On the analysis of heat haze effects with spacetime DIC

Abstract

Long image series (i.e., movies) can be analyzed via digital image correlation (DIC) techniques. A new spacetime implementation is introduced to extend instantaneous DIC with minimal implementation complexity. With this procedure, a denoised reference image is built considering a series of more than 200 images. Such spacetime approach is very interesting when high temperature experiments are performed since it provides a very eective temporal regularization. The analysis of heat haze eects is carried out on a thermomechanical test in which the sample made of ceramic matrix composite is subjected to temperatures greater than 1,200°C in the heat affected zone of the laser beam. It is shown that the addition of a small fan reduces the spurious displacement uctuations.

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

“…However, because all spatial dimensions are kept together without separation, the problem requires a different setting than that introduced in Refs. [39,40], and is closer to other spacetime frameworks [46,47,41].…”

“…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. Cubic splines provide a higher (i.e., C 2 ) regularity.…”

“…Few tens of Matlab code lines were needed to implement this variant. This non intrusiveness allows standard (stereo)DIC kernels to be used [47,41]. To remove PGD from the previous algorithm, it suffices to consider in the "for" loop (i.e., lines 14-21) a number of modes N m equal to the number of temporal basis functions, and in this case the singular value decomposition that would be used as a mere change of basis is no longer needed.…”

“…When the residual is normalized by the dynamic range of the images (i.e., gray level difference between maximum and minimum gray levels in the ROI), the minimum change is chosen to be 10 −4 in the following. This level is a standard practice for spacetime DIC [47] and is not specific to the PGD variant presented herein. Such a parameter is chosen very small (i.e., less that the noise level) just as a safe detection of convergence, and choosing a smaller value increases the number of iterations without any detectable difference.…”

“…-The reference state is not (as usual) the first image in the time series but it is computed from the determination of the kinematics. This peculiar formulation is not specific to PGD-DIC [47] but is introduced here and shown to be fully compatible with the proposed methodology.…”