Evaluation of volume deformation from surface DIC measurement

Stereo-DIC allows to track with a high accuracy the shape change and the surface displacement field of objects during deformation processes. When multiple camera arrangements are used, the shape and deformation measurement can be performed over the whole surface of the object. We submit that, in the case of intact specimens, with no internal defects and/or discontinuities, such boundary information can be used to estimate the internal displacement field by using proper interpolation functions. This calculation could serve, for instance, to evaluate the strain localization that occurs in metal specimens subjected to plastic deformation, hence allowing to get a better insight in the necking initiation and fracture propagation processes. In this paper, an interpolation method based on Bézier curves is developed and tested using simulated and real experiments on specimens with flat and cylindrical geometries. In particular, the deformation behaviour in the necking zone was investigated in the case of highly ductile and anisotropic materials. Numerical models were used to validate the method while the application to two real experiments demonstrated its feasibility in practical cases. The applicability of the method to more complex loading cases (e.g., bending, torsion, mixed-loads) or different initial shapes (e.g., curved beams, notches) will be investigated in future studies.