Identification of Materials Mechanical Properties from Full-Field Measurements: Latest Advances in the Virtual Fields Method

This paper presents a short overview of the state of the art and future challenges of the use of full-field measurements and inverse procedures to identify the constitutive mechanical parameters of a wide range of materials. It concentrates on the so-called Virtual Fields Method (VFM) which is a tool fully dedicated to the processing of full-field measurements. Some of the future challenges are briefly covered here, namely the design of test configurations and the application to damage assessment, high strain rate testing and biomaterials. Some examples are given and the main scientific issues briefly discussed. Introduction The fast development of full-field optical methods (digital image correlation, grid methods, speckle Interferometry, etc.) has dramatically changed the prospects of the mechanical testing of materials and structures. Whereas local strain measurements over a limited number of points with strain gauges or LVDTs restricted the testing configurations to simple shapes and load cases (uniaxial tension or compression, three-point bending etc.), kinematic measurements over tens to hundreds of thousands of points now enable much more complex situations to be addressed. In particular, one can perform the simultaneous determination of several stiffness parameters for anisotropic materials, complex elasto-plastic laws and heterogeneous materials (including welds and damaged composites). Recently, this area of research has dramatically expanded and significant progress has been made towards the production of automated tools to help testing engineers and scientists. The objective of the present paper is to provide an overview of the state of the art and the future challenges awaiting the Virtual Fields Method (VFM), which is an inverse identification procedure fully dedicated to full-field measurements. State of the art The main objective of the VFM is to process full-field kinematic measurements to identify parameters driving the constitutive behaviour of materials. The present paper will not review the method in detail, this can be found in [1,2]. Only the main features will be highlighted. It must also be noted that the VFM is an alternative to a more general approach often referred to as Finite Element Model Updating (FEMU). Again, more details can be found in [2,3]. In short, the main difference between the two is that FEMU recalculates the mechanical fields iteratively from external load and geometry whereas the VFM processes the measured fields directly to assess equilibrium, hence leading to much shorter computation times. Homogeneous linear elasticity. Historically, this was the first application of the VFM. The motivation was aimed at the simultaneous identification of all the anisotropic in-plane or bending stiffness components of composite plates [4-5]. Basically, the idea is to write the principle of virtual work, as in Eq. 1 below (in the static case of static and without body forces): Applied Mechanics and Materials Online: 2008-07-11 ISSN: 1662-7482, Vols. 13-14, pp 3-9 doi:10.4028/www.scientific.net/AMM.13-14.3 © 2008 Trans Tech Publications, Switzerland This is an open access article under the CC-BY 4.0 license (https://creativecommons.org/licenses/by/4.0/)