Measurement and Identification Techniques for Evolving Discontinuities

Abstract

Digital Image Correlation allows one to estimate displacement fields based on a series of digital images of the surface of a specimen subjected to a specific loading history. Recent advances have been achieved through a novel formulation that enables one to decompose the searched displacement field onto a suited library of such fields. The latter are either finite element shape functions, which open the way to a further identification step, or mechanically significant fields such as analytic displacement fields for cracks. The interest of this experimental tool is that it provides full kinematic fields. For heterogeneous tests or discontinuities, it thus gives access to a wealth of data that are exploited to estimate mechanical properties. A first route is given by post-processing measured displacements. Two options are followed. First, by using a standard least squares technique, stress intensity factors and crack tip locations are determined by using a known displacement basis. Second, an integral interaction formulation gives also access to stress intensity factors. By choosing suitable test functions, the minimization of a scalar product with respect to measurement noise yields the optimal basis to extract stress intensity factors. An alternative route consists in using the displacement basis directly at the measurement stage. The different procedures are applied to the analysis of a high cycle fatigue experiment on a cracked sample.