Full‐Field Strain Rate Measurement by White‐Light Speckle Image Correlation

Abstract

This paper explains a numerical procedure to process sequences of digital images and to return a full‐field evaluation of the strain rate. The processing procedure is based on a nonlinear least squares fitting performed globally, on the whole image, and simultaneously on several images. The use of a highly optimised code allows the analysis of long sequences in a few minutes. The results of calculations are presented as movies built by blending the colour maps of the measured strain field with the specimen pictures used in the correlation procedure. Our application is focused on studying the plastic behaviour of metals and, in particular, on highlighting any transient phenomena that might occur during yielding and strain‐hardening phases on thin sheets used in the manufacture of sheet metals. A typical example for such phenomena is the Portevin‐Le Châtelier effect, a repetitive yielding of alloys during plastic deformation.

Detection of the Strains Induced in Murine Tibias by Ex Vivo Uniaxial Loading with Different Sensors

“…Once having demonstrated the linear relationship between F and ɛ x , and between F and Δl/l, the highest force value sums up all the others. Moreover, the correlation algorithm employed in this work [32] was specifically designed to extend the correlation procedure to a set of consecutive pictures, across which the path of a point was considered to be smooth according to a chosen spatiotemporal function. In view of this, the algorithm predictions for 1 N, 3 N, and 6 N would be more accurate when loading the specimens up to 9 N. The bone samples were therefore subjected to the following protocol: one step signal of 9 N of amplitude followed by 4 triangular compressions of 9 N at the following frequencies, 0.1 Hz, 0.5 Hz, 1 Hz, and 4 Hz, which correspond to loading speeds of 120 N/s (for the step signal), 1.8 N/s, 9 N/s, 18 N/s, and 72 N/s respectively.…”

Mode-enhanced space-time DIC: applications to ultra-high-speed imaging

“…The first route, which is referred to as 'hard regularization', consists in choosing a basis of functions in time, rich enough to provide a fair representation of the actual time history but with the required regularity or smoothness. For instance linear [53] or tent shaped functions (i.e. 1D regular linear finite elements [46,47]) will provide C 0 continuity.…”

White-light speckle image correlation applied to large-strain material characterization

“…This data are obtained by processing the sequence of digital images jointly and synchronously acquired along with the load during the uniaxial test. The local deformation field is then computed exploiting the white-light speckle image correlation technique (Broggiato, 2004(Broggiato, , 2008 that will be explained in the next section.…”