Application of digital speckle correlation for strain measurement in composites

With the aim of improving the understanding of composite mechanical behaviour, a remote-sensing non-contact strain measurement technique has been developed and tested. In fact, due to the intrinsic nature of composites, the adoption of optical devices for strain measurement can be very desirable and often mandatory (for instance in the characterisation of foams or ceramic coatings). The proposed methodology, called Digital Speckle Correlation, is based on a numerical evaluation of the correlation between digital white light speckle images. In particular, the development of a new algorithm for the maximisation of the cross-correlation between each couple of sub-images allows to process quickly a large amount of data so that thousands of high-resolution pictures can be treated during each test. In the present study, two different set-ups have been chosen. The first, that hereafter will be called single point, has been developed to reach a high strain accuracy (+/-20 microstrain) and it is proposed to be used when a uniform strain field is expected (such as in a tensile test). The single point device may substitute a standard clip gauge in most of its usual applications; moreover, the digital speckle correlation method has also the advantage that it can return all three plane strain components (epsilon(x), epsilon(y), gamma(xy)). The second set-up, suitable to return a full field response, differs from single point in image processing procedure. This second approach is mainly addressed to study specimens where strain gradients are possible.