CN111458243A - Experimental method for measuring metal mechanical property by using indentation instrument - Google Patents

Abstract

The invention provides an experimental method for determining metal mechanical properties by using an indenter, which is characterized in that a prepared sample is fixed after the mechanical error measurement of an indentation test is finished; carrying out an indentation test on a metal or alloy sample; converting the corresponding load-displacement value into a true stress-true plastic strain data point through a formula; and performing yield strength conversion and tensile strength estimation of the tested material: the invention utilizes the nanoindentor to measure the mechanical properties of metal, and obtains the inertia, yield strength and engineering ultimate tensile strength of a true stress-true strain curve of the metal material. The method carries out automatic ball indentation test on the tested material by using an indenter method, carries out whole-course mapping on each loading and unloading cycle in the test process, avoids the occurrence of theoretical ideal values, has an error compensation link, and can accurately and directly measure the load-displacement data, the true stress-true strain curve, the yield strength and the engineering ultimate tensile strength of the tested material.

Description

Experimental method for measuring metal mechanical property by using indentation instrument

Technical Field

The invention relates to an experimental method for measuring mechanical properties of metal by using an indenter, belonging to the technical field of nondestructive mechanical experiments.

Background

Socioeconomic and rapidly developing scientific technologies have led to increased attention being paid to the safety of industrial production and equipment service. Therefore, material quality inspection and life evaluation of industrial equipment have also become one of the focuses of attention. In order to accurately evaluate the residual life of industrial equipment and test the performance of in-service equipment, it is necessary to grasp the change of mechanical properties of materials. However, the conventional mechanical property test needs to damage the test sample in most cases, which means that the conventional mechanical property test method cannot perform online evaluation on the in-service equipment, so that the ball indentation test plays an extremely important role in online evaluation of the in-service equipment as a nondestructive test method.

As shown in fig. 1 and 2, the indentation tester comprises a portal frame 1, a sensor 2, a testing head 3, a spherical indenter 31, a clamp 4, an operating platform 5, a base 6 and the like,

the Chinese patent No. CN107860671A discloses a device and a method for measuring yield strength and strain hardening index of a metal material by an indentation method, wherein a servo motor is used for loading a load, repeated loading and unloading processes are carried out, the loading is stopped when the indentation depth reaches the set total indentation depth, the obtained load-displacement curve is composed of a plurality of loading and unloading cyclic curves, but each unloading process is carried out by partial unloading, the load applied in each cycle cannot be completely unloaded, and obviously, the tested data can bring about not small errors. In addition, in the process of carrying out an indentation test, the indentation tester can generate measurement misdetection due to structural deformation and structural gaps, but the existing indentation test method does not contain a misdetection compensation link, and currently, no indentation test standard exists internationally, so that whether the methods can accurately reflect the authenticity of the tested material or not can not be accurately judged.

Therefore, the test data of each link is more real and reliable in the test process, and the prevention of ideal measurement is very important.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the experimental method for measuring the mechanical property of the metal by using the indenter is more real and reliable in test data.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows: an experimental method for measuring metal mechanical properties by using an indenter comprises the following steps:

step 1, after sampling of a blank is completed, grinding and polishing the surface of the blank to enable the blank to meet the requirements of the size and the surface roughness of a sample, and then completing preparation of a metal or alloy material sample;

step 2, detaching the spherical test head of the indenter, mounting the flat-bottom test head, and detaching the indenter clamp from the operating table;

and 3, carrying out indentation test mechanical error measurement:

3.1 setting the rate of indenter test head depression to V in the indenter control System₀；

3.2 continuous loading and unloading are carried out at the same position in the measuring process, the process of one-time loading and complete unloading is a period, firstly, the number I, I of the loading and unloading periods is set in the indentation instrument control system>1 and setting the peak value of the applied load of the ith cycle toP_iWhere I is 1, 2, 3 … I, unit: n;

3.3 control the start of mechanical error measurement in the indenter control system:

3.3.1 making i equal to 1, firstly, carrying out a first loading and unloading period; under the loading of motor drive, the flat-bottom test head presses the speed V according to settlement₀And the flat-bottom test head is gradually pressed downwards, and the flat-bottom test head cannot be pressed into the operating table because the contact area of the flat-bottom test head and the operating table of the indentation instrument is large. After the flat-bottom test head contacts the operation table of the indenter, the value of the load applied by the indenter is gradually increased from 0 until the flat-bottom test head is loaded to the set P₁Value, at which point the indentor system records P₁Corresponding sensor displacement value lambda under load₁₁. The indenter then starts the unloading of the force and the load will be from P₁Slowly unload to 0, at which time the indentor system will record the displacement value λ of the sensor under load of 0₂₁；

3.3.2 making i equal to i +1, the indenter will continue the ith loading and unloading cycle, and the value of the load applied by the indenter will gradually increase from 0 again until the preset value of P is applied_iValue, at which point the indentor system will record P_iCorresponding sensor displacement value lambda under load_1i. The indenter then starts the unloading of the force and the load will be from P_iAgain slowly unload to 0, at which time the indentor system will record the displacement value λ of the sensor under load of 0_2i

3.3.3 if I < I, jumping back to step 3.3.2, and if I ≧ I, continuing to execute step 3.3.4 downwards;

3.3.4 at this time, the displacement value recorded in the indenter system is the deformation of the whole mechanical structure of the indenter under the corresponding load; after the I cycle is finished, the measurement is finished, and the lambda is recorded in the indentation instrument system_1i(I ═ 1, 2, 3 … … I) and λ_2i(I ═ 1, 2, 3 … … I) two sets of data;

step 4, after the mechanical error measurement of the indentation test is completed, controlling a force sensor and a displacement sensor to rise in an indentation instrument system, detaching a flat-bottom testing head, mounting the indentation instrument testing head, mounting a clamp on an indentation instrument operating platform, and fixing the sample prepared in the step 1 through the clamp;

and 5, performing an indentation test on the metal or alloy sample:

5.1 the pressing speed is kept constant, and the pressing speed of the indenter test head is still set to V in the indenter control system₀；

5.2 the number of load and unload cycles remains unchanged and is still set to I (I)>1) The peak value of the applied load of the i-th period is also kept unchanged and is still set as P_iWhere I is 1, 2, 3 … I, unit: n;

5.3 controlling the indentation test to start in an indentation instrument control system;

5.3.1 let i equal to 1, first the first loading and unloading cycle is performed: under the loading of motor drive, the test head presses down the speed V according to the settlement₀Slowly pressing down, and vertically pressing a spherical pressure head at the tail end of the test head into the surface of the sample; when the spherical indenter contacts the metal sample, the indenter system begins to use the displacement value of the spherical indenter as the abscissa, and the displacement value unit: mm, the value of the load applied by the indenter as ordinate, the unit of the load value: n, drawing a synchronous curve in a Cartesian coordinate system; the value of the applied load is gradually increased from 0 until the set value P is loaded₁Value, at which point the indentor system will record P₁Corresponding displacement value h of spherical pressure head under load_t1. The indenter then starts the unloading of the force and the load will be from P₁Slowly unloading to 0, and recording the displacement value h of the spherical indenter under the load of 0 by the indenter system_p1；

5.3.2 making i equal to i +1, the indenter will continue the ith loading and unloading cycle, and the value of the load applied by the indenter will gradually increase from 0 again until the preset value of P is applied_iValue, at which point the indentor system will record P_iCorresponding displacement value h of spherical pressure head under load_ti. The indenter then starts the unloading of the force and the load will be from P_iSlowly unloading to 0 again, and recording the displacement value h of the spherical indenter under the load of 0 by the indenter system at the moment_pi；

5.3.3, if I < I, jumping back to step 5.3.2, and if I ≧ I, continuing to execute step 5.3.4 downwards;

5.3.4 at this time, a load-displacement curve of the indentation test process was generated in the indentor system and P was recorded_iWherein i is 1, 2, 3 … … I, h_tiWherein I is 1, 2, 3 … … I and h_piWhere I is 1, 2, 3 … … I three groups of data, P_iI.e. the peak value of the applied load for the ith cycle, h_tiI.e. the total penetration depth, h, of the ith period_piNamely the residual indentation depth of the ith period;

and 6, converting the corresponding load-displacement value into a true stress-true plastic strain data point through a formula:

6.1 in step 3 and step 5 lambda has been obtained_2i、h_piValue and P_iValues according to the following formula:

the residual diameter d of the indentation in the ith loading and unloading period can be calculated_piThe unit: mm; in the above formula E₁Elastic modulus of a spherical indenter, unit: mpa; e₂Is the elastic modulus of the material to be tested, unit: mpa; d is the diameter of the spherical indenter in units: mm;

6.2 d has been calculated in step 6.1_piBy the following formula:

the true strain value of the ith loading and unloading period can be calculated_piD in the above formula is the diameter of the spherical pressure head;

6.3 the indentation residual diameter d for the ith load and unload cycle has been calculated in step 6.1 and step 6.2_piSum true strain value_piN sets of true stress-true strain sigma can be calculated and obtained by the following calculation procedure_t—_pA data point;

6.3.1 making i equal to 0;

6.3.2 let i ═ i +1, calculate

Then checking

If the condition is satisfied, recording and storing sigma_tiThen, step 6.3.5 is carried out, if the condition is not met, step 6.3.3 is carried out;

6.3.3 calculation

Formula (III) α_mTaking the value of the low strain rate sensitive material as 1 as a constraint factor index, and checking

If the condition is satisfied, recording and storing sigma_tiThen step 6.3.5 is performed, if the condition is not met, step 6.3.4 is performed;

6.3.4 calculation

Formula (III) α_mThe value of the constraint factor index for the low strain rate sensitive material is 1; e₂The elastic modulus of the tested material is recorded and stored_tiThen, a step 6.3.5 is carried out,

6.3.5 judging whether I is I; if yes, performing step 6.4; if not, returning to the step 6.3.2;

6.4 in step 6.2N sets of true stress-true strain σ have been obtained_t—_pData points, the indentor system plots the N data points on the abscissa as true plastic strain_pThe ordinate is the true stress sigma_tIn a cartesian coordinate system of (3), the true stress σ_tUnit: mpa, and obtaining a true stress-true plastic strain curve measured by an indentation test through fitting;

and 7, converting the yield strength of the tested material:

7.1 in step 3 and step 5 already the i-th cycle λ is obtained_2i(i＝1、2、3……I)、Peak value P of applied load in i-th cycle_i(I ═ 1, 2, 3 … … I) and total indentation depth h for the ith cycle_ti(I ═ 1, 2, 3 … … I) by the following formula:

d can be calculated_ti(i＝1、2、3……I)，d_ti(mm) is the total indentation diameter of the ith period;

7.2 treatment of P_i(I-1, 2, 3 … … I) and d_ti(I ═ 1, 2, 3 … … I) were transformed as follows to obtain I yield strength conversion data points:

(d_ti/D；

)

in the above formula, β_mIs the yield coefficient of the material, B is the yield strength deviation parameter, in Mpa;

plot the I point at d_tithe/D is the abscissa, and the D is the abscissa,

in a Cartesian coordinate system of a vertical coordinate, a yield strength conversion curve can be obtained through fitting, and when d is_tiThe longitudinal coordinate value corresponding to the value 1/D is the yield strength value α of the measured material_yIn Mpa;

and 8, estimating the tensile strength of the tested material:

8.1 in step 6, a true stress-true plastic strain curve of the material to be measured is obtained by fitting, and the system can determine the power relation equation of the curve, such as y-KxⁿSo as to obtain a strength coefficient K and a strain hardening index n, the strength coefficient K unit: mpa;

8.2 substituting the strength coefficient K and the strain hardening index n value obtained in the step 8.1 into an engineering ultimate tensile strength value expression:

the ultimate tensile strength value sigma of the project can be calculated_uThe unit: mpa; sigma_uThe value is similar to the tensile strength value of the tested material, so the engineering ultimate tensile strength value sigma can be used_u(Mpa) to estimate the tensile strength of the material under test;

step 9, controlling the indenter test head to ascend for a certain distance in the indenter control system so as to separate the indenter head from the tested material sample;

and step 10, loosening the clamp of the indentation instrument, adjusting the position of the tested material sample, then clamping and fixing again, repeating the steps 5 to 9, and starting to perform the next indentation test.

The invention has the beneficial effects that: the method utilizes the indenter to carry out automatic ball indentation test on the material to be tested, carries out whole-course mapping on each loading and unloading cycle in the test process, avoids the occurrence of theoretical ideal value, has error compensation link, and can accurately and directly measure the load-displacement data of the material to be tested, thereby further obtaining the true stress-true strain curve, the yield strength and the engineering ultimate tensile strength of the material to be tested.

Drawings

Fig. 1 is a schematic structural diagram of an indenter.

Fig. 2 is an enlarged view of the indenter test head mechanism.

FIG. 3 is a drawing showing the requirements of a sample of the metal Q345.

Fig. 4 is a schematic diagram of the load loading and unloading period of the indentation test.

Fig. 5 is a load-displacement graph of metal Q345 obtained by the indentation test.

Fig. 6 is an estimated curve of indentation parameters of the metal Q345 obtained by the indentation test.

Fig. 7 is a graph of the true stress-true plastic strain curve and engineering ultimate tensile strength estimate of the metal Q345 obtained by the indentation test.

In the figure: 1-an indentor portal frame; 2-force, displacement sensors; 3-an indentor test head; 4-a clamp body; 5-an indentor console; 6-an indentor base; 31-spherical indenter.

Detailed Description

Specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in fig. 1-6, an experimental method for measuring mechanical properties of metals by using an indenter comprises the following specific operation steps:

step 1, firstly, preparing a sample of an indenter, wherein the size of the sample of the metal Q345 is 40 × 25 × 15(mm), and certain parallelism and flatness requirements are met, and the specific requirements are shown in figure 3;

step 2, detaching the spherical test head of the indenter, mounting the flat-bottom test head, and detaching the indenter clamp from the operating table;

step 3, carrying out indentation test mechanical error measurement before Q345 indentation test:

3.1 setting the rate of indenter test head depression to V in the indenter control System₀，V₀＝3mm/min；

3.2 in the measurement process, continuous loading and unloading of the load are required to be carried out at the same position, and the process of one-time loading and complete unloading is one cycle, so that the number I of loading and unloading cycles is firstly set to be 8 in the indenter control system, and the peak value of the applied load of the ith cycle is set to be P_i(N)(i＝1、2、3、4、5、6、7、8)，P₁＝100N、P₂＝200N、P₃＝300N、P₄＝400N、P₅＝500N、P₆＝600N、P₇＝700N、P₈＝800N；

3.3 control the beginning of mechanical error measurement in the indentation instrument control system, under the loading of motor drive, the force and displacement sensor 2 according to the set pressing speed V₀Since the test head 3 is removed and the force/displacement sensor 2 is directly pressed on the surface of the indenter table 5, the contact area between the force/displacement sensor 2 and the indenter table 5 is large, the sensor 2 is not pressed into the indenter table 5. At the moment when the force and displacement sensor 2 contacts the indenter console 5, the value of the load applied by the indenter gradually increases from 0 to P₁100N, this time indentor systemWill record P₁Corresponding sensor displacement value lambda under 100N₁₁In this embodiment λ₁₁0.000055 mm. The indenter then starts the unloading of the force and the load will be from P₁100N is slowly unloaded to 0N, at which time the indenter system will register a displacement value λ of the sensor 2 under a load of 0N₂₁In this embodiment λ₂₁0.51 μm. After the first loading and unloading period is finished, the indenter continues to perform the second loading and unloading period, and the load value gradually increases from 0N again until the preset P is loaded₂At 200N, the indentor system will now record P₂Corresponding sensor 2 displacement value λ at 200N₁₂In this embodiment λ₁₂0.61 μm, and the indenter then starts the unloading of the force, the load will be from P₂Again slowly unload to 0N at 200N, at which time the indenter system will register a displacement value λ of the sensor 2 under a load of 0N₂₂In this embodiment λ₂₂When the loading and unloading cycle is sequentially carried out to the I (8 th) loading and unloading cycle at 0.56 mu m … …, the load value is gradually increased from 0N again until the set P is loaded₈At this point the indentor system will record P as 800N₈Sensor 2 displacement value λ corresponding to 800N₁₈In this embodiment λ₁₈4.1 μm. The indenter then starts the unloading of the force and the load will be from P₈Again slowly unload to 0N 800N, at which time the indentor system will register a displacement value λ of the sensor 2 under a load of 0N₂₈In this embodiment λ₂₈3.4 μm. Since the sensor 2 is not pressed into the indenter console 5, the displacement value recorded in the indenter system is the deformation of the entire mechanical structure of the indenter under the corresponding load. And the measurement is finished after the I (i.e. 8) th period is finished. At this time, lambda has been recorded in the indentor system_1i(i ═ 1, 2, 3, 4, 5, 6, 7, 8) and λ_2i(i ═ 1, 2, 3, 4, 5, 6, 7, 8) two sets of data, in this example λ₁₁＝0.000055μm、λ₁₂＝0.61μm、λ₁₃＝0.69μm、λ₁₄＝0.79μm、λ₁₅＝0.94μm、λ₁₆＝1.6μm、λ₁₇＝2.7μm、λ₁₈＝4.1μm；λ₂₁＝0.51μm、λ₂₂＝0.56μm、λ₂₃＝0.63μm、λ₂₄＝0.72μm、λ₂₅＝0.86μm、λ₂₆＝1.2μm、λ₂₇＝2.3μm、λ₂₈＝3.4μm。

Step 4, after the mechanical error measurement of the indentation test is completed, controlling the force and the displacement sensor 2 to rise in an indentation instrument system, detaching the flat-bottom testing head, reinstalling the spherical testing head 3 of the indentation instrument, reinstalling the clamp 4 on the operation table 5 of the indentation instrument, and fixing the Q345 sample prepared in the step 1 through the clamp 4;

and 5, carrying out an indentation test on the Q345 sample:

5.1 the pressing speed is kept constant, and the pressing speed of the indenter test head 3 is still set to V in the indenter control system₀，V₀＝3mm/min；

5.2 continuous loading and unloading of load at the same point are required in the measurement process, as shown in fig. 4, the process of one loading and complete unloading is one cycle, the number of loading and unloading cycles is kept unchanged, I (I ═ 8) are still set, and the peak value of the applied load in the ith cycle is also kept unchanged, P is still set_i(N)(i＝1、2、3、4、5、6、7、8)，P₁＝100N、P₂＝200N、P₃＝300N、P₄＝400N、P₅＝500N、P₆＝600N、P₇＝700N、P₈＝800N；

5.3 controlling the indentation test to start in the indentation instrument control system, and under the drive loading of the motor, the test head 3 controls the indentation test to start according to the set indentation speed V₀ A ball indenter 31 at the end of the test head 3 is pressed vertically into the surface of the Q345 sample by slowly pressing down at 3 mm/min. At the moment when the spherical indenter 31 comes into contact with the Q345 sample, the indenter system starts to perform synchronous curve plotting in a cartesian coordinate system with the displacement value (mm) of the spherical indenter 31 as the abscissa and the load value (N) applied by the indenter as the ordinate. The value of the applied load is gradually increased from 0N until the set value P is loaded₁At 100N, the indentor system will record P₁Displacement h of spherical indenter 31 corresponding to 100N_t1In aIn this example, h_t10.01734mm, the indenter then starts the unloading of the force, the load will be from P₁100N is slowly unloaded to 0N, at which time the indenter system will register a displacement value h for the spherical indenter 31 at 0N_p1In this embodiment, h_p10.01625 mm. After the first loading and unloading period is finished, the indenter continues to perform the second loading and unloading period, and the load value gradually increases from 0N again until the preset P is loaded₂At 200N, the indentor system will now record P₂Displacement h of spherical indenter 31 corresponding to 200N_t2In this embodiment, h_t20.03011 mm. The indenter then starts the unloading of the force and the load will be from P₂Again 200N is slowly unloaded to 0N, at which point the indenter system will record the displacement h of the ball indenter 31 at 0 load_p2In this embodiment, h_p2When the 8 th loading and unloading period is carried out in turn at 0.02634mm … …, the load value gradually increases from 0N again until the set P is loaded₈At this point the indentor system will record P as 800N₈Spherical indenter displacement value h corresponding to 800N_t8In this embodiment, h_t80.13258mm, the indenter then starts the unloading of the force, the load will be from P₈Again, 800N is slowly unloaded to 0N, at which point the indenter system will record the displacement h of the ball indenter 31 at a load of 0N_p8In this embodiment, h_p80.120812 mm. And the measurement is finished after the 8 th period is finished. As shown in FIG. 5, a load-displacement curve of the indentation test process is generated in the indenter system, and P is recorded in the indenter system_i(i＝1、2、3、4、5、6、7、8)、h_ti(i-1, 2, 3, 4, 5, 6, 7, 8) and h_pi(i ═ 1, 2, 3, 4, 5, 6, 7, and 8) three sets of data, P_iI.e., the peak of the applied load for the ith cycle, P in this embodiment₁＝100N、P₂＝200N、P₃＝300N、P₄＝400N、P₅＝500N、P₆＝600N、P₇＝700N、P₈＝800N；h_tiI.e. the total penetration depth of the ith cycle, whereIn the examples, h_t1＝0.01734mm、h_t2＝0.03011mm、h_t3＝0.04667mm、h_t4＝0.06321mm、h_t5＝0.08001mm、h_t6＝0.09592mm、h_t7＝0.11321mm、h_t8＝0.13258mm；h_piI.e. the residual indentation depth of the i-th cycle, in this embodiment, h_p1＝0.01625mm、h_p2＝0.02634mm、h_p3＝0.04435mm、h_p4＝0.06023mm、h_p5＝0.07382mm、h_p6＝0.09156mm、h_p7＝0.10867mm、h_p8＝0.120812mm；

And 6, converting the corresponding load-displacement value into a true stress-true plastic strain data point through a formula:

6.1 in step 3 and step 5 lambda has been obtained_2i、h_piValue and P_iValues according to the following formula:

the residual diameter d of the indentation in the ith loading and unloading period can be calculated_pi(mm). In the above formula E₁(Mpa) is the modulus of elasticity of the spherical indenter, E₁＝510000Mpa；E₂(Mpa) is the modulus of elasticity of the material to be tested, E₂206000 MPa; d (mm) is the diameter of the spherical indenter, D ═ 1 mm. By substituting the values, d in this embodiment can be obtained_p1＝0.25287mm，d_p2＝0.32029mm，d_p3＝0.41174mm，d_p4＝0.47582mm，d_p5＝0.52296mm，d_p6＝0.57681mm，d_p7＝0.62245mm，d_p8＝0.65182mm。

6.2 in step 6.1 the indentation residual diameter d for the ith load and unload cycle has been calculated_piBy the following formula:

the true strain value of the ith loading and unloading period can be calculated_pi(i＝1. 2, 3, 4, 5, 6, 7, 8), wherein D is the diameter of the spherical indenter, D is 1mm, and D is_piThe value substitution results in the true strain value for the ith load and unload cycle, which, in this embodiment,_p1＝0.05162，_p2＝0.07103，_p3＝0.08712，_p4＝0.10013，_p5＝0.11012，_p6＝0.12157，_p7＝0.13312，_p8＝0.14311。

6.3 the indentation residual diameter d for the ith load and unload cycle has been calculated in step 6.1 and step 6.2_piSum true strain value_piBy the following calculation procedure:

6.3.1 making i equal to 0;

6.3.2 let i ═ i +1, calculate

Then checking

If the condition is satisfied, recording and storing sigma_tiThen, step 6.3.5 is carried out, if the condition is not met, step 6.3.3 is carried out;

6.3.3 calculation

Formula (III) α_mTaking the value of the low strain rate sensitive material as 1 as a constraint factor index, and checking

If the condition is satisfied, recording and storing sigma_tiThen step 6.3.5 is performed, if the condition is not met, step 6.3.4 is performed;

6.3.4 calculation

Formula (III) α_mThe value of the constraint factor index for the low strain rate sensitive material is 1; e₂The elastic modulus of the tested material is recorded and stored_tiThen, a step 6.3.5 is carried out,

6.3.5 judging whether I is I; if yes, performing step 6.4; if not, returning to the step 6.3.2;

the true stress value sigma of the ith loading and unloading period can be calculated_ti(Mpa), in this example,. sigma._t1＝645.91241Mpa，σ_t2＝689.89564Mpa，σ_t3＝702.67565Mpa，σ_t4＝706.85345Mpa，σ_t5＝710.01274Mpa，σ_t6＝715.47847Mpa，σ_t7＝719.02374Mpa，σ_t8＝720.10637Mpa。

Thus 8 groups of sigma can be obtained_ti—_pi(true stress-true strain) data points, (0.05162, 645.91241), (0.07103, 689.89564), (0.08712, 702.67565), (0.10013, 706.85345), (0.11012, 710.01274), (0.12157, 715.47847), (0.13312, 719.02374), (0.14311, 720.10637)

6.3 at step 6.2 already 8 groups of σ have been obtained_t—_p(true stress-true strain) data points, as shown in FIG. 7, the indentor system plots these 8 data points on the abscissa as true plastic strain_pThe ordinate is the true stress sigma_tIn a Cartesian coordinate system of (Mpa), a true stress-true plastic strain curve measured by an indentation test can be obtained through fitting;

and 7, conversion of yield strength of Q345:

7.1 in step 3 and step 5 already the i-th cycle λ is obtained_2i(i is 1, 2, 3, 4, 5, 6, 7, 8), the peak value P of the i-th cycle applied load_i(i ═ 1, 2, 3, 4, 5, 6, 7, 8) and total indentation depth h for the ith cycle_ti(i ═ 1, 2, 3, 4, 5, 6, 7, 8) by the following formula:

d can be calculated_ti(i＝1、2、3、4、5、6、7、8)，d_ti(mm) is the total indentation diameter of the ith period. Changing D to 1mm and lambda_2iValue and h_tiSubstituting the value to obtain d_t1＝0.25343mm、d_t2＝0.34823mm、d_t3＝0.42112mm、d_t4＝0.48341mm、d_t5＝0.54572mm、d_t6＝0.59019mm、d_t7＝0.63087mm、d_t8＝0.68831mm。

7.2 treatment of P_i(i is 1, 2, 3, 4, 5, 6, 7, 8) and d_ti(i ═ 1, 2, 3, 4, 5, 6, 7, 8) were transformed as follows to yield strength conversion data points of 8:

(d_ti/D；

)

in the above formula, β_mFor the material yield coefficient, B (mpa) is the yield strength offset parameter, in this example β_m＝0.22，B＝0。

These 8 points are plotted as d in FIG. 6_tithe/D is the abscissa, and the D is the abscissa,

in a Cartesian coordinate system of a vertical coordinate, a yield strength conversion curve can be obtained through fitting, and when d is_tiThe longitudinal coordinate value corresponding to the value 1/D is the yield strength value α of the measured material_y(Mpa), yield strength value α for the Q345 sample in this example_y＝400.41Mpa。

Step 8, estimating the tensile strength of Q345:

8.1 in step 6, a true stress-true plastic strain curve of the material to be measured is obtained by fitting, and the system can determine the power relation equation of the curve, such as y-KxⁿThereby obtaining the strength coefficient K (mpa) and the strain hardening index n. As shown in fig. 7, the system can have solved the power relation equation for this curve, which is:

y＝883.17x^0.0997

as shown in fig. 7, the obtained strength factor K was 883,17Mpa, and the strain hardening index n was 0.0997

8.2 substituting the strength factor K of 883,17Mpa, the strain hardening index n of 0.0997 and e of 2.71828 obtained in step 8.1 into the engineering ultimate tensile strength value expression:

the ultimate tensile strength value sigma of the project can be calculated_u(Mpa)。σ_uThe value (Mpa) is similar to the tensile strength value of the material to be tested, so that the engineering ultimate tensile strength value sigma can be used_u(Mpa) to estimate the tensile strength of the material under test. As shown in FIG. 7, in this embodiment, σ_u＝635.2067213Mpa

Step 9, controlling the indenter test head 3 to ascend for a certain distance in the indenter control system to separate the indenter pressure head from the Q345 test sample;

and step 10, loosening the indenter clamp 4, adjusting the position of the Q345 sample, then clamping and fixing again, repeating the steps 5 to 9, and starting to perform the next indentation test.

The above-mentioned embodiments are merely illustrative of the principles and effects of the present invention, and some embodiments may be used, not restrictive; it should be noted that, for those skilled in the art, various changes and modifications can be made without departing from the inventive concept of the present invention, and these changes and modifications belong to the protection scope of the present invention.

Claims (1)

1. An experimental method for measuring metal mechanical properties by using an indenter comprises the following steps:

step 1, after sampling of a blank is completed, grinding and polishing the surface of the blank to enable the blank to meet the requirements of the size and the surface roughness of a sample, and then completing preparation of a metal or alloy material sample;

step 2, detaching the spherical test head of the indenter, mounting the flat-bottom test head, and detaching the indenter clamp from the operating table;

and 3, carrying out indentation test mechanical error measurement:

3.1 setting the rate of indenter test head depression to V in the indenter control System₀；

3.2 continuous loading at the same location during the measurementThe process of loading and unloading, one-time loading and complete unloading is a period, firstly, the number I, I of loading and unloading periods is set in the indentation instrument control system>1 and setting the peak value of the applied load of the ith period as P_iWhere I is 1, 2, 3 … I, unit: n;

3.3 control the start of mechanical error measurement in the indenter control system:

3.3.1 making i equal to 1, firstly, carrying out a first loading and unloading period; under the loading of motor drive, the flat-bottom test head presses the speed V according to settlement₀And the flat-bottom test head is gradually pressed downwards, and the flat-bottom test head cannot be pressed into the operating table because the contact area of the flat-bottom test head and the operating table of the indentation instrument is large. After the flat-bottom test head contacts the operation table of the indenter, the value of the load applied by the indenter is gradually increased from 0 until the flat-bottom test head is loaded to the set P₁Value, at which point the indentor system records P₁Corresponding sensor displacement value lambda under load₁₁. The indenter then starts the unloading of the force and the load will be from P₁Slowly unload to 0, at which time the indentor system will record the displacement value λ of the sensor under load of 0₂₁；

3.3.2 making i equal to i +1, the indenter will continue the ith loading and unloading cycle, and the value of the load applied by the indenter will gradually increase from 0 again until the preset value of P is applied_iValue, at which point the indentor system will record P_iCorresponding sensor displacement value lambda under load_1i. The indenter then starts the unloading of the force and the load will be from P_iAgain slowly unload to 0, at which time the indentor system will record the displacement value λ of the sensor under load of 0_2i

3.3.3 if I < I, jumping back to step 3.3.2, and if I ≧ I, continuing to execute step 3.3.4 downwards;

3.3.4 at this time, the displacement value recorded in the indenter system is the deformation of the whole mechanical structure of the indenter under the corresponding load; after the I cycle is finished, the measurement is finished, and the lambda is recorded in the indentation instrument system_1i(I ═ 1, 2, 3 … … I) and λ_2i(I ═ 1, 2, 3 … … I) two sets of data;

step 4, after the mechanical error measurement of the indentation test is completed, controlling a force sensor and a displacement sensor to rise in an indentation instrument system, detaching a flat-bottom testing head, mounting the indentation instrument testing head, mounting a clamp on an indentation instrument operating platform, and fixing the sample prepared in the step 1 through the clamp;

and 5, performing an indentation test on the metal or alloy sample:

5.1 the pressing speed is kept constant, and the pressing speed of the indenter test head is still set to V in the indenter control system₀；

5.2 the number of load and unload cycles remains unchanged and is still set to I (I)>1) The peak value of the applied load of the i-th period is also kept unchanged and is still set as P_iWhere I is 1, 2, 3 … I, unit: n;

5.3 controlling the indentation test to start in an indentation instrument control system;

5.3.1 let i equal to 1, first the first loading and unloading cycle is performed: under the loading of motor drive, the test head presses down the speed V according to the settlement₀Slowly pressing down, and vertically pressing a spherical pressure head at the tail end of the test head into the surface of the sample; when the spherical indenter contacts the metal sample, the indenter system begins to use the displacement value of the spherical indenter as the abscissa, and the displacement value unit: mm, the value of the load applied by the indenter as ordinate, the unit of the load value: n, drawing a synchronous curve in a Cartesian coordinate system; the value of the applied load is gradually increased from 0 until the set value P is loaded₁Value, at which point the indentor system will record P₁Corresponding displacement value h of spherical pressure head under load_t1. The indenter then starts the unloading of the force and the load will be from P₁Slowly unloading to 0, and recording the displacement value h of the spherical indenter under the load of 0 by the indenter system_p1；

5.3.2 making i equal to i +1, the indenter will continue the ith loading and unloading cycle, and the value of the load applied by the indenter will gradually increase from 0 again until the preset value of P is applied_iValue, at which point the indentor system will record P_iCorresponding displacement value h of spherical pressure head under load_ti. The indenter then starts the unloading of the force and the load will be from P_iAgain slowly unload to 0, at which time the indentor systemWill record the displacement h of the spherical indenter under a load of 0_pi；

5.3.3, if I < I, jumping back to step 5.3.2, and if I ≧ I, continuing to execute step 5.3.4 downwards;

5.3.4 at this time, a load-displacement curve of the indentation test process was generated in the indentor system and P was recorded_iWherein i is 1, 2, 3 … … I, h_tiWherein I is 1, 2, 3 … … I and h_piWhere I is 1, 2, 3 … … I three groups of data, P_iI.e. the peak value of the applied load for the ith cycle, h_tiI.e. the total penetration depth, h, of the ith period_piNamely the residual indentation depth of the ith period;

and 6, converting the corresponding load-displacement value into a true stress-true plastic strain data point through a formula:

6.1 in step 3 and step 5 lambda has been obtained_2i、h_piValue and P_iValues according to the following formula:

the residual diameter d of the indentation in the ith loading and unloading period can be calculated_piThe unit: mm; in the above formula E₁Elastic modulus of a spherical indenter, unit: mpa; e₂Is the elastic modulus of the material to be tested, unit: mpa; d is the diameter of the spherical indenter in units: mm;

6.2 d has been calculated in step 6.1_piBy the following formula:

the true strain value of the ith loading and unloading period can be calculated_piD in the above formula is the diameter of the spherical pressure head;

6.3 the indentation residual diameter d for the ith load and unload cycle has been calculated in step 6.1 and step 6.2_piSum true strain value_piN can be calculated and obtained by the following calculation procedureGroup true stress-true strain sigma_t—_pA data point;

6.3.1 making i equal to 0;

6.3.2 let i ═ i +1, calculate

Then checking

If the condition is satisfied, recording and storing sigma_tiThen, step 6.3.5 is carried out, if the condition is not met, step 6.3.3 is carried out;

6.3.3 calculation

Formula (III) α_mTaking the value of the low strain rate sensitive material as 1 as a constraint factor index, and checking

If the condition is satisfied, recording and storing sigma_tiThen step 6.3.5 is performed, if the condition is not met, step 6.3.4 is performed;

6.3.4 calculation

Formula (III) α_mThe value of the constraint factor index for the low strain rate sensitive material is 1; e₂The elastic modulus of the tested material is recorded and stored_tiThen, a step 6.3.5 is carried out,

6.3.5 judging whether I is I; if yes, performing step 6.4; if not, returning to the step 6.3.2;

6.4 in step 6.2N sets of true stress-true strain σ have been obtained_t—_pData points, the indentor system plots the N data points on the abscissa as true plastic strain_pThe ordinate is the true stress sigma_tIn a cartesian coordinate system of (3), the true stress σ_tUnit: mpa, the true stress-true determined by indentation test can be obtained by fittingA plastic strain curve;

and 7, converting the yield strength of the tested material:

7.1 in step 3 and step 5 already the i-th cycle λ is obtained_2i(I1, 2, 3 … … I), peak value P of I-th cycle applied load_i(I ═ 1, 2, 3 … … I) and total indentation depth h for the ith cycle_ti(I ═ 1, 2, 3 … … I) by the following formula:

d can be calculated_ti(i＝1、2、3……I)，d_ti(mm) is the total indentation diameter of the ith period;

7.2 treatment of P_i(I-1, 2, 3 … … I) and d_ti(I ═ 1, 2, 3 … … I) were transformed as follows to obtain I yield strength conversion data points:

in the above formula, β_mIs the yield coefficient of the material, B is the yield strength deviation parameter, in Mpa;

plot the I point at d_tithe/D is the abscissa, and the D is the abscissa,

in a Cartesian coordinate system of a vertical coordinate, a yield strength conversion curve can be obtained through fitting, and when d is_tiThe longitudinal coordinate value corresponding to the value 1/D is the yield strength value α of the measured material_yIn Mpa;

and 8, estimating the tensile strength of the tested material:

8.1 in step 6, a true stress-true plastic strain curve of the material to be measured is obtained by fitting, and the system can determine the power relation equation of the curve, such as y-KxⁿSo as to obtain a strength coefficient K and a strain hardening index n, the strength coefficient K unit: mpa;

8.2 substituting the strength coefficient K and the strain hardening index n value obtained in the step 8.1 into an engineering ultimate tensile strength value expression:

the ultimate tensile strength value sigma of the project can be calculated_uThe unit: mpa;

step 9, controlling the indenter test head to ascend for a certain distance in the indenter control system so as to separate the indenter head from the tested material sample;

and step 10, loosening the clamp of the indentation instrument, adjusting the position of the tested material sample, then clamping and fixing again, repeating the steps 5 to 9, and starting to perform the next indentation test.

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