Ajit D. Kelkar

A review: carbon nanofibers from electrospun polyacrylonitrile and their applications

Carbon nanofibers with diameters that fall into submicron and nanometer range have attracted growing attention in recent years due to their superior chemical, electrical, and mechanical properties in combination with their unique 1D nanostructures. Unlike catalytic synthesis, electrospinning polyacrylonitrile (PAN) followed by stabilization and carbonization has become a straightforward and convenient route to make continuous carbon nanofibers. This paper is a comprehensive and state-of-the-art review of the latest advances made in development and application of electrospun PAN-based carbon nanofibers. Our goal is to demonstrate an objective and overall picture of current research work on both functional carbon nanofibers and high-strength carbon nanofibers from the viewpoint of a materials scientist. Strategies to make a variety of carbon nanofibrous materials for energy conversion and storage, catalysis, sensor, adsorption/separation, and biomedical applications as well as attempts to achieve high-strength carbon nanofibers are addressed.

Effect of Various Parameters on Effective Engineering Properties of 2 × 2 Braided Composites

Abstract Textile composites can be tailored to meet specific thermomechanical requirements for structural applications. Textile composites have good stiffness and strength properties; moreover, they have potentially better impact and fatigue resistance than laminated composites. Along with good properties, they have reduced manufacturing cost because much of the fabrication can be automated. To exploit these benefits, thorough understanding of the effect of various factors on their material behavior is necessary. Dominant forms of textiles are weaves, braids, and knits. The focus of this research is on 2 × 2 biaxial braided composites, which are considered a potential material for lightweight aircraft and have a wide variety of applications in the recreational, medical, and aerospace industries. Obtaining effective mechanical properties is the first order of concern in any structural analysis. This work presents an investigation of the effect of various parameters such as braid angle, waviness ratio, material properties, and cross-sectional shape on the effective engineering properties of the 2 × 2 braids. To achieve this goal, three-dimensional finite element micromechanics models were developed. Extensive parametric studies were conducted for two material systems: (1) glass fiber/epoxy matrix (S2/SC-15) and (2) carbon fiber/epoxy matrix (AS4/411-350). Equivalent laminated materials with angle plies and a resin layer were also analyzed to compare the difference in predictions from full three-dimensional finite element analyses of the 2 × 2 braided composites. The predictions are also compared with experimental results for a carbon/epoxy material system.

Static and Fatigue Behavior of Epoxy/Fiberglass Composites Hybridized with Alumina Nanoparticles

Advanced composites are hybridized by the integration of alumina nanoparticles into the matrix and onto the fabric surface. Alumina was pre-dispersed by sonication. Pre-processing of alumina was carried out by resin modification or fiber modification, prior to the consolidation into composite laminates. Alumina nano-particles were also functionalized by silane coupling agent tris-2-metoxyethoxy vinyl silane (T2MEVS). Vacuum assisted resin transfer molding (VARTM) was used to fabricate the composite panels for mechanical performance evaluation and characterization. Material property characterization for tensile, fatigue life and inter-laminar fracture toughness were determined for these hybrid composites and compared with the traditional fiber—matrix composite system as a baseline. Experimental characterization indicated that mode-I fracture toughness was significantly improved with the inclusion of alumina nanoparticles as well as by the functionalization of alumina nano-particles. However, the influences on the tensile behavior and tension/tension un-notched fatigue behavior in [0/90] configuration were not significant. In applications that involve only tension/tension fatigue loading, hybrid composites with nano-particulate inclusions can provide improved delamination failure characteristics without impacting the fatigue life and tensile behavior significantly.

Classical laminate theory model for twill weave fabric composites

Abstract The present paper is concerned with the development of an analytical model, based on Classical Lamination Theory, to predict the stiffness of twill in composites. A very simple yet quite general model is developed to obtain the elastic properties, i.e. extensional stiffness (A), extension–bending coupling stiffness (B) and bending stiffness (D). The model takes into account effects of the fabric structure by considering tow undulations and continuity along both the fill and warp directions. Various tow cross sections are possible and can be easily incorporated for a particular application. The model results in simple formulae for calculating in-plane and bending elastic constants, which can be used further in structural analysis.

Large deflections of circular isotropic membranes subjected to arbitrary axisymmetric loading

Abstract Circular membranes with fixed peripheral edges, subjected to arbitrary axisymmetric loading were analyzed. A single governing differential equation in terms of radial stress was used. This nonlinear governing equation was solved using the finite difference method in conjunction with Newton-Raphson method. Three loading cases, namely (1) uniformly loaded membrane, (2) a membrane with uniform load over an inner portion, and (3) a membrane with ring load, were analyzed. Calculated central displacement and the central and edge radial stresses for uniformly loaded membrane, agreed extremely well with the classical solution.

Thermal-Mechanical Effects of Ceramic Thermal Barrier Coatings on Diesel Engine Piston

The purpose of this paper is to investigate the piston temperature and stress distribution resulting from varying coating thicknesses of Partially Stabilized Zirconia (PSZ) thermal barrier coatings for the performance in diesel engine applications. This analysis is based on the premise that coating thickness affects the heat transfer and temperature distribution in the piston. A gas dynamic engine cycle simulation code was used to obtain thermal boundary conditions on the piston then, a 2-D axisymmetric Finite Element Analysis (FEA) using ANSYS was performed to evaluate the temperature and stress distributions in the piston as a function of coating thickness. Coating thicknesses studied include 0.1, 0.2, 0.3, 0.5, 1.0, 1.5, and 2.0mm. The results indicate increased piston surface temperature with increasing coating thickness. The maximum stress on the coated piston surface was high while the substrate stress was less than the coating yield stress for all coating thicknesses. Further, the analysis showed that the interface stress at all coated conditions is low enough such that no separation of the coating is expected. The FEA results suggest an optimum coating thickness of 0.1 to 1.5 mm for diesel engine application to avoid unduly high stress in the ceramic.

PERFORMANCE EVALUATION AND MODELING OF BRAIDED COMPOSITES

Braided composites have good properties in mutually orthogonal directions; more balanced properties than unidirectional laminates and have better impact resistance. These composites are being considered as a primary load carrying components in place of conventional laminated composites. They are being manufactured by using new process such as Vacuum Assisted Resin Transfer Molding (VARTM). This new process is a low cost, affordable and suitable for high volume manufacturing environment. In VARTM process, the flow of resin occurs in-plane as well as in the transverse directions to the preform. The permeability of the preform, fiber architecture and fabric crimp has an influence on the wetting of the fabric. This paper addresses a detailed study of VARTM manufactured 2x2 biaxial braided composites. These composites are fabricated using AS4 carbon fibers and vinyl ester resin system components. These braided composites are being evaluated for structural applications. To assess the feasibility of this material manufactured through VARTM, it is very important to understand the tensile behavior of these composite materials. Numbers of tests are performed to evaluate the basic properties such as modulus, ultimate tensile strength, and Poissons ratio of VARTM manufactured braided composites. This paper also presents, a finite element model for the 2 x 2 biaxial braided composites to predict the mechanical properties.

On the Behavior of Fiberglass Epoxy Composites under Low Velocity Impact Loading

Response of fiberglass epoxy composite laminates under low velocity impact loading is investigated using LS-DYNA®, and the results are compared with experimental analysis performed using an instrumented impact test setup (Instron dynatup 8250). The composite laminates are manufactured using H-VARTM© process with basket weave E-Glass fabrics. Epon 862 is used as a resin system and Epicure-W as a hardening agent. Composite laminates, with 10 layers of fiberglass fabrics, are modeled using 3D solid elements in a mosaic fashion to represent basket weave pattern. Mechanical properties are calculated by using classical micromechanical theory and assigned to the elements using ORTHOTROPIC ELASTIC material model. The damage occurred since increasing impact energy is incorporated using ADVANCED COMPOSITE DAMAGE material model in LS-DYNA®. Good agreements are obtained with the failure damage results in LS-DYNA® and experimental results. Main considerations for comparison are given to the impact load carrying capacity and the amount of impact energy absorbed by the laminates.

Microstructure evolution accompanying high temperature; uniaxial tensile creep of self-reinforced silicon nitride ceramics

Extensive transmission electron microscopy (TEM) has been performed to study the microstructure evolution of a self-reinforced silicon nitride associated with high temperature creep. A large population of strain whorls is observed in samples crept at relatively high temperatures and the strain whorls are not necessarily asymmetrical with respect to the grain boundary normal. Large angle convergent beam electron diffraction (LACBED) at the grain boundaries where strain whorl contrast is visible reveals severely curved Bragg lines, implying large residual strains. This indicates that grain boundary interlocking might be effective to enhance the creep resistance at high temperatures. Dislocation pile-ups, arrays and tangles are present in certain silicon nitride grains. However, a simple analysis rules out dislocations as the major creep mechanism. Most dislocations started from grain boundaries. The role of dislocations is to relieve the stress concentrations at the strain whorls. This adds to the diffusion mechanism of stress relaxation at the strain whorls and facilitates other creep mechanisms such as grain boundary sliding. A large density of multiple-junction cavities is observed in the samples crept at relatively high temperatures. It is proposed that grain boundary sliding and cavity formation, in addition to stress relaxation through nucleation of dislocations at the strain whorls act together to produce a much shorter life to failure at high temperatures. While at lower temperatures, the creep is more diffusion controlled which gives a stress exponent of unity.

Experimental and Numerical Investigations of Textile Hybrid Composites Subjected to Low Velocity Impact Loadings

In the present study experimental and numerical investigations were carried out to predict the low velocity impact response of four symmetric configurations: 10 ply E Glass, 10 ply AS4 Carbon, and two Hybrid combinations with 1 and 2 outer plies of E Glass and 8 and 6 inner plies of Carbon. All numerical investigations were performed using commercial finite element software, LS-DYNA. The test coupons were manufactured using the low cost Heated Vacuum Assisted Resin Transfer Molding (H-VARTM©) technique. Low velocity impact testing was carried out using an Instron Dynatup 8250 impact testing machine. Standard 6 × 6 Boeing fixture was used for all impact experiments. Impact experiments were performed over progressive damage, that is, from incipient damage till complete failure of the laminate in six successive impact energy levels for each configuration. The simulation results for the impact loading were compared with the experimental results. For both nonhybrid configurations, it was observed that the simulated results were in good agreement with the experimental results, whereas, for hybrid configurations, the simulated impact response was softer than the experimental response. Maximum impact load carrying capacity was also compared for all four configurations based on their areal density. It was observed that Hybrid262 configuration has superior impact load to areal density ratio.