Stereo Image Correlation Research Articles

Accurate 3D Shape, Displacement and Deformation Measurement Using a Smartphone.

The stereo-digital image correlation technique using two synchronized industrial-grade cameras has been extensively used for full-field 3D shape, displacement and deformation measurements. However, its use in resource-limited institutions and field settings is inhibited by the need for relatively expensive, bulky and complicated experimental set-ups. To mitigate this problem, we established a cost-effective and ultra-portable smartphone-based stereo-digital image correlation system, which only uses a smartphone and an optical attachment. This optical attachment is composed of four planar mirrors and a 3D-printed mirror support, and can split the incoming scene into two sub-images, simulating a stereovision system using two virtual smartphones. Although such a mirror-based system has already been used for stereo-image correlation, this is the first time it has been combined with a commercial smartphone. This publication explores the potential and limitations of such a configuration. We first verified the effectiveness and accuracy of this system in 3D shape and displacement measurement through shape measurement and in-plane and out-of-plane translation tests. Severe thermal-induced virtual strains (up to 15,000 με) were found in the measured results due to the smartphone heating. The mechanism for the generation of the temperature-dependent errors in this system was clearly and reasonably explained. After a simple preheating process, the smartphone-based system was demonstrated to be accurate in measuring the strain on the surface of a loaded composite specimen, with comparable accuracy to a strain gauge. Measurements of 3D deformation are illustrated by tracking the deformation on the surface of a deflating ball. This cost-effective and ultra-portable smartphone-based system not only greatly decreases the hardware investment in the system construction, but also increases convenience and efficiency of 3D deformation measurements, thus demonstrating a large potential in resource-limited and field settings.

Damage characterisation of flax fibre fabric reinforced epoxy composites during low velocity impacts using high-speed imaging and Stereo Image Correlation

This work aims at studying the low velocity impact and post-impact behaviour of flax/epoxy composites. The samples were composed of 8 plies of 2/2 twill weave fabric. The experimental device used for impact tests was a drop tower with impact energies varying from 5 to 34 J (perforation). Pictures of the sample were captured during the test with two high-speed cameras in order to perform high-speed imaging and Stereo Image Correlation (SIC) analysis. Residual properties have been assessed by bending after impact tests. High-speed imaging easily detected the back face cracks at maximum deflection while some were almost invisible after impact. The results also highlighted the material’s behaviour in three-stages. At low energy, no damage was observed. The cracks and the recovered energy evolved proportionally with the impact energy between 5 and 25 J, and then reached a threshold. The post-impact tests showed that even a 5 J impact resulted in a degradation of composite performance, with a loss of approximately 10% in stiffness and 21% in strength.

Evaluating Stress Triaxiality of High Strength Steel Using Stereo Image Correlation

Evaluating Stress Triaxiality of High Strength Steel Using Stereo Image Correlation

CORRECTING SPACECRAFT JITTER IN HIRISE IMAGES

Abstract. Mechanical oscillations or vibrations on spacecraft, also called pointing jitter, cause geometric distortions and/or smear in high resolution digital images acquired from orbit. Geometric distortion is especially a problem with pushbroom type sensors, such as the High Resolution Imaging Science Experiment (HiRISE) instrument on board the Mars Reconnaissance Orbiter (MRO). Geometric distortions occur at a range of frequencies that may not be obvious in the image products, but can cause problems with stereo image correlation in the production of digital elevation models, and in measuring surface changes over time in orthorectified images. The HiRISE focal plane comprises a staggered array of fourteen charge-coupled devices (CCDs) with pixel IFOV of 1 microradian. The high spatial resolution of HiRISE makes it both sensitive to, and an excellent recorder of jitter. We present an algorithm using Fourier analysis to resolve the jitter function for a HiRISE image that is then used to update instrument pointing information to remove geometric distortions from the image. Implementation of the jitter analysis and image correction is performed on selected HiRISE images. Resulting corrected images and updated pointing information are made available to the public. Results show marked reduction of geometric distortions. This work has applications to similar cameras operating now, and to the design of future instruments (such as the Europa Imaging System).

Experimental identification of the overall elastic rigidities of superconducting windings

This study deals with an experimental methodology developed in order to identify the elastic properties of super- conducting ring-shaped windings, constituents of the main coil winding of a magnetic resonance imaging magnet (MRI). Mechanical tensile tests were conducted on real scale specimens associated to an optical full-field displacement measurement technique (stereo image correlation). Strain fields were then obtained from the measured displacement fields by numerical dif- ferentiation. Finally, the four in-plane orthotropic stiffnesses of the windings were determined using the Virtual Fields Method (VFM). Experimental set-up also allows detecting a possible occurrence of a delamination thanks to in-plane displacement fields.

Fast and Dense 2D and 3D Displacement Field Estimation by a Highly Parallel Image Correlation Algorithm

AbstractThis paper describes a fast method for estimation of dense 2D and 3D displacement fields from image correlation. It is based on a previously published local, or window‐based, optical flow algorithm which is ideally suited for parallel processors. We describe the algorithm, its extension to stereo image correlation and its implementation on Graphical Processing Unit (GPU). We present the properties of the estimated displacement fields on simulated images and evaluate their accuracy on real data from a rigid body movement experiment. The main features of the method are a dense output (i.e. a 2D or 3D displacement vector per pixel) and a highly parallel structure which allows very high computational performance. A pair of 4 megapixels stereoscopic images is processed in less than 0.2 s. on a Titan GPU. Finally, we present and comment several experimental results obtained with the proposed method during mechanical experiments conducted at ONERA.

ステレオ画像相関法を用いた高張力鋼板の応力3軸度および破断ひずみの評価方法の確立

ステレオ画像相関法を用いた高張力鋼板の応力3軸度および破断ひずみの評価方法の確立

Failure analysis based on microvoid growth for sheet metal during uniaxial and biaxial tensile tests

The aim of the presented investigations is to perform an analysis of fracture and instability during simple and complex load testing by addressing the influence of ductile damage evolution in necking processes. In this context, an improved experimental methodology was developed and successfully used to evaluate localization of deformation during uniaxial and biaxial tensile tests. The biaxial tensile tests are carried out using cruciform specimen loaded using a biaxial testing machine. In this experimental investigation, Stereo-Image Correlation technique has is used to produce the heterogeneous deformations map within the specimen surface. Scanning electron microscope is used to evaluate the fracture mechanism and the micro-voids growth. A finite element model of uniaxial and biaxial tensile tests are developed, where a ductile damage model Gurson–Tvergaard–Needleman (GTN) is used to describe material deformation involving damage evolution. Comparison between the experimental and the simulation results show the accuracy of the finite element model to predict the instability phenomenon. The advanced measurement techniques contribute to understand better the ductile fracture mechanism.

Characterizing elastic properties of superconducting windings by simulations andexperiments

The objective of the present study is to identify the equivalent stiffness components ofsuperconducting windings. First, a 3D multi-material finite element (FE) model wasestablished to investigate the influence of the winding structural behavior on the identifiedequivalent stiffnesses. Then, the ideas resulting from this FE analysis were applied to theexperiment. Four in-plane stiffnesses of the double pancake were retrieved successfully withthe virtual fields method using the actual strain fields obtained from stereo imagecorrelation.

Out-of-plane Testing Procedure for Inverse Identification Purpose: Application in Sheet Metal Plasticity

Many recent works in inverse identification of constitutive parameters have pointed to the need of tests which exhibit heterogeneous strain paths. The present study details a new testing procedure based on out-of-plane motion capture by Stereo-Image Correlation (SIC). With the original test proposed hereby, a unique sample is deformed on a tensile machine along two perpendicular tensile directions, two perpendicular shear directions and one expansion area. The choice of the sample shape is discussed along with the stereo imaging device, 3D reconstruction and measurement uncertainties. The test sample is made from a sheet of commercially pure titanium. A Finite-Element updating inverse method is applied in order to identify six parameters of an anisotropic plastic constitutive model. Results show that this new testing procedure allows every constitutive parameter of the model to be identified from one and only one test.

Identification of the Mechanical Properties of Superconducting Windings Using the Virtual Fields Method

Tensile tests were performed in order to identify the stiffness components of superconducting windings in the shape of rings (also called ‘double pancakes’). The stereo image correlation technique was used for full-field displacement measurements. The strain components were then obtained from the measured displacement fields by numerical differentiation. Because differentiation is very sensitive to spatial noise, the displacement maps were fitted by polynomials before differentiation using a linear least-square method. Then, in the orthotropy basis, the four in-plane stiffnesses of the double pancake were determined using the Virtual Fields Method.

Identification of the Mechanical Properties of Superconducting Windings Using the Virtual Fields Method

Tensile tests were performed in order to identify the stiffness components of superconducting coils. The stereo image correlation technique was used for full-field displacement measurements. The strain components were then obtained from the measured displacement fields by numerical differentiation. Because differentiation is very sensitive to spatial noise, the displacement maps were fitted by polynomials before differentiation using a linear least-square method. Then, the four in-plane stiffnesses of the double pancake were determined using the Virtual Fields Method in the orthotropy basis.