Heaviside-Digital Image Correlation

In polycrystalline metallic materials, quantitative assessment of the plasticity in relation to the microstructure provides insight on the deformation processes that occur during mechanical loading.Plastic deformation for this class of materials is often localized into physical slip bands, persistent slip bands (PSBs) or sub-grain scale shear bands. Understanding of the relationship between plastic localization and its evolution during loading as a function of the underlying microstructure has been used to identify the detrimental microstructural configurations that promote damage. Predictive models for monotonic and cyclic loading that relate the local mechanical behavior to the global loading condition require quantitative assessment of the microscopic scale heterogeneous strain distribution. Existing measurements of the average strain fields at the grain scale insufficiently capture the cyclic plastic localization and resulting damage, which evolve during the early stages of plasticity at the sub-grain scale through the generation of persistent slip or shear bands (PSBs).

Increases in spatial resolution in DIC by the use of SEM imaging has enabled the observation of discrete slip events. However the conventional DIC codes need to be developed and adapted to correctly handle the physics of the discontinuities present at the sharp localization of plasticity. Emerging DIC codes with discontinuity implementations have been recently applied in fracture mechanics. These new DIC codes are significantly improved in their ability to identify and characterize cracks or large shear band character.

In this study, the properties of Heaviside functions are employed to solve the problem of kinematical discontinuities. The capabilities of this new DIC code for the measurement of strain fields and strain localization in a polycrystalline sample will be shown, comparing the classical DIC code to Heaviside-DIC. The Heaviside-DIC method has been extended for the automated identification of slip system types. Furthermore, the out-of-plane displacements and in-plane shearing resulting from slip events are also calculated using the new code and have been validated using SEM and atomic force microscopy (AFM) measurements. The discontinuity-tolerant DIC code provides for the first time physical full-field measurements of the magnitude of plastic localization at slip bands.