Inverse material characterization from 360-deg DIC measurements on steel samples

A tension-torsion testing machine is used for the elasto-plastic characterization of an isotropic steel grade for offshore pipeline applications. The machine is capable of performing multiaxial tests using different sample geometries and load combinations. It is equipped with an optical rig for stereo digital image correlation (DIC) such that, while load is hold at different steps up to final fracture, a set of high resolution images is taken framing the full 360_ view of the specimen. Image capturing is synchronized with standard load-displacement and torque-rotation acquisition. Illustrative results obtained from tensile tests are presented. In particular, the 3D shape and surface deformation are retrieved with DIC throughout a stepwise axial test of an isotropic steel sample. Different constitutive models, previously calibrated with the aid of FE analysis using multiaxial global experimental data (i.e. load-displacement curves), are here validated based on tension-torsion tests results. Afterwards, the local information from DIC (i.e. cross-section dimensions at different axial positions, deformation along significant surface paths) is compared with the corresponding FEM data implementing the validated material model. Evidence on the accuracy of the optical measurements, and on the benefits in terms of material characterization deriving from the additional local information available with such approach are here presented and discussed in detail.