Papers by Author: Matti Ristinmaa

Abstract: A thermomechanically coupled constitutive model for finite strain elasto-plasticity is formulated and numerically implemented. The model gives a physically sound description of an initially austenitic material influenced by martensitic phase transformation. The heat dissipated by plastic slip deformation and by phase transformation is allowed to influence the material behavior and appears as a key influencing factor on the growth of the martensitic phase. The model is calibrated using a common stainless steel as prototype material, allowing numerical simulations of crack propagation to be performed. Alterations of the crack growth behavior are observed as different simulation scenarios are compared.

Abstract: A constitutive model for ductile porous material is formulated within the thermodynamic framework. A yield function based on the lower-bound solution for a cylindrical void model embedded in a plastic matrix is proposed. The new yield function is compared to the classical Gurson yield function using cell model calculations. The results reveal that the proposed yield function agreed well with the plastic region found from the cell model calculations. In addition to the influence of the void-volume ratio, the elastic part of the free energy is dependent on a scalar damage field which allows the elasticity to be influenced by the void-volume fraction. The degradation is controlled by a scalar valued damage field and enters the formulation via the Helmholtz's free energy. This dependence allows the elastic properties to naturally depend upon the damage accumulation. The numerical treatment of the model is derived and the capability of the model is demonstrated via numerical simulation of the necking of an axi-symmetric bar.

Abstract: Hydrogen in excess of solid solubility precipitates as hydride phase of plate shaped morphology in hcp α-Zr with the broad face of the hydride plate coinciding with certain crystallographic plane of α-Zr crystal called habit plane. The objective of the present investigation is to predict the habit plane of δ-hydride precipitating in α-Zr at 298 K using strain energy minimization technique. The δ-hydride phase is modeled to undergo isotropic elasto-plastic deformation. The α-Zr phase was modeled to undergo transverse isotropic elastic deformation but isotropic plastic deformation. Accommodation strain energy of δ-hydride forming in α-Zr crystal was computed using initial strain method as a function of hydride nuclei orientation. Hydride was modeled as disk with round edge. Contrary to several habit planes reported in literature for δ- hydrides precipitating in α-Zr crystal, the total accommodation energy minima at 298 K suggests only basal plane i.e. (0001) as the habit plane.