An instrumented ball indentation method for detecting arbitrary residual stresses without reference specimens

Abstract

An instrumented ball indentation method for detecting any residual stress without a reference specimen comprises the following steps: 1) establishing a three-dimensional finite element model of a spherical indentation specimen, establishing analysis parameters and residual stress components σ R ,τ R 1) The relationship between normalized unloading work and material parameters without residual stress is established through numerical simulation; 2) A spherical indenter is used to perform an indentation test on a sample with arbitrary residual stress, and the relative change of normalized unloading work M is calculated; 3) An optical microscope is used to measure the major and minor axes of the elliptical indentation on the surface of the sample with residual stress, and the ellipticity η of the residual indentation is calculated; 4) The parameters M and lnη obtained by the instrumented indentation test are substituted into equations (3) and (4) to calculate the residual stress component σ of the material under test. R ,τ R The present invention does not require a press-in test on a reference sample, thereby improving efficiency and engineering applicability. At the same time, the magnitudes of two principal stresses of any residual stress can be obtained with only one press-in.

Description

Technical Field

[0001] The invention relates to a method for detecting residual stress on a metal surface, and in particular to an instrumented spherical pressing method for detecting any residual stress on a metal surface without a reference sample. Background Art

[0002] During the processing of engineering materials or components, different degrees of residual stress will be introduced into them due to reasons such as uneven plastic deformation or uneven temperature field. The existence of residual stress will affect the service life of materials or components, and residual stress needs to be detected. Compared with traditional detection methods, instrumented indentation detection methods have developed rapidly due to the characteristics of micro-area, micro-damage and a wide range of test materials. This type of detection method can be applied to portable micron indentation instruments to achieve in-situ detection of service structures such as oil and gas pipelines, and has good application prospects.

[0003] The residual stress indentation test method can be divided into two categories according to whether a reference specimen is required: comparative analysis method and direct analysis method. The comparative analysis method requires a specimen without residual stress as a reference, which is subject to certain limitations in practical applications; the direct analysis method can directly detect the residual stress in the specimen or component without the need for a reference specimen. The residual stress indentation test method can be divided into the equiaxed residual stress test method and the arbitrary residual stress test method according to the research object. Any plane residual stress can be equivalent to two orthogonal principal stress components. When the two principal stresses are equal in magnitude and tension / compression state, it is called equiaxial residual stress; otherwise it is called non-equiaxial residual stress. is the maximum principal stress, and the two principal stress components It can be further decomposed into the equiaxed residual stress part σ R The shear residual stress part τ R ,Right now:

[0004]

[0005] In actual engineering, welding or thermal spraying usually lead to non-equiaxed residual stress in the material. At present, the existing direct analysis method is mainly aimed at equiaxed residual stress, which is not convenient for practical application. The direct analysis method for non-equiaxed residual stress uses the characteristic that the Knoop indenter is sensitive to the direction of the principal stress, and requires multiple indentations into the material, which is inconvenient to operate and has low efficiency. Summary of the invention

[0006] In order to overcome the shortcomings of existing instrumented indentation detection methods such as being limited by reference specimens and inconvenient operation, the present application proposes an instrumented spherical indentation method that does not require a reference specimen to detect any residual stress on the metal surface. This method only requires one indentation test to simultaneously detect the magnitude of the two principal stress components of the residual stress without a reference specimen.

[0007] To achieve the above object, the present invention provides the following technical solution: an instrumented ball indentation method for detecting any residual stress without a reference sample, comprising the following steps:

[0008] 1) Establish a three-dimensional finite element model of the spherical indentation specimen, fix the relative indentation depth h / R = 0.01, R is the radius of the spherical indenter, h is the indentation depth, and obtain the load-depth (Fh) curve; select the relative change of normalized unloading work M and the logarithm of the residual indentation ellipticity lnη as analysis parameters, and establish the relationship between the analysis parameters and the residual stress component σ by means of dimensional analysis and numerical simulation. R ,τ R The relationship between

[0009] The calculation formula of M is as follows:

[0010] M=(W u / Eh 3 -W u0 / Eh 3 ) / W u0 / Eh 3 (2)

[0011] In the above formula, W u0 / Eh 3 is the normalized unloading work without residual stress, W u / Eh 3 is the normalized unloading work under residual stress; in addition, η is given by (d Ⅰ -d Ⅱ ) / (d Ⅰ +d Ⅱ ) is calculated, d Ⅰ is the major axis diameter of the elliptical indentation, d Ⅱ is the minor axis diameter of the elliptical indentation;

[0012] Analysis parameters and residual stress components σ R ,τ R The relationship between them is as follows:

[0013]

[0014]

[0015] In the above formula, σ y is the yield strength, ε y is the yield strain, n is the hardening exponent;

[0016] 2) The relationship between normalized unloading work and material parameters without residual stress is established through numerical simulation, that is,

[0017]

[0018] Substituting the known material mechanical parameters into equation (5), we can obtain the normalized unloading work W without residual stress: u0 / Eh 3 ;

[0019] 3) A spherical indenter was used to perform an indentation test on a sample containing any residual stress. The spherical indenter was pressed into the sample to a relative indentation depth of h / R = 0.01, and the normalized unloading work W of the sample containing residual stress was obtained. u / Eh 3 , calculate the relative change of normalized unloading work M;

[0020] 4) Use an optical microscope to measure the major axis and minor axis of the elliptical indentation on the surface of the sample containing residual stress, and calculate the ellipticity η of the residual indentation;

[0021] 5) Substitute the parameters M and lnη obtained by the instrumented indentation test into equations (3) and (4) to calculate the residual stress component σ of the material under test: R and τ R .

[0022] In the present invention, the relationship between the unloading work and the elastic-plastic properties of the material without residual stress is constructed by numerical simulation, thereby avoiding the need for a reference sample.

[0023] The beneficial effects of the present invention are: no need to perform a press-in test on a reference sample (i.e. a sample without residual stress), thereby improving efficiency and engineering applicability; at the same time, the magnitudes of two principal stresses of any residual stress can be obtained with only one press-in. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the relationship between the normalized unloading work, yield strain and hardening index when there is no residual stress. DETAILED DESCRIPTION

[0025] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of 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.

[0026] In the description of the present invention, it should be noted that the orientations or positional relationships indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0027] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0028] Refer to the following Figure 1 , the present invention is further described.

[0029] Three typical metals (AA7075, AA2014 and TC4) were selected as finite element simulation materials to verify the residual stress detection method.

[0030] In the first step, a spherical indentation model without residual stress was established with the help of numerical simulation, and the normalized unloading work and ε y ,n, such as Figure 1 Substituting the material mechanical parameters into equation (5), the normalized unloading work W of the sample without residual stress at the relative indentation depth h / R = 0.01 is obtained: u0 / Eh 3 .

[0031] In the third step, the finite element software was used to simulate the spherical indentation of three materials, AA7075, AA2014, and TC4, with arbitrary plane residual stress, and the normalized unloading work W at the relative indentation depth h / R = 0.01 was obtained. u / Eh 3 , calculate the relative change of normalized unloading work M, see Table 1. Apply two principal stress components to the specimen to equalize any residual stress, and convert the two principal stress components As the conventional true value, the specific value is shown in Table 1.

[0032] The fourth step is to measure the major and minor axis diameters of the elliptical indentation on the sample surface and calculate the logarithm of the ellipticity lnη. The calculation results are shown in Table 1.

[0033] Table 1

[0034]

[0035] The fifth step is to transform the material parameters (σ y ,ε y ,n) and the measured normalized unloading work relative change M and the logarithm of the ellipticity lnη are substituted into equations (3) and (4) to calculate the equiaxial stress part and the shear stress part. From equation (1), the two principal stress components can be obtained: The calculation results are shown in Table 1. It can be seen from the calculation results that the detection error in direction I and direction II can be controlled within ±15MPa, so this method can detect any residual stress in the plane more accurately.

[0036] The contents described in the embodiments of this specification are merely an enumeration of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms described in the embodiments. The protection scope of the present invention also extends to equivalent technical means that can be conceived by those skilled in the art based on the inventive concept.

Claims

1. An instrumented ball indentation method for detecting any residual stress without a reference specimen, characterized in that: The following steps are involved: 1) Establish a three-dimensional finite element model of the spherical indentation specimen, fix the relative indentation depth to h / R = 0.01, R is the radius of the spherical indenter, h is the indentation depth, and obtain the load-depth (Fh) curve; select the relative change of the normalized unloading work M and the logarithm of the residual indentation ellipticity lnη as analysis parameters, and establish the relationship between the analysis parameters and the residual stress component σ by means of dimensional analysis and numerical simulation. R , The relationship between The calculation formula of M is as follows: M=(W u / Eh 3 -W u0 / Eh 3 )W u0 / Eh 3 (1) In the above formula, W u0 / Eh 3 is the normalized unloading work without residual stress, W u / Eh 3 is the normalized unloading work under residual stress; in addition, η is given by (d Ⅰ -d Ⅱ )(d Ⅰ +d Ⅱ ) is calculated, d Ⅰ is the major axis diameter of the elliptical indentation, d Ⅱ is the minor axis diameter of the elliptical indentation; Analysis parameters and residual stress components σ R , The relationship between them is as follows: In the above formula, σ y is the yield strength, ε y is the yield strain, n is the hardening exponent; 2) The relationship between normalized unloading work and material parameters without residual stress is established through numerical simulation, that is, Substituting the known material mechanical parameters into equation (4), we can obtain the normalized unloading work W without residual stress: u0 / Eh 3 ; 3) A spherical indenter is used to perform an indentation test on a sample containing any residual stress. The spherical indenter is pressed into the sample to a relative indentation depth of h / R = 0.01, and the normalized unloading work W of the sample containing residual stress is obtained. u / Eh 3 , calculate the relative change of normalized unloading work M; 4) Use an optical microscope to measure the major axis and minor axis of the elliptical indentation on the surface of the sample containing residual stress, and calculate the ellipticity η of the residual indentation; 5) Substitute the parameters M and lnη obtained by the instrumented indentation test into equations (2) and (3) to calculate the residual stress component σ of the material under test: R and .

Images