KR102218594B1 - Device and Method for measuring material properties and stress state by digital image correlation(DIC) near micro-indentation mark - Google Patents

Abstract

The present invention relates to an apparatus and method for measuring the physical properties and stress state of a target material using a surface displacement field near a microindentation determined by digital imaging technology, and forming an indentation by pressing the surface of the target material with a constant load. Indentation test unit; An image acquisition unit for photographing a surface state before and after pressing the target material; An image analysis unit that generates surface displacement field data around the indentation using an image of the surface state before and after the indentation; A comparison data storage unit that stores surface displacement field data around the indentation according to various material properties and stress state variables when a material is indented based on at least one flow stress model, calculated through simulation using elastoplastic finite element analysis; And comparing the surface displacement field data of the target material generated by the image analysis unit with the surface displacement field data for various material properties and stress state variables stored in the comparison data storage unit, and the material property value and the stress state of the target material. It consists of a configuration including a central processing unit to determine.

Description

Device and Method for measuring material properties and stress state by digital image correlation (DIC) near micro-indentation mark by applying digital image fitting technology around micro-indentation mark

The present invention relates to an apparatus and method for simply and accurately measuring material properties and stress states, and more specifically, to digitally measure a surface displacement field around an indentation mark generated by pressing micro-indentation particles into the surface of a target material. Physical properties and stress states such as yield stress, tensile stress, flow stress, and anisotropy coefficient (Lankford value) are measured by digital image correlation (DIC). It relates to an apparatus and method for measuring.

Various equipments are used to measure the material properties and stress state of various mechanical and structural parts or surface coatings. However, there is no device that can easily measure material properties or stress conditions directly in the field for products or parts in use without preparing separate specimens.

Among the mechanical properties of the material, properties such as elastic modulus and Poisson's ratio are approximately constant, but properties such as yield stress, tensile strength, flow stress, and anisotropy coefficient continue to change depending on processing history, heat treatment, and use conditions. Therefore, it is very likely that the material properties of the prepared specimen and the actual properties of the part will be different. If a load is applied to the object and a stress is applied, it is reasonable to consider that the stress state is also different from the value in the prepared specimen.

Existing devices for measuring material properties or stress conditions are based on a destructive test or a non-destructive test, and have the following limitations.

Material property measurement technology

(1) Tensile test: This is a fracture test performed by a tensile tester by preparing a strict standard specimen. As it causes destruction of the material due to tension, it cannot be applied to the part in use.

(2) Compression test: This is a fracture test performed by a compression tester after preparing a standard specimen. The reliability of the measurement is insufficient due to the occurrence of secondary stress due to friction and cannot be applied in the field.

(3) Hardness test: Depending on the test method, various types of hardness testers are used. There are restrictions on the type and thickness of materials, and it cannot be applied on-site except for the Shore hardness tester. In addition, other physical properties other than the hardness value cannot be grasped.

(4) Nano-indentation test: An expensive measuring device is required, and the accuracy of the material properties or stress state determined based on the load-displacement curve is low.

Stress state measurement technology

(1) Strain gauge: This is a technology that measures the strain from the change in gauge resistance according to the change in surface length before and after the load is applied. The process of mounting the gauge on the specimen surface is inconvenient, and the stress state or residual stress already applied cannot be measured.

(2) Photoelastic test: As it is limited to photoelastic materials, only qualitative stress observation according to shape is possible. It is difficult to measure the direction of the principal stress, the specimen fabrication and test methods are very complex, and the results are very sensitive to the modulus of elasticity.

(3) X-ray diffraction test: As it is a non-destructive test method using a separate expensive measuring device, it cannot be applied to large structures. Since it is a non-contact, indirect measurement method, the analysis method is difficult and the measurement reliability is low.

(4) Hole drilling: This is a fracture test method in which a strain gauge is attached to the target surface, and the residual stress is measured by reading the change in strain after removing the material by drilling. Although it has high precision, it is difficult to use it in actual industrial sites because the target part cannot be reused due to damage to the material to be evaluated.

(5) Layer-by-layer removal, section division method: Although it is a technique for measuring residual stress with high precision and a clear analysis method, the target part cannot be reused, and a separate analysis method is required to know the local value.

Therefore, it is possible to easily measure various material properties and stress states with high accuracy while being easily applied to the component materials used in the field, and a measurement technology that can be used continuously even after the measurement target material is required.

Korean Patent Registration 10-1332264 European Patent 1 771 717

The present invention is to solve the problems of the prior art described above, the surface displacement field around the indentation generated by the micro-indentation test is read with digital image matching technology (DIC), and the displacement is calculated and stored in advance through elastoplastic finite element analysis. By determining the physical properties and stress state of the material to be measured by comparing it with the field big data, it is possible to easily measure the material of the part being used in the field with high accuracy, and the target material used for the measurement is not required to prepare a separate specimen. An object of the present invention is to provide an apparatus and method for measuring material properties and stress states that can be used continuously.

The problems of the present invention are not limited to those mentioned above, and other problems that are not mentioned will be clearly understood by those skilled in the art from the following description.

The present inventors have found that the surface displacement field caused by the microindentation caused by a constant load varies depending on the material properties and the stress state, and accordingly, the surface around the indentation is converted into a high-resolution digital image before and after the microindentation test on the surface of the object. After shooting, digital image matching technique (DIC) is applied to the images before and after deformation to create surface displacement field data by tracking a number of surface feature points, and this is applied to the surface displacement field obtained by using elastoplastic finite element analysis in advance. The technology to determine the material properties and stress state was devised by comparing it with Korean big data and artificial intelligence.

The material properties to be measured in the present invention include yield stress, tensile strength, flow stress, anisotropic coefficient, etc., and the stress state is the applied stress caused by the load acting on the object or the residual stress remaining in the object after the load is removed. stress). The present invention reads the surface displacement field around the indentation produced by the micro-indentation test using spherical, conical, and pyramidal indentations with digital image adaptation technology, and compares it with the displacement field big data stored in advance by elastoplastic finite element analysis. Thus, it provides the technology to determine the material properties and stress state.

The technology of the present invention can be applied to both mechanical and structural parts and surface coating materials made of metals, ceramics, and plastics, that is, objects in which indentations are generated when pressed with high-hardness micro-indentation particles such as diamond. The size of the micro-indentation can be controlled by the press-in load, and if the size is not an issue, reliability evaluation and safety security for parts or surface coatings in use, such as vehicles and transport machinery such as aircraft and structures such as bridges, are possible. In addition, it can be usefully used for online quality control of mass-produced plate press products or plastic injection products.

Specifically, an apparatus for measuring a material property value and a stress state according to an aspect of the present invention includes: a press-fitting test unit for forming an indent by pressing the surface of a target material with a constant load; An image acquisition unit for photographing a surface state before and after pressing the target material; An image analysis unit that generates surface displacement field data around the indentation using an image of the surface state before and after the indentation; A comparison data storage unit that stores surface displacement field data around the indentation according to various material properties and stress state variables when a material is indented based on at least one flow stress model, calculated through simulation using elastoplastic finite element analysis; And comparing the surface displacement field data of the target material generated by the image analysis unit with the surface displacement field data for various material properties and stress state variables stored in the comparison data storage unit, and the material property value and the stress state of the target material. It may be configured to include a central processing unit to determine. In this case, the indentation formed on the surface of the target material is preferably 0.1 mm or less in diameter.

The image acquisition unit may include at least one CCD camera.

The image analysis unit receives an image of the surface state before and after the press-fitting of the target material from the image acquisition unit, extracts a number of feature points from the image before press-fitting, and tracks the positions of the feature points in the image after the press-fitting to determine displacement. The displacement field data can be formed by calculation.

In addition, the simulation using the elastoplastic finite element analysis, when dividing the specimen material based on at least one flow stress model into a plurality of elements based on the indentation point and pressing the indentation point with a constant load, the anisotropy of the material. It is a method of simulating the reaction of the specimen material by considering the type of indenter, the direction of the applied stress, and measuring the displacement between the nodes between the elements, and the surface displacement field data around the indentation according to the material properties and the indentation according to the stress state of the material. It is possible to generate the surrounding surface displacement field data. Here, the flow stress model may be selected from Swift type, Hollomon type, Ludwick type, Voce type, Johnson-Cook type, and the like.

In addition, the apparatus of the present invention may apply differently the indentation load in the indentation test unit and the resolution in the image acquisition unit and analysis unit according to a value to be measured among material properties and stress states. Specifically, for the measurement of the physical properties of steel materials, 0.05 to 0.1 micron/pixel resolution is used for 100-200N press-in load, and 0.02 to 0.05 micron/pixel resolution for 20-40N press-in load is used for stress state measurement. Practical measurement results can be obtained from the resolution. In the case of other materials, the indentation load can be adjusted in proportion to the yield stress or tensile strength when measuring physical properties, and in proportion to the modulus of elasticity when measuring the stress state.

Meanwhile, in the apparatus of the present invention, the image analysis unit, the comparison data storage unit, and the central processing unit may be provided on an upper portion of the apparatus, and the press-fit test unit may be mounted vertically downward from the upper bottom surface of the apparatus. At this time, while supporting the upper portion of the device, the press-fit test unit is spaced apart from the surface of the target material around the press-fit test unit, and is fixed to the surface of the target material so that it can be fixed to the material surface of the part in use. A fixed support may be further provided.

In addition, the apparatus of the present invention may further include a display unit that displays to the user the surface displacement field data of the target material generated by the image analysis unit or the physical properties and stress states of the target material determined by the central processing unit.

In addition, the press-fit test unit may include a drive device including a servo motor; It is connected to the driving device through a connection mechanism and moves in a direction perpendicular to the surface of the target material according to the operation of the driving device to press-fit the surface of the target material, and is formed in any one shape selected from conical, spherical, and pyramidal shapes. , At least one indenter installed to be replaced; And a load cell mounted between the connection mechanism and the indenter to measure the indentation load.

On the other hand, according to another aspect of the present invention, there is provided a method of measuring a material property value and a stress state by a surface displacement field near an indentation determined by a digital image adaptation technique, the method comprising: acquiring an image of a surface of a target material; A press-fitting test step of forming an indent by pressing the surface of the target material with a constant load; Acquiring an image around the indentation on the surface of the target material after the indentation test; Surface displacement field data around the indentation before and after the indentation of the target material by extracting a plurality of feature points from the image on the surface of the target material before the indentation test and tracking the positions of the feature points from the image on the surface of the target material after the indentation test Generating a; Comparing the surface displacement field data generated for the target material with pre-stored surface displacement field data for various material properties and stress state variables, and determining a material property value and a stress state of the target material; A method of measuring a material property value and a stress state including a is provided.

Here, the pre-stored surface displacement field data for various material properties and stress state variables are sample materials based on at least one flow stress model through simulation using elastoplastic finite element analysis, and multiple elements based on the indentation point. When dividing into and pressing the indentation point with a constant load, it is generated by simulating the reaction of the specimen material and measuring the displacement between the nodes between elements in consideration of the anisotropy of the material, the type of the indentation, and the direction of the applied stress. It includes data on the surface displacement field around the indentation according to the material properties and data on the surface displacement field around the indentation according to the stress state of the material.

In addition, in the method of measuring the material property value and the stress state of the present invention, the indentation load and the resolution may be differently applied according to the value to be measured among the material property value and the stress state. In addition, the indentation test step may be performed using indenters having one or two or more shapes selected from conical, spherical, and pyramidal shapes.

On the other hand, the determining of the material property value and the stress state of the target material may include: a first step of setting the material property value of the target material to an arbitrary value; In the step of generating surface displacement field data of the target material by searching for surface displacement field data for a stress state variable corresponding to the set material property from among the previously stored surface displacement field data for various material properties and stress state variables. A second step of searching for surface displacement field data closest to the generated actual surface displacement field data, and obtaining a stress state condition corresponding to the searched surface displacement field data; The surface displacement field data for the material property variable corresponding to the obtained stress state is searched for the surface displacement field data closest to the actual surface displacement field data from among the previously stored surface displacement field data for various material properties and stress state variables. A third step of searching for field data and obtaining a material property value corresponding to the retrieved surface displacement field data; It is determined whether the material properties obtained in the third step match the material properties set in the first step, and if they do not match, the material properties obtained in the third step are reset to the material properties of the target material. , Repeating the second and third steps, and if they match, the stress state obtained in the second step and the material property value obtained in the third step are determined as the stress state and the physical property value of the target material. It can include 4 steps.

According to another aspect of the present invention, a method for constructing big data of a surface displacement field for measuring material properties and stress states is provided. The big data construction method may include generating a specimen material based on at least one flow stress model; Dividing the specimen material into a plurality of elements based on a point to be pressed; When indenting the point to be pressed into indenters of various sizes and shapes and applying various indentation loads, the reaction by indentation of the specimen material having various material constants and various stress conditions according to the flow stress model Simulating; Measuring the displacement between nodes between the elements before and after the press-fitting of the simulated specimen material, for each type of the indenter and the press-fitting load, and a material constant and stress condition according to the flow stress model; And generating surface displacement field data around the indentation for various material properties and stress state variables as a function of a displacement magnitude according to a distance from the indentation center using the measured displacement data. At this time, the flow stress model may be selected from Swift type, Hollomon type, Ludwick type, Voce type, Johnson-Cook type, and the like.

Embodiments of the disclosed technology may have effects including the following advantages. However, since it does not mean that embodiments of the disclosed technology should include all of them, it should not be understood that the scope of the rights of the disclosed technology is limited thereby.

According to an apparatus and method for measuring material properties and stress states with a surface displacement field near a microindentation determined by the digital imaging technology according to the present invention, a microindentation test is performed on the surface of an object and the resulting surface around the microindentation By measuring the material properties and stress state using the displacement field, damage to the target object can be minimized, so it is applied to measure the material properties and stress state of the target object or surface coating directly in the field without preparing a separate specimen. In addition, if microindentation is not a problem, the object can be reused, and there is an advantage of being able to directly check whether the structure or component is safe by evaluating the material properties or stress state of the object currently in use.

In addition, without using inaccurate and difficult to obtain micro-indentation load-displacement curves, the displacement field before and after micro-indentation is extracted from the high-resolution image around the micro-indentation, and the material properties and stress state are determined by comparing it with the already established big data. As a result, there is an advantage in that it is possible to quickly and easily obtain accurate material properties and stress states, whereas the existing non-destructive tests are difficult to analyze by indirect measurement methods and require special training and qualifications.

In addition, without using laboratory measurement equipment such as a tensile tester, a portable device easily acquires high-resolution images of the surface before and after the occurrence of indentations due to micro-indentation, and processes them through resources provided in the device or laptops that can communicate with the device. Therefore, by measuring the material properties and the stress state, there is an advantage that it can be used for real-time quality control of mass-produced products or parts, which was not possible with conventional test methods requiring a lot of time and preparation.

1 is a block diagram showing a functional configuration of an apparatus for measuring material properties and stress states according to an embodiment of the present invention;
2 is a block diagram showing a functional configuration of an apparatus for measuring material properties and stress states according to another embodiment of the present invention;
3 is a view showing the structure of a device for measuring material properties and stress states according to an embodiment of the present invention having the functional configuration shown in FIGS. 1 and 2;
Fig. 4(a) is a view showing a material surface image of a target object before a micro-indentation test obtained through an image acquisition unit according to the present invention, and Fig. 4(b) is a view showing a material surface image of a target object after a micro-indentation test.
FIG. 5(a) is a diagram showing the displacement vectors of surface feature points extracted from the image around the indentation using the image of FIG. 4, and FIG. 5(b) is a distance away from the indentation center using FIG. 5(a). A graph corresponding to the corresponding displacement value,
6 is a diagram illustrating a three-dimensional elastoplastic finite element analysis model for a Vickers-type indenter according to an embodiment of the present invention;
7 is a diagram illustrating surface displacement field data around an indentation according to material properties (K, n, a) when a Swift type flow stress model material is exemplarily pressed in;
FIG. 8 is a diagram showing surface displacement field data around an indentation according to an applied stress (RS) when various stress states are applied to a material having a specific material property value when a Swift type flow stress model material is exemplarily pressed in;
9 is a flow chart showing a method of measuring material properties and stress states using an apparatus according to an embodiment of the present invention;
10 is a comparison of the surface displacement field data actually generated for the target material in an embodiment of the present invention with various pre-stored material properties and big data of the surface displacement field for the stress state variables to compare the material properties and the stress state of the target material. A flowchart illustratively showing how to determine.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In the drawings, the shapes and sizes of elements may be exaggerated for clearer explanation, and elements indicated by the same reference numerals in the drawings are the same elements.

Further, the present invention includes all possible combinations of features and aspects described herein. Thus, for example, when one feature is described in connection with one embodiment and another feature is described in connection with another embodiment, it is to be understood that the present invention includes embodiments having a combination of these features.

And throughout the specification, when a part is said to be "connected" to another part, it is not only "directly connected", but also "electrically or mechanically connected" with another element in the middle. Include. In addition, when a part "includes" or "includes" a certain component, it means that other components are not excluded and other components may be further included or provided unless otherwise stated. .

In addition, in the present specification, the term “target material” may specifically mean a material of an object to be measured in use in the field, but it may be interpreted as including “specimen material” unless otherwise noted.

FIG. 1 is a block diagram showing a functional configuration of a device for measuring material properties and a stress state according to an embodiment of the present invention, and FIG. 3 is a diagram showing the external structure of the device having the functional configuration shown in FIG. . An apparatus for measuring material properties and stress states according to an exemplary embodiment of the present invention will be described with reference to FIGS. 1 and 3.

As shown, the apparatus for measuring material properties and stress states according to an embodiment of the present invention includes a mechanism unit 100 that performs a press-fitting test on a target material and acquires a surface state image of the target material before and after the press-fitting. , The control unit 200 may generate surface displacement field data around the indentation from the obtained image and compare it with previously stored surface displacement field big data to determine the material properties and stress state of the target material.

Specifically, the mechanism unit 100 includes a press-fitting test unit 110 for forming an indentation by press-fitting the surface of a target material with a predetermined load, and an image acquisition unit 120 for photographing a surface state before and after press-fitting for the target material. Can include.

The indentation test unit 110 is configured to perform a micro-indentation test on the surface of a target object, and is connected to the driving device 111 including a servo motor through a connection mechanism to the target object according to the operation of the driving device. It may include an indenter 115 for pressing the surface of the material, and a load cell 113 mounted between the connection mechanism and the indenter to measure the press-in load.

The load cell 113 for measuring a load applied during press fitting may have a maximum capacity of 500N and a resolution of 1N in consideration of elastoplastic finite element analysis data for a general metal material. During press-fitting of the metal material, the load cell 113 is configured to press-fit the material to be measured 10 with a constant press-in load in units of 10 N that can create a small displacement field in the metal material by press-fitting. On the other hand, when applied to a non-metallic material, a load cell having a lower press-in load and resolution than when applied to a metallic material can be used.

The indenter 10 may press-fit the surface of the target object to form an indent, and move in a direction perpendicular to the surface of the target object to press-fit the material surface of the target object. The present invention is a technology to measure the material properties and stress state by using the surface displacement field data by a micro indentation with a diameter of 0.1 mm or less, and accordingly, the size of the indenter is also 0.5 to 1.0 mm in diameter based on the spherical indentation. It is configured to have.

According to a preferred embodiment of the present invention, the shape of the indenter 10 is selected from a conical shape, a spherical shape, and a pyramid shape, and may be installed interchangeably in the indentation test unit 110. In addition, in the case of the conventional nano indentation test, in consideration of an error problem due to self-similarity in which materials having different material property values due to the characteristics of the indenter when the conical indenter is used alone exhibit the same indentation load-displacement curve, the present invention In the case of a conical indenter, it can be configured to apply two types of indenters together with a spherical indenter. Even in the case of a pyramid-shaped indenter, when the size is reduced, it can act similarly to a conical indenter, so it can be configured to be used with a spherical indenter.

When the two types of indenter 115 are applied, the mechanical part 100 of the present invention has two types of indenter separately applied to the indentation test unit 110 to perform the indentation test, and then image for each case. To obtain or configure the press-fit test unit 110 to be installed at two adjacent points spaced apart by 1 mm or more where the two types of indenters do not interfere with each other, the pressure test is performed to simultaneously press-fit the surface of the target object and respond to the indenter. It can be configured to acquire each image.

On the other hand, the driving device 111 of the indentation test unit 110 according to the present invention may include a servo motor to precisely control the amount of mechanical movement due to the characteristics of the micro-indentation test according to the present invention, through which repeatability and controllability This high motor control system can be implemented.

The image acquisition unit 120 is configured to photograph the surface of the target material before and after the indentation test, and is configured to include at least one high-performance CCD camera. As shown in Figure 3, the image acquisition unit 120 including the CCD camera is located on both sides of the indentation test unit 110 can take an image around the indentation on the material surface 10, if necessary, the pixel The number and specifications of CCD cameras can be replaced to improve performance, such as area expansion.

As will be described later, in the present invention, since the displacement of the singular point of the material surface before and after the indentation must be calculated by using the image of the surface before and after the occurrence of indentation due to the micro-indentation of the target material surface, a high-resolution image is required. The resolution of the image may vary depending on the value to be measured (i.e., material property value or stress state), but as described later, it is approximately 0.02 to 0.1 micron/micron in consideration of the big data of the surface displacement field constructed through elastoplastic finite element analysis. The resolution can be set in a range of pixels.

Specifically, the control unit 200 uses an image analysis unit 210 that generates surface displacement field data around the indentation using an image of the surface state of the material before and after indentation from the image acquisition unit 120, and finite element analysis. Thus, the comparison data storage unit 220 that stores big data of the surface displacement field around the indentation according to various material property values and stress state variables, and the surface displacement field data of the target material generated by the image analysis unit are stored in the comparison data storage unit. It may include a central processing unit 230 for determining the material property value and the stress state of the target object by comparing the material property value and the surface displacement field big data for the stress state variable.

The image analysis unit 210 uses a digital image matching technique (DIC) to generate surface displacement field data around the micro indentation by using images before and after the indentation of the target material surface. The DIC technique is a method of analyzing the movement information of a material point according to the deformation of a material surface through an algorithm that extracts and tracks information representing regional characteristics such as the location, size, and direction of the feature points of an image acquired through a camera. In this device, the micro-indentation test and DIC technique are combined to determine the surface displacement field around the indentation from the movement information of the mass before and after the micro-indentation.

4 is a diagram showing a material surface image of a target object before and after a micro-indentation test obtained through an image acquisition unit according to an embodiment of the present invention. Illustratively, an aluminum material surface is pressed into a Vickers-type indenter, a kind of pyramidal indentation. It shows a high-resolution image (resolution 0.028 microns/pixel) of 0.129 mm X 0.092 mm around the second quadrant of the formed indentation. Here, FIG. 4(a) shows the image before micro-indentation, and FIG. 4(b) is the image after micro-indentation, and the area from the dotted line inclined 45 degrees to the lower right corner corresponds to the Vickers-type indentation. In this way, a random pattern shape is shown in the image before and after the micro-press fitting.

The image analysis unit 210 exemplarily extracts a number of points, that is, features that are distinguished from the periphery from the image before micro-indentation as shown in FIG. The displacement field data is formed by calculating displacement by tracking the positions of the feature points in the image after press-fitting as shown in 4(b). In the present invention, it is possible to measure the material properties and the stress state using the surface displacement field data of the feature points in the image before and after the micro-indentation. Is preserved.

When the pixel coordinates of the feature points before and after the indentation extracted through image analysis ( _before x _indentation , y _{before indentation} ) and ( _after x _indentation , y _{after indentation} ) are extracted, displacement vectors of each feature point (dx, dy) Is calculated as follows.

(dx, dy) = ( _After x _indentation -x _{before indentation} , y _{after indentation before} -y _indentation ) (Equation 1)

5(a) shows the displacement vectors of surface feature points extracted from the image around the indentation using the image of FIG. 4 as described above, and FIG. 5(b) shows the indentation center using FIG. 5(a). A graph corresponding to the displacement value according to the distance away from is shown. As shown in FIG. 5, the number of singular points extracted from the image before and after the press-fitting may reach tens of thousands to tens of thousands, and in the present invention, the displacement field data is statistically averaged for each displacement value for a considerable number of feature points. Can be created.

As described above, the indenter used for the microindentation of the present invention can use all of the generally used spherical, conical, and pyramidal shapes. Accordingly, a plurality of surface displacement field data obtained from a plurality of indentations and a plurality of indentations can be obtained. When used, the reliability of the measurement results can be further increased. To this end, in the comparison data storage unit to be described later, big data on displacement fields obtained in various ways according to the indentation load or the type of indenter are separately calculated and stored, and may be compared with the generated plurality of surface displacement field data.

In the present invention, instead of the conventional method of continuously measuring the load-displacement for micro-indentation, which is difficult to measure and control, the indentation test unit press-fits the material surface with a predetermined constant load as described above to obtain micro-indentation marks with a diameter of 0.1 mm or less. The advantage of being able to reflect all detailed shapes such as pile-up and sink-in as high-resolution images around the indentation are provided and used from the image acquisition unit while minimizing damage to the target object and increasing the accuracy of the test. There is this. Therefore, a vast amount of information can be used compared to the indentation load-displacement curve measured in the conventional nanoindentation tester, and accordingly, the problem of similar physical property values showing the same indentation load-displacement curve with different material properties can be completely solved. There will be. Moreover, instead of accurately grasping the material properties or stress conditions of the object to be measured on average across the entire specimen, the use of multiple microindentation results allows the local distribution of the material properties and stress conditions to be known.

On the other hand, in the embodiment of the present invention, since a high-resolution image was used, there were a number of singular points that were distinguished from the surroundings, such as a prominent contrast value displayed on the surface of the target material, so that the pattern was separately printed and printed as has been used in the conventional DIC technique. I didn't use a method of tracking the pattern. However, it is also possible to print a separate pattern and track it to generate a surface displacement field.

The comparison data storage unit 220 is an object to be compared with the surface displacement field data of the target material provided from the image analysis unit 210, and corresponds to a configuration in which surface displacement field big data for various material property values and stress state variables are stored. do. The comparison data storage unit 220 stores big data of the surface displacement field represented by the material according to various material properties and stress state variables when a material is indented according to various flow stress models through simulation using elastoplastic finite element analysis in advance. . The comparison data storage unit 220 may be embedded in the device of the present invention, but may optionally be provided in a memory included in an external device such as a notebook computer or a computer.

Big data constructed through elastoplastic finite element analysis includes commonly used flow stress models and displacement field data around the indentation using initial yield stress and applied stress as variables. At this time, the displacement field data is calculated and stored based on the three-dimensional analysis result considering the anisotropy of the material, the type of indentation, and the direction of the applied stress.

6 shows an exemplary three-dimensional elastoplastic finite element analysis model for a Vickers-type indenter according to an embodiment of the present invention. Specifically, the simulation using elastoplastic finite element analysis divides the specimen material based on a predetermined flow stress model into a plurality of elements based on the indentation point, and when the indentation point is pressed with a constant load, the anisotropy of the material and the indentation. It simulates the response of the specimen material by considering the type of ruler and the direction of the applied stress. As a result, by measuring the displacement between the nodes between the elements due to the indentation, it is possible to generate surface displacement field data around the indentation according to the material properties and the surface displacement field data around the indentation according to the stress state of the material. .

A method of constructing big data according to an embodiment of the present invention includes the steps of: generating a specimen material based on at least one flow stress model; Dividing the specimen material horizontally and vertically into a plurality of elements based on a point to be pressed; When indenting the point to be pressed into indenters of various sizes and shapes and applying various indentation loads, the reaction by indentation of the specimen material having various material constants and various stress conditions according to the flow stress model Simulating; Measuring the displacement between nodes between the elements before and after the press-fitting of the simulated specimen material, for each type of the indenter and the press-fitting load, and a material constant and stress condition according to the flow stress model; Using the measured displacement data, generating surface displacement field data around the indentation for various material properties and stress state variables as a function of a displacement magnitude according to a distance from the indentation center.

7 and 8 exemplarily show surface displacement field data around the indentation according to the result of micro-indentation elastoplastic finite element analysis for steel materials when the Swift type flow stress model represented by the following (Equation 2) is applied.

... (식 2)

... (Equation 2)

(Where σ is the flow stress, K, a, n are material constants, and ε is the total effective strain.)

The material constant a can be expressed as (Equation 3) when Y is the initial yield stress of the target material and E is the elastic modulus.

... (식 3)

... (Equation 3)

[MPa]에서 재료상수 K, n, a에 따른 압흔 주변의 변위장 데이터 곡선 그래프를 나타낸다. Specifically, FIG. 7 shows surface displacement field data around the indentation according to the material properties (K, n, a) when the Swift type flow stress model material is micro-pressed. Here, the horizontal axis represents the distance from the indentation center (r/D), and the vertical axis represents the displacement magnitude (u _r ), and is represented by a relative value divided by the size of the indenter (D=1mm), respectively. Fig. 7 shows a Swift type flow stress when micro-pressing a steel material conforming to the Swift type flow stress model with a 200N press-in load

In [MPa], a graph of the displacement field data curve around the indentation according to the material constants K, n, a is shown.

The comparison data storage unit 220 stores surface displacement field data around the indentation according to various material property values (K, n, a, ,,) generated in the manner described above. Therefore, using the surface displacement field data generated by the image analysis unit 210 of the present invention, for example, when a specific material property value of a steel material as a material of the target object is to be measured, the generated surface displacement field data is compared. The specific material properties (i.e., K, n, a values) of the target object are determined by reading and comparing the surface displacement field data around the indentation according to the material properties corresponding to FIG. 7 of the steel material stored in the data storage unit 220 You will be able to.

After measuring the radial displacement field at the center of the indentation by several angles to obtain the physical properties, compare the values and if they are the same, the target material is isotropic. If the material properties obtained by angle, that is, flow stress are different, the anisotropy coefficient can be determined from the flow stress ratio by direction.

In addition, FIG. 8 shows surface displacement field data around the indentation according to the applied stress (RS[MPa]) when various stress states act on the material having a specific material property value when microindentation of a Swift type flow stress model material. Here, the horizontal axis represents the distance from the indentation center (r/D), and the vertical axis represents the displacement magnitude (u _r ), and is represented by a relative value divided by the size of the indenter (D=1mm), respectively. 8 shows a graph of the displacement field data curve around the indentation according to the applied stress when a steel material conforming to the Swift type flow stress model is micro-pressed with a press-in load of 27N.

The comparison data storage unit 220 stores surface displacement field data around the indentation according to the applied stress when various stress states are applied to materials with various material property values generated in the manner described above. Therefore, in the case of measuring a specific stress state acting on a steel material, which is a material of a target object, using the surface displacement field data generated by the image analysis unit 210 of the present invention, the generated surface displacement field data By reading and comparing the surface displacement field data around the indentation according to the stress state corresponding to FIG. 8 of the steel material stored in the comparison data storage unit 220, it is possible to determine the specific stress state of the target object.

In the above embodiment, the case of applying the Swift type flow stress model was described, but there is no particular limitation on the applicable flow stress model, so that the model such as Hollomon type, Ludwick type, Voce type, Johnson-Cook type, or a combination thereof It can be extended and applied.

On the other hand, when considering the finite element analysis big data of the steel materials of Figs. 7 and 8, when using the apparatus of the present invention to measure the material properties and stress state of the target object, appropriate indentation load and It is possible to derive the range of application of the resolution. For example, referring to FIG. 7, when measuring material properties of steel materials, using a resolution of 0.05 to 0.1 microns/pixel at an indentation load of approximately 100 to 200 N can obtain engineering useful data. In addition, referring to FIG. 8, when using a resolution of 0.02 to 0.5 microns/pixel at an indentation load of approximately 20 to 40 N for the measurement of the stress state, this corresponds to approximately 15 MPa/pixel, and thus an engineering useful value can be measured. At this time, the resolution of the stress state increases in inverse proportion to the Young's modulus of the material. That is, since the aluminum material has a stiffness of about 1/3 of that of steel, the resolution of 0.02 micron/pixel corresponds to approximately 5 MPa/pixel, thus increasing the measurement accuracy. If the indentation load and resolution are set based on a steel material with high rigidity as described above, material properties and stress conditions can be measured with sufficient accuracy even for other materials with less rigidity than steel.

Optionally, in the case of materials other than steel materials, the indentation load can be adjusted in proportion to the yield stress or tensile strength for measuring physical properties, and in proportion to the modulus of elasticity for measuring the physical properties. The reason why the indentation load is set differently for each material is to keep the size of the microindentation around 0.1mm. Since the current commercial CCD comes out up to 48 million pixels (8,000 X 6,000 pixels), if the size around the micro indentation is 0.16 mm as 8,000 pixels, it becomes 0.02 micron/pixel, so that the stress state of the material can be understood practically. In this way, the setting relationship between the indentation load and the resolution is set in consideration of the current CCD resolution, and if a higher resolution CCD is developed in the future, the indentation load (i.e., the indentation size) can be adjusted more widely and the measurement accuracy is Will be higher.

Typically, the resolution is a value obtained by dividing the image range by the number of pixels, and when photographing a range of 0.4mm X 0.3mm at 48 million pixels, the resolution is 0.05 microns/pixel. However, as shown in FIG. 5, the apparatus of the present invention tracks thousands to tens of thousands of feature points to obtain displacement vectors and fits them into the stored displacement field big data. Therefore, it is not a pixel-based image resolution, but a statistically much higher reliability accuracy. It has the advantage of being able to determine the material properties or stress state.

The central processing unit 230 matches the surface displacement field data of the target material generated by the image analysis unit 210 with various material property values stored in the comparison data storage unit 220 and/or the surface displacement field big data for stress state variables. It determines the material properties and/or stress state of the target object by detecting the nearest surface displacement field data.

As described above, the comparative data storage unit 220 systematically stores a vast amount of database obtained by finite element analysis, and the material properties such as yield stress and strain hardening index and the stress state of each axis are used as variables. It includes a built-in function that shows the correlation between material properties, stress state and displacement field through displacement field data of element analysis indentation model. Therefore, the displacement field data generated from the indentation test on the material surface of the actual target object obtained through the image analysis unit 210 is compared with the analysis results according to the variables using the built-in function of the comparison data storage unit 220. Through this, it is possible to obtain a material property value and/or a stress state value of the material to be evaluated 10. A specific method of obtaining the material property value and/or the stress state value of the target object using the surface displacement field data around the indentation obtained through the micro indentation test will be described later.

2 is a block diagram showing a functional configuration of an apparatus for measuring material properties and stress states according to another embodiment of the present invention. In addition, although FIG. 3 is a view showing the external structure of the device having the functional configuration shown in FIG. 1, the external structure of FIG. 3 may be applied to the embodiment of FIG. 2 as well. In the embodiment of FIG. 2, since the same description as in the embodiment of FIG. 1 may be applied to the configuration partially overlapping with the embodiment of FIG. 1, a detailed description will be omitted. Conversely, in the description related to the embodiment of FIG. 2, the configuration overlapping with that of FIG. 1 may be similarly applied to the embodiment of FIG. 1.

As shown in FIGS. 2 and 3, the apparatus for measuring material properties and stress states according to another exemplary embodiment of the present invention includes a fixed support 130 and a display 310 as compared to the exemplary embodiment of FIG. 1. And a configuration of the power supply unit 410.

The fixing support 130 is a configuration for fixing and supporting the device of the present invention on the surface of a target object in use. Although not particularly limited, as shown in FIG. 3, a control unit 200 including an image analysis unit 210, a comparison data storage unit 220, and a central processing unit 230 is provided on the top of the device, and The press-fitting test unit 110 may be mounted vertically downward from the upper bottom surface. At this time, the fixed support part 130 supports the upper part of the device and at the same time supports the press-fitting test part apart from the material surface 10 of the target object around the press-fitting test part 110, and It can be fixed to the material surface. There is no particular limitation on the manner in which the fixed support 130 is fixedly supported on the material surface 10 of the target object, so the fixed support 130 may be fixedly supported on the material surface 10 by the weight of the device itself. It can also be fixed by the operator's hand. In addition, by providing a separate fixing mechanism such as, for example, an adsorption mechanism on the contact surface of the fixing support 130 with the material surface 10, it is possible to improve convenience when applied to an object whose surface is not horizontally flat. . In either case, it is not necessary to perform separate processing or adhesion to the surface of the object to be measured.

The display unit 310 may display the displacement field data around the indentation on the material surface of the target object generated by the image analysis unit 210 or the material properties and the stress state of the target object determined by the central processing unit 230 to the user. I can. In addition, the display unit 310 is a component that provides a user interface for manipulating each part of the device. That is, the display unit 310 may be configured to allow a user to conveniently use the device through a GUI, and display the displacement field data or the final material result value extracted through the indentation test in the form of a graph, a table, or a single number. can do. In addition, since it is connected to all parts of the device such as the power supply of the device, the press-fitting test part, and the image acquisition part such as a CCD camera, the operator can operate the device based on the information displayed on the display, and separate power and computer as needed. It can assist in charging and data acquisition through connection to.

In addition, the power supply unit 410 is a configuration that supplies power to drive each component of the device, and is not particularly limited, but is charged or replaced to facilitate application to evaluation of materials in use by securing the portability of the device. It can be composed of a battery capable of.

A series of processes of acquiring information on material properties and stress states using the apparatus according to the above embodiment will be described as follows.

First, in order to evaluate the material to be measured, the fixing support 130 of the device is fixed to the material surface 10, and images such as the press-in test unit 110 and the CCD camera using the power provided from the power supply unit 410 such as a battery. The acquisition unit 120 is operated, and after acquiring an image of the material surface using a CCD camera or the like 120 before the press-fitting test, a micro-press test is performed by operating the motor of the press-fitting test unit 110. After removing the indenter 115 from the surface of the material, the image of the surface displacement field around the indentation generated through the micro-indentation test is photographed with a CCD camera or the like, and the acquired image is the image analysis unit 210 of the controller 200 It is transferred to and the displacement field data around the indentation is formed. The central processing unit 230 identifies material properties and stress states of the material using the formed data and the finite element analysis data of the comparison data storage unit 220 and displays the result on the display unit 310.

Optionally, in the apparatus according to the embodiment of the present invention described above, the image analysis unit 210 for generating surface displacement field data around the indentation, the comparison data storage unit 220 and determining the physical properties and stress states of the target material The control unit 200 including the central processing unit 230 is physically separated from the mechanical unit 100 including the press-fitting test unit 110 and the image acquisition unit 120, etc., to be provided in an external device such as a laptop computer or a computer. May be. In this case, the external device is configured to receive the material surface displacement field data of the target object generated from the mechanism unit 100 online to measure and display the material property value and the stress state.

9 shows a method of measuring a material property value and a stress state of a target object according to an embodiment of the present invention. Each illustrated step may be performed by an apparatus for measuring material properties and stress states according to an exemplary embodiment of the present invention. Therefore, since the specific operating conditions of the device required in performing each step are the same as those described in relation to each component of the device above, detailed reference will be omitted, and an embodiment will be described below based on a process aspect. .

As shown in FIG. 9, in the method of measuring the material property value and the stress state according to the present invention, first, a high-resolution image of the material surface of the target object is obtained (S100). Then, a press-fitting test is performed to form an indentation by pressing the material surface of the target object with a constant load (S200), and after the indentation test, an image around the indentation is obtained with respect to the material surface of the target object (S300). Next, using the DIC technique, a plurality of feature points are extracted from the image of the surface of the target material before the indentation test, and the position of the feature points is tracked from the image of the surface of the target material after the indentation test to press-fit the target material. The front and rear surface displacement field data is generated (S400). Subsequently, by comparing the surface displacement field data generated for the material of the target object with previously stored surface displacement field data for various material property values and stress state variables, the material property value and the stress state of the target object are determined (S500). .

The indentation test step (S200) and the image acquisition and analysis steps (S100, S300) may be applied by varying the indentation load and the resolution according to a value to be measured among material properties and stress states. Although not particularly limited, a resolution of about 0.1 micron/pixel can be applied to an indentation load of 100 to 200 N for material property measurement, and a resolution of about 0.02 micron/pixel for an indentation load of 20 to 40 N for stress state measurement. Resolution can be applied.

Meanwhile, the pre-stored big data of the surface displacement field for various material properties and stress state variables, as described above, divides the specimen material based on at least one flow stress model into a plurality of elements based on the indentation point, and the A method of simulating the reaction of the specimen material using elastoplastic finite element analysis, taking into account the anisotropy of the material, the type of the indentation, and the direction of the applied stress when indenting the indentation point with a constant load, and measuring the displacement between nodes between elements. It was created with.

Specifically, in the case of obtaining a stress state for a target object whose material property value is known, the surface displacement field data generated for the target object is used, and the surface displacement field data according to various stress state variables using the material property value (Fig. 8), the stress state variable corresponding to the actual displacement field data can be obtained. In addition, when trying to obtain material properties for an object whose stress state is known (mainly, stress state = 0), surface displacement field data according to various material property values using built-in stress conditions (stress state = 0) (see Fig. 7 Correspondence), it is possible to obtain the material properties corresponding to the actual displacement field data.

In addition, in the case of simultaneously measuring the material properties and the stress state, both the material properties and the stress state can be derived by an iterative method by estimating the material properties or the stress state. For example, after assuming a material property value arbitrarily, the stress state condition is obtained through the stress evaluation process of a target whose material property value is known from the stored surface displacement field big data, and then the stress state condition is known again using the stress state condition. It is determined whether or not the material property values obtained through the process of evaluating the material properties of an existing object and the initial arbitrary material property values converge, and the process is repeated while updating the material constant values until convergence. The material properties and stress conditions at the time of convergence are determined by the material properties and stress conditions of the target material.

FIG. 10 exemplarily shows a method of deriving a material property value and a stress state by an iterative method by estimating material property values.

As shown in FIG. 10, the method of the present invention for determining a stress state together with a material property value of a target object by an iterative method by estimating a material property value is a first step of setting the material property value of the target object to an arbitrary value (S610). ; In the step of generating surface displacement field data of the target material by searching for surface displacement field data for a stress state variable corresponding to the set material property value from among various previously stored surface displacement field data for a variety of material property values and stress state variables. A second step (S620) of reading the surface displacement field data closest to the generated actual surface displacement field data, and obtaining a stress state value corresponding to the read surface displacement field data; The surface displacement field data closest to the actual surface displacement field data is searched for the surface displacement field data for the material property value variable corresponding to the obtained stress condition from among the previously stored various material property values and the surface displacement field data for the stress state variable. A third step of reading field data and obtaining a material property value corresponding to the read surface displacement field data (S630); It is determined whether the material property values obtained in the third step match the material properties set in the first step, and if they do not match, the material properties obtained in the third step are reset to the material properties of the target material. The second step (S620) and the third step (S630) are repeated, and if they match, the stress state obtained in the second step and the material property value obtained in the third step are used as the stress state and the material property value of the target material. A fourth step of determining (S640) may be included.

On the other hand, in the same way, it is possible to determine the material property value and the stress state of the target object by an iterative method by estimating the stress state. After obtaining the material property value through the evaluation process, the stress condition obtained through the process of evaluating the stress condition of the object of which the material property value is known using the above material property value and the initial arbitrary stress condition condition are determined. , Repeat the process while updating the stress state condition until convergence. The condition of the stress state and the material properties at the time of convergence become the final result.

110 Press-fit test section
120 image acquisition unit
130 fixed support
210 image analysis unit
220 Comparison data storage unit
230 Central processing unit
310 Display
410 power supply

Claims (20)

A press-fitting test unit for forming an indent by pressing the surface of the target material with a constant load;
An image acquisition unit for photographing a surface state before and after pressing the target material;
An image analysis unit that generates surface displacement field data around the indentation using an image of the surface state before and after the indentation;
A comparison data storage unit that stores surface displacement field data around the indentation according to various material properties and stress state variables when a material is indented based on at least one flow stress model, calculated through simulation using elastoplastic finite element analysis; And
By comparing the surface displacement field data of the target material generated by the image analysis unit with the surface displacement field data for various material properties and stress state variables stored in the comparison data storage unit, the material property value and the stress state of the target material Central processing unit to decide
Including,
The image analysis unit,
A material property value that generates the displacement field data by extracting a plurality of feature points, which are points that are distinguished from the surroundings, from the image of the surface state before indentation, and then calculating the displacement by tracking the position of the feature point in the image of the surface state after indentation. And a device that measures the state of stress. The method according to claim 1,
An apparatus for measuring material properties and stress conditions, wherein the indentation formed on the surface of the target material is 0.1 mm or less in diameter.
The method according to claim 1,
The image analysis unit receives an image of the surface state before and after the press-fitting of the target material from the image acquisition unit, extracts a number of feature points from the image before press-fitting, and tracks the positions of the feature points in the image after the press-fitting to determine displacement. A device for measuring material property values and stress states, characterized in that the displacement field data is formed by calculation.
The method according to claim 1,
Simulation using the elastoplastic finite element analysis,
When the specimen material based on at least one flow stress model is divided into a number of elements based on the indentation point and the indentation point is pressed with a certain load, the specimen is considered to be the anisotropy of the material, the type of the indentation, and the direction of the applied stress. A method of simulating the reaction of the material and measuring the displacement between nodes between elements, characterized in that it generates surface displacement field data around the indentation according to the material properties and surface displacement field data around the indentation according to the stress state of the material. A device that measures material properties and stress conditions.
The method of claim 4,
The flow stress model,
A device that measures material properties and stress states, characterized in that it is selected from Swift type, Hollomon type, Ludwick type, Voce type, Johnson-Cook type, or a combination thereof.
The method according to claim 1,
A device for measuring material property values and stress states, characterized in that differently applying the press-in load in the indentation test unit and the resolution in the image acquisition unit and the analysis unit according to a value to be measured among material property values and stress states .
The method of claim 6,
For material property measurement, a resolution of 0.05 to 0.1 microns/pixel is used for 100-200N press-in load based on steel materials.
A device for measuring material properties and stress state, characterized in that 0.02-0.05 micron/pixel resolution is used for a 20-40N press-in load for stress state measurement.
The method of claim 6,
Based on the case of steel materials, the press-in load is adjusted in proportion to the yield stress or tensile strength for material property measurement, and the press-in load is adjusted in proportion to the modulus of elasticity to measure the stress state. A device that measures the state of stress.
The method according to claim 1,
The image analysis unit, the comparison data storage unit, and the central processing unit are provided on the top of the device,
The press-fit test unit is mounted vertically downward from the upper bottom surface of the device,
A fixing support fixed to the surface of the target material so as to support the upper part of the device and support the press-in test part apart from the surface of the target material around the press-fitting test part, and to be fixed to the material surface of the part in use A device for measuring material properties and stress state further comprising a.
The method according to claim 1,
The image acquisition unit is a device for measuring material properties and stress state including at least one CCD camera.
The method according to claim 1,
A device for measuring material property values and stress states, further comprising a display unit that displays to a user the surface displacement field data of the target material generated by the image analysis unit or the physical properties and stress states of the target material determined by the central processing unit.
The method according to claim 1,
The indentation test unit,
A drive device including a servo motor;
It is connected to the driving device through a connection mechanism and moves in a direction perpendicular to the surface of the target material according to the operation of the driving device to press-fit the surface of the target material, and is formed in any one shape selected from conical, spherical, and pyramidal shapes. , At least one indenter installed to be replaced; And
A device for measuring material properties and a stress state, including a load cell mounted between the connection mechanism and the indenter to measure the indentation load.
As a method of measuring material properties and stress state by the surface displacement field near the indentation determined by digital imaging technology,
Obtaining an image of the surface of the target material;
A press-fitting test step of forming an indent by pressing the surface of the target material with a constant load;
Acquiring an image around the indentation on the surface of the target material after the indentation test;
By extracting a plurality of feature points from the image of the surface of the target material before the indentation test, tracking the position of the feature points from the image of the surface of the target material after the indentation test, and calculating displacement, around the indentation before and after the indentation of the target material. Generating surface displacement field data;
Comparing the surface displacement field data generated for the target material with pre-stored surface displacement field data for various material properties and stress state variables, and determining a material property value and a stress state of the target material;
Including,
The plurality of feature points are points having features that are distinguished from surroundings in an image of the surface of the target material before the indentation test, a method of measuring a material property value and a stress state.
The method of claim 13,
The previously stored surface displacement field data for various material properties and stress state variables is divided into a plurality of elements based on the indentation point of the specimen material based on at least one flow stress model through simulation using elastoplastic finite element analysis. And when the indentation point is pressed with a certain load, the material property value generated by simulating the reaction of the specimen material and measuring the displacement between the nodes between elements in consideration of the anisotropy of the material, the type of the indentation, and the direction of the applied stress. A method of measuring material properties and stress conditions, characterized in that the data of the surface displacement field around the indentation according to the indentation and the surface displacement field data around the indentation according to the stress state of the material.
The method of claim 14,
The flow stress model,
A method of measuring material properties and stress states, characterized in that it is selected from Swift type, Hollomon type, Ludwick type, Voce type, Johnson-Cook type, or a combination type thereof.
The method of claim 13,
A method of measuring material property values and stress conditions, characterized in that differently applied indentation loads and resolutions according to values to be measured among material properties and stress states.
The method of claim 13,
The indentation test step is a method of measuring a material property value and a stress state, characterized in that it is carried out using an indenter having one or two or more shapes selected from a conical shape, a spherical shape, and a pyramid shape.
The method of claim 13,
The step of determining the material property value and the stress state of the target material,
A first step of setting a property value of the target material to an arbitrary value;
In the step of generating surface displacement field data of the target material by searching for surface displacement field data for a stress state variable corresponding to the set material property from among the previously stored surface displacement field data for various material properties and stress state variables. A second step of searching for surface displacement field data closest to the generated actual surface displacement field data, and obtaining a stress state condition corresponding to the searched surface displacement field data;
The surface displacement field data for the material property variable corresponding to the obtained stress state is searched for the surface displacement field data closest to the actual surface displacement field data from among the previously stored surface displacement field data for various material properties and stress state variables. A third step of searching for field data and obtaining a material property value corresponding to the retrieved surface displacement field data;
It is determined whether the material properties obtained in the third step coincide with the material properties set in the first step,
If they do not match, after resetting the material properties obtained in the third step to the material properties of the target material, repeating the second and third steps,
In the case of coincidence, measuring the material property value and the stress state including the fourth step of determining the stress state obtained in the second step and the material property value obtained in the third step as the stress state and the physical property value of the target material. Way.
Generating a specimen material based on at least one flow stress model;
Dividing the specimen material into a plurality of elements based on a point to be pressed;
When indenting the point to be pressed into indenters of various sizes and shapes and applying various indentation loads, the reaction by indentation of the specimen material having various material constants and various stress conditions according to the flow stress model Simulating;
Measuring the displacement between nodes between the elements before and after the press-fitting of the simulated specimen material, for each type of the indenter and the press-fitting load, and a material constant and stress condition according to the flow stress model;
Generating surface displacement field data around the indentation for various material properties and stress state variables as a function of the displacement magnitude according to the distance from the indentation center, using the measured displacement data;
Including,
The surface displacement field data,
A material property value, characterized in that it is generated by extracting a plurality of feature points, which are points that have features distinguished from the surroundings, from the image of the surface state before indentation, and then calculating the displacement by tracking the position of the feature points in the image of the surface state after indentation. A method of constructing big data of the surface displacement field for measuring the stress state. The method of claim 19,
The flow stress model,
Swift type, Hollomon type, Ludwick type, Voce type, Johnson-Cook type, or a combination of these, characterized in that selected from the material properties and stress state measurement of the surface displacement field big data construction method.

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