Photoelastic stress measurement system and method based on three-wavelength illumination light source
阅读说明：本技术 一种基于三波长照明光源的光弹性应力测量系统与方法 (Photoelastic stress measurement system and method based on three-wavelength illumination light source ) 是由 陆雨洁 刘诚 张源哲 邢童璐 高夏立 于 2021-04-16 设计创作，主要内容包括：本发明公开了一种基于三波长照明光源的光弹性应力测量系统与方法,属于光弹性测量的技术领域。所述光弹性应力测量系统包括图像获取模块,用于分别获取波长λ-(1)、波长λ-(2)和波长λ-(3)光源照明下消除背景光的条纹强度分布；主应力差计算模块,用于采集图像获取模块获得的图像条纹信息并求解待测样品的主应力差；主应力方向计算模块,用于采集图像获取模块获得的图像条纹信息并求解待测样品主应力方向。本发明使用方便,测量速度快；极大减小了高级次条纹处理时的误差,拓展了光弹性应力测量的范围；能够对含有误差的曲线段进行处理,准确测得样品的主应力方向和主应力差大小,显著消除了双波长测量方法产生的误差,具有测量精度高的优势。(The invention discloses a photoelastic stress measurement system and method based on a three-wavelength illumination light source, and belongs to the technical field of photoelastic measurement. The photoelastic stress measurement system comprises an image acquisition module for respectively acquiring wavelength lambda 1 Wavelength lambda of 2 And wavelength lambda 3 Eliminating the fringe intensity distribution of background light under the illumination of a light source; the main stress difference calculation module is used for acquiring the image stripe information obtained by the image acquisition module and solving the main stress difference of the sample to be detected; and the main stress direction calculation module is used for acquiring the image stripe information obtained by the image acquisition module and solving the main stress direction of the sample to be tested. The invention has convenient use and high measuring speed; the error in the high-level stripe processing is greatly reduced, and the measuring range of photoelastic stress is expanded; the method can process the curve segment containing errors, accurately measure the main stress direction and the main stress difference of the sample, obviously eliminate the errors generated by the dual-wavelength measuring method, and has the advantage of high measuring precision.)
1. A photoelastic stress measurement method based on a three-wavelength illumination light source is characterized in that a photoelastic stress measurement system based on a three-wavelength illumination light source is applied for measurement, and the photoelastic stress measurement system based on the three-wavelength illumination light source comprises the following steps:
the image acquisition module (11) comprises a three-wavelength surface light source (10), wherein a polarizer (2), an analyzer (8), an imaging lens (6) and a photoelectric detector (9) are sequentially arranged on a propagation path of emergent light of the three-wavelength surface light source (10) and used for respectively acquiring wavelength lambda1Wavelength lambda of2And wavelength lambda3The light source illuminates the sample (5) to be detected to eliminate the stripe intensity distribution of the background light;
the main stress difference calculation module (12) is used for acquiring the image stripe information acquired by the image acquisition module (11) and solving the main stress difference of the sample (5) to be detected;
the main stress direction calculation module (13) is used for acquiring the image stripe information obtained by the image acquisition module (11) and solving the main stress direction of the sample (5) to be detected;
the photoelastic stress measuring method based on the three-wavelength illumination light source comprises the following steps:
the method comprises the following steps: the polarizer (2) is orthogonal to the analyzer (8) at a wavelength lambda1Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained1(ii) a At a wavelength λ2Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained2(ii) a At a wavelength λ3Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained3(ii) a The included angle between the polarizer (2) and the analyzer (8) is 45 DEG at the wavelength lambda2Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained4At a wavelength λ3Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained5(ii) a The polarizer (2) and the analyzer (8) are orthogonal and synchronously rotated for 45 DEG at the wavelength lambda2Under the illumination of the light source, the elimination back is obtainedFringe intensity distribution Img after influence of scene light6At a wavelength λ3Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained7；
Step two: acquiring image stripe information obtained in the first step, and solving a main stress difference sigma (x, y) of a sample (5) to be detected by using Fourier transform, low-pass filtering and phase unwrapping techniques;
step three: and (3) acquiring the image stripe information obtained in the first step, and solving the main stress direction theta of the sample (5) to be detected by using a phase unwrapping technique.
2. Photoelastic stress measurement system based on three-wavelength illumination light source according to claim 1, characterized in that the three-wavelength surface light source (10) is controlled by a switch to emit wavelengths λ at different times1Wavelength lambda of2And wavelength lambda3Light of three frequencies.
3. The photoelastic stress measurement system based on a three-wavelength illumination light source according to claim 1 or 2, characterized in that the photodetector (9) is configured to convert the light information transmitted from the imaging lens (6) into an electrical signal and collect a fringe pattern generated by the illumination of the sample (5) to be measured by the three-wavelength surface light source (10).
4. The system of claim 2, wherein the three illumination sources have wavelengths λ1Wavelength lambda of2Wavelength lambda of3The following relationship is satisfied:
λ1>λ2>λ3。
5. the method for measuring photoelastic stress based on three-wavelength illumination source according to claim 1, wherein the second step of solving the main stress difference σ (x, y) of the sample (5) to be measured by using fourier transform, low-pass filtering and phase unwrapping techniques specifically includes the following steps:
step1, under an illumination light source with three wavelengths, the normalized fringe intensity distribution after eliminating the influence of background light is respectively as follows:
wherein c is the stress optical coefficient of the material and is a known constant, and t is the thickness of the sample (5) to be measured and is a measurable constant;
step2. for Img respectively1、Img2、Img3Using the formula of multiple angles to obtain I1(x,y)、I2(x,y)、I3(x,y)：
Step3. mixing I2(x, y) are each independently of I1(x, y) and I3(x, y) are compared at the same stripe level to give arccos [ I2(x,y)]In an increasing or decreasing range, arccos [ I ]2(x,y)]The increasing and decreasing turning parts are preliminarily distinguished, and two images { arccos [ I ] which have jumping discontinuities and still contain partial turning parts are obtained respectively2(x,y)]}21And { arccos[I2(x,y)]}23；
Step4, { arccos [ I ] obtained by Step32(x,y)]}21And { arccos [ I ]2(x,y)]}23Calculating the error curve interval, and assigning the value in the error interval as 0 to obtain the function { arccos [ I [ ]2(x,y)]}230；
Step5. pair { arccos [ I ]2(x,y)]}230Fourier transform is carried out to obtain the spectrum distribution of the spectrum, a pass band is selected for low-pass filtering, and the filtering result is converted into space domain information by inverse Fourier transform and is marked as { arccos [ I ]2(x,y)]}230′；
Step6. by judging { arccos [ I ]2(x,y)]}230The positive and negative values of the segment value of the median error curve construct a reversal judgment matrix, which is marked as L (x, y), and the L (x, y) and { arccos [ I ]2(x,y)]}23Multiplication to make { arccos [ I ]2(x,y)]}23Corrected error of (2) to obtain an image { arccos [ I ] having only jumping discontinuities2(x,y)]}23′；
Step7. pair { arccos [ I ]2(x,y)]}23' phase unwrapping processing is performed by adopting a phase unwrapping algorithm, and continuous stripe distribution can be spliced and recorded as { arccos [ I2(x,y)]}23FI.e., [ arccos (1-2 Img)2)]23F。
6. The photoelastic stress measurement method based on a three-wavelength illumination light source according to claim 5, wherein the principal stress difference σ (x, y) of the sample (5) to be measured satisfies the following relation:
7. the method for measuring photoelastic stress based on three-wavelength illumination light source according to claim 1, wherein the solving of the principal stress direction θ of the sample (5) to be measured using the phase unwrapping technique in the third step includes the following processes:
wherein, I20、I30Respectively represent the wavelength lambda2And wavelength lambda3The intensity of the illumination when illuminated.
8. The photoelastic stress measurement method based on the three-wavelength illumination light source of claim 7, wherein the principal stress direction θ of the sample (5) satisfies the following relation:and (5) obtaining the actual stress direction of the sample (5) to be detected by adopting a phase unwrapping algorithm on the theta.
Technical Field
The invention relates to a photoelastic stress measurement system and method based on a three-wavelength illumination light source, and belongs to the technical field of photoelastic measurement.
Background
The photoelastic measurement technology is a stress measurement technology based on the birefringence characteristics of a light-transmitting material, and can intuitively reflect the stress distribution condition in the material through a fringe pattern. In addition to the conventional phase shift method, the dual-wavelength method for performing series analysis on photoelastic fringes provides a new idea for quantitative analysis of the magnitude and direction of the internal stress of the material, but still has certain limitations. Errors caused by frequency difference of illumination light are ignored in the calculation process of the dual-wavelength method, when the measured fringe level is low, the error between the result and the actual result is small, but for some samples with large stress variation, the calculation errors are also cumulatively increased along with the increase of the measurement level, so that the measurement accuracy of the samples cannot meet the requirement.
FIG. 1 shows a photoelastic measurement optical path of a two-wavelength method (Chen T Y. digital determination of photoelastic bifurcating using two-wavelength wavelet hs [ J]Experimental Mechanics 1997,37(3): 232-. Chen proposes to place 5 x 5 cross-shaped points within one stripe width of the detected stripe pattern for accurate stripe search. The fringe order error of each point under the irradiation of two kinds of wavelength light is calculated respectively, the accurate fringe order of the central point is determined firstly by searching the minimum error value, and then the accurate fringe order of other points is determined by tree search by taking the minimal error value as the seed point. Thus, the value of Nf equals t (σ)1-σ2) The value (sigma) of the difference in principal stresses at each point can be determined1-σ2) Wherein N is the accurate fringe order, f is the fringe value under the corresponding wavelength, and t is the sample thickness. Although the method does not need special treatment of phase unwrapping, the method is only suitable for judging the fringe level in a certain level, and when the fringe density is higher, a larger error is generated (Hanyong, Zhangong Sheng, Ro\2815638]Engineering mechanics, 2008,25(002):62-65.), thusResulting in a limited range of applications.
Disclosure of Invention
[ problem ] to
The invention aims to solve the problems that: the existing dual-wavelength photoelasticity measuring method is only suitable for judging the fringe order in a certain order, and when the fringe density is higher, a larger error is generated, so that the application range is limited.
[ solution ]
The invention provides a photoelastic stress measurement system and method based on a three-wavelength illumination light source, which are applicable to the technical field of photoelastic measurement.
The photoelastic stress measuring method based on the three-wavelength illumination light source is used for measuring by using a photoelastic stress measuring system based on the three-wavelength illumination light source, and the photoelastic stress measuring system based on the three-wavelength illumination light source comprises the following steps:
the image acquisition module comprises a three-wavelength surface light source, wherein a polarizer, an analyzer, an imaging lens and a photoelectric detector are sequentially arranged on a propagation path of emergent light of the three-wavelength surface light source and are used for respectively acquiring wavelength lambda1Wavelength lambda of2And wavelength lambda3The light source illuminates the sample to be detected to eliminate the stripe intensity distribution of the background light;
the main stress difference calculation module is used for acquiring the image stripe information obtained by the image acquisition module and solving the main stress difference of the sample to be detected;
the main stress direction calculation module is used for acquiring the image stripe information obtained by the image acquisition module and solving the main stress direction of the sample to be tested;
the photoelastic stress measuring method based on the three-wavelength illumination light source comprises the following steps:
the method comprises the following steps: making the polarizer orthogonal to the analyzer at wavelength lambda1Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained1(ii) a At a wavelength λ2Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained2(ii) a At a wavelength λ3Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained3(ii) a The included angle between the polarizer and the analyzer is 45 DEG at wavelength lambda2Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained4At a wavelength λ3Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained5(ii) a The polarizer and the analyzer are orthogonal and synchronously rotated by 45 DEG at the wavelength lambda2Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained6At a wavelength λ3Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained7；
Step two: acquiring image stripe information obtained in the first step, and solving a main stress difference sigma (x, y) of a sample to be detected by using Fourier transform, low-pass filtering and phase unwrapping techniques;
step three: and (4) acquiring the image stripe information obtained in the first step, and solving the main stress direction theta of the sample to be tested by using a phase unwrapping technique.
In one embodiment of the present invention, the three-wavelength surface light source is controlled by a switch to emit the wavelength λ at different timings1Wavelength lambda of2And wavelength lambda3Light of three frequencies.
In an embodiment of the present invention, the photodetector is configured to convert optical information transmitted from the imaging lens into an electrical signal, and collect a fringe pattern generated by the three-wavelength surface light source irradiating the sample to be measured.
In one embodiment of the present invention, the wavelengths λ of the three illumination light sources1Wavelength lambda of2Wavelength lambda of3The following relationship is satisfied:
λ1＞λ2＞λ3。
in an embodiment of the present invention, the second step of solving the main stress difference σ (x, y) of the sample to be measured by using fourier transform, low-pass filtering and phase unwrapping techniques specifically includes the following steps:
step1, under an illumination light source with three wavelengths, the normalized fringe intensity distribution after eliminating the influence of background light is respectively as follows:
wherein c is the stress optical coefficient of the material and is a known constant, and t is the thickness of the sample to be measured and is a measurable constant;
step2. for Img respectively1、Img2、Img3Using the formula of multiple angles to obtain I1(x,y)、I2(x,y)、I3(x,y)：
Step3. mixing I2(x, y) are each independently of I1(x, y) and I3(x, y) are compared at the same stripe level to give arccos [ I2(x,y)]In an increasing or decreasing range, arccos [ I ]2(x,y)]Preliminarily distinguishing the increasing and decreasing turning parts to obtain two partsImage with jumping discontinuities and still containing partially turned portions { arccos [ I ]2(x,y)]}21And { arccos [ I ]2(x,y)]}23；
Step4, { arccos [ I ] obtained by Step32(x,y)]}21And { arccos [ I ]2(x,y)]}23Calculating the error curve interval, and assigning the value in the error interval as 0 to obtain the function { arccos [ I [ ]2(x,y)]}230；
Step5. pair { arccos [ I ]2(x,y)]}230Fourier transform is carried out to obtain the spectrum distribution of the spectrum, a pass band is selected for low-pass filtering, and the filtering result is converted into space domain information by inverse Fourier transform and is marked as { arccos [ I ]2(x,y)]}230′；
Step6. by judging { arccos [ I ]2(x,y)]}230The positive and negative values of the segment value of the median error curve construct a reversal judgment matrix, which is marked as L (x, y), and the L (x, y) and { arccos [ I ]2(x,y)]}23Multiplication to make { arccos [ I ]2(x,y)]}23Corrected error of (2) to obtain an image { arccos [ I ] having only jumping discontinuities2(x,y)]}23′；
Step7. pair { arccos]I2(x,y)]}23' phase unwrapping processing is performed by adopting a phase unwrapping algorithm, and continuous stripe distribution can be spliced and recorded as { arccos [ I2(x,y)]}23FI.e., [ arccos (1-2 Img)2)]23F。
In an embodiment of the present invention, the principal stress difference σ (x, y) of the sample to be tested satisfies the following relation:
in an embodiment of the present invention, the step three of solving the principal stress direction θ of the sample to be tested by using the phase unwrapping technique includes the following steps:
wherein, I20、I30Respectively represent the wavelength lambda2And wavelength lambda3The intensity of the illumination when illuminated.
In an embodiment of the present invention, the principal stress direction θ of the sample to be measured satisfies the following relation:and (5) obtaining the actual stress direction of the sample to be detected by adopting a phase unwrapping algorithm on the theta.
Advantageous effects
(1) According to the photoelastic stress measurement system based on the three-wavelength illumination light source, the illumination light sources with three frequencies are used for respectively irradiating a sample to be measured to obtain a fringe pattern containing sample stress information, so that the stress magnitude and direction can be measured, and the photoelastic stress measurement system based on the three-wavelength illumination light source has the advantages of convenience in use and high measurement speed.
(2) The photoelastic stress measuring system based on the three-wavelength illumination light source disclosed by the invention can measure a large-caliber sample under the condition of not obviously increasing the weight of an instrument, greatly reduces the error in advanced stripe processing, and expands the measurement range of photoelastic stress.
(3) When the dual-wavelength method is used for solving the main stress difference, an error interval is generated along with the increase of the fringe order, and the error interval is increased along with the increase of the fringe order (see c in fig. 5), so that the principle of measuring the stress by the dual-wavelength method cannot be corrected. The invention disclosed is based onThe photoelastic stress measuring method of wavelength lighting source includes introducing one additional wavelength λ3The illumination light source can process the curve segment containing errors by applying the modern digital image processing technology, accurately measure the main stress direction and the main stress difference of the sample, obviously eliminate the errors generated by the dual-wavelength measuring method (see c in figure 6), and has the advantage of high measuring precision.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a schematic structural diagram of a dual-wavelength stress measuring instrument in the prior art of photoelasticity measurement;
FIG. 2 is a schematic structural diagram of a three-wavelength illumination-based photoelastic stress measurement system according to the present invention;
FIG. 3 is a schematic structural diagram of an image acquisition module according to the present invention;
FIG. 4 is a flowchart of a method for measuring photoelastic stress based on a three-wavelength illumination source according to the present invention;
FIG. 5 is a diagram illustrating a simulation result of stress calculation by a dual-wavelength method in the prior art photoelastic measurement;
FIG. 6 is a schematic diagram of a simulation result of stress calculation using a three-wavelength illumination light source-based photoelastic stress measurement system and method of the present invention;
FIG. 7 shows a schematic view of the present invention2(x, y) are each independently of I1(x, y) and I3(x, y) comparison at the same stripe level, { arccos [ I ] with jumping discontinuities and still containing partial breakover2(x,y)]}21And { arccos [ I ]2(x,y)]}23A simulation result schematic diagram;
FIG. 8 is a diagram illustrating simulation results of screening error curve segment positions according to the present invention;
FIG. 9 is a diagram illustrating simulation results of processing the error curve segment filtered in FIG. 8 using Fourier transform, inverse Fourier transform and low pass filtering techniques in accordance with the present invention;
FIG. 10 is a diagram illustrating simulation results of the symbol decision matrix L (x, y) according to the present invention;
FIG. 11 shows { arccos [ I ] according to the present invention2(x,y)]}23The increasing and decreasing turning parts are further differentiated to obtain { arccos [ I ] with jumping discontinuities only2(x,y)]}23' simulation result schematic diagram;
FIG. 12 is a graph of the present invention vs { arccos [ I ] in FIG. 112(x,y)]}23' A schematic diagram of a simulation result of true phase distribution obtained by unwrapping processing is made.
In the figure, 1, a dual-wavelength laser light source; 2. a polarizer; 3. a first quarter wave plate; 4. a collimating lens; 5. a sample to be tested; 6. an imaging lens; 7. a second quarter wave plate; 8. an analyzer; 9. a photodetector; 10. a three-wavelength surface light source; 11. an image acquisition module; 12. a main stress difference calculation module; 13. and a principal stress direction calculation module.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Example 1:
as shown in fig. 2, the present embodiment provides a photoelastic stress measurement system based on a three-wavelength illumination light source, which includes an image acquisition module 11, configured to acquire a wavelength λ when a relative direction of deflection axes of a polarizer 2 and an analyzer 8 is changed1、λ2And λ3Eliminating the fringe intensity distribution of background light under the illumination of a light source; the main stress difference calculating module 12 is used for solving the main stress difference of the image stripe information obtained by the image obtaining module 11 by using modern digital image processing technologies such as Fourier transform, low-pass filtering, phase unwrapping and the like; and the main stress direction calculation module 13 is configured to collect the image stripe information obtained by the image obtaining module 11, and solve the main stress direction by using a phase unwrapping technique.
As shown in fig. 3, the image acquisition module 11 includes a three-wavelength surface light source 10, and the three-wavelength surface light source 10 is controlled by a switch to emit a wavelength λ at different times1Wavelength lambda of2And wavelength lambda3Light of three frequencies; a polarizer 2, an analyzer 8, an imaging lens 6 and a photoelectric detector 9 are sequentially arranged on a propagation path of the light emitted by the three-wavelength surface light source 10; a sample 5 to be tested is placed between the polarizer 2 and the analyzer 8; the photoelectric detector 9 converts the light information transmitted from the imaging lens 6 into an electric signal, and collects a fringe pattern generated by irradiating the sample 5 to be measured by the three-wavelength surface light source 10.
Example 2:
as shown in fig. 4, the present embodiment discloses a method for measuring photoelastic stress based on a three-wavelength illumination light source, where the method for measuring photoelastic stress based on a three-wavelength illumination light source uses the system for measuring photoelastic stress based on a three-wavelength illumination light source disclosed in embodiment 1 to perform measurement, and includes the following steps:
the method comprises the following steps: making the polarizer 2 orthogonal to the analyzer 8 at the wavelength lambda1Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained1(ii) a At a wavelength λ2Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained2(ii) a At a wavelength λ3Under the illumination of the light source, the normalized fringe intensity distribution Img after eliminating the influence of background light is obtained3(ii) a The included angle between the polarizer 2 and the analyzer 8 is 45 DEG at the wavelength lambda2Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained4At a wavelength λ3Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained5(ii) a The polarizer 2 and the analyzer 8 are orthogonal and synchronously rotated by 45 DEG at the wavelength lambda2Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained6At a wavelength λ3Under the illumination of the light source, the fringe intensity distribution Img after eliminating the influence of the background light is obtained7；
Step two: acquiring image stripe information obtained in the first step, and solving a main stress difference sigma (x, y) of the sample 5 to be detected by using Fourier transform, low-pass filtering and phase unwrapping techniques;
step three: and (4) acquiring the image stripe information obtained in the first step, and solving the main stress direction theta of the sample 5 to be tested by using a phase unwrapping technique.
In the second step, the main stress difference sigma (x, y) of the sample 5 to be measured is solved by using Fourier transform, low-pass filtering and phase unwrapping technologies, and the method specifically comprises the following steps:
step1, under an illumination light source with three wavelengths, the normalized fringe intensity distribution after eliminating the influence of background light is respectively as follows:
wherein c is the stress optical coefficient of the material and is a known constant, and t is the thickness of the sample 5 to be measured and is a measurable constant;
step2. for Img respectively1、Img2、Img3Using the formula of multiple angles to obtain I1(x,y)、I2(x,y)、I3(x,y)：
The normalized stripe intensity distribution Img after the background light influence is eliminated, which is obtained in the step one1The square of the sine function with the principal stress difference σ (x, y) as the argument cannot be applied directly to Img1The real main stress difference distribution is obtained through root-opening processing, so a multiple angle formula is used to obtain a cosine function with the main stress difference sigma (x, y) as an independent variable: i is1(x, y) in this case, the two-wavelength method by introducing Img2To obtain I2(x, y) to determine I1(x, y) and I2(x, y) relative size at the same stripe order, to obtain arccos [ I2(x,y)]The main stress difference is obtained by phase unwrapping technique, but as the fringe order increases, I1(x, y) and I2The relative size of (x, y) does not sufficiently guarantee arccos [ I ]2(x,y)]Is monotonically increasing or monotonically decreasing, as shown by a in FIG. 5, at [280, 310]Within the interval, I2(x,y)>I1(x, y) but arccos [ I2(x,y)]Instead of decreasing monotonically, the area marked in the rectangular box increases monotonically (see b in fig. 5), causing this area that otherwise does not need to be flipped (see c in fig. 5) to also flip, resulting in a downward concave curve segment, i.e., an error zone, which increases with the increase in the fringe order and cannot be corrected from the principle of measuring stress by the two-wavelength method. In order to solve the error and accurately obtain the main stress difference, the invention applies two wavelengths which are lambda respectively in a dual-wavelength method1And λ2On the basis of the illumination light source, an additional introduction of a wavelength of lambda3And let the wavelengths λ of the three illumination light sources1、λ2、λ3Successively smaller, i.e. λ1>λ2>λ3To 1, pair1(x,y)、I2(x,y)、I3(x, y) processing. The simulation result of stress calculation by the photoelastic stress measurement method based on the three-wavelength illumination light source provided by the invention is shown in fig. 6, wherein a in fig. 6 is I1(x,y)、I2(x,y)、I3(x, y) simulation diagram, b in FIG. 6 is arccos [ I ]1(x,y)]、arccos[I2(x,y)]、arccos[I3(x,y)]C in fig. 6 is the phase distribution simulation result of the unwrapped phase distribution which can reflect the main stress difference distribution, and it can be found that c in fig. 6 does not have the concave curve segment, i.e. the error interval, similar to c in fig. 5. By adopting the photoelastic stress measurement method based on the three-wavelength illumination light source, inevitable errors in a dual-wavelength method are obviously eliminated. Further, continue to I1(x,y)、I2(x,y)、I3(x, y) processing;
step3. mixing I2(x, y) are each independently of I1(x, y) and I3(x, y) the comparisons are made at the same stripe order:
for I2(x, y) and I1(x, y), the same stripe order needs to be satisfiedNamely, it isIf I1(x,y)>I2(x, y) then I2(x, y) can be regarded as [0, π/2 ]]Internal decreasing function, its inverse arccos [ I2(x,y)]Can be regarded as [0, pi/2 ]]Is increased function ofIf I1(x,y)<I2(x, y) then I2(x, y) can be regarded as [ pi/2, pi]Increasing function of inner, its inverse function arccos [ I2(x,y)]Can be regarded as [ pi/2, pi]Is a decreasing function ofAfter recording treatmentThe fringe image is { arccos [ I ]2(x,y)]}21As shown in fig. 7 a;
for I2(x, y) and I3(x, y), the same stripe order needs to be satisfiedNamely, it isIf I2(x,y)>I3(x, y) then I2(x, y) can be regarded as [0, π/2 ]]Internal decreasing function, its inverse arccos [ I2(x,y)]Can be regarded as [0, pi/2 ]]An increasing function of the inside, anIf I2(x,y)<I3(x, y) then I2(x, y) can be regarded as [ pi/2, pi]Increasing function of inner, its inverse function arccos [ I2(x,y)]Can be regarded as [ pi/2, pi]Within a decreasing function, anAfter recording treatmentThe fringe image is { arccos [ I ]2(x,y)]}23B in fig. 7;
thus by separately judging I2(x, y) and I1(x, y) and I2(x, y) and I3(x, y) the relationship of the sizes of the same stripe order, arccos [ I ] can be obtained2(x,y)]Coarse range of increase and decrease, thereby decreasing arccos [ I ]2(x,y)]The increasing and decreasing turning parts are preliminarily distinguished to respectively obtain two images { arccos [ I ] with jumping discontinuities and still containing partial turning parts2(x,y)]}21And { arccos [ I ]2(x,y)]}23(see FIG. 7);
step4, { arccos [ I ] obtained by Step32(x,y)]}21And { arccos [ I ]2(x,y)]}23Calculating the error curve interval, and assigning the value in the error interval as 0 to obtain the function { arccos [ I [ ]2(x,y)]}230；
Due to three wavelengths lambda1、λ2、λ3With a certain difference, I2(x, y) are each independently of I1(x,y)、I3(x, y) obtained by comparison with respect to I2The increasing part of the curve (x, y) includes the decreasing partA small curve segment with respect to I2The part of the curve with decreasing (x, y) comprises the tiny curve segments with increasing (x, y) and simultaneously satisfies I corresponding to the same stripe level1(x,y)<I2(x, y) and I2(x,y)>I3The curve segment of (x, y) and the same stripe order satisfy I simultaneously1(x,y)>I2(x, y) and I2(x,y)<I3(x, y), which is unavoidable, and these errors are larger as the fringe order increases;
let { arccos [ I ]2(x,y)]}21The error curve segment of the ith-level stripe is [ Ei]21，{arccos[I2(x,y)]}23The error curve segment of the ith-level stripe is [ Ei]23I is 1,2,3 …, byCan determine the position of these error curve segments: { arccos [ I2(x,y)]}21And { arccos [ I ]2(x,y)]}23In satisfyIs error-free and satisfiesIs erroneous, so that [ Ei ] can be selected]21And [ Ei]23I is 1,2,3 …, where only [ Ei ] needs to be screened out]21Or [ Ei]23. To screen [ Ei]23For example, the value in the error interval is set to 0, and the obtained fringe image is recorded as { arccos [ I [ ]2(x,y)]}230The result is shown in FIG. 8, where the positions of a series of line segments parallel to the x-axis represent the error curve segment [ Ei ]]23I is the position of 1,2,3 …;
step5. pair { arccos [ I ]2(x,y)]}230Fourier transform is carried out to obtain the spectrum distribution of the spectrum, a proper passband is selected for low-pass filtering, and the filtering result is converted into space domain information by inverse Fourier transform and is marked as { arccos [ I ]2(x,y)]}230′；
For { arccos [ I ]2(x,y)]}230Fourier transform is carried out to obtain a in figure 9, a proper passband is selected according to a measuring object, low-pass filtering is carried out on a in figure 9, and inverse Fourier transform is carried out on the filtering result to obtain a space domain image which is marked as { arccos [ I ]2(x,y)]}230' as shown in b of fig. 9, wherein all the descending curve segments are processed error curve segments, and the straight line parallel to the x-axis is the reference line in the figure. The intersection point of the ith descending curve segment b in FIG. 9 and the reference line is the error interval [ Ei]23(b in fig. 9 marks the middle of the partial error interval with an arrow). From b in FIG. 9, error interval [ Ei ]]23The curve value of the part on the left side of the middle point is larger than zero, the part needs to be turned, the curve value of the part on the right side of the middle point is smaller than zero, and the part does not need to be turned;
step6. by judging { arccos [ I ]2(x,y)]}230' Positive and negative of the segment value of the medium error curve, constructing a reversal judgment matrix, marked as L (x, y), for judging { arccos [ I [ ]2(x,y)]}23Whether the curve in each interval needs to be inverted.
B error interval [ Ei ] in FIG. 9]23The curve value of the left part of the midpoint is larger than zero, the part originally needs to be overturned, but cannot be overturned when the dual-wavelength method is adopted for judgment, so that overturning correction is needed, and an error interval [ Ei ]]23The value of the curve in the portion to the right of the midpoint is less than zero and the portion does not need to be flipped. If { arccos [ I ]2(x,y)]}230If the value of the middle error curve segment is positive, let the value of the corresponding position in the L (x, y) matrix be-1, and the remaining position values be 1, then L (x, y) is obtained as shown in fig. 10. Mixing L (x, y) with { arccos [ I ]2(x,y)]}23Multiplying by the error interval [ Ei]23The left part of the midpoint is flipped and the right part remains unchanged, so that { arccos [ I2(x,y)]}23Corrected error of (2) to obtain an image { arccos [ I ] having only jumping discontinuities2(x,y)]}23', as shown in FIG. 11;
step7. pair { arccos [ I ]2(x,y)]}23' phase unwrapping Using phase unwrapping AlgorithmWrapping, splicing to obtain continuous stripe distribution, and recording as { arccos [ I ]2(x,y)]}23FI.e., [ arccos (1-2 Img)2)]23F(see fig. 12), the magnitude of the primary stress difference σ (x, y) of the sample 5 to be measured satisfies the following relation:
thirdly, solving the main stress direction theta of the sample 5 to be tested by using a phase unwrapping technology, wherein the method comprises the following specific steps:
wherein, I20、I30Respectively represent the wavelength lambda2And wavelength lambda3The intensity of the illumination when illuminated. Therefore, it is not only easy to useTherefore, it isAnd (5) processing the theta by adopting a phase unwrapping algorithm to obtain the actual stress direction of the sample to be measured.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.