CA1255795A - Apparatus and method for determining stress and strain in pipes, pressure vessels, structural members and other deformable bodies - Google Patents
Apparatus and method for determining stress and strain in pipes, pressure vessels, structural members and other deformable bodiesInfo
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- CA1255795A CA1255795A CA000509846A CA509846A CA1255795A CA 1255795 A CA1255795 A CA 1255795A CA 000509846 A CA000509846 A CA 000509846A CA 509846 A CA509846 A CA 509846A CA 1255795 A CA1255795 A CA 1255795A
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Abstract
ABSTRACT OF THE DISCLOSURE
A method and apparatus for measuring stress and strain associated with a pipe, pressurized vessel, structural member or deformable body contain-ing a flaw or stress concentration utilizes a laser beam to illuminate a surface being analyzed and an optical data digitizer to sense a signal provided by a speckle pattern produced by the light beam reflected from the illuminated surface. One signal is received from the surface in a reference condition and subsequent signals are received from the surface after surface deformation. The optical data digitizer provides the received signal to an image processor, and the processor stores the signals and correlates the deformed image received with the reference image and then sends this correlated information to a minicomputer which performs mathematical analyses of the signal to determine stress and strain associated with the surface. The apparatus is constructed as one integral unit, and further includes a digital and tape display, as well as a television monitor and an electro-optic range indicator.
A method and apparatus for measuring stress and strain associated with a pipe, pressurized vessel, structural member or deformable body contain-ing a flaw or stress concentration utilizes a laser beam to illuminate a surface being analyzed and an optical data digitizer to sense a signal provided by a speckle pattern produced by the light beam reflected from the illuminated surface. One signal is received from the surface in a reference condition and subsequent signals are received from the surface after surface deformation. The optical data digitizer provides the received signal to an image processor, and the processor stores the signals and correlates the deformed image received with the reference image and then sends this correlated information to a minicomputer which performs mathematical analyses of the signal to determine stress and strain associated with the surface. The apparatus is constructed as one integral unit, and further includes a digital and tape display, as well as a television monitor and an electro-optic range indicator.
Description
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The present invention relates to a method and apparatus for deter-mining the deformation which occurs on a surface of an object as a result of stressing the object, and more particularly pertains to the invention of a rapid non-destructive, non-contacting device for determining strain and stress in developmental or operating systems, such device utili~ing the prlnciple of laser speckle interferometry or an extension of laser speckle interferometry as the device has these alternative capabilities and both are im~ovations in this invention. The extension of laser speckle interferometry is Ltself an entirely new concept that is not inter~erometric in nature but 1() rather a recordi~g, dlgiti~ing, correlation and analysis of laser speckle patterns to yield strain and stress. The new concept, althaugh not inter-ferometric in nature, is an extension of the laser speckle interferometric technique in that the same mathematical basis for analysis is employed.
Laser ~peckLe interferometry is the most recent advance in coherent optics used in engineering applicatinns to measure stresses and strains in bodies, and shows promise of alleviating many difficult problems in experi-mental mechanics. The basic method utilizes simple high-resolution photo-graphs of a surface which is illuminated by coherent light~ The result is a real time or permanently stored whole-field record through interference ~() fril~es of a deformed surface. This record yields a map of displacements In the object. It has been shown that if two identical speckle patterns are superimposed on a photographic plate tran~lated laterally by a short distance between exposures, then the diffraction halo generated by the processed plate will consist of a pattern of parallel straight fringes. The diffraction halo observed through a small area of the recorded image will correspond to the local displacement at the corresponding point on the object, and the direction of the fringes will be orthogonal to the direction of the local displacement vector.
Add~tionally, by optically illuminating the developed photographic plate with a converging spherical wave, the entire surface can be analy~ed at one time to 3~ deterluine the dlsplacement field of the surface.
~2~ 5 In additlon to the above, another technique utili~es the laser speckle effect for measuring either normal or in-plane components of dis-placement over an elltire surface a~ one time. Discussion of in-plane measurements follow to illustrate the general approach. For measurement of the in-plane components of displacement, a surface is illuminated by two beams of coheren~ laser light, symmetrically disposed about the normal to the surface. These two speckle patterns are superimposed and their re~ultant speckle pattern is recorded on film. The intensity distributLon nf the resultant speckle pattern depends on the relative phase of the com-ponent patterns. Then one or both speckle patterns is changed and again,the resultant speckle pattern is recorded on the same photographic film.
By measuring correlation between the resultant pattern at two different times, a change of relative phase is detected, which in turn gives a measure of ~urface displacements. These correlation fringes are observed either in real-time or by combining two transparencies having resultant speckle patterns at two different times and illuminating the pattern in a Fourier filter system.
A major drawback of this technique is that the path length difference between the two iLluminating beams has to be less than the coherent length of the ~ight used to generate correlation fringes.
The present invention relates to a method and apparatus for deter-mining the deformation which occurs on a surface of an object as a result of stressing the object, and more particularly pertains to the invention of a rapid non-destructive, non-contacting device for determining strain and stress in developmental or operating systems, such device utili~ing the prlnciple of laser speckle interferometry or an extension of laser speckle interferometry as the device has these alternative capabilities and both are im~ovations in this invention. The extension of laser speckle interferometry is Ltself an entirely new concept that is not inter~erometric in nature but 1() rather a recordi~g, dlgiti~ing, correlation and analysis of laser speckle patterns to yield strain and stress. The new concept, althaugh not inter-ferometric in nature, is an extension of the laser speckle interferometric technique in that the same mathematical basis for analysis is employed.
Laser ~peckLe interferometry is the most recent advance in coherent optics used in engineering applicatinns to measure stresses and strains in bodies, and shows promise of alleviating many difficult problems in experi-mental mechanics. The basic method utilizes simple high-resolution photo-graphs of a surface which is illuminated by coherent light~ The result is a real time or permanently stored whole-field record through interference ~() fril~es of a deformed surface. This record yields a map of displacements In the object. It has been shown that if two identical speckle patterns are superimposed on a photographic plate tran~lated laterally by a short distance between exposures, then the diffraction halo generated by the processed plate will consist of a pattern of parallel straight fringes. The diffraction halo observed through a small area of the recorded image will correspond to the local displacement at the corresponding point on the object, and the direction of the fringes will be orthogonal to the direction of the local displacement vector.
Add~tionally, by optically illuminating the developed photographic plate with a converging spherical wave, the entire surface can be analy~ed at one time to 3~ deterluine the dlsplacement field of the surface.
~2~ 5 In additlon to the above, another technique utili~es the laser speckle effect for measuring either normal or in-plane components of dis-placement over an elltire surface a~ one time. Discussion of in-plane measurements follow to illustrate the general approach. For measurement of the in-plane components of displacement, a surface is illuminated by two beams of coheren~ laser light, symmetrically disposed about the normal to the surface. These two speckle patterns are superimposed and their re~ultant speckle pattern is recorded on film. The intensity distributLon nf the resultant speckle pattern depends on the relative phase of the com-ponent patterns. Then one or both speckle patterns is changed and again,the resultant speckle pattern is recorded on the same photographic film.
By measuring correlation between the resultant pattern at two different times, a change of relative phase is detected, which in turn gives a measure of ~urface displacements. These correlation fringes are observed either in real-time or by combining two transparencies having resultant speckle patterns at two different times and illuminating the pattern in a Fourier filter system.
A major drawback of this technique is that the path length difference between the two iLluminating beams has to be less than the coherent length of the ~ight used to generate correlation fringes.
2~ Two different variations of the dual beam approach for measuring ln-plane surface displacements exist. In the first method, the displacement is determined by photographing a coherently illuminated object through two laterally displaced apertures. The displacement is displayed as a pattern of ~oire' fringes over the image of the surface. Thus, there is no need for scanning of the beam on a point-by-point basis. As the surface is illumirated by only a single laser be~ , the implementation problems associated with the dual-beam techr~ ue (mechanical stability and equal path lengths between the various optical components) are minimized. In the second method, the object is illuminated using a single laser beam and photographed via a double exposure
3~ be~ore and after displacement~ The Fourier transforlD of the doubLy exposed 2S~7~
transparency ls obtained optically by illuminating the photographlc plate with a converging spherical wave. The main advantage of this procedure is that the whole-field displacement can be analyzed and, by appropriate po6ition of a set of apertures in the transform plane, any component of the displacement normal to the line of sight can be detected with variable sensitivity.
Speckle interferometry does have some limitations however, such as the fact that measurements are not as accurate as those made with strain gages. However, the inaccuracies usually associated with the laser speckle techn~que involve numerical error in calculating the derivatives, and not the ~llecrology of the laser speckle technique per se. Strain gage measurements with accuracies of 1~ can be obtained while strain calculations using the spe~kle data technique result in accuracies of approximately 5%. The accura-cies associated with the numerical analysis technique used with the speckle approach will be improved as a result of the computeri~ed approach employed by the present invention. Also, fringe analysis is time consu~ing in that the analyst must view the photographic plates, make the necessary measurements and then calculate the results. ~owever, this limitation can be overcome by u~ing an optical data digitizer system, including an image storage device and computer for data correlation and analysis, as proposed by the pre6ent ~1) lnvention.
Laser speckle interferometry is not dependent on the use of models and ls in fact applicable to full scale systems. ~any of the recent advances in coherent optics have suggested the engineering applications to prototype systems;
however, with all of the techniques developed thus far, the recording medium has been photographic film. Therefore, data analysis has required the use of a specialist for interpretation. Even with this limitation, ~hough, the great potential o~ coherent optics technl~lues for engineering analysis has been clearly demonstrated.
The laser speckle effect, which is the basis of ~he new concept of :10 this invention, prololses to be the optical technique whereby the photographic 579~
film can be eliminated ln the data acquistlon process. The ellminatlon of the photographlc film means that interference fringe patterns classlcally assocl-ciated with laser speckle interferometry need not be employed to yleld data.
These fringe patterns can be generated in the electronic imaging system of this invention and analyzed as in the classical case of interferometry and this capability is one claim of the invention. ~owever, the invention is cabnple o~ eliminating this step by introducing a laser speckle technlque based on the digltal correlation of successive laser speckle patterns beEore and after object surface deformation. Thus, laser speckle and dlgltal correlation, as the technique is proposed to be termed, has its fo~mdatlon in laser speckle interferometry and has all the attributes of laser speckle interferometry without the necessity of photographs and fringe measurements.
The laser speckle and digital correlation will be accomplished through the development of electronic video systems capable of high resolution of the speckle patterns. The compatibility of the speckle technique with image processing offers a user-oriented system for a wide range of engineering applications. Finally, it should be pointed out that there is commercially available an Electronic Speckle Pattern Interferometry (ESPI) device for time veraged holographic stress analysis, as reported in ~aterials Evaluation (May, 1979). However, this device, ~hile demonstrating the fact that speckle p.ltternS can be digitiæed~ does not include the use of a computer for dat~
analysis and management aud does not have the capability to by-pass the step of creating in~erference fringe patterns, as its sole purpose is to create these patterns. Furthermore, the ESPI device requires that the body under investigation be vibrated in order to generate the fringe patterns as opposed to illumination alone by a coherent light source.
The present invention9 provides for an apparatus and method for determining whole-field, regional or pointwise, stress and strain associated wlth pipes, pressure vessels, structural members and deformable bodies ~3~ (including both isoropic and anisotropic materials) and provides for a direct read out, non-destructive, non-contacting device to determine ~train and stress in pipes, pressure vessels or other bodies on a real-time basis.
The device is optical in nature ond can operate at some distance from the body under investigation, and thus, the device i8 insensitive to hostile environments. rhe present invention is based on the application of the speckle effect produced by illumlnalion of a diffuse surface by a coherent source of llght. ~aslcally, this effect is accomplished by illuminating a ~urface which is under examination through the use of a laser. The illuminated surface reflects the laser beam, an~l this reflected signal is recorded by an optical data digitizer which can record interference fringes or laser speckle patterns. If the surface e~hibits deformation or strain subsequent to a previous recording of the fringe pattern or laser speckle patterns, a new recording of fringe or speckle patterns will result in a measurement of object movements, i~e., a difference b~tween the original and subsequent patterns. The measured differences in these patterns can be mathematically related to the actual deformation and hence stress of the body being examined. The optical data digitizer i~ not only employed to directly receive the reflected light beam but to also send a signal representative of the reflected beam directly to a minicomputer for mathe~atical analysis~
An object of the present invention is to provide a method and apparatus for rapidly d~termining stress and strain relationships in a pressurized syste~, structural members or deformable bodies which effectively tnakes use of speckle interferometry or laser speckle and digital correlation, which eliminates the need for photographic recordings of surfacé displacemen~
data and an interferometrist to read and analyse the photographic data, which will give real-time indications of strain and stress directly and which will be applicable to nuclear pressure vessels, synfuel generation plants and any system or structure operating under pressure or static and 3~ dytallllc loading~ whlch can be effectuated at a safe stand off distance ;7~
from a hostilc environmellt of temperature, toxicity and radiation, or remotely within the environment, which is based on the development of the theory of fringe formation in laser speckle interferometry and the extension thereto referred to as laser speckle and digital correlation to include thermal transients, and which is based on the development of laboratory apparatus for electronically acquiring displacement data for a body undergoing deformation, and storing such data, which is non-destructive and requires no contact of the levice.
Figure 1 is a schematic illustration of the basic system forming the 1() present inventlon as used for measuring or monitoring stress and strain in a region of an operating system.
Figure 2 is an enlarged view of the data recording system forming the present invention as shown in Figure 1 and employing laser speckle inter~erontetry and/or laser speckle and data correlation.
Figure 3 schematically represents the apparatus and system of Figure 2 in a form whereby all of the components of the system are combined in one integrated unit.
Figure 4 schematically represents the system of data recording as utilized in laser speckle interferometry.
~() Figure 5 schematically represents the system of data analysls utllLzed in laser speckle interferometry.
Figure 6 is an expanded schematic illustration of the apparatus and method for determining stress and strain relationships in pressurized systems as employed in laser s~eckle interferometry.
Figure 7 is an i]lustration of a typical speckle photography fringe pattern obtained iu an optical data analysis system.
Figure 8 is a graphical illustration of the original data ob-talned from the average of a firæt scan line and a second scan line image prior to and subsequent to deformation.
`~ Figure '3 ls a graphical lllustration of the filtered data of the ~:~S5~
averaged scan lines of Figure 8.
Figure 10 is a block diagram illustrating the method associated with the apparatus of the present invention.
Figure 11 is a schematic illustration of the object coordinates ma~helnatically obtainable at a surface point, P.
Figure 12 is a schematic of the reference and deformed surface ~or correlation of laser speckle patterns.
Figure 13 ls the digitized optical surface resulting from the ~peckle pattern of the surface under investigation.
1~ ~igure 14 is a schematic o~ the reference and deformed subsets of the original and displaced surfaces Figure 15 is a schematic illustration of a region on the object under investigation containing a crack.
Reference is now made to the drawings and, in particular, to ~`igl1res 1 and 2 wherein there is schematically illustrated a system which may be employed to determine the stress and strain relationships forming the present invention and being generally designated by the reference numeral 1(). In this respectJ the stress and strain measuring system 10 can be used to measure the stress and strain associated with any type of pressurized 2~ vessel 12 through the projection and focusing of a light beam on a critical re~ion 14 assvciated with the vessel. However, the device is not limited to use with pressure vessels but a pressure vessel is used for illustrative purposes. As shown, the system 10 includes a laser light source 16 for projecting a llght beam 18 against a critical region 14, and an optical data digitizer 20 for receiving the light beam 22 being reflected from the critical region. In this regard, the optical data digitizer 20 might typically be a television camera, and the signal received by the digitizer may then be directed to a computer interface device 24 which serves to process the received signal and dlrect the same to a television monitor 26 a~ welL as to a minicomputer 2B.
3~) The television monitor 26 is effectively a higll-resolution monltor and provides a graphlcal data display. On the other hand, the mini-computer 28 serves to take the same signal as provided to the television monitor 26 and further enhance the data provlded, thereby to display the results on a dlsplay unit 30. In this connection, the mlnicomputer 28 can provide a display.
Additionally, an electro-optic or other range indicating device 23 is employed to indicate ~he distance between the digitizer 20 anc1 the critical re~on 14.
Figure 3 has been provided solely for the purpose of illustrating the various components sho~n in Figure 2 in a systelD whereby the same are 1~ operatively and functionally combined into one integral unit 34. In this regard, it can be seen that the integral unit 34 could be positioned external to a nuclear radiation or thermal ~one 32 and would include a laser light source 36 fixedly secured thereto and being utili~able to direct the light b~am 18 against the critical region 14 in the manner aforedescribed. The reflected light beam 22 then would be directed to an optical data digitizer 38, also integrally a part of the unit 34, and a television monitor 40 inte-grnlly a part of the unit 34 could then be used to view the received data.
An e1ectro-optic range indicating device 35 would perform the distance measuring function, while the unit 34 would perform the computing function ~0 so n~ to provide a data read out on the digital and tape display 42 associated therewith.
Laser Interfero!lletry A better understanding of the technique of using laser interferometry can be ascertained with reference to Figure 4. Typically, laser interferometry can be used to analy~e a structure 12 through the use of a laser light source 16 having an expandlng lens whereby the light beam 1~ directed against a critical area 14 expands outwardly from the l~gllt source. A camera 44 may then be employed to image the light 22 reflected from the critical surface 14 and then to record the surface in 3~ two configurations. The two configurations are a representation of the 7~;
deformatioll of the structure 12 resulting Erom an applied loa~l or thermal e~fect. A photographic plate 48 withln the camera 44 records an im~ge 46, which is separately illustrated in Figure 4, such image including a surface point on the body 12 at position P representative of the first exposure. The same point is imaged at point P' after deformation, and of course, lf there is no deformation, the point P' will be imaged at P during the second exposure.
Once a film record 48 is o.ade and is complete, it must be processed nnd ana1ysed to determllle the magnitude of deformation of the area 14 photo-~raplltd. Convelltionally, as il1ustrated in Figure 5, each developed plate 48 is illuminated by a laser 50 for the purpose of performing an analysis of the provided data. This method of data analysis, known a~ pointwise filtering, projects a set of parallel interference fringes 52 on a viewing screen 54. In this respect, the laser 50 is provided with all expanding lens 56 so as to project a light beam 58 through the film plate 48 to thereby project che elllarged image 52 onto the viewing screen 54. The distance 60 between re~pective fringes on the viewing screen 54 is proportional to the displacement of point P, and there is a mathematical correlation of the fringe pattern 52 and displacement, and hence strain/stress. Thus, the analyst must view the pro~jected image (such as illustrated in Figure 7) of each plate, make measure-mellts ~1nd then use these measurements as input to a mathematical procedure to yiel<l strain and then stress relative to the original point P.
Present Automated ~`ilm Analysis Techl1ique for Laser Speckle Interferometric System To reduce the complexity and time consumption associated with data analysis as performed in the manner illustrated in Figure 5, a present laser speckle interferometric system makes use of the data analysis system 62 illustrated in Figure 6. Specifically, the film 48 having the double exposure thereon including the points P and P', is positionable between a pair of mirrors 64, 66. A laser 68 directs a beam of light 70 against the reflective surface of the first mirror 66 whereby it is reflected up through the film ~2~57~;
plate 48 against the reflective surface of the second mirror 6~ The image 52 whlch is projected from the second mirror 64 is then displayed on a viewing screen 5~ in a manner similar to that illustrated in Flgure 5. A television camera 72 then views the image 52 and directs a picture of the same to a video computer interface 74. A minicomputer 76 having a conventional terminal 78 may then be used to analyze the image 52 as provided to the video computer interface 74. Specifically, the minicomputer 76 can be utilized to perform a conventional mathematical analysis which would normally be computed by hand so as to substantially reduce the time consumption and complexity of such nnalysis. The analysis which would be performed by the minicomputer 76 can be understood by reference to Figure 11 of the drawings ~herein the object coordinates at a surface point P are illustrated, i.e.~ the dlsplacement of the surface of an object is completely described by a vector ~ (x,y,~,t) Recognizing that the coordinates x,y,z are restricted to the surface of the body shown in Figure 11, the tangential components ~ and the angle ~
are the data that are recorded and analyzed by the laser speckle photography.
Thus, at each prescribed surface point P, the tangential components of the dlsplacement vector U can be stored in the memory of the minicomputer 76.
~0 These data ehen can be used to numerically calculate the stresses and strains as desired.
Specifically, a vector gradient Uij ~ ~-U may be separated into symmetric and skew symmetric parts, where V~-U- aaU~
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is the symmetric part, and ij = 2 ( ~Ui + ~UJ) ~ii ~ 2 ( a~i _ a~J) is the skew symmetric part. When the values of ij 1 wij are small as compared to unity, then the symmetric part ij is the strain matrix and~ij is the rotatic matrLc. The quantity of the primary ~atrix is the strain matrix ~,;j which is calculated from the experimental data.
For elastic deterMination of an isotropic material, the stress/strain relations are ~ii = Ç [ 6ij ~ j) Sij] + ~c aT
where E is the modulus of elasticity, ~ is Poissons ratio, and o~ is the coef~icient of thermal expansionO Generally, the surface point P is located on the free surface of a body, and therefore, additional relationships are obtained from the boundary conditions and the known mechanical loading on the sur~ace In this connection~ T~ =ni 6 j~ may be calculated, wh~re the surface traction T~ is known, and in most cases ~ . This implies that 6 ~ j - O , and the result now becomes Uy~ = -U~x ~U _ ~ ~ (aux ~ au"
x ay ~s~
Thus, with the measured value of the laser speckle and the known mechAnical loading, the surEace stress and strain components can be calculated at any described surface polnt P by the minicomputer 76. The expressions or the strain components are = ~Ux ~Jy ~ ( 3y ~ ~x ) xy ~ !2 (~UX ~ auy x~ ~ y~ ~ O
Similarly, the components of the rotation vector ~ are oy, = W;!y _ ~Ui~
~)y = Wx~
~ Jyx - 2 ( U y _ UX ) Th~ stress components ~ij can now be determined from the strain components throu~h the stress/strain relationships as stated previously.
With respect to the data transmitted from the video computer inter-face 74 to the minicomputer 76, it should be noted that the typical interfer-ence pattern 80> as shown in Figure 7, will be converted to a data analysis record 82, as shown in Figure 8, by the video computer interface. Effectively then, the data analysis record 82 having fringe spacing 84 will be filtered digitally (numerically within the computer) so as to produce the result 86 shown in Figure 9 and to provide the same to a storage display screen 88.
Tllis process signal 86 is then a measure of the fringe spacing 84 as shown in Figure 8.
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Also lllustrated ln the data analysls system 62 of Figure 6 is the use of a scan converter 90 in conjunction wlth the video computer interface 74. The scan converter 90 is of a conventional construction and ls essentially used to sample images provided, while use might also be made of an ~-Y controller 92 in combination with a X-Y table 94 to facilitate a permanent graphical recording of the calculated stress and strain relationships. While Figllre 8 illustrates the typical display viewable on the storage display screen 88, a monltor 96 mlght be provided for visually vlewing the filtered display 86 as shown in Figure 9.
Speckle an~ Di~tal Correlation Device of This Invention It can be seen from the discussion relative to the data analysis system 62 illustrated in Figure 6 that while the described technique and apparatus can be used and does work, the use of photographic plates 48, with the required intermedlate development of these plates, is tlme consuming.
Furthermore, present techniques require one plate 48 for ea&h double exposure.
As such, the present invention further envisions utilizing the optlcal dat;;
digitizer 20, as shown in l~igure 2, along with the same basic single beam laser technique, so as to replace the film 48 and the image storage system coupled CO a computerized analysis technique, as shown in Figure 6, so as to provide ~or a direct read out of the resulting stress and strain relationships.
Furthermore, the present invention does not rely on tlle determination of fringe pattern spacing to determine amplitude displacement and hence strain/
stress, as is the case with the technique of Figure 6. The present invention has the capability of eliminatin~ the photographic process and determining strain/stress by electronically constructing and analyzing fringe patterns and this is an option of this invention, but the present invention goes a step beyond. Specifically, a new concept is introduced, A laser speckle pattern constituting the reflected optical signal from the surface in questlon is recorded. A subsequent laser speckle pattern from the same surface location subsequent to deformation of the surface is recorded.
The displacement of the second pattern relative to the first is determined, A mathematical analysis correlating thls displacement to surface strain and stress is carried ont by the computer software of Lhe present lnvention.
The basic theory used in this correlation is predicated on the theory of data acyuisition in polntwise filterlng employed in single beam laser inter-f~rometry as presented earlier.
The mathematical basis for the present invention is as followa.
A diffuse surface (Figure 12) is illuminated with a laser beam and the r~sulting intensity pattern of the reflected optical signal is digitized and stored in memory of a minicomputer. The digitized intensity pattern constltuting this stored date is shown in Figure 13. This digiti~ed inten-sity pattern of the reference surface is denoted as P(Xl' X2) (Figure 12), where Xl X~ are the coordinates of the illuminated surface. When the diffuse surface is deformed relative to the reference configuration, a com-plex fleld ~(X'l, X'2) represents the intensity of the surface in the deformed position (Figure 12). The measurement of the displacement (Ul and U2 which represents movement of P to P in Figure 12) of the sllrface is obtai~ed by correlating P(Xl, X2) and P(Xl', X2'). This corr~lation is determined using the auto correlation function of the two 2~ signals, C(U1 . U2) ~ SM P(X1~ X2) P(Xl ' X2 ) 1' 2 where M = area of the illuminated surface with the laser.
1 Xl + U2 (anid U2 is )the displacement in the X2 dlrection X2' - X2 + U2 and U2 is the displacement in the X2 direction (Figure 12).
The approach to correlating P(Xl, X2) with P(Xl', X2') is to assume the ~5~7~
basic theory of pointwise filtering used in slngle beam laser speckle interferometry. The restrictions in pointwise filtering optics dictates the following approach in laser speckle and digital correlation. The reference signal P(Xl " X2) is recorded over some area M of an illuminated object. Within some small area Po' contained in P(Xl', X2') the displace-ment components Ul and U2 are uniform. This restriction in the data analy~is allows the auto correlation function to be expressed in the following ~orm C (l~" U2~ - S~ P~X, ~X2) ~P (X, + U"X2 ~U~) dX ~ dX j~
where = ~X~ 2 + ~2~
is a small area of the deformed laser speckle pattern where Ul and U2 are uniform ~Figure 14).
The correct values of Ul and U2 corresponding to the displacement will result in a maximuln value of the correlation function C(Ul, U~). Thls correlation procadure corresponds to the data analysis procedure in pointwise filtering Ln sLngle beam laser speckle interferometry and thus completes the data analysis at a point.
Since this analysLs is automated, the displacements at any desired 2~ number of locations can be calculated. The principle of operation of the present invention around regions of surface cracks or stress concentrations is carried out as illustrated using Figure 15. A closed boundary denoted as S surrounds a crack of length 2a. The optical (Figure 3) digitizer records the laser speckle digital signal before and after the object is deformed (Region 14 of Figure 3). A typical digitized laser pattern is shown in Figure 13. The optical data digitizer records this pactern and stores in luemory the pattern referred to as the reference signal. At desired points around the boundary S denoted as points 1 through N, the deformed signal is ~L2~
correlated with ihe reference signal 110 in Figure 10, as was just explained.
At each desired boundary point on S ~he displacement components (Figures 12 and 14) are calculated from the correlation of reference and deformed images.
Yhis displacement data is then used as input to the solution oE the integral equations for stress and strain (112, Figure 10). The operator (114) then can specify the calculation of stress and strain at any desired location inside the closed region R (Figure 15). In other words, the need for the film 48 can be eliminated, and a direct computation of the strain/stress via the computer 76 and optical data digitizer 20 can be made. Again, reference is made to Figure 3, where it is to be understood that the system 10 might be combined into one integral unit 34 so as to further enllance the ease of computation required for obtaining the stress and strain relationships.
In this connection, reference is made to Figure 10 of the drawings which is a block diagram of the process associated with the apparatus illus-trated in Figures 2 and 3. Specifically, the optical data digltizer 20 senses the input signal, whlch is effectively the reflected light beam 22 shown in Figures 2 and 3, and then digitizes the signal. E~uipment to accoluplish this step exists, and there are basically two types of equipment --a vidicon television camera and an image disection camera. The choice of cameru types is a function of the resolution required. Current conventional vidicoll cameras have a 512 x 512 resolution, and current image disection cameras have a maximum 4000 x 4000 resolution. The time required to digitize the signal increases as the resolution increases. The preseni~ invention can employ either camera or an improved camera or cameras depending, as indicated, on the resolution and response time desired.
An image processor 98, which is utilized for sorting and storing data, receives the digitized data from the optical data digitizer 20 and sends the data to the host computer and central controller 100 for storage and manipulation. In this respect, the host computer 100 controls the image processor 9~.
~2~
Since the optical digiti~er 20 can be a conventional camera wlth 512 x 512 resolution or better or an image disection camera with up to 4000 x 4000 resolution or better, there are two concepts for the image processor. One concept applicable to the use of conventional camera employs a commercially available hardware item which can digitally process a picture frame at least in 1/30 of a second. This device is a high speed image preprocessor when the same is connected to a computer. Connection hetween this device and the compu~er is through an IEEE (Institute of Electrlcal and Electronics Engineers) interface bus or other appropriate connector~ The other concept employed with a disection camera is to connect the camera directly to the computer 100 and process the signal with computer software.
~andom access control of the image processor 98 in both cases is accomplisled with computer so~tware. ~ to host computer 10(), there are many computers ~vuilnble which can be utilized as the host computer and centraL controller.
~ typlcal computer would be a DEC (Digital Equipment Corporatlon) Model PDP-11 series having mass storage capabilities or other appropriate computer.
Additionally~ a console 102 for interaction with the computer 100 would be available depending on the computer selected. Figure 10 further illustrates the use of a monitor 96, and furtl1er an optional digitized lmage display 104 2l) might be employed.
While this portion of the process described has been directed to the system hardware 106, lt should be noted that the process further includes a software portlon 108. Effectlvely, the software portion 10~ includes a corre-latlon of the reference and deformed lmages 110 and a mathematlcal analysis of the same, and then further includes the numerical solution of integral equations for stress and strain 112 which are effectively the experimental boundary integral equation techniques which form the mathematical foundation o~ the numerical analysis. Finally, the software portion 108 involves the calculation of stress and strain at desired locations 114 which, of course, 3l) nu1y be speclfled by the operator.
~s~
In summary, it should be realized that the current laser speckle interferometry technique, while valid, is time coDsu~ing and costly, even on a laboratory basis. The present invention, while based on the proven technique employing photographic film and visual observstion, coupled with analysis, represents a significant advancement ln the state of the art.
The present invention, as illustrated in the Figures of the drawings, may be used in a manner which replaces the photographic plates with an optical dl1tn digitizer camera and image processor coupled to a computer. This form o~ the invention permlts a rapid sequence of test records of laser speckle 1() patten1~ or interference fringes to be acquired over a critical region of interest, as well as permitting a determination of the difference in laser speckle patterns or fringe patterns to be measured and a converting of these measurements into a digital display on a real-time basis. Furthermore, the computer can store the data for display as required.
Thus, the lnvention as illustrated not only presents an advance-ment in the state of the art, but also translates a laboratory proven technique into a commercial device. The inver.tion will be utilizable by a trained technician, as opposed to requiring a laser speckle interferome-rist/stress analyst or highly trained engineer. Further, it will per~lit ~() n real-time analysis of a full-scale operating system and will permit sequential observation of critical points in the system as a function of time. In addition, the invention will not be limited to applications in-volving pressuri~ed systems, e.g., the device could be used to review structural members such as the pilons on a DC-10 aircraft. In this respect, the present invention could be applied to nuclear reactor components operat-ing in hostile environments, pressure vessels, pipe lines and pipe systems, structural members having regions of high stress gradients such as geometrical discontinuities (holes, cut-outs, fillets and grooves), aircraft bodies and associated components, turbine blades (fillet area where peak stresses occur), energy conversion plants, chemical processing plants, and data ~2~ g5 analysis in non-destructlve testing. As can be appreclated, there are many more specific and general applications which could apply in addition to those above listed. As indicated, the present invention gains its utility from the fact that it is a direct read out, non-destructive, non-coneacting device which yields strain/stress data on a real-time basis for full-scale systems. Furthermore, the output of the present invention could constitute input to m(re complex analysis programs, thus further increasing its versatility.
transparency ls obtained optically by illuminating the photographlc plate with a converging spherical wave. The main advantage of this procedure is that the whole-field displacement can be analyzed and, by appropriate po6ition of a set of apertures in the transform plane, any component of the displacement normal to the line of sight can be detected with variable sensitivity.
Speckle interferometry does have some limitations however, such as the fact that measurements are not as accurate as those made with strain gages. However, the inaccuracies usually associated with the laser speckle techn~que involve numerical error in calculating the derivatives, and not the ~llecrology of the laser speckle technique per se. Strain gage measurements with accuracies of 1~ can be obtained while strain calculations using the spe~kle data technique result in accuracies of approximately 5%. The accura-cies associated with the numerical analysis technique used with the speckle approach will be improved as a result of the computeri~ed approach employed by the present invention. Also, fringe analysis is time consu~ing in that the analyst must view the photographic plates, make the necessary measurements and then calculate the results. ~owever, this limitation can be overcome by u~ing an optical data digitizer system, including an image storage device and computer for data correlation and analysis, as proposed by the pre6ent ~1) lnvention.
Laser speckle interferometry is not dependent on the use of models and ls in fact applicable to full scale systems. ~any of the recent advances in coherent optics have suggested the engineering applications to prototype systems;
however, with all of the techniques developed thus far, the recording medium has been photographic film. Therefore, data analysis has required the use of a specialist for interpretation. Even with this limitation, ~hough, the great potential o~ coherent optics technl~lues for engineering analysis has been clearly demonstrated.
The laser speckle effect, which is the basis of ~he new concept of :10 this invention, prololses to be the optical technique whereby the photographic 579~
film can be eliminated ln the data acquistlon process. The ellminatlon of the photographlc film means that interference fringe patterns classlcally assocl-ciated with laser speckle interferometry need not be employed to yleld data.
These fringe patterns can be generated in the electronic imaging system of this invention and analyzed as in the classical case of interferometry and this capability is one claim of the invention. ~owever, the invention is cabnple o~ eliminating this step by introducing a laser speckle technlque based on the digltal correlation of successive laser speckle patterns beEore and after object surface deformation. Thus, laser speckle and dlgltal correlation, as the technique is proposed to be termed, has its fo~mdatlon in laser speckle interferometry and has all the attributes of laser speckle interferometry without the necessity of photographs and fringe measurements.
The laser speckle and digital correlation will be accomplished through the development of electronic video systems capable of high resolution of the speckle patterns. The compatibility of the speckle technique with image processing offers a user-oriented system for a wide range of engineering applications. Finally, it should be pointed out that there is commercially available an Electronic Speckle Pattern Interferometry (ESPI) device for time veraged holographic stress analysis, as reported in ~aterials Evaluation (May, 1979). However, this device, ~hile demonstrating the fact that speckle p.ltternS can be digitiæed~ does not include the use of a computer for dat~
analysis and management aud does not have the capability to by-pass the step of creating in~erference fringe patterns, as its sole purpose is to create these patterns. Furthermore, the ESPI device requires that the body under investigation be vibrated in order to generate the fringe patterns as opposed to illumination alone by a coherent light source.
The present invention9 provides for an apparatus and method for determining whole-field, regional or pointwise, stress and strain associated wlth pipes, pressure vessels, structural members and deformable bodies ~3~ (including both isoropic and anisotropic materials) and provides for a direct read out, non-destructive, non-contacting device to determine ~train and stress in pipes, pressure vessels or other bodies on a real-time basis.
The device is optical in nature ond can operate at some distance from the body under investigation, and thus, the device i8 insensitive to hostile environments. rhe present invention is based on the application of the speckle effect produced by illumlnalion of a diffuse surface by a coherent source of llght. ~aslcally, this effect is accomplished by illuminating a ~urface which is under examination through the use of a laser. The illuminated surface reflects the laser beam, an~l this reflected signal is recorded by an optical data digitizer which can record interference fringes or laser speckle patterns. If the surface e~hibits deformation or strain subsequent to a previous recording of the fringe pattern or laser speckle patterns, a new recording of fringe or speckle patterns will result in a measurement of object movements, i~e., a difference b~tween the original and subsequent patterns. The measured differences in these patterns can be mathematically related to the actual deformation and hence stress of the body being examined. The optical data digitizer i~ not only employed to directly receive the reflected light beam but to also send a signal representative of the reflected beam directly to a minicomputer for mathe~atical analysis~
An object of the present invention is to provide a method and apparatus for rapidly d~termining stress and strain relationships in a pressurized syste~, structural members or deformable bodies which effectively tnakes use of speckle interferometry or laser speckle and digital correlation, which eliminates the need for photographic recordings of surfacé displacemen~
data and an interferometrist to read and analyse the photographic data, which will give real-time indications of strain and stress directly and which will be applicable to nuclear pressure vessels, synfuel generation plants and any system or structure operating under pressure or static and 3~ dytallllc loading~ whlch can be effectuated at a safe stand off distance ;7~
from a hostilc environmellt of temperature, toxicity and radiation, or remotely within the environment, which is based on the development of the theory of fringe formation in laser speckle interferometry and the extension thereto referred to as laser speckle and digital correlation to include thermal transients, and which is based on the development of laboratory apparatus for electronically acquiring displacement data for a body undergoing deformation, and storing such data, which is non-destructive and requires no contact of the levice.
Figure 1 is a schematic illustration of the basic system forming the 1() present inventlon as used for measuring or monitoring stress and strain in a region of an operating system.
Figure 2 is an enlarged view of the data recording system forming the present invention as shown in Figure 1 and employing laser speckle inter~erontetry and/or laser speckle and data correlation.
Figure 3 schematically represents the apparatus and system of Figure 2 in a form whereby all of the components of the system are combined in one integrated unit.
Figure 4 schematically represents the system of data recording as utilized in laser speckle interferometry.
~() Figure 5 schematically represents the system of data analysls utllLzed in laser speckle interferometry.
Figure 6 is an expanded schematic illustration of the apparatus and method for determining stress and strain relationships in pressurized systems as employed in laser s~eckle interferometry.
Figure 7 is an i]lustration of a typical speckle photography fringe pattern obtained iu an optical data analysis system.
Figure 8 is a graphical illustration of the original data ob-talned from the average of a firæt scan line and a second scan line image prior to and subsequent to deformation.
`~ Figure '3 ls a graphical lllustration of the filtered data of the ~:~S5~
averaged scan lines of Figure 8.
Figure 10 is a block diagram illustrating the method associated with the apparatus of the present invention.
Figure 11 is a schematic illustration of the object coordinates ma~helnatically obtainable at a surface point, P.
Figure 12 is a schematic of the reference and deformed surface ~or correlation of laser speckle patterns.
Figure 13 ls the digitized optical surface resulting from the ~peckle pattern of the surface under investigation.
1~ ~igure 14 is a schematic o~ the reference and deformed subsets of the original and displaced surfaces Figure 15 is a schematic illustration of a region on the object under investigation containing a crack.
Reference is now made to the drawings and, in particular, to ~`igl1res 1 and 2 wherein there is schematically illustrated a system which may be employed to determine the stress and strain relationships forming the present invention and being generally designated by the reference numeral 1(). In this respectJ the stress and strain measuring system 10 can be used to measure the stress and strain associated with any type of pressurized 2~ vessel 12 through the projection and focusing of a light beam on a critical re~ion 14 assvciated with the vessel. However, the device is not limited to use with pressure vessels but a pressure vessel is used for illustrative purposes. As shown, the system 10 includes a laser light source 16 for projecting a llght beam 18 against a critical region 14, and an optical data digitizer 20 for receiving the light beam 22 being reflected from the critical region. In this regard, the optical data digitizer 20 might typically be a television camera, and the signal received by the digitizer may then be directed to a computer interface device 24 which serves to process the received signal and dlrect the same to a television monitor 26 a~ welL as to a minicomputer 2B.
3~) The television monitor 26 is effectively a higll-resolution monltor and provides a graphlcal data display. On the other hand, the mini-computer 28 serves to take the same signal as provided to the television monitor 26 and further enhance the data provlded, thereby to display the results on a dlsplay unit 30. In this connection, the mlnicomputer 28 can provide a display.
Additionally, an electro-optic or other range indicating device 23 is employed to indicate ~he distance between the digitizer 20 anc1 the critical re~on 14.
Figure 3 has been provided solely for the purpose of illustrating the various components sho~n in Figure 2 in a systelD whereby the same are 1~ operatively and functionally combined into one integral unit 34. In this regard, it can be seen that the integral unit 34 could be positioned external to a nuclear radiation or thermal ~one 32 and would include a laser light source 36 fixedly secured thereto and being utili~able to direct the light b~am 18 against the critical region 14 in the manner aforedescribed. The reflected light beam 22 then would be directed to an optical data digitizer 38, also integrally a part of the unit 34, and a television monitor 40 inte-grnlly a part of the unit 34 could then be used to view the received data.
An e1ectro-optic range indicating device 35 would perform the distance measuring function, while the unit 34 would perform the computing function ~0 so n~ to provide a data read out on the digital and tape display 42 associated therewith.
Laser Interfero!lletry A better understanding of the technique of using laser interferometry can be ascertained with reference to Figure 4. Typically, laser interferometry can be used to analy~e a structure 12 through the use of a laser light source 16 having an expandlng lens whereby the light beam 1~ directed against a critical area 14 expands outwardly from the l~gllt source. A camera 44 may then be employed to image the light 22 reflected from the critical surface 14 and then to record the surface in 3~ two configurations. The two configurations are a representation of the 7~;
deformatioll of the structure 12 resulting Erom an applied loa~l or thermal e~fect. A photographic plate 48 withln the camera 44 records an im~ge 46, which is separately illustrated in Figure 4, such image including a surface point on the body 12 at position P representative of the first exposure. The same point is imaged at point P' after deformation, and of course, lf there is no deformation, the point P' will be imaged at P during the second exposure.
Once a film record 48 is o.ade and is complete, it must be processed nnd ana1ysed to determllle the magnitude of deformation of the area 14 photo-~raplltd. Convelltionally, as il1ustrated in Figure 5, each developed plate 48 is illuminated by a laser 50 for the purpose of performing an analysis of the provided data. This method of data analysis, known a~ pointwise filtering, projects a set of parallel interference fringes 52 on a viewing screen 54. In this respect, the laser 50 is provided with all expanding lens 56 so as to project a light beam 58 through the film plate 48 to thereby project che elllarged image 52 onto the viewing screen 54. The distance 60 between re~pective fringes on the viewing screen 54 is proportional to the displacement of point P, and there is a mathematical correlation of the fringe pattern 52 and displacement, and hence strain/stress. Thus, the analyst must view the pro~jected image (such as illustrated in Figure 7) of each plate, make measure-mellts ~1nd then use these measurements as input to a mathematical procedure to yiel<l strain and then stress relative to the original point P.
Present Automated ~`ilm Analysis Techl1ique for Laser Speckle Interferometric System To reduce the complexity and time consumption associated with data analysis as performed in the manner illustrated in Figure 5, a present laser speckle interferometric system makes use of the data analysis system 62 illustrated in Figure 6. Specifically, the film 48 having the double exposure thereon including the points P and P', is positionable between a pair of mirrors 64, 66. A laser 68 directs a beam of light 70 against the reflective surface of the first mirror 66 whereby it is reflected up through the film ~2~57~;
plate 48 against the reflective surface of the second mirror 6~ The image 52 whlch is projected from the second mirror 64 is then displayed on a viewing screen 5~ in a manner similar to that illustrated in Flgure 5. A television camera 72 then views the image 52 and directs a picture of the same to a video computer interface 74. A minicomputer 76 having a conventional terminal 78 may then be used to analyze the image 52 as provided to the video computer interface 74. Specifically, the minicomputer 76 can be utilized to perform a conventional mathematical analysis which would normally be computed by hand so as to substantially reduce the time consumption and complexity of such nnalysis. The analysis which would be performed by the minicomputer 76 can be understood by reference to Figure 11 of the drawings ~herein the object coordinates at a surface point P are illustrated, i.e.~ the dlsplacement of the surface of an object is completely described by a vector ~ (x,y,~,t) Recognizing that the coordinates x,y,z are restricted to the surface of the body shown in Figure 11, the tangential components ~ and the angle ~
are the data that are recorded and analyzed by the laser speckle photography.
Thus, at each prescribed surface point P, the tangential components of the dlsplacement vector U can be stored in the memory of the minicomputer 76.
~0 These data ehen can be used to numerically calculate the stresses and strains as desired.
Specifically, a vector gradient Uij ~ ~-U may be separated into symmetric and skew symmetric parts, where V~-U- aaU~
~Z5~7~
is the symmetric part, and ij = 2 ( ~Ui + ~UJ) ~ii ~ 2 ( a~i _ a~J) is the skew symmetric part. When the values of ij 1 wij are small as compared to unity, then the symmetric part ij is the strain matrix and~ij is the rotatic matrLc. The quantity of the primary ~atrix is the strain matrix ~,;j which is calculated from the experimental data.
For elastic deterMination of an isotropic material, the stress/strain relations are ~ii = Ç [ 6ij ~ j) Sij] + ~c aT
where E is the modulus of elasticity, ~ is Poissons ratio, and o~ is the coef~icient of thermal expansionO Generally, the surface point P is located on the free surface of a body, and therefore, additional relationships are obtained from the boundary conditions and the known mechanical loading on the sur~ace In this connection~ T~ =ni 6 j~ may be calculated, wh~re the surface traction T~ is known, and in most cases ~ . This implies that 6 ~ j - O , and the result now becomes Uy~ = -U~x ~U _ ~ ~ (aux ~ au"
x ay ~s~
Thus, with the measured value of the laser speckle and the known mechAnical loading, the surEace stress and strain components can be calculated at any described surface polnt P by the minicomputer 76. The expressions or the strain components are = ~Ux ~Jy ~ ( 3y ~ ~x ) xy ~ !2 (~UX ~ auy x~ ~ y~ ~ O
Similarly, the components of the rotation vector ~ are oy, = W;!y _ ~Ui~
~)y = Wx~
~ Jyx - 2 ( U y _ UX ) Th~ stress components ~ij can now be determined from the strain components throu~h the stress/strain relationships as stated previously.
With respect to the data transmitted from the video computer inter-face 74 to the minicomputer 76, it should be noted that the typical interfer-ence pattern 80> as shown in Figure 7, will be converted to a data analysis record 82, as shown in Figure 8, by the video computer interface. Effectively then, the data analysis record 82 having fringe spacing 84 will be filtered digitally (numerically within the computer) so as to produce the result 86 shown in Figure 9 and to provide the same to a storage display screen 88.
Tllis process signal 86 is then a measure of the fringe spacing 84 as shown in Figure 8.
~ZSS7~.~
Also lllustrated ln the data analysls system 62 of Figure 6 is the use of a scan converter 90 in conjunction wlth the video computer interface 74. The scan converter 90 is of a conventional construction and ls essentially used to sample images provided, while use might also be made of an ~-Y controller 92 in combination with a X-Y table 94 to facilitate a permanent graphical recording of the calculated stress and strain relationships. While Figllre 8 illustrates the typical display viewable on the storage display screen 88, a monltor 96 mlght be provided for visually vlewing the filtered display 86 as shown in Figure 9.
Speckle an~ Di~tal Correlation Device of This Invention It can be seen from the discussion relative to the data analysis system 62 illustrated in Figure 6 that while the described technique and apparatus can be used and does work, the use of photographic plates 48, with the required intermedlate development of these plates, is tlme consuming.
Furthermore, present techniques require one plate 48 for ea&h double exposure.
As such, the present invention further envisions utilizing the optlcal dat;;
digitizer 20, as shown in l~igure 2, along with the same basic single beam laser technique, so as to replace the film 48 and the image storage system coupled CO a computerized analysis technique, as shown in Figure 6, so as to provide ~or a direct read out of the resulting stress and strain relationships.
Furthermore, the present invention does not rely on tlle determination of fringe pattern spacing to determine amplitude displacement and hence strain/
stress, as is the case with the technique of Figure 6. The present invention has the capability of eliminatin~ the photographic process and determining strain/stress by electronically constructing and analyzing fringe patterns and this is an option of this invention, but the present invention goes a step beyond. Specifically, a new concept is introduced, A laser speckle pattern constituting the reflected optical signal from the surface in questlon is recorded. A subsequent laser speckle pattern from the same surface location subsequent to deformation of the surface is recorded.
The displacement of the second pattern relative to the first is determined, A mathematical analysis correlating thls displacement to surface strain and stress is carried ont by the computer software of Lhe present lnvention.
The basic theory used in this correlation is predicated on the theory of data acyuisition in polntwise filterlng employed in single beam laser inter-f~rometry as presented earlier.
The mathematical basis for the present invention is as followa.
A diffuse surface (Figure 12) is illuminated with a laser beam and the r~sulting intensity pattern of the reflected optical signal is digitized and stored in memory of a minicomputer. The digitized intensity pattern constltuting this stored date is shown in Figure 13. This digiti~ed inten-sity pattern of the reference surface is denoted as P(Xl' X2) (Figure 12), where Xl X~ are the coordinates of the illuminated surface. When the diffuse surface is deformed relative to the reference configuration, a com-plex fleld ~(X'l, X'2) represents the intensity of the surface in the deformed position (Figure 12). The measurement of the displacement (Ul and U2 which represents movement of P to P in Figure 12) of the sllrface is obtai~ed by correlating P(Xl, X2) and P(Xl', X2'). This corr~lation is determined using the auto correlation function of the two 2~ signals, C(U1 . U2) ~ SM P(X1~ X2) P(Xl ' X2 ) 1' 2 where M = area of the illuminated surface with the laser.
1 Xl + U2 (anid U2 is )the displacement in the X2 dlrection X2' - X2 + U2 and U2 is the displacement in the X2 direction (Figure 12).
The approach to correlating P(Xl, X2) with P(Xl', X2') is to assume the ~5~7~
basic theory of pointwise filtering used in slngle beam laser speckle interferometry. The restrictions in pointwise filtering optics dictates the following approach in laser speckle and digital correlation. The reference signal P(Xl " X2) is recorded over some area M of an illuminated object. Within some small area Po' contained in P(Xl', X2') the displace-ment components Ul and U2 are uniform. This restriction in the data analy~is allows the auto correlation function to be expressed in the following ~orm C (l~" U2~ - S~ P~X, ~X2) ~P (X, + U"X2 ~U~) dX ~ dX j~
where = ~X~ 2 + ~2~
is a small area of the deformed laser speckle pattern where Ul and U2 are uniform ~Figure 14).
The correct values of Ul and U2 corresponding to the displacement will result in a maximuln value of the correlation function C(Ul, U~). Thls correlation procadure corresponds to the data analysis procedure in pointwise filtering Ln sLngle beam laser speckle interferometry and thus completes the data analysis at a point.
Since this analysLs is automated, the displacements at any desired 2~ number of locations can be calculated. The principle of operation of the present invention around regions of surface cracks or stress concentrations is carried out as illustrated using Figure 15. A closed boundary denoted as S surrounds a crack of length 2a. The optical (Figure 3) digitizer records the laser speckle digital signal before and after the object is deformed (Region 14 of Figure 3). A typical digitized laser pattern is shown in Figure 13. The optical data digitizer records this pactern and stores in luemory the pattern referred to as the reference signal. At desired points around the boundary S denoted as points 1 through N, the deformed signal is ~L2~
correlated with ihe reference signal 110 in Figure 10, as was just explained.
At each desired boundary point on S ~he displacement components (Figures 12 and 14) are calculated from the correlation of reference and deformed images.
Yhis displacement data is then used as input to the solution oE the integral equations for stress and strain (112, Figure 10). The operator (114) then can specify the calculation of stress and strain at any desired location inside the closed region R (Figure 15). In other words, the need for the film 48 can be eliminated, and a direct computation of the strain/stress via the computer 76 and optical data digitizer 20 can be made. Again, reference is made to Figure 3, where it is to be understood that the system 10 might be combined into one integral unit 34 so as to further enllance the ease of computation required for obtaining the stress and strain relationships.
In this connection, reference is made to Figure 10 of the drawings which is a block diagram of the process associated with the apparatus illus-trated in Figures 2 and 3. Specifically, the optical data digltizer 20 senses the input signal, whlch is effectively the reflected light beam 22 shown in Figures 2 and 3, and then digitizes the signal. E~uipment to accoluplish this step exists, and there are basically two types of equipment --a vidicon television camera and an image disection camera. The choice of cameru types is a function of the resolution required. Current conventional vidicoll cameras have a 512 x 512 resolution, and current image disection cameras have a maximum 4000 x 4000 resolution. The time required to digitize the signal increases as the resolution increases. The preseni~ invention can employ either camera or an improved camera or cameras depending, as indicated, on the resolution and response time desired.
An image processor 98, which is utilized for sorting and storing data, receives the digitized data from the optical data digitizer 20 and sends the data to the host computer and central controller 100 for storage and manipulation. In this respect, the host computer 100 controls the image processor 9~.
~2~
Since the optical digiti~er 20 can be a conventional camera wlth 512 x 512 resolution or better or an image disection camera with up to 4000 x 4000 resolution or better, there are two concepts for the image processor. One concept applicable to the use of conventional camera employs a commercially available hardware item which can digitally process a picture frame at least in 1/30 of a second. This device is a high speed image preprocessor when the same is connected to a computer. Connection hetween this device and the compu~er is through an IEEE (Institute of Electrlcal and Electronics Engineers) interface bus or other appropriate connector~ The other concept employed with a disection camera is to connect the camera directly to the computer 100 and process the signal with computer software.
~andom access control of the image processor 98 in both cases is accomplisled with computer so~tware. ~ to host computer 10(), there are many computers ~vuilnble which can be utilized as the host computer and centraL controller.
~ typlcal computer would be a DEC (Digital Equipment Corporatlon) Model PDP-11 series having mass storage capabilities or other appropriate computer.
Additionally~ a console 102 for interaction with the computer 100 would be available depending on the computer selected. Figure 10 further illustrates the use of a monitor 96, and furtl1er an optional digitized lmage display 104 2l) might be employed.
While this portion of the process described has been directed to the system hardware 106, lt should be noted that the process further includes a software portlon 108. Effectlvely, the software portion 10~ includes a corre-latlon of the reference and deformed lmages 110 and a mathematlcal analysis of the same, and then further includes the numerical solution of integral equations for stress and strain 112 which are effectively the experimental boundary integral equation techniques which form the mathematical foundation o~ the numerical analysis. Finally, the software portion 108 involves the calculation of stress and strain at desired locations 114 which, of course, 3l) nu1y be speclfled by the operator.
~s~
In summary, it should be realized that the current laser speckle interferometry technique, while valid, is time coDsu~ing and costly, even on a laboratory basis. The present invention, while based on the proven technique employing photographic film and visual observstion, coupled with analysis, represents a significant advancement ln the state of the art.
The present invention, as illustrated in the Figures of the drawings, may be used in a manner which replaces the photographic plates with an optical dl1tn digitizer camera and image processor coupled to a computer. This form o~ the invention permlts a rapid sequence of test records of laser speckle 1() patten1~ or interference fringes to be acquired over a critical region of interest, as well as permitting a determination of the difference in laser speckle patterns or fringe patterns to be measured and a converting of these measurements into a digital display on a real-time basis. Furthermore, the computer can store the data for display as required.
Thus, the lnvention as illustrated not only presents an advance-ment in the state of the art, but also translates a laboratory proven technique into a commercial device. The inver.tion will be utilizable by a trained technician, as opposed to requiring a laser speckle interferome-rist/stress analyst or highly trained engineer. Further, it will per~lit ~() n real-time analysis of a full-scale operating system and will permit sequential observation of critical points in the system as a function of time. In addition, the invention will not be limited to applications in-volving pressuri~ed systems, e.g., the device could be used to review structural members such as the pilons on a DC-10 aircraft. In this respect, the present invention could be applied to nuclear reactor components operat-ing in hostile environments, pressure vessels, pipe lines and pipe systems, structural members having regions of high stress gradients such as geometrical discontinuities (holes, cut-outs, fillets and grooves), aircraft bodies and associated components, turbine blades (fillet area where peak stresses occur), energy conversion plants, chemical processing plants, and data ~2~ g5 analysis in non-destructlve testing. As can be appreclated, there are many more specific and general applications which could apply in addition to those above listed. As indicated, the present invention gains its utility from the fact that it is a direct read out, non-destructive, non-coneacting device which yields strain/stress data on a real-time basis for full-scale systems. Furthermore, the output of the present invention could constitute input to m(re complex analysis programs, thus further increasing its versatility.
Claims (14)
1. An apparatus for determining stress and strain associated with a surface, said apparatus comprising:
light source means to provide a light beam for illuminating said surface optical data receiving means for sensing a reference speckle pattern and a succeeding speckle pattern produced by a reflected light source reflected from an illuminated area of said surface when undergoing deformation, said optical data receiving means further serving to digitize signals provided by said reflected speckle patterns; and computation means for mathematically analyzing the amplitude of signals to determine displacement between the speckle pattern reflected from the surface in a reference state and the speckle pattern reflected from the surface when undergoing deformation and thereby to determine stress and strain associated with said surface.
light source means to provide a light beam for illuminating said surface optical data receiving means for sensing a reference speckle pattern and a succeeding speckle pattern produced by a reflected light source reflected from an illuminated area of said surface when undergoing deformation, said optical data receiving means further serving to digitize signals provided by said reflected speckle patterns; and computation means for mathematically analyzing the amplitude of signals to determine displacement between the speckle pattern reflected from the surface in a reference state and the speckle pattern reflected from the surface when undergoing deformation and thereby to determine stress and strain associated with said surface.
2. The apparatus for determining stress and strain associated with a surface as defined in claim 1, wherein said computation means includes a computer programmed to calculate said stress and strain asso-ciated with said surface.
3. The apparatus for determining stress and strain associated with a surface as defined in claim 2, wherein a monitoring means is included in combination with said computer so as to permit a visual viewing of a laser speckle pattern or fringe pattern associated with said surface.
4. The apparatus for determining stress and strain associated with a surface as defined in claim 3, wherein a display means is provided so that stress and strain calculations computed by said computer may be made readily accessible to a user.
5. The apparatus for determining stress and strain associated with a surface as defined in claim 4, wherein said light source means, said optical data receiving means, said computation means, said monitoring means and said display means are all contained in one integral unit.
6. The apparatus for determining stress and strain associated with a surface as defined in claim 5, wherein said light source means includes a laser.
7. The apparatus for determining stress and strain associated with a surface as defined in claim 6, wherein said optical data means includes the use of an optical data digitizer.
8. The apparatus for determining stress associated with a surface as defined in claim 7, wherein an electro-optic or other range indicating device is used to determine the distance between said digitizer and the surface being examined.
9. The apparatus for determining stress and strain associated with a surface as defined in claim 3, wherein said monitoring means includes 8 television monitor.
10. The apparatus for determining stress and strain associated with a surface as defined i? claim 4, wherein said display means includes a visual display.
11. The method of correlating speckle pattern displacement reflected from a surface undergoing deformation, said method including the steps of utilizing a light source means for illuminating said surface with a light beam;
using an optical data digitizer for sensing a speckle pattern produced by at least a portion of said light beam being reflected from said surface when in a reference state, forming a signal in said optical data digitizer in response to said sensing of said reflected speckle pattern wh?n the surface is in a reflected state, using the optical data digitizer for sensing a subsequent speckle pattern produced by at least a portion of said light beam being reflected from said surface when undergoing deformation;
forming signals in said optical data digitizer in response to said sensing of said succeeding speckle patterns; and providing the amplitude of said signals to a computation means for correlating displacement of said speckle patterns.
using an optical data digitizer for sensing a speckle pattern produced by at least a portion of said light beam being reflected from said surface when in a reference state, forming a signal in said optical data digitizer in response to said sensing of said reflected speckle pattern wh?n the surface is in a reflected state, using the optical data digitizer for sensing a subsequent speckle pattern produced by at least a portion of said light beam being reflected from said surface when undergoing deformation;
forming signals in said optical data digitizer in response to said sensing of said succeeding speckle patterns; and providing the amplitude of said signals to a computation means for correlating displacement of said speckle patterns.
12. An apparatus for determining stress and strain associated with a body, said apparatus comprising:
means associated with said body to cause said body to produce a speckle pattern;
data receiving means for sensing a reference speckle pattern and a succeedin speckle pattern produced by the body when undergoing deformation, said data receiving means further serving to digitize signals provided by said speckle patterns; and computation means for mathematically analyzing the amplitude of signals to determine displacement between the speckle pattern produced by the body in a reference state and the speckle pattern produced by the body when deformed and thereby to determine stress and strain associated with said surface.
means associated with said body to cause said body to produce a speckle pattern;
data receiving means for sensing a reference speckle pattern and a succeedin speckle pattern produced by the body when undergoing deformation, said data receiving means further serving to digitize signals provided by said speckle patterns; and computation means for mathematically analyzing the amplitude of signals to determine displacement between the speckle pattern produced by the body in a reference state and the speckle pattern produced by the body when deformed and thereby to determine stress and strain associated with said surface.
13. The apparatus for determining stress and strain associated with a body as defined in claim 12, wherein said means includes means emitting a receivable signal from the body.
14. The method of correlating speckle pattern displacement produced by a body undergoing deformation, said method including the steps of utilizing means associated with said body to produce a speckle pattern;
using a data digitizer for sensing a speckle pattern produced by said body when in a reference state, forming a signal in said data digitizer in response to said sensing of said speckle pattern when the body is in a reference state, using the data digitizer for sensing a subsequent speckle pattern produced by said body when undergoing deformation;
forming signals in said data digitizer in response to said sensing of said succeeding speckle patterns; and providing the amplitude of said signals to a computation many for correlating displacement of said speckle patterns.
using a data digitizer for sensing a speckle pattern produced by said body when in a reference state, forming a signal in said data digitizer in response to said sensing of said speckle pattern when the body is in a reference state, using the data digitizer for sensing a subsequent speckle pattern produced by said body when undergoing deformation;
forming signals in said data digitizer in response to said sensing of said succeeding speckle patterns; and providing the amplitude of said signals to a computation many for correlating displacement of said speckle patterns.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US767,663 | 1985-08-21 | ||
| US06/767,663 US4591996A (en) | 1981-05-18 | 1985-08-21 | Apparatus and method for determining stress and strain in pipes, pressure vessels, structural members and other deformable bodies |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1255795A true CA1255795A (en) | 1989-06-13 |
Family
ID=25080179
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000509846A Expired CA1255795A (en) | 1985-08-21 | 1986-05-23 | Apparatus and method for determining stress and strain in pipes, pressure vessels, structural members and other deformable bodies |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1255795A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10234354B2 (en) | 2014-03-28 | 2019-03-19 | Intelliview Technologies Inc. | Leak detection |
| US10373470B2 (en) | 2013-04-29 | 2019-08-06 | Intelliview Technologies, Inc. | Object detection |
| US10943357B2 (en) | 2014-08-19 | 2021-03-09 | Intelliview Technologies Inc. | Video based indoor leak detection |
-
1986
- 1986-05-23 CA CA000509846A patent/CA1255795A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10373470B2 (en) | 2013-04-29 | 2019-08-06 | Intelliview Technologies, Inc. | Object detection |
| US10234354B2 (en) | 2014-03-28 | 2019-03-19 | Intelliview Technologies Inc. | Leak detection |
| US10943357B2 (en) | 2014-08-19 | 2021-03-09 | Intelliview Technologies Inc. | Video based indoor leak detection |
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