Ali Naghashpour

A technique for real-time detection, location and quantification of damage in large polymer composite structures made of electrically non-conductive fibers and carbon nanotube networks

In this work, we have developed a novel, practical and real-time structural health monitoring (SHM) technique to detect, locate and quantify damage that occurs at one or more locations in large polymer composite structures (LPCSs) made of electrically non-conductive fibers and carbon nanotube networks. Our technique exploits the piezoresistive effect of multiwalled carbon nanotubes (MWCNTs) in epoxy resin. The electrically conductive epoxy resin was used to prepare glass fiber reinforced composite plates. The plates were marked with grid points where electrically conductive silver-epoxy pastes were deposited. The electrical resistances between the grid points were measured and used as a reference set. Two new concepts are introduced. One is uniformity of MWCNT distribution which gives rise to uniformity in electrical conductivity. The second is maximum sensitivity to change in electrical resistance due to the occurrence of damage. These issues are demonstrated as criteria to determine the optimal quantity of MWCNTs. This optimal quantity is used to assure damage detectability at any region in the large plates. Drilled holes and impact testing were conducted to simulate damage. The damage causes the electrical resistance between the contact points surrounding the damage to increase. This increase is used to detect, locate and quantify damage.

A technique for real-time detecting, locating, and quantifying damage in large polymer composite structures made of carbon fibers and carbon nanotube networks

A significant safety concern preventing extensive use of composite materials for large polymer composite structures is the ability to detect, locate, and quantify damages that occur at one or several locations in large polymer composite structures. Real-time health monitoring of large polymer composite structures improves their performance, durability, and reliability while minimizing the life cycle cost. In this article, we present a new, practical, and real-time structural health monitoring technique for detecting, locating, and quantifying damages in large polymer composite structures made of carbon fibers and carbon nanotube networks. In this technique, electrically conductive epoxy resin was prepared by dispersing multiwalled carbon nanotubes into epoxy matrix. This modified epoxy matrix was then incorporated with long carbon fibers to make large composite plates. Two sets of grid points made from silver-epoxy paste were mounted on the surface of the large plates. The first set was used to apply the constant electric current, and the second set was utilized to measure the electric potential. The electric potentials across the second set of grid points on the undamaged plate were measured and used as a reference set. Two different damages were created by drilling holes and by applying impact loading on the large plates. It is found that the electric potential between the contact points surrounding the damage changes. The significant change in electric potential corresponds to the damage location in the plates. As such, drilled holes, impact damages, and barely visible impact damages are detected, located, and quantified.

Investigation of Through-Thickness Stresses in Composite Laminates Using Layerwise Theory

In this study, an analytical method is developed to exactly obtain the interlaminar stresses near the free edges of laminated composite plates under the bending moment based on the reduced form of elasticity displacement field for a long laminate. The analytical and numerical studies were performed based on the Reddy’s layerwise theory for the boundary layer stresses within cross-ply, symmetric, angle-ply, and general composite laminates. Finally, a variety of numerical results are presented for the interlaminar normal and shear stresses along the interfaces and through thickness of laminates near the free edges. The results showed high stress gradient of interlaminar normal and shear stresses near the edges of laminates.

Analysis of Free Edge Stresses in Composite Laminates Using Higher Order Theories

This paper presents the determination of the interlaminar stresses close to the free edges of general cross-ply composite laminates based on higher order equivalent single-layer theory (HESL). The laminates with finite dimensions were subjected to a bending moment, an axial force, and/or a torque for investigation. Full three-dimensional stresses in the interior and the boundary-layer regions were determined. The computed results were compared with those obtained from Reddy’s layerwise theory. It was found that HESL theory predicts precisely the interlaminar stresses near the free edges of laminates. Besides, high efficiency in terms of computational time is obtainable when HESL theory is used as compared with layerwise theory. Finally, various numerical results were presented for the cross-ply laminates. Also design guidelines were proposed to minimize the edge-effect problems in composite laminates.

Curved nanotube structures under mechanical loading

Configuration of carbon nanotube (CNT) has been the subject of research to perform theoretical development for analyzing nanocomposites. A new theoretical solution is developed to study curved nanotube structures subjected to mechanical loadings. A curved nanotube structure is considered. A nonlocal displacement-based solution is proposed by using a displacement approach of Toroidal Elasticity based on Eringens theory of nonlocal continuum mechanics. The governing equations of curved nanotube structures are developed in toroidal coordinate system. The method of successive approximation is used to discretize the displacement-based governing equations and find the general solution subjected to bending moment. The numerical results show that all displacement components increase with increasing the nonlocal parameter. The present theoretical study highlights the significance of the geometry and nonlocal parameter effects on mechanical behavior of nanotube structures.

Nonlocal theory to analyse nanotube structures under tension

Because nanocomposites have found augmented use in many industries, the analytical solutions are required to be developed. This paper presents the development of a new analytical method for studying nanotube structures under tension using layer-wise and Eringen theories. Two opposite ends of tubes are subjected to normal forces. Nonlocal governing differential equations are derived and presented. The theoretical developments determine the effect of the geometric and nonlocal constitutive relations for single-walled nanotubes (SWNTs), double-walled nanotubes (DWNTs), and multiwalled nanotubes (MWNTs) under tension loading. It is observed that all displacement components increase with the increase in the nonlocal parameter.

Prediction of Interlaminar Stresses of an Unsymmetric Cross-Ply Laminate Using Layerwise and Higher-Order Equivalent Single-Layer Theories

In the present study, the layerwise theory (LWT) and the first-order shear deformation theory (FSDT) are used to analyze analytically the interlaminar stresses near the freeedges of a general (symmetric and unsymmetric) cross-ply composite laminate subjected to bending. The FSDT is employed to determine the global deformation parameters appearing in the displacement fields. The LWT is then utilized to determine the local interlaminar stresses within the boundary layer regions of laminates. To assess the obtained results from the LWT in conjunction with FSDT, the interlaminar stresses are also computed using higher-order equivalent single-layer theory (HESL) available in the literature. Comparison of results for edge-effect problems of several cross-ply laminates indicates high stress gradients of interlaminar normal and shear stresses near the edge of laminates. Also, LWT can predict interlaminar stresses more accurate than HESL.