Shailendra P. Joshi

Modeling the constitutive response of bimodal metals

The mechanical response of metals with a bimodal grain-size distribution is modeled using the secant Mori-Tanaka (M-T) mean-field approach. The actual microstructure of bimodal metals involves a grain size distribution in the ultrafine and coarse regimes; the model approximates this in terms of two phases with distinct grain sizes and with specific volume fractions. The model is applied to two bimodal materials: the Al-5083 alloys of Laverniaet al. and the Cu of Wanget al. In both the materials, the predictions agree well with the experiments. In the bimodal Al alloy, the effect of extrusion on the anisotropy in yield strength and flow behavior is also addressed. Finally, based on the model predictions, an empirical expression of the Voce form is proposed to describe the overall flow behavior of both bimodal metals.

A transition from localized shear banding to homogeneous superplastic flow in nanoglass

A promising remedy to the failure of metallic glasses (MGs) by shear banding is the use of a dense network of glass-glass interfaces, i.e., a nanoglass (NG). Here we investigate the effect of grain size (d) on the failure of NG by performing molecular dynamics simulations of tensile-loading on Cu50Zr50 NG with d = 5 to 15 nm. Our results reveal a drastic change in deformation mode from a single shear band (d ∼ 15 to 10 nm), to cooperative shear failure (d ∼ 10 to 5 nm), to homogeneous superplastic flow (d ≤ 5 nm). Our results suggest that grain size can be an effective design parameter to tune the mechanical properties of MGs.

Piezoelectric Sensor and Actuator Spatial Design for Shape Control of Piezolaminated Plates

Controlled actuation and sensing of structures by spatially distributing the piezoelectric material has been a topic of interest in recent years. We present an iterative technique to design the shape of piezoelectric actuators in order to achieve the desired shape of the structure. A gradientless shape design procedure based on the residual voltages is developed. It aims at minimizing the quadratic measure of global displacement residual error between the desired and the current structural configuration. The actuators gradually adapt to a shape that is most efficient in resisting the external excitation. The present technique can be well suited for any static and time-varying excitation. In vibration control it is often necessary to create modal sensors and actuators in order to observe or excite some specific modes. Such modal sensors and actuators alleviate spillover problems, and thus they avoid exhaustive signal processing. Several numerical examples for static as well as dynamic cases are presented to demonstrate the efficacy of the present technique.

Composition and grain size effects on the structural and mechanical properties of CuZr nanoglasses

Nanoglasses (NGs), metallic glasses (MGs) with a nanoscale grain structure, have the potential to considerably increase the ductility of traditional MGs while retaining their outstanding mechanical properties. We investigated the effects of composition on the structural and mechanical properties of CuZr NG films with grain sizes between 3 to 15 nm using molecular dynamics simulations. Results indicate a transition from localized shear banding to homogeneous superplastic flow with decreasing grain size, although the critical average grain size depends on composition: 5 nm for Cu36Zr64 and 3 nm for Cu64Zr36. The flow stress of the superplastic NG at different compositions follows the trend of the yield stress of the parent MG, i.e., Cu36Zr64 yield/flow stress: 2.54 GPa/1.29 GPa and Cu64Zr36 yield/flow stress: 3.57 GPa /1.58 GPa. Structural analysis indicates that the differences in mechanical behavior as a function of composition are rooted at the distinct statistics of prominent atomic Voronoi polyhedra. T...

Developing a light weight lithium ion battery – an effective material and electrode design for high performance conversion anodes

The process of lithium storage by conversion reaction is a subject of intense research in the field of lithium ion batteries as it opens up the possibility of storing more than one mole of lithium per formula unit, leading to very high storage capacities. For instance, lithium storage by conversion reaction in hematite (α-Fe2O3) results in high theoretical capacity of 1005 mAh g−1. Despite numerous attempts, the first cycle reversibility and cyclability achieved in this material have been disappointingly low. To overcome these limitations, we report here an effective “active material-electrode design” incorporating the following features: (i) well-connected active material particles; (ii) adequate active material surface area; (iii) strong particle-current collector adhesion and (iv) superior degree of electrode drying. Incorporating these features in α-Fe2O3 electrodes enhances its overall electrochemical performance. For the first time, a high first cycle reversibility of 90% is reported for lithium storage via conversion reaction in α-Fe2O3. The long term cyclability over 800 cycles demonstrated here is one of highest reported values for this material. Even at high current densities of 5.025 A g−1 (12 mins of charge/discharge), this tailored α-Fe2O3 delivers capacities (446 mAh g−1) in excess of graphite (372 mAh g−1). Most importantly, this anode material shows feasible operation in a full cell containing olivine LiMn0.8Fe0.2PO4 cathode. It is believed that this simple design approach could also be extended to other material systems such as phosphides, sulphides, nitrides and fluorides that store lithium via conversion mechanism.

Stochastic rate-dependent elasticity and failure of soft fibrous networks

This work focuses on modeling the rate-sensitive stiffening-to-softening transition in fibrous architectures mimicking crosslinked fibrous actin (F-actin) networks induced by crosslink unbinding. Using finite element based discrete network (DN) modeling combined with stochastic crosslink scission kinetics, we correlate the microstructural damage evolution with the macroscopic stress–strain responses of these networks as a function of applied deformation rate. Simulations of multiple DN realizations for fixed filament density indicate that an incubation strain exists, which characterizes the minimum macroscopic deformation that a network should accrue before damage initiates. This incubation strain exhibits a direct relationship with the applied strain rate. Simulations predict that the critical damage fraction corresponding to colossal softening is quite low, which may be ascribed to the network non-affinity and filament reorientation. Furthermore, this critical fraction appears to be independent of applied strain rate. Based on these characteristics, we propose a phenomenological damage evolution law mimicking scission kinetics in an average sense. This law is embedded within an existing continuum model that is extended to include non-affine effects induced by filament bending.

Suppression of Shear Banding and Transition to Necking and Homogeneous Flow in Nanoglass Nanopillars

In order to improve the properties of metallic glasses (MG) a new type of MG structure, composed of nanoscale grains, referred to as nanoglass (NG), has been recently proposed. Here, we use large-scale molecular dynamics (MD) simulations of tensile loading to investigate the deformation and failure mechanisms of Cu64Zr36 NG nanopillars with large, experimentally accessible, 50 nm diameter. Our results reveal NG ductility and failure by necking below the average glassy grain size of 20 nm, in contrast to brittle failure by shear band propagation in MG nanopillars. Moreover, the results predict substantially larger ductility in NG nanopillars compared with previous predictions of MD simulations of bulk NG models with columnar grains. The results, in excellent agreement with experimental data, highlight the substantial enhancement of plasticity induced in experimentally relevant MG samples by the use of nanoglass architectures and point out to exciting novel applications of these materials.

Stochastic size-dependent slip-twinning competition in hexagonal close packed single crystals

In this paper, we present an analytical model to investigate stochastic size effects on the critical stress for the activation of non-basal slip and contraction twinning (CT) in miniaturized hexagonal-close packed (HCP) single crystals. The analysis is based on the basic model developed by Parthasarathy et al (Scr. Mater. 56 313–6), but extends it to account for the mechanism-based description of pyramidal slip and CT in HCP metals. The results demonstrate that for a given initial dislocation density, a probabilistic competition exists among CT nucleation, CT growth and pyramidal slip. Based on the stochastic model, we construct a mechanism map that provides regimes of dominant deformation modes that govern the macroscopic yield stress in HCP single crystals.

Cohesive zone modeling of 3D delamination in encapsulated silicon devices

Interfacial delamination in encapsulated silicon devices has been a great reliability concern in IC packaging. Experimental testing of a transparent Quad Flat No Leads Package (QFN) was carried out with the goal of studying delamination characteristics and investigating the viability of cohesive zone modeling (CZM) in simulating delamination patterns and trends. To simplify the study, the package was molded without the die. The pattern of initiation and propagation of delamination under thermal loading is the focus of this study. A video camera was focused on the interface between the pad and the encapsulant. When the temperature has reached a critical value, delaminations were seen to initiate and propagate in a certain pattern. The experimental setup was then modeled within the finite element framework with the failure of the interface described through a cohesive-zone surface interaction approach. The cohesive-zone approach is ideal as, unlike other fracture mechanics methods, it does not require prior specification of any initial delamination. It was found that the 3D numerical model was able to capture the experimentally observed delamination pattern satisfactorily.

Experiments and Three-Dimensional Modeling of Delamination in an Encapsulated Microelectronic Package Under Thermal Loading

Interfacial delamination in encapsulated silicon devices has been a great reliability concern in IC packaging. Experimental testing of a transparent quad flat no leads package is carried out with the goal of studying the delamination characteristics and investigating the viability of cohesive zone modeling in simulating delamination patterns and trends. The pattern of initiation and propagation of delamination under thermal loading is the focus of this paper. A microscope is focused on the interface between the pad and the encapsulant to capture the progressive delamination in a package that was molded without a die. When the temperature reaches a critical value, delaminations are observed to initiate and propagate in a certain pattern. The experimental setup is then modeled within the finite element framework, with the failure of the interface described through a cohesive-zone surface interaction approach. With a slight modification to the experimental procedure and through a separate finite element model, the fracture energy of the interface is estimated. It is found that the 3-D numerical model is able to capture the experimentally observed delamination pattern satisfactorily.