Torki Harchegani, Mohammad Ebrahim (2019-04). Ductile Fracture under Combined Tension and Shear: Theory and Applications. Doctoral Dissertation.
Thesis
Fracture causes enormous material and energy waste per annum, with large economical, industrial and environmental impact. In particular, ductile failure under shear-dominated loading pervades in many areas of manufacturing, load-bearing structures and impact protection systems. However, failure in shear remain elusive there being no complete theory of ductile fracture without a physics-based model. A robust micromechanics-based constitutive framework, founded on mechanism-based yield criteria for materials with evolution laws accounting for microstructural evolution, is essential to this end. Experimental observations reveal cell-level plastic deformation as homogeneous or inhomogeneous, the latter being idealized with plasticity confined within intervoid ligaments or occasionally within intervoid plugs. The present thesis is partly targeted to the development of analytical yield functions that predict yielding by either mechanism, attained by limit analysis over a cylindrical cell containing a coaxial void. Nonetheless, existing outcomes indicate the shear-dominated deformation process at early stages as an intermediate state between a homogeneous and an ideally localized one. Correspondingly, a hybrid model is adopted consisting of simple modifications to both an existing homogeneous yield criterion as well as a derived localized yield function. Upon current limitations of a highly complex physical process, a surrogate microstructure, tied to a possible localization plane, is invoked. The next missing link to the constitutive framework calls for microstructural evolution equations during localized deformation, which sets the second objective of the present work. The body of existing and derived yield criteria supplemented with available and derived evolution equations sets enough grounds for the numerical simulation of ductile fracture, thus the third milestone. The hybrid model predictions are firstly borne out by existing numerical outcomes under combined loading. The parametric studies are then carried through a complete range of loading combinations from uniaxial to pure shear loading. The effects of initial porosity, void shape, relative spacing, void misalignment with the principal loading directions, and matrix plastic anisotropy are accounted for. Furthermore, the strain to failure is evaluated vs. a complete scope of triaxialities. The thesis closes with proposed extensions to 3D voids, void coalescence along columns and other potential prospects for more robust numerical implementation. Key Words: Ductile fracture; Void coalescence; Combined tension and shear; Homogeneous/ Inhomogeneous Yielding; Strain localization; Simple/Pure shear.
