Fatigue of a Nanocomposite Laminate

A multi-scale carbon fiber reinforced polymer nanocomposite laminate, with strategically incorporated fluorine functionalized carbon nanotubes at 0.2 weight percent, is studied for improvements in strength, stiffness and fatigue life under both tension-tension fatigue (R = +0.1) and tension-compression fatigue (R = −0.1) loading. The nanotubes were incorporated into the carbon fabric, and laminates were fabricated using a high temperature vacuum assisted resin transfer molding process. The influence of the fluorinated functionalized carbon nanotubes on the evolution of damage and the resistance to catastrophic failure is credited for these mechanical property improvements.

Approximate Macroscale Constitutive Relations Using a Finite Element Based Constitutive Equations for Microscale Contact

Three dimensional elastic-plastic contact of two nominally flat rough surfaces is considered. Equations governing the shoulder-shoulder contact of asperities are derived based on the asperity-asperity constitutive relations from a finite element model of their elastic-plastic interaction. Shoulder-shoulder asperity contact yields a slanted contact force consisting of both tangential (parallel to mean plane) and normal components. Multiscale modeling of the elastic-plastic rough surface contact is presented in which asperity-level FE-based constitutive relations are statistically summed to obtain total force in the normal and tangential direction. The equations derived are in the form of integral functions and provide expectation of contact force components between two rough surfaces. An analytical fusion technique is developed to combine the piecewise asperity level constitutive relations. This is shown to yield upon statistical summation the cumulative effect resulting in the contact force between two rough surfaces with two components, one in the normal direction and a half-plane tangential component.

A Stabilized B-Splines FEM Formulation for the Solution of an Inverse Elasticity Problem Arising in Medical Imaging

Soft tissue pathologies are often associated with changes in mechanical properties. For example, breast and other tumors usually present as stiff lumps. Imaging the spatial distribution of the mechanical properties of tissues thus reveals information of diagnostic value. Doing so, however, typically requires the solution of an inverse elasticity problem. In this work we consider the inverse elasticity problem for an incompressible material in plane stress, formulated and solved as a constrained optimization problem. We formulate this inverse problem enforcing high order continuity for our variables. Driven by the requirements for the strong and weak solutions to this problem, we assume that our data field (i.e. the measured displacement) is in H2 and our parameter distribution (i.e. the sought shear modulus distribution) is in H1. This high order regularity requirement for the data is incompatible with standard FEM. We solve this problem using a FEM formulation that is novel in two respects. First, we employ quadratic b-splines that enforce C1 continuity in our displacement field, consistent with the variational requirements of the continuous problem. Second, we include Galerkin-least-squares (GLS) stabilization in the iterative optimization formulation. GLS adds consistent stability to the discrete formulation that otherwise violates an ellipticity condition that is satisfied by the continuous problem. Computational examples validate this formulation and demonstrate numerical convergence with mesh refinement.

Modeling Two-Phase Flow in Porous Media Including Fluid-Fluid Interfacial Area

We present a new numerical model for macro-scale two-phase flow in porous media which is based on a physically consistent theory of multi-phase flow. The standard approach for modeling the flow of two fluid phases in a porous medium consists of a continuity equation for each phase, an extended form of Darcy’s law as well as constitutive relationships for relative permeability and capillary pressure. This approach is known to have a number of important shortcomings and, in particular, it does not account for the presence and role of fluid–fluid interfaces. An alternative is to use an extended model which is founded on thermodynamic principles and is physically consistent. In addition to the standard equations, the model uses a balance equation for specific interfacial area. The constitutive relationship for capillary pressure involves not only saturation, but also specific interfacial area. We show how parameters can be obtained for the alternative model using experimental data from a new kind of flow cell and present results of a numerical modeling study.

On the Vibration of Layered Sandwich Elastic Plates

Sandwich elastic plates have found increasing applications in civil, aerospace, military and offshore industries to enhance superior resistance to fatigue crack propagation, impact damage, local buckling and are very effective for vibration damping and noise reduction. Such structural application has significantly led to reduction in vulnerability of warships to blasts, ballistics, bomb and fire attacks. In engineering structures, one of the effective ways of damping vibration and noise attenuation, is to exploit the occurrence of slip at the interface of structural laminates where such members are held together in a pressurised environment. Recent analysis and experimental investigation of vibration characteristics and damping properties of layered sandwich structures, are mostly limited to elastic beams. This paper is an attempt to extend such analytical investigations to layered sandwich plates. By employing contact mechanics and laminated thin plate theory, the generalised equation governing the vibration of two layered sandwich plates that are held together in pressurised environment is presented. In particular, by invoking operational methods for the case of linear interface pressure distribution, closed form analytical results for the system natural frequency and dynamic response under external excitation are reported for design analysis and applications.

A Novel Perspective in the Design of Cable-Driven Systems

This paper deals with a novel approach to the design of cable-driven systems. This kind of robots possesses several desirable features that distinguish them from common manipulators, such as: low-inertia, cost-effectiveness, safety, easy reconfiguration and transportability. One key-issue that arises from the unilateral actuation is the design for workspace optimization. Most previous researches on cable-driven systems design focused their attention on workspace analysis for existing devices. Conversely, we introduce a new approach for improving workspace by design, introducing movable pulley-blocks rather than increasing the number of cables. By properly moving the pulley-blocks, the end-effector can be always maintained in the best part of the working space, thus enhancing robot capabilities without the need for additional cables. Furthermore, the eventuality of cable interference is strongly reduced. In this paper, the novel design concept is applied to different planar point-mass cable-driven robots, with one or more translating pulley-blocks. The maximum feasible isotropic force, along with the power dissipation and the effective mass at the end-effector are employed to compare the performances of different configurations.

Nondimensional Analysis of Interfacial Stresses of Wavy CNT/Polymer Composites

In this paper, with introducing a new simplified 3-D Representative Volume Element (RVE) for a wavy carbon nanotube (CNT), an analytical model has been developed to study the stress transfer in single-walled carbon nanotube (SWNT) reinforced polymer composites (NRPCs). The model is capable of predicting axial as well as interfacial shear stresses, along a wavy CNT embedded in a matrix. Based on the pullout modeling technique, the effects of waviness, wavelength and matrix modulus on axial and interfacial shear stresses have also been analyzed in details also using the statistical multiple non-linear regression method, the best-fitted functions for the interfacial stresses of CNT/polymer composites are obtained. The results of the present analytical model are in good agreements when compared with the corresponding results for straight NTs.

Mechanical Properties of α-Helices Estimated Using Molecular Dynamics and Finite Element Simulations

Mechanical properties of biomolecules and their response to mechanical forces may be studied using Molecular Dynamics (MD) simulations. However, high computational cost is a primary drawback of MD simulations. This paper presents a computational framework based on the integration of the Finite Element Method (FEM) with MD simulations to calculate the mechanical properties of polyalanine α-helix proteins. In this method, proteins are treated as continuum elastic solids with molecular volume defined exclusively by their atomic surface. Therefore, all solid mechanics theories would be applicable for the presumed elastic media. All-atom normal mode analysis is used to calculate protein’s elastic stiffness as input to the FEM. In addition, constant force molecular dynamics (CFMD) simulations can be used to predict other effective mechanical properties, such as the Poisson’s Ratio. Force versus strain data help elucidate the mechanical behavior of α-helices upon application of constant load. The proposed method may be useful in identifying the mechanical properties of any protein or protein assembly with known atomic structure.

Modeling Unstable Flow Dynamics in Porous Media Associated With CO2 Sequestration

Multiphase flow in porous media is fundamentally a microscopic process that governs the behavior of geologic scale processes. The application of existing (standard) macroscopic models to problems of geologic scale multiphase flow has proved to be unsatisfactory within a wide range of governing parameters. Our objective is to develop the missing link between the fundamental physics of multiphase flow at the pore-scale and the phenomenological representation of dynamic behaviors across a hierarchy of geologic scales. An essential prerequisite to such an analysis is a qualitative understanding of the flow behavior in terms of flow structures that exist for various parameter combination within the regime of CO2 sequestration. An experimental study addressing these objectives is presented. Experiments are carried out at the laboratory scale in a vertical glass-bead pack, in the parameter range of sequestration flows. Experimental results are interpreted with the help of invasion percolation models.

A Mesoscale Model for Molecular Interaction in Functionalized Nanopores

Nanopores have been used to detect DNA translocation and gene detection. However, the interaction between DNA and nanopore is still not well understood due to the small size of DNA/nanopore and dynamic translocation process. Very recently, various chemical modifications have been applied on nanopore surface for improved signal yield and selective detection. Thus, it is important to characterize the interaction between DNA and chemically modified nanopores. This paper intends to develop an understanding of the interaction between DNA and chemically modified nanopore surface and the translocation process of DNA by probing the DNA-nanopore interaction mechanisms through computational modeling. The DNA-nanopore interaction will be explored through a model that links atomistic DNA-nanopore interaction to meso-scale particle dynamics. Critical interrelationships between physical properties of the nanopore (surface properties, sizes, roughness etc.), electric field strength, and translocation kinetics will be established. This research not only advances the molecular-level understanding of the DNA-nanopore interface, but would also help design lab-on-chip devices for molecule based diagnosis.