Raphaël Moulart

Performances and Limitations of Three Ultra High-Speed Imaging Cameras for Full-Field Deformation Measurements

This paper compares the technology and the performances of three ultra high speed cameras for full-field deformation measurements with Digital image correlation or the grid method. The three cameras are based on multiple CCD sensors (Cordin 550-62, with rotating mirror or DRS IMACON 200 with gated intensified CCDs) or dedicated chip (Shimadzu HPV). The advantages and limitations of these cameras are critically reviewed.

On the use of a penalized least squares method to process kinematic full-field measurements

This work is aimed at exploring the performances of an alternative procedure to smooth and differentiate full-field displacement measurements. After recalling the strategies currently used by the experimental mechanics community, a short overview of the available smoothing algorithms is drawn up and the requirements that such an algorithm has to fulfil to be applicable to process kinematic measurements are listed. A comparative study of the chosen algorithm is performed including the 2D penalized least squares method and two other commonly implemented strategies. The results obtained by penalized least squares are comparable in terms of quality to those produced by the two other algorithms, while the penalized least squares method appears to be the fastest and the most flexible. Unlike both the other considered methods, it is possible with penalized least squares to automatically choose the parameter governing the amount of smoothing to apply. Unfortunately, it appears that this automation is not suitable for the proposed application since it does not lead to optimal strain maps. Finally, it is possible with this technique to perform the derivation to obtain strain maps before smoothing them (while the smoothing is normally applied to displacement maps before the differentiation), which can lead in some cases to a more effective reconstruction of the strain fields.

Development of a Full-Field Displacement Measurement Technique at the Microscale and Application to the Study of Strain Fields in a Tensile Steel Specimen

This work deals with the development of a full-field extensometric method at a micrometric scale in order to precisely identify the local features of a metallic alloy at the scale of the grains. The full-field method that has been chosen is the grid method that applies a spatial phase-shifting algorithm to a periodic pattern. To mark the sample, direct interferometric photolithography was used. The paper presents the basic features of the technique and first mechanical test results are commented.

Identification of the Through-Thickness Orthotropic Stiffness of Composite Tubes from Full-Field Measurements

This paper deals with the simultaneous identification of the four through-thickness orthotropic rigidities of a thick composite tube. A diametral compression test has been carried out on a glass/epoxy ring cut from the tube. The full strain field has been measured over one face of the sample with the grid method. The measured fields have been processed with the Virtual Fields Method to identify the rigidities of the material. At the beginning, discrepancies and significant variations occurred in the identified moduli due to inhomogeneous distribution of the strains through the thickness. A method based on a simultaneous measurement on both sides of the ring has been adopted. Very satisfactory results have been obtained using this methodology.

Evaluation of the Penalized Least Squares Method for Strain Computation

This work proposes an alternative procedure to smooth and differentiate experimental full-field displacement measurements to get strain fields. This one, the penalized least squares method, relies on the balance between the fidelity to original raw data and the smoothness of the reconstructed ones. To characterize its performance, a comparative study between this algorithm and two other commonly implemented strategies (the ‘diffuse approximation’ and the Savitzky-Golay filter) is achieved. The results obtained by the penalized least squares method are comparable in terms of quality of the reconstruction to those produced by the two other algorithms, while the proposed technique is the fastest as its computation time is totally independent from the asked amount of smoothing. Moreover, unlike both other considered methods, it is possible with this technique to perform the derivation to obtain strain maps before smoothing them (while the smoothing is normally applied to displacement maps before the differentiation) which can lead in some cases to a more effective reconstruction of the strain fields.

Experimental identification of the overall elastic rigidities of superconducting windings

This study deals with an experimental methodology developed in order to identify the elastic properties of super- conducting ring-shaped windings, constituents of the main coil winding of a magnetic resonance imaging magnet (MRI). Mechanical tensile tests were conducted on real scale specimens associated to an optical full-field displacement measurement technique (stereo image correlation). Strain fields were then obtained from the measured displacement fields by numerical dif- ferentiation. Finally, the four in-plane orthotropic stiffnesses of the windings were determined using the Virtual Fields Method (VFM). Experimental set-up also allows detecting a possible occurrence of a delamination thanks to in-plane displacement fields.

Optical full-field measurement of strain at a microscopic scale with the grid method

In this work, a new micro-extensometric technique to study the local heterogeneities of strain fields in metallic alloys is introduced. It is based on the optical full-field measurement method called the grid method adapted to the micrometric scale. In the first part of the paper, the making of a periodic grating at the surface of the sample by direct interferometric photolithography is explained. The optimization of the grids in terms of phase noise is then addressed.