Inspection of Surface Strain in Materials using Optical Flow

We have developed optical flow based techniques for finding the surface strain of an untreated sample of material from a sequence of images taken during the application of force from a test rig. Not all optical flow algorithms have suitable functional characteristics for this task, as image sequences are characterised by both shortand long-range displacements, non-rigid deformations, as well as a low signal-to-noise ratio and methodological artefacts. We show how optical flow can be used as a high confidence estimator of the strain tensor characterising the deformation of the material. We give results from three studies: Aluminium alloy under axial tension, the formation of kink bands in compressed carbon-fibre composite, and non-homogeneous strain fields in welded alloy. These studies have demonstrated the capabilities of a new optical flow algorithm. Because the algorithm offers increased accuracy near motion contrast boundaries, its application has resulted in new observations of interest to materials scientists.