René Rotinat

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.

Full-Field Strain Measurement On Titanium Welds And Local Elasto-Plastic Identification With The Virtual Fields Method

One of the main problems in welding is the microstructural transformation within the area affected by the thermal history. The resulting heterogeneous microstructure within the weld nugget and the heat affected zones is often associated with changes in local material properties. The present work deals with the identification of material parameters governing the elasto—plastic behaviour of the fused and heat affected zones as well as the base material for titanium hybrid welded joints (Ti6Al4V alloy). The material parameters are identified from heterogeneous strain fields with the Virtual Fields Method. This method is based on a relevant use of the principle of virtual work and it has been shown to be useful and much less time consuming than classical finite element model updating approaches applied to similar problems. The paper will present results and discuss the problem of selection of the weld zones for the identification.

Optimization of the Unnotched Iosipescu Test on Composites for Identification from Full-Field Measurements

The paper presents the use of an optimization routine in order to find the best combination of free length and fibre angle in an unnotched Iosipescu test processed with the virtual fields method. The sensitivity to noise coefficients arising from the special virtual fields procedure are used to build up a cost function aiming at balancing out the different coefficients of the three main orthotropic stiffnesses so that the same confidence level can be reached on these three stiffnesses. Then, experimental validation was performed using an ESPI system. Full-fields were measured and stiffnesses identified and compared to the usual 0◦, 30 mm configuration. The outcome of the optimization was confirmed by testing the same specimen several times and comparing scatter between the two configurations. This is a first promising result on the way to the design of a new test for orthotropic stiffness identification on a single specimen from full-field measurements. Introduction Full-field measurements are becoming more and more widespread both in academia and industry. The recent outcome of the SPOTS project (http://www.opticalstrain.org) is a first step towards future standardization and there is no doubt that these techniques will be used routinely in a few years time. Nevertheless, the existing mechanical test configurations have not been designed to take full advantage of the great amount of experimental information now available. For instance, producing a uniform (tensile or compressive test of a rectangular specimen) or simple (bending or torsion of a rectangular beam) strain state in a specimen is useful when only a few local strain measurements are available (strain gauge, extensometer...) that cannot represent the complexity of a non uniform strain field. But the drawback of such simple tests is that only limited information is available. For instance, a longitudinal tensile test on a unidirectional composite can only provide identification on the longitudinal modulus and major Poisson’s ratio because only the longitudinal stress is present in the specimen. No information on the transverse and shear moduli can be obtained with this test. Moreover, the data processing of such tests relies on stringent assumptions on the shape and the boundary conditions that cannot always be met. This is the case for off-axis tests on unidirectional composites, for instance [22]. The availability of full-field data gives the opportunity to resort to more complex test configurations with the idea of activating more parameters of the material constitutive equations in Applied Mechanics and Materials Online: 2006-10-15 ISSN: 1662-7482, Vols. 5-6, pp 125-134 doi:10.4028/www.scientific.net/AMM.5-6.125

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.

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.

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.