A residual stress testing method based on cylindrical plane indentation technology

Abstract

The present invention provides a residual stress testing method based on cylindrical plane indentation technology, which belongs to the field of residual stress technology and comprises the following steps: Step 1: Calculate the load value P0| corresponding to the cylindrical plane indentation test displacement h=0.2D of the material under test in a state without residual stress h＝0.2D ; Step 2: Obtain the load P corresponding to the residual stress specimen at the indentation depth h / D = 0.2 R │ h＝0.2D , and calculate the load value P0 corresponding to the sample with no residual stress h=0.2D│ h＝0.2D The relative change value (P R │ h＝0.2D ‑P0│ h＝0.2D ) / P0│ h＝0.2D ; Step 3: Material mechanical properties parameters and load relative difference (P R │ h＝0.2D ‑P0│ h＝0.2D ) / P0│ h＝0.2D Substitute into formula (1) to calculate the residual stress σ of the material being tested R , positive values ​​indicate tensile residual stress, and negative values ​​indicate compressive residual stress; #imgabs0# In the formula, σ y and ε y are the nominal yield strength and yield strain respectively, and σ y0 =Eε y0 , E is the elastic modulus of the material, n is the strain hardening index, D is the diameter of the flat indenter, α, a i (i=0,1,2),b i (i=0,1,2) and c i (i=0,1) is the model constant.

Description

Technical Field

[0001] The invention relates to the technical field of residual stress, and in particular to a residual stress testing method based on cylindrical plane indentation technology. Background Art

[0002] During the manufacturing and service process of engineering components, residual stress is inevitably generated due to the effects and influence of factors such as thermal stress, phase change stress and shrinkage stress, which affects the service performance (fatigue strength, resistance to brittle fracture, resistance to stress corrosion cracking and high temperature creep cracking), structural stiffness and stability of the structure or component. Therefore, it is particularly important to develop a reliable residual stress test method to accurately test the residual stress state and size of key components in order to timely evaluate their service performance and ensure service safety. Compared with traditional destructive residual stress detection methods (drilling method, ring core method, grooving method and layer-by-layer milling method, etc.), the press-in test method has attracted widespread attention due to its advantages of micro-damage and micro-regionalization, and has a good application prospect.

[0003] At present, the residual stress detection methods based on the indentation technology include the conical indentation method and the spherical indentation method according to the geometric shape of the indenter. The existing conical indentation method often uses the indentation contact area that cannot be directly measured as the residual stress analysis parameter, which affects its test accuracy; while the existing spherical indentation method mainly relies on empirical observation, lacks theoretical support, and is not yet convenient for practical engineering applications. Summary of the invention

[0004] The purpose of the present invention is to provide a residual stress testing method based on cylindrical plane indentation technology to solve the technical problems in the prior art such as low testing accuracy of the conical indentation method, lack of theoretical support for the spherical indentation method, and inconvenience for practical engineering applications.

[0005] The present invention discloses a residual stress testing method based on cylindrical plane indentation technology, comprising the following steps:

[0006] Step 1: Calculate the load value P0 corresponding to the displacement h = 0.2D of the cylindrical plane indentation test of the material under no residual stress. h＝0.2D ;

[0007] Step 2: Obtain the load P corresponding to the indentation depth h / D = 0.2 of the residual stress specimen R │ h＝0.2D , and calculate the load value P0 corresponding to the sample with no residual stress h=0.2D│ h＝0.2D The relative change value (P R │ h＝0.2D -P0│ h＝0.2D ) / P0│ h＝0.2D ;

[0008] Step 3: The material mechanical properties parameters and the relative difference of load (P R │ h＝0.2D -P0│ h＝0.2D ) / P0│ h＝0.2D Substitute into formula (1) to calculate the residual stress σ of the material being tested R , positive values ​​indicate tensile residual stress, and negative values ​​indicate compressive residual stress;

[0009]

[0010] In the formula, σ y and ε y are the nominal yield strength and yield strain respectively, and σ y0 =Eε y0 , E is the elastic modulus of the material, n is the strain hardening index, D is the diameter of the flat indenter, α, a i (i=0,1,2),b i (i=0,1,2) and c i (i=0,1) is the model constant.

[0011] Furthermore, the mechanical property parameters of the material are obtained by plane indentation technology.

[0012] Furthermore, the mechanical property parameters of the material are obtained by a standard uniaxial tensile test.

[0013] Furthermore, the load-displacement curve of the cylindrical plane compression elastic-plastic stage in the state of no residual stress of the tested material satisfies a good power law, that is,

[0014]

[0015] In the formula, k1~k4 and d ij (i=0,1,2;j=0,1) are model constants, C and m are loading coefficient and loading index respectively.

[0016] Furthermore, h=0.2D and material mechanical properties parameters (E,σ y ,ε y and n) into formula (2) to calculate P0│ h＝0.2D .

[0017] Furthermore, the load-displacement curve P of the residual stress specimen is obtained. R ~h R .

[0018] Furthermore, through the load-displacement curve P R ~h R , and get P R │ h＝0.2D .

[0019] Furthermore, the indentation load-displacement curve P R ~h R The residual stress of the specimens made of the same material is obtained by indentation test using a cylindrical flat indenter.

[0020] Furthermore, in the indentation test, the indentation depth is 0.2 times the diameter D of the cylindrical plane indenter.

[0021] The second purpose of the present invention is to protect the application of a residual stress testing method based on cylindrical plane indentation technology for obtaining residual stress of service structures in the fields of aerospace, nuclear power or oil and gas transportation.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The method of the present invention uses a single cylindrical plane indenter to perform a quasi-static indentation test on the surface of a material containing local uniform residual stress to obtain a continuous load-displacement curve, and the residual stress on the surface of the material component can be obtained after simple processing, which overcomes the shortcomings of the existing indentation residual stress test, the contact area of ​​the existing cone pressure residual stress determination method is difficult to measure directly, and the ball pressure residual stress relies on complex multi-level empirical fitting equations. The model has fewer parameters, a simple and convenient solution method, good prediction effect and universal applicability, and is easy to popularize and apply.

[0024] 2. It is of great significance to obtain the residual stress of service structures which are widely present in engineering fields such as aerospace, nuclear power, oil and gas transportation. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only represent some embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0026] Figure 1 It is a schematic diagram of the pressing method of the cylindrical plane pressure head adopted in the present invention.

[0027] Figure 2 This is a typical load-displacement curve of a cylindrical flat indenter.

[0028] Figure 3 It is the load-displacement curve of the plane compression test of #45 steel column in the specific embodiment. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, 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.

[0030] A residual stress testing method based on cylindrical plane indentation technology comprises the following steps:

[0031] Step 1: Use plane indentation technology or standard uniaxial tensile test to obtain material mechanical properties parameters, including elastic modulus E, yield strength σ y , yield strain ε y and strain hardening coefficient n; if the above material parameters are known, this step can be ignored;

[0032] Step 2: Calculate the load value P0 corresponding to the displacement h = 0.2D according to the column plane compression elastic-plastic load-displacement model under the state of no residual stress in the material. h＝0.2D , the load-displacement model table is

[0033]

[0034] In the formula, σ y and ε y are the nominal yield strength and yield strain respectively, and σ y0 =Eε y0 , E is the elastic modulus of the material, n is the strain hardening index, D is the diameter of the flat indenter, C and m are the loading coefficient and loading index, respectively, k1~k4 and d ij (i=0,1,2; j=0,1) are model constants, listed in Table 1;

[0035] Step 3: Use a cylindrical plane indenter to perform an indentation test on the residual stress specimen of the same material to obtain the indentation load-displacement curve P R ~h R , the indentation depth is fixed at 0.2 times the diameter D of the cylindrical plane indenter;

[0036] Step 4: Obtain the load P corresponding to the indentation depth h / D = 0.2 of the residual stress specimen R │ h＝0.2D , and calculate the load value P0 corresponding to the sample with no residual stress h=0.2D│ h＝0.2D The relative change value (P R │ h＝0.2D -P0│ h＝0.2D ) / P0│ h＝0.2D ;

[0037] Step 5: The mechanical properties parameters (E, σ y ,εy and n) and relative load difference (P R │ h＝0.2D -P0│ h＝0.2D ) / P0│ h＝0.2D Substitute into formula (4) to calculate the residual stress σ of the material being tested R , positive values ​​indicate tensile residual stress, and negative values ​​indicate compressive residual stress.

[0038]

[0039] In the formula, σ y and ε y are the nominal yield strength and yield strain respectively, and σ y0 =Eε y0 , E is the elastic modulus of the material, n is the strain hardening exponent, α, a i (i=0,1,2),b i (i=0,1,2) and c i (i=0,1) are model constants, listed in Table 2.

[0040] The primary condition of the technical solution of the present invention is to obtain an accurate indentation load-displacement test curve of the residual stress sample surface through a cylindrical plane indentation test (taking a cylindrical indenter with a diameter of 0.5 mm as an example); at this time, the surface of the material or structure to be tested needs to be ground and polished so that the surface roughness is better than 0.32 μm before the cylindrical plane indentation test can be carried out. The loading method is as follows: Figure 1 As shown in the figure, the compressive residual stress causes the indentation load-displacement curve to increase compared with the load-displacement curve of the specimen without residual stress, while the tensile residual stress causes the indentation load-displacement curve to decrease compared with the load-displacement curve of the specimen without residual stress, as shown in the figure. Figure 2 shown.

[0041] The present invention uses the material mechanical property parameters (E, σ y ,ε y and n) and the load-displacement model (3) of the plane indentation of the specimen without residual stress to obtain the load value P0 corresponding to the indentation displacement h = 0.2D of the specimen without residual stress. h＝0.2D ; Quasi-statically press the material surface with a cylindrical flat indenter to obtain a continuous load P R ~Displacement h R Curve, and then obtain the load value P corresponding to the corresponding indentation displacement h = 0.2D R │ h＝0.2D ;P0│ h＝0.2D , P R │ h＝0.2D and material mechanical properties parameters (E,σ y ,ε ySubstituting σ and n) into formula (4), the residual stress σ of the material or component under test can be predicted. R .

[0042] Example 1

[0043] The following is an example of #45 steel. A quasi-static indentation test is performed on #45 steel using a cylindrical flat indenter to obtain its residual stress. Table 3 lists the material mechanical properties parameters obtained by a uniaxial tensile test on #45 steel using a material testing machine. According to the material mechanical properties parameters (E, σ y ,ε y and n) and the plane compression load-displacement model of the column without residual stress to obtain P0│ h＝0.2D ; Figure 3 The load-displacement curve obtained from the plane indentation test of the specimen with residual stress is given. The load value P corresponding to the indentation displacement h = 0.2D is taken. R │ h＝0.2D , P0│ h＝0.2D , P R │ h＝0.2D and material mechanical properties parameters (E,σ y ,ε y Substituting the residual stress σ into equation (4) R Table 4 is a comparison of the residual stress value predicted by the scheme of the present invention and the actual residual stress value applied in advance. The absolute error between the two is better than 25MPa, and the relative error does not exceed 10%. The scheme of the present invention has good prediction accuracy.

[0044] Table 1 Model parameters of formula (3)

[0045]

[0046] Table 2 Model parameters of formula (4)

[0047]

[0048] Table 3 Uniaxial tensile mechanical properties parameters of #45 steel

[0049]

[0050] Table 4 Prediction results of residual stress of #45 steel

[0051]

[0052] Note: “-” indicates compressive residual stress.

[0053] The above are the implementation methods listed in this embodiment, but this embodiment is not limited to the above optional implementation methods. Those skilled in the art can arbitrarily combine the above methods to obtain other various implementation methods. Anyone can derive other various forms of implementation methods under the inspiration of this embodiment. The above specific implementation methods should not be understood as limiting the scope of protection of this embodiment. The scope of protection of this embodiment shall be based on the definition in the claims, and the description can be used to interpret the claims.

Claims

1. A residual stress testing method based on cylindrical plane indentation technology, characterized in that: The following steps are involved: Step 1: Calculate the load value P0 corresponding to the displacement h = 0.2D of the cylindrical plane indentation test of the material under no residual stress. h＝0.2D ; Step 2: Obtain the load P corresponding to the indentation depth h / D = 0.2 of the residual stress specimen R │ h＝0.2D , and calculate the load value P0 corresponding to the sample with no residual stress h=0.2D│ h＝0.2D The relative change value (P R │ h＝0.2D -P0│ h＝0.2D ) / P0│ h＝0.2D ; Step 3: The material mechanical properties parameters and the relative difference of load (P R │ h＝0.2D -P0│ h＝0.2D ) / P0│ h＝0.2D Substitute into formula (1) to calculate the residual stress σ of the material being tested R , positive values ​​indicate tensile residual stress, and negative values ​​indicate compressive residual stress; In the formula, σ y and ε y are the nominal yield strength and yield strain respectively, and σ y0 =Eε y0 , E is the elastic modulus of the material, n is the strain hardening index, D is the diameter of the flat indenter, α, a i (i=0,1,2),b i (i=0,1,2) and c i (i=0,1) is the model constant; The load-displacement curve of the cylindrical plane compression elastic-plastic stage in the state of no residual stress of the tested material satisfies a good power law, that is, In the formula, k1~k4 and d ij (i=0,1,2;j=0,1) are model constants, C and m are loading coefficient and loading index respectively.

2. The residual stress testing method based on cylindrical plane indentation technology according to claim 1 is characterized in that: The mechanical property parameters of the material are obtained by plane pressing technology.

3. The residual stress testing method based on cylindrical plane indentation technology according to claim 1 is characterized in that: The mechanical property parameters of the material are obtained by a standard uniaxial tensile test.

4. The residual stress testing method based on cylindrical plane indentation technology according to claim 1 is characterized in that: Substituting h = 0.2D and the material mechanical properties parameters into formula (2), we can calculate P0│ h＝0.2D .

5. The residual stress testing method based on cylindrical plane indentation technology according to claim 1 is characterized in that: Obtain the load-displacement curve P of the sample with residual stress R ~h R .

6. The residual stress testing method based on cylindrical plane indentation technology according to claim 5 is characterized in that: Through the load-displacement curve P R ~h R , and get P R │ h＝0.2D .

7. The residual stress testing method based on cylindrical plane indentation technology according to claim 5 is characterized in that: The indentation load-displacement curve P R ~h R The residual stress of the specimens made of the same material is obtained by indentation test using a cylindrical flat indenter.

8. The residual stress testing method based on cylindrical plane indentation technology according to claim 7 is characterized in that: In the indentation test, the indentation depth is 0.2 times the diameter D of the cylindrical plane indenter.

9. Application of a residual stress testing method based on cylindrical plane indentation technology according to any one of claims 1 to 8, characterized in that: Used to obtain residual stress of service structures in aerospace, nuclear power or oil and gas transportation fields.

Images