Extension of the virtual fields method to elasto-plastic material identification with cyclic loads and kinematic hardening

International audienceThe virtual fields method (VFM) has been specifically developed for solving inverse problems from dense full-field data. This paper explores recent improvements regarding the identification of elasto-plastic models. The procedure has been extended to cyclic loads and combined kinematic/isotropic hardening. A specific attention has also been given to the effect of noise in the data. Indeed, noise in experimental data may significantly affect the robustness of the VFM for solving such inverse problems. The concept of optimized virtual fields that minimize the noise effects, previously developed for linear elasticity, is extended to plasticity in this study. Numerical examples with models combining isotropic and kinematic hardening have been considered for the validation. Different load paths (tension, compression, notched specimen) have shown that this new procedure is robust when applied to elasto-plastic material identification. Finally, the procedure is validated on experimental data

Estimation of the strain field from full-field displacement noisy data. Comparing finite elements global least squares and polynomial diffuse approximation

International audienceIn this study, the issue of reconstructing strain fields from corrupted full-field displacement data is addressed. Two approaches are proposed, a global one based on Finite Element Approximation (FEA) and a local one based on Diffuse Approximation (DA). Both approaches are compared on a case study which is supposed difficult (open-hole tensile test). DA provides more stable results, but is more CPU time consuming. Eventually, it is proposed to monitor locally the filtering effect of both approaches, the prospects being an impending improvement of the reconstruction for both approaches

Correction of refraction induced distortion in optical coherence tomography corneal reconstructions for volume deformation measurements

In this project, the depth-resolved full-field deformation of the porcine cornea under changing intraocular pressure was investigated by performing digital volume correlation (DVC) on the reconstructed volume images generated through swept source optical coherence tomography (SS-OCT). Posterior inflation test of porcine cornea sample for two load steps were performed and the distribution patterns of displacement and strain fields were produced. The error sources for the measurements were analyzed. The refraction induced OCT image distortion is a main error source for the measurement results. Then, a methodology was developed to correct the OCT distortion based on the Fermat’s principle

Viscoelastic properties identification through innovative Image-Based DMTA strategy

The identification of the high strain-rate properties of materials is an important topic for many engineering applications such as crash worthiness, blast loading, industrial forming, among others. It is also a very challenging experimental task mainly because of the difficulty in measuring impact loads accurately in regimes where inertia effects are significant as well as in presence of heterogeneous deformation states. We present here an innovative identification strategy, using high power ultrasonic loadings together with both InfraRed Thermography, Ultra-High-Speed Imaging and grid method, able to simultaneously characterize the viscoelastic behaviour of polymer materials over a large loading spectrum. The main originalities lies in the fact that heterogeneous stress fields are experimentally reconstructed through acceleration fields measurement and that contrary to conventional DMA, no frequency or temperature sweep is required since the experiment is designed to simultaneously produce both a heterogeneous strain-rate state (up to 400 s-1) and a heterogeneous temperature state (up to the glassy transition point) allowing a local and ?spectral? identification. Moreover, by cooling the sample down, the apparent strain-rate loading range can be significantly increased to fill the gap between servo-hydraulic (10 3 s-1) tests. The present work falls within an effort to invent new high-strain test methodologies based on full field imaging and inverse identification, to both overcome the limits of standard experimental strategies and take advantage of the deformation heterogeneities to achieve a full-characterization of a material from a ?one-shot? test

On the use of simulated experiments in designing tests for material characterization from full-field measurements

The present paper deals with the use of simulated experiments to improve the design of an actual mechanical test. The analysis focused on the identification of the orthotropic properties of composites using the unnotched Iosipescu test and a full-field optical technique, the grid method. The experimental test was reproduced numerically by finite element analysis and the recording of deformed grey level images by a CCD camera was simulated trying to take into account the most significant parameters that can play a role during an actual test, e.g. the noise, the failure of the specimen, the size of the grid printed on the surface, etc. The grid method then was applied to the generated synthetic images in order to extract the displacement and strain fields and the Virtual Fields Method was finally used to identify the material properties and a cost function was devised to evaluate the error in the identification. The developed procedure was used to study different features of the test such as the aspect ratio and the fibre orientation of the specimen, the use of smoothing functions in the strain reconstruction from noisy data, the influence of missing data on the identification. Four different composite materials were considered and, for each of them, a set of optimized design variables was found by minimization of the cost function

A computational approach to design new tests for viscoplasticity characterization at high strain-rates

International audienceRate-dependent behaviour characterization of metals at high strain rate remains challenging mainly because of the strong hypotheses when tests are processed with statically determinate approaches. As a non-standard methodology, Image-Based Inertial Impact (IBII) test has been proposed to take advantage of the dynamic Virtual Fields Method (VFM) which enables the identification of constitutive parameters with strain and acceleration fields. However, most of the test parameters (e.g. projectile velocity, specimen geometry) are not constrained. Therefore, an FE-based approach is addressed to optimize the identification over a wide range of strain and strain-rate, according to two design criteria: (1)-the characterized viscoplastic spectra (2)-the identifiability of the parameters. Whereas the first criterion is assessed by processing the FEA simulations, the second is rated extracting material parameters using synthetic images to input the VFM. Finally, uncertainties regarding the identification of material constants are quantified for each IBII test configuration and different camera performances