Jean-Christophe Dupré

Calibration of a system of projection moiré for relief measuring: biomechanical applications

The projection moire method which belongs to the family of structured light projection allows to obtain the relief of an object or the out-of-plane displacements by the difference in relief between two prompting states. Its principle consists in projecting a sinusoidal grating of vertical lines onto the surface of the object, and the relief is then deduced from the geometric parameters of the set-up and the analysis of the grating observed by a numeric camera. Several implementations are possible, some of them allowing simplifying assumptions for the calculation of the relief. This paper presents this measurement method in any case. A calibration process has been developed to determine the necessary geometric parameters influencing the calculation of the relief.

Photoelastic analysis of a three-dimensional specimen by optical slicing and digital image processing

The authors show a nondestructive method for obtaining the isochromatic and isoclinic fringes in a three-dimensional photoelastic specimen. The basic idea is to delimit a slice between two plane laser beams. The properties of polarization of the scattered light (Rayleighs law) and the interference possibilities of the diffused beams are used. By introducing speckle pattern properties, the correlation factor of the two scattered beams is similar to the illumination given in a plane polariscope for the investigation of a slice (in a classical frozen-stress technique). The authors use a monochromatic laser beam, a CCD camera and a personal computer. Because they cannot obtain the correlation factor directly, they do a statistical analysis of the speckle patterns. The variance (function of the correlation factor) is computed from the light intensities of three images corresponding to the speckle pattern for plane 1 alone, plane 2 alone, and both planes together.

Numerical spectral analysis of a grid: Application to strain measurements

Abstract We present a nondestructive and noncontact extensometry technique, which has been developed in order to study the behavior of different materials such as polycarbonate, araldite, urethane, etc. A spectral analysis of a crossed grating marked on the surface of a specimen and recorded by a charge coupled device camera is presented. The procedure is based on the use of a bidimensional fast Fourier transform algorithm coupled with an interpolation process. It leads to the accurate determination of frequencies. Numerical simulations have been made to evaluate the strain sensitivity; it depends on both the initial phase and the number of cycles characterizing the grid. Theoretically, the strain sensitivity can reach 10 -5 in best cases, but practically it is notbetter than 10 -4 . Several applications on different materials are presented: polyurethane, paper and cloth.

Grating interrogations: from small to large strain measurement

Our purpose is the direct strain measurement from the interrogation of a crossed grating marked on the surface of a specimen. The observation of the object through a master grid (as in moiré method) is replaced by a direct characterization of the pattern using a Fourier transform. This gives direct access to the modification of the pitches which leads to quantification of the strain without the intermediary of a fringe pattern. The comparison between the undeformed and deformed states allows the determination of the magnitude and orientation of principal strains and of the local rigid-body rotation. We describe three analysis techniques, one using the diffraction phenomenon, another utilizing a numerical spectral evaluation and third combining diffraction and phase-shifting procedure. These grid interrogations have different domains of application and a suitable choice of these analysis techniques allows a very large measurement range (10−5 to high strain according to the grating resistance).

Separation of isochromatic and isoclinic patterns of a slice optically isolated in a 3-D photoelastic medium

Abstract We show a nondestructive experimental method which gives the isochromatic and isostatic patterns of slice optically isolated in a three dimensional photoelastic model. We used a nondestructive method of optical slicing developed in our laboratory. In this method, the photoelastic model is optically sliced by two plane laser beams. The analyze of the scattered light gives fringes equivalent to the ones obtained in a plane polariscope (isochromatic and isoclinic fringes). In order to separate the isochromatic and the isoclinic fringes, we recorded several images for different polarization orientations and we separated the isoclinic and isochromatic fringes using the Fourier transform. Then the isostatics pattern is plotted. We show a test of a punctual loading on the top of a prismatic specimen and the possibilities of our method in an industrial case of a model realized by stereolithography technique.

Refinement of digital image correlation technique to investigate the fracture behaviour of refractory materials

Refractory materials exhibit a heterogeneous microstructure consisting in coarse aggregates surrounded by fine grains that form an aggregate/matrix composite. This heterogeneous microstructure often leads to a complex mechanical behaviour during loading. This paper is devoted to the study, thanks to an optical method, Digital Image Correlation (DIC), of the fracture behaviour of two industrial refractory materials in relation with their microstructure resulting from both the chosen constituents and the sintering process. The aim is here, specifically, to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. DIC is indeed a helpful and effective tool, in the topic of experimental mechanics, for the measurement of deformation in a planar sample surface. This non-contact optical method directly provides full-field displacements by comparing the digital images of the sample surface obtained before and during loading. In the present study, DIC has been improved to take into account the occurrence of cracks and performed so as to better identify the early stage of the cracking behaviour. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex and a discontinuity of displacement should be taken into account. Then each subset which crosses a crack can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening).

Characterization of biological materials by means of optical methods of measurement

This study presents the different possibilities of measuring mechanical quantities on biological materials by means of optical methods. These methods allow to perform non-contact and non-disturbing evaluations. The goal is to determine the mechanical characteristics of human organs to be able to model them in order to carry out digital simulations closer to reality. We will set out a method which measures the relief, another one which measures a field of displacements and the last one strain tensors.

Detection of cracks in refractory materials by an enhanced digital image correlation technique

This paper is devoted to the study of the fracture behaviour of two industrial refractory materials thanks to the development of a new technique of digital image correlation (DIC). DIC, already known as a helpful and effective tool for the measurement of displacement and deformation fields in materials, has been adapted to take into account displacement discontinuities as cracks. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex, while each subset can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening) during the loading with a higher spatial resolution than the one obtained by standard DIC. After having presented the principle of the new technique, its metrological performances are assessed from synthetic images and the choice of crack detection criterion is discussed. The capacity of this new technique is shown through a comparative study with standard DIC. Its application is led on magnesia-spinel refractory materials, specifically to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. We show that refractories with aggregates of iron aluminate spinel present a fracture mechanism with crack branching and can dissipate more energy thanks to a longer crack network.

Improvement of local strain measurement by grid method: new optical device and quasi-heterodyne technique

Among optical strain measurement methods, the grid method is a powerful one. Progresses in the technique of information processing have contributed to develop classical analysis (optical and numerical Fourier transform). We propose a new optical device that allows the interference of diffracted beams from two crossed gratings of parallel lines marked on the specimen surface. The analysis of the interference fringes during loading provides the geometry of the gratings and so leads to the strain determination. The proposed method is insensitive to the specimen translations and presents a range from 105, with the use of a phase-shifting technique, to a few 102. Using experimental traction tests, we compare this method with the classical ones and we develop the performances of the proposed method (sensitivity and large measurement range).

In vitro production and biomechanical experimental analysis of thoracolumbar burst fractures

Spinal traumatisms constitute a current pathology in traumatology (10%) and thoracolumbar burst fractures are the most frequent and generally concern young patients. Burst fractures are characterised by compression loadings involved lesion of the vertebrae endplate between the anterior and the posterior walls (Magerl et al. 1994). Determinations of instability and treatment indications remain particular points of questioning. Various treatment solutions exist: immobilisation by an aircast, surgical treatment by posterior or anterior instrumentation and, recently, kyphoplasty with cement injection after vertebral reduction. Choice of the treatment can be delicate and few biomechanical studies have been performed to establish the mechanical response of each option. For that, a cadaveric model of the burst fracture is helpful. This study concerns the development of experiments to generate burst fractures.