CN100427930C - Method for extracting order value of coherent gradient sensing interference fringe - Google Patents

Abstract

The present invention relates to a method for extracting coherent gradient sensitive interference fringe level numbers, which belongs to the technical field of fracture mechanics parameter measurements. Firstly, before a test piece is loaded, the test piece is shot to obtain the background light intensity a of an interference fringe image; two coherent gradient sensitive interference fringe images are respectively shot under the condition that the test piece loads P1 and P2; the lighting intensity of the first interference fringe image and the lighting intensity of the second interference fringe image are respectively I1 and I2 obtained by shooting; and the fringe level number m 1 of the first interference fringe image is calculated according to the load and the light intensity. The method has the advantages that calculation accuracy of a coherent gradient sensitive method, particularly the calculation result of fringe points of low level numbers can be obviously enhanced; the method is simple and does not need optical devices beyond the prior art; and the method can carry out corresponding processing with fringe images on any other load in homogeneous fracture experiments to extract the fringe level numbers.

Description

A kind of method of extracting order value of coherent gradient sensing interference fringe

Technical field

The present invention relates to a kind of method of extracting order value of coherent gradient sensing interference fringe, belong to the fracture mechanics parameter technical field of measuring.

Background technology

Coherent gradient sensing method (Coherent Gradient Sensing, hereinafter to be referred as CGS) be a kind of horizontal double grid shear interference of noncontact, whole audience optical measurement method with online spatial filtering effect, thereby it utilizes the reorganization of two parallel gratings to produce interference fringe by the distortion light beam that test piece deformation causes, and sets up relation between in-plane stress gradient or acoplanarity displacement gradient and the fringe order by the optical interference governing equation.This method can be respectively applied for the deformation information of the transparent and nontransparent object of research under transmission and reflection case, its striped distribution has directly reflected in-plane stress gradient fields under the transmission situation and the acoplanarity displacement gradient fields under the reflection case.Because this method counter stress or displacement gradient sensitivity are so be particularly suitable for studying the breakage problem with high stress concentrations phenomenon.The coherent gradient sensing method is divided into transmission-type and reflective two kinds, Figure 1 shows that typical coherent gradient sensing interference fringe picture, and black streaking is represented half progression striped in the actual photographed image, and outmost turns is 0.5 grade of striped.

The weak point of above-mentioned prior art is, similar with other optical measurement method of handling bar graph, obtain fracture mechanics parameter and must extract order value of coherent gradient sensing interference fringe earlier, and will determine that order value of coherent gradient sensing interference fringe just must accurately extract stripe centerline.Interference fringe is thicker as can be seen from Figure 1, and out-of-shape, is difficult to satisfy accuracy requirement iff relying on image process method to extract stripe centerline, even can brings very big error, especially for the harmonic series striped.

Summary of the invention

The objective of the invention is to propose a kind of method of extracting order value of coherent gradient sensing interference fringe, improve the computational accuracy of coherent gradient sensing method.

The method of the extraction order value of coherent gradient sensing interference fringe that the present invention proposes comprises following each step:

(1) before not loading, test specimen is taken, obtained the background light intensity a of interference fringe picture;

(2) utilize existing coherent gradient sensing method light path to take above-mentioned test specimen respectively in load p ₁, P ₂Under two width of cloth coherent gradient sensing interference fringe pictures, take first width of cloth obtain and the light intensity of second width of cloth interference fringe picture and be respectively: I ₁, I ₂

(3), calculate the fringe order m of first width of cloth interference fringe picture according to above-mentioned load and light intensity ₁For: $\frac{I_1-a}{I_2-a}=\frac{\cos \left(2\mathrm{\π}{m}_1\right)}{\cos \left(2\mathrm{\π}{m}_1\frac{P_2}{P_1}\right)},$ Wherein a is the background light intensity of the above-mentioned interference fringe picture of test specimen being taken before loading.

The method of the extraction order value of coherent gradient sensing interference fringe that the present invention proposes, its advantage is: the fringe order that can accurately extract coherent gradient sensing interference fringe picture optional position point, experimental result shows that this method can significantly reduce because striped extracts the error of bringing, thereby improves the precision of coherent gradient sensing method.Concrete advantage is: a) can obviously improve the computational accuracy of coherent gradient sensing method, especially for the result of calculation of harmonic series striped point; B) method is simple, does not need prior art optical devices in addition; C) can carry out respective handling with the bar graph of other any load in the homogeneous experiment and extract fringe order.

Description of drawings

Fig. 1 is existing typical coherent gradient sensing interference fringe picture, and wherein (a) is theoretical modeling figure; (b) be the actual photographed bar graph.

Fig. 2 is the index path that produces the interference fringe of the inventive method.

Fig. 3 is the schematic diagram that produces the interference fringe of the inventive method.

Fig. 4 adopts the inventive method, the crack tip transmission coherent gradient sensing interference fringe picture that on same test specimen, produces during different loads, and wherein (a) is load p=40N, (b) is load p=80N.

Fig. 5 is the stress intensity factor that calculates of a inventive method and prior art comparison diagram as a result, and the longitudinal axis is the relative error of experimental calculation value and theoretical value, and solid line is represented the result of calculation of prior art, and dotted line is represented the result of calculation of the inventive method.Wherein (a) is the result of calculation comparison diagram between interference fringe picture-40 °～-30 °, (b) is the result of calculation comparison diagram between interference fringe Figure 30 °～40 °.

Among Fig. 2 and Fig. 3, the 1st, laser instrument, the 2nd, beam expanding lens, the 3rd, convex lens, the 4th, test specimen, 5 is first gratings, 6 is second gratings, the 7th, filter lens, the 8th, diaphragm, the 9th, video camera, the 10th, computing machine.

Embodiment

The method of the extraction order value of coherent gradient sensing interference fringe that the present invention proposes at first before not loading, can be utilized existing coherent gradient sensing method light path as shown in Figure 2, and test specimen is taken, and obtains the background light intensity a of interference fringe picture; Utilize existing coherent gradient sensing method light path to take this test specimen respectively in load p ₁, P ₂Under two width of cloth coherent gradient sensing interference fringe pictures, take first width of cloth obtain and the light intensity of second width of cloth interference fringe picture and be respectively: I ₁, I ₂According to above-mentioned load and light intensity, calculate the fringe order m of first width of cloth interference fringe picture then ₁For: $\frac{I_1-a}{I_2-a}=\frac{\cos \left(2\mathrm{\π}{m}_1\right)}{\cos \left(2\mathrm{\π}{m}_1\frac{P_2}{P_1}\right)},$ Wherein a is the background light intensity of the above-mentioned interference fringe picture of test specimen being taken before loading.

Below introduce the principle of the inventive method in detail:

The light path of the coherent gradient sensing method of the inventive method institute foundation is as shown in Figure 2: a branch of coherent laser beam forms directional lights through beam expanding lens 2 backs that have the filtering hole by optical lenses 3, parallel beam is vertically by being with the test specimen 4 of crackle, be subjected to the influence of test piece deformation and stress field, the direction of parallel beam and phase place will change.Though the light beam by the test specimen outgoing no longer is a parallel beam, because deviation in driction is very little, for simplicity, the light beam that still will have test piece deformation information is considered as parallel beam.By (x, y) (x, y), similarly the light by (x, y+ ε) some outgoing on the test specimen has optical path difference δ S (x, y+ ε) owing to test piece deformation to the light of some outgoing because test piece deformation has optical path difference δ S on the test specimen.As shown in Figure 3, the high density Ronchi grating 5 and 6 (pitch is all p mutually) that these parallel beams are parallel to each other through two after the test specimen outgoing, spacing is Δ, produce diffraction, if behind diaphragm, place video camera 9, can photograph the interference fringe that causes optical path difference to cause owing to test piece deformation, and handle by 10 pairs of stripeds of computing machine and can extract relevant mechanics parameter.

If transmission coherent gradient sensing method, optical path difference δ S (x, y) specimen thickness that causes of test specimen variations in refractive index that is mainly caused by stress changes and poisson effect changes two factors and produces, so the photodynamics governing equation of transmission coherent gradient sensing method is:

$\mathrm{cd}\frac{\∂({\widetilde{\mathrm{\σ}}}_x+{\widetilde{\mathrm{\σ}}}_y)}{\∂x}=\frac{\mathrm{mp}}{\mathrm{\Δ}}---\left(1\right)$

Wherein m is a fringe order, and p is a raster pitch, and Δ is two spacings between the grating, and c is the optical constant of material, and d is a specimen thickness,

With

Represent the mean stress component on the specimen thickness direction.

Photodynamics governing equation (1) shows when grating principal direction is parallel to the x axle, and the physical significance of coherent gradient sensing striped representative is a test specimen principle stress and to the isoline of x direction gradient under the transmission situation.

For reflection coherent gradient sensing method, optical path difference δ S (x, y) specimen thickness that is only caused by poisson effect changes generation, so the photodynamics governing equation of reflection coherent gradient sensing method is:

$\frac{\&PartialD;w}{\&PartialD;x}=\frac{\mathrm{<figure-callout\; id="2"\; label="mp"\; filenames="C20061000136500072.png"\; state="\{\{state\}\}">mp</figure-callout>}}{2\mathrm{\&Delta;}}---\left(2\right)$

Wherein m is a fringe order, and p is a raster pitch, and Δ is two spacings between the grating, and w represents the acoplanarity displacement of test specimen.

Photodynamics governing equation (2) shows that when grating principal direction is parallel to the x axle physical significance of coherent gradient sensing interference fringe representative is the isoline of test specimen acoplanarity displacement to the x direction gradient under reflection case.

The method of the extraction order value of coherent gradient sensing interference fringe that the present invention proposes is as follows:

Under the linear elasticity situation, the test specimen stress field

Or acoplanarity displacement field w is directly proportional with external applied load P.According to the photodynamics governing equation (1) and (2) of coherent gradient sensing method, test specimen stress field

Or the gradient of acoplanarity displacement field w is directly proportional with interference fringe progression m, so external applied load P also is directly proportional with coherent gradient sensing method interference fringe progression m:

$\frac{P_1}{{\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="C20061000136500072.png"\; state="\{\{state\}\}">P</figure-callout>}}_2}=\frac{m_1}{m_2}---\left(3\right)$

The light distribution of coherent gradient sensing method interference fringe picture is:

I＝a+bcos2πm (4)

Wherein a represents the background light intensity, and bar graph changes according to cosine rule bcos2 π m.After taking interference fringe picture with CCD and being converted to digital signal, light intensity I is directly proportional with the gray-scale value of digital picture, and light intensity all is to use the gray-scale value representative of digital picture in follow-up analysis.

If take two width of cloth coherent gradient sensing interference fringe pictures under the same test specimen different loads, convolution (3) can obtain twice the load p and the light intensity I and first width of cloth interference fringe picture fringe order m of two width of cloth interference fringe pictures with (4) ₁Between relation:

$\frac{I_1-a}{I_2-a}=\frac{\cos \left(2\mathrm{\&pi;}{m}_1\right)}{\cos \left(2\mathrm{\&pi;}{m}_1\frac{P_2}{P_1}\right)}---\left(5\right)$

If load p when taking two width of cloth bar graphs and two width of cloth interference fringe picture light distribution I are known, just can obtain the accurate fringe order m of any point on first width of cloth bar graph according to (5) formula ₁, shot digital images did not obtain when wherein background light intensity a loaded by test specimen.

With the experiment of transmission coherent gradient sensing method is that the effect that example is extracted fringe order to the load method is tested.Figure 4 shows that same test specimen load is respectively P ₁=40N and P ₂The coherent gradient sensing interference fringe picture of taking during=80N, at this moment according to (5) Shi Kede:

$\frac{I_1-a}{I_2-a}=\frac{\cos \left(2\mathrm{\&pi;}{m}_1\right)}{\cos \left(4\mathrm{\&pi;}{m}_1\right)}---\left(6\right)$

According to the light distribution of two width of cloth interference fringe images among Fig. 4, (6) formula of utilization can obtain P ₁Any fringe order of any in bar graph during=40N.In order to compare with direct result by Flame Image Process extraction stripe centerline, Fig. 5 has shown the error condition that adopts load method front and back.Can see that by Fig. 5 the method for extracting stripe centerline with direct employing Flame Image Process unit load method has mutually significantly reduced striped and extracts the error of bringing, and has improved the computational accuracy of coherent gradient sensing method.The load method is especially obvious for the harmonic series stripe effect, and this is less in load, has great importance in the time of must adopting harmonic series striped calculating stress strength factor.

Claims (1)

1, a kind of method of extracting order value of coherent gradient sensing interference fringe is characterized in that this method comprises following each step:

(1) before not loading, test specimen is taken, obtained the background light intensity a of interference fringe picture;

(2) utilize existing coherent gradient sensing method light path to take above-mentioned test specimen respectively in load p ₁, P ₂Under two width of cloth coherent gradient sensing interference fringe pictures, take first width of cloth obtain and the light intensity of second width of cloth interference fringe picture and be respectively: I ₁, I ₂

(3), calculate the fringe order m of first width of cloth interference fringe picture according to above-mentioned load and light intensity ₁For: $\frac{I_1-a}{I_2-a}=\frac{\cos \left({2\mathrm{\&pi;m}}_1\right)}{\cos \left({2\mathrm{\&pi;m}}_1\frac{P_2}{P_1}\right)}.$

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