[show abstract][hide abstract] ABSTRACT: Optical full-field measurement methods such as Digital Image Correlation (DIC) are increasingly used in the field of experimental mechanics, but they still suffer from a lack of information about their metrological performances. To assess the performance of DIC techniques and give some practical rules for users, a collaborative work has been carried out by the Workgroup “Metrology” of the French CNRS research network 2519 “MCIMS”1. A methodology is proposed to assess the metrological performances of the image processing algorithms that constitute their main component, the knowledge of which being required for a global assessment of the whole measurement system. The study is based on displacement error assessment from synthetic speckle images. Series of synthetic reference and deformed images with random patterns have been generated, assuming a sinusoidal displacement field with various frequencies and amplitudes. Displacements are evaluated by several DIC packages based on various formulations and used in the French community. Evaluated displacements are compared with the exact imposed values and errors are statistically analyzed. Results show general trends rather independent of the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the uncertainty with the parameters of the algorithms, such as subset size, gray level interpolation or shape functions, is discussed.
[show abstract][hide abstract] ABSTRACT: It is demonstrated that two grating interferometers with high spatial resolutions can successfully be applied for the mechanical characterisation of the advanced fabric composite materials. Based on these two techniques, the mechanical properties of two kinds of fabric laminates are obtained without assumption of uniform strain fields to be used in the characterisation approaches using the local strain sensors. The degree of the yarn crimp effects of the two laminates is compared in terms of the out-of-plane displacement derivatives. Especially, it is shown that the grating shearing interferometer is appropriate for the crimped fabric structure requiring a three-dimensional analysis. The modification from moiré interferometer to grating shearing interferometer is performed by introducing a Michelson interferometer modified for image shearing.
[show abstract][hide abstract] ABSTRACT: The paper aims at introducing basics on full-field optical methods applied to composites since those methods highly enhance the experimental characterisation of material and structure behaviour. Different classical methods are first introduced, focusing on their relative performances. The special interest of the grating shearography method is then emphasised. It is applied to a one-ply fabric coupon in uniaxial tensile loading. The comparison with strain gauges of different effective length shows the flexibility of the technique to perform adapted analysis. Since the method is capable to measure the surface slope and in-plane strain fields, it is shown that both those data are necessary to catch an in-depth understanding of the mechanical behaviour of the fabrics. Finally, the basic interest of these techniques coupled to numerical analysis is underlined. It is believed that such a coupling can lead to a complete renewal of the mechanical characterisation of materials and systems.
[show abstract][hide abstract] ABSTRACT: Grating shearography is an extension of moire interferometry where a shearing element is included in the imaging part of the setup. It is possible to obtain all three components of the in-plane strains, the in-plane rotation and the two out-of-plane slopes. The presented application is an experimental investigation of the mechanical behavior of a plain woven graphite fiber reinforced polymer composite lamina under tensile loading.