Single‐camera special stereo‐digital image correlation applied for accurate virtual fields method identification

A single‐camera special stereo‐DIC (SS‐DIC) is proposed for accurate virtual fields method (VFM) identification. The single‐camera SS‐DIC allows accurate surface 3D deformation measurements using a single colour camera and a specially designed colour separation device. It not only effectively eliminates the unavoidable out‐of‐plane movement/rotation due to unideal in‐plane loading but also delivers uniformly distributed measurement points that brings great simplicity and convenience for internal virtual work calculation in VFM. In addition, since only a single camera is used for stereovision, the proposed SS‐DIC system requires no complicated synchronisation devices. The effectiveness and practicality of the proposed method are evaluated by heterogeneous deformation experiments of a holed aluminium alloy and 304 stainless steel plate. Combined with a high‐speed colour camera, the proposed method is expected to be a simple and practical method for the calibration of material constitutive model under intermediate and high strain rate conditions using VFM.     

A single camera three‐dimensional digital image correlation system for the study of adiabatic shear bands

We describe the capability of a high‐resolution three‐dimensional digital image correlation (DIC) system specifically designed for high strain‐rate experiments. Utilising open‐source camera calibration and two‐dimensional DIC tools within the MATLAB framework, a single camera three‐dimensional DIC system with submicron displacement resolution is demonstrated. The system has a displacement accuracy of up to 200 times the optical spatial resolution, matching that achievable with commercial systems. The surface strain calculations are benchmarked against commercially available software before being deployed on quasi‐static tests showcasing the ability to detect both in‐ and out‐of‐plane motion. Finally, a high strain‐rate (1.2×10³ s^?1) test was performed on a top‐hat sample compressed in a split‐Hopkinson pressure bar in order to highlight the inherent camera synchronisation and ability to resolve the adiabatic shear band phenomenon.     

Application of Stereo‐Digital Image Correlation to Full‐Field 3‐D Deformation Measurement of Intervertebral Disc

Abstract: This paper presents the application of a Stereo‐Digital Image Correlation (Stereo‐DIC) based approach for full‐field deformation measurements on a porcine intervertebral disc (IVD) under in‐vitro loading. Full‐field capabilities of Stereo‐DIC provide useful information on the IVD structure‐function relationship needed for designing novel disc replacement devices on the basis of biomimetic concepts. In this work, the use of a multi‐view Stereo‐DIC system allows full‐field measurement over more than 180° of the IVD surface. This is achieved by sequentially moving a single camera through seven fixed positions in order to cover the desired angle of vision. Ad hoc data processing and merging procedures are calibrated for a cylindrical sample. Strain maps are determined for a rubber cylinder subjected to rigid‐body motions and then to uniform compression. These preliminary procedures serve to evaluate strain‐mapping errors as well as to assess repeatability and accuracy of camera re‐positioning. Once the entire procedure is verified and calibrated, a fresh porcine functional spine unit is loaded under anterior, posterior and lateral compression. From displacement maps obtained with the multi‐view set‐up, it is possible to gather information on both IVD bulging and surface strains. Results are consistent with the IVD global behaviour under standard testing protocols reported in the recent literature. Furthermore, detailed information on local variations in structural response and stiffness properties occurring when load is applied in different regions of the IVD are obtained. The present approach allows to correlate structural response with the entire 3‐D deformation and strain field and not just with a single displacement/strain component and thus it could be effectively used for explaining in‐homogeneity observed in human discs degenerative patterns.     

Application of a Multi‐Camera Stereo DIC Set‐up to Assess Strain Fields in an Erichsen Test: Methodology and Validation

Abstract: A multi‐camera stereo digital image correlation (MC‐DIC) set‐up is presented to obtain full displacement and strain fields of a sheet‐metal specimen subjected to an Erichsen test. The set‐up is composed of several conventional stereo DIC systems (two camera set‐up), each of which tracks the deformation of an aspect of the specimen. The individual measurements, including the geometries and the displacements, are then converted to the same reference frame to integrate into a global view. Afterwards, the strain is calculated based on the composed displacement field. It is found that the geometry and the displacement fields of the bulged specimen are ideally stitched, and smooth strain fields are obtained. The influences of the reference frame transformation and the stitching procedure on the MC‐DIC measurement are investigated. A rigid motion test is performed to validate the displacement measurement. It is discussed that the global field is more reliable than the individual measurements for this test set‐up.     

Recent Developments in Measuring Creep Strain in High Temperature Plant Components

Accurate measurements of creep strain are necessary to evaluate the condition and predict the remaining life of power plant constituent materials. Optical techniques are appropriate for this purpose as they are a non‐contact method and can therefore be used to measure strain without requiring direct access to the surface. Within this class of techniques, the Auto‐Reference Creep Management And Control (ARCMAC) camera system can be used to calculate the strain between two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two iterations in system design, the Conventional ARCMAC and Digital Single‐Lens Reflex (DSLR) ARCMAC. Experiments are conducted to determine the absolute limit of accuracy of the systems in comparison to a strain gauge, and the relative accuracy across several orders of magnitude until specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated temperatures to evaluate the effect of temperature on the gauges and to investigate whether its accuracy diminishes in creep conditions. It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 με or less. In accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable displacement transducer when used to measure creep strain. Strain variations (under 500 με) were noted on a steel plate subjected only to operational temperature and no stress. This error is very reasonable compared to a critical strain value of 93 000 με in a given high temperature‐service material. Digital image correlation (DIC) results using the DSLR ARCMAC system show approximately 4% error in measurement for plastic strains in the specimen. The two measures of strain measurement (using ARCMAC and DIC) can serve to complement each other.     

Liquid-crystal-display projector-based modulation transfer function measurements of charge-coupled-device video camera systems

We demonstrate the ability to measure the system modulation transfer function (MTF) of both color and monochrome charge-coupled-device (CCD) video camera systems with a liquid-crystal-display (LCD) projector. Test matrices programmed to the LCD projector were chosen primarily to have a flat power spectral density (PSD) when averaged along one dimension. We explored several matrices and present results for a matrix produced with a random-number generator, a matrix of sequency-ordered Walsh functions, a pseudorandom Hadamard matrix, and a pseudorandom uniformly redundant array. All results are in agreement with expected filtering. The Walsh matrix and the Hadamard matrix show excellent agreement with the matrix from the random-number generator. We show that shift-variant effects between the LCD array and the CCD array can be kept small. This projector test method offers convenient measurement of the MTF of a low-cost video system. Such characterization is useful for an increasing number of machine vision applications and metrology applications.     

基于三次插值坐标变换的反向条纹投影技术

反向条纹投影技术是一种应用于在线或批量检测的快速而稳定的光学三维面形检测技术。本文提出了一种利用分片二元三次多项式插值来确定摄像机和投影器坐标变换关系的方法,它通过在摄像机10个像素点区域内拟和两个二元三次多项式,来求得该区域内投影器像素点对应的摄像机坐标值。计算机模拟和实物的对比实验表明,该方法能有效地提高反向条纹投影的精度。     

Non-destructive testing of industrial structures with the use of multi-camera Digital Image Correlation method

Although Digital Image Correlation (DIC) provides capabilities of scaling a field-of-view (FOV), dimensions of some industrial installations in many cases are too big to be measured with DIC based on a single camera pair. In this paper we present two strategies of spatial stitching of data obtained with multi-camera DIC systems that can be used for engineering failure analysis, namely strategy for the case when there are overlapping FOVs of 3D DIC setups and strategy for the case when 3D DIC setups are distributed and not necessarily have overlapping FOVs. Data obtained with the presented methodologies can be used to verify or update FEM models of the investigated objects. Two practical applications of the multi-camera DIC system are described in order to show its feasibility in engineering failure analysis.     

A human‐like numerical technique for design of engineering systems

Because of the necessity for considering various creative and engineering design criteria, optimal design of an engineering system results in a highly‐constrained multi‐objective optimization problem. Major numerical approaches to such optimal design are to force the problem into a single objective function by introducing unjustifiable additional parameters and solve it using a single‐objective optimization method. Due to its difference from human design in process, the resulting design often becomes completely different from that by a human designer. This paper presents a novel numerical design approach, which resembles the human design process. Similar to the human design process, the approach consists of two steps: (1) search for the solution space of the highly‐constrained multi‐objective optimization problem and (2) derivation of a final design solution from the solution space. Multi‐objective gradient‐based method with Lagrangian multipliers (MOGM‐LM) and centre‐of‐gravity method (CoGM) are further proposed as numerical methods for each step. The proposed approach was first applied to problems with test functions where the exact solutions are known, and results demonstrate that the proposed approach can find robust solutions, which cannot be found by conventional numerical design approaches. The approach was then applied to two practical design problems. Successful design in both the examples concludes that the proposed approach can be used for various design problems that involve both the creative and engineering design criteria. Copyright © 2005 John Wiley & Sons, Ltd.     

Toward dynamic recalibration and three-dimensional reconstruction in a structured light system

We present a method for dynamic recalibration and 3D reconstruction via a structured light system. Assuming that the light planes cast from the digital light projector have been calibrated off-line, we show that the focal length, aspect ratio, and all motion parameters of the camera can be determined on-line. Then the 3D reconstruction can be carried out by either a traditional triangulation method or a more efficient transformation-based method. In the latter method, a single image is sufficient for the whole process of calibration and reconstruction. Thus a hand-held camera can be used. Computer simulation and real data experiments were carried out to validate the method.     

Hybrid method combining orthogonal projection Fourier transform profilometry and laser speckle imaging for 3D visualization of flow profile

This paper reports a straightforward technique for three-dimensional (3D) visualization of a flow profile by a hybrid algorithm combining Fourier transform orthogonal fringe projection and laser speckle imaging techniques. The use of orthogonal projection aims to suppress the zero order allowing surface reconstruction with high spatial resolution and accuracy while analyzing the intensity fluctuations of diffuse backscattered laser light providing 2D flow information. Once both are achieved, 3D flow visualization can be displayed. The method is experimentally validated first with a plastic tube filled with scattering liquid (milk) running at various controlled flow rates and then with the tube embedded under scattering layers (chicken breast) of varying thickness. The system includes a single, common camera, a commercial projector (profilometry channel), a laser light source (flow channel), and a computer station. In addition, orthogonal projection processing was combined with Hilbert transform, increasing the visualization and resolution of the measured flow profile.     

Visible-band testbed projector with a replicated diffractive optical element

Raytheon has designed, fabricated, and tested a diffractive-optical-element-based (DOE-based) testbed projector for direct and indirect visual optical applications. By use of a low-cost replicated DOE surface from Rochester Photonics Corporation for color correction the projector optics bettered the modular transfer function of an equivalent commercial camera lens. The testbed demonstrates that a practical DOE-based optical system is suitable for both visual applications (e.g., head-mounted displays) and visual projection (e.g., tactical sensors). The need for and the proper application of DOE's in visual optical systems, the nature and the performance of the projector optical design, and test results are described.     

Determination of three-dimensional displacement using two-dimensional digital image correlation

A novel method that uses a two-dimensional (2D) digital image correlation (DIC) based on a single CCD camera to measure three-dimensional (3D) displacement and deformation is proposed. Rigid-body displacement in 3D space consists of both in-plane and out-of-plane components. The presence of an in-plane displacement component results in a shift of the center of the image displacement vector, while the slope of the image displacement vector is related to the out-of-plane displacement component. Global DIC is employed to determine the displaced position of each point on an object based on a linear distribution characteristic of the displacement vector. Speckle images with deformation introduced by 3D displacement are generated to demonstrate the feasibility of the proposed method. In the 3D rigid-body displacement, both in-plane and out-of-plane displacement components are separated by determining the intercept and slope of the image displacement vector. In the 3D deformation, a zero order displacement (pure rigid-body displacement) mode is assumed in a small subset of pixels. Simulated and experimental results demonstrate that both in-plane and out-of-plane displacements can be accurately retrieved using the proposed method.     

Digital Image Correlation and Noise‐filtering Approach for the Cracking Assessment of Massive Reinforced Concrete Structures

This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non‐contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC‐based methods may lead to erroneous results. In this study a post‐processing procedure is presented to cope with noisy full‐field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full‐scale rectangular beams and then on mock‐ups scaled down by 1/3. In addition, fours RC walls are tested under in‐plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in‐plane displacement and strain fields. Full‐field measurements are post‐processed by the noise‐filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point‐based sensors (linear variable differential transformer LVDT and fibre‐optic FO transducers). The proposed DIC‐based post‐processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale‐effect on the cracks width.     

光栅投影式三维摄影测量仪的几何标定方法

光栅投影式三维摄影测量仪利用了时域结构光投影技术和立体视觉测量原理获得三维点坐标。针对传统标定方法易受镜头畸变影响和标定约束方程少导致精度下降的问题,采用了非线性的摄像机和投影机模型,并提出了二维的投影机模型;使用多平面法标定了系统测量所需的摄像机和投影机几何参数;为进一步提高参数精度,采用Levenberg-Marquardt算法优化了摄像机和投影机模型。实验结果表明,该方法操作简单,无需精确的位置和姿态调整,标定的绝对精度为0.2pixel,相对精度为1/5000。     

Displacement Measurement Through Digital Image Correlation and Digital Speckle Pattern Interferometry Techniques in Cold‐Expanded Holes

Abstract: In this paper, the displacement field induced by the split‐sleeve cold expansion of holes was measured using both digital image correlation (DIC) and digital speckle pattern interferometry (DSPI) techniques. Thus, the experimental results, which were evaluated on the inlet surface of a 6082‐T6 aluminium plate, were compared with those from theoretical prediction. DIC provided accurate measurements up to the elastic–plastic boundary, whereas the DSPI technique highlighted the changes of displacement in the elastic domain. Prediction of the displacement based on the existing analytical model agreed with the experimental results achieved with both techniques. Possible explanations for the differences are discussed.     

Absolute phase-assisted three-dimensional data registration for a dual-camera structured light system

For a three-dimensional shape measurement system with a single projector and multiple cameras, registering patches from different cameras is crucial. Registration usually involves a complicated and time-consuming procedure. We propose a new method that can robustly match different patches via absolute phase without significantly increasing its cost. For y and z coordinates, the transformations from one camera to the other are approximated as third-order polynomial functions of the absolute phase. The x coordinates involve only translations and scalings. These functions are calibrated and only need to be determined once. Experiments demonstrated that the alignment error is within RMS 0.7 mm.     

Characterization of dynamic microgyroscopes by use of temporal digital image correlation

The advanced mechanical testing of microelectromechanical systems (MEMS) is necessary to provide feedback of measurements that can help the designer optimize MEMS structures and improve the reliability and stability of MEMS. We describe a digital image correlation (DIC) method for dynamic characterization of MEMS using an optical microscope with a high-speed complementary metaloxide semiconductor-based camera. The mechanical performance of a series of microgyroscopes is tested. The DIC method is employed to measure the microgyroscope in-plane displacement with subpixel accuracy. Use of the DIC method is less restrictive on the surface quality of the specimen and simplifies the measurement system. On the basis of a series of temporal digital images grabbed by a high-speed camera, the stability characteristic of the microgyroscopes is analyzed. In addition, the quality factors of the microgyroscopes are determined and agree well with other experimental methods.     

Low-cost digital image correlation and strain measurement for geotechnical applications

Particle image velocimetry (PIV), or digital image correlation (DIC), is a widely used technique to measure soil displacements and strains in small-scale geotechnical models. Arrays of single-board computers (SBCs) produced by Raspberry Pi, and their associated 8-MP cameras, are being used at the University of Cambridge to capture the images required for DIC analysis. This alternative to more expensive camera set-ups has numerous advantages. A single expensive and large camera can be replaced—at low cost—by multiple cameras, adding flexibility and affordability to any experimental set-up. Traditionally, the alignment of multiple cameras to each other and the referencing to a known coordinate system required painted or machined markers to be located on the observation windows through which the experiments are viewed. This can obstruct localised soil grain displacement measurements in those areas of the model where such markers are placed. To complement the Raspberry Pi camera system, a markerless calibration method was used during image acquisition. This paper outlines the set-up of four of these small computers and associated cameras, provides an overview of the use of the markerless referencing system and reviews two different experimental apparatus used to measure soil displacement and strain. When the cost of additional cabling, connectors and mounting hardware is considered for this system, the total cost to implement was approximately $125 USD per camera plus one-time costs of $175 USD for system peripherals, which represents outstanding value and enables practically all geotechnical laboratories to develop similar capabilities.     

Assessing the Feasibility of Monitoring Strain in Historical Tapestries Using Digital Image Correlation

Abstract:  Digital image correlation (DIC) is used to monitor strain in a representative textile material and an historic tapestry. The validity of a 'map function' that allows 3D DIC displacement measurements to be obtained when the reference data are collected with a camera set-up different from that of the deformed data is assessed. An experiment was devised to study the effects of DIC processing parameters (interrogation cell size and overlap) on strain measurements, and to investigate if the textile contains adequate contrast for DIC to operate. The study shows that the textile's weave pattern can be used as the device for correlation. Long-term tests for monitoring creep strain using DIC both in the laboratory and in situ are presented. The results show good correspondence between strain changes in the tapestry and relative humidity.