M. A. Siddiq Qidwai

Quality Improvement of Non-manifold Hexahedral Meshes for Critical Feature Determination of Microstructure Materials

This paper describes a novel approach to improve the quality of non-manifold hexahedral meshes with feature preservation for microstructure materials. In earlier works, we developed an octree-based isocontouring method to construct unstructured hexahedral meshes for domains with multiple materials by introducing the notion of material change edge to identify the interface between two or more materials. However, quality improvement of non-manifold hexahedral meshes is still a challenge. In the present algorithm, all the vertices are categorized into seven groups, and then a comprehensive method based on pillowing, geometric flow and optimization techniques is developed for mesh quality improvement. The shrink set in the modified pillowing technique is defined automatically as the boundary of each material region with the exception of local non-manifolds. In the relaxation-based smoothing process, non-manifold points are identified and fixed. Planar boundary curves and interior spatial curves are distinguished, and then regularized using B-spline interpolation and resampling. Grain boundary surface patches and interior vertices are improved as well. Finally, the local optimization method eliminates negative Jacobians of all the vertices. We have applied our algorithms to two beta titanium datasets, and the constructed meshes are validated via a statistics study. Finite element analysis of the 92-grain titanium is carried out based on the improved mesh, and compared with the direct voxel-to-element technique.

Performance Characterization of Multifunctional Structure-Battery Composites for Marine Applications

The aim of our work is to design, fabricate and characterize multifunctional structure-power composites for marine applications such as in unmanned underwater vehicles. Three types of structure-power (or structure-battery (SB)) specimens were fabricated using fiber-reinforced polymers and closed-cell foam as the structural components, and commercial-off-the-shelf lithium-polymer cells as the power-plus-structure component. This paper details the mechanical and electrical characterization of the S-B composites while a companion paper deals with the design and fabrication issues. The three multifunctional designs are: integrated SB laminate with lithium-polymer pouch cells embedded on one side, SB sandwich with cells embedded within a closed-cell polymeric foam along the neutral axis, and a SB modular stiffener that can be attached and removed from a host-structure. Unifunctional composites (i.e. without embedded cells) were also fabricated for comparison with the multifunctional composites. The embedded cells show identical charge-discharge electrical performance as their un-embedded counterparts, thus, indicating that the composite fabrication procedures did not adversely affect their electrical performance. Ragone curves (energy density vs. power density) of the S-B composites show that the targeted energy density of 50 Wh/L was achieved in the SB modular stiffener design. The bending stiffnesses of the integrated SB and SB modular stiffener designs were ∼7x greater than the unifunctional design while the multifunctional sandwich specimens were ∼17% stiffer than their unifunctional counterparts. Tests are currently being conducted to determine the affect of mechanical flexure (constant displacement) on embedded cell discharge and charge characteristics, and conversely, cell discharge and charge on the load and deflection during flexure.Copyright

Numerical Implementation of an SMA Thermomechanical Constitutive Model Using return Mapping Algorithms

In the previous chapter we described the derivation of a 3-D SMA thermomechanical constitutive model. We now address the numerical implementation of this model and the development of numerical tools to support the process of designing SMA devices for use in load bearing 3-D structures. In this chapter, the numerical implementation of SMA thermomechanical constitutive response is presented using return mapping algorithms appropriate for rate-independent inelastic constitutive models. The closest point projection return mapping algorithm and the convex cutting plane return mapping algorithm are discussed, and finite element analysis examples are provided.

Design and Fabrication of Multifunctional Structure-Power Composites for Marine Applications

Various subsystems in marine applications, especially unmanned underwater vehicles, compete for space. Multifunctional structure-battery (SB) composites combine structure and power functions through the use of high-performance fiber reinforced polymer layers and lithium ion cell batteries, to create volumetric opportunities for increase in overall power generation capacity and/or payload. This paper focuses on the design and fabrication aspects of the SB composites. The design objectives are to achieve or exceed structural performance of traditional marine composites while attaining a volumetric energy density of 50 Wh/L with similar buoyancy levels and dimensional sizes. The design process reveals that all four objectives can be achieved only if the components related to energy storage have the same mechanical and physical properties as the material being replaced. With commercially available batteries, at least three out of four objectives are met for the proposed SB designs. Selection of materials and fabrication methods are heavily influenced by the temperature limit of the battery cells, cell surface preparation for adhesion and load transfer, and power bussing layout. A companion paper addresses multifunctional performance characterization of these composites.Copyright

An Analysis of Composite Piezoelectric Actuators Incorporating Nonlinear Material Behavior

Piezoelectric actuators of various composite designs have been proposed during the last few years including extension and shear bimorphs, tubular composites and multilayered actuators. These designs exploit the ability to define the actuation direction by varying the alignment between poling direction and applied electric field. Considerable research effort has been put to accurately model these actuators in order to attain predictive capability. The common factor in almost all of these studies is the assumption that the poled material behaves linearly under applied electric field. However, this assumption may only be accurate for the limited case of a homogenous actuator under relatively unconstrained environment, such as that of simply supported boundary conditions. In the case of composite structures, the actuation can potentially be restricted by non-actuating constituents resulting in multi-dimensional loading states, which may cause domain switching. The same argument can be made for most boundary conditions that are imposed in practical applications, such as when the actuator is clamped or fixed. Another point of concern is the presence of discontinuities and minor defects in the actuator. Both of these would promote non-uniform electric field causing domain switching, and hence, unexpected actuator output. Unless proven otherwise, these concerns directly affect the credibility of life cycle estimates based upon linear models. In this paper linear and nonlinear material models will be used to determine actuator performance using an established constitutive model in a commercial finite element code. Actuator performance for both material cases will be calculated and compared with existing analytical predictions under the same set of boundary conditions.Copyright

Numerical Implementation of Nonlinear Piezoelectric Models in Commercially Available Tools

Commercially available finite element programs currently provide only linear piezoelectric models for device analysis, precluding their use in understanding domain-switching based failure mechanisms, and thus, limiting their usefulness in predicting accurate life cycle estimates for devices. This work is an initial attempt to bridge this gap by slightly modifying and implementing an existing macromechanical theoretical framework, which represents a broader class of nonlinear model development, in commercial software. The rate-independent evolution equations of remanent variables in the original model are replaced by their rate-dependent form which better imitates physical reality and facilitate quick implementation. Decoupled ferroelectric and ferroelastic versions of the model have been successfully realized and tested. Ongoing work is focused on incorporating the fully coupled model, which will then be used to simulate switching near cracks in PMN-PT single crystal material.Copyright

Effect of Crystal Orientation on Polarization in Piezoelectric Materials

This work highlights the computational application of a nonlinear constitutive model to determine the impact of crystal orientation with respect to both initial poling direction and applied electrical fields. Misalignment between material axes and loading axes can occur during the fabrication process and may exhibit desirable performance feature for actuator design. The analysis showed that small angles of loading and material axes misalignment such as may occur in fabrication (less than 5 degrees) have minor impact on material performance, and that the liner response range of these materials can be expanded by increased levels of constraint at the cost of maximum actuation strain. For larger angles, variations in loading and material axes misalignment angles can have a significant impact on performance.© 2008 ASME

Single Crystal PMN-PT Electric Fatigue and Fracture Behavior

Piezoelectric single crystals are being incorporated into many new devices because of the superior properties they exhibit. Not much attention has been paid to the mechanical robustness of these materials. In this work the fracture and fatigue behavior of Pb(Mg1/3 Nb2/3 )O3 −29 mol% PbTiO3 (PMN-29PT) is studied. Vickers indentation result show that the KIC of this material is 0.3 MPa m1/2 . Intentional defects from the indentation show that the direction is the weakest direction and that electrical fatigue is fastest in the direction. By looking at the hysteresis behavior of this material before and after the fatigue runs, the material ages over time.Copyright

High-Fidelity Reconstruction and Computational Modeling of Metallic Microstructure

The end-objective of this research is to identify critical microstructural features in metals that precipitate plastic flow, and therefore, cause degradation of mechanical performance at higher scales. The material focus is a titanium alloy-β21s. The three-dimensional (3D) microstructure in the mesoscale range was obtained using serial sectioning, optical microscopy, electron backscatter diffraction (EBSD) and computerized 3D reconstruction techniques. The reconstructed volumes, comprising hundreds of beta-Ti grains, provide information on morphology and crystallography. This data was used as input into 3D finite element models to analyze the spatial evolution of state variables, such as stress, strain and crystallographic slip under simple loading conditions. Single crystal hypoelasticity and the assumption of resolved shear stress causing crystal slip were used to represent microstructural material behavior. Evolution of plastic flow with applied loading was analyzed at grain boundary interfaces where most flow occurred. Rendering of large reconstructions into faithful but lean finite element meshes was identified to be the critical issue in simulating accurate material behavior near grain boundaries, establishing definite structure-property relationships at mesoscale and reducing the computational cost.© 2007 ASME