Mehdi Hojjati

Review article: Polymer-matrix Nanocomposites, Processing, Manufacturing, and Application: An Overview

This review is designed to be a comprehensive source for polymer nanocomposite research, including fundamental structure/property relationships, manufacturing techniques, and applications of polymer nanocomposite materials. In addition to presenting the scientific framework for the advances in polymer nanocomposite research, this review focuses on the scientific principles and mechanisms in relation to the methods of processing and manufacturing with a discussion on commercial applications and health/safety concerns (a critical issue for production and scale-up). Hence, this review offers a comprehensive discussion on technology, modeling, characterization, processing, manufacturing, applications, and health/safety concerns for polymer nanocomposites.

Fusion Bonding/Welding of Thermoplastic Composites

Joining of thermoplastic composites is an important step in the manufacturing of aerospace thermoplastic composite structures. Therefore, several joining methods for thermoplastic composite components have been under investigation and development. In general, joining of thermoplastic composites can be categorized into mechanical fastening, adhesive bonding, solvent bonding, co-consolidation, and fusion bonding or welding. Fusion bonding or welding has great potential for the joining, assembly, and repair of thermoplastic composite components and also offers many advantages over other joining techniques. The process of fusion-bonding involves heating and melting the polymer on the bond surfaces of the components and then pressing these surfaces together for polymer solidification and consolidation. The focus of this paper is to review the different fusion-bonding methods for thermoplastic composite components and present recent developments in this area. The various welding techniques and the corresponding manufacturing methodologies, the required equipment, the effects of processing parameters on weld performance and quality, the advantages/disadvantages of each technique, and the applications are described.

Curing simulation of thick thermosetting composites

Abstract A one-dimensional through-the-thickness simulation is developed in which heat conduction, kinetic, viscosity and flow (squeezed sponge model) equations are solved as a coupled system of equations. The control-volume method combined with the alternating direction explicit method is employed in the solution. Heat capacity, density, thermal conductivity and permeability of the composite are considered as functions of fibre volume fraction. It is shown that either temperature overshoot at the middle of the composite or incomplete through-the-thickness consolidation can put a restriction on the application of each cure cycle up to a certain thickness. Although the recommended cure cycles for a thick composite can satisfy the temperature conditions, complete through-the-thickness consolidation mostly cannot be achieved. Pressure effect, bleeding from the top and bottom and prebleeding are studied as possible solutions for incomplete consolidation. It seems that the prebleeding technique is the most promising method of fabrication for thick composites.

Design of domes for polymeric composite pressure vessels

Abstract The role of mechanical properties of composite material in dome contour design has been investigated. It has been shown that the optimal dome profile depends on the ratio of longitudinal Youngs modulus to transverse Youngs modulus in individual laminae ( E x / E y ). This profile for any type of composite material always lies between the Netting Analysis solution ( E y = 0, E x / E y =∞) and a semi-circle which is the solution for the square symmetric ( E x / E y = 1) and isotropic materials. The shells of revolution of double curvature, classical lamination theory (CLT) and the geodesic-isotensoid assumptions have been used.

Investigation of Intra/ply Shear Behavior of Out-of-Autoclave Carbon/Epoxy Prepreg

The successful formation of composite parts without defects remains a challenging issue due to the complexity of the forming process. A better understanding of the factors that cause these flaws is necessary to optimize the operation. The present work investigates the in-plane shear behavior of out-of-autoclave carbon epoxy thermoset prepregs OOA and its effect on wrinkling using the picture frame test. The deformability of OOA at the real processing conditions helps to understand the applicability of such material for forming processes such as the double-diaphragm forming technique with aims to minimize overall manufacturing time and cost. Tests were performed at varying temperatures and displacement rates in order to determine their contribution to the fabric deformability. Digital image correlation was used to take sequential images at various stages of deformation and capture the onset of wrinkling. It was found that the processing temperature (resin viscosity), displacement rate, and layer counts (layer interactions) are the three most important parameters that influence the wrinkling. Presence of resin between the layers makes them interact with each other and therefore has an impact on the each layer shear angle. These parameters were then analyzed using the Taguchi and analysis of variance techniques to determine which factor has the most significant influence on the wrinkling.

The Effect of Displacement Field on Bending, Buckling, and Vibration of Cross-Ply Circular Cylindrical Shells

The effect of assumed displacement field is investigated on the bending, buckling, and natural frequencies of cross-ply circular cylindrical shells using first-order shear deformation theory through an analytical solution. Linear strain-displacement relation is assumed. The governing equations are derived from Hamiltons principle. Assuming Levy-type solution, the governing equations are then converted to ordinary differential equations and changed to state-space form introducing ten unknown variables and solved for displacements. Different lamination sequences, including symmetric and asymmetric laminate, are studied and compared. The effect of various boundary conditions (i.e., clamped, simply supported, and free edge), radius-to-thickness, and radius-to-length ratio on the displacement of mid-surface is investigated.

Model Laws for Curing of Thermosetting Composites

Model laws for curing of thermosetting composites are established. The transient heat conduction coupled with the kinetic equation and the initial and boundary conditions are non-dimensionalized. Dimensionless parameters are extracted from the non-dimensional governing equations and boundary conditions. Model laws are constructed based on these dimensionless parameters. It is shown how one can use the model laws to design and fabricate a model composite structure for a prototype one. The model can be employed for the experimental purposes. Since both model and prototype have the same behavior at the homologous points and times, the model experimental results can be interpreted for the prototype one. The use of different systems of materials in the design process of a model is investigated. It is shown that due to the different kinetic equations for the different resin systems, a feasible way to design a model is to use the same resin, but the different fibers. For this case, it can be shown that for the same initial temperature and cure cycle for both model and prototype, the temperature and degree of cure at the homologous points are the same. Also to transform the model experimental results to the prototype, it is not necessary to know the resin kinetic equation. Therefore, one can use a less expensive model to determine the optimum cure cycle by performing experiments, and then the results can be used directly for the more expensive prototype.

Stress analysis of thick orthotropic cantilever tubes under transverse loading

In this work, a new high-order displacement-based method is proposed to investigate stresses and strains in thick arbitrary laminated orthotropic cantilever straight tubes under transverse loading. The most general displacement field of elasticity for an arbitrary thick laminated orthotropic straight tube is developed. A layer-wise method is employed to analytically determine the local displacement functions and stresses under transverse loading. The accuracy of the proposed method is subsequently verified by comparing the theoretical results with experimental data, finite-element method (FEM), and Lekhnitskii solution. The results show good agreement. In addition, high efficiency in terms of computational time is shown when the proposed method is used as compared with FEM. Finally, several numerical examples for stress and strain distributions in various thick cantilever composite straight tubes subjected to transverse loading are discussed.

The effect of the percentage of steered plies on the bending-induced buckling performance of a variable stiffness composite cylinder

Abstract The fiber steering capability of automated fiber placement machines offers the designers more room to fully exploit the directional properties of composite materials. Circumferential stiffness tailoring by fiber steering can considerably increase the bending-induced buckling performance of laminated composite cylinders. The potential structural improvement resulting from fiber steering depends on different design parameters such as the number of plies considered for fiber steering in a laminate. In this study, the buckling performance improvement of a variable stiffness (VS) composite cylinder is investigated for different percentages of plies considered for fiber steering in a multilayered composite laminate. A surrogate-based modeling along with a multi-step optimization is used in the design procedure of this study. The improvements in the buckling performance are shown and verified using finite element analysis in ABAQUS software. The mechanisms leading to buckling performance improvement of VS composites are also investigated and presented for different percentages of fiber-steered plies.

Composite Tube Exhibiting Large Deformation under Bending

Polymer matrix composites provide high specific strength and stiffness. However, their brittleness has prevented them from structural applications that require large deformation. One such application is the cross tubes of helicopter landing gears, which, until now, are solely made of high-grade aluminum alloys. This article shows that with strategic placement of fibers at proper orientations in different layers, it is possible to fabricate composite tubes that exhibit large deformation when subjected to bending loads. A composite tube has been designed, fabricated, and tested. This composite tube was able to support the same load and exhibit large deformation as a high-grade aluminum 7075-T6 tube with similar dimensions. Rationale for the strategy of stacking sequence is presented.