stress region. The energy per unit area required for fracture in these two regions differs. Only when the plane strain region is much larger than the plane stress region can the measured fracture toughness be called plane strain fracture toughness K. IC The method for estimating the plane strain fracture toughness of materials described in this invention combines the energy principle and small-sample fracture toughness tests. This method, based on small samples and the energy principle, offers advantages over existing technologies, including wide applicability, simple operation, high efficiency, and good economy. Specifically, it is reflected in the following aspects:

[0114] (1) The method for estimating the plane strain fracture toughness of materials described in this invention has a wide range of applications: For structures of commonly used metallic materials, such as turbine disks of aero engines, wheels and rails of airplanes and high-speed trains, hulls of ships, petrochemical pipelines and high-pressure containers, it is necessary to obtain the plane strain fracture toughness of the materials used. Often, the specimen size required to measure the plane strain fracture toughness of these materials is very large. When it is difficult to obtain the plane strain fracture toughness of the materials using conventional inspection methods, this method can be used to estimate the plane strain fracture toughness.

[0115] (2) The method for estimating the plane strain fracture toughness of materials described in this invention is simple to operate: This method adds only one step to the traditional fracture toughness measurement test, namely, taking a three-dimensional topographic point cloud map of the fracture surface, and there are already relevant devices that can intelligently process this step, so the operation process is simple and easy to handle.

[0116] (3) The method for estimating the plane strain fracture toughness of materials described in this invention is highly efficient: After obtaining the test results, this invention only needs to solve a three-element linear equation to obtain the test results, which is fast and efficient.

[0117] (4) The method for estimating the plane strain fracture toughness of materials described in this invention is economical: This invention avoids the use of large-sized specimens to measure plane strain fracture toughness, which saves materials on the one hand and energy consumption during the test on the other hand, thus making it economical.

[0118] In summary, the method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles described in this invention can significantly improve the efficiency of obtaining plane strain fracture toughness and save the cost of related experiments. Moreover, the entire process is simple, convenient, fast, and has strong engineering practicality and applicability.

[0119] The embodiments of this application have been described above with reference to the accompanying drawings. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. This application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of this application.

Claims

1. A method for estimating the plane strain fracture toughness of materials based on small specimens and energy principles, characterized in that, Including the following steps: S1, Design small specimens for fracture toughness testing: Design at least three different thicknesses of small specimens based on the material's mechanical properties and testing standards; S2, Conduct fracture toughness tests: Conduct fracture toughness tests on the small specimens obtained in step S1 according to the test standards, obtain the force-displacement curves of the small specimens, and the small specimens after fracture; S3, Small specimen fracture morphology analysis: Three-dimensional morphology images of the small specimen fracture surface were taken to obtain three-dimensional morphology point cloud images of the fracture surface. According to the different fracture surface forms, the fracture surface area was divided into plane strain fracture area and plane stress fracture area, and the area of ​​different fracture areas was calculated. S4, Calculate the energy consumed per unit area in each region: Based on the energy principle, using the force-displacement curve obtained in step S2, calculate the energy consumed per unit area in the plane strain fracture region, the energy consumed per unit area in the plane stress fracture region, and the dissipation work per unit area in the entire fracture region during the fracture process. S5, Calculate the plane strain fracture toughness of the material: Based on the energy consumed per unit area of ​​each region calculated in step S4, and combining the relationship between the energy principle and the plane strain fracture toughness, the plane strain fracture toughness is calculated.

2. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S1, the relevant dimensions of the small specimen, except for the thickness, are designed according to the test standards.

3. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S1, the thickness of each small specimen should be selected so that the load required for the small specimen in the fracture toughness test does not exceed the maximum tensile force of the tensile machine.

4. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, The small specimen is a compact tensile specimen or a three-point bend specimen.

5. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S2, the fracture toughness test process is divided into two stages: Pre-crack stage: The amplitude, loading frequency, and pre-crack length on the surface are set according to the size of the specimen; Fracture toughness tensile stage: Based on the specimen size, the rate of increase of the stress intensity factor is controlled by controlling the stress loading rate. The loading continues until the specimen fractures. At the same time, the force and notch-opening displacement data of the specimen are collected using the testing machine software to obtain the force-displacement curve.

6. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S3, a three-dimensional morphological image of the fracture surface is captured using a super depth-of-field three-dimensional video microscope.

7. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S3, the position information of each point on the fracture surface is obtained by using the three-dimensional topographic point cloud map of the fracture surface, and the area of ​​each fracture region is calculated based on the position information of each point.

8. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S3, the fracture region is divided according to the different fracture surface types as follows: The fracture surface near the surface is under plane stress. The fracture surface at this point forms an angle of approximately 45° with the surface, and the fracture is shear. This region is designated as the plane stress fracture region, and its area is denoted as A. s ; The fracture regions other than the plane stress fracture region are in a plane strain state. The fracture surface in this region undergoes normal fracture. This region is denoted as the plane strain fracture region, and its area is labeled A. b ; In addition, the entire three-dimensional area of ​​the fracture region is labeled as A. t All planar areas are labeled A. p .

9. The method for estimating the plane strain fracture toughness of a material according to claim 1, characterized in that, In step S4, the process of calculating the energy per unit area of ​​each region is as follows: Based on the energy principle and the force-displacement curve obtained in step S2, the following equation (5) is obtained: Moreover, c s ,c b ,c d >0; Where, γ s γ represents the energy consumed per unit area in the plane stress fracture zone. b γ represents the energy consumed per unit area of ​​the corresponding plane strain fracture zone. d A represents the work dissipated per unit area during the fracture process; s i A b i A t i represent the sum of the areas of the plane stress fracture zone, the plane strain fracture zone, and all fracture regions in each small specimen, respectively; i represents different small specimens; W t This represents the work done by external forces during the fracture toughness test. Then, using samples of various thicknesses, at least three different equations are obtained according to equation (5), and three of these equations are selected to form a set of equations. Based on the system of equations, γ is obtained by solving the equations. s γ b and γ d The value of .

10. The method for estimating the plane strain fracture toughness of a material according to claim 9, characterized in that, In step S5: plane strain fracture toughness K of the material IC The following formula (7) is used to calculate: Where E is the elastic modulus of the material; v is the Poisson's ratio of the material.

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