White-light digital image cross-correlation (DICC) analysis of the deformation of composite materials with random microstructure

A sophisticated image cross-correlation algorithm (J. Appl. Opt. 33 (1994) 6667) has been used to measure both components of in-plane displacement at the surface of a deforming composite material. The natural random pattern present on the surface of a polished polymer bonded explosive (PBX) sample is photographed using a high-resolution digital camera mounted on an optical microscope. Frames are taken as the sample deforms and small subimages from each frame are compared with a base-frame recorded before loading. The resulting displacement information reveals the average motion present from the centre of each of the small subimages used in the analysis, relative to the original position. Currently, there is considerable interest in monitoring the safety of PBXs over the medium to long term and in response to accidental stimuli. Knowledge of the ways in which cracks are created and propagate within the material is a key aspect of understanding. Experiment shows that the accuracy of this technique is 0.1 pixels corresponding to 50 nm in the experimental arrangement used. The technique has wide application to many small to medium strain experimental situations due to the ease of data recording. In the experiment presented here, the natural random microstructure of the sample is used to make measurements, however, any manufactured random pattern applied to a sample would enable the method to be successful.