Danial Faghihi

Analytical and Experimental Determination of Rate- and Temperature-Dependent Length Scales Using Nanoindentation Experiments

This work addresses the temperature and rate indentation size effects (TRISE) encountered in nanoindentation experiments and the corresponding material intrinsic length scales at different strain rates. The same value for the material length scale cannot be used for different rate, temperature, and accumulated plastic-strain conditions. A variable length scale is introduced in this work and used on two different face-centered cubic (FCC) metals. Indentation experiments are performed on copper and aluminum polycrystalline samples for different strain rates. To check the validity of the assumed concept for local hardening in nanoindentation, additional experiments are conducted on single-crystal materials. The existing theories describing the indentation size effects and length scales are reviewed, and a physically based model that depends on strain rate, accumulated plastic strain, and temperature that were scaled with hardness experiments results is proposed for length scales. Furthermore, numerical simul...

Gradient plasticity for thermo-mechanical processes in metals with length and time scales

A thermodynamically consistent framework is developed in order to characterize the mechanical and thermal behavior of metals in small volume and on the fast transient time. In this regard, an enhanced gradient plasticity theory is coupled with the application of a micromorphic approach to the temperature variable. A physically based yield function based on the concept of thermal activation energy and the dislocation interaction mechanisms including nonlinear hardening is taken into consideration in the derivation. The effect of the material microstructural interface between two materials is also incorporated in the formulation with both temperature and rate effects. In order to accurately address the strengthening and hardening mechanisms, the theory is developed based on the decomposition of the mechanical state variables into energetic and dissipative counterparts which endowed the constitutive equations to have both energetic and dissipative gradient length scales for the bulk material and the interface. Moreover, the microstructural interaction effect in the fast transient process is addressed by incorporating two time scales into the microscopic heat equation. The numerical example of thin film on elastic substrate or a single phase bicrystal under uniform tension is addressed here. The effects of individual counterparts of the framework on the thermal and mechanical responses are investigated. The model is also compared with experimental results.

The Effect of Temperature on Interfacial Gradient Plasticity in Metallic Thin Films

The material microstructural interfaces have a profound impact on the scale-dependent yield strength and strain hardening when the surface-to-volume ratio of the medium increases such as in micro and nanosystems. In this paper, the framework of higher-order strain gradient plasticity with interfacial energy effect is used to investigate the coupling of thermal and mechanical responses of materials in small scales and fast transient processes. In addition to the nonlocal yield condition for the material bulk, a temperature and rate dependent microscopic yield condition for the interface is presented, which determines the stress at which the interface begins to deform plastically and harden. In order to address the strengthening and hardening mechanisms, the theory is developed based on the decomposition of the mechanical state variables into energetic and dissipative counterparts. This, consecutively, provides the constitutive equations to have both energetic and dissipative gradient length scales \(\ell _{en}\) and \(\ell _{dis}\) respectively. Hence four material length scales are introduced: two for the bulk and the other two for the interface. In addition, the effect of temperature on the yield strength and hardening of the interface is included in the formulation by postulating that the interfacial energy decreases as temperature increases. Finally the developed framework is solved numerically to investigate the size effect of unaxial loading of a film substrate system.

Thermal and Mechanical Responses of BCC Metals to the Fast-Transient Process in Small Volumes

AbstractPlasticity in heterogeneous metallic materials with small volumes is governed by the interactions of dislocations within the bulk. Energy (heat) transfer, on the other hand, is micromechanically related to the interaction within phonons, electrons, or photons. To address the experimentally observed characteristics of small-volume metallic components such as thin films, these microstructural interactions need to be included in modeling. This gives rise to a large variety of generalized multiscale models that are developed on the continuum level and used to bridge the gap between the micromechanical and classical continuum models by means of certain characteristic time and length scales. A higher-order strain gradient model accounting for the size effect is combined in this paper with the generalized heat equation to identify the coupling effect of thermal and mechanical behavior of body-centered cubic (BCC) materials with small volumes and in transient time. A fully thermodynamically consistent fra...

Analytical solution for shear bands in cold-rolled 1018 steel

Abstract Cold-rolled 1018 (CR-1018) carbon steel has been well known for its susceptibility to adiabatic shear banding under dynamic loadings. Analysis of these localizations highly depends on the selection of the constitutive model. To deal with this issue, a constitutive model that takes temperature and strain rate effect into account is proposed. The model is motivated by two physical-based models: the Zerilli and Armstrong and the Voyiadjis and Abed models. This material model, however, incorporates a simple softening term that is capable of simulating the softening behavior of CR-1018 steel. Instability, localization, and evolution of adiabatic shear bands are discussed and presented graphically. In addition, the effect of hydrostatic pressure is illustrated.