Experimental investigation of the systematic error on photomechanic methods induced by camera self-heating
TL;DR: The experimental results showed that the thermal expansion of the camera outer case and lens mount, instead of mechanical components within the camera, were the main reason for image expansion.
Abstract: The systematic error for photomechanic methods caused by self-heating induced image expansion when using a digital camera was systematically studied, and a new physical model to explain the mechanism has been proposed and verified. The experimental results showed that the thermal expansion of the camera outer case and lens mount, instead of mechanical components within the camera, were the main reason for image expansion. The corresponding systematic error for both image analysis and fringe analysis based photomechanic methods were analyzed and measured, then error compensation techniques were proposed and verified.
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TL;DR: In this article, the displacement of the reference sample rigidly attached behind the test sample is first measured using 2D-DIC, and then it is fitted using a parametric model to correct the displacements of the deformed sample to remove the influences of these unfavorable factors.
Abstract: The ideal pinhole imaging model commonly assumed for an ordinary two-dimensional digital image correlation (2D-DIC) system is neither perfect nor stable because of the existence of small out-of-plane motion of the test sample surface that occurred after loading, small out-of-plane motion of the sensor target due to temperature variation of a camera and unavoidable geometric distortion of an imaging lens. In certain cases, these disadvantages can lead to significant errors in the measured displacements and strains. Although a high-quality bilateral telecentric lens has been strongly recommended to be used in the 2D-DIC system as an essential optical component to achieve high-accuracy measurement, it is not generally applicable due to its fixed field of view, limited depth of focus and high cost. To minimize the errors associated with the imperfectness and instability of a common 2D-DIC system using a low-cost imaging lens, a generalized compensation method using a non-deformable reference sample is proposed in this work. With the proposed method, the displacement of the reference sample rigidly attached behind the test sample is first measured using 2D-DIC, and then it is fitted using a parametric model. The fitted parametric model is then used to correct the displacements of the deformed sample to remove the influences of these unfavorable factors. The validity of the proposed compensation method is first verified using out-of-plane translation, out-of-plane rotation, in-plane translation tests and their combinations. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively examine the strain accuracy of the proposed compensation method. Experiments show that the proposed compensation method is an easy-to-implement yet effective technique for achieving high-accuracy deformation measurement using an ordinary 2D-DIC system.
70 citations
TL;DR: In this article, the second-order displacement gradients were used to evaluate the displacement and strain systematic errors of the digital image correlation (DIC) method in local deformations such as the Portevin-Le Chatelier bands, the deformations near the gap and the crack tips.
57 citations
TL;DR: In this paper, the authors applied a pre-filtering to the images prior to the correlation so that the high-frequency contents of the speckle pattern were suppressed. Butterworth and Wiener filters were used to reduce the systematic error.
44 citations
TL;DR: In this paper, a 3D-DIC system was used to measure the thermal deformation of a tungsten sample above 3000 °C, and three schemes based on blue light sources, self-radiating light sources and a dual wavelength optical filter were designed for three temperature ranges from room temperature to 3067 °C.
42 citations
TL;DR: This paper aims to give a comprehensive overview and more insights of the work on thermal error analysis and compensation for DIC/DVC measurements.
30 citations
References
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TL;DR: In this article, a review of the 2D digital image correlation (2D DIC) technique for displacement field measurement and strain field estimation is presented, and detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided.
Abstract: As a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface, two-dimensional digital image correlation (2D DIC) is now widely accepted and commonly used in the field of experimental mechanics. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. In this review, methodologies of the 2D DIC technique for displacement field measurement and strain field estimation are systematically reviewed and discussed. Detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided. Measures for achieving high accuracy deformation measurement using the 2D DIC technique are also recommended. Since microscale and nanoscale deformation measurement can easily be realized by combining the 2D DIC technique with high-spatial-resolution microscopes, the 2D DIC technique should find more applications in broad areas.
2,530 citations
TL;DR: In this paper, the authors developed and limited experimental verification of a method which can determine displacements and gradients using the Newton-Raphson method of partial corrections, which was shown to be accurate in determining displacement and certain gradients, while using significantly less CPU time than the current coarse-fine search method.
Abstract: Digital image correlation is finding wider use in the field of mechanics. One area of weakness in the current technique is the lack of available displacement gradient terms. This technique, based on a coarse-fine search method, is capable of calculating the gradients. However the speed at which it does so has prevented widespread use. Presented in this paper is the development and limited experimental verification of a method which can determine displacements and gradients using the Newton-Raphson method of partial corrections. It will be shown that this method is accurate in determining displacements and certain gradients, while using significantly less CPU time than the current coarse-fine search method.
1,304 citations
TL;DR: Two algorithms, one based on filtering and the other based on similarity measure, are developed and some applications based on these two algorithms are explored, including strain determination, phase unwrapping, phase-shifter calibration, fault detection, edge detection and fringe segmentation.
620 citations
01 Jan 1992
TL;DR: The task was to investigate the effects of cameras, signal transmission techniques, and frame grabbers as they are widely available on the market as off-the-shelf equipment.
Abstract: The advent of filmless imaging systems, especially of Charge-Coupled Devices (CCD), have created manifold opportunities and new applications which have led to significant changes in Photogrammetry. The exacting demands of Photogrammetry on the radiomet¬ ric and geometric characteristics of the imaging sensor and all other elements involved in the acquisition of imagery with solid-state sensors, require a detailed analysis of the fac¬ tors affecting the performance. This analysis can build on extensive knowledge developed over the last twenty years for the calibration of film based cameras. The modelling of systematic errors introduced by lenses is identical for both systems. Calibration and analysis techniques, such as the bun¬ dle adjustment with self-calibration, are important tools. Another source of a great deal of information are applications of solid-state sensors in astronomy. Exacting radiometric requirements have led to the development of specialized sensors, cameras, image acqui¬ sition systems, and calibration techniques. These techniques are currently only partially applicable to photogrammetric tasks though. They are tuned to be used with extremely specialized hardware (cameras, image acquisition systems, testing arrangements) which is typically not available for photogrammetric tasks. The task was thus to investigate the effects of cameras, signal transmission techniques, and frame grabbers as they are widely available on the market as off-the-shelf equipment. The individual components were in¬ vestigated first, then the combined radiometric and geometric performance of the system was analyzed, and finally the performance of a system for three-dimensional measure¬ ments was verified. This in turn required the installation of an image acquisition system with advanced capabilities to perform various comparative investigations, to set up a three-dimensional testfield, and to write a large amount of software for analysis and cali¬ bration purposes. The geometric regularity and the excellent radiometric characteristics of solid-state sen¬ sors make them ideal measurement devices. A number of investigations have shown that the regularity of the sensor element spacing is in the order of l/100th of the spacing. The uniformity of response from sensor element to sensor element is 1 % and better for many off-the-shelf cameras.The geometric regularity of the sensor would thus allow to mea¬ sure positions of targets imaged on such a sensor with a precision of 1/100 of the spac¬ ing and better. This is actually achieved and surpassed with special sensors in star tracking applications. The mechanical design and the electronics of CCD-cameras are usually not designed for photogrammetric purposes. The mechanical design is often not sufficiently stable, i.e. the assembly of lens, housing and sensor is not rigid. The camera electronics are designed for visual purposes (e.g. surveillance), thus potentially introduc¬ ing significant degradations due to a number of factors such a low-pass-filters, gamma correction, addition of video signals, to name but a few. The largest drawback with respect to the radiometric and geometric performance, but an advantage with respect to costs, is the use of standard analog video signals for the trans¬ mission of the imagery acquired by the sensor. These standards were developed in the 1950's for broadcasting and are not at all tuned for the requirements of precise measure¬ ments. The radiometric and geometric properties of these signals were analyzed, draw¬ backs discussed, methods for determination and compensation and/or elimination of deficiencies investigated and performed. Frame grabbers are another critical component of the image acquisition system. They must convert the analog transmitted imagery into matrices of numbers, the digital image.
146 citations
TL;DR: A method is presented that permits a vast increase in the capability of projection moire topography by simultaneous translation of the reference and object gratings to generate high-definition contour fringes without any of the artifacts associated with ordinary moire.
Abstract: A method is presented that permits a vast increase in the capability of projection moire topography. It involves simultaneous translation of the reference and object gratings to generate high-definition contour fringes without any of the artifacts associated with ordinary moire. Experimental results are presented for single- and double-projection arrangements for a general 3-D object and an inclined-plane test surface. Clear contouring fringes for angles of inclination up to 80° are shown, and fringe densities higher than the grating frequency are obtained. Coupled with the flexibility of projection moire, this technique is shown to be particularly suited for computer-assisted moire fringe pattern analysis. Means of practical implementation in on-line processing system are proposed.
97 citations