CN104266900A - CTOD (crack-tip opening displacement) measurement method based on high-temperature image processing - Google Patents

CTOD (crack-tip opening displacement) measurement method based on high-temperature image processing Download PDF

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CN104266900A
CN104266900A CN201410539452.7A CN201410539452A CN104266900A CN 104266900 A CN104266900 A CN 104266900A CN 201410539452 A CN201410539452 A CN 201410539452A CN 104266900 A CN104266900 A CN 104266900A
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crack tip
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crack
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CN104266900B (en
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冯雪
方旭飞
张长兴
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Tsinghua University
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Abstract

The invention discloses a CTOD (crack-tip opening displacement) measurement method based on high-temperature image processing, and belongs to the technical field of engineering materials, structure deformation and mechanical experiments. The CTOD measurement method is technically characterized in that a test piece provided with precracks is subjected to high-temperature mechanical loading experiment, a camera equipped with a filter device is used for shooting the propagation process of cracks on the surface of the test piece, , cracks of a notch of the test piece are subjected to positioning detection with an edge detection method on an acquired image, and the open displacement of a crack tip is calculated, so that CTOD in the high-temperature environment of the material is acquired. According to the method, the CTOD is directly calculated through an image processing technology; and compared with a conventional method for obtaining the CTOD of the material through a stress-displacement curve, the method is simple and direct, a result can be directly obtained through auxiliary calculation of software, and conventional errors of the stress-displacement curve due to human judgment are reduced.

Description

A kind of crack tip opening displacement measuring method based on high temperature image procossing
Technical field
The present invention relates to a kind of crack tip opening displacement measuring method based on high temperature image procossing, belong to construction material, structural deformation and Experiments of Machanics technical field.
Background technology
Construction material fracture toughness important indicator under elasto-plastic is weighed in crack tip opening displacement (crack-tip opening displacement, CTOD), and it characterizes the ability of material repels crack initiation or expansion.Crack tip opening displacement refers to after cracks in body is stressed, in the displacement that former crack tip produces along vertical crack direction.Nineteen sixty-five, Wells proposes the CTOD criterion of elastic-plastic fracture mechanics.This criterion can be expressed as: when crack tip opening displacement δ reaches critical value δ ctime, crackle will ftracture, i.e. δ=δ c.Crack tip opening displacement in formula is by measuring.In view of crack tip opening displacement method be weigh Materials Fracture performance one very effectively and simple method and by engineering actual accept and apply, China has made measures critical crack tip opening displacement δ cmethod (GB/T 2358-80; GB/T 2358-94), test for the crack tip opening displacement of metal material, but be only limitted to the test of room temperature and low-temperature cracks tip opening displacement.
But along with the raising of the demand for development of engineering, the working temperature of corresponding material and structure is also more and more higher.Be very different under the deformation mechanism of material under high temperature and fracture behaviour and normal temperature.Meanwhile, material and structural member thereof are in the fracture parameter test under high-temperature work environment is a difficult point always.In order to realize the sign that material under high temperature fracture toughness is measured, effective approach is a crack tip opening displacement for material under measurement hot conditions, and this then will seek development further and manufacture high-temperature extensometer to realize the measurement of the crack tip opening displacement under hot environment.Most advanced at present and probe temperature that the is more ripe high-temperature extensometer (Epsilon company of the U.S.) of application can reach 1200 DEG C, part can meet the test of 1600 DEG C.But use the defect of high-temperature extensometer to be: high-temperature extensometer price is very expensive on the one hand, on the other hand, installing and using of high-temperature extensometer is more complicated, be vulnerable to the impact of manual operation, and then cause larger measuring error, and cannot to realize under hot test condition, to the online observation of test specimen and record, the distortion of test specimen in process of the test and the real-time detailed process of evolution cannot being disclosed further.
Online observation experimental technique aspect, is generally undertaken by high precision industrial camera CCD, for Crack Extension etc. then high speed dynamic process can adopt high-speed camera.Hambury-Harburg technology university of Germany utilizes the high-temperature stability of camera head to CMC to study, real-time monitored Crack Extension, acquisition and processing data.California Institute of Technology utilizes high-speed CCD camera with the speed acquisition image of ten thousand width/second, the dynamic expansion process etc. of research material crackle.Aforementioned two class technology are only limitted to the application under normal temperature condition.Chinese patent literature 201310739173.0 (examination state) utilizes high-speed camera and industrial camera to coordinate filtering technique to carry out quantitative examination to test specimen surface oxidation under the high temperature conditions, ablation process.But said method all accurately cannot be measured the crack tip opening displacement under hot environment.
Summary of the invention
The object of this invention is to provide a kind of crack tip opening displacement measuring method based on high temperature image procossing, the method is by carrying out the experiment of high temperature constant temperature Mechanical loading to the material test specimen of band precrack, by being equipped with the expansion process of the crackle of the video camera shooting surface of test piece of filter plate, further the high temperature image obtained is processed, thus obtain material crack tip opening displacement in high temperature environments (crack-tip opening displacement, CTOD).
Technical scheme of the present invention is as follows: a kind of crack tip opening displacement measuring method based on high temperature image procossing, is characterized in that the method comprises the steps:
1) Mechanical loading is carried out under the test specimen with precrack being put into high temperature constant temperature environment, this test specimen is crack tip generation selective oxidation in hot environment, crack tip image local gray-scale value Spline smoothing, video camera is used to take the change procedure of test specimen crack tip in loading procedure continuously, obtain test specimen notch cracks under same temperature in not expanded images in the same time, and at least one high temperature filter plate is installed on camera lens;
2) second derivative is adopted to detect, calculate crack tip edge, as shown in the formula calculating:
The Laplacian values of definition two dimensional image gamma function f (x, y) is:
▿ 2 f ( x , y ) = ∂ 2 f ( x , y ) ∂ x 2 + ∂ 2 f ( x , y ) ∂ y 2 - - - ( 1 )
Wherein obtaining the face internal coordinate of image is (x, y), the horizontal ordinate of x, y correspondence image pixel and ordinate, at point (x, y) place in above formula differential representation be respectively:
∂ 2 f ( x , y ) ∂ x 2 = f ( x + 1 , y ) - 2 f ( x , y ) + f ( x - 1 , y ) - - - ( 2 )
∂ 2 f ( x , y ) ∂ x 2 = f ( x , y + 1 ) - 2 f ( x , y ) + f ( x , y - 1 ) - - - ( 3 )
Formula (2) and (3) are substituted into formula (1), obtain following formula (4):
▿ 2 f ( x , y ) = ∂ 2 f ( x , y ) ∂ x 2 + ∂ 2 f ( x , y ) ∂ y 2 = f ( x + 1 , y ) + f ( x - 1 , y ) + f ( x , y + 1 ) + f ( x , y - 1 ) - 4 f ( x , y )
The local maximum of the Laplacian values of corresponding two dimensional image gamma function f (x, y) is obtained according to formula (4);
3) according to the expanding change of crack tip edge particular location, Calculating material crack tip opening displacement:
Defined function g (x max, y max) represent and corresponding not position in the same time, crack tip edge detected, x max, y maxcorrespondence image does not obtain the local maximum of the Laplacian values of two dimensional image gamma function f (x, y) in the same time horizontal ordinate and ordinate by the method for rim detection; Moment t 0corresponding crack tip position is g 0(x max, y max), moment t 1corresponding crack tip position is g 1(x max, y max); Document tests the time of carrying out Δ t=t 1-t 0, obtain crack tip opening displacement δ=g 1(x max, y max)-g 0(x max, y max).
In technique scheme, step 1) in high temperature power load and adopt the three-point bending test method with high-temperature heater to load; Under referring to 600 DEG C of-1600 DEG C of temperature environments under described high temperature constant temperature environment.
The present invention compared with prior art, have the following advantages and high-lighting effect: the present invention realizes carrying out online observation and image record to the material test specimen be out of shape by load effect under high temperature (600 DEG C-1600 DEG C) environment, by the method for high temperature image procossing, the expansion of test specimen notch cracks and evolution are calculated, simple and convenient, directly can obtain result of calculation, the artificial error judged in conventional stress-displacement curve can be reduced simultaneously.
Accompanying drawing explanation
Fig. 1 is the test operation process flow diagram of a kind of crack tip opening displacement measuring method based on high temperature image procossing provided by the invention.
Fig. 2 is the result of calculation of being carried out the crack tip opening displacement that high temperature alloy Nb521 material carries out under 1200 DEG C of high temperature experiment conditions by this method.
Embodiment
The specific embodiment of the present invention is further illustrated below in conjunction with drawings and Examples.
Fig. 1 is the test operation process flow diagram of a kind of crack tip opening displacement measuring method based on high temperature image procossing provided by the invention, and its concrete operation step is as follows:
A. precrack test material preparation is with:
Sample dimensions and crack size should design preparation according to standard GB/T/T 2358-94, make test material preparation meet national Specification;
B. high temperature loading experiment is carried out: under the test specimen with precrack being put into high temperature constant temperature environment, carry out Mechanical loading, this test specimen is crack tip generation selective oxidation in hot environment, crack tip image local gray-scale value Spline smoothing, video camera is used to take the change procedure of test specimen crack tip in loading procedure continuously, obtain test specimen notch cracks under same temperature in not expanded images in the same time, and at least one high temperature filter plate is installed on camera lens.High temperature loading experiment can carry out with on the three point bending test machine of high-temperature heater, testing machine record load, time and the change of test specimen amount of deflection in process of the test; Furnace wall has viewport to heating furnace, places high quartz glass, prevent flow perturbation in viewport in viewport, improves subsequent image capture quality; Described high temperature constant temperature environment refers to and carry out loading experiment under 600 DEG C of-1600 DEG C of temperature environment.
C. the video camera shooting by installing filtering system obtains test specimen notch cracks expansion picture: shooting process is shooting continuously, time interval when carrying out edge detection calculation to the image obtained can be selected between 10s to 20s, avoid because the time too short Crack Extension caused is not obvious, avoid because the Crack Extension information that overlong time causes is missed simultaneously; By installing the filtering system be made up of a series of filter plate on camera lens, the High temperature radiation of filtering high temperature viewport, reduces optical noise, improves the sharpness of the dynamic process of video camera shooting Crack Extension;
D. technique of image edge detection is adopted to detect crack tip edge:
The image of the test specimen obtained in high temperature experiment crack tip expansion in hot environment is in crack tip generation selective oxidation, the change of crack tip local image intensity belongs to Spline smoothing function, and namely image intensity has significant difference at the grey scale pixel value on the both sides at discontinuous place.Theoretical by Image Edge-Detection, adopt second derivative to detect, calculate the geometrical morphology of crack tip.Computing Principle is:
The Laplacian values of definition two dimensional image gamma function f (x, y) is:
▿ 2 f ( x , y ) = ∂ 2 f ( x , y ) ∂ x 2 + ∂ 2 f ( x , y ) ∂ y 2 - - - ( 1 )
Wherein obtaining the face internal coordinate of image is (x, y), the horizontal ordinate of x, y correspondence image pixel and ordinate, at point (x, y) place in above formula differential representation be respectively:
∂ 2 f ( x , y ) ∂ x 2 = f ( x + 1 , y ) - 2 f ( x , y ) + f ( x - 1 , y ) - - - ( 2 )
∂ 2 f ( x , y ) ∂ x 2 = f ( x , y + 1 ) - 2 f ( x , y ) + f ( x , y - 1 ) - - - ( 3 )
Formula (2) and formula (3) are substituted into formula (1) obtain:
▿ 2 f ( x , y ) = ∂ 2 f ( x , y ) ∂ x 2 + ∂ 2 f ( x , y ) ∂ y 2 = f ( x + 1 , y ) + f ( x - 1 , y ) + f ( x , y + 1 ) + f ( x , y - 1 ) - 4 f ( x , y ) - - - ( 4 )
Based on above-mentioned principle, the two dimensional image gamma function f (x at the crack tip edge of corresponding different time in image, y) Laplacian values has step, calculates corresponding two dimensional image gamma function and obtains local maximum, the position of the corresponding different time of crack tip namely detected;
E. according to the expanding change of crack tip edge particular location, Calculating material crack tip opening displacement: defined function g (x max, y max) represent and corresponding not position in the same time, crack tip edge detected, x max, y maxcorrespondence image does not obtain the local maximum of the Laplacian values of two dimensional image gamma function f (x, y) in the same time horizontal ordinate and ordinate by the method for rim detection; Moment t 0corresponding crack tip position is g 0(x max, y max), moment t 1corresponding crack tip position is g 1(x max, y max); Document tests the time of carrying out Δ t=t 1-t 0, corresponding crack tip opening displacement δ=g 1(x max, y max)-g 0(x max, y max).
The present invention is understood further below by specific embodiment.
Embodiment:
This method has carried out the experiment of 600 DEG C, 800 DEG C, 1000 DEG C, 1200 DEG C, 1400 DEG C and 1600 DEG C respectively, wherein chooses the high temperature experimental result specific explanations of 1200 DEG C:
A. high temperature alloy Nb521 is made SEN beam test specimen (concrete dimensional details meets the requirement of GB/T 2358-94);
B. carry out high temperature three-point bending test to the Nb521 test specimen prepared, experimental temperature is constant temperature 1200 DEG C;
C. in this example, high temperature three-point bending test oven wall has the viewport that diameter is 12mm, by the high-speed camera being equipped with high temperature filtering system, whole experimentation is taken, obtain test specimen notch cracks expansion picture;
D. technique of image edge detection is adopted to obtain crack tip edge, calculate crack tip opening displacement (selecting crack opening displacement to be less than 0.05mm is that crackle starts expansion time), the measured value of crack tip opening displacement actual (tube) length angle value that following table provides not in the same time (time interval is 20s), result of calculation is respectively see Figure of description 2.

Claims (3)

1., based on a crack tip opening displacement measuring method for high temperature image procossing, it is characterized in that the method comprises the steps:
1) Mechanical loading is carried out under the test specimen with precrack being put into high temperature constant temperature environment, this test specimen is crack tip generation selective oxidation in hot environment, crack tip image local gray-scale value Spline smoothing, video camera is used to take the change procedure of test specimen crack tip in loading procedure continuously, obtain test specimen notch cracks under same temperature in not expanded images in the same time, and at least one high temperature filter plate is installed on camera lens;
2) second derivative is adopted to detect, calculate crack tip edge, as shown in the formula calculating:
The Laplacian values of definition two dimensional image gamma function f (x, y) is:
▿ 2 f ( x , y ) = ∂ 2 f ( x , y ) ∂ x 2 + ∂ 2 f ( x , y ) ∂ y 2 - - - ( 1 ) Wherein obtaining the face internal coordinate of image is (x, y), the horizontal ordinate of x, y correspondence image pixel and ordinate, at point (x, y) place in above formula differential representation be respectively:
∂ 2 f ( x , y ) ∂ x 2 = f ( x + 1 , y ) - 2 f ( x , y ) + f ( x - 1 , y ) - - - ( 2 )
∂ 2 f ( x , y ) ∂ x 2 = f ( x , y + 1 ) - 2 f ( x , y ) + f ( x , y - 1 ) - - - ( 3 )
Formula (2) and (3) are substituted into formula (1), obtain following formula (4):
▿ 2 f ( x , y ) = ∂ 2 f ( x , y ) ∂ x 2 + ∂ 2 f ( x , y ) ∂ y 2 = f ( x + 1 , y ) + f ( x - 1 , y ) + f ( x , y + 1 ) + f ( x , y - 1 ) - 4 f ( x , y )
The local maximum of the Laplacian values of corresponding two dimensional image gamma function f (x, y) is obtained according to formula (4);
3) according to the expanding change of crack tip edge particular location, Calculating material crack tip opening displacement:
Defined function g (x max, y max) represent and corresponding not position in the same time, crack tip edge detected, x max, y maxcorrespondence image does not obtain the local maximum of the Laplacian values of two dimensional image gamma function f (x, y) in the same time horizontal ordinate and ordinate by the method for rim detection; Moment t 0corresponding crack tip position is g 0(x max, y max), moment t 1corresponding crack tip position is g 1(x max, y max); Document tests the time of carrying out Δ t=t 1-t 0, obtain crack tip opening displacement δ=g 1(x max, y max)-g 0(x max, y max).
2., according to a kind of crack tip opening displacement measuring method based on high temperature image procossing according to claim 1, it is characterized in that: in step 1) in high temperature power load to adopt and load with the three-point bending test method of high-temperature heater.
3., according to a kind of crack tip opening displacement measuring method based on high temperature image procossing described in claim 1 or 2, it is characterized in that: step 1) refer to 600 DEG C of-1600 DEG C of temperature environments under described high temperature constant temperature environment under.
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CN108760546A (en) * 2018-08-14 2018-11-06 湖南大学 A kind of fatigue crack growth rate measurement method based on Infrared Thermography Technology
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CN112903443A (en) * 2021-01-28 2021-06-04 中国科学院武汉岩土力学研究所 Method and device for determining propagation speed of fracture process zone of rock material crack tip
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CN114441337A (en) * 2022-01-14 2022-05-06 中国石油大学(北京) Detection method for opening displacement of metal fracture toughness crack tip

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN105371769A (en) * 2015-11-04 2016-03-02 西北工业大学 Method and device of measuring dynamic crack lengths
CN105371769B (en) * 2015-11-04 2018-06-12 西北工业大学 A kind of method and device for measuring Dynamic Crack length
CN106841206A (en) * 2016-12-19 2017-06-13 大连理工大学 Untouched online inspection method is cut in heavy parts chemical milling
CN106841206B (en) * 2016-12-19 2018-07-24 大连理工大学 Untouched online inspection method is cut in heavy parts chemical milling
CN109751955A (en) * 2017-11-01 2019-05-14 北京振兴计量测试研究所 Non-contact object displacement measuring device and the measurement method for using it
CN108760546A (en) * 2018-08-14 2018-11-06 湖南大学 A kind of fatigue crack growth rate measurement method based on Infrared Thermography Technology
CN112903443A (en) * 2021-01-28 2021-06-04 中国科学院武汉岩土力学研究所 Method and device for determining propagation speed of fracture process zone of rock material crack tip
CN112903443B (en) * 2021-01-28 2021-12-14 中国科学院武汉岩土力学研究所 Method and device for determining propagation speed of fracture process zone of rock material crack tip
CN113446932A (en) * 2021-05-18 2021-09-28 西北工业大学 Non-contact crack measuring method and system
CN114441337A (en) * 2022-01-14 2022-05-06 中国石油大学(北京) Detection method for opening displacement of metal fracture toughness crack tip

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