Christopher Bayley

Bio: Christopher Bayley is an academic researcher from Defence Research and Development Canada. The author has contributed to research in topics: Welding & Heat-affected zone. The author has an hindex of 11, co-authored 21 publications receiving 500 citations. Previous affiliations of Christopher Bayley include Queen's University & Eindhoven University of Technology.

Second-order crystal plasticity: internal stress effects and cyclic loading

Abstract: This paper studies dislocation induced internal stresses as introduced in a strain gradient crystal plasticity approach. The need for a second-order crystal plasticity approach is motivated in the context of the analysis of free-standing metal films used in radio-frequency micro-electro-mechanical systems (RF-MEMS). The presented second-order framework incorporates the influence of inter- and intragranular deformation inhomogeneities in the constitutive description, generally assumed to be the basic origin of scale dependent behaviour and associated size effects. Focusing on the dynamic loading of thin RF-MEMS films, attention is given to dislocation induced kinematical hardening effects. The corresponding internal stresses are discussed with respect to their contribution to the solution of the underlying elasto-plastic framework which implicitly accounts for elastic incompatibilities. Within the loading regime of interest, the response in loading–unloading–reloading and cyclic loading is assessed.

A three-dimensional dislocation field crystal plasticity approach applied to miniaturized structures

Abstract: Material size effects are predicted for idealized planar micromanufactured structures, as a consequence of the competitive contributions of strain gradient strengthening and loss of microstructural constraints with diminishing dimensions, assuming a constant grain size. Simulations are carried out using a three-dimensional strain gradient crystal plasticity model which intrinsically accounts for the influence of differently oriented crystals within the material. By distinguishing between different crystallographic slip boundary conditions, the influences of surface layer passivity, internal grain boundaries and back stresses are assessed under externally applied in-plane tension and through-thickness bending loading conditions. Analyses are carried out on samples with a size that is representative of micromanufacturing processes. The simulations reveal a competitive process between first-order constraints, generally inducing a weakening behaviour as the number of grains decreases, and second-order strengt...

Microstructural evolution during simulated OLAC processing of a low-carbon microalloyed steel

Abstract: The microstructural evolution during simulated on-line accelerated cooling (OLAC) of a commercial Grade 80 pipe steel was studied using a quench deformation dilatometer. The transformed matrix microstructure contains various amounts of polygonal ferrite, granular bainite and acicular ferrite, depending mainly on the accelerated-cooling interrupt temperature. The final microstructure is predicted well by drawing the OLAC schedule on the appropriate CCT diagram. Three distinct groups of precipitates are found in the final microstructure, which form during reheat, austenite deformation, and cooling, respectively. The distribution and composition of the precipitates varies widely with steel composition and processing schedule. The microstructure of industrially processed plate agrees well with that of corresponding laboratory simulations.

Micromorphic Approach for Gradient Elasticity, Viscoplasticity, and Damage

Abstract: A unifying thermomechanical framework is presented that reconciles several classes of gradient elastoviscoplasticity and damage models proposed in the literature during the last 40 years . It is based on the introduction of the micromorphic counterpart ϕχ of a selected state or internal variable ϕ in a standard constitutive model. In addition to the classical balance of momentum equation, a balance of micromorphic momentum is derived that involves generalized stress tensors. The corresponding additional boundary conditions are also deduced from the procedure. The power of generalized forces is assumed to contribute to the energy balance equation. The free energy density function is then chosen to depend on a relative generalized strain, typically ϕ- ϕχ , and the microstrain gradient ∇ ϕχ . When applied to the deformation gradient itself, ϕ≡ F , the method yields the micromorphic theory of Eringen and Mindlin together with its extension to finite deformation elastoviscoplasticity by Forest and Sievert. If...

Mechanical properties and deformation mechanisms of gradient nanostructured metals and alloys

Abstract: Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolaminated nd nanotwinned metals and alloys, has provided new opportunities to understand gradient-related mechanical behaviour. These emerging gradient materials often exhibit unprecedented mechanical properties, such as strength–ductility synergy, extraordinary strain hardening, enhanced fracture and fatigue resistance, and remarkable resistance to wear and corrosion, which are not found in materials with homogeneous or random microstructures. This Review critically assesses the state of the art in the field of gradient nanostructured metallic materials, covering topics ranging from the fabrication and characterization of mechanical properties to underlying deformation mechanisms. We discuss various deformation behaviours induced by structural gradients, including stress and strain gradients, the accumulation and interaction of new dislocation structures, and unique interfacial behaviour, as well as providing insight into future directions for the development of gradient structured materials. Gradient nanostructured metals and alloys are an emerging class of materials that exhibit a combination of excellent mechanical properties that are not possessed by their homogeneous counterparts. This Review assesses the fabrication, characterization and deformation behaviour of these materials, as well as the challenges and future directions of the field.