Interlayer material mechanical property testing method and system combined with digital image technology

Abstract

The invention provides an interlayer material mechanical property testing method and system combined with a digital image technology. The interlayer material mechanical property testing method comprises the steps that a sample is pasted to a statics three-point bending testing device; speckles need to be sprayed on the whole surface of the side face of the sandwich test piece, and a deformation measurement area is selected; the clamping end of the upper clamp is clamped and restrained, and the clamping end of the lower clamp loads downward displacement; recording images of the ROI before and after loading deformation to obtain displacement and strain data under different loads; establishing a three-dimensional finite element model consistent with the sandwich structure test piece, and solving to obtain stress and strain distribution of the side surface of the sandwich structure; constructing an objective function of parameter identification according to the variance between the numerical calculation strain at the node of the finite element model and the DIC actually measured strain at the same position; and carrying out inversion identification on the linear elasticity constitutive parameters of the material. According to the technical scheme, the problems that in the prior art, the number of tests is large, and the test cost is high are solved; and in the test process, local extrusion failure of the sample is easy to occur, and the data precision is relatively low.

Description

Technical Field

[0001] The present invention relates to the technical field of material mechanical property characterization, and in particular to a sandwich material mechanical property testing method and system combined with digital imaging technology. Background Art

[0002] Sandwich structure composite materials have been successfully applied to load-bearing structures of aerospace vehicles due to their advantages of light weight, high stiffness, strong designability, and heat insulation, sound insulation, and fatigue resistance. Obtaining comprehensive and reliable mechanical properties parameters of sandwich materials is of great significance for the application and analysis of sandwich materials.

[0003] The core of a sandwich structure can be divided into several categories, such as web core, honeycomb core, foam and end-grain balsa. The core material is usually a macroscopic orthotropic material with different mechanical properties in the in-plane and out-of-plane directions. In a sandwich structure, the out-of-plane tension, shear stiffness and strength of the core are the most important performance parameters. At the same time, since the face plate has a fixing effect on the core, it is usually impossible and meaningless to test the core performance alone without the face plate. Conventional mechanical property test tests for core materials all use contact measurement methods, and it is necessary to carry out mechanical tests with multiple different loading methods (out-of-plane tension / compression, out-of-plane shear, etc.) to obtain the elastic constants in different directions. This method has a large number of tests and high test costs; during the test, the specimen is prone to local extrusion failure and the data accuracy is low. Summary of the invention

[0004] The present invention provides a sandwich material mechanical property testing method and system combined with digital image technology, which can solve the technical problems in the prior art of large number of tests and high test cost; local extrusion failure of the sample during the test and low data accuracy.

[0005] According to one aspect of the present invention, a method for testing the mechanical properties of sandwich materials combined with digital image technology is provided, and the method for testing the mechanical properties of sandwich materials combined with digital image technology comprises: step A, designing the size parameters of the sample according to the actual size of the sandwich material in place, and pasting the sample to a static three-point bending test device; step B, spraying the entire surface of the side surface (thickness direction) of the sandwich test piece, and selecting an area in the middle of the side surface as a deformation measurement area ROI (Region of Interest); step C, the upper clamp holding end of the three-point bending test device is fixedly supported and constrained, and the lower clamp holding end is loaded with downward displacement; step D, during the experiment, the sampling frequency of the image acquisition device is 1 Hz, that is, an image is obtained every 1 second, and the experimental load size is recorded while collecting each image until the end of the experiment, so as to obtain the experimental loading history; step E, recording the images of the ROI area on the side of the test piece before and after loading and deformation, and calculating the displacement and strain data under different loads through digital image correlation software (VIC-3D); step F, establishing a three-point bending test consistent with the sandwich structure test piece A finite element model is constructed, and the constraints and loading conditions of the model are highly consistent with the test, and the stress and strain distribution on the side of the sandwich structure is obtained by solving; in step G, the objective function of parameter identification is constructed by the variance between the numerically calculated strain at the node of the finite element model and the DIC measured strain at the same position; in step H, the inverse identification of the linear elastic constitutive parameters of the material is carried out by using the optimization algorithm; in step H, the specific implementation process is as follows: the initial value of the finite element model correction process is obtained by material mechanics estimation or by using the linear elastic material constants provided by the material preparation party, and the minimization of the objective function is achieved by iteratively updating multiple constitutive relationship parameters of the material.

[0006] Furthermore, the static three-point bending test device in step A consists of an upper fixture (1), a metal plate (2) and a lower fixture (3); the upper fixture (1) is located above the metal plate (2), and the lower surfaces on the left and right sides of the upper fixture (1) are designed with mounting bosses for fixed connection with the metal plate (2), and the height of the bosses should be at least greater than the thickness of the sandwich test piece; a clamping structure is designed in the middle of the upper surface of the upper fixture (1) for connection with the upper chuck of the testing machine during the test; the lower fixture (3) is located below the metal plate (2) and is fixedly connected to the middle of the metal plate (2); the lower fixture (3) is used to connect with the lower chuck of the testing machine during the test.

[0007] Furthermore, the material of the metal plate (2) is selected from metal materials with relatively high yield strength, including titanium alloy, with a thickness ranging from 3 mm to 5 mm, and the positions on the left and right sides and the middle that need to be connected to the upper and lower clamps need to be locally reinforced.

[0008] Furthermore, in the static three-point bending test in step A, the sandwich material test piece is adhered to the upper surface of the metal plate (2).

[0009] Furthermore, step F specifically includes: in order to obtain the distribution of stress and strain along the thickness direction of the structure, especially the side of the core material, each layer of the model is simulated by solid units; the constitutive relationship of the core material is established, the core thickness direction is taken as the material 3 direction, the speckle spraying surface is the material main direction 13 surface, and a linear elastic constitutive model of the core material with in-plane transverse isotropy (E11=E22) is constructed; the boundary conditions of the model are highly consistent with the mechanical experiment, that is, the upper chuck is fixed, the lower chuck constrains the three rotational degrees of freedom and the translational degrees of freedom in the x and y directions, and a non-zero displacement vertically downward along the z direction is applied as the load; multiple analysis steps are defined in the finite element model, the number of analysis steps is the same as the number of images collected by VIC-3D, and the size of the load is defined in each analysis step. According to the experimental loading sequence, the load size defined in each analysis step is the same as the loading load corresponding to the moment when VIC-3D collects each image.

[0010] Furthermore, in step G, the initially assumed material constitutive parameters are first substituted into the finite element model, the strain data at the unit nodes are calculated, and the measured strain data obtained by VIC-3D correlation calculation is interpolated to the same position as the unit node of the finite element model by bilinear interpolation. The variance between the finite element numerical calculation strain and the measured strain at the same position is used to construct the objective function of parameter identification: in, represents a set of unknown material parameters of the core material, and They represent the numerical strain calculated by the finite element model and the strain measured by the digital image correlation method at node i, respectively. M is the number of nodes of the finite element model contained in the ROI area in each image. N is the number of loading steps in the finite element model, which corresponds to the number of images collected by the digital image correlation method equipment before the failure of the specimen during the experiment.

[0011] According to another aspect of the present invention, a sandwich material mechanical properties testing system combined with digital image technology is provided. The sandwich material mechanical properties testing system combined with digital image technology uses the sandwich material mechanical properties testing method combined with digital image technology as described above to perform sandwich material mechanical properties testing.

[0012] The technical solution of the present invention provides a method for testing the mechanical properties of sandwich materials in combination with digital image technology. The method is targeted at sandwich composite material structures. Based on a new three-point bending test device, the method uses digital image correlation technology combined with finite element model correction technology through a three-point bending test to construct the variance of numerically calculated strain and measured strain data as the objective function, and uses an optimization algorithm to simultaneously identify multiple mechanical property parameters in the main direction of the core material 13. Compared with the prior art, the method for testing the mechanical properties of sandwich materials provided by the present invention has the following advantages: (1) It can simultaneously identify multiple engineering elastic constants in the main direction of the core material 13 through a three-point bending test, which can simplify the number of tests and reduce the test cost; (2) A new three-point bending test fixture is designed to avoid the occurrence of local extrusion failure of the sample under the loading head in the conventional three-point bending test in advance; (3) The in-situ mechanical properties of the core material and the panel after molding can be directly measured, and the data accuracy is higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The included drawings are used to provide a further understanding of the embodiments of the present invention, which constitute a part of the specification, are used to illustrate the embodiments of the present invention, and together with the text description, explain the principles of the present invention. Obviously, the drawings in the following description are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0014] Figure 1 A flow chart showing a method for testing mechanical properties of sandwich materials in combination with digital image technology provided according to a specific embodiment of the present invention;

[0015] Figure 2 It shows a schematic structural diagram of a three-point bending test device provided according to a specific embodiment of the present invention;

[0016] Figure 3 A schematic diagram of a three-point bending finite element simulation model provided according to a specific embodiment of the present invention is shown.

[0017] The above drawings include the following reference numerals:

[0018] 1. Upper clamp; 2. Metal plate; 3. Lower clamp. DETAILED DESCRIPTION

[0019] It should be noted that, in the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0021] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be regarded as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0022] like Figures 1 to 3As shown, according to a specific embodiment of the present invention, a method for testing the mechanical properties of sandwich materials combined with digital image technology is provided, and the method for testing the mechanical properties of sandwich materials combined with digital image technology includes: step A, designing the size parameters of the sample according to the actual size of the sandwich material in place, and pasting the sample to a static three-point bending test device; step B, spraying the speckle on the entire surface of the side surface (thickness direction) of the sandwich test piece, and selecting an area in the middle of the side surface as the deformation measurement area ROI (Region of Interest); step C, the upper clamping end of the three-point bending test device is fixedly supported and constrained, and the lower clamping end is loaded with downward displacement; step D, during the experiment, the sampling frequency of the image acquisition device is 1Hz, that is, an image is obtained every 1 second, and the experimental load size is recorded while collecting each image until the end of the experiment, so as to obtain the experimental loading history; step E, recording the images of the ROI area on the side of the test piece before and after loading and deformation, and calculating the displacement and strain data under different loads through digital image correlation software (VIC-3D); step F, establishing a three-dimensional three-point bending test consistent with the sandwich structure test piece The finite element model, the constraint conditions and loading conditions of the model are highly consistent with the test, and the stress and strain distribution on the side of the sandwich structure is solved; step G, the objective function of parameter identification is constructed by the variance between the numerically calculated strain at the node of the finite element model and the DIC measured strain at the same position; step H, the inverse identification of the linear elastic constitutive parameters of the material is carried out using the optimization algorithm; the specific implementation process in step H is: the initial value of the finite element model correction process is obtained by material mechanics estimation or using the linear elastic material constants provided by the material preparation party, and the minimization of the objective function is achieved by iteratively updating multiple constitutive relationship parameters of the material.

[0023] By using this configuration, a sandwich material mechanical property testing method combined with digital image technology is provided. The method is targeted at sandwich composite material structures, based on a new three-point bending test device, and through a three-point bending test, digital image correlation technology is used in combination with finite element model correction technology to construct the variance of numerically calculated strain and measured strain data as the objective function, and an optimization algorithm is used to simultaneously identify multiple mechanical property parameters of the main direction of the core material 13. Compared with the prior art, the sandwich material mechanical property testing method provided by the present invention has the following advantages:

[0024] (1) It is possible to simultaneously identify multiple engineering elastic constants of the core material 13 in the main direction through a three-point bending test, which can simplify the number of tests and reduce the test cost;

[0025] (2) Design a new three-point bending test fixture to avoid the premature local extrusion failure of the specimen under the loading head in the conventional three-point bending test;

[0026] (3) The in-situ mechanical properties of the core material and the panel after molding can be directly measured with higher data accuracy.

[0027] Furthermore, in the present invention, the static three-point bending test device in step A consists of an upper fixture (1), a metal plate (2) and a lower fixture (3); the upper fixture (1) is located above the metal plate (2), and the lower surfaces on the left and right sides of the upper fixture (1) are designed with mounting bosses for fixed connection with the metal plate (2), and the height of the bosses should be at least greater than the thickness of the sandwich test piece; a clamping structure is designed in the middle of the upper surface of the upper fixture (1) for connection with the upper chuck of the testing machine during the test; the lower fixture (3) is located below the metal plate (2) and is fixedly connected to the middle of the metal plate (2); the lower fixture (3) is used to connect with the lower chuck of the testing machine during the test.

[0028] In the present invention, the metal plate (2) is made of a metal material with a high yield strength, including titanium alloy, with a thickness ranging from 3 mm to 5 mm, and the left and right sides and the middle where the upper and lower clamps need to be connected need to be locally reinforced. In the static three-point bending test in step A, the sandwich material test piece is pasted to the upper surface of the metal plate (2).

[0029] Furthermore, in the present invention, step F specifically includes: in order to obtain the distribution of stress and strain along the thickness direction of the structure, especially the side of the core material, each layer of the model is simulated by solid units; the constitutive relationship of the core material is established, the core thickness direction is taken as the material 3 direction, the speckle spraying surface is the material main direction 13 surface, and a linear elastic constitutive model of the core material with in-plane transverse isotropy (E11=E22) is constructed; the boundary conditions of the model are highly consistent with the mechanical experiment, that is, the upper chuck is fixed, the lower chuck constrains the three rotational degrees of freedom and the translational degrees of freedom in the x and y directions, and a non-zero displacement vertically downward along the z direction is applied as a load; multiple analysis steps are defined in the finite element model, the number of analysis steps is the same as the number of images collected by VIC-3D, and the size of the load is defined in each analysis step. According to the experimental loading sequence, the load size defined in each analysis step is the same as the loading load corresponding to the moment when VIC-3D collects each image.

[0030] In step G, the initially assumed material constitutive parameters are first substituted into the finite element model, the strain data at the unit nodes are calculated, and the measured strain data obtained by VIC-3D correlation calculation is interpolated to the same position as the unit node of the finite element model through bilinear interpolation. The variance between the finite element numerical calculation strain and the measured strain at the same position is used to construct the objective function of parameter identification: in, represents a set of unknown material parameters of the core material, and They represent the numerical strain calculated by the finite element model and the strain measured by the digital image correlation method at node i, respectively. M is the number of nodes of the finite element model contained in the ROI area in each image. N is the number of loading steps in the finite element model, which corresponds to the number of images collected by the digital image correlation method equipment before the failure of the specimen during the experiment.

[0031] According to another aspect of the present invention, a sandwich material mechanical properties testing system combined with digital image technology is provided. The sandwich material mechanical properties testing system combined with digital image technology uses the sandwich material mechanical properties testing method combined with digital image technology as described above to perform sandwich material mechanical properties testing.

[0032] By using this configuration, a sandwich material mechanical properties testing system combined with digital image technology is provided. The system is aimed at sandwich composite material structures. Based on a new three-point bending test device, a three-point bending test is conducted. The digital image correlation technology is combined with the finite element model correction technology to construct the variance of the numerically calculated strain and the measured strain data as the objective function. The optimization algorithm is used to simultaneously identify multiple mechanical properties parameters of the main direction of the core material 13. Compared with the prior art, the sandwich material mechanical properties testing system provided by the present invention has the following advantages:

[0033] (1) It is possible to simultaneously identify multiple engineering elastic constants of the core material 13 in the main direction through a three-point bending test, which can simplify the number of tests and reduce the test cost;

[0034] (2) Design a new three-point bending test fixture to avoid the premature local extrusion failure of the specimen under the loading head in the conventional three-point bending test;

[0035] (3) The in-situ mechanical properties of the core material and the panel after molding can be directly measured with higher data accuracy.

[0036] In order to further understand the present invention, the following Figures 1 to 3 The method for testing mechanical properties of sandwich materials combined with digital imaging technology provided by the present invention is described in detail.

[0037] like Figures 1 to 3 As shown, according to a specific embodiment of the present invention, a method for testing the mechanical properties of sandwich materials combined with digital imaging technology is provided, and the method specifically includes the following steps.

[0038] Step A, design the size parameters of the sample according to the actual size of the sandwich material, and stick the sample to the static three-point bending test device (test device such as Figure 2As shown); the static three-point bending test device in step A is composed of an upper fixture (1), a metal plate (2) and a lower fixture (3); the upper fixture (1) is located above the metal plate (2), and the lower surfaces of the left and right sides of the upper fixture (1) are designed with mounting bosses for fixed connection with the metal plate (2), and the height of the bosses should be at least greater than the thickness of the sandwich test piece; the middle part of the upper surface of the upper fixture (1) is designed with a clamping structure for connecting with the upper chuck of the testing machine during the test; the lower fixture (3) is located below the metal plate (2) and is fixedly connected to the middle part of the metal plate (2); the lower fixture (3) is used to connect with the lower chuck of the testing machine during the test; the material of the metal plate (2) is recommended to be a metal material with a higher yield strength, such as titanium alloy, with a thickness of 3mm to 5mm, and the left and right sides and the middle where the upper and lower fixtures need to be connected need to be locally strengthened. In the static three-point bending test in step A, the sandwich material test piece should be adhered to the upper surface of the metal plate (2) (the side of the metal plate away from the lower fixture (3) is defined as the upper surface).

[0039] Step B: Speckles need to be sprayed on the entire surface of the side surface (thickness direction) of the sandwich test piece, and an area in the middle of the side surface is selected as the deformation measurement area ROI (Region of Interest);

[0040] Step C, the upper clamping end of the three-point bending test device is fixedly supported and constrained, and the lower clamping end is loaded with downward displacement;

[0041] Step D: During the experiment, the sampling frequency of the image acquisition device is 1 Hz, that is, an image is acquired every 1 second. When each image is acquired, the experimental load size is recorded until the end of the experiment to obtain the experimental loading history;

[0042] Step E, recording the images of the ROI area on the side of the test piece before and after loading and deformation, and calculating the displacement and strain data under different loads through digital image correlation software (VIC-3D);

[0043] Step F, establish a three-point bending test three-dimensional finite element model consistent with the sandwich structure test piece (such as Figure 3 The constraints and loading conditions of the model are highly consistent with the test, and the stress and strain distribution on the side of the sandwich structure are obtained.

[0044] Step F specifically includes: in order to obtain the distribution of stress and strain of the structure, especially the side of the core material along the thickness direction, each layer of the model is simulated by solid units; the constitutive relationship of the core material is established, the core thickness direction is taken as the material 3 direction, the speckle spraying surface is the material main direction 13 surface, and the linear elastic constitutive model of the core material with in-plane transverse isotropy (E11=E22) is constructed:

[0045]

[0046] Among them, E 11 , E 33 , G 13 and v 13 is the engineering constant of the core material to be identified, E 11 , E 33 Respectively represent the elastic modulus along the 1 direction and 3 direction of the material, G 13 represents the shear modulus of the material 13 in the plane, v 13 Represents the normal stress σ acting alone in the 1 direction 11 When there are no other stress components, the negative value of the ratio of the normal strain in the 3-direction to the normal strain in the 1-direction is called Poisson's ratio; They are respectively represented as the normal stress components of the finite element model along the material 1 direction and 3 direction, It is expressed as the shear stress component of the finite element model within the material 13 surface, They are respectively expressed as the normal strain components of the finite element model along the 1st and 3rd directions of the material, It is expressed as the shear strain component of the finite element model within the material 13 surface.

[0047] The boundary conditions of the model are highly consistent with the mechanical experiment, that is, the upper chuck is fixed, the lower chuck constrains the three rotational degrees of freedom and the translational degrees of freedom in the x and y directions, and a non-zero displacement vertically downward in the z direction is applied as the load; multiple analysis steps are defined in the finite element model, the number of analysis steps is the same as the number of images collected by VIC-3D, and the load size is defined in each analysis step. According to the experimental loading sequence, the load size defined in each analysis step is the same as the loading load corresponding to the moment when VIC-3D collects each image.

[0048] Step G, constructing an objective function for parameter identification based on the variance between the numerically calculated strain at the node of the finite element model and the DIC measured strain at the same position;

[0049] In step H, an optimization algorithm is used to perform inverse identification of the linear elastic constitutive parameters of the material. The specific implementation process in step H is as follows: the initial value of the finite element model correction process is obtained by material mechanics estimation or by using the linear elastic material constants provided by the material preparation party, and the objective function is minimized by iteratively updating multiple constitutive relationship parameters of the material.

[0050] The present invention aims at sandwich composite material structures, based on digital image related experimental data combined with finite element numerical analysis, and through a three-point bending experiment, multiple engineering elastic constants of anisotropic core materials are identified, which has the advantages of simplifying the number of tests and reducing the test cost. By designing a new three-point bending test fixture, the local extrusion failure of the sample under the loading head of the conventional three-point bending test is avoided in advance, and it is ensured that the test can obtain effective data on the performance of the core material.

[0051] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" may include both "above" and "below". The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0052] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for testing mechanical properties of sandwich materials in combination with digital imaging technology, characterized in that: The sandwich material mechanical property testing method combined with digital image technology includes: Step A, designing the size parameters of the sample according to the actual size of the sandwich material, and pasting the sample to the static three-point bending test device; Step B: Speckles need to be sprayed on the entire surface of the side surface (thickness direction) of the sandwich test piece, and an area in the middle of the side surface is selected as the deformation measurement area ROI (Region of Interest); Step C, the upper clamping end of the three-point bending test device is fixedly supported and constrained, and the lower clamping end is loaded with downward displacement; Step D: During the experiment, the sampling frequency of the image acquisition device is 1 Hz, that is, an image is acquired every 1 second. When each image is acquired, the experimental load size is recorded until the end of the experiment to obtain the experimental loading history; Step E, recording the images of the ROI area on the side of the test piece before and after loading and deformation, and calculating the displacement and strain data under different loads through digital image correlation software (VIC-3D); Step F, establishing a three-dimensional finite element model of a three-point bending test that is consistent with the sandwich structure test piece, wherein the constraint conditions and loading conditions of the model are highly consistent with the test, and the stress and strain distribution on the side of the sandwich structure is obtained by solving; Step G, constructing an objective function for parameter identification based on the variance between the numerically calculated strain at the node of the finite element model and the DIC measured strain at the same position; Step H, using an optimization algorithm to carry out inverse identification of the linear elastic constitutive parameters of the material; the specific implementation process in said step H is: the initial value of the finite element model correction process is obtained by material mechanics estimation or by using the linear elastic material constants provided by the material preparation party, and the objective function is minimized by iteratively updating multiple constitutive relationship parameters of the material.

2. The method for testing mechanical properties of sandwich materials combined with digital image technology according to claim 1, characterized in that: The static three-point bending test device in step A is composed of an upper fixture (1), a metal plate (2) and a lower fixture (3); the upper fixture (1) is located above the metal plate (2), and the lower surfaces on the left and right sides of the upper fixture (1) are designed with mounting bosses for fixed connection with the metal plate (2), and the height of the bosses should be at least greater than the thickness of the sandwich test piece; a clamping structure is designed in the middle of the upper surface of the upper fixture (1) for connection with the upper chuck of the testing machine during the test; the lower fixture (3) is located below the metal plate (2) and is fixedly connected to the middle of the metal plate (2); the lower fixture (3) is used to connect with the lower chuck of the testing machine during the test.

3. The method for testing mechanical properties of sandwich materials combined with digital image technology according to claim 2, characterized in that: The material of the metal plate (2) is a metal material with a relatively high yield strength, including titanium alloy, with a thickness ranging from 3 mm to 5 mm. The left and right sides and the middle where the upper and lower clamps need to be connected need to be locally reinforced.

4. The method for testing mechanical properties of sandwich materials in combination with digital image technology according to claim 3, characterized in that: In the static three-point bending test in step A, the sandwich material test piece is adhered to the upper surface of the metal plate (2).

5. The method for testing mechanical properties of sandwich materials in combination with digital image technology according to claim 4, characterized in that: The step F specifically comprises: In order to obtain the distribution of stress and strain along the thickness direction of the structure, especially the side of the core material, each layer of the model is simulated by solid elements; The constitutive relation of the core material is established, the core thickness direction is taken as the material 3 direction, the speckle spraying surface is taken as the material main direction 13 surface, and the linear elastic constitutive model of the core material with in-plane transverse isotropy (E11=E22) is constructed; The boundary conditions of the model are highly consistent with the mechanical experiment, that is, the upper clamp is fixed, the lower clamp constrains the three rotational degrees of freedom and the translational degrees of freedom in the x and y directions, and a non-zero displacement vertically downward in the z direction is applied as the load; Multiple analysis steps are defined in the finite element model. The number of analysis steps is the same as the number of images collected by VIC-3D. The load size is defined in each analysis step. According to the experimental loading sequence, the load size defined in each analysis step is the same as the loading load corresponding to the moment when VIC-3D collects each image.

6. The method for testing mechanical properties of sandwich materials in combination with digital image technology according to claim 5, characterized in that: In step G, the initially assumed material constitutive parameters are first substituted into the finite element model, the strain data at the unit nodes are calculated, and the measured strain data obtained by VIC-3D correlation calculation is interpolated to the same position as the unit node of the finite element model by bilinear interpolation. The variance between the finite element numerical calculation strain and the measured strain at the same position is used to construct the objective function of parameter identification: in, represents a set of unknown material parameters of the core material, and They represent the numerical strain calculated by the finite element model and the strain measured by the digital image correlation method at node i, respectively. M is the number of nodes of the finite element model contained in the ROI area in each image. N is the number of loading steps in the finite element model, which corresponds to the number of images collected by the digital image correlation method equipment before the failure of the specimen during the experiment.

7. A sandwich material mechanical properties testing system combined with digital image technology, characterized in that: The sandwich material mechanical properties testing system combined with digital image technology uses the sandwich material mechanical properties testing method combined with digital image technology as described in any one of claims 1 to 6 to perform sandwich material mechanical properties testing.

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