The present disclosure relates to methods and systems for measuring stress, strain, and/or deformation. Strain measurement is often used in a variety of applications, including manufacturing, biomedical, and microelectronics, among others. For example, aircrafts are often analyzed using shearography.
Conventional shearography systems typically have several drawbacks that prevent effective use for analyzing dynamic systems. For example, conventional systems may require acquisition of at least two images before loading and at least two images after loading. Acquisition of multiple images before and after loading may undesirably increase processing time.
Additionally, conventional shearography systems may include complicated structures that prohibit wide commercial adoption, particularly for field applications.
The present disclosure includes a shearography system that may include a plurality of light sources each configured to produce a beam of light to illuminate a test area, each of the beams of light having a different wavelength. In embodiments, a shearography system may include a camera configured to obtain intensity information corresponding to reflections of the plurality of lights off of the test area. In embodiments, a shearography system may include an optical shearing device disposed in an optical path between the plurality of light sources and the camera, the optical shearing device configured to provide a shearing angle. In embodiments, the camera may be configured to obtain intensity information corresponding to simultaneous reflections of the plurality of lights off of the test area. In embodiments, captured intensity information may allow for the calculation of in-plane normal strain, in-plane shear strain, and/or pure out-of-plane shear strain from a single testing image.
In embodiments, a method of determining strain may include illuminating, via a plurality of light sources, a test area of a test object, capturing, via a camera, a first plurality of interferograms corresponding to the test area, the first plurality of interferograms being captured in a reference image, determining a reference phase difference from the reference image, capturing, via the camera, a second plurality of interferograms corresponding to the test area, the second plurality of interferograms being captured in a testing image, determining a testing phase difference from the testing image, and determining a strain measurement according to a relative phase difference between the reference phase difference and the testing phase difference. In embodiments, each of the light sources of the plurality of light sources may be configured to produce a beam of light, and the beam of light produced by each light source may have a different wavelength than the lights produced by the other light sources.
Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.
Reference will now be made in detail to embodiments of the present disclosure, examples of which are described herein and illustrated in the accompanying drawings. While the invention will be described in conjunction with embodiments and examples, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by appended claims.
The present disclosure includes a shearography system 100. Shearography system 100 may be referred to as a digital speckle pattern shearing interferometry system and may be used for non-destructive testing (NDT) to analyze properties of various materials, such as, for example, composite materials. Shearography system 100 may be configured laser-based, full field, non-contact optical measurement of strain (e.g., in-plane strain) with a sensitivity of several microstrains.
As generally illustrated in
In embodiments, first light source 110 may illuminate test object 50, which may allow detector 70 to capture a reference image (e.g., a reference shearogram). A reference shearogram may correspond to a first beam 112 from first light source 110 illuminating test object 50 and may be captured by detector 70 via optical shearing device 60. First light source 110 may then illuminate test object 50 again, but with test object 50 in second state 54, which may allow for generating a corresponding testing image (e.g., a testing shearogram). Comparing a reference image with a testing image may allow for a determination of relative phase difference information, which may be used to measure stress, strain, and/or deformation, and/or locate faults in a material.
In embodiments, a light source (e.g., first light source 110) may be configured to emit convergent, generally convergent, and/or partially convergent light, such as, for example, first beam 112. In embodiments, a light source may be configured to emit coherent, generally coherent, and/or partially coherent light. In embodiments, a light source may include a laser, first beam 112 may be configured as a laser beam, and/or first beam of light 112 may be referred to herein as first laser beam 112. In embodiments, a light source may include a helium-neon (HeNe) laser, which may be configured to emit a laser beam including a wavelength of about 630 nm, for example, 632.8 nm. For example, and without limitation, first beam of light 112 may be configured as a laser with a wavelength of about 632.8 nm. Additionally or alternatively, a light source may include a green laser, which may be configured to emit a laser beam including a wavelength of about 532 nm. For example, and without limitation, first beam of light 112 may be configured as a laser with a wavelength of about 532 nm. In embodiments, a light source may be configured to illuminate test object 50 and/or may be configured to direct first beam of light 112 toward test object 50.
Shearography system 100 may include one or more beam expanders 120. A beam expander 120 may be configured to expand a beam of light into an expanded beam of light, such as, for example, first beam of light 112 into expanded first beam of light 112A. Beam expander 120 may be disposed in an optical path 90 between a light source (e.g., first light source 110) and testing object 50.
Detector 70 may be configured to detect, receive, capture, and/or measure light and/or the intensity of light. Detector 70 may also be referred to herein as camera 70. In embodiments, camera 70 may include a charge-coupled device (CCD). A CCD may be configured to determine a value of light intensity provided to it. In embodiments, for example only, intensity may be measured on a scale of 0 to 255. In embodiments, a shearography system (e.g., shearography systems 100, 200, 300), may include a single camera 70 or a plurality of cameras.
In embodiments, camera 70 may include a high speed camera, such as, for example, a camera capable of capturing at least 15,000 frames per second (fps). A high speed camera may allow a shearography system to include a dynamic measurement range of up to and/or exceeding 7.5 kHz.
In embodiments, shearography system 100 may include aspects that generally correspond to a Michelson interferometer and/or may include a Michelson-based spatial phase-shift shearography system. For example, optical shearing device 60 may include an interferometer, such as, for example, a modified Michelson interferometer. In embodiments, camera 70 may include portions and/or all of optical shearing device 60. As generally illustrated in
In embodiments, a mirror (e.g., mirror 64) disposed at angle β may be configured to introduce a frequency component to a beam of light and/or angle may be referred to as shearing angle β. For example, and without limitation, mirror 64 disposed at angle β may be configured to introduce a frequency component to first beam of light 112. A frequency component introduced into a beam of light may correspond to a wavelength of the beam of light. For example, and without limitation, frequency component f1 may correspond to a wavelength of first beam of light. Frequency components may be introduced in such a way that after a Fourier transform, phase information may be extracted or derived from a resulting spectrum. Extracting phase information may be accomplished via a windowed inverse Fourier transform (WIFT) and/or via a filter, such as a band pass filter, a gate function filter, and/or a normal function filter. In embodiments, for example, phase information may include a range from 0 to 2π.
In embodiments, third element 66 may include a beam splitter and/or may be referred to herein as beam splitter 66. Beam splitter 66 may include one or more of a variety of configurations. For example, and without limitation, beam splitter 66 may include a cube, which may include two triangular prisms joined together, and/or beam splitter may include a half-silvered element. Beam splitter 66 may be configured such that all of, a portion of, or none of the light that is directed to beam splitter 66 passes through beam splitter 66. In embodiments, beam splitter 66 may be configured to reflect light that does not pass through it. In embodiments, beam splitter 66 may be configured to reflect a first portion of first beam 112B toward mirror 62. Additionally or alternatively, beam splitter 66 may be configured to receive first beam 112 and allow a second portion 112C of first beam 112 to pass through to mirror 64. In embodiments, beam splitter 66 may be configured to allow first portion 112B of first beam 112, which may include about half of first beam 112, to pass through to mirror 64, and/or beam splitter 66 may be configured to reflect second portion 112C, which may include about half of first beam 112, toward mirror 62. Beam splitter 66 may, additionally or alternatively, be configured to allow light reflected from mirror 62 (e.g., first beam second portion 112C) to pass through toward camera 70 as a first wave front 114A and/or reflect light reflected from mirror 64 (e.g., second portion 112C) toward camera 70 as a second wave front 114B.
In embodiments, camera 70 may be configured to capture first wave front 114A corresponding to first portion 112B of first beam 112 and/or capture second wave front 114B corresponding to second portion 112C of first beam 112. The first and second wave fronts 114A, 114B may be represented via the following equations, respectively:
u
1(x,y)=|u1(x,y)|exp[iφ(x,y)] Eq. 1.
u
2(x,y)=|u2(x+Δx,y)|exp{iφ(x+Δx,y)+2πif0·x} Eq. 2.
where u1 corresponds to first wave front 114A, u2 corresponds to second wave front 114B, and Δx corresponds to the shearing distance in the x direction, which may correspond to shearing angle β.
The f0 value may represent the spatial frequency component introduced by second mirror 64 being disposed at shearing angle β. The f0 value may be represented by the following equation:
f
0=(sin β/λ) Eq. 3.
where λ corresponds to a wavelength of first light source 110.
An intensity of light provided to and/or captured by camera 70 may be represented by:
I=(u1+u2)(u1*+u2*)=u1u1*+u2u2*+u1u2*+u2u1* Eq. 4.
where * corresponds to a complex conjugate of u1 (e.g., u1* may correspond to a complex conjugate of u1).
A Fourier transform of the speckle interferogram (e.g., of the intensity equation, Eq. 4), may convert the captured image into the frequency (Fourier) domain from the spatial domain. A Fourier transform of Eq. 4 may be represented by:
where corresponds to a convolution operation, U1(fx, fy) corresponds to a Fourier transform of first wave front 114A (u1), and U2(fx+f0, fy) corresponds to a Fourier transform of second wave front 114B (u2).
In embodiments, an ideal version 158 of spectrum 150 that may result from a Fourier transform corresponding to Equation 5 is generally illustrated in
In practice, spectrum 150 may not be as easily distinguishable as ideal spectrum 158 and/or may more closely resemble the spectrum illustrated in
In embodiments, first portion 152 and/or second portion 154 of spectrum may contain phase information of the captured interferogram (e.g., reference interferogram and/or testing interferogram). A windowed inverse Fourier transform (WIFT) may be applied to spectrum 150 to extract phase information from first portion 152 and/or second portion 154. In embodiments, a window of the windowed inverse Fourier transform may correspond to (e.g., be centered at) frequency component f0 which may correspond to the wavelength of first beam 112 and the shearing angle (see, e.g., Equation 3) and/or the window may include a width of 2fc1 (see, e.g., Equation 11, below). Applying a WIFT may result in the following equation:
where Im may correspond to an imaginary portion of a complex number and Re may correspond to a real portion of the complex number. The Φ term may correspond to a phase difference between wave fronts u1 (unsheared) and u2 (sheared). This phase difference Φ may correspond to an unloaded, non-deformed, and/or reference state of test object. To calculate strain, it may be desirable to compare reference phase difference Φ with a loaded, deformed, and/or testing phase difference Φ′.
In embodiments, a testing phase difference Φ′ may be obtained by similar operations as applied to obtain reference phase difference Φ. For example, a Fourier transform may be applied to an intensity equation of a second image (e.g., corresponding to a test object 50 in second state 54) and a WIFT may be applied to the resulting spectrum to obtain the testing phase difference Φ′. The relative phase difference ΔΦ between the reference and loaded test object may be represented as:
ΔΦ=Φ−Φ′ Eq. 7.
where, Δx corresponds to a shearing amount, d may correspond to an x component
of a gradient of deformation vector,
The s term may correspond to a sensitivity vector, which may correspond to s=ki−k0. The ki term may correspond to a unit vector along the illumination direction and k0 may correspond to a unit vector in the observation direction. Thus, s may be represented according to:
s=(sin θ,0,1+cos α) Eq. 9.
where θ corresponds to an illumination angle. Equation 8 may be solved for
to determine strain in the x-direction.
In embodiments, it may be desirable for a speckle to cover at least six pixels. In embodiments, it may also be desirable to increase spatial resolution to attempt to achieve or to achieve an ideal spectrum, which may allow for easier and/or more accurate extraction of phase information. Larger speckle size may reduce spatial resolution, so it may be desirable to reduce the size of the pixels so that a relatively small speckle may still be able to cover at least six pixels.
In embodiments, a high resolution camera may include a relatively small pixel size, which may help limit the size of the speckles. For example, a 5 Megapixel camera (e.g., a model ICL-B2520M camera available from IMPERX) may include about 2456 pixels by about 2058 pixels, with a pixel size of about 3.4 μm, and a 1.5 Megapixel camera (e.g., a model STC-CL152A available from SENTECH) may include about 1392 pixels by about 1040 pixels, with a pixel size of about 4.7 μm. The reduced pixel size of the 5 Megapixel camera may allow for a reduced speckle size Δs and increased spatial resolution. In embodiments, the difference between a phase map generated via a 1.5 Megapixel camera (e.g., as generally illustrated in
In embodiments, the focus length Lf may affect the speckle size. For example, the speckle size may be represented by:
Δs=λ1Lf/D Eq. 10.
where Δs corresponds to the speckle size, λ1 corresponds to the wavelength of first light source, Lf corresponds to the focus length of an imaging lens and D corresponds to the aperture size. Thus, as focus length Lf decreases, speckle size Δs may also decrease, proportionately.
In embodiments, focus length Lf may be relatively short compared to a conventional Mach-Zehdner Interferometer. In embodiments of a shearography system (e.g., system 100 and/or system 200), additional lenses may be included to increase a field of view of a test object. For example, as generally illustrated in
Aperture 72 may be located between test object 50 and optical shearing device 60. In practical applications of embodiments, certain variables may be relatively fixed and/or constrained (e.g., maximum resolution of CCD camera, focus length, etc.), and the configuration of aperture 72 may be controlled to achieve certain desired system effects. For example, the size D of aperture 72 may be adjusted to control the speckle size in the system. In embodiments, the size D of aperture may be increased to decrease speckle size Δs and/or the size D of aperture may be decreased to increase speckle size Δs.
Additionally or alternatively, the size D of aperture 72 may be configured to act as a spatial frequency filter, which may limit a maximum spatial frequency that can be captured (e.g., the size of the aperture may correspond to a cutoff spatial frequency). A relationship between the size of aperture and the cutoff spatial frequency may include:
f
c1
=D/2λ1Lf Eq. 11.
where fc1 corresponds to a cutoff spatial frequency, D corresponds to aperture size (e.g., a diameter) on a focus plane, λ1 corresponds to a wavelength of first light source 112 and Lf corresponds to a focus length of image lens 132.
In embodiments, a sufficiently large spatial frequency shift f0 may make it easier and/or possible to separate various portions of spectrum from each other in the frequency domain. In embodiments, it may be desirable to include a spatial frequency shift f0 that is at least twice the cutoff frequency fc. Thus, it may be desirable for the following relationship to be true:
Thus, it may be desirable to introduce shearing angle β according to the following relationship:
The relationship of Equation 13 may be derived from Equations 3, 11, and 12.
In embodiments, a spatial frequency shift f0 may be introduced by disposing at least one of the mirrors of an interferometer (e.g., mirror 64) in an orientation corresponding to the shearing angle β. Shearing angle β may also have a maximum desired value, which may correspond to the maximum spatial frequency that can be captured by camera 70. A maximum spatial frequency that may be captured by a camera may correspond to one half of the pixel size of the camera. A maximum desired value of shearing angle β may be determined according to:
Equation 14 may also be represented as:
Thus, it may be desirable if the shearing angle β satisfies the following:
Equation 16 may also be represented as:
In embodiments, increasing aperture size D as much as possible may provide a greater signal to noise (S/N) ratio, which may be desirable. As the S/N ratio increases, it may be possible to use a greater portion of spectrum 150 resulting from the Fourier transformation. However, it may be desirable to configure aperture size D to accommodate for increased speckle size Δs (e.g., Eq. 10) and non-frequency aliasing (e.g., Eq. 16). A smaller aperture size D may correspond to increased speckle size Δs, and/or may prevent and/or reduce non-frequency aliasing.
In embodiments, it may be desirable to configure aperture size D to balance improving signal-to-noise ratio S/N with increasing speckle size Δs and/or with reducing non-frequency aliasing. For example, in embodiments, shearing angle β may be determined first according to a desired measurement sensitivity, as measurement sensitivity may be proportional to the shearing angle β. Once shearing angle β is selected, the largest aperture size D permitted according to Equation 17 may be selected to achieve as high of a S/N ratio as possible.
In embodiments, system may include one or more WIFT filters 170. WIFT filter 170 may be configured to extract and/or filter phase information from a spectrum, such as, for example, a spectrum corresponding to a Fourier transform of an image. It may be desirable to configure an algorithm of WIFT filter 170 to be relatively simple so that the algorithm may be carried out relatively quickly, which may allow for use of WIFT filter 170 in conjunction with dynamic measurements.
In embodiments, WIFT filter 170 may comprise a gate function 172. A plot of a gate function 172 is generally illustrated in
In embodiments, WIFT filter 170 may comprise a normal function filter, which may be represented as:
As the inverse Fourier transformation of a normal function (e.g., a normal function filter) may still be a normal function, a WIFT filter 170 comprising a normal function filter may not introduce extra fringes and/or as many extra fringes as embodiments in which WIFT filter 170 comprises a gate function. As generally illustrated in
As generally illustrated in
In embodiments, shearography system 200 may include a second light source 210. Second light source 210 may be configured to provide a second light and/or laser, which may include a second wavelength. In embodiments, first wavelength of first light source 110 and second wavelength of second light source 210 may be equal or may be unequal. Second light source 210 may be disposed on an opposite side of optical shearing device 70 relative to first light source 110 (e.g., first light source and second light source 210 may be separated by about 180 degrees when viewed from test object 50). Second light source 210 may be configured to illuminate (e.g., direct a second beam 212 toward) test object 50. In embodiments, first light source 110 and second light source 210 may be configured to simultaneously illuminate test object 50 test object. In embodiments, second beam 212 may be expanded into an expanded second beam 212A via a second beam expander 220. In embodiments, first and second light sources 110, 210 may be configured to direct first beam 112 and second beam 212, respectively, toward the same location and/or area of test object 50 (e.g., illuminate test object 50). In embodiments, first and second light sources 110, 210 may be configured to simultaneously direct first beam 112 and second beam 212, respectively, toward the same location and/or area of test object 50 (e.g., simultaneously illuminate test object 50). In embodiments, it may be desirable for first wavelength λ1 and the second wavelength λ2 to be unequal, which may reduce and/or eliminate interference between first light source 110 and second light source 210 (e.g., relative to the first and second wavelengths being equal). In embodiments, first light source 110 and second light source 210 may be aligned with each other or, as generally illustrated in
In embodiments, shearography system 200 may allow for simultaneously measuring individual components (e.g., in the x-direction, in the y-direction, and/or the z-direction) of strain. Camera 70 may be configured to obtain intensity information corresponding to interferograms of sheared and unsheared versions of beams of light (e.g., first beam 112 and second beam 212). Intensity information may then be used to determine phase difference information between loaded and unloaded states of test object 50 for the first beam 112 and/or the second beam 212. Phase difference information may then be used to calculate strain, which may include calculating individual components of pure in-plane normal strain, pure in-plane shear strain, and pure out-of-plane shear strain.
In embodiments, first beam expander 120 may be configured to expand first beam 112, which may result in illumination of a larger area of test object 50. In embodiments, second beam expander 220 may be configured to expand second beam 212, which may result in illumination of a larger area of test object 50 (e.g., relative to second beam 212 illuminating test object 50 without second beam expander 220). First and second beams 112, 212 may illuminate generally the same area of testing object. First beam 112 and/or second beam 212 may reflect off of testing object 50 in a direction toward aperture 72.
In embodiments, a mirror (e.g., mirror 62 and/or mirror 64) may be tilted to introduce a first frequency component f1 to first beam 112 and/or a second frequency component f2 to second beam 212. The introduction of a frequency component may be referred to as shearing. First and second spatial frequency components f1, f2 may be represented, respectively, as:
f
1=(sin β/λ1) Eq. 19.
f
2=(sin β/λ2) Eq. 20.
where β corresponds to the shearing angle (e.g., the tilting angle of mirror 64), λ1 corresponds to a wavelength of first light source, and λ2 corresponds to a wavelength of second light source. Thus, an introduced spatial frequency components (e.g., f1, f2) may correspond to a ratio of the shearing angle β and the wavelength (e.g., λ1, λ2) of the respective light source.
In embodiments, a plurality of wave fronts may result from optical shearing device 60. For example, in embodiments, first wave front 114A may correspond to (e.g., originate from) the direction and/or first light source 110, and/or may reflect off of mirror 62, and may be represented by the following equation:
u
11(x,y)=|u11(x,y)|exp[iφ1(x,y)] Eq. 21.
In embodiments, second wave front 114B may correspond to the ki1 direction, first light source 110, and/or mirror 64, and may be represented by the following equation:
u
12(x,y)=|u12(x+Δx,y)|exp{φ1(x+Δx,y)+2πif1·x} Eq. 22.
In embodiments, a third wave front 214A corresponding to the ki2 direction, second light source 210, and/or mirror 62 may be represented by the following equation:
u
21(x,y)=|u21(x,y)|exp[iφ2(x,y)] Eq. 23.
In embodiments, a fourth wave front 214B corresponding to the ki2 direction, second light source 210, and/or mirror 64 may be represented by the following equation:
u
22(x,y)=|u22(x+Δx,y)|exp{iφ2(x+Δx,y)+2πif2·x} Eq. 24.
Camera 70 may be configured to detect, receive, capture, and/or measure the light and/or the intensity of the light output from optical shearing device 60, such as, for example, wavefronts 114A, 114B, 214A, 214B, which may include light reflections corresponding to first light source 110 and/or second light source 210. In embodiments, camera 70 may be configured to obtain intensity information while first light source 110 and second light source 210 are simultaneously illuminating test object 50. For example, camera 70 may be configured to record the intensity of wavefronts 114A, 114B, 214A, 214B. In embodiments, a recorded intensity may be represented by:
where * corresponds to a complex conjugate of ui (e.g., u1* corresponds to a complex conjugate of u1).
The recorded intensity may be processed by processing unit 80. For example, and without limitation, camera 70 may be configured generate one or more electrical signals corresponding to measured intensity and processing unit 80 may be configured to receive and/or process the signal or signals. Processing unit 80 may be configured to apply a Fourier transform to an intensity information (e.g., Equation 25). The result of a Fourier transform applied to Equation 25 may be represented by:
where is the convolution operation, U11(fx, fy)=FT(u11), U12(fx+f1, fy)=FT(u12), U21(fx, fy)=FT(u21), U22(fx+f2, fy)=FT(u22).
In embodiments, as generally illustrated in
In embodiments, the remaining terms may include phase information. The U12U11* term may correspond to portion 238 of spectrum 230, which may be located at or near (f1, 0). The U12U11* term may correspond to portion 236 of spectrum 230, which may be located at or near (−f1, 0). The U12U11* and U12U11* terms may each include/correspond to a spectrum width of 2fc1. These two terms may include phase information of the recorded speckle interferogram in the ki1 direction.
The U22U21* term may correspond to portion 240 of spectrum 230, which may be located at or near (f2, 0). The U21U22* term may correspond to portion 234 of spectrum 230, which may be located at or near (−f2, 0). The U22U21* term and/or the U21U22* term may correspond to a spectrum width of 2fc2. One or both of the U22U21* term and/or the U21U22* term may include phase information related to the recorded speckle interferogram in the ki2 direction.
In embodiments, values of cutoff frequencies fc1 and fc2 may correspond to the size of aperture D. For example, and without limitation,
f
c1
=D/2λ1Lf Eq. 27.
f
c2
=D/2λ2Lf Eq. 28.
where D corresponds to an aperture size (e.g., a diameter) on a focus plane, λ1 corresponds to a wavelength of first light source 112, λ2 corresponds to a wavelength of second light source 212, and Lf corresponds to a focus length of image lens 132.
A Windowed Inverse Fourier Transform (WIFT) may be applied to the Fourier transformed intensity information. In embodiments, a first window of the windowed inverse Fourier transform may correspond to (e.g., be centered at) frequency component f1 which may correspond to the wavelength of first beam 112 and the shearing angle (see, e.g., Equation 19) and/or the first window may include a width of 2fc1. A second window of the windowed inverse Fourier transform may correspond to frequency component f2 which may correspond to the wavelength of second beam 212 and the shearing angle (see, e.g., Equation 20), and/or the second window may include a width of 2fc2. Applying a WIFT to the Fourier transformed intensity information (e.g., to spectrum portion 238 and/or spectrum portion 240) may allow for the phase information (e.g., distributions) to be determined, such as via the complex amplitudes. For example, applying a WIFT to a Fourier transformed captured intensity may result in the following:
where Im and Re correspond to the imaginary and real part of the complex numbers, respectively. In embodiments, φi may correspond to a phase difference between a sheared portion of a beam and an unsheared portion of a beam. In embodiments, φ1 may correspond to a phase difference between first (e.g., unsheared) portion of first beam (φ1(x, y)) and second (e.g., sheared) portion of first beam (φ1(x+Δx, y)):
φ1=φ1(x,y)−φ1(x+Δx,y) Eq. 31.
In embodiments, φ2 may correspond to a phase difference between first (e.g., unsheared) portion 212B of second beam 212 (φ2(x, y)) and second (e.g., sheared) portion 212C of second beam 212 (φ2(x+Δx, y)):
φ2=φ2(x,y)−φ2(x+Δx,y) Eq. 32.
Phase differences φ1 and φ2 may correspond to a reference state 52 of test object 50.
In embodiments, a second measurement may, additionally or alternatively, be taken for second state 54 of test object 50. A second measurement may be similar to the reference measurement. For example, applying a Fourier transform to the captured intensity from object 50 in second state 54 and then applying a WIFT may result in the following:
where φ1′ and φ2′ represent the phase differences corresponding to the second state 54 (e.g., a testing state) of test object 50. A first relative phase difference Δ1 may be calculated for light corresponding to first light source φ1′ (second state 54) and φ1 (first state 52) and/or a second relative phase difference Δ2 may be calculated for light corresponding to second light source φ2′ (second state 54) and φ2 (first state 52).
Δ1=φ1′−φ1 Eq. 35.
Δ2=φ2′−φ2 Eq. 36.
Relative phase differences Δ1 and/or Δ2 may correspond to the deformation and/or gradient of deformation, if any, of test object 50 in second state 54 relative to test object 50 in first state 52. Based on the relationship between relative phase difference and strain, Equations 35 and 36 may be solved to obtain at least two strain components. For example, and without limitation, for a configuration including x, z illumination and shearing in the x-direction (e.g., via second mirror 64 tilting with respect to the x axis, such as generally illustrated in
(pure normal in-plane strain) and
(pure shear out-of-plane strain) may be solved for via the following equations:
where θ corresponds to an illumination angle. Equations 37 and 38 may then result in the following equation that may be solved/computed (e.g., all of λ1, λ2, Δ1, Δ2, and Δx are known) for pure in-plane strain in the x-direction:
A similar calculation may be used to determine
and/or the calculated
may be inserted into Equation 37 or Equation 38. Thus, via a single setup/configuration, pure in-plane normal strain and pure out-of-plane shear strain may be calculated. If the illumination plane is changed to the y-z plane (e.g., via rotating first light source 110 and second light source 210 in a clockwise or counterclockwise direction 90 degrees), a y-direction component of pure in-plane shear strain with shearing in the x-direction may be calculated via the following equation:
In embodiments, a shearing direction may be changed. For example, and without limitation, the following equations may correspond to changes in shearing direction. An x-direction component of pure in-plane shear strain,
with shearing in the y-direction (e.g., via tilting second mirror 64 with respect to the y-axis), may be determined from the following equation:
A y-direction component of pure in-plane normal strain may be calculated via the following equation:
If the shearing direction is changed to the y-direction, a y-component of pure out-of-plane shear strain,
may be calculated similarly to the calculation of
above.
An exemplary schematic of a method of measuring strain conducted in embodiments of a shearography system (e.g., shearography system 200) is generally illustrated in
In embodiments, test object 50 may then be loaded and, in step 260, testing information (e.g., testing intensity information and/or a testing image), which may include one or more spatial frequency components, may be acquired of the loaded test object 50 via camera 70. In step 262, a Fourier transform may then be applied to the testing image (loaded) to obtain a testing spectrum. In step 264 and/or step 266, a WIFT may be applied to the testing information and/or spectrum according to the one or more spatial frequency components to obtain a testing phase difference or testing phase differences for the loaded test object. For example, and without limitation, a first WIFT, which may correspond to spatial frequency component f1, and a second WIFT, which may correspond to spatial frequency component f2, may be applied to the testing spectrum.
In steps 270 and 272, relative differences between phase differences corresponding to each spatial frequency component may then be determined with respect to the reference information and the testing information. For example, and without limitation, a difference between phase difference φ1 and phase difference φ1′ may correspond to frequency component f1, and/or a difference between phase difference φ2 vs. φ2′ may correspond to frequency component f2. In step 280, a strain measurement may then be determined from the relative phase differences.
In embodiments, as generally illustrated in
In embodiments, including third light source 320 and/or fourth light source 420 may allow for illuminating test object 50 in more than one direction (e.g., in the x-direction and in the y-direction). Illuminating test object 50 in more than one direction may allow shearography system to simultaneously measure strain in a plurality of directions. For example and without limitation, all of a plurality of light sources (e.g., first light source 110, second light source 210, third light source 310, and fourth light source 410) may be configured to simultaneously illuminate an area of test object 50. First light source 110 and second light source 210 may provide illumination in the x-z plane (e.g., as generally illustrated in
a y-direction component in-plane shear strain,
and/or an x-direction component of out-of-plane shear strain,
simultaneously and/or from a single image. In embodiments, using at least three light sources with shearing in the y-direction may allow for measuring an y-direction component of pure in-plane normal strain,
a x-direction component pure in-plane shear strain,
and/or an y-direction component of pure out-of-plane shear strain,
simultaneously and/or from a single image.
In embodiments, first light source 110, second light source 210, third light source 310, and fourth light source 410 may simultaneously illuminate an area of test object 50 with first beam 112, second beam 212, third beam 312, and fourth beam 412, respectively. First beam 112 and second beam 212 may include wavelengths λ1 and λ2, respectively, and may be set in the X-Z plane. Third beam 312 and fourth beam 412 may include wavelengths λ3 and λ4, respectively, and may be set in the Y-Z plane. The illumination angle θ may be the same for each beam, and the following equations may represent the phase difference for each beam:
A resulting relationship for four beam illumination and shearing in the x direction may be:
A resulting value for
and/or
may be inserted into Equation 43 and/or Equation 44 to obtain
If the shearing direction is the y direction, a resulting relationship for a four light source illumination setup:
A resulting value for
and/or
may be inserted into Equation 45 and/or Equation 46 to obtain
For example, and without limitation, in a four light source illumination setup (e.g., as generally illustrated in
and
in one measurement (e.g., not just a total strain measurement). In embodiments, a y-direction shearing configuration may permit measuring three individual strain components:
and
in one measurement.
In embodiments, camera 70 may be configured to obtain intensity information corresponding to each of the plurality of light sources illuminating test object 50. The intensity information may include reference intensity information that may be obtained while test object 50 is in a reference state and may include testing intensity information that may be obtained while test object is in a testing state. The reference intensity information may include a plurality of interferograms that may each correspond to a particular light source. For example, and without limitation, a plurality of reference interferograms may include a first interferogram that may correspond to first light source, a second interferogram that may correspond to second light source 210, a third interferogram that may correspond to third light source, 310, and/or a fourth interferogram that may correspond to fourth light source. Camera 70 may be configured to capture some or all of the first interferogram, the second interferogram, the third interferogram, and/or the fourth interferogram in a single reference image.
In embodiments, testing intensity information may include a plurality of interferograms that may each correspond to a particular light source. A plurality of testing interferograms may include a fifth interferogram that may correspond to first light source, a sixth interferogram that may correspond to second light source 210, a seventh interferogram that may correspond to third light source, 310, and/or an eighth interferogram that may correspond to fourth light source. Camera 70 may be configured to capture some or all of the fifth interferogram, the sixth interferogram, the seventh interferogram, and/or the eighth interferogram in a single testing image.
In embodiments, a shearography system may be configured to determine a strain measurement from only a single reference image and a single testing image.
In embodiments, an interferogram may include one or more image components. Image components may correspond to a particular light source, the wavelength of a light produced by the particular light source, and/or the shearing angle. For example, and without limitation, a first interferogram may correspond to first light source 110 (e.g., result from first beam 112 reflecting off of test object 50) and/or may include two image components that have been sheared by shearing angle β. As described above, an amount of shearing between the two image components may correspond to the wavelength of first beam 112 and shearing angle β. In embodiments, image components may be identical.
In embodiments, a reference image may be acquired before testing begins. One or more testing images may be captured and processed generally according to the process shown and/or described in connection with
In embodiments, a reference image may be updated with the most recent testing image and then compared to a new test image. For example, and without limitation, a reference image may be obtained of an unloaded test object and a first testing image may be obtained of the test object under a first load. The reference image may then be compared to the first testing image, which may permit a determination of strain. Then, the reference image may be updated with the first testing image and compared with a second testing image that may be obtained of the test object under a second load, which may permit a second determination of strain.
It should be understood that embodiments (e.g., shearography system 100 and/or shearography system 200) do not require a temporal phase shift to determine strain. It should also be understood that switching between light sources located in difference places may not be required. For example, different wavelengths of first, second, third, and/or fourth light sources 110, 210, 310, 410 may allow for all light sources to be simultaneously illuminated and/or directed toward a test object 50, which may allow for camera 70 to acquire a single image containing multiple distinguishable speckle inteferograms. Thus, during testing, embodiments of a shearography system may be configured to process a single testing image acquired, without requiring a shutter operation, and compare it to a single reference image to determine strain (e.g., shearography system 100 and/or shearography system 200 may use as few as two total images, which may each contain a plurality of speckle interferograms, to determine strain).
In contrast, in conventional shearography systems 1000, as generally illustrated in
Embodiments of the present disclosure may include one or more advantages relative to conventional systems, such as the system 1000 generally represented in
Another advantage of embodiments of a shearography system 100, 200 may include a simpler structure with better resulting image quality relative to conventional systems, such as conventional Mach-Zehnder interferometer-based shearography. For example, embodiments of shearography system 100 may include fewer beam splitters (e.g., one for each light source), fewer mirrors (e.g., two), and/or a shorter image distance, which may permit higher quality phase maps.
Another advantage of embodiments of a shearography system according to the present disclosure (e.g., shearography system 100 and/or shearography system 200) may include an adjustable field of view. For example, and without limitation, in embodiments including a 4f system, a field of view may be adjusted not only by an image lens 132, but may also be adjusted via lens 134 and/or lens 136.
Another advantage of embodiments of a shearography system according to the present disclosure may include the ability to measure pure out-of-plane strain components
In conventional systems, measurement of out-of-plane strain components are not pure because such measurements inevitably include at least some elements of other strain components (e.g., in-plane strain components).
It should be understood that, in embodiments, at least some of the steps of determining a strain measurement could be completed in real-time, near real-time, or offline. For example, a series of images of a dynamic test object could be collected over a period of time and later analyzed to determine strain. Additionally or alternatively, the efficiency of analyzing a single image before loading and a single image after loading may allow for faster processing times that may approach real-time or effectively be real-time.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and various modifications and variations are possible in light of the above teaching. It should be understood that references to a single element are also intended to include embodiments that may include more than one of that element or zero of that element. For example, references to a light source are intended to include embodiments with one light source or more than light source. Also, references to a light source are not limited to a particular type of light source or laser and are intended to include any type of light source or other component with similar functionality.
The embodiments and examples were chosen and described to explain the principles of the invention and a practical application, to thereby enable others skilled in the art to utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.
This application claims the benefit of priority to U.S. Provisional application No. 61/896,391 filed 28 Oct. 2013, the entire disclosure of which is hereby incorporated by reference as though fully set forth herein.
Filing Document | Filing Date | Country | Kind |
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PCT/US14/62610 | 10/28/2014 | WO | 00 |
Number | Date | Country | |
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61896391 | Oct 2013 | US |