Direct Detection Method for Equiaxed Residual Stress Based on Instrumented Conical Indentation Technology

Abstract

The present invention discloses a direct detection method for equiaxed residual stress based on instrumented conical indentation technology, comprising the following steps: first, a conical indenter is used to perform an indentation test on a sample with residual stress, the maximum indentation load is fixed as F, and the load-depth curve is obtained by fitting h m 、h m ‑h f and α; the projection area A of the residual indentation is measured using an ordinary optical microscope p Calculate the contact depth h c ; The second step is to calculate the maximum indentation depth h m , the difference between the maximum indentation depth and the residual depth after unloading h m ‑h f , contact depth h c Combined with the indenter contact depth function, the maximum indentation depth h corresponding to the state without residual stress under the same load is reconstructed. m0 ; The third step is to reconstruct the maximum indentation depth h without residual stress m0 Substitute F=Ch α , and obtain the loading curvature C0 of the loading curve without residual stress; the fourth step is to integrate the loading curve at the same depth to obtain the loading work U L and U L0 , combined with the known material plastic parameters, the residual stress σ of the material being tested is calculated R .

Description

Technical Field

[0001] The present invention relates to the technical field of material residual stress detection, and in particular to a method for directly detecting equiaxed residual stress on a material surface by utilizing an instrumented conical indentation technique without a reference sample. Background Art

[0002] Engineering materials are subject to mechanical processing, uneven temperature fields (such as welding and thermal spraying), and other influences, which can generate residual stress on their surfaces, thereby affecting their mechanical properties. Measuring residual stress in engineering structures is crucial for analyzing mechanical properties and assessing service safety. Compared with traditional testing methods (such as drilling and ring core methods), instrumented indentation technology offers advantages such as micro-area detection, minimal damage, and rapid detection. Combined with a portable indentation instrument, it enables in-situ online testing and holds great promise for future applications.

[0003] Currently, instrumented indentation testing methods can be divided into two categories based on the technical route for solving residual stress: comparative testing and direct testing. Existing comparative testing methods compare the indentation response parameters of specimens with and without residual stress (such as load and loading curvature at the same depth), and inversely identify residual stress based on the changes in these indentation response parameters. In some practical engineering applications (such as pipeline residual stress testing), in-situ online testing is achieved in conjunction with a portable indentation instrument. In order to obtain a reference specimen without residual stress, the pipeline needs to be cut to release residual stress, thereby destroying the structural integrity. This contradicts the characteristics of instrumented indentation technology: micro-area, minimal damage, and rapid testing, making it inconvenient for practical application. Summary of the Invention

[0004] In order to overcome the shortcomings of existing equiaxed residual stress indentation detection methods, which are limited in application and poor in practical engineering, the present invention proposes an instrumented conical indentation direct detection method for equiaxed residual stress. This method has wide applicability and can accurately detect equiaxed residual stress without the need for indentation testing under reference sample conditions.

[0005] In order to solve the above technical problems, the present invention proposes the following technical solutions:

[0006] A method for directly detecting equiaxed residual stress based on an instrumented conical indentation technique, the method comprising the following steps:

[0007] In the first step, a conical indenter is used to perform an indentation test on a sample with residual stress. The maximum indentation load is fixed as F. The load-depth curve is obtained by fitting h. m 、h m -h f and α, where h m is the maximum indentation depth, h fis the residual depth after unloading, α is the loading index of the load-depth curve; the projected area A of the residual indentation of the sample is measured using an ordinary optical microscope p ,pass Calculate the contact depth h c ;

[0008] The second step is to calculate the maximum indentation depth h m , the difference between the maximum indentation depth and the residual depth after unloading h m -h f , contact depth h c Substituting into formula (1), using the subscript “0” to represent the parameters in the state without residual stress; combining with the indenter contact depth function of formula (2), the maximum indentation depth h corresponding to the state without residual stress under the same load is reconstructed. m0 ;

[0009]

[0010]

[0011] In formula (2), κ and η are constants related to the pressure head.

[0012] In the third step, the maximum indentation depth h without residual stress is reconstructed. m0 Substitute F=Ch α , where the loading index does not change with the change of residual stress, that is: α0 = α, and the loading curvature C0 of the loading curve without residual stress is obtained; fixed at the same depth, the loading work U of the sample with and without residual stress is calculated by formula (3) L and U L0 ;

[0013]

[0014] The fourth step is to convert the above-obtained parameter U L and U L0 , combined with the known material plasticity parameters (yield strength σ y , yield strain ε y and power hardening index n); Substitute into formula (4) to calculate the residual stress σ of the tested material R ;

[0015]

[0016] In formula (4), σ R A positive value indicates residual tensile stress, and a negative value indicates residual compressive stress.

[0017] The technical concept of this invention is as follows: Numerical simulations reveal that, compared with samples without residual stress, the changes in some parameters during the conical indentation process of materials with residual stress are related to the degree of indentation ridge and depression in the sample indentation morphology. Based on this finding and in combination with the contact depth function of the conical indenter without residual stress, the key parameters of the indentation loading curve of the sample without residual stress can be deduced from the conical indentation results of any equiaxed residual stress sample, allowing the loading curve of the sample without residual stress to be reconstructed. Finally, combined with a comparative detection method, the magnitude of the equiaxed residual stress at the same depth can be calculated based on the loading work.

[0018] The present invention has the beneficial effect of directly detecting the equiaxed residual stress in a material through a single instrumented conical indentation test, without requiring a reference specimen free of residual stress. Compared to comparative instrumented indentation residual stress detection methods, the present invention is more applicable in practical engineering applications where obtaining a reference specimen free of residual stress is difficult. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the indentation load-depth curve with residual stress and without residual stress obtained by reconstruction under the same indentation load. DETAILED DESCRIPTION

[0020] The present invention will be further described below with reference to the accompanying drawings.

[0021] Reference Figure 1 A direct detection method for equiaxed residual stress based on instrumented conical indentation is proposed. Three commonly used metals in engineering (aluminum alloy AA2014, aluminum alloy AA7075 and titanium alloy TC4) are selected as simulation materials, and finite element simulation is used to verify the residual stress detection method.

[0022] In the first step, the plasticity parameters σ of three materials, aluminum alloy AA2014, aluminum alloy AA7075 and titanium alloy TC4, are known. y , ε y , n, the instrumented conical indentation process with residual stress was simulated in the commercial finite element software ABAQUS, and the obtained indentation load-depth curves were fitted to obtain the difference h between the maximum indentation depth and the residual unloading depth. m -h f , loading index α; obtain the projected contact area of the residual indentation by ordinary optical microscope measurement and calculate the contact depth h c The equiaxed residual stress σ of the simulation input T R rue As the conventional true value of residual stress, the specific values of input residual stress of the three materials are shown in Table 1.

[0023] In the second step, the difference h between the maximum indentation depth and the residual unloading depth is calculated. m -h f , contact depth h c Substituting into formula (1) and combining with formula (2) we can get the maximum indentation depth h corresponding to the reference specimen without residual stress under the same load: m0 , the obtained h m0 Substitute F=Ch α , where α0 = α, and the loading curvature C0 of the loading curve of the reference specimen without residual stress is reconstructed.

[0024] In the third step, the same indentation depth (20 μm) is fixed and the loading work U of the specimens with and without residual stress is calculated using formula (3): L and U L0 ,The specific results of each material are shown in Table 1.

[0025] The fourth step is to convert the known material plasticity parameters σ into y , ε y , n and the calculated loading work U L and U L0 Substituting into formula (4), the magnitude of the equiaxed residual stress σ is calculated R The final calculation results are shown in Table 1. The calculated residual stress values are compared with the agreed true values, and the maximum relative error is controlled within ±10%.

[0026] Table 1

[0027]

Claims

1. A direct detection method for equiaxed residual stress based on instrumented conical indentation technology, characterized in that: The following steps are involved: 1) A conical indenter is used to perform an indentation test on a sample with residual stress. The maximum indentation load is fixed as F. The load-depth curve is obtained by fitting h. m 、h m -h f and α, where h m is the maximum indentation depth, h f is the residual depth after unloading, α is the loading index of the load-depth curve; the projected area A of the residual indentation of the sample is measured using an ordinary optical microscope p ,pass Calculate the contact depth h c ; 2) The maximum indentation depth h obtained by fitting calculation m , the difference between the maximum indentation depth and the residual depth after unloading h m -h f , contact depth h c Substitute into formula (1), use subscript 0 to represent the parameters under the state of no residual stress; combine with formula (2) the contact depth function of the indenter, and reconstruct the maximum indentation depth h corresponding to the state of no residual stress under the same load m0 ; In formula (2), κ and η are constants related to the pressure head; 3) The maximum indentation depth h obtained without residual stress is reconstructed m0 Substitute F=Ch α , where the loading index does not change with the change of residual stress, that is: α0 = α, and the loading curvature C0 of the loading curve without residual stress is obtained; fixed at the same depth, the loading work U of the sample with and without residual stress is calculated by formula (3) L and U L0 ; 4) The parameter U obtained above L and U L0 , combined with known material plasticity parameters, including yield strength σ y , yield strain ε y and power hardening index n; Substitute into formula (4) to calculate the residual stress σ of the tested material R ; In formula (4), σ R A positive value indicates residual tensile stress, and a negative value indicates residual compressive stress.

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