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.
The use of full-field displacement measurements in mechanical testing has increased dramatically over the last two decades. This is a result of the very rich information they provide, which is enabling new possibilities for the characterization of material constitutive parameters for inhomogeneous tests often based upon inverse approaches. Nonetheless, the measurement errors limit the accuracy of the identification of the constitutive parameters and their possible spatial resolution. The question addressed by this work is the following: can a filtering of the displacement measurement improve the results of the identification of elastic properties? The discussion is based on the study of a numerical example where the elastic parameters of an elastic structure with inhomogeneous properties are sought from synthetic data representative of in-plane full-field data. The displacement data are first filtered through a diffuse approximation algorithm, based on a moving least-squares approximation. Then, the identification of the elastic parameters is performed by an inverse approach based on the minimization of a cost function, defined as the least-squares gap between the experimental data and their numerical counterpart (finite element model updating). Within this framework, a first-order analysis is proposed in order to characterize the errors in the identified parameters, the measurement error characteristics being known. Results from raw and filtered displacement data are compared and discussed, filtering improving the identification for lower spatial resolution. The choice of the norm to define the gap between the experiment and the calculation is also discussed. For practical use and to take advantage of the proposed first-order methodology, two different ways can be considered: applying the methodology to a numerical example, representative of the experimental setup, to determine whether or not a filtering is valuable, and estimating the uncertainties of the identified parameters at the end of the identification process of an experimental characterization.
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