Ante Buljac

Evaluation of measurement uncertainties of digital volume correlation applied to laminography data

The effect of different data acquisition parameters on the displacement and strain uncertainties of kinematic fields is assessed via digital volume correlation applied to three-dimensional data sets imaged by synchrotron laminography. The measurement uncertainty is estimated for (1) different materials and associated varying contrast; (2) repeated scans, that is, uncertainty due to the imaging system and three-dimensional reconstruction; (3) rigid body motions between two subsequent scans; (4) rescaling of the gray level histogram during 32-bit floating point to 8-bit integer data conversion; (5) changes in the beam properties, that is, use of a monochromator or an undulator; and (6) changes in camera/detector characteristics. It is found that the amount of image contrast is not the only parameter that controls uncertainties for different materials. Furthermore, the rigid body motion procedure should be preferred over repeated scans as it provides more conservative measurement uncertainty values. The applied 32- to 8-bit conversion procedure, beam tuning and detector characteristics hardly affect the measurement uncertainty.

Experimental-Numerical Validation Framework for Micromechanical Simulations

A combined experimental-numerical framework is presented in order to validate computations at the microscale. It is illustrated for a flat specimen with two holes, which is made of cast iron and imaged via in situ synchrotron laminography at micrometer resolution during a tensile test. The region in the reconstructed volume between the two holes is analyzed via Digital Volume Correlation (DVC) to measure displacement fields. Finite Element (FE) simulations, whose mesh is made consistent with the studied material microstructure, are driven by measured Dirichlet boundary conditions. Damage levels and gray level residuals for DVC measurements and FE simulations are assessed for validation purposes.

Early Strain Localization in Strong Work Hardening Aluminum Alloy (2198 T3): 3D Laminography and DVC Measurement

The effect of strain hardening on localization in front of a notch is assessed by following the interactions between strain concentrations, damage, initial microstructure and grain orientations. A CT-like specimen made of strong work hardening 2198 T3 aluminum alloy is subjected to an in situ synchrotron laminography experiment. Kinematic fields are measured via digital volume correlation. The final results are bulk displacement and strain fields including their corresponding resolutions. The reported results refer to the portion of the specimen around 1 mm away from the notch root. With the selected spatial resolution, damage nucleation and growth is evaluated in strained bands until the very end of the loading process.

Numerical modeling of ductile fracture at the microscale combined with x-ray laminography and digital volume correlation.

Predicting ductile fracture for complex loading paths is essential within the framework of metal forming processes. Most models are developed and used at the macroscopic scale and do not account explicitly for material microstructures. This paper describes a methodology aiming at understanding and modeling ductile damage mechanisms at the microscale. This methodology relies on the acquisition of X-Ray laminography pictures during in-situ tensile tests, digital volume correlation (DVC) to measure 3D displacement and strain fields in the bulk and 3D finite element (FE) modeling of the heterogeneous microstructure including ductile damage mechanisms. The methodology is illustrated on nodular graphite cast iron. FE simulations of the heterogeneous microstructure are conducted and compared with DVC results and the influence of boundary conditions is discussed.