CN104729928A - Testing device and testing method for forming limit diagram under high-temperature constant-strain rate condition - Google Patents

Abstract

The invention relates to a testing device and a testing method for a forming limit diagram under a high-temperature constant-strain rate condition. The device comprises a sample deformation pressure device, a real-time strain measurement device and a controller; based on a testing method for the forming limit diagram under the high-temperature constant-strain rate condition built by the testing device, the constancy of the strain rate of a sample in a central point is realized under the condition of constant forming temperature, and a forming limit diagram under the obtained slab isothermal condition contains a forming limit curve under a constant-strain rate condition in a certain strain rate range. The testing device and the testing method have the advantages of high control accuracy and accuracy in measurement, and are significant in theoretical research and practical application of light alloys and high-strength steel warm-hot forming.

Description

The proving installation of forming limit diagram and test method under high-temperature constant strain rate condition

Technical field

The present invention relates to metal sheet forming performance testing device, be specifically related to proving installation and the test method of forming limit diagram under a kind of high-temperature constant strain rate condition.

Background technology

The current whole world faces the pressure of energy shortage and environmental pollution, and lightweight is the main trend of development of automobile, is turned to the component material on the lightweighting materials such as high-strength steel, light-alloy body of a motor car and chassis just gradually, be used for alleviating vehicle weight by conventional steel plates.Expedite the emergence of the Board Forming process that some are emerging thus, the superplastic forming (SPF) of such as aluminium alloy, magnesium alloy plate, Plastic Forming (QPF) fast, warm drawing (WSF), and hot press-formed (HSF) of the high strength steel plate grown up in recent years etc.The common feature of these techniques is exactly that sheet material is shaped under tepid state, and the forming property studying warm state sheet material is most important for the development of these emerging forming technologies.

Forming limit diagram (FLD, Forming Limit Diagram) is important evidence and the index of evaluating sheet material forming performance, and widespread use in actual Board Forming process.Shaping limit curve is not only in actual Board Forming process the important evidence assessed formability He select sheet material, and is the major criterion judging in finite element analogy sheet material forming that sheet material lost efficacy and broke.For the research relative maturity of toughness For The Forming Limit of Sheet Metals under normal temperature, wherein the most frequently used FLC test method is Nakazima method (hemispherical head rigidity punch bulging test) and Marciniak method (tack rigidity punch bulging test), and ASTM E2218 (2002) and ISO12004 (1997) and China GB/T15825.8 (2008) all defines forming limit standard test code for toughness sheet material.

The general not strained velocity factor impact of normal temperature toughness sheet material forming, and sheet material presents sensitivity of strain rate under tepid state, therefore the experimental study of sheet material warm forming performance should consider strain rate.Warm state sheet material may not be suitable for for the conventional test device of toughness For The Forming Limit of Sheet Metals and test method.Application number be 201110082554.7,201210238542.3 and 201310337979.7 patent all relate to hot forming limit testing device and how to obtain hot forming limiting figure, but existing sheet material hot forming limiting device and test method all adopt constant punch (or die) cavity rate, the forming limit diagram obtained like this comprises the impact of strain rate change on formability to some extent.

Along with high temperature sheet forming technique field is to the raising of For The Forming Limit of Sheet Metals figure accuracy requirement, need invention badly and a kind ofly can realize the constant forming limit proving installation of sample central spot strain rate, so just can eliminate the strain rate factor in forming process, by obtaining the shaping limit curve under differently strained rate conditions, just can grasp the affecting laws of strain rate to sheet material warm forming performance, and set up sheet material warm forming Limit Surface figure ( strain Rate Forming Limit Diagram).

Summary of the invention

Object of the present invention, above-mentioned at present to the technical matters that the technology setting up For The Forming Limit of Sheet Metals figure exists in order to solve, proving installation and the test method of forming limit diagram under a kind of high-temperature constant strain rate condition are proposed.

The proving installation of forming limit diagram under high-temperature constant strain rate condition of the present invention, comprises sample deformation pressure device, in real time strain gauge means and controller;

Described sample deformation pressure device, comprise base, the worktable be installed in by column on base, be installed in base lower surface main hydraulic cylinder and be installed in base upper surface secondary hydraulic cylinder, be set on base and worktable and with it in the pull bar be slidably matched, the die being connected in pull bar upper end, pull bar lower end is connected with the main piston rod of described main hydraulic cylinder; Be arranged on the blank holder being provided with bead of die lower end by being connected with the auxiliary piston bar of described secondary hydraulic cylinder in the push rod through worktable be slidably matched; The punch coaxially set firmly with die and blank holder on the table; Sample is positioned between die and blank holder; Also be installed with lower heating furnace and upper heating furnace on the table, the high-temperature seal ring between upper and lower heating furnace, form the work space holding described concave, convex mould;

Described strain gauge means, comprise and be installed in the video camera with supporting high speed image analysis process system above described upper heating furnace upper surface and following eyepiece, object lens, optical filter by support, the center pit that quartz glass is housed coaxial with described punch and matrix (namely video camera, described eyepiece, described object lens and described optical filter and sample center ensure on the same axis) that the optical axis of video camera, eyepiece, object lens and optical filter is arranged by upper heating furnace upper surface; The data analysis processor be connected with the image analysis processing system communication of video camera;

Described data analysis processor is connected with described controller communication, and transmission strain signal is to controller; Controller, respectively by servo-driver and pressure limiting valve and described main hydraulic cylinder and described secondary hydraulic cylinder control linkage, does straight reciprocating motion to drive described die and blank holder.

Described upper heating furnace upper surface is provided with lighting source by the inclined hole that quartz glass is housed.

Be provided with inside described upper heating furnace upper surface and contact with described pull bar upper surface the pole coordinated, moving up and down and the self gravitation of upper heating furnace by means of pull bar, realizes the folding of upper and lower heating furnace.

Be provided with flexible bellow in the lower surface of described lower heating furnace, corrugated tube and upper heating furnace and lower heating furnace surround confined space; Lower heating furnace is provided with the outlet be connected with vacuum extractor, for realizing the vacuum environment of heating arrangement inside, is also provided with the inlet tube be connected with argon bottle, provides protection against oxidation for passing into argon gas for mould described after punching press and sample and cools.

Set up a test method for forming limit diagram under high-temperature constant strain rate condition based on proving installation of the present invention, comprise the following steps:

A. sample central spot being marked with cruciform pattern is placed on the blank holder described in proving installation, make that the point of crossing of its cruciform pattern overlaps with mold axis, cruciform pattern line segment is consistent with the direction of first principal stress and second principal stress respectively, start and be heated to probe temperature and be incubated;

B. ensureing, under the condition that sample center strain rate is constant, to carry out drawing to sample, when sample occurs constriction or breaks, test is stopped;

C. to the sample deformation data analysis after punching press and measurement, the first principal strain and second principal strain of constriction or burst region is obtained;

D. change the sample of different specification size, repeat step a-c, obtain the sheet material warm forming limit curve under this strain rate condition corresponding;

E. change sample center strain rate values according to setting interval, repeat step a-d, obtain the hot forming limit curve of corresponding differently strained speed, obtain the sheet material warm forming Limit Surface under this probe temperature corresponding by matching.

Described sample is selected from GB/T15825.8 (2008), specimen length is 180mm, width is respectively 20, 40, 60, 80, 100, 120, 140, 160 and 9 kinds of specifications of 180mm, contain in forming limit diagram from unilateral stretching to the stress state scope of drawing such as two-way, laser marking machine is adopted to carry out circular grill drafting to sample, circular grill diameter is 2.5mm, circular grill arranged tangential, circular grill array is consistent with initial principal stress orientation, and wherein a circular grill the center of circle must with sample center superposition, described cruciform pattern is drawn in the circular grill of sample center, orthogonal line segment point of crossing is sample center, line segment direction is respectively along first principal stress and second principal stress direction.

First principal strain of constriction described in step c or burst region and the acquisition of the second principal strain, be that the major axis of circular grill of sample constriction or burst region and the change of minor axis are measured, then adopt following formulae discovery to change the first principal strain and second principal strain of grid:

${\mathrm{\ϵ}}_1=\ln \frac{d_1}{d_0}$

${\mathrm{\ϵ}}_2=\ln \frac{d_2}{d_0}$

Wherein, ε ₁be the first principal strain, ε ₂be the first principal strain, d ₀for the initial diameter of circular grill, d ₁for the long axis length of measured circular grill, d ₂for the minor axis length of measured circular grill.

The analyzing and processing of sample deformation data described in step c, is carry out analyzing and processing by the sample center cross-shaped image of described data analysis processor to camera Real-time Collection being shot, adopts the equivalent strain in following formulae discovery sample central spot region:

$\stackrel{\‾}{\mathrm{\ϵ}}=\frac{\sqrt{2}}{3}\sqrt{{\left(\ln \frac{l_2}{l_1}\right)}^2+{\left(\ln \frac{l_1{l}_2^2}{l_0^3}\right)}^2+{\left(\ln \frac{l_2{l}_1^2}{l_0^3}\right)}^2}$

Wherein, for sample central spot equivalent strain, l ₀for the initial length of each line segment of cruciform pattern, l ₁for cruciform pattern is parallel to the length of first principal stress line segment, l ₂for cruciform pattern is perpendicular to the length of first principal stress line segment; Then following formulae discovery sample central spot strain rate is adopted,

$\stackrel{.}{\stackrel{\‾}{\mathrm{\ϵ}}}=\frac{\stackrel{\‾}{\mathrm{\ϵ}}{|}_t-\stackrel{\‾}{\mathrm{\ϵ}}{|}_{t-\mathrm{\Δt}}}{\mathrm{\Δt}}$

Wherein, for sample central spot strain rate, for current time equivalent strain, for previous moment equivalent strain (initial time equivalent strain is zero), Δ t is the time interval of described data analysis processor sampling.

The sample central spot strain rate measured value that described controller provides real-time strain gauge means and strain rate setting value compare, then the movement rate of execution unit is regulated in real time based on feedback control principle, the i.e. movement rate of described die specific punch, is used for ensureing the constant of sample central spot strain rate.

Heating and thermal insulation and follow-up punching course carry out under sealed vacuum environment, so not only avoid sample to be oxidized, and more can avoid because light reflects the scalloping caused in flowing hot-air.

The sample central spot strain rate measured value that described controller provides real-time strain gauge means and strain rate setting value compare, then the movement rate of described execution unit is regulated in real time based on feedback control principle, the i.e. movement rate of the relatively described punch of described die, is used for ensureing the constant of sample central spot strain rate.

Compared with prior art, the present invention has the following advantages:

1. proving installation provided by the invention can realize the distortion of sample central spot constant strain rate, so just eliminates the strain rate factor in plate stamping process, is convenient to research strain rate to the affecting laws of sheet material warm forming;

2. test method provided by the invention can obtain sheet material warm forming Limit Surface figure relate to the affecting laws rule of strain rate to sheet material forming, can be used for light-duty alloy and the analyses of high-strength steel sheet material hot forming such as current magnesium alloy, aluminium alloy, titanium alloy;

3. device and method provided by the invention is applicable to Nakazima method (hemispherical head rigidity punch bulging test) and Marciniak method (tack rigidity punch bulging test);

4. device provided by the invention adopts punch to maintain static the structure of die motion, ensures sample central spot invariant position, increases the stability of sample central spot image acquisition;

5. the real-time strain gauge means of device provided by the present invention uses metallographic micro-imaging principle, adopt and cast oblique rays on the structure such as light source and optical filter, obtain clear black and white contrast image, be convenient to image express-analysis process, the distortion of sample central spot tiny area is after optical amplifier simultaneously, effectively can improve measuring accuracy;

6. the cruciform pattern of sample central spot mark of the present invention, not only be convenient to the calculating of central spot equivalent strain, and due to pattern succinct, information processing capacity is little, improve the speed of image analysis processing, thus increase the precision of FEEDBACK CONTROL, ensure the constancy of sample central spot strain rate.

Accompanying drawing explanation

Fig. 1 is the structure principle chart of proving installation of the present invention;

Fig. 2 is the structural representation of proving installation of the present invention;

Fig. 3 is data acquisition and the control principle block diagram of proving installation provided by the invention;

Fig. 4 is that the sample circular grill that test method of the present invention adopts draws a design;

Fig. 5 is sample central spot cruciform pattern and the deformation pattern of test method of the present invention employing.

In figure: 1, video camera; 2, support; 3, eyepiece; 4, object lens; 5, optical filter; 6, quartz glass; 7, inlet tube; 8, worktable; 9, corrugated tube; 10, column; 11, base; 12, sliding sleeve; 13, main hydraulic cylinder; 14, pull bar; 15, main limit switch; 16, main piston rod; 17, secondary hydraulic cylinder; 18, secondary limit switch; 19, auxiliary piston bar; 20, push rod; 21, punch; 22, outlet; 23, lower heating furnace; 24, high-temperature seal ring; 25, upper heating furnace; 26, die; 27, sample; 28, pole; 29, blank holder; 30, lighting source; 31, data analysis processor; 32, controller.

Embodiment

Below in conjunction with the drawings and specific embodiments, the invention will be further described.

With reference to Fig. 1 ~ 3, the proving installation of forming limit diagram under a kind of high-temperature constant strain rate condition, comprise sample deformation pressure device, in real time strain gauge means and controller;

Described sample deformation pressure device, comprise base 11, set firmly worktable 8 on the pedestal 11 by column 10, be installed in the main hydraulic cylinder 13 of base 11 lower surface and be installed in base 11 upper surface secondary hydraulic cylinder 17, be set on base 11 and worktable 8 and with it in the pull bar 14 be slidably matched, the die 26 being connected in pull bar 14 upper end, pull bar 14 lower end is connected with the main piston rod 16 of described main hydraulic cylinder 13; Be arranged on the blank holder 29 being provided with bead of die 26 lower end by being connected with the auxiliary piston bar 19 of described secondary hydraulic cylinder 17 in the push rod 20 through worktable 8 be slidably matched; The punch 21 that worktable 8 coaxially sets firmly with die 26 and blank holder 29; Described sample 27 is positioned between die 26 and blank holder 29; Worktable 8 is also installed with lower heating furnace 23 and upper heating furnace 25, the high-temperature seal ring 24 between upper and lower heating furnace, forms the work space holding described concave, convex mould;

Described strain gauge means, comprise and be installed in the video camera 1 with supporting high speed image analysis process system above described upper heating furnace 25 upper surface and following eyepiece 3, object lens 4, optical filter 5 by support 2, the center pit 6 that quartz glass is housed that the optical axis of video camera 1, eyepiece 3, object lens 4 and optical filter 5 is arranged by upper heating furnace 25 upper surface is coaxial with described punch and matrix; The data analysis processor 31 be connected with the image analysis processing system communication of video camera 1;

Described data analysis processor 31 is connected with described controller 32 communication, and transmission strain signal is to controller; Controller 32, by servo-driver (omit in figure and do not mark) and described main hydraulic cylinder 13 control linkage, to drive described die 26 to do straight reciprocating motion, realizes sample 27 drawing; Controller 32, by pressure limiting valve (omit in figure and do not mark) and described secondary hydraulic cylinder 17 control linkage, to drive described blank holder 29 to do straight reciprocating motion, and ensures that the pressure-pad-force putting on sample 27 in stamping process is invariable.

Described upper heating furnace 25 upper surface is provided with lighting source 30 by the inclined hole that quartz glass is housed.

Be provided with inside described upper heating furnace 25 upper surface and contact with described pull bar 14 upper surface the pole 27 coordinated, moving up and down and the self gravitation of upper heating furnace 25 by means of pull bar 14, realizes the folding of upper and lower heating furnace.

Described upper heating furnace 25 upper surface and inwall and lower heating furnace 23 lower surface and inwall are provided with heating element, and upper heating furnace 25 and lower heating furnace 23 lateral wall and end face are made up of thermofin and betal can.

Be provided with flexible bellow 9 in the lower surface of described lower heating furnace 23, corrugated tube 9 surrounds confined space with upper heating furnace 25 and lower heating furnace 23; Lower heating furnace 23 is provided with the outlet 22 be connected with vacuum extractor; for realizing the vacuum environment of heating arrangement inside; also be provided with the inlet tube 7 be connected with argon bottle, protection against oxidation be provided for passing into argon gas for mould described after punching press and sample and cool.

Described main hydraulic cylinder 13 and secondary hydraulic cylinder 17 are respectively equipped with main limit switch 15 and secondary limit switch 18.

Set up the test method of forming limit diagram under high-temperature constant strain rate condition based on above-mentioned proving installation, comprise the following steps:

A. the sample 27 central spot being marked with cruciform pattern is placed on the blank holder 29 described in proving installation, make that the point of crossing of its cruciform pattern overlaps with mold axis, cruciform pattern line segment is consistent with the direction of first principal stress and second principal stress respectively, start and be heated to probe temperature and be incubated;

B. ensureing, under the condition that sample center strain rate is constant, to carry out drawing to sample, when sample occurs constriction or breaks, test is stopped;

C. to the sample deformation data analysis after punching press and measurement, the first principal strain and second principal strain of constriction or burst region is obtained;

D. change the sample of different specification size, repeat step a-c, obtain the sheet material warm forming limit curve under this strain rate condition corresponding;

E. change sample center strain rate values according to setting interval, repeat step a-d, obtain the hot forming limit curve of corresponding differently strained speed, obtain the sheet material warm forming Limit Surface under this probe temperature corresponding by matching.

The basis for selecting GB/T15825.8 (2008) of sample 27, specimen length is 180mm, width is respectively 9 kinds of specifications of 20,40,60,80,100,120,140,160 and 180mm, contains in forming limit diagram from unilateral stretching to the stress state scope of drawing such as two-way.Adopt laser marking machine to carry out circular grill drafting to sample 27, circular grill diameter is 2.5mm.As shown in Figure 4, circular grill arranged tangential, circular grill array should be consistent with initial principal stress orientation, and wherein a circular grill the center of circle must with sample 27 center superposition, orthogonal line segment is drawn in the circular grill of sample 27 center, line segment point of crossing is sample center, and line segment direction is respectively along first principal stress and second principal stress direction.

With reference to Fig. 5, calculated the distortion of sample 27 central spot by the change of cruciform pattern.The following deformation analysis of concrete basis:

The computing formula of sample central spot first principal strain and the second principal strain is,

${\mathrm{\ϵ}}_1^{\′}=\ln \frac{l_1}{l_0}$

${\mathrm{\ϵ}}_2^{\′}=\ln \frac{l_2}{l_0}$

The computing formula of plastic yield equivalent stress is,

$\stackrel{\‾}{\mathrm{\ϵ}}=\frac{\sqrt{2}}{3}\sqrt{{({\mathrm{\ϵ}}_1^{\′}-{\mathrm{\ϵ}}_2^{\′})}^2+{({\mathrm{\ϵ}}_2^{\′}-{\mathrm{\ϵ}}_3^{\′})}^2+{({\mathrm{\ϵ}}_3^{\′}-{\mathrm{\ϵ}}_1^{\′})}^2}$

Wherein, for equivalent stress, ε ' ₁, ε ' ₂with ε ' ₃be respectively sample central spot first, second, and third principal strain, l ₀for the initial length of each line segment of cruciform pattern, l ₁for cruciform pattern is parallel to the length of first principal stress line segment, l ₂for cruciform pattern is perpendicular to the length of first principal stress line segment.Meanwhile, constancy of volume principle is followed in plastic yield, therefore has following relation,

ε′ ₁+ε′ ₂+ε′ ₃＝0

In conjunction with above 4 formula, through the equivalent strain computing formula that can obtain sample central spot of deriving,

$\stackrel{\‾}{\mathrm{\ϵ}}=\frac{\sqrt{2}}{3}\sqrt{{\left(\ln \frac{l_2}{l_1}\right)}^2+{\left(\ln \frac{l_1{l}_2^2}{l_0^3}\right)}^2+{\left(\ln \frac{l_2{l}_1^2}{l_0^3}\right)}^2}$

The data analysis processor of described real-time strain gauge means carries out analyzing and processing to cross-shaped image, and according to the equivalent strain in above formulae discovery sample central spot region.Because the definition of strain rate is data analysis processor adopts following formulae discovery sample central spot strain rate,

$\stackrel{.}{\stackrel{\‾}{\mathrm{\ϵ}}}=\frac{\stackrel{\‾}{\mathrm{\ϵ}}{|}_t-\stackrel{\‾}{\mathrm{\ϵ}}{|}_{t-\mathrm{\Δt}}}{\mathrm{\Δt}}$

Wherein, for sample central spot strain rate, for current time equivalent strain, for previous moment equivalent strain (initial time equivalent strain is zero), Δ t is the time interval of data analysis processor sampling.

With reference to Fig. 3, the sample 27 central spot strain rate measured value that the controller of described drive unit provides data analysis processor and strain rate test setting value compare, based on feedback control principle, main hydraulic cylinder 13 is regulated by servo-driver, thus through main piston rod 16 and pull bar 14, real-time adjustment die 26 moves down speed, and in guarantee punching course, sample central spot strain rate is constant.Controller, according to the test setting value of pressure-pad-force, regulates secondary hydraulic cylinder 17 to keep constant oil pressure by pressure limiting valve, through auxiliary piston bar 19 and push rod 20, makes blank holder 29 apply constant blank holder force to sample 27 in punching course.Controller is also according to test related setting, control the unloaded action of main hydraulic cylinder 13 and secondary hydraulic cylinder 17, and the restriction of acceptor's limit switch 15 and secondary limit switch 18, realize die 26 and blank holder 29 moving up and down in respective scope of activities, the needs taken with heating arrangement folding and sample before and after satisfied test.

Be provided with high-temperature seal ring 24 between upper heating furnace 25 and lower heating furnace 23, worktable 8 lower surface is provided with flexible bellow 9, and corrugated tube 9 surrounds confined space with upper heating furnace 25 and lower heating furnace 23; Lower heating furnace 23 is provided with outlet 22 and is connected with vacuum extractor, and for realizing the vacuum environment of heating arrangement inside, lower heating furnace 23 is provided with inlet tube 7 and is connected with argon bottle, and passing into argon gas for mould described after punching press and sample provides protection against oxidation and cool.

The present invention is open a kind of acquisition sheet material warm forming Limit Surface figure also test method, described method relates to strain rate under obtaining isothermy ---the first principal strain (ε ₁)---the second principal strain (ε ₂) the forming limit curved surface of triadic relation, forming limit curved surface contains the shaping limit curve within the scope of certain strain rate under constant strain rate condition.

Obtain sheet material warm forming Limit Surface figure the concrete practice of test as follows:

(1) control main hydraulic cylinder 13 and secondary hydraulic cylinder 17 drive die 26 and blank holder 29 to rise to respective extreme position, now go up heating furnace 25 to separate with lower heating furnace 23, uniform application high temperature lubricant on punch 21, then sample 27 is placed on blank holder 29, sample 27 center and blank holder 29 center are at same axis, controlling main hydraulic cylinder 13 afterwards drives die 26 to decline certain fixed position, now go up heating furnace 25 to close due to Action of Gravity Field and lower heating furnace 23, remain in a gap between die 26 and sample 27.Heating furnace 25 and lower heating furnace 23 in locking.

(2) start vacuum extractor, make heating arrangement internal vacuum reach certain value and maintain.

(3) control heating devices heat sample and mold temperature are to probe temperature, and held for some time.Adjustment video camera 1 and eyepiece 3 and object lens 4 etc., making sample 27 center gather clear picture can reflect, visual field centering and size to fit.Set sample center strain rate values and flanging force value simultaneously.

(4) starting characteristics test, controller drives die 26 to move downward by FEEDBACK CONTROL main hydraulic cylinder 13, sample 27 is out of shape, and keep sample 27 central spot strain rate constant, make secondary hydraulic cylinder 17 keep constant oil pressure with Time Controller by pressure limiting valve, ensure that blank holder 29 applies constant blank holder force to sample 27 all the time in passive folding process like this.When sample 27 occurs constriction or breaks, stop punching press.

(5) close vacuum extractor, pass into argon gas, wait sample 27 to be cooled to uniform temperature, close ar gas passage.

(6) control main hydraulic cylinder 13 and secondary hydraulic cylinder 17 drive die 26 and blank holder 29 to rise to respective extreme position, open heating arrangement, take out sample 27.Adopt camera acquisition sample 27 constriction or burst region image, then apply strain measurement analytic system to process image, the major axis of circular grill and the change of minor axis that calculate sample constriction or burst region are measured, according to the first principal strain and second principal strain of following formulae discovery change grid

${\mathrm{\ϵ}}_1=\ln \frac{d_1}{d_0}$

${\mathrm{\ϵ}}_2=\ln \frac{d_2}{d_0}$

Wherein, ε ₁be the first principal strain, ε ₂be the first principal strain, d ₀for the initial diameter of circular grill, d ₁for the long axis length of measured circular grill, d ₂for the minor axis length of measured circular grill.

(7) keep sample center strain rate setting value constant, for 9 kinds of different size samples, complete forming limit test according to step (1) ~ (5) respectively, obtain the ultimate strain point that per sample (p.s.) is corresponding.By being smoothly connected or curve, obtain the sheet material warm forming limit curve under this strain rate condition corresponding.

(8) within the scope of certain strain rate, the strain rate values setting of sample center is changed according to certain intervals, repeat step (1) ~ (6), obtain the hot forming limit curve of corresponding differently strained speed, obtain the sheet material warm forming Limit Surface under this probe temperature corresponding by surface fitting

Claims (12)

1. the proving installation of forming limit diagram under high-temperature constant strain rate condition, comprises sample deformation pressure device, in real time strain gauge means and controller (32), it is characterized in that:

Described sample deformation pressure device, comprise base (11), the worktable (8) on base (11) is installed in by column (10), be installed in the main hydraulic cylinder (13) of base (11) lower surface and be installed in the secondary hydraulic cylinder (17) of base (11) upper surface, be set in base (11) and worktable (8) upper and with it in the pull bar be slidably matched (14), be connected in the die (26) of pull bar (14) upper end, pull bar (14) lower end is connected with the main piston rod (16) of described main hydraulic cylinder (13), be arranged on the blank holder (29) being provided with bead of die (26) lower end by being connected with the auxiliary piston bar (19) of described secondary hydraulic cylinder (17) in the push rod (20) through worktable (8) be slidably matched, the punch (21) that upper and die (26) and blank holder (29) coaxially set firmly at worktable (8), described sample (27) is positioned between die (26) and blank holder (29), worktable (8) is also installed with lower heating furnace (23) and upper heating furnace (25), the high-temperature seal ring (24) between upper and lower heating furnace, forms the work space holding described concave, convex mould,

Described strain gauge means, comprise and be installed in the video camera (1) with supporting high speed image analysis process system above described upper heating furnace (25) upper surface and following eyepiece (3), object lens (4), optical filter (5) by support (2), the center pit (6) that quartz glass is housed that the optical axis of video camera (1), eyepiece (3), object lens (4) and optical filter (5) is arranged by upper heating furnace (25) upper surface is coaxial with described punch and matrix; The data analysis processor (31) be connected with the image analysis processing system communication of video camera (1);

Described data analysis processor (31) is connected with described controller (32) communication, and transmission strain signal is to controller; Controller (32), respectively by servo-driver and pressure limiting valve and described main hydraulic cylinder (13) and described secondary hydraulic cylinder (17) control linkage, does straight reciprocating motion to drive described die (26) and blank holder (29).

2. the proving installation of forming limit diagram under high-temperature constant strain rate condition according to claim 1, is characterized in that, described upper heating furnace (25) upper surface is provided with lighting source (30) by the inclined hole that quartz glass is housed.

3. the proving installation of forming limit diagram under high-temperature constant strain rate condition according to claim 1, it is characterized in that, the pole (27) contacting with described pull bar (14) upper surface and coordinate is provided with inside described upper heating furnace (25) upper surface, moving up and down and the self gravitation of upper heating furnace (25) by means of pull bar (14), realizes the folding of upper and lower heating furnace.

4. the proving installation of forming limit diagram under high-temperature constant strain rate condition according to claim 1, it is characterized in that, described upper heating furnace (25) upper surface and inwall and lower heating furnace (23) lower surface and inwall are provided with heating element, and upper heating furnace (25) and lower heating furnace (23) lateral wall and end face are made up of thermofin and betal can.

5. the proving installation of forming limit diagram under high-temperature constant strain rate condition according to claim 1, it is characterized in that, be provided with flexible bellow (9) in the lower surface of described lower heating furnace (23), corrugated tube (9) surrounds confined space with upper heating furnace (25) and lower heating furnace (23); Lower heating furnace (23) is provided with the outlet (22) be connected with vacuum extractor; for realizing the vacuum environment of heating arrangement inside; also be provided with the inlet tube (7) be connected with argon bottle, protection against oxidation be provided for passing into argon gas for mould described after punching press and sample and cool.

6. the proving installation of forming limit diagram under high-temperature constant strain rate condition according to claim 1, it is characterized in that, described main hydraulic cylinder (13) and secondary hydraulic cylinder (17) are respectively equipped with main limit switch (15) and secondary limit switch (18).

7. set up a test method for forming limit diagram under high-temperature constant strain rate condition based on proving installation according to claim 1, it is characterized in that comprising the following steps:

A. the sample (27) central spot being marked with cruciform pattern is placed on the blank holder (29) described in proving installation, make that the point of crossing of its cruciform pattern overlaps with mold axis, cruciform pattern line segment is consistent with the direction of first principal stress and second principal stress respectively, start and be heated to probe temperature and be incubated;

B. ensureing, under the condition that sample center strain rate is constant, to carry out drawing to sample, when sample occurs constriction or breaks, test is stopped;

C. to the sample deformation data analysis after punching press and measurement, the first principal strain and second principal strain of constriction or burst region is obtained;

D. change the sample of different specification size, repeat step a-c, obtain the sheet material warm forming limit curve under this strain rate condition corresponding;

E. change sample center strain rate values according to setting interval, repeat step a-d, obtain the hot forming limit curve of corresponding differently strained speed, obtain the sheet material warm forming Limit Surface under this probe temperature corresponding by matching.

8. the test method of forming limit diagram under high-temperature constant strain rate condition according to claim 7, it is characterized in that, described sample (27) is selected from GB/T15825.8 (2008), specimen length is 180mm, width is respectively 20, 40, 60, 80, 100, 120, 140, 160 and 9 kinds of specifications of 180mm, contain in forming limit diagram from unilateral stretching to the stress state scope of drawing such as two-way, laser marking machine is adopted to carry out circular grill drafting to sample (27), circular grill diameter is 2.5mm, circular grill arranged tangential, circular grill array is consistent with initial principal stress orientation, and wherein a circular grill the center of circle must with sample (27) center superposition, described cruciform pattern is drawn in the circular grill of sample (27) center, orthogonal line segment point of crossing is sample center, line segment direction is respectively along first principal stress and second principal stress direction.

9. the test method of forming limit diagram under high-temperature constant strain rate condition according to claim 7, it is characterized in that, first principal strain of constriction described in step c or burst region and the acquisition of the second principal strain, be that the major axis of circular grill of sample constriction or burst region and the change of minor axis are measured, then adopt following formulae discovery to change the first principal strain and second principal strain of grid:

${\mathrm{\ϵ}}_1=\ln \frac{d_1}{d_0}$

${\mathrm{\ϵ}}_2=\ln \frac{d_2}{d_0}$

Wherein, ε ₁be the first principal strain, ε ₂be the first principal strain, d ₀for the initial diameter of circular grill, d ₁for the long axis length of measured circular grill, d ₂for the minor axis length of measured circular grill.

10. the test method of forming limit diagram under high-temperature constant strain rate condition according to claim 7, it is characterized in that, the analyzing and processing of sample deformation data described in step c, be carry out analyzing and processing by sample (27) the center cross-shaped image of described data analysis processor to camera Real-time Collection being shot, adopt the equivalent strain in following formulae discovery sample central spot region:

$\stackrel{\‾}{\mathrm{\ϵ}}=\frac{\sqrt{2}}{3}\sqrt{{\left(\ln \frac{l_2}{l_1}\right)}^2+{\left(\ln \frac{l_1{l}_2^2}{l_0^3}\right)}^2+{\left(\ln \frac{l_2{l}_1^2}{l_0^3}\right)}^2}$

Wherein, for sample central spot equivalent strain, l ₀for the initial length of each line segment of cruciform pattern, l ₁for cruciform pattern is parallel to the length of first principal stress line segment, l ₂for cruciform pattern is perpendicular to the length of first principal stress line segment; Then following formulae discovery sample central spot strain rate is adopted,

$\stackrel{.}{\stackrel{\‾}{\mathrm{\ϵ}}}=\frac{\stackrel{\‾}{\mathrm{\ϵ}}{|}_t-\stackrel{\‾}{\mathrm{\ϵ}}{|}_{t-\mathrm{\Δt}}}{\mathrm{\Δt}}$

Wherein, for sample central spot strain rate, for current time equivalent strain, for previous moment equivalent strain (initial time equivalent strain is zero), Δ t is the time interval of described data analysis processor sampling.

The test method of forming limit diagram under 11. high-temperature constant strain rate conditions according to claim 7, it is characterized in that, the sample central spot strain rate measured value that described controller provides real-time strain gauge means and strain rate setting value compare, then the movement rate of execution unit is regulated in real time based on feedback control principle, the i.e. movement rate of described die specific punch, is used for ensureing the constant of sample central spot strain rate.

12. test methods setting up high-temperature molding limiting figure under constant strain rate condition according to claim 7, it is characterized in that, heating and thermal insulation and follow-up punching course carry out under sealed vacuum environment, so not only avoid sample to be oxidized, more can avoid because light reflects the scalloping caused in flowing hot-air.