Luis Felipe-Sesé

Integrating fringe projection and digital image correlation for high-quality measurements of shape changes

Abstract. An approach for the measurement of surface displacement fields in three dimensions is presented based on the combination of two-dimensional digital image correlation with fringe projection. Only a single RGB image is required at each deformation state, thereby allowing real-time data acquisition, which is achieved using red speckle and projected blue fringes that are captured in the single image and separated using a Bayer filter. The approach allows both a perpendicular alignment relative to a flat reference surface and self-calibration, i.e., no calibration object is employed. The minimum measurement uncertainty of such a system is found to be 0.0083±0.00239 and 0.0238±0.0068  mm, respectively, for the in-plane and out-of-plane displacements. The potential of the approach is demonstrated for an elastic membrane undergoing large (5 to 20 mm) applied out-of-plane displacements, and the results show no significant difference (<1%) in the measured in-plane displacement fields compared with a commercially available system for stereoscopic digital image correlation.

Optical low-cost and portable arrangement for full field 3D displacement measurement using a single camera

In the current paper, an optical low-cost system for 3D displacement measurement based on a single camera and 3D digital image correlation is presented. The conventional 3D-DIC set-up based on a two-synchronized-cameras system is compared with a proposed pseudo-stereo portable system that employs a mirror system integrated in a device for a straightforward application achieving a novel handle and flexible device for its use in many scenarios. The proposed optical system splits the image by the camera into two stereo images of the object. In order to validate this new approach and quantify its uncertainty compared to traditional 3D-DIC systems, solid rigid in and out-of-plane displacements experiments have been performed and analyzed. The differences between both systems have been studied employing an image decomposition technique which performs a full image comparison. Therefore, results of all field of view are compared with those using a stereoscopy system and 3D-DIC, discussing the accurate results obtained with the proposed device not having influence any distortion or aberration produced by the mirrors. Finally, the adaptability of the proposed system and its accuracy has been tested performing quasi-static and dynamic experiments using a silicon specimen under high deformation. Results have been compared and validated with those obtained from a conventional stereoscopy system showing an excellent level of agreement.

A Validation Approach for Quasistatic Numerical/Experimental Indentation Analysis in Soft Materials Using 3D Digital Image Correlation

A quasistatic indentation numerical analysis in a round section specimen made of soft material has been performed and validated with a full field experimental technique, i.e., Digital Image Correlation 3D. The contact experiment specifically consisted of loading a 25 mm diameter rubber cylinder of up to a 5 mm indentation and then unloading. Experimental strains fields measured at the surface of the specimen during the experiment were compared with those obtained by performing two numerical analyses employing two different hyperplastic material models. The comparison was performed using an Image Decomposition new methodology that makes a direct comparison of full-field data independently of their scale or orientation possible. Numerical results show a good level of agreement with those measured during the experiments. However, since image decomposition allows for the differences to be quantified, it was observed that one of the adopted material models reproduces lower differences compared to experimental results.

Modal Identification in an Automotive Multi-Component System Using HS 3D-DIC

The modal characterization of automotive lighting systems becomes difficult using sensors due to the light weight of the elements which compose the component as well as the intricate access to allocate them. In experimental modal analysis, high speed 3D digital image correlation (HS 3D-DIC) is attracting the attention since it provides full-field contactless measurements of 3D displacements as main advantage over other techniques. Different methodologies have been published that perform modal identification, i.e., natural frequencies, damping ratios, and mode shapes using the full-field information. In this work, experimental modal analysis has been performed in a multi-component automotive lighting system using HS 3D-DIC. Base motion excitation was applied to simulate operating conditions. A recently validated methodology has been employed for modal identification using transmissibility functions, i.e., the transfer functions from base motion tests. Results make it possible to identify local and global behavior of the different elements of injected polymeric and metallic materials.

Modal Parameters Evaluation in a Full-Scale Aircraft Demonstrator under Different Environmental Conditions Using HS 3D-DIC

In real aircraft structures the comfort and the occupational performance of crewmembers and passengers are affected by the presence of noise. In this sense, special attention is focused on mechanical and material design for isolation and vibration control. Experimental characterization and, in particular, experimental modal analysis, provides information for adequate cabin noise control. Traditional sensors employed in the aircraft industry for this purpose are invasive and provide a low spatial resolution. This paper presents a methodology for experimental modal characterization of a front fuselage full-scale demonstrator using high-speed 3D digital image correlation, which is non-invasive, ensuring that the structural response is unperturbed by the instrumentation mass. Specifically, full-field measurements on the passenger window area were conducted when the structure was excited using an electrodynamic shaker. The spectral analysis of the measured time-domain displacements made it possible to identify natural frequencies and full-field operational deflection shapes. Changes in the modal parameters due to cabin pressurization and the behavior of different local structural modifications were assessed using this methodology. The proposed full-field methodology allowed the characterization of relevant dynamic response patterns, complementing the capabilities provided by accelerometers.

An automated calibration system that combines fringe projection and 2D digital image correlation

An optical non-contact and full-field system that allows large displacement measurements in x-, y- and z-direction is presented. The system combines 2-dimentional digital image correlation (for in-plane measurements) and fringe projection (for out-of-plane displacements) and uses only one camera. The in- and out-of-plane displacements are obtained at the same instant allowing real-time measurements thanks to a color encoding filtering procedure. The out-of-plane measurement allows the correction of the in-plane measurements and the system has to be precisely aligned by following an established alignment procedure. Furthermore, a calibration has to be done to obtain a fringe parameter k for each pixel of the specimen surface image necessary to relate the shifted phase with the out-of-plane displacements. The presented system obtains different values of k for each pixel because of the divergent and non-normal incidence of the fringe beam onto the sample surface (non zero incidence angle). The calibration is performed automatically and only has to be done once for each configuration of the system. The system is portable and can be easily adapted to measure large displacements and wide areas (using small incidence angle) or smaller distances but with higher resolutions (when increasing the incidence angle).

RGB Colour Encoding Improvement for Three-Dimensional Shapes and Displacement Measurement Using the Integration of Fringe Projection and Digital Image Correlation

Three-dimensional digital image correlation (3D-DIC) has become the most popular full-field optical technique for measuring 3D shapes and displacements in experimental mechanics. The integration of fringe projection (FP) and two-dimensional digital image correlation (FP + DIC) has been recently established as an intelligent low-cost alternative to 3D-DIC, overcoming the drawbacks of a stereoscopic system. Its experimentation is based on the colour encoding of the characterized fringe and speckle patterns required for FP and DIC implementation, respectively. In the present work, innovations in experimentation using FP + DIC for more accurate results are presented. Specifically, they are based on the improvement of the colour pattern encoding. To achieve this, in this work, a multisensor camera and/or laser structural illumination were employed. Both alternatives are analysed and evaluated. Results show that improvements both in three-dimensional and in-plane displacement are obtained with the proposed alternatives. Nonetheless, multisensor high-speed cameras are uncommon, and laser structural illumination is established as an important improvement when low uncertainty is required for 2D-displacement measurement. Hence, the uncertainty has been demonstrated to be reduced by up to 50% compared with results obtained in previous experimental approaches of FP + DIC.

Integration of Fringe Projection and 2D Digital Image Correlation for the measurement of 3D displacements and strains

espanolDiversas aplicaciones de ingenieria requieren la medida de desplazamientos las tres direcciones espaciales de un objeto sometido a cargas mecanicas, por ejemplo, en pruebas estructurales o en controles de calidad de fabricacion. En la actualidad, se emplea comunmente la tecnica Correlacion Digital de Imagenes (3D-DIC). Esta tecnica utiliza dos camaras que adquieren imagenes del objeto que esta siendo deformado desde diferentes angulos de vision. Las camaras deben estar perfectamente sincronizadas y calibradas para poder llevar a cabo la reconstruccion de formas en 3D y el seguimiento de la posicion de cada elemento de la superficie del objeto mientras se produce la deformacion. El proceso de calibracion de esta tecnica requiere de la adquisicion de una secuencia de imagenes de un objeto de calibracion y de una alta cantidad de recursos computacionales. En esta tesis se presenta un novedoso sistema que permite medir los desplazamientos en las direcciones X-, Y- y Z ocurridos en la superficie de un objeto sometido a deformacion mediante una sola camara. El metodo empleado se basa en combinar las tecnicas 2D-DIC y FP para obtener los mapas de desplazamiento medidos en el plano y fuera de este (3D). Este sistema propuesto obtiene dichos resultados a partir de la adquisicion de tan solo una imagen por cada etapa de la deformacion del objeto, lo que permite la adquisicion de datos en tiempo real durante ensayos dinamicos. Uno de los intereses de esta investigacion reside en que el sistema permite una alineacion perpendicular precisa de la camara con respecto a un superficie plana de referencia, y tambien permite la auto-calibracion (no se emplea ningun objeto de calibracion). Ademas, permite la determinacion de una constante de calibracion de proyeccion de franjas para cada pixel, asi como todos los parametros necesarios para la correccion de los desplazamientos medidos en el plano. Para ilustrar el potencial del sistema propuesto, se ha realizado una serie de experimentos estaticos y dinamicos. El mayor desplazamiento medido fuera de plano ha sido de 20 mm con una incertidumbre de 0,023 mm y de 0,0083 mm para la medida de desplazamientos en el plano. Los resultados se han comparado con los obtenidos empleando un sistema Correlacion Digital de Imagenes 3D comercial, manifestando un nivel de concordancia muy alto. EnglishIn many engineering applications, the measurement of displacements maps in all three spatial directions over the surface of a loaded object is often required, such an example is, structural testing or manufacturing quality control. At present, for this proposes, typically 3-dimensional digital image correlation (3D-DIC) is often used. This technique employs two cameras acquiring images from different viewing angles of the object while it is deformed. The cameras have to be perfectly synchronized and calibrated for both: 3D reconstruction and for tracking each surface element while the deformation occurs. The calibration process of the technique requires acquiring a sequence of several images of a calibration object and a high amount of computational resources. Alternative techniques have been proposed to obtain 3D displacement maps by combining Fringe Projection (FP) with two-dimensional DIC (2D-DIC). The hybrid technique only requires one camera and a fringe projector. In this thesis it is presented a novel device that allows obtaining the maps of displacements in X-, Y- and Z- direction at the surface of an object during deformation. The system is based on a method that combines 2D-DIC and FP to obtain the in- and out-of-plane components of displacement during deformation. The device operates by acquiring only one image of the studied object at each deformation state desired to analyze during the total test, thereby allowing real time data acquisition. The goal of the presented work is that the device allows both: a precise perpendicular alignment respect to a flat reference surface, and a self-calibration (i.e. no calibration object is employed). Thus, a fringe calibration constant is estimated for each pixel as well as all the required parameters for the in-plane displacement correction. To illustrate the potential of the proposed device, a set of static and dynamic experiments have been conducted using hyperelastic materials. The maximum out-of-plane displacement achieved was 20 mm with an uncertainty of 0,023 mm and an in-plane displacement uncertainty of 0,0083 mm. Results have been compared with those obtained using a commercial three dimensional digital image correlation system showing a very high level of agreement.

Integration of fringe projection and two-dimensional digital image correlation for three-dimensional displacements measurements

Abstract. A low-cost approach for three-dimensional (3-D) full-field displacement measurement is applied for the analysis of large displacements involved in two different mechanical events. The method is based on a combination of fringe projection and two-dimensional digital image correlation (DIC) techniques. The two techniques have been employed simultaneously using an RGB camera and a color encoding method; therefore, it is possible to measure in-plane and out-of-plane displacements at the same time with only one camera even at high speed rates. The potential of the proposed methodology has been employed for the analysis of large displacements during contact experiments in a soft material block. Displacement results have been successfully compared with those obtained using a 3D-DIC commercial system. Moreover, the analysis of displacements during an impact test on a metal plate was performed to emphasize the application of the methodology for dynamics events. Results show a good level of agreement, highlighting the potential of FP + 2D DIC as low-cost alternative for the analysis of large deformations problems.