Sub-pixel Digital Image Correlation Research Articles

High-precision and high-accuracy stereophotogrammetric image analysis for small to large strain deformation measurement in triaxial apparatus

A deformation measurement system for laboratory soil testing based on stereophotogrammetric image analysis was developed with a purpose of characterising soil behaviour continuously from small to large strains. The system is applicable to triaxial tests with pressure chamber and confining fluid by correcting the refraction-induced distortion of images by ray tracing. In comparison to similar existing studies, the present study put priority in precision and accuracy in relative displacement tracking, which is the key feature in resolving small strain in soil specimen and characterising its stiffness. The displacement tracking is performed with a hybrid algorithm combining Particle Tracking Velocimetry (PTV) for primary target matching and subpixel Digital Image Correlation (DIC) for more precise image matching. The deformation of clay samples with and without end lubrication was compared by visualising the deformation at different levels of strain. This paper is basically a summary of Nishimura (2022). Another example of application in laboratory testing involving fine particle suffusion is also shown. Some notes on implementation of the technique for good practice and on future development are additionally described.

Real-Time Vibration Visualization Using GPU-Based High-Speed Vision

In this study, we developed a real-time vibration visualization system that can estimate and display vibration distributions at all frequencies in real time through parallel implementation of subpixel digital image correlation (DIC) computations with short-time Fourier transforms on a GPU-based high-speed vision platform. To help operators intuitively monitor high-speed motion, we introduced a two-step framework of high-speed video processing to obtain vibration distributions at hundreds of hertz and video conversion processing for the visualization of vibration distribution at dozens of hertz. The proposed system can estimate the full-field vibration displacements of 1920 × 1080 images in real time at 1000 fps and display their frequency responses in the range of 0–500 Hz on a computer at dozens of frames per second by accelerating phase-only DICs for full-field displacement measurement and video conversion. The effectiveness of this system for real-time vibration monitoring and visualization was demonstrated by conducting experiments on objects vibrating at dozens or hundreds of hertz.

Characterisation of soil deformation over wide strain ranges in triaxial test with high-precision stereophotogrammetry

Stereophotogrammetry was adopted as a means to measure three-dimensional displacements of a soil specimen surface in triaxial tests. New developments include an efficient algorithm that resolves the relative displacements with high precision to the order of 10−3 mm, and strains in common-sized soil specimens to the order of 10−3%, while correcting for ray refraction effects caused by the pressure cell wall and water. The system, requiring only sets of compact-type digital cameras as hardware, allows a stiffness–strain curve to be determined over wide strain ranges spanning from 10−3% to virtually any large strain with a fixed configuration. This paper explains the proposed image analysis processes, which combine a ray tracing formulation by Zhang and co-workers, particle tracking velocimetry and sub-pixel digital image correlation in efficiently deriving accurate and precise relative displacements. Rigorous assessment of the accuracy and precision was conducted. As a demonstration, two undrained triaxial compression tests on reconstituted clay were performed with and without end lubrication. Both for small-strain (<0·05%) axial loading–unloading cycles and for monotonic loading to large axial strain (15%), the strain development was tracked and the specimen behaviour was characterised. These tests demonstrate that the new technique can be a useful option in a soil laboratory both for research and practice.

Heterogeneous parallel computing accelerated iterative subpixel digital image correlation

Parallel computing techniques have been introduced into digital image correlation (DIC) in recent years and leads to a surge in computation speed. The graphics processing unit (GPU)-based parallel computing demonstrated a surprising effect on accelerating the iterative subpixel DIC, compared with CPU-based parallel computing. In this paper, the performances of the two kinds of parallel computing techniques are compared for the previously proposed path-independent DIC method, in which the initial guess for the inverse compositional Gauss-Newton (IC-GN) algorithm at each point of interest (POI) is estimated through the fast Fourier transform-based cross-correlation (FFT-CC) algorithm. Based on the performance evaluation, a heterogeneous parallel computing (HPC) model is proposed with hybrid mode of parallelisms in order to combine the computing power of GPU and multicore CPU. A scheme of trial computation test is developed to optimize the configuration of the HPC model on a specific computer. The proposed HPC model shows excellent performance on a middle-end desktop computer for real-time subpixel DIC with high resolution of more than 10000 POIs per frame.

High accuracy digital image correlation powered by GPU-based parallel computing

A sub-pixel digital image correlation (DIC) method with a path-independent displacement tracking strategy has been implemented on NVIDIA compute unified device architecture (CUDA) for graphics processing unit (GPU) devices. Powered by parallel computing technology, this parallel DIC (paDIC) method, combining an inverse compositional Gauss–Newton (IC-GN) algorithm for sub-pixel registration with a fast Fourier transform-based cross correlation (FFT-CC) algorithm for integer-pixel initial guess estimation, achieves a superior computation efficiency over the DIC method purely running on CPU. In the experiments using simulated and real speckle images, the paDIC reaches a computation speed of 1.66×105POI/s (points of interest per second) and 1.13×105POI/s respectively, 57–76 times faster than its sequential counterpart, without the sacrifice of accuracy and precision. To the best of our knowledge, it is the fastest computation speed of a sub-pixel DIC method reported heretofore.

基于数字散斑照相术测量刚体面内位移

Two common methods based on digital speckle photography———digital image correlation and the Fourier transform are studied for measuring in-plane displacement of rigid body.The algorithm of gradient-based subpixel digital image correlation method is described,and the size of the subset selection is analyzed,so that the measurement accuracy is improved.The speckle patterns are processed using Fourier transform and mathematical morphology operations to calculate fringe spaces and in-plane displacement accurately.Experimental results indicate that the Fourier transform method is precise.

Study on Rolling Processing Evaluation Characteristics Using Sub-DIC Method

It is extremely important to evaluate the rolling deformation and large strain distribution conditions in the surface of the processed materials after machining. On the other hand, the sub-pixel digital image correlation (Sub-DIC) method enables to measure the full-field strain distribution only using the photographs taken by the commercially available digital camera. This study aims to develop the rolling deformation and the large strain distribution conditions using the Sub-DIC method. Here, it is well known that the measurement accuracy deteriorates in large strain conditions when the conventional Sub-DIC method is used. In this paper, we proposed the facility technique for measuring the large strain distribution by the Sub-DIC method in which the sub-set area are applied to the strain regarding the reduction in thickness of rolling processing. And, we demonstrated the measurement characteristics using the facility Sub-DIC method through a series of experiments from the viewpoint of changes in the test piece surface strain distribution conditions. Moreover, we evaluated the curve of test piece after rolling processing with a radius of curvature. Finally, we discussed measurement characteristics of the steady/ unsteady conditions in the rolling processing using the facility Sub-DIC method.

Coming Full Circle: The Application of Microtechnology Techniques to Evaluate Bulk Materials

ABSTRACTA tensile test procedure that accommodates specimens with gage section 25 μm thick, 70 μm wide and 360 μm long was developed and demonstrated. The instrumentation and technique were adapted from those previously developed and used to test thin films, by increasing both the force capacity of the load cell and the stiffness of the pull rod. Specimens with bow-tie geometry were fabricated by photolithography from nominally 25 μm thick full hard stainless steel 302 foil. A silicon test frame fabricated by bulk micromachining techniques included tapered grips in the form of recesses in its top surface that accepted and retained the specimen grip sections. One grip was on the fixed outer portion of the frame. The other grip was on a plate suspended in the center of the frame by long slender silicon beams. Force was imposed on this plate by pin loading. The force was measured by use of a custom load cell. The displacement was measured by sub-pixel digital image correlation to surface features on the two ends of the gage section, applied to images with a resolution of approximately 0.8 μm per pixel. Yield and ultimate strengths and elongation values consistent with vendor-provided information were obtained. The values of Young’s modulus were scattered but within the range of expected behavior for the specimen material.

デジタル画像相関法を用いた全視野変形・ひずみ測定の精度評価研究(機械力学,計測,自動制御)

Sub-pixel digital image correlation (Sub-DIC) method is effective for health monitoring of structures, because the Sub-DIC method can measure full-field deformation and strain distribution. However, the effect of analysis parameters of Sub-DIC method on measurement accuracy is not clarified. In this paper, we discussed the relation between the analysis parameters and the measurement accuracy of the Sub-DIC method. We demonstrated the analysis parameters such as a sub-set size, scan-step interval, and averaged point size on the standard deviation of measured deform and strain error distributions. Moreover, we found measurement accuracy is correlation related to the half width of spatial FFT spectrum of measured object and strain distribution. Finally, it is basically confirmed the measurement accuracy of Sub-DIC method depends on the central limit theorem.

Studies on Strain Distribution Measurement using Digital Image Correlation Method

In order to inspect the strength and reliability of structures, measurement using a strain gauge is widely adopted. However the measurement of real structures using a strain gauge requires dangerous preparatory work such as fixing strain gauges onto the structure and connecting them via cables to strain meters located in a remote place. Moreover, strain gauges detect strain only at the points where they are attached. Therefore there is a growing demand for a more reliable and accurate strain distribution measurement of structures. The purpose of this study is to develop a strain distribution measurement system based on the sub-pixel digital image correlation (Sub-DIC) method. However, the Sub-DIC method has several problems concerning accuracy; an out-of-plane deformation of structures has an adverse effect on the measurement accuracy. In this study, the effect of the out-of-plane deformation is taken into account. We formulate a model of the out-of-plane deformation, and discuss the effect of the theoretical equation given on the strain measurement accuracy. Furthermore, we show the effectiveness of the theory by a model experiment. Moreover, a compensation method for the out-of-plane deformation is proposed. It is found that the Sub-DIC method with the proposed compensation system is superior to existing strain distribution measurement systems, on the basis of a series of experimental examinations using the four-point bending test.

Focus errors and their correction in microscopic deformation analysis using correlation

Subpixel digital image correlation has been applied to microscope images to analyze surface deformation. Nonintegral pixel shifting and successive approximation are used to calculate the subpixel element of the sample displacement without introducing systematic interpolation errors. Although in-plane displacement precision of better than 2% of a pixel, or < 15 nm at x10 magnification, is shown to be achievable, the use of even moderate numerical aperture microscope objectives render the technique sensitive to errors or variations in sample focusing. The magnitude of this effect is determined experimentally and a focus compensation method is described and demonstrated.