CN107741368B - Test system for three-dimensional structure internal crack propagation test - Google Patents
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Abstract
The invention provides a test system for a three-dimensional structure internal crack propagation test, which comprises a fatigue testing machine (3), a clamp (2), first DIC equipment (4) and second DIC equipment (5), and is characterized in that: the fatigue testing machine (3) is used for fatigue loading; the double-cantilever beam test sample (1) is used as a test piece for detecting cracks; the clamp (2) is used for connecting and fixing the double-cantilever beam sample (1) and the fatigue testing machine (3); the first lens (401) of the first DIC device (4), the second lens (402) of the first DIC device (4), the first lens (501) of the second DIC device (5) and the second lens (502) of the second DIC device (5) are respectively aligned to the upper surface and the lower surface of the double-cantilever sample (1) and used for observing coordinate changes of points on the upper surface and the lower surface of the double-cantilever sample (1). The method can enable the test process to be closer to the actual loading process, so the measured crack propagation behavior is closer to the actual situation, and the method has more guiding significance.
Description
Technical Field
The invention belongs to the field of material performance detection methods, and particularly relates to a test system for a three-dimensional structure internal crack propagation test.
Background
In recent years, fatigue failure of machine parts has frequently occurred. According to relevant statistics, 60% -80% of the failures in aeronautical engineering related projects are due to fatigue failure of materials, and about 85% of the failures in automotive parts related projects are due to fatigue failure. This has forced further research into fatigue damage, and fatigue crack detection is particularly important.
The method for detecting fatigue cracks is limited by equipment and technology at first, people have to visually observe whether cracks are generated on the surface of a workpiece or not, the fatigue test and the surface observation are carried out separately, and after a certain number of times of loading, a sample is unloaded from a fatigue testing machine and then is directly observed under various microscopes. The detection mode greatly differentiates the loading process of the workpiece from the loading process under the real condition, and possibly ignores certain information of fatigue crack propagation, so that the dislocation mode cannot continuously track and record the whole process of crack propagation in real time. Later methods for in situ detection of fatigue cracks were explored to allow continuous information on fatigue crack propagation to be recorded. Surface Replica technique (Replica method), ultrasonic method, ray method, magnetic powder method, acoustic emission method, and the like are generally used. However, most of them are suitable for detecting the crack propagation behavior of the surface, and the internal crack propagation behavior is difficult to be detected accurately, and these methods have certain disadvantages, such as complicated operation, influence on human health, high requirements on environmental conditions, and the like
Disclosure of Invention
Aiming at the problem that the mechanical equipment parts in various engineering fields often generate internal cracks, the invention designs a test system for the internal crack propagation test of a three-dimensional structure, which monitors the internal crack propagation behavior in real time, calculates the crack propagation rate and can obtain the contour line of the crack tip.
The technical scheme of the invention provides a test system for a crack propagation test in a three-dimensional structure, which comprises a fatigue testing machine, a clamp, a first DIC device and a second DIC device, and is characterized in that:
the fatigue testing machine is used for fatigue loading;
testing the double cantilever beams as test pieces for detecting cracks;
the clamp is used for connecting and fixing the double-cantilever beam sample fatigue testing machine;
the first lens, the first lens and the second lens of the first DIC device are arranged on the first DIC, and are used for detecting the crack tip and the crack contour on the double-cantilever sample;
and the first lens of the first DIC device, the second lens of the first DIC device, the first lens of the second DIC device and the second lens of the second DIC device are respectively aligned to the upper surface and the lower surface of the double-cantilever sample and used for observing coordinate changes of points on the upper surface and the lower surface of the double-cantilever sample.
The invention also provides a test method for the internal crack propagation test of the three-dimensional structure, which specifically comprises the following steps:
step 6, determining test fatigue test parameters, wherein the parameters comprise load, frequency, cycle times and sectional area of a working part of the test piece;
step 7, starting the test, and setting the fatigue test parameters determined in the step 6 in a computer software interface of the fatigue testing machine; the load applied to the double-cantilever beam sample by the fatigue testing machine is positive-sine wave constant amplitude loading, and a pull-pull fatigue test is carried out; simultaneously operating computers of the first DIC device and the second DIC device and corresponding software of a computer of a fatigue testing machine until the sample is broken;
and 8, calculating the crack length a through the crack tip detected by the first DIC device and the second DIC device through coordinate change and coordinates thereof according to the cycle number N obtained by the fatigue testing machine, obtaining a plurality of data points of the crack length a and the cycle number N, drawing a scatter diagram by using MAT L AB software, fitting an exponential curve to obtain the change condition of the crack propagation rate da/dN, and inputting the data points obtained through the test into MAT L AB software to obtain a profile diagram of the crack tip.
Compared with the prior art, the invention has the advantages that:
1. the method can detect the crack propagation behavior in the sample under the condition of not interrupting the test loading process, so that the test process is closer to the actual loading process, and the measured crack propagation behavior is closer to the actual condition and has more guiding significance.
2. The method can obtain the profile of crack tip propagation by detecting the cracks in all directions in the sample, and can provide more effective basis for researching the propagation behavior of the internal cracks.
3. The invention can detect the crack propagation behavior in the sample by a non-contact measurement method, so that the reliability of the test result is higher.
4. The invention can detect the crack change in the whole field of the sample by using DIC technology (the abbreviation of Digital Image Correlation means Digital Image Correlation technology), and provides a basis for theoretical analysis of crack propagation.
5. The method has the advantages of simple operation, good environmental adaptability, more accurate detection result than the traditional detection method, higher practical value and stronger engineering significance.
Drawings
FIG. 1 is a schematic diagram of the overall structure of the method for testing the internal crack propagation of a three-dimensional structure according to the present invention;
FIG. 2 is a schematic flow chart of a testing method of the present invention;
FIG. 3 is a schematic view of the profile of a crack tip measurable in accordance with the present invention;
FIG. 4 is a schematic diagram of a dual cantilever beam test sample structure for use in the three-dimensional internal crack propagation test method of the present invention;
FIG. 5 is a schematic structural view of a part of the fixture for internal crack propagation testing of a three-dimensional structure according to the present invention;
FIG. 6 is a schematic view of a fixture mounting specimen for internal crack propagation testing of three-dimensional structures according to the present invention;
FIG. 7 is a schematic view of a clamping structure of a fatigue testing machine for a three-dimensional internal crack propagation test according to the present invention;
in the figure: 1-double cantilever sample, 2-clamp for double cantilever sample, 3-fatigue testing machine, 4-first DIC device, 5-second DIC device, 101-upper clamping block for double cantilever sample, 102-lower clamping block for double cantilever sample, 103-upper pin for double cantilever sample, 104-lower pin for double cantilever sample, 105-pre-crack, 210-upper chuck of double cantilever sample clamp, 211-upper chuck pin hole, 212-upper chuck thread, 220-lower chuck of double cantilever sample clamp, 221-lower chuck pin hole, 222-lower chuck thread, 230-first positioning pin, 240-second positioning pin, 401-first lens of first DIC device, 402-second lens of first DIC device, 501-first lens of second DIC device, 502-a second lens of a second DIC apparatus.
Detailed Description
The invention will be described in further detail below with reference to the drawings and specific examples.
As shown in fig. 1, the embodiment provides a test system for a three-dimensional structure internal crack propagation test, which includes a fatigue tester 3, a clamp 2, a first DIC apparatus 4, and a second DIC apparatus 5, and is characterized in that:
the fatigue testing machine 3 is used for fatigue loading;
the double-cantilever beam sample 1 is used as a test piece for detecting cracks;
the clamp 2 is used for connecting and fixing the double-cantilever beam sample 1 and the fatigue testing machine 3;
the first lens 401 of the first DIC device 4, the second lens 402 of the first DIC device, the first lens 501 of the second DIC device 5 and the second lens 502 of the second DIC device 5 are used for detecting crack tips and crack contours on the double-cantilever sample 1;
the method measures the internal fatigue crack propagation behavior of the double-cantilever beam sample 1 by reasonably arranging the first DIC device 4, the second DIC device 5, the fatigue testing machine 3, the clamp 2 and the double-cantilever beam sample 1.
Their relative positional relationship is as follows: the double-cantilever beam sample 1 is arranged on the clamp 2, and the clamp 2 is directly connected with the fatigue testing machine 3. The first lens 401 of the first DIC apparatus 4, the second lens 402 of the first DIC apparatus 4, the first lens 501 of the second DIC apparatus 5, and the second lens 502 of the second DIC apparatus 5 are respectively aligned to the upper and lower surfaces of the double cantilever sample 1 for observing the coordinate change of each point on the upper and lower surfaces of the double cantilever sample 1.
As shown in fig. 2, this embodiment provides a test method for a three-dimensional structure internal crack propagation test, specifically including:
As shown in fig. 4, the double cantilever sample 1 comprises clamping blocks 101 and 102 for clamping and fixing, the clamping blocks are provided with upper pin holes 103 and lower pin holes 104, the clamping blocks are fixed on the upper and lower surfaces of the sample by glue, and the double cantilever sample 1 further comprises a prefabricated crack 105.
and 3.1, powering on and starting up the computer, and starting the software. Firstly, hardware calibration is carried out, a camera is started to start calibration, a measuring distance is determined, the relative positions of a left camera and a right camera are determined, an aperture is adjusted, and a proper aperture value is selected according to test requirements;
and 3.2, then carrying out software Calibration, clicking a Calibration button (Calibration) in the software, reasonably placing the position of a Calibration plate according to software prompt, sequentially collecting 13 photos at different positions, and automatically calculating by the software to finish final Calibration.
When the calculation result shows that the Calibration degree is less than 0.3pixels and the Scale degree is less than 0.03mm, the Calibration result is considered to be reasonable, and then the experiment collection experiment can be carried out.
And 4, integrally calibrating the first DIC device 4 and the second DIC device 5, and converting local coordinate systems of the first DIC device 4 and the second DIC device 5 into a whole coordinate system through rotation transformation, displacement transformation and coordinate system scale change.
Initial local coordinates (x ″) measured on the first DIC apparatus 4 and the second DIC apparatus 5i,y″i,z″i)，(x″j,y″j,z″j) Conversion to global coordinates (x)i,yi,zi)，(xj,yj,zj) The method is completed by formula (1) and formula (2);
local coordinates (x' ″) during the test measured on the first DIC apparatus 4 and the second DIC apparatus 5i,y′″i,z′″i)，(x′″j,y′″j,z′″j) Conversion to Overall coordinates (x'i,y′i,z′i)，(x′j,y′j,z′j) The method is completed by the formulas (3) and (4):
wherein m isi，mj，m′i，m′jFor the scale-varying parameter, the rotation angle of rectangular coordinate transformation in three-dimensional space, also called the Ocler angle, Deltaxi，Δxj，Δyi，Δyj，Δzi，Δzj，Δx′i，Δx′j，Δy′i，Δy′jΔz′i，Δz′jThe parameter is changed for translation.
And 5, aligning the first lens 401 and the second lens 402 of the first DIC device 4 with the upper surface of the sample, and aligning the first lens 501 and the second lens 502 of the second DIC device 5 with the lower surface of the sample, wherein the first lens and the second lens are used for measuring coordinates and changes of each point on the upper surface and the lower surface of the sample.
Firstly, the local coordinates (x ″) of each point on the upper and lower surfaces of the sample in the initial state in the local coordinate system of the first DIC apparatus 4 and the second DIC apparatus 5 are measuredi,y″i,z″i)，(x″j,y″j,z″j) (i, j ═ 1, 2, 3.), and the local coordinates (x' ") of each point in the local coordinate system during the testi,y′″i,z′″i)，(x′″j,y′″j,z′″j)(i，j＝1，2，3...)；
And (4) obtaining the corresponding coordinates (x) of the initial state under the whole coordinate system through coordinate conversion in the step (4)i,yi,zi)，(xj,yj,zj) And coordinates during the test (x'i,y′i,z′i)，(x′j,y′j,z′j) (i, j ═ 1, 2, 3.), and then two points A corresponding to the upper and lower surfaces are setiAnd Aj(i, j ═ 1, 2, 3.), h is obtained by subtracting the Z coordinate in the initial state under the global coordinate system, and a corresponding point A detected along with the testiAnd AjThe crack propagation length a and the crack propagation rate da/dN can be calculated based on the difference h ' between the Z coordinates in the global coordinate system, wherein the difference h ' is equal to h ' -h, and the crack is considered to appear when the difference h is larger than a certain value (the value is determined according to materials).
And 6, determining test fatigue test parameters, wherein the parameters comprise load, frequency, cycle times and the sectional area of a working part of the test piece.
And 7, starting the test, and setting the fatigue test parameters determined in the step 6 in a computer software interface of the fatigue testing machine 3. The load applied to the sample by the fatigue testing machine is positive-sine wave constant amplitude loading, and a pull-pull fatigue test is carried out. The computers of the first and second DIC apparatuses 4 and 5 and the corresponding software of the computer of the fatigue tester 3 are run simultaneously until the specimen breaks.
And 8, calculating the crack length a through the crack tip detected by the first DIC device 4 and the second DIC device 5 through coordinate change and coordinates thereof according to the cycle number N obtained by the fatigue testing machine 3 to obtain a plurality of data points of the crack length a and the cycle number N, drawing a scatter diagram by using MAT L AB software, fitting an exponential curve to obtain a change condition of the crack propagation rate da/dN, and inputting the data points obtained through the test into MAT L AB software to obtain a profile diagram of the crack tip, as shown in FIG. 3.
Although the present invention has been described in terms of the preferred embodiment, it is not intended that the invention be limited to the embodiment. Any equivalent changes or modifications made without departing from the spirit and scope of the present invention also belong to the protection scope of the present invention. The scope of the invention should therefore be determined with reference to the appended claims.
Claims (3)
1. A test method for a three-dimensional structure internal crack propagation test, wherein: the test system for the internal crack propagation test of the three-dimensional structure comprises a fatigue testing machine (3), a clamp (2), first DIC equipment (4) and second DIC equipment (5), wherein the fatigue testing machine (3) is used for fatigue loading; the double-cantilever beam test sample (1) is used as a test piece for detecting cracks; the clamp (2) is used for connecting and fixing the double-cantilever beam sample (1) and the fatigue testing machine (3); a first lens (401) of the first DIC device (4), a second lens (402) of the first DIC device, a first lens (501) of the second DIC device (5) and a second lens (502) of the second DIC device (5) are used for detecting crack tips and crack contours on the double-cantilever sample (1); a first lens (401) of the first DIC device (4), a second lens (402) of the first DIC device (4), a first lens (501) of the second DIC device (5) and a second lens (502) of the second DIC device (5) are respectively aligned to the upper surface and the lower surface of the double-cantilever sample (1) and used for observing coordinate changes of points on the upper surface and the lower surface of the double-cantilever sample (1);
the test method for the internal crack propagation test of the three-dimensional structure specifically comprises the following steps:
step 1, preparing a double-cantilever beam sample (1) for detecting the internal crack propagation behavior;
step 2, an upper clamp head (210) and a lower clamp head (220) of the clamp are respectively connected to the upper end and the lower end of a fatigue testing machine (3) through an upper clamp head thread (212) and a lower clamp head thread (222), one end of a double-cantilever beam sample (1) is connected to the lower clamp head (220) through a lower pin hole (104), a lower clamp head pin hole (221) and a first positioning pin (230), the relative position is adjusted through a cross beam of the lifting fatigue testing machine (3), and the other end of the double-cantilever beam sample (1) is connected to the upper clamp head (210) through an upper pin hole (103), an upper clamp head pin hole (211) and a second positioning pin (240);
step 3, calibrating the first DIC device (4) and the second DIC device (5) separately:
step 4, converting local coordinate systems of the first DIC device (4) and the second DIC device (5) into a global coordinate system through rotation transformation, displacement transformation and coordinate system scale change by integrally calibrating the first DIC device (4) and the second DIC device (5);
initial local coordinates (x ″) measured on a first DIC device (4) and a second DIC device (5)i,y″i,z″i)，(x″j,y″j,z″j) Conversion to global coordinates (x)i,yi,zi)，(xj,yj,zj) The method is completed by formula (1) and formula (2);
local coordinates (x ″) during testing measured on a first DIC apparatus (4) and a second DIC apparatus (5)'i,y″′i,z″′i)，(x″′j,y″′j,z″′j) Conversion to Overall coordinates (x'i,y′i,z′i)，(x′j,y′j,z′j) The method is completed by the formulas (3) and (4):
wherein m isi，mj，m′i，m′jFor the scale-varying parameter, the rotation angle of rectangular coordinate transformation in three-dimensional space, also called the Ocler angle, Deltaxi，Δxj，Δyi，Δyj，Δzi，Δzj，Δx′i，Δx′j，Δy′i，Δy′jΔz′i，Δz′jIs a translation variation parameter;
step 5, aligning a first lens (401) of a first DIC device (4) and a second lens (402) of the first DIC device (4) with the upper surface of the sample, and aligning a first lens (501) of a second DIC device (5) and a second lens (502) of the second DIC device (5) with the lower surface of the sample, wherein the first lens and the second lens are used for measuring coordinates and changes of points on the upper surface and the lower surface of the sample;
firstly, the local coordinates (x ″) of each point on the upper and lower surfaces of the sample in the initial state in the local coordinate system of the first DIC apparatus 4 and the second DIC apparatus 5 are measuredi,y″i,z″i)，(x″j,y″j,z″j) (i, j-1, 2, 3.) and local coordinates (x ″) of each point in the local coordinate system during the test'i，y″′i，z″′i)，(x″′j，y″′j，z″′j)(i，j＝1，2，3...)；
Then obtaining the corresponding coordinates (x) of the initial state under the whole coordinate system through the coordinate conversion in the step 4i,yi,zi)，(xj,yj,zj) And coordinates during the test (x'i,y′i,z′i)，(x′j,y′j,z′j) (i, j ═ 1, 2, 3.), and then two points A corresponding to the upper and lower surfaces are setiAnd Aj(i, j ═ 1, 2, 3.), h is obtained by subtracting the Z coordinate in the initial state under the global coordinate system, and a corresponding point A detected along with the testiAnd AjThe difference h 'of Z coordinates in the overall coordinate system is made to be h' -h, when the delta h is larger than a certain specific value, the crack is considered to appear, the value is determined according to materials, and the crack propagation length a and the crack propagation rate da/dN can be calculated based on the value;
step 6, determining test fatigue test parameters, wherein the parameters comprise load, frequency, cycle times and sectional area of a working part of the test piece;
step 7, starting the test, and setting the fatigue test parameters determined in the step 6 in a computer software interface of the fatigue testing machine (3); the load applied to the double-cantilever beam sample (1) by the fatigue testing machine is positive-sine wave constant amplitude loading, and a pull-pull fatigue test is carried out; simultaneously operating computers of the first DIC device (4) and the second DIC device (5) and corresponding software of a computer of the fatigue testing machine (3) until the sample is broken;
and 8, calculating the crack length a through the crack tip detected by the first DIC device (4) and the second DIC device (5) through coordinate change and coordinates of the crack tip according to the cycle number N obtained by the fatigue testing machine (3), obtaining a plurality of data points of the crack length a and the cycle number N, drawing a scatter diagram by using MAT L AB software, fitting a curve by using an exponential curve, further obtaining the change condition of the crack propagation rate da/dN, and inputting the data points obtained through the test in the MAT L AB software to obtain a profile diagram of the crack tip.
2. The method for testing the internal crack propagation test of the three-dimensional structure according to claim 1, wherein: the double-cantilever beam sample (1) comprises a first clamping block (101) and a second clamping block (102) which play a role in clamping and fixing, an upper pin hole (103) and a lower pin hole (104) are formed in the clamping blocks, the clamping blocks are fixed on the upper surface and the lower surface of the double-cantilever beam sample (1) through glue, and the double-cantilever beam sample (1) further comprises a prefabricated crack (105).
3. The method for testing the internal crack propagation test of the three-dimensional structure according to claim 1, wherein: the step 3 specifically comprises the following steps:
step 3.1, powering on and starting up the computer, and starting up the software; firstly, hardware calibration is carried out, a camera is started to start calibration, a measuring distance is determined, the relative positions of a left camera and a right camera are determined, an aperture is adjusted, and a proper aperture value is selected according to test requirements;
and 3.2, then performing software calibration, clicking a calibration button in the software, reasonably placing the position of a calibration plate according to software prompt, sequentially collecting 13 photos at different positions, and automatically calculating by the software to finish final calibration.
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| JP2013011504A (en) * | 2011-06-29 | 2013-01-17 | Ihi Corp | Method for evaluating delamination strength of composite material |
| CN103884603A (en) * | 2014-04-02 | 2014-06-25 | 华东理工大学 | Creep deformation-fatigue crack growth testing device and corresponding testing method |
| RU163521U1 (en) * | 2015-12-29 | 2016-07-20 | Федеральное государственное бюджетное образовательное учреждение высшего профессионального образования "Казанский национальный исследовательский технический университет им. А.Н. Туполева - КАИ" (КНИТУ-КАИ) | EASY CHASSIS CHASSIS |
| CN105842062A (en) * | 2016-06-02 | 2016-08-10 | 江西洪都航空工业集团有限责任公司 | Real-time crack propagation monitoring device and real-time crack propagation monitoring method |
| CN106896025B (en) * | 2017-04-25 | 2019-07-23 | 湖南大学 | Test method for cemented joint subsurface fatigue crack expanding test test macro |
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