Contrast patterns with prescribed frequency content for image analysis

Abstract

A contrast pattern, that can, for example, be applied to an object, such as, but not limited to, an object that is subjected to analysis using digital image correlation, is provided. The contrast pattern includes a plurality of superposed gratings. Each of the plurality of superposed gratings includes a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency. Further, methods of making and applying the contrast pattern to a substrate or adhesive layer are also provided.

Description

[0001] USC Tech ID 1739

[0002] PCT Patent Application

[0003] CONTRAST PATTERNS WITH PRESCRIBED FREQUENCY CONTENT FOR IMAGE ANALYSIS

[0004] Government Support Clause

[0005] This invention was made with government support under Grant Number D23AP00190-00, awarded by DARPA (U.S. Department of Interior). The government has certain rights in the invention.

[0006] Related Application

[0007] The present application claims priority to U.S. Provisional Patent Application No. 63 / 764,667, filed February 28, 2025, the entire contents of which are incorporated herein by reference.

[0008] Technical Field

[0009] The subject matter disclosed herein is generally directed to a pattern and methods of making a pattern to increase digital image correlation accuracy, increase spatial resolution, prevent loss of correlation / data loss, and decrease computation time.

[0010] Background

[0011] Digital image correlation (DIC) is a non-destructive and non-contact optical technique for image analysis that is often used to measure the mechanical behavior (e.g., deformation, strain, etc.) of a specimen by tracking a pattern on the surface of the specimen via a digital camera. DIC is one of the most popular methods used by scientists and engineers to measure the motion and deformation of objects (e.g., test specimens). For this technique to work, the object being photographed is typically painted with a random speckle pattern that usually consists of many small paint splotches. A digital image correlation algorithm is then implemented in software and tracks small groups of these paint splotches through multiple images to provide a map of deformation over the entire surface of the test specimen. Creating a reliable and high quality speckle pattern for digital image correlation is often a challenge.

[0012] Creating a random contrast pattern for the purposes of digital image correlation is often done through spray painting. This process makes it difficult toUSC Tech ID 1739

[0013] PCT Patent Application

[0014] control speckle sizes, density, and uniformity. In recent years, stamp and roller kits have been brought to market to bring greater control and repeatability to the task of applying dots to a surface of a test specimen. However, this is simply an improved method to produce a quasi-random pattern of dots, and dot patterns do not directly control the frequency content of the image and require the use of rather large dots (e.g., dots that are 3-5 pixels in diameter) in order to avoid aliasing errors. Thus, in order to avoid aliasing, which leads to inaccurate correlation, the desired high intensity gradients between many pixels have to be reduced.

[0015] As such, a need currently exists for an improved contrast pattern to increase the accuracy of digital image correlation in various applications including, but not limited to, mechanical testing of materials.

[0016] Summary

[0017] Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or can be learned from the description, or can be learned through practice of the embodiments.

[0018] In one aspect of the present disclosure, a contrast pattern for application to an object, such as, but not limited to, an object being analyzed using digital image correlation, is provided. The contrast pattern includes a plurality of superposed gratings, each of the plurality of superposed gratings comprising a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency.

[0019] In another aspect, the plurality of superposed gratings can include at least two pairs of superposed gratings.

[0020] In still another aspect, the function facilitates a DIC subset size of less than about 21 pixels by 21 pixels.

[0021] In yet another aspect, the function can include a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a square wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof. For example, the function can include one or more sine functions.USC Tech ID 1739

[0022] PCT Patent Application

[0023] Further, in the case of sine functions, each of the plurality of superposed gratings can have an intensity determined according to the following equation, where A refers to an amplitude, A refers to a wavelength, 6 refers to a rotation angle, 0 refers to a phase shift, and Ao refers to an initial background intensity to which the contrast pattern is superposed:

[0024] & >

[0025]

[0026] In addition, the amplitude can range from 0 to about 255 for 8-bit imaging, from 0 to about 4,095 for 12-bit imaging, from 0 to about 16,383 for 14-bit imaging, and from 0 to about 65,535 for 16-bit imaging.

[0027] Additionally, the wavelength can range from about 2 pixels to about 200 pixels.

[0028] In one aspect, the rotation angle can range from about 0° to about 90°, although it is to be understood that the rotation angle can have any value.

[0029] In another embodiment of the present disclosure, a substrate is provided. The substrate includes the contrast pattern of any of the foregoing claims, wherein the contrast pattern is applied to a surface of the substrate.

[0030] In yet another embodiment, an adhesive sheet is contemplated, wherein the adhesive sheet includes the contrast pattern described above, wherein the contrast pattern is applied to a surface of the adhesive sheet.

[0031] In still another embodiment, a test specimen is contemplated, wherein the contrast pattern described above is applied to one or more surfaces of the test specimen.

[0032] A method of making a digital image correlation contrast pattern for application to an object is also contemplated by the present disclosure. The method includes obtaining a substrate; applying an adhesive to a lower surface of the substrate; and applying a contrast pattern to an upper surface of the substrate, wherein the contrast pattern comprises a plurality of superposed gratings, each of the plurality of superposed gratings comprising a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency.

[0033] In one aspect, the method can further include attaching the adhesive to the object.USC Tech ID 1739

[0034] PCT Patent Application

[0035] In still another aspect, the plurality of superposed gratings can include at least two pairs of superposed gratings.

[0036] In one more aspect, the function can facilitate a pixel subset size of less than about 21 pixels by 21 pixels.

[0037] In yet another aspect, the function can include a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a square wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof.

[0038] Another method of making a digital image correlation contrast pattern application to an object is also provided. The method includes obtaining a substrate; applying a contrast pattern to a surface of the substrate; contacting the surface of the substrate on which the contrast pattern is applied to an upper surface of an adhesive layer; and transferring the contrast pattern onto the upper surface of the adhesive layer, wherein the contrast pattern comprises a plurality of superposed gratings, each of the plurality of superposed gratings comprising a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency.

[0039] In one aspect, the method can further include attaching a lower surface of the adhesive layer to a test specimen.

[0040] In still another aspect, the plurality of superposed gratings can include at least two pairs of superposed gratings.

[0041] In one more aspect, the function can facilitate a pixel subset size of less than about 21 pixels by 21 pixels.

[0042] In yet another aspect, the function can include a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a square wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof.

[0043] These and other features, aspects, and advantages of various embodiments of the present disclosure will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exampleUSC Tech ID 1739

[0044] PCT Patent Application

[0045] embodiments of the present disclosure and, together with the description, serve to explain the related principles.

[0046] Brief Description of the Drawings

[0047] A full and enabling disclosure of the present disclosure to one skilled in the art, including the best mode thereof, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:

[0048] FIG. 1 is a schematic representation of a mechanical testing system utilizing a sample on which the contrast pattern contemplated by the present disclosure is applied;

[0049] FIG. 2 is a schematic representation of a sample on which the contrast pattern contemplated by the present disclosure is applied when loaded into the mechanical testing system of FIG. 1;

[0050] FIG. 3 is a schematic representation of a sample on which the contrast pattern contemplated by the present disclosure is applied;

[0051] FIG. 4 is an example of a contrast pattern generated from square waves that has a known frequency spectrum but lacks the pseudo-randomness required by the contrast patterns contemplated by the present disclosure;

[0052] FIG. 5 illustrates the formation of a contrast pattern that exhibits a single prescribed value of frequency content but without the pseudo-randomness required by the contrast patterns contemplated by the present disclosure;

[0053] FIGs. 6A-6D illustrate contrast patterns that exhibit the prescribed frequency content and the pseudo-randomness required by the contrast patterns contemplated by the present disclosure as they would be recorded by a digital imaging device with the optimally matched spatial resolution, where the actual contrast pattern has smoothly varying contrast as shown in FIG. 5, where each contrast pattern has been optimized for the expected signal to noise ratio (SNR) of the imaging device;

[0054] FIGs. 7A-7D are graphs of the mean displacement RMSE (in mPx) for rigid body translation on the y-axis as a function of the minimum prescribed grating wavelength on x-axis, evaluated across multiple subset sizes (9, 11, 13, 17, 25, 45) and optimized for four noise conditions: (A) SNR = 30 dB, (B) SNR = 40 dB, (C)USC Tech ID 1739

[0055] PCT Patent Application

[0056] SNR = 50 dB, and (D) noise-free, as shown in FIGs. 7A, 7B, 7C, and 7D, respectively.

[0057] FIGs. 8A-8D are graphs of the mean displacement RMSE (in mPx) on the y-axis as a function of the maximum amplitude of the grating waves on axis, evaluated across multiple subset sizes (9, 11, 13, 17, 25, 45) and optimized for four noise conditions: (A) SNR = 30 dB, (B) SNR = 40 dB, (C) SNR = 50 dB, and (D) noise-free, as shown in FIGs. 8A, 8B, 8C, and 8D, respectively.

[0058] FIG. 9. is an image showing the Fourier transform of the image and validating that the frequency content of the pattern is at the prescribed values concentrated near the Nyquist frequency and approximately 200 times less than the Nyquist frequency.

[0059] FIGs. 10A and 10B are images showing the full field displacement analysis of a synthetic image with rigid motion of (-2.058 (see FIG. 10A), 5.731 (see FIG. 10B)) px to which the contrast pattern of the present disclosure has been applied;

[0060] FIG. 11 is a zoomed-in photograph of a contrast pattern contemplated by the present disclosure after application to a test sample;

[0061] FIGs. 12A-12D are images showing (A) application of the pattern of the present disclosure for analysis with 3D-DIC at small strain levels and (B) application of the pattern of the present disclosure for analysis with 3D-DIC at large strain levels compared to (C) application of a standard dot pattern with 3D-DIC at small strain levels and (D) application of a standard dot pattern with 3D-DIC at large strain levels.

[0062] FIG. 13 is a cross-sectional view of the contrast pattern of the present disclosure after application to a test specimen;

[0063] FIG. 14 is a cross-sectional view of another contrast pattern contemplated by the present disclosure after application to a test specimen;

[0064] FIG. 15 is a flow chart outlining a method for applying the contrast pattern to the test specimen; and

[0065] FIG. 16 is a flow chart outlining another method for applying the contrast pattern to the test specimen.USC Tech ID 1739

[0066] PCT Patent Application

[0067] Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present disclosure.

[0068] Detailed Description of Representative Embodiments It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present disclosure. Any of the features, components, or details of any of the arrangements or embodiments disclosed in this application are interchangeably combinable with any other features, components, or details of any of the arrangements or embodiments disclosed herein to form new arrangements and embodiments.

[0069] Generally speaking, the present disclosure is directed to a contrast pattern for application to, by way of example only, a test specimen undergoing mechanical testing using digital image correlation. However, it is to be understood that the contrast pattern is contemplated for use in various situations, including, but not limited to, making measurements on in-service structures, bridges, aircraft, the human body, or in any situation in which image analysis of an object is desired. The contrast pattern includes a plurality of superposed gratings. Each of the plurality of superposed gratings includes a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency. Further, methods of making and applying the contrast pattern to a substrate or adhesive layer are also contemplated by the present disclosure.

[0070] Without intending to be limited by any particular theory, the present inventors have found that the contrast patterns contemplated by the present disclosure greatly improve the quality of the patterns to apply to test specimens that are analyzed via DIC to allow for higher spatial resolution digital image correlation measurements and increased reliability of the pattern application process to reduce data loss. The present disclosure allows for the frequency content of the contrast pattern to be specified in its design. With the disclosed pattern generation process, high intensity gradients between all pixels can be achieved without creating aliasing errors. It is generally known in the art that aliasing is a phenomenon that occurs in signalUSC Tech ID 1739

[0071] PCT Patent Application

[0072] processing, particularly in digital signal processing (DSP), when a continuous signal is sampled at a frequency that is too low to accurately represent the original signal. In the digital imaging context, aliasing occurs when the object being imaged contains features smaller than the spatial resolution of the image. In general digital photography, aliasing results in image artifacts such as the appearance of moire patterns, and in DIC aliasing causes bias error from subpixel interpolation. In simple terms, the Nyquist rate, also known as the Nyquist sampling theorem, is the minimum sampling rate required to accurately sample and reconstruct a signal or pattern without distortion. It is defined as twice the maximum frequency present in the signal. Typically, sampling at a rate below the Nyquist rate can lead to aliasing. However, the disclosed pattern generation process allows for higher data density by allowing for high intensity gradients between all pixels without aliasing. For example, the frequency content of the contrast patterns contemplated by the present disclosure can be about 200 times lower than the Nyquist frequency to about the Nyquist frequency, such as from about 100 times lower than the Nyquist frequency to about 1.5 times lower than the Nyquist frequency, such as from about 50 times lower than the Nyquist frequency to about 3 times lower than the Nyquist frequency. Further, by controlling the frequency content, the various functions utilized to form the contrast patterns of the present disclosure allow for the use of a pixel subset size of less than about 21x21, such as less than about 17x17, such as less than about 15x15, such as less than about 13x13, such as less than about 11x11, such as less than about 9x9. In any event, the pixel subset size is less than the typical 21x21 pixel subset size for DIC. The use of a smaller subset size has the benefit of allowing for higher spatial resolution of the quantities measured with DIC. Smaller subset sizes also decrease the amount of computation time to conduct DIC. Finally, the new contrast pattern can be used to reduce measurement uncertainty with the larger subset sizes (e.g., subset sizes of 21x21 or 49x49) typically used in DIC. Specifically, it should be understood that features of the present disclosure can be used with larger DIC subset sizes, such as subset sizes of 65 x 65 or even larger.

[0073] In one embodiment, the contrast pattern can be based on a superposition of a plurality of gratings. The plurality of gratings can be based on a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a squareUSC Tech ID 1739

[0074] PCT Patent Application

[0075] wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof. In one embodiment, the wavelength, amplitude, rotation angle, phase offset, ora combination thereof for each of the plurality of gratings can be specified to avoid aliasing, maximize intensity gradients, and / or maximize the spatial randomness of the contrast pattern. For instance, the wavelength can be selected to avoid aliasing, while the amplitude, rotation angle, and phase offset can be selected to maximize the intensity gradients and spatial randomness of the resulting contrast pattern. Thus, the contrast patterns contemplated by the present disclosure can achieve high intensity gradients without sharp black to white transitions, allowing the contrast patterns to be printed with higher accuracy compared to the dot patterns used in the current state of the art. The various features of the periodic and non-periodic functions utilized for the plurality of superposed gratings used to form the contrast patterns contemplated by the present disclosure are discussed in detail below.

[0076] Regardless of the particular function used to form the contrast pattern, the resulting contrast pattern can enhance the efficacy and / or efficiency of DIG, resulting in a higher spatial density of data; the ability to better resolve large deformation gradients; faster computation time; decreases in experimental uncertainty; faster application of the contrast pattern, simpler patterning procedures; and decreased risk of losing data from correlation errors.

[0077] As mentioned above, the contrast patterns contemplated by the present disclosure achieve high intensity gradients without sharp black to white transitions, thus the contrast patterns can be printed with higher accuracy than the dot patterns that are the current state of the art.

[0078] Referring now to FIG. 1, a mechanical testing system 100 that can be used to subject a test specimen 114 to one or more mechanical testing protocols via a mechanical testing apparatus 118 and monitor and track the movement of a contrast pattern 120 applied to the test specimen 114 via the methods of the present disclosure to determine various mechanical properties of the test specimen is illustrated. The system 100 can include a special purpose computer control system 104 for implementing the mechanical testing protocols. More particularly, the exemplary system 100 can include at least one image capture device 102 in theUSC Tech ID 1739

[0079] PCT Patent Application

[0080] form of a digital camera that can include a charge coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS). Further, a computer control system 104 may generally include such components as at least one memory / media element or database for storing data and software instructions as well as at least one processor 106. In the particular example of FIG. 1, the processor 106 and associated memory / media elements 108a, 108b, and 108c can be configured to perform a variety of computer-implemented functions (i.e., software-based data services). At least one memory / media element 108b is dedicated to electronically storing software and / or firmware in the form of computer-readable and executable instructions that will be implemented by the processor 106. Other memory / media elements (e.g., memory / media elements 108a and 108c) can be used to store data which will also be accessible by the processor 106 and which will be acted on per the software instructions stored in memory / media element 108b.

[0081] The various memory / media elements of FIG. 1 may be provided as a single or multiple portions of one or more varieties of computer-readable media, such as but not limited to any combination of volatile memory (e.g., random access memory (RAM, such as DRAM, SRAM, etc.)) and nonvolatile memory (e.g., ROM, flash, hard drives, magnetic tapes, CD-ROM, DVD-ROM, etc.), or any other memory devices including diskettes, drives, other magnetic-based storage media, optical storage media and others. Although FIG. 1 shows three separate memory / media elements 108a, 108b, and 108c, the content dedicated to such devices may actually be stored in one memory / media element or in multiple elements, or any other variations of data storage as will be appreciated by one of ordinary skill in the art.

[0082] In one particular embodiment of the present subject matter, a first portion of memory / media 108a can be configured to store data corresponding to a plurality of images obtained from the image capture device 102 as well as measured positions of selected image points determined from the plurality of images. As such, the input data stored in the first memory / media element 108a may include captured images, extracted data associated with selected image points from the captured images used to monitor the displacement of one or more pattern 120 components present on the test specimen 114 in the form of the random speckle pattern, etc.USC Tech ID 1739

[0083] PCT Patent Application

[0084] Further, some of the input data stored in the memory / media element 108a may include information related to the test specimen 114 that can be entered as input data from a user accessing an input device 110, which may correspond to one or more peripheral devices configured to operate as a user interface with the computer control system 104. Exemplary input devices may include but are not limited to a keyboard, touch-screen monitor, display, microphone, mouse and the like.

[0085] Meanwhile, the second memory element 108b can include computerexecutable software instructions that can be read and executed by the processor 106 to act on the data stored in memory / media element 108a to create new output data (e.g., processed image data, measured displacements, calculated mechanical properties, etc.) for storage in a third memory / media element 108c. Such output data may be provided to a peripheral output device 112, such as a monitor, display, printer, or other device for visually depicting the output data, or as control signals to still further components. The processor 106 may be adapted to operate as a special-purpose machine by executing the software instructions rendered in a computer-readable form stored in memory / media element 108b. When software is used, any suitable programming, scripting, or other type of language or combinations of languages may be used to implement the teachings contained herein. In other embodiments, the methods disclosed herein may alternatively be implemented by hard-wired logic or other circuitry, including, but not limited to application-specific circuits.

[0086] Referring now to FIGs. 2 and 3, a test specimen 114 is shown in more detail with respect to its insertion into the system 100 and the contrast pattern 120 that is applied to the test specimen 114, although it is to be understood that the present disclosure contemplates that the contrast pattern 120 can be applied to any object on which image analysis can be conducted. Further, it is also to be understood that the present disclosure does not require the use of a test system. For instance, the test specimen 114 can be positioned between opposing wedge grips 130 that are coupled to an adjustable upper crosshead 124 and an adjustable lower crosshead 128 of the mechanical testing apparatus 118. Further, the contrast pattern 120 can be applied to the test specimen 114 in at least an area definedUSC Tech ID 1739

[0087] PCT Patent Application

[0088] between the wedge grips 130. Then, as one or both of the adjustable upper crosshead 124 and adjustable lower crosshead 126 move vertically along the screw column 132, the test specimen 114 can subjected to deformation testing in the form of, for example, tensile, compression, etc., to analyze its mechanical properties in conjunction with DIC, which tracks the changes in the contrast pattern 120 to then determine the various mechanical properties of the test specimen 114. More particularly, the test specimen 114 can have an overall length Lo and an overall diameter referred to as the overall diameter Do, and in some embodiments, the contrast pattern 120 can be applied only to an area associated with the test length LT of the test specimen 114 on one or both sides of the test specimen, where the test length LT, which is the smaller length in the area defined between the opposing wedge grips 130. The test specimen 114 can also have a reduced test diameter DT as compared to the crimping diameter De. However, it is also to be understood that the contrast pattern 120 can, in some embodiments, be applied to the entire test specimen 114 on one or both sides.

[0089] Regardless of the particular arrangement of the system 100, the system 100 includes or can be used in conjunction with a contrast pattern 120 configured for application to the test specimen 114.

[0090] The details and characteristics of the contrast pattern 120 and a method of applying the contrast pattern to the test specimen 114 are discussed in more detail below.

[0091] According to one embodiment of the present invention, the contrast pattern 120 can be in the form of a plurality of superposed gratings that can be applied to the test specimen 114. Referring to FIG. 5, a first pattern grating 120a and a second pattern grating 120b can be superposed to form contrast pattern 120, although it is to be understood that any number of superposed gratings are contemplated by the present disclosure. For example, in some embodiments and although not required, the gratings can be superposed in pairs. For instance, the present disclosure contemplates 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or more pairs of gratings that are superposed to form the contrast pattern 120. In one example and referring to FIGs. 6A-6D, the resulting contrast pattern 120 can include three pairs of superposed gratings, for six gratings total. The plurality of superposedUSC Tech ID 1739

[0092] PCT Patent Application

[0093] gratings can have a predetermined frequency content that can allow for improved contrast pattern 120 quality, leading to more accurate DIC, and thus, more accurate mechanical property or any other image analysis-based determinations for the material from which the test specimen 114 is formed. However, it is also to be understood that the present disclosure does not require pairs of gratings, and individual gratings or gratings that are superposed in other arrangements besides pairs are also contemplated by the present disclosure. By way of example, the contrast patterns 120 contemplated by the present disclosure are distinct from the pattern 119 shown in FIG. 4, which is a standard checkerboard pattern that does not allow for correlation because its zero mean normalized cross correlation (ZNCC) is periodically equal to zero (i.e. , neighboring subsets are not unique from each other).

[0094] In one particular embodiment represented by FIGs. 6A-6D, the plurality of superposed gratings can be in the form of a sine or cosine function, where the intensity l(x) or “gray level” of a grating is set forth below in the following equation, where A refers to the amplitude, A refers to the wavelength, 9 refers to the rotation angle, and <t> refers to the phase shift:

[0095] & <

[0096]

[0097] Further, Ao refers to the mean intensity of the image and ranges from 0 for black to 255 for white for an 8-bit image, where Ao is generally associated with the background intensity of the contrast pattern and is only needed for one grating. In some embodiments, the amplitude A for a specific grating for 8-bit imaging can range in intensity from about 1 to about 255, such as from about 25 to about 200, such as from about 50 to about 275, such as from about 100 to about 250, and any intensity value therebetween. Additionally, the amplitude A for a specific grating can range from 0 to about 4,095 for 12-bit imaging, from 0 to about 16,383 for 14-bit imaging, and from 0 to about 65,535 for 16-bit imaging, and any ranges therebetween.

[0098] Further, the wavelength can range from about 2 pixels to about 200 pixels. For example, contemplated wavelengths can range from about 2 pixels to about 100 pixels, such as from about 4 pixels to about 50 pixels and any ranges orUSC Tech ID 1739

[0099] PCT Patent Application

[0100] individual values therebetween so long as each wavelength for each of the plurality of superposed gratings is different from the other wavelengths, or has a different rotation angle from a superposed grating with an equal wavelength, where it is understood that the Nyquist frequency has a wavelength of about 2 pixels, where it is noted that any wavelengths below 2 pixels could result in aliasing. In addition, the rotation angle between each pair of gratings can range from about 0° to about 90°, such as from about 15° to about 80°, such as from about 30° to about to about 70°, and any ranges therebetween. Further, the phase shift can range from about 0 radians to about 1.6 radian, such as from about 0 radians to about 1 radian, such as from about 0 radians to about 0.4 radians, and any ranges therebetween.

[0101] In the particular embodiment of the contrast pattern shown in Fig. 6A-D, three pairs of superposed sinusoidal gratings are employed. The patterns correspond to different optical noise conditions, including signal-to-noise ratio (SNR) levels of 30 dB, 40 dB, 50 dB, as well as a noise-free case, with the optimized parameters summarized in Table 1 below. For all optimization cases, the mean intensity Aowas fixed at 127 and the phase offset <T>, was set to zero. These contrast patterns were developed through an optimization process combined with parameterization of the grating wavelengths and amplitudes, providing improved insight into how grating parameters influence the overall performance and robustness of the contrast pattern during digital image correlation (DIC) under varying optical noise conditions.

[0102] Table 1 - Optimized Parameters

[0103] "

[0104]

[0105] Next FIGs. 7A-7D and FIGs. 8A-D represent the results of the parameterization of the wavelength and amplitude for the contrast pattern. It can be seen that with the increase in the minimum allowable wavelength the RMSE error increases for all the noise levels. Contrary to that the increase in the maximumUSC Tech ID 1739

[0106] PCT Patent Application

[0107] allowable amplitude lowers the RMSE calculated from rigid body motions. As the subset size increase the RMSE decreases for all the cases of wavelength and amplitude.

[0108] FIG. 9 represents the fast Fourier transform (FFT) of the contrast pattern developed for SNR 40 as mentioned in FIG 7B. The FFT plot confirms the optimized contrast pattern contains the prescribed frequency content , including a frequency that is near to the Nyquist frequency to generate high pixel-to-pixel contrast and another frequency that is much lower to introduce pseudo-randomness to the pattern.

[0109] Next, FIGs. 10A and 10B illustrate the full field displacement analysis of a test sample to which the contrast pattern of the present disclosure has been applied with 9x9 pixel subsets instead of the standard 21x21 pixel or larger subsets in most speckling patterns used in the art currently, with an analytically applied 2D rigid motion of x,y = -2.058 px, 5.731 px in FIGs. 10A and 10B. As shown, based on DIC analysis, the mean displacement in x and y in FIGs. 10A-10B was (-2.0580354 px (See FIG. 10A), 5.7310235 px (see FIG. 10B)), with a mean displacement error of just 35.47 ppx and a 95% confidence interval of 39.50 ppx in x, while the y axis processes with a mean displacement error of just 23.57 ppx and a 95% confidence interval of 39.63 ppx in y. Furthermore the RMSE for x, y and resultant directions is 2.096 mpx, 2.103 mpx and 2.96 mpx respectively. This demonstrates that the subset size can be reduced to 9x9 pixels and the DIC algorithm can still be accurate without increasing experimental uncertainty beyond values typical for DIC. This also demonstrates the increased spatial resolution and decreased computational cost enabled by the contrast pattern.

[0110] Next, an actual reduction to practice for a tensile test was executed using the contrast pattern 120 applied to a test specimen 114 as shown in FIG. 11 , after which strain analysis was conducted upon loading the test specimen 114 into grips of a mechanical testing device. After tensile testing, the DIC results are shown superimposed on the test specimen 114 with the contrast pattern applied thereto in FIGs. 12A and 12B. A subset size of 9x9 pixels as utilized, showing that the contrast patterns contemplated by the present disclosure can be used on real experimental specimens. FIGs. 12C and 12D show the DIC results superimposedUSC Tech ID 1739

[0111] PCT Patent Application

[0112] on the test specimen 114 using the current state of the art dot-based pattern with identical camera setup and DIG parameters as used in FIGS. 12A and 12B. It can be seen that the conventional pattern has a large amount of data loss (loss of correlation).

[0113] Turning now to FIG. 13, a cross-sectional view of the contrast pattern 120 according to one embodiment of the present disclosure after application to a test specimen 114 is shown, such as via an adhesive 134. For example, a substrate 136 can be provided that has a lower surface 138 and an upper surface 140. The adhesive 134 can be applied to the lower surface 138 of the substrate 136, while the contrast pattern 120 can be applied to the upper surface 140. Further, the adhesive 134 can be sprayed, coated, dipped, or otherwise applied to the lower surface 138 of the substrate 136 via methods known in the art, and a release liner (not shown) may be attached to the exposed portion of the adhesive layer 134 until the substrate 136 is ready to be applied to the test specimen 114. In some embodiments, the adhesive layer 134 can be a tape or a pressure sensitive adhesive. In addition, the contrast pattern 120 can be applied to the upper surface 140 of the substrate 136 by printing, spraying, coating, dipping, or any other suitable method. In one embodiment, the contrast pattern 120 can be applied to an upper surface 140 of the substrate 136 via laser printing of ink. The substrate 136 itself can be a paper, film, or any other suitable material. Further, the substrate 136 can be flexible, which can allow for easier, simpler, faster, and more accurate application to the test specimen 114. In one embodiment, for example, the contrast pattern 120 and substrate 136 can be in the form of a flexible sticker that can be applied to the test specimen 114 on which the DIC measurements will be made.

[0114] In another embodiment, as shown in FIG. 14, a printer, such as, but not limited to, a laser printer can be used to apply the contrast pattern 120 onto a substrate such as tattoo paper (not shown). The contrast pattern 120 can then be transferred to an upper surface 142 of an adhesive layer 134 such as an adhesive sheet so that there is no substrate present between the contrast pattern 120 and the adhesive layer 134. Then, a lower surface 144 of the adhesive layer 134 (with the contrast pattern 120 applied to the supper surface 142) can then be adhered to the test specimen 114 on which the DIC measurements will be made. However, itUSC Tech ID 1739

[0115] PCT Patent Application

[0116] should also be understood that the contrast pattern 120 can, in some embodiments, be directly printed onto one or more surfaces of a test specimen 114 for DIC testing.

[0117] FIG. 15 is a flow chart outlining a method 200 for applying the contrast pattern to the test specimen with respect to FIG. 13. It is to be understood that the steps of the method can be performed in any order known to one of skill in the art and that the steps described are not required to be performed in the order in which they are described. In one embodiment of the method 200, a substrate can be obtained in step 202. Then, in step 204, an adhesive can be applied to a lower surface of the substrate. Meanwhile, in step 206, a contrast pattern can be applied to the upper surface of the substrate. Further, in step 208, the adhesive can be attached to a test specimen, after which the test specimen can be subjected to mechanical testing and DIC can be utilized to determine the mechanical properties of the test specimen. The substrate with the contrast pattern applied thereto can also be trimmed to fit the size of the test specimen if the substrate shape does not correspond exactly with the shape of the test specimen (e.g., if the substrate is a rectangle and the test specimen is in the shape of a dog-bone). Alternatively, the substrate can be precut or otherwise formed so that its shape corresponds exactly with the shape of the test specimen.

[0118] FIG. 16 is a flow chart outlining another method 300 for applying the contrast paper to the test specimen with respect to FIG. 14. It is to be understood that the steps of the method can be performed in any order known to one of skill in the art and that the steps described are not required to be performed in the order in which they are described. First, a substrate can be obtained in step 302. Then, in step 304, a contrast pattern can be applied to a surface of the substrate. Next, in step 306, the surface of the substrate on which the contrast pattern has been applied can be placed in contact with an upper surface of an adhesive layer. Further, in step 308, the contrast pattern can be transferred from the substrate to the upper surface of the adhesive layer. This can be done with heat, pressure, or a combination thereof, or any other known method of transferring ink from one surface to another. Thereafter, in step 310, after the substrate has been removed from contacting the upper surface of the adhesive layer, the lower surface of the adhesive layer can beUSC Tech ID 1739

[0119] PCT Patent Application

[0120] attached to a test specimen, after which the test specimen can be subjected to mechanical testing and DIC can be utilized to determine the mechanical properties of the test specimen. The adhesive layer with the contrast pattern applied thereto can also be trimmed to fit the size of the test specimen if the adhesive layer shape does not correspond exactly with the shape of the test specimen (e.g., if the adhesive layer is a rectangle and the test specimen is in the shape of a dog-bone).

[0121] Alternatively, the adhesive layer can be precut or otherwise formed so that its shape corresponds exactly with the shape of the test specimen.

[0122] While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. Likewise, the various diagrams may depict an example architectural or other configuration for the disclosure, which is done to aid in understanding the features and functionality that can be included in the disclosure. The disclosure is not restricted to the illustrated example architectures or configurations but can be implemented using a variety of alternative architectures and configurations. Additionally, although the disclosure is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described. They instead can be applied, alone or in some combination, to one or more of the other embodiments of the disclosure, whether or not such embodiments are described, and whether or not such features are presented as being a part of a described embodiment. Thus, the breadth and scope of the present disclosure should not be limited by any of the abovedescribed exemplary embodiments.

[0123] Unless otherwise defined, all terms (including technical and scientific terms) are to be given their ordinary and customary meaning to a person of ordinary skill in the art, and are not to be limited to a special or customized meaning unless expressly so defined herein. It should be noted that the use of particular terminology when describing certain features or aspects of the disclosure should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the disclosureUSC Tech ID 1739

[0124] PCT Patent Application

[0125] with which that terminology is associated. Terms and phrases used in this application, and variations thereof, especially in the appended claims, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing, the term ‘including’ should be read to mean ‘including, without limitation,’ ‘including but not limited to,’ or the like; the term ‘comprising’ as used herein is synonymous with ‘including,’ ‘containing,’ or ‘characterized by,’ and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps; the term ‘having’ should be interpreted as ‘having at least;’ the term ‘includes’ should be interpreted as ‘includes but is not limited to;’ the term ‘example’ is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; adjectives such as ‘known’, ‘normal’, ‘standard’, and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass known, normal, or standard technologies that may be available or known now or at any time in the future; and use of terms like ‘preferably,’ ‘preferred,’ ‘desired,’ or ‘desirable,’ and words of similar meaning should not be understood as implying that certain features are critical, essential, or even important to the structure or function of the present disclosure, but instead as merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure. Likewise, a group of items linked with the conjunction ‘and’ should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as ‘and / or’ unless expressly stated otherwise. Similarly, a group of items linked with the conjunction ‘or’ should not be read as requiring mutual exclusivity among that group, but rather should be read as ‘and / or’ unless expressly stated otherwise.

[0126] Where a range of values is provided, it is understood that the upper and lower limit, and each intervening value between the upper and lower limit of the range is encompassed within the embodiments. For instance, when a plurality of ranges are provided, any combination of a minimum value and a maximum value described in the plurality of ranges are contemplated by the present disclosure. For example, if ranges of ‘from about 20% to about 80%’ and ‘from about 30% to aboutUSC Tech ID 1739

[0127] PCT Patent Application

[0128] 70%’ are described, a range of ‘from about 20% to about 70%’ or a range of ‘from about 30% to about 80%’ are also contemplated by the present disclosure.

[0129] With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity. The indefinite article ‘a’ or ‘an’ does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

[0130] It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases ‘at least one’ and ‘one or more’ to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles ‘a’ or ‘an’ limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases ‘one or more” or ‘at least one’ and indefinite articles such as ‘a’ or ‘an’ (e.g., ‘a’ and / or ‘an’ should typically be interpreted to mean ‘at least one’ or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of ‘two recitations,’ without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to ‘at least one of A, B, and C, etc.’ is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., ‘a system having at least one of A, B, andUSC Tech ID 1739

[0131] PCT Patent Application

[0132] C’ would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). In those instances where a convention analogous to ‘at least one of A, B, or C, etc.’ is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., ‘a system having at least one of A, B, or C’ would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase ‘A or B’ will be understood to include the possibilities of ‘A’ or ‘B’ or ‘A and B.’

[0133] All numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification are to be understood as being modified in all instances by the terms ‘about,’ ‘approximately,’ or ‘generally.’ Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of any claims in any application claiming priority to the present application, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches. As used herein, the terms ‘about,’ ‘approximately,’ or ‘generally,’ when used to modify a value, indicate that the value can be raised or lowered by 5% and remain within the disclosed embodiment.

[0134] All of the features disclosed in this specification (including any accompanying exhibits, claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims,USC Tech ID 1739

[0135] PCT Patent Application

[0136] abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0137] While the present subject matter has been described in detail with respect to various specific example embodiments thereof, each example is provided by way of explanation, not limitation of the disclosure. Those skilled in the art, upon attaining an understanding of the foregoing, can readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the subject disclosure does not preclude inclusion of such modifications, variations and / or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such alterations, variations, and equivalents.

Claims

USC Tech ID 1739PCT Patent ApplicationWHAT IS CLAIMED IS:

1. A contrast pattern comprising:a plurality of superposed gratings, each of the plurality of superposed gratings comprising a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency.

2. The contrast pattern of claim 1 , wherein the plurality of superposed gratings comprises at least two pairs of superposed gratings.

3. The contrast pattern of claim 1 , wherein the function facilitates a digital image correlation subset size of less than about 21 pixels by 21 pixels.

4. The contrast pattern of claim 1 , wherein the function comprises a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a square wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof.

5. The contrast pattern of claim 4, wherein the function comprises a sine function.

6. The contrast pattern of claim 5, wherein each of the plurality of superposed gratings has an intensity determined according to the following equation, where A refers to an amplitude, A refers to a wavelength, 0 refers to a rotation angle, refers to a phase shift, and Ao refers to an initial background intensity to which the contrast pattern is superposed:

7. The contrast pattern of claim 6, wherein the amplitude ranges from 0 to about 255 for 8-bit imaging, from 0 to about 4,095 for 12-bit imaging, from 0 to about 16,383 for 14-bit imaging, and from 0 to about 65,535 for 16-bit imaging.

8. The contrast pattern of claim 6, wherein the wavelength ranges from about 2 pixels to about 200 pixels.

9. The contrast pattern of claim 6, wherein the rotation angle ranges from about 0° to about 90°.

10. A substrate, wherein the substrate includes the contrast pattern of any of the foregoing claims, wherein the contrast pattern is applied to a surface of the substrate.USC Tech ID 1739PCT Patent Application11. An adhesive sheet, wherein the adhesive sheet includes the contrast pattern of any one of claims 1 to 9, wherein the contrast pattern is applied to a surface of the adhesive sheet.

12. A test specimen, wherein the contrast pattern of any one of claims 1 to 9 is applied to one or more surfaces of the test specimen.

13. A method of making a digital image correlation contrast pattern for application to an object, the method comprising:obtaining a substrate;applying an adhesive to a lower surface of the substrate; andapplying a contrast pattern to an upper surface of the substrate, wherein the contrast pattern comprises a plurality of superposed gratings, each of the plurality of superposed gratings comprising a function having a frequency content ranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency.

14. The method of claim 13, further comprising attaching the adhesive to the object.

15. The method of claim 13, wherein the plurality of superposed gratings comprises at least two pairs of superposed gratings.

16. The method of claim 13, wherein the function facilitates a digital image correlation subset size of less than about 21 pixels by 21 pixels.

17. The method of claim 13, wherein the function comprises a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a square wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof.

18. A method of making a digital image correlation contrast pattern for application to an object, the method comprising:obtaining a substrate;applying a contrast pattern to a surface of the substrate;contacting the surface of the substrate on which the contrast pattern is applied to an upper surface of an adhesive layer; andtransferring the contrast pattern onto the upper surface of the adhesive layer, wherein the contrast pattern comprises a plurality of superposed gratings, each of the plurality of superposed gratings comprising a function having a frequency contentUSC Tech ID 1739PCT Patent Applicationranging from about 200 times lower than the Nyquist frequency to about the Nyquist frequency.

19. The method of claim 18, further comprising attaching a lower surface of the adhesive layer to the object.

20. The method of claim 18, wherein the plurality of superposed gratings comprises at least two pairs of superposed gratings.

21. The method of claim 18, wherein the function facilitates a digital image correlation subset size of less than about 21 pixels by 21 pixels.

22. The method of claim 18, wherein the function comprises a sine function, a cosine function, a triangular wave function, a trapezoidal wave function, a square wave function, a rectangular wave function, a Gaussian pulse wave function, a Ricker wavelet function, or a combination thereof.

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