Valéry Valle

Experimental determination and empirical representation of out-of-plane displacements in a cracked elastic plate loaded in mode I

A Michelson interferometer is applied to a mode I fracture mechanic problem to investigate the out-of-plane displacement near the crack tip. For an elastic cracked plate, it appears that the two-dimensional theory predicts correctly the free-stress surface displacement only in a region greater than half the specimen thickness. Combining this two-dimensional solution and the measurements provided by the interferometer, the out-of-plane displacement is likewise accessible inside the previous region where the state of stress is three-dimensional. A mathematical formulation, using an exponential integral function is then proposed to describe the out-of-plane surface displacement in the measurement area around the crack tip.

Phase demodulation method from a single fringe pattern based on correlation with a polynomial form.

The method presented extracts the demodulated phase from only one fringe pattern. Locally, this method approaches the fringe pattern morphology with the help of a mathematical model. The degree of similarity between the mathematical model and the real fringe is estimated by minimizing a correlation function. To use an optimization process, we have chosen a polynomial form such as a mathematical model. However, the use of a polynomial form induces an identification procedure with the purpose of retrieving the demodulated phase. This method, polynomial modulated phase correlation, is tested on several examples. Its performance, in terms of speed and precision, is presented on very noised fringe patterns.

Phase demodulation from a single fringe pattern based on a correlation technique

We present a method for determining the demodulated phase from a single fringe pattern. This method, based on a correlation technique, searches in a zone of interest for the degree of similarity between a real fringe pattern and a mathematical model. This method, named modulated phase correlation, is tested with different examples.

Demodulation of spatial carrier images: Performance analysis of several algorithms using a single image

Optical full-field techniques have a great importance in modern experimental mechanics. Even if they are reasonably spread among the university laboratories, their diffusion in industrial companies remains very narrow for several reasons, especially a lack of metrological performance assessment. A full-field measurement can be characterized by its resolution, bias, measuring range, and by a specific quantity, the spatial resolution. The present paper proposes an original procedure to estimate in one single step the resolution, bias and spatial resolution for a given operator (decoding algorithms such as image correlation, low-pass filters, derivation tools …). This procedure is based on the construction of a particular multi-frequential field, and a Bode diagram representation of the results. This analysis is applied to various phase demodulating algorithms suited to estimate in-plane displacements.

High Speed Local Strain Determination from Grating Diffraction

The spectral analysis of grating allows, for static loading, the direct measurement of local strains at the surface of a body. This grating analysis is achieved by two ways (optical diffraction phenomenon or numerical Fourier transform) in order to determine at each step of load pitches and orientations of crossed grating. Our purpose is to extend this strain measurement method to investigate dynamic problems. The grating interrogation is performed using optical diffraction of a laser beam with an oblique incidence. In order to separate the diffracted beams during the dynamic event, we associate with each strain state a specific angle of the incident laser beam. This procedure allows to record 23 strain states at a maximum frequency equal to 1 MHz. The diffracted spots can be stored by two ways (film and CCD camera) and their analysis gives a strain sensitivity of 2.10−4.

Determination of Asphalt Pavements Fracture Parameters Using Optical Approaches

This paper deals with the characterization of mode I fracture parameters using a kinematic approach integrating the experimental displacement measured by optical techniques. Tests are carried out using a wedge splitting sample made in asphalt concrete. The WST-cube specimen is a bi-layered asphalt concrete AC12 with Carbon Fiber grid at the interface. The tested material was provided in the RILEM-SIB. During the test the displacement field evolution close to the crack tip was recorded by using the optical techniques. An adjustment procedure was also used to improve the displacement fields and avoid experimental noise. Based on the experimental optical measurements, the energy release rate was performed by means the Crack Relative Displacement Factor and Stress In-tensity Factor. The Mark Tracking method is employed in order to measure the Crack Opening Displacement. This approach allows to consider the assessment of fracture parameters for the real structures.

Optical Techniques for Relief Study of Mona Lisa’s Wooden Support

We present in this paper an original application of the use of optical methods for the study of the famous painting: Mona Lisa. Several laboratories [1] have participated to the study of this painting to evaluate the degradation risk, especially in relation with the existing crack and to optimise the conservation conditions, regarding both the humidity regulation and the design of the frame. Mona Lisa is painting on a poplar support, so the aim of our study, is to obtain a whole field 3D profile of the panel on front and back face of Mona Lisa [2]. Furthermore, these values allow us to understand the mechanical impact by the frame in which the panel is maintained attached and are used to describe the hygromechanical behaviour of the wooden painting by many measurements realized during several hours. These experimental data have been used to realize a numerical model of the panel, and also to validate the numerical simulations of the mechanical behavior of the panel [1,3].

Experimental Study of the Out-Of-Plane Displacement Fields for Different Crack Propagation Velocities

The fundamental aim of this study is to determine the influence of crack propagation speed on the out-of-plane displacement fields in the neighbourhood of a crack tip (i.e. on the size of the 3D effects zone). As the crack propagation is a complex process that involves the deformation mechanisms, the out-of-plane displacement measurement gives pertinent information about the 3D effects. We propose in this paper, to analyse the out-of-plane displacements fields for different crack velocities in brittle materials.

Phase demodulation from a single fringes pattern: Application to DSPI fringes

The aim of this paper concerns the phase demodulation of a fringes pattern in dynamic regime. During a dynamic loading, we can not use a phase shifting technique because the mechanical parameters involve according to the time. This problem can be avoided with the development of algorithms which can extract the information with the help of only one fringes pattern. In this way, we present two algorithms, the MPC [1] (Modulated Phase Correlation) and the pMPC [2] (polynomial Modulated Phase Correlation). These algorithms allow us to extract the phase from a single fringes pattern obtained, for example, by shadow moire, photoelasticity or interferometry. These processes are based on the use of the virtual fringes pattern which locally approaches the real fringes morphology. The similarity degree between real and virtual fringes pattern is estimated by digital correlation technique. When the best similarity is obtained, we suppose that the virtual phase function is very near of the real phase function. One particularity of these algorithms is theirs few sensitivity to noises. So, we propose to use the MPC or the pMPC algorithms in order to extract the relief in dynamic from a single interferogram obtained by digital speckle pattern interferometry. In this paper we present an experimental test of impact loading on textile specimen. The frame rate is equal to 6000 frames per second. The figure 1 shows one of the fringes patterns recorded in dynamic and the result of wrapped phase demodulated with one of ours algorithms.

Holographic Recording of Gratings for Dynamic Strain Field Measurement

A new development of holographic recording for strain field evaluation during a transient event is proposed. The suggested metrology is a method associating holographic recording with numerical analysis of grating deposed on the specimen surface. An holographic camera allowing the storage of 6 grating images is presented. This process is based on the temporal coding of each hologram, which is realised by fast variation of the reference beam orientation. Characteristics of this dynamic holographic recording technique are exposed, and the experimental prototype and its characteristics are reported.