Digital Image Correlation Cameras Research Articles

Experimental investigation of transient strains of GGBS-FA-SF blended geopolymer concrete at elevated temperatures

Fire is one of the most severe conditions encountered during the lifetime of a structure. Consequently, the provision of proper fire safety measures for structural members is a major safety requirement in building design. In this paper an experimental study is reported on the axial deformation of GGBS-FA-SF blended geopolymer mortar and geopolymer concrete with and without steel fibre when they are subjected to both mechanical and thermal loadings. The transient tests were conducted in Instron machine with additional heating facility. During the test the specimen was first subjected to a pre-defined mechanical load and followed by thermal heating. The transient axial deformations of the tested specimens were recorded using digital image correlation camera. By using the experimentally obtained temperature-dependent thermal strains of the specimen with different preloads the transient strains of the geopolymer mortar and geopolymer concrete with and without steel fibre are analysed and evaluated. Finally, empirical formulas are also proposed to reproduce the influence of the preload, heating temperature and constituents of the mix on the transient strain of geopolymer mortar and geopolymer concrete.

Measuring the motion of the eardrum using Digital Image Correlation

The human eardrum is a translucent curved membrane and is in direct contact with the middle ear bones. Studying this complicated biomechanical system remains challenging. Digital image correlation (DIC) is an optical technique that allows measuring the shape and displacement of curved surfaces in 3D. In this work, we discuss the advantages and disadvantages of using DIC to study the eardrum at frequencies around 1 kHz. Special attention is given to the application of speckle patterns required to perform DIC on the translucent eardrum. Additionally, we detail an alternative approach to high-speed cameras: phase-locked light pulses. Using light pulses synced to the phase of the sound, regular DIC cameras can be used at their full resolution. This enables a high spatial resolution to thoroughly investigate the motion of the eardrum at the relevant frequencies.

Influence of the Approach Direction on the Repeatability of an Industrial Robot

The article aims to prove the hypothesis, that an approach direction influences repeatability at target point of a trajectory. Unlike most researches that deal with absolute accuracy, this paper is focused on determining the achievable repeatability and the influence of the direction of approach on it. To prove the hypothesis, several measurements are performed under different conditions, on industrial robot ABB IRB1200. To verify and confirm the result obtained from the resolvers located on the individual axes of the robot, the measurements are replicated using high-speed digital image correlation cameras. Using an external measuring device, the real repeatability of the robot endpoint is determined. The measurement proved the correctness of the hypothesis, i.e., the dependence of the approach direction on repeatability was proved. Furthermore, real deviations were measured and the extent of this influence on the robot repeatability was determined.

Development of a Camera Localization System for Three-Dimensional Digital Image Correlation Camera Triangulation

Advancements in computer-vision methods have made stereophotogrammetry and three-dimensional digital image correlation (3D-DIC) attractive tools for Structural Health Monitoring (SHM). Nonetheless, optical approaches are not always well suited for quantitative measurement on large-scale structures due to the challenging calibration process of the cameras. In this study, a novel multi-sensor system is presented to compute the stereophotogrammetry system’s extrinsic parameters. This system can extend 3D-DIC’s capability and allow for easier calibration and measurement. The proposed system integrates a MEMS-based Inertial Measurement Unit (IMU) for determining the spatial orientation of the cameras (i.e., roll, pitch, and yaw angles) and a 77 GHz radar sensor for measuring the relative distance of the stereo cameras. The prototype’s effectiveness is evaluated through laboratory and analytical tests. Its accuracy in calculating the extrinsic parameters of a stereophotogrammetry system is demonstrated by direct comparison with data obtained from a traditional calibration. Results show that the multi-sensor system can determine the separation angle between the two cameras with an error below 2% when the comparison is made.

A Generalized Stress Inversion Approach With Application to Residual Stress Estimation

Abstract We develop a generalized stress inversion technique (or the generalized inversion method) capable of recovering stresses in linear elastic bodies subjected to arbitrary cuts. Specifically, given a set of displacement measurements found experimentally from digital image correlation (DIC), we formulate a stress estimation inverse problem as a partial differential equation-constrained optimization problem. We use gradient-based optimization methods, and we accordingly derive the necessary gradient and Hessian information in a matrix-free form to allow for parallel, large-scale operations. By using a combination of finite elements, DIC, and a matrix-free optimization framework, the generalized inversion method can be used on any arbitrary geometry, provided that the DIC camera can view a sufficient part of the surface. We present numerical simulations and experiments, and we demonstrate that the generalized inversion method can be applied to estimate residual stress.

Comparison of Formability and Microstructural Evolution of C106 Copper and 316L Stainless Steel

Strain-hardenable materials achieve their strength due to the build-up of defects and dislocation tangles within the microstructure brought about by strain induced through processing or forming. This strength can be significantly reduced and negatively impacted if the material is used in service at elevated temperatures, which causes the grain structure to recover and recrystallize to a more organized, lower strength state. Forming limit curves of C106 DHP copper and 316L stainless steel were established using an Interlaken hydraulic press with Nakajima punch tooling and GOM digital image correlation cameras. These formed samples were used as an analog to current formed parts, and the effect of extant elevated operating temperatures was then investigated by means of microhardness testing. High-resolution electron backscatter diffraction (EBSD) analysis using a JEOL JSM-7800F field-emission gun scanning electron microscope was employed to understand the various mechanisms responsible for the physical and microstructural changes during forming and at elevated temperatures.

Robust reference system for Digital Image Correlation camera recalibration in fieldwork

Digital Image Correlation (DIC) is a modern powerful tool to acquire displacement fields in both laboratory and field work. Still, a number of severe limitations persist that can hinder the application of the method to objects in field work, where perfect stability of the cameras or the object itself cannot be enforced, or image acquisition is to be performed during long periods and the equipment has to be moved in the meantime. A common example of this problem is found in large structure monitoring, such as auto or railway bridges, landslides, peers or any other structure that requires displacement monitoring due to damage, often associated with fatigue and fracture in metallic structures. Accurate detection of camera movement across images of the same surface enable the computation of 3D point clouds with correct camera calibration values and geometrically aligned with other point clouds from previous time instants. An algorithm, which uses feature detection to recalibrate a stereo camera-pair after movement, was developed in this work. The cameras are originally calibrated in their initial position, and the world coordinates of detected features in the background are computed to serve as reference. In subsequent image capture following camera setup movement, the new camera positions are computed using the new coordinates of the featured reference points, as well as newly detected point correspondences between images from both cameras.

Durability Analysis and Experimental Validation of Environmental Barrier Coating (EBC) Performance Using Combined Digital Image Correlation and NDE

To understand the failure mechanism or to predict the spallation life of environmental barrier coatings (EBC) on fiber reinforced ceramic matrix composites, the fracture strength of EBC and the process of the crack growth in EBC layers need to be experimentally determined under standard or simulated engine operating conditions. The current work considers a multi layered barium strontium aluminum silicate (BSAS)-based EBC-coated, melt infiltrated silicon carbide fiber reinforced silicon carbide matrix composite (MI SiC/SiC) specimen that was tensile tested at room temperature. Numerous tests were performed under tensile loading conditions, and the specimen was loaded until failure under pre-determined stress levels. The specimen was examined with optical microscopy, scanning electron microscopy (SEM), computed tomography (CT) scan, and digital image correlation (DIC) camera. Observation from the computed tomography scanning, the SEM, and the optical microscopy did not offer conclusive information concerning the cracks that spawned during the tests. However, inspection with the DIC camera offered some indication that cracks had developed and allowed their detection and the location of their initiation site. Thus, this study provides detailed discussion of the results obtained from the experimental investigation and the nondestructive evaluation (NDE), and it also includes assessment of the stress response predicted by analytical modeling and their impact on EBC durability and crack growth formation under complex loading settings.