A nondestructive measurement method for residual stress gradient

Abstract

The present invention relates to a non-destructive measurement method for residual stress gradient. The experimental platform mainly includes a through-light X-ray source with an energy of not less than 15keV, a surface detector and a sample loading device, and the whole adopts a transmission geometry mode. In the unloaded state, the incident X-ray is scanned from one side of the sample to the other side to obtain the interplanar spacing d value of the crystal plane {hkl} in the X and Y directions in different layers of the sample. The interplanar spacing d0 value of the {hkl} crystal plane in the stress-free state at different layers of the sample is calculated according to the d-sin 2 The intersection of the ψ relationship is determined. Based on Hooke's law, the residual stress gradients in the X and Y directions of the material at different layers are ultimately calculated. This invention can rapidly and non-destructively measure the residual stress gradients in two directions within the material, accurately evaluating the changes in residual stress within the material, helping to improve material design and strengthening processes, and ultimately extending the service life of material components.

Description

Technical Field

[0001] The present invention relates to the technical field of residual stress measurement, in particular to a non-destructive measurement method of residual stress gradient. Background Art

[0002] Residual stress refers to the stress that still exists and maintains its own balance inside the material after the external force or uneven temperature field is eliminated. The size, distribution and gradient of the residual stress inside the materials and components will have a significant impact on their fatigue strength, dimensional stability, corrosion resistance and service life. Accurately evaluating the residual stress inside the material is of great theoretical and engineering significance. At present, there are dozens of residual stress measurement technologies, which are mainly divided into destructive measurement, micro-destructive measurement and non-destructive measurement. Destructive measurement and micro-destructive measurement generally cause certain damage to the material or component, while the non-destructive measurement technology of residual stress represented by X-ray diffraction will not cause physical damage to the material or component during the detection process. Its principle is mainly to use X-ray diffraction to measure the elastic strain caused by the change in crystal plane spacing caused by residual stress inside the material.

[0003] One of the hallmarks of additively manufactured materials, surface-strengthened materials, and surface-coated materials is the presence of residual stress gradients within the materials, generated during the manufacturing process. Currently, existing methods for measuring residual stress rely primarily on fixed-point measurements, and non-destructive methods for measuring residual stress gradients within materials have yet to be established. Consequently, it is impossible to quickly and accurately assess the changes in residual stress gradients within materials. Therefore, a non-destructive measurement method for measuring residual stress gradients within materials is urgently needed to rapidly and accurately assess these changes, thereby improving material design and strengthening processes and ultimately extending the material's service life. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a non-destructive measurement method of residual stress gradient, which is used to quickly and accurately obtain and evaluate the gradient change of residual stress inside a material.

[0005] To achieve the above objectives, the present invention provides the following technical solutions:

[0006] A non-destructive measurement method for residual stress gradient mainly includes the following steps:

[0007] S1. Experimental platform construction: including a direct light X-ray source, a surface detector and a sample loading device.

[0008] S2. Sample preparation: Process the material with residual stress gradient into a tensile specimen that matches the sample loading device.

[0009] S3. Residual stress gradient measurement: In the unloaded state, the incident X-ray is scanned from one side of the tensile specimen to the other side to obtain the interplanar spacing d value of the {hkl} crystal planes in the X and Y directions in different layers of the specimen.

[0010] S4. Obtaining the {hkl} interplanar spacing d0 at different layers of the material in a stress-free state: Use the sample loading device to gradually load the tensile specimen to obtain the d-sin{hkl} interplanar spacing d0 at different layers of the sample in different stress states. 2 The ψ relationship (ψ is the angle between the diffraction vector direction and the normal direction of the sample surface), and the intersection value is taken as the d0 value of the {hkl} crystal plane in the stress-free state at this layer.

[0011] S5. Calculation of residual stress gradient: Calculate the lattice strain ε of the {hkl} crystal plane in the X and Y directions at different layers of the specimen using the following formula:

[0012] The elastic modulus E at different layers of the specimen is fitted, and the residual stress σ in the X and Y directions at different layers of the specimen is calculated according to Hooke's law: σ = E·ε.

[0013] Preferably, in step S1, the energy of the direct light X-ray source is generally not less than 15 keV to ensure that the incident X-rays can penetrate the sample to be tested. The X-ray source and the area detector are respectively located on both sides of the sample to be tested, that is, a transmission geometry mode is adopted.

[0014] Preferably, in step S2, the width direction of the tensile specimen is the residual stress gradient direction of the material, the length direction of the tensile specimen is the loading direction, and the thickness of the tensile specimen needs to be calculated based on the energy of the incident X-ray to ensure that the incident X-ray can penetrate the sample.

[0015] Preferably, in step S3, when measuring the residual stress gradient, the tensile specimen is translated so that the incident X-rays are scanned along the width direction of the specimen (i.e., the direction of the residual stress gradient), and the interplanar spacing d values of the crystal planes {hkl} in the X and Y directions in different layers of the tensile specimen are calculated by the Bragg equation.

[0016] Preferably, in step S4, during the elastic deformation stage, the {hkl} crystal plane sin of the sample under different stress states is obtained. 2 The relationship between ψ and the interplanar spacing d is obtained, and the intersection value is selected as the d0 value of the {hkl} crystal plane in the stress-free state of this layer.

[0017] Preferably, in step S5, when calculating the residual stress gradient, the elastic modulus E at different layers of the sample is obtained by fitting the tensile curve of the tensile sample during the elastic deformation loading process.

[0018] The beneficial effects of the above technical solution of the present invention are as follows:

[0019] In the above scheme, the present invention employs an area detector and transmission geometry, utilizing X-ray diffraction to rapidly and accurately nondestructively measure residual stresses at different layers of a material exhibiting a residual stress gradient. This method simultaneously obtains the residual stress gradient distribution in both the X and Y directions within the material under test. By using different d0 and E values at different layers of the gradient material, the residual stress gradient obtained is highly accurate. This nondestructive testing method accurately assesses residual stress gradient variations within a material. It is simple, easy to promote, and implement, and is of great significance for improving material design, strengthening processes, and extending material service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the process of non-destructive measurement of residual stress gradient;

[0021] Figure 2 It is a schematic diagram of the principle of non-destructive measurement of residual stress gradient. DETAILED DESCRIPTION

[0022] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0023] The present invention provides a non-destructive measurement method for residual stress gradient, the measurement flow diagram is as follows: Figure 1 The measurement principle diagram is shown in Figure 2 As shown, the following steps are included:

[0024] Step S1, build the experimental platform: mainly including a direct light X-ray source with energy not less than 15keV, a surface detector and a sample loading device. Figure 2 As shown, the through-light X-ray source and the surface detector are respectively located on both sides of the sample to be measured, that is, a transmission geometry mode is adopted.

[0025] Step S2, prepare the sample: process the material with residual stress gradient into a tensile specimen that matches the sample loading device. Figure 2 As shown, the width direction of the sample is the residual stress gradient direction of the sample, the length direction of the tensile sample is the loading direction, and the sample thickness needs to be calculated according to the energy of the incident X-ray to ensure that the incident X-ray can penetrate the sample.

[0026] Step S3, residual stress gradient measurement: Figure 2 As shown in the figure, in the unloaded state, by translating the tensile specimen, the incident X-rays are scanned from one side of the specimen width direction (i.e., the residual stress gradient direction) to the other side, and the diffraction angle 2θ of the {hkl} crystal plane in the X and Y directions in different layers of the specimen is obtained. The interplanar spacing d value is calculated according to the Bragg equation.

[0027] Step S4, obtaining the {hkl} interplanar spacing d0 values of different layers of the material under stress-free conditions: using a sample loading device to gradually load the tensile specimen during the elastic deformation stage, and after each loading, scanning the incident X-ray from one side of the specimen width direction (i.e., the residual stress gradient direction) to the other side, to obtain the {hkl} interplanar spacing d0 values of different layers of the tensile specimen under different stress conditions. 2 The relationship between ψ (ψ is the angle between the diffraction vector direction and the normal direction of the sample surface) and the interplanar spacing d, and the intersection value is taken as the interplanar spacing d0 value of the {hkl} crystal plane in this layer in a stress-free state.

[0028] Step S5, residual stress gradient calculation: Calculate the lattice strain ε of the {hkl} crystal plane in the X and Y directions at different layers of the sample using the following formula:

[0029] The elastic modulus E is obtained by fitting the tensile curves of the elastic deformation stage at different layers of the specimen. According to Hooke's law, the residual stress σ in the X and Y directions of different layers of the specimen is calculated: σ = E ·ε .

[0030] This invention employs an area detector and transmission geometry, using X-ray diffraction to rapidly and accurately nondestructively measure residual stresses at different layers of a material exhibiting a residual stress gradient. This method simultaneously obtains the residual stress gradient distribution in both the X and Y directions within the material under test. By using different d0 and E values at different layers of the gradient material, the residual stress gradient obtained is highly accurate. This nondestructive testing method accurately assesses residual stress gradient variations within a material. It is simple, easy to promote, and implement, and is of great significance for improving material design, strengthening processes, and increasing material service life.

[0031] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A non-destructive measurement method for residual stress gradient of a material, characterized in that: The following steps are involved: S1. Experimental platform construction: including a direct light X-ray source, a surface detector and a sample loading device; S2. Specimen preparation: Process the material with residual stress gradient into a tensile specimen that matches the sample loading device; S3. Residual stress gradient measurement: In the unloaded state, the incident X-ray is scanned from one side of the tensile specimen to the other side to obtain the interplanar spacing d values of the crystal planes {hkl} in the X and Y directions in different layers of the sample; S4. Obtain the d0 value of the {hkl} interplanar spacing at different layers of the material under stress-free conditions: Use the sample loading device to gradually load the tensile specimen to obtain the d-sin value of the {hkl} interplanar spacing at different layers of the material under different stress conditions. 2 ψ relationship, the intersection value is taken as the d0 value of the {hkl} crystal plane in the layer under stress-free state, where ψ is the angle between the diffraction vector direction and the normal direction of the sample surface; S5. Calculation of residual stress gradient: Calculate the lattice strain ε of the {hkl} crystal plane in the X and Y directions at different layers of the specimen using the following formula: Fit the elastic modulus E at different layers of the specimen and calculate the residual stress σ in the X and Y directions at different layers of the specimen according to Hooke's law: σ = E·ε; To obtain the {hkl} interplanar spacing d0 values of different layers of the material in a stress-free state, the tensile specimen needs to be loaded step by step during the elastic deformation stage, and after each loading, the incident X-ray is scanned along the residual stress gradient direction of the tensile specimen. Different d0 values and E values are used in different layers of the gradient material.

2. The non-destructive measurement method of residual stress gradient of a material according to claim 1, characterized in that: When the experimental platform was set up, the direct light X-ray source and the surface detector were located on both sides of the sample, and the overall transmission geometry mode was adopted.

3. The non-destructive measurement method of residual stress gradient of a material according to claim 1, characterized in that: The energy of the direct light X-ray source is not less than 15keV to ensure that the incident X-ray can penetrate the sample to be tested.

4. The non-destructive measurement method of residual stress gradient of a material according to claim 1, characterized in that: When preparing the specimen, the width direction of the tensile specimen is the residual stress gradient direction of the material, and the length direction of the tensile specimen is the loading direction.

5. The non-destructive measurement method of residual stress gradient of a material according to claim 1, characterized in that: The thickness of the tensile specimen needs to be calculated based on the energy of the incident X-rays to ensure that the incident X-rays can penetrate the sample to be tested.

6. The non-destructive measurement method of residual stress gradient of a material according to claim 1, characterized in that: When measuring the residual stress gradient, the incident X-ray is scanned along the residual stress gradient direction of the sample, and the interplanar spacing d value of the crystal plane {hkl} in the X and Y directions in different layers of the tensile sample is calculated by the Bragg equation.

7. The non-destructive measurement method of residual stress gradient of a material according to claim 6, characterized in that: When measuring the residual stress gradient, the tensile specimen is translated so that the incident X-rays scan along the direction of the residual stress gradient of the specimen.

8. The non-destructive measurement method of residual stress gradient of a material according to claim 1, characterized in that: When calculating the residual stress gradient, the elastic modulus E at different layers of the specimen is obtained by fitting the tensile curve of the tensile specimen during the elastic deformation loading process.

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