CN103969108A - Method for measuring residual stress on surface of metal sample based on micro indentation - Google Patents

Abstract

The invention discloses a method for measuring residual stress on surface of a metal sample based on micro indentation. The method comprises the following steps: performing a micro indentation test on the surface of the sample by utilizing a Vickers microhardness tester, recording the indentation morphology by adopting a scanning electron microscope, and obtaining the nominal projection area and actual projection area of the indentation on the surface of the sample through calculation for diagonal length and side length of the micro indentation; obtaining a strength coefficient and strain hardening index of the material through a standard tensile test; and obtaining a theoretical formula for measuring the residual stress through an indentation method and containing the ratio of the actual projection area to the nominal projection area of the micro indentation, the strength coefficient and strain hardening index based on the hypothesis for the two-dimension isometric residual stress field on the surface of the sample, and further obtaining the residual strain and residual stress on the surface of the sample. The measurement method belongs to basically nondestructive or micro-damage measurement and has the advantages of low measurement cost, simplicity and convenience in operation and high measurement efficiency. If being combined with a portable high-power optical microscope with a distance measurement function, the method can be popularized and applied in the field of engineering.

Description

A kind of method of the measurement test button surface residual stress based on micro-impression

Technical field

The present invention relates to a kind of method that metal structure surface residual stress is measured, mainly by micro Vickers, micro-impression is resided in to specimen surface, by the ratio of the actual projected area of this micro-impression and nominal projected area with in sample, be associated because unrelieved stress exists the overstrain producing, and then just can determine overstrain and the unrelieved stress in sample according to the formula of the indentation method measurement unrelieved stress based on being pressed into response and elasto-plastic Contact theory.This invention belongs to elasto-plastic Contact mechanics, construction material and impression residual stress measurement field.

Background technology

Unrelieved stress is the result that inhomogeneous elastic deformation or inhomogeneous elastic-plastic deformation have occurred in material, or perhaps the elastic anisotropy of material and plastic anisotropy's reflection.All processes that can cause inhomogeneous elasticity, plastic yield in material all can make material production unrelieved stress, and as cold and hot when distortion, inhomogeneous inhomogeneous Temperature Distribution when inhomogeneous, thermal treatment of expanding with heat and contract with cold causing of its interior temperature distribution causes the asynchronism of phase transition process during along cross section elastoplasticity nonaffine deformation, workpiece heating and cooling.No matter in machinery, water conservancy and hydropower, Aero-Space, thermoelectricity nuclear power field, or in industries such as petrochemical complex, metallurgic railway, communications and transportation, residual stress problems receives much concern always, so measuring technology and the applied research thereof of every country to unrelieved stress all takes much count of.

The method of testing of unrelieved stress is a lot, whether has destructiveness can be divided into and damage method of testing and the large class of non-destructive testing method two by it for tested member.The cardinal principle that damages method of testing is destructive stress relief, make its release portion stress produce corresponding displacement and strain, measure these displacements and strain, obtain the original stress of member through converting, conventional method has boring method, peeling method, gets bar method, grooving method etc.Non-destructive testing method is the method that scientific research personnel explores always for many years, and the method for non-destructive testing has x-ray method, neutron diffraction method, supercritical ultrasonics technology, method of magnetic etc. at present.But in view of the application of these methods has certain limitation, using is in practice boring method and X-ray diffraction method comparatively widely, although boring method development is comparatively ripe, but boring method need to destroy tested sample, thereby unrelieved stress is discharged and records unrelieved stress numerical value, so can not use in many important engineering structures, simultaneously, because foil gauge is for responsive to temperature, strain measurement precision depends on the technology of bonding, and boring centering process is had relatively high expectations, be therefore not suitable for measuring under complex environment; In the time utilizing X-ray diffraction method to measure sample unrelieved stress, test value costliness, requires strictly the grain size of sample, and sample crystal grain can not be excessive, and sample can not have higher texture.Therefore the research of unrelieved stress measuring technology, no matter for fundamental research, or application is all of great significance for engineering.[Zhang Dingquan. the ABC lecture [J] of residual stress analysis. physical and chemical inspection: physics fascicle, 2007, 4:1-5. Wang Qing is bright, Sun Yuan. the progress of unrelieved stress measuring technology and trend [J]. electromechanical engineering, 2011, 28 (001): 11-15. Xu Hong, Teng Hongchun, Cui Bo, Deng. the current situation brief introduction [J] of unrelieved stress non-destroyed measurement technology. physical and chemical inspection: physics fascicle, 2003, 39 (11): 595-598.Chen X, Yan J, Karlsson A M.On thedetermination of residual stress and mechanical properties by indentation[J] .Materials Science and Engineering:A, 2006, 416 (1): 139-149.]

While using the unrelieved stress in Drawing Process test sample, test process belongs to basic harmless or micro-damage, and various permission removed and caused the engineering component of potential safety hazard to have important practical significance because of material; The physical background of Drawing Process is comparatively clear, and correlation theory maturation has the advantages such as testing tool is light, flexible and convenient operation, if the Powerful Light Microscope of combined type portable band distance measurement function can be promoted the use of at engineering field.

Summary of the invention

The object of the invention is to, a kind of method of the measurement test button surface residual stress based on micro-impression is provided, the method is simple to operate, and correlation theory maturation has important value to fundamental research and engineering application.

Technical scheme of the present invention is: a kind of method of the measurement test button surface residual stress based on micro-impression, specifically comprises the following steps:

Step 1:

1.1 are processed into standard tensile sample according to national standard by the sample that has unrelieved stress;

1.2 according to the stress-strain diagram that standard tensile sample is carried out stretching experiment and obtained this material on cupping machine, tries to achieve the strength factor σ of material by stress-strain diagram ₀with strain hardening exponent n, for subsequent use;

Step 2:

2.1 process the specimen surface of processing through step 1 that has unrelieved stress to obtain smooth, the smooth surface that meets vickers microhardness experiment;

2.2 adopt vickers microhardness testing machine sample to be carried out to micro-indentation test, suitable loaded load and the load time of attentional selection in micro-indentation test process; Record catercorner length D and the vickers microhardness value HV of micro-impression;

2.3 utilize scanning electron microscopy measurement to obtain the length of side a of micro-impression;

2.4 calculate the nominal projected area A of micro-impression according to catercorner length D _nom, A _nom=D*D/2, is obtained the actual projected area A of micro-impression by length of side a, if impression is convex shape, and A=a*a*0.96, wherein 0.96 is form factor, and then obtains the ratio c of two areas ²;

Step 3:

3.1 measure unrelieved stress according to the indentation method based on being pressed into response and elasto-plastic Contact theory, obtain overstrain value ε by formula (1) _res, as follows:

Wherein, ε in formula _resit is overstrain; σ ₀it is the strength factor of material; N is the strain hardening exponent of material; HV is the measured value of vickers microhardness;

The overstrain ε that the formula (1) of 3.2 bases obtains _res, substitution formula (2) calculates the axle unrelieved stress σ such as the two dimension of specimen surface _res, as follows:

${\mathrm{\σ}}_{\mathrm{res}}={\mathrm{\σ}}_0\left[\right|{\mathrm{\ϵ}}_{\mathrm{res}}|{]}^n\×[\exp \left(\frac{1-c^2}{0.32}\right)-1]---\left(2\right)$

Wherein, ε in formula _resit is overstrain; σ _resit is unrelieved stress; σ ₀it is the strength factor of material; n

It is the strain hardening exponent of material; HV is the measured value of vickers microhardness; c ²for the actual projected area A of impression and nominal projected area A _nomratio.

The present invention has the following advantages and high-lighting effect: the method that the present invention proposes is based on elasto-plastic Contact mechanics and the mechanical meaurement method that is pressed into response, in conjunction with harmless micro-impression, realized more accurately measuring of unrelieved stress, and operating process is convenient and simple, testing tool is light flexibly.

Brief description of the drawings

Fig. 1 is used for measuring unrelieved stress 2A02 aluminium alloy, 7050 aluminium alloy turriform part labeled graphs.

The impression shape appearance figure that Fig. 2 turriform part surface obtains after micro Vickers is got ready.

The impression length of side signature that Fig. 3 impression obtains by the tape measure under scanning electron microscope.

Figure 42 A02 aluminium alloy turriform part is without the residual stress distribution figure of restraint layer.

Figure 57 050 aluminium alloy turriform part is without the residual stress distribution figure of restraint layer.

Embodiment

Further illustrate the specific embodiment of the present invention below in conjunction with accompanying drawing.

Embodiment 1

The test of 2A02 aluminium alloy turriform part quenching unrelieved stress.

After 2A02 aluminium alloy turriform part is incubated to 2h at 470 DEG C of temperature, carry out rapidly normal temperature shrend and introduce quenching unrelieved stress.

1) 2A02 aluminium alloy one way tensile test

Material is processed into standard tensile sample according to national standard, on MTS810 universal testing machine, carry out normal temperature one directional tensile test, obtain the normal temperature stress-strain diagram of 2A02 aluminum alloy materials, can obtain its strength factor=899.19MPa, strain hardening exponent n=0.25 through calculating with matching.

2) surface treatment of turriform part and label

Turriform part pattern as shown in Figure 1.Its surface is polished and polishing, and in the time of sanding and polishing, must be noted that can not be firmly excessive, avoids destroying original residual stress field and produces larger new unrelieved stress.And then, the upper surface of turriform part is carried out to mark, upper surface when thickness is 30mm, 20mm, 10mm is labeled as respectively ground floor, the second layer, the 3rd layer, as shown in Figure 1.Every one deck longitudinally axis of symmetry direction is row, lists and selects three square region marking pens to mark at each, is labeled as respectively A, B, C.There are five row as the 3rd layer, are designated as respectively the 3rd layer of the first from left row A, B, C, the 3rd layer of the second from left row A, B, C, a 3rd layer of left side three is listed as A, B, C, and a 3rd layer of left side four is listed as A, B, C and a 3rd layer of left side five and is listed as A, B, C.

3) measure microhardness

Utilize vickers microhardness testing machine, carry out hardness test at the label face of turriform part.Impression point pattern is similar to Fig. 2.

4) measure and calculate name and actual projected area

The actual length of side a of impression ₁, a ₂obtain by the tape measure under scanning electron microscope or high-power microscope, as shown in Figure 3, and impression catercorner length D ₁, D ₂directly measure by vickers microhardness testing machine.

5) data gather and processing

The indentation hardness measuring, the actual length of side, catercorner length are gathered, adopt formula

${\mathrm{\σ}}_{\mathrm{res}}={\mathrm{\σ}}_0\left[\right|{\mathrm{\ϵ}}_{\mathrm{res}}|{]}^n\×[\exp \left(\frac{1-c^2}{0.32}\right)-1]$

Calculate the corresponding unrelieved stress numerical value of each impression in table 2.

Each layer of unrelieved stress numerical tabular of table 1 2A02 aluminium alloy turriform part

2A02 aluminium alloy turriform part is shown in Fig. 4 without the residual stress distribution of restraint layer, and the residual-stress value that this method records distributes consistent with actual turriform part theoretical stress.

Embodiment 2

The test of 7050 aluminium alloy turriform part quenching unrelieved stresss.

After 7050 aluminium alloy turriform parts are incubated to 2h at 470 DEG C of temperature, carry out rapidly normal temperature shrend and introduce quenching unrelieved stress.

1) 7050 aluminium alloy one way tensile tests

Material is processed into standard tensile sample according to national standard, on MTS810 universal testing machine, carry out normal temperature one directional tensile test, obtain the normal temperature stress-strain diagram of 7050 aluminum alloy materials, can obtain its strength factor=846.92MPa, strain hardening exponent n=0.22 through calculating with matching.

2) surface treatment of turriform part and label

Turriform part pattern as shown in Figure 1.Its surface is polished and polishing, and in the time of sanding and polishing, must be noted that can not be firmly excessive, avoids destroying original residual stress field and produces larger new unrelieved stress.And then, the upper surface of turriform part is carried out to mark, upper surface when thickness is 30mm, 20mm, 10mm is labeled as respectively ground floor, the second layer, the 3rd layer, as shown in Figure 1.Every one deck longitudinally axis of symmetry direction is row, lists and selects three square region marking pens to mark at each, is labeled as respectively A, B, C.There are five row as the 3rd layer, are designated as respectively the 3rd layer of the first from left row A, B, C, the 3rd layer of the second from left row A, B, C, a 3rd layer of left side three is listed as A, B, C, and a 3rd layer of left side four is listed as A, B, C and a 3rd layer of left side five and is listed as A, B, C.

3) measure microhardness

Utilize vickers microhardness testing machine, carry out hardness test at the label face of turriform part.Impression point pattern is similar to Fig. 2.

4) measure and calculate name and actual projected area

The actual length of side a of impression ₁, a ₂obtain by the tape measure under scanning electron microscope or high-power microscope, as shown in Figure 3, and impression catercorner length D ₁, D ₂directly measure by vickers microhardness testing machine.

5) data gather and processing

The indentation hardness measuring, the actual length of side, catercorner length are gathered, adopt formula

${\mathrm{\σ}}_{\mathrm{res}}={\mathrm{\σ}}_0\left[\right|{\mathrm{\ϵ}}_{\mathrm{res}}|{]}^n\×[\exp \left(\frac{1-c^2}{0.32}\right)-1]$

Calculate the corresponding unrelieved stress numerical value of each impression in table 2.

Each layer of unrelieved stress numerical tabular of table 27050 aluminium alloy turriform part

7050 aluminium alloy turriform parts are shown in Fig. 5 without the residual stress distribution of containment surfaces, and the residual-stress value that this method records distributes consistent with actual turriform part theoretical stress.

Claims (1)

1. a method for the measurement test button surface residual stress based on micro-impression, specifically comprises the following steps:

Step 1:

1.1 are processed into standard tensile sample according to national standard by the sample that has unrelieved stress;

1.2 according to the stress-strain diagram that standard tensile sample is carried out stretching experiment and obtained this material on cupping machine, tries to achieve the strength factor σ of material by stress-strain diagram ₀with strain hardening exponent n, for subsequent use;

Step 2:

2.1 process the specimen surface of processing through step 1 that has unrelieved stress to obtain smooth, the smooth surface that meets vickers microhardness experiment;

2.2 adopt vickers microhardness testing machine sample to be carried out to micro-indentation test, suitable loaded load and the load time of attentional selection in micro-indentation test process; Record catercorner length D and the vickers microhardness value HV of micro-impression;

2.3 utilize scanning electron microscopy measurement to obtain the length of side a of micro-impression;

2.4 calculate the nominal projected area A of micro-impression according to catercorner length D _nom, A _nom=D*D/2, is obtained the actual projected area A of micro-impression by length of side a, if impression is convex shape, and A=a*a*0.96, wherein 0.96 is form factor, and then obtains the ratio c of two areas ²;

Step 3:

3.1 measure unrelieved stress according to the indentation method based on being pressed into response and elasto-plastic Contact theory, obtain overstrain value ε by formula (1) _res, as follows:

Wherein, ε in formula _resit is overstrain; σ ₀it is the strength factor of material; N is the strain hardening exponent of material; HV is the measured value of vickers microhardness;

The overstrain ε that the formula (1) of 3.2 bases obtains _res, substitution formula (2) calculates the axle unrelieved stress σ such as the two dimension of specimen surface _res, as follows:

${\mathrm{\σ}}_{\mathrm{res}}={\mathrm{\σ}}_0\left[\right|{\mathrm{\ϵ}}_{\mathrm{res}}|{]}^n\×[\exp \left(\frac{1-c^2}{0.32}\right)-1]---\left(2\right)$

Wherein, ε in formula _resit is overstrain; σ _resit is unrelieved stress; σ ₀it is the strength factor of material;

N is the strain hardening exponent of material; HV is the measured value of vickers microhardness; c ²for the actual projected area A of impression and nominal projected area A _nomratio.