| dc.description.abstract | In recent years, numerous studies have shown the complexity of plastic deformation in metals. The accuracy in modelling inelastic behaviour holds great significance for engineers as metals and metallic alloys are extensively used in various industries (e.g., aerospace, automobile, defence, chemical, and energy), and arriving at such models remains a challenging task. Both phenomenological and physically motivated models are available in the literature to describe the thermo-mechanical behaviour of metals under complex loading conditions. The phenomenological models typically require a few parameters with physically opaque origin to incorporate strain rate and temperature dependence of flow stress. On the other hand, dislocation based models have also been proposed by drawing upon the physics of plastic deformation in metals.
Despite visible progresses in constitutive modelling, many such models are still not applicable over a wide range of strain rates and temperatures. Furthermore, many models do not describe the change in stress under sudden changes in strain rate, e.g. during an impact. A consistent model is thus required to capture such important features during large plastic deformation.
The main objective of this thesis is to develop viscoplasticity models consistent with a certain framework of non-equilibrium thermodynamics for metals and metallic alloys over a wide range of temperature and strain rate. The thermodynamic framework is built with two weakly interacting subsystems, viz. a kinetic-vibrational subsystem and a configurational subsystem. The first describes the fast thermal vibration of atoms, whereas the second comprises the slower degrees of freedom of dislocation motion, grain boundary movement, etc. The separation of time scales is the basis to define these two weakly interacting subsystems, each of which has its own temperature. These two temperatures evolve differently during plastic deformation and establish a heat current from the configurational subsystem to the other.
In Chapter 1, we offer a brief review of the literature and a discussion motivating the work undertaken in this thesis. In Chapter 2, different dislocation densities (e.g. mobile, forest) are used as internal variables to model visco-plastic deformation in metals with a body-centered cubic (BCC) crystal structure. Starting with an idealized set-up that involves only homogeneous deformation, a full-fledged three-dimensional (3D) continuum formulation is derived. This study includes the numerical implementation and validation of the viscoplasticity model in Abaqus using the user material subroutine (VUMAT). Material response is checked through a series of numerical experiments on different boundary value problems (Taylor impact on 4340 steel, Tantalum, and dynamic tensile test on 430 steel).
In the third chapter, the viscoplasticity model is phenomenologically coupled with a ductile damage model to study material degradation. Borvik's shear plugging test is performed on Weldox 460E steel for different plate thicknesses, and an accurate prediction of the ballistic limit is achieved.
In Chapter 4, the model is extended to include the grain boundary evolution and its effect in the visco-plastic response in BCC metals. The configurational temperature evolution equation is modified considering grain boundary as well as dislocation densities. The model is validated over a wide range of strain rates and temperatures for two BCC metals (HSLA steel and Low carbon steel). Additionally, flat rolling simulation is performed in Abaqus on HSLA steel.
In Chapter 5, the development of a thermo-viscoplasticity model for face-centered cubic (FCC) metals is reported. Unlike the preceding chapters, wherein the models involved different types of dislocations, the model in Chapter 4 considers only one dislocation density and a density for grain boundary. Validations are performed against homogeneous deformation in a 7075 aluminum alloy (AA 7075). The deep-drawing process is also simulated and checked against experimental evidence.
The thesis concludes in Chapter 6 with a summary of the reported developments and a brief discussion on the scope of future research. | en_US |
