"Strain Characterization of Superelastic Wire in Bending Using Digital Image Correlation"

Full-field surface strain measurement of thin superelastic wire in bending, using microscopic 3D Digital Image Correlation (DIC), has been undertaken in order to characterise the behaviour under representative loading as experienced in-service. Test rigs have been designed for 3-point bending, 4-point bending and free bending of the wire, with measurement of the strain field within the microscope field of view. The set-up allows characterization of the maximum inner surface strains, outer surface strains and neutral axis position as the wire is loaded and unloaded in bending. Thereafter, this is used to investigate tensile-compressive strain asymmetry and the effect of cyclic load-unload bending on the local strains. Together with 3- and 4-point load-deflection bend tests on the wire, the results can be used for validation of an FEA superelastic wire model. The aim is to apply this validated wire model to ring stent simulation in FEA for stent graft analysis and design. The paper gives an overview of the DIC technique as applied to thin superelastic wires, including necessary equipment and sample preparation. Thereafter, details of the test set-up used are given along with the resulting strain distribution across the wire surface when subjected to a bending regime. Finally, the experimentally measured results are  compared with those obtained by FEA simulation of superelastic wire and comparisons made.