Jean-Noël Périé

Analysis of a Multiaxial Test on a C/C Composite by Using Digital Image Correlation and a Damage Model

The “planar” digital image correlation technique needs a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states. With these two images, one can determine in-plane displacement and strain fields. The digital image correlation technique used herein is based on Fast Fourier Transforms, which are very effective in reducing the computation cost. Its performance is assessed and discussed on artificial signals and in a real experimental situation. The technique is utilized to analyze experimental results of a plane shear experiment and validate a damage meso-model describing different degradations in a C/C composite material.

Integrated Digital Image Correlation for the Identification of Mechanical Properties

Digital Image Correlation (DIC) is a powerful technique to provide full-field displacement measurements for mechanical tests of materials and structures. The displacement fields may be further processed as an entry for identification procedures giving access to parameters of constitutive laws. A new implementation of a Finite Element based Integrated Digital Image Correlation (I-DIC) method is presented, where the two stages (image correlation and mechanical identification) are coupled. This coupling allows one to minimize information losses, even in case of low signal-to-noise ratios. A case study for elastic properties of a composite material illustrates the approach, and highlights the accuracy of the results. Implementations on GPUs (using CUDA) leads to high speed performance while preserving the versatility of the methodology.

3D analysis from micro-MRI during in situ compression on cancellous bone

A mini-compression jig was built to perform in situ tests on bovine trabecular bone monitored by micro-MRI. The MRI antenna provided an isotropic resolution of 78 microm that allows for a volume correlation method to be used. Three-dimensional displacement fields are then evaluated within the bone sample during the compression test. The performances of the correlation method are evaluated and discussed to validate the technique on trabecular bone. By considering correlation residuals and estimates of acquisition noise, the measured results are shown to be trustworthy. By analyzing average strain levels for different interrogation volumes along the loading direction, it is shown that the sample size is less than that of a representative volume element. This study shows the feasibility of the 3D-displacement and strain field analyses from micro-MRI images. Other biological tissues could be considered in future work.

On the Identification and Validation of an Anisotropic Damage Model Using Full-field Measurements

Two different mechanical tests are performed on a laminated composite coupon to induce an anisotropic damage affecting essentially shear modulus softening. The first test is a uniaxial tension loading on a straight coupon, which is used to evaluate the damage law using a conventional approach, while the second contains a notch that enhances dramatically the strain (and hence damage) heterogeneity. A global digital image correlation approach is used to quantify the kinematic fields all along the loading path of the second experiment. Displacement fields are hence evaluated based on a finite element type discretization. A further exploitation based on the reconditioned equilibrium gap method (and without any further information) gives access to a quantitative measurement of the damage law. The latter approach makes use of a finite element model based on the very same mesh and element shape function. This full-field-based identification method compares very well with traditional techniques, up to the stage where macroscopic localization prevents their subsequent exploitations. Moreover, it is shown that neither the type of mechanical test, nor the discretization of the displacement field, affects the identification of the damage law.

Space-resolved diffusing wave spectroscopy measurements of the macroscopic deformation and the microscopic dynamics in tensile strain tests

We couple a laser-based, space-resolved dynamic light scattering apparatus to a universal testing machine for mechanical extensional tests. We perform simultaneous optical and mechanical measurements on polyether ether ketone, a semi-crystalline polymer widely used in the industry. Due to the high turbidity of the sample, light is multiply scattered by the sample and the diffusing wave spectroscopy (DWS) formalism is used to interpret the data. Space-resolved DWS yields spatial maps of the sample strain and of the microscopic dynamics. An excellent agreement is found between the strain maps thus obtained and those measured by a conventional stereo-correlation bench. The microscopic dynamics reveals both affine motion and plastic rearrangements. Thanks to the extreme sensitivity of DWS to displacements as small as 1 nm, plastic activity and its spatial localization can be detected at an early stage of the sample deformation, making the technique presented here a valuable complement to existing material characterization methods.

Combined Approach for the Characterization of Composites Manufactured by RFI and Industrial Application

A combined approach is proposed here to characterize composite structures made of a recent semi-preg called HexFIT® developed by Hexcel Composites® for resin film infusion (RFI). Physicochemical and mechanical tests were first carried out on specimens obtained by oven cured vacuum bag molding and then compared with autoclave cured specimens. Non-destructive evaluation techniques (US, IR, and RX) were used to sample specimens in manufactured plates for mechanical testing taking into account the effects of the process conditions. Multi-instrumented tensile tests undertaken on specimens with different widths allowed us to highlight limitations in classical tensile tests. The manufacture of a structural part called corner fitting is also presented taking into account the effects of the stacking sequence and the cure cycle. The preliminary results highlight the need to correctly define experimental protocols taking into account manufacture conditions and the inner architecture of the reinforcements.

Experiment / Computation Interactions by Using Digital Image Correlation

The validation of the constitutive equations describing the mechanical behavior of structures is obtained through the comparisons between independent experimental results and numerical computations. In the latter, the boundary conditions are very often postulated. It is proposed to use the experimentally measured displacement on the boundary as an input for the simulations. Consequently, digital image correlation technique has been developed. It gives access to displacement maps and strain fields with an accuracy of 5 10-5. Various applications have been considered. Each one is dealing with one particular aspect i.e. (i) effect of the boundary conditions on the homogeneity of the strain field, (ii) inception of cracking and (iii) comparison between experimental results and numerical predictions.

Finite Element Stereo Digital Image Correlation Measurement for Plate Model

The aim of this study is to measure accurately the boundary conditions, namely the displacements and rotations, along the edges of a large part of a composite panel subjected to complex loadings. With classical Stereo Digital Image Correlation techniques (Stereo DIC), the displacement field can only be measured on the upper skin. The rotations are usually estimated a posteriori by numerical differentiation and smoothing. Unfortunately additional uncertainties may arise with these steps. Conversely, the use of a Finite Element plate model is proposed here to regularize a Stereo Finite Element-DIC measurement (Stereo FE-DIC). The idea is to use the 3D displacement as the primary unknown of the correlation problem. This approach thus requires projection operators (here based on a pinhole camera model). It allows taking into account distortions directly.

Identification of a macroscopic anisotropic damage model using digital image correlation and the equilibrium gap method

The aim of the work is to demonstrate how an anisotropic damage model may be identified from full field measurements retrieved during a heterogeneous test. The example of a biaxial test performed on a 3D C / C composite is used. In a first step, the displacement fields measured by classical Digital Image Correlation are used as input data of a finite difference version of the Equilibrium Gap Method. A benefit from unloadings (assumed to be elastic) is shown to retrieve a damage law. In a second step, inelastic strains can be assessed from the total measured strain and the elastic estimated strains. The constitutive parameters relative to the inelastic part of the model are then identified.

From digital image correlation to damage law identification

In this paper, it is proposed to identify a damage law based upon kinematic fields. The equilibrium gap method is used to determine the parameters of an anisotropic damage model. The approach is applied to a practical biaxial test performed on a C/C composite.