Direct strain and slope measurement using 3D DSPSI

This communication presents a new implementation of DSPSI. Its main features are 1. an advanced model taking into account the beam divergence, 2. the coupling with a surface shape measurement in order to generalize DSPSI to nonplanar surfaces 3. the use of small shear distance made possible using a precise measurement procedure. A first application on a modified Iosipescu shear test is presented and compared to classical DIC measurements

Direct strain and slope measurement using 2D DSPSI Title

Large variety of optical full-field measurement techniques are being developed and applied to solve mechanical problems. Since each technique possess its own merits, it is important to know the capabilities and limitations of such techniques. Among these optical full-field methods, interferometry techniques take an important place. They are based on illumination with coherent light (laser). In shearing interferometry the difference of the out of-plane displacement in two neighboring object points is directly measured. Since object displacement does not result in interferometry fringes, the method is suited for localization of strain concentrations and is indeed used in industry for this purpose. Used quantitatively DSPSI possesses the advantage over conventional out-of-plane displacement-sensitive interferometry that only a single difference of the unwrapped phase map is required to obtain flexural strains, thereby relieving problems with noise and reduction in the field of view. The first publication on (DSPSI) was made in 1973, but the emergence of a system providing quantitative measurements is more recent. This work aims to present the results of strain measurements using digital speckle pattern shearing interferometry (DSPSI).Comment: Communication \`a la Conf\'erence M\'editerran\'eenne sur les Mat\'eriaux Innovants et Applications -CIMA, Beyrouth, Liban-Mars 15-17, 2011; JM-CIMA.pd

Estimation directe des déformations à partir d'une paire d'images de motif aléatoire par analyse fréquencielle

Colloque avec actes et comité de lecture. Internationale.National audienceUne approche novatrice pour extraire les déplacements à partir d'images de surfaces encodées avec un mouchetis a récemment été présentée et validée. Mais un enjeu essentiel pour les ingénieurs mécaniciens est l'estimation optimale des déformations. Une approche novatrice permettant l'estimation directe des déformations sans dérivation d'un champ de déplacement est proposée. Les premières estimations métrologiques sont encourageantes : la résolution, proportionnelle à la déformation, est de 9% de la valeur nominale, pour une résolution spatiale de 9 pixels (ZOI 64×64 pixels²). Un exemple numérique montre l'intérêt de l'approche. Puis une première étude concerne un composite renforcé par un fibres longues en carbone. Les déformations liées au tissage sont clairement visibles, et les résultats corroborent bien des études précédentes

QUANTITATIVE ANALYSIS OF HETEROGENEITIES OF DEFORMATION WITH THE GRID METHOD

http://www.imim.pl/files/archiwum/Vol1_2009/7-18.pdfInternational audienceThe article deals with the measurement of heterogeneous deformations during the plastic flow of metals. To encode de surface of the test-pieces, transferable carbon grids were used. They proved a reliable technique in the conditions of the experiments (channel-die, step of 100 mu m), even for highly localized deformations. To calculate the displacement field, the grid method was chosen. Up to now it has been developed for elastic structures. It is extended here to large displacement situations. Results are analysed from a metrological point of view and the resolution is kept at 1/100(th) fringe, the final spatial resolution being at two fringes. The mean deformation is 15% between each loading step. Displacement maps, or better, principle shear strain maps, derived from the previous ones, show the presence of a preferential direction even at low deformation levels. They put forward the decisive role played in the development of heterogeneities by the corners of the test-piece, sensitive in channel-die compression because two of their faces are submitted to friction while the third is free. This first attempt of using the grid method for large displacements will be optimized in the future by automating the iterative procedure and reducing the calculation time

Uncertainty on fringe projection technique: a Monte-Carlo-based approach

International audienceError estimation on optical full field techniques (OFFT) is millstone in the diffusion of OFFT. The present work describes a generic way to estimate overall error in fringe projection, either due to random sources (phase error, basically related to the quality of the camera and of the fringe extraction algorithm) or the bias (calibration errors). Here, a high level calibration procedure based on pinhole model has been implemented. This model compensates for the divergence effects of both the video-projector and the camera. The work is based on a Monte Carlo procedure. So far, the complete models of the calibration procedure and of a reference experiment are necessary. Here, the reference experiment consists in multiple step out-of-plane displacement of a plane surface. Main conclusions of this work are: 1/ the uncertainties in the calibration procedure lead to a global rotation of the plane, 2/ the overall error has been calculated in two situations; the overall error ranges from 104 µm down to 10 µm, 3/ the main error source is the phase error even if errors due to the calibration are not always negligible

Numerical and experimental analyses of resin infusion manufacturing processes of composite materials

Liquid resin infusion (LRI) processes are promising manufacturing routes to produce large, thick, or complex structural parts. They are based on the resin flow induced, across its thickness, by a pressure applied onto a preform/resin stacking. However, both thickness and fiber volume fraction of the final piece are not well controlled since they result from complex mechanisms which drive the transient mechanical equilibrium leading to the final geometrical configuration. In order to optimize both design and manufacturing parameters, but also to monitor the LRI process, an isothermal numerical model has been developed which describes the mechanical interaction between the deformations of the porous medium and the resin flow during infusion.1, 2 With this numerical model, it is possible to investigate the LRI process of classical industrial part shapes. To validate the numerical model, first in 2D, and to improve the knowledge of the LRI process, this study details a comparison between numerical simulations and an experimental study of a plate infusion test carried out by LRI process under industrial conditions. From the numerical prediction, the filling time, the resin mass and the thickness of the preform can be determined. On another hand, the resin flow and the preform response can be monitored by experimental methods during the filling stage. One key issue of this research study is to highlight the changes in major process parameters during the resin infusion stage, such as the temperature of the preform and resin, and the variations of both thickness and fiber volume fraction of the preform. Moreover, this numerical/experimental approach is the best way to improve our knowledge on the resin infusion processes, and finally, to develop simulation tools for the design of advanced composite parts

Frequency-based image analysis of random patterns: an alternative way to classical stereocorrelation

The paper presents an alternative way to classical stereocorrelation. First, 2D image processing of random patterns is described. Sub-pixel displacements are determined using phase analysis. Then distortion evaluation is presented. The distortion is identified without any assumption on the lens model because of the use of a grid technique approach. Last, shape measurement and shape variation is caught by fringe projection. Analysis is based on two pin-hole assumptions for the video-projector and the camera. Then, fringe projection is coupled to in-plane displacement to give rise to 3D measurement set-up. Metrological characterization shows a resolution comparable to classical (stereo) correlation technique (1/100th pixel). Spatial resolution seems to be an advantage of the method, because of the use of temporal phase stepping (shape measurement, 1 pixel) and windowed Fourier transform (in plane displacements measurement, 9 pixels). Two examples are given. First one is the study of skin properties; second one is a study on leather fabric. In both cases, results are convincing, and have been exploited to give mechanical interpretation

Bayesian Identification of Elastic Constants in Multi-Directional Laminate from Moir\'e Interferometry Displacement Fields

The ply elastic constants needed for classical lamination theory analysis of multi-directional laminates may differ from those obtained from unidirectional laminates because of three dimensional effects. In addition, the unidirectional laminates may not be available for testing. In such cases, full-field displacement measurements offer the potential of identifying several material properties simultaneously. For that, it is desirable to create complex displacement fields that are strongly influenced by all the elastic constants. In this work, we explore the potential of using a laminated plate with an open-hole under traction loading to achieve that and identify all four ply elastic constants (E 1, E 2, 12, G 12) at once. However, the accuracy of the identified properties may not be as good as properties measured from individual tests due to the complexity of the experiment, the relative insensitivity of the measured quantities to some of the properties and the various possible sources of uncertainty. It is thus important to quantify the uncertainty (or confidence) with which these properties are identified. Here, Bayesian identification is used for this purpose, because it can readily model all the uncertainties in the analysis and measurements, and because it provides the full coupled probability distribution of the identified material properties. In addition, it offers the potential to combine properties identified based on substantially different experiments. The full-field measurement is obtained by moir\'e interferometry. For computational efficiency the Bayesian approach was applied to a proper orthogonal decomposition (POD) of the displacement fields. The analysis showed that the four orthotropic elastic constants are determined with quite different confidence levels as well as with significant correlation. Comparison with manufacturing specifications showed substantial difference in one constant, and this conclusion agreed with earlier measurement of that constant by a traditional four-point bending test. It is possible that the POD approach did not take full advantage of the copious data provided by the full field measurements, and for that reason that data is provided for others to use (as on line material attached to the article)