Wael G. Abdelrahman

SOLAR-POWERED AIR CONDITIONING SYSTEM

The development of renewable energy is on the rise worldwide because of the growing demand on energy, high oil prices, and concerns of environmental impacts. In recent years, progress on solar-powered air conditioning has increased as nowadays, air conditioning system is almost a must in every building if we want to have a good indoor comfort inside the building. Therefore, this paper focuses in the design and construction of a direct current (DC) air conditioning system integrated with photovoltaic (PV) system which consists of PV panels, solar charger, inverter and batteries. The air conditioning system can be operated on solar and can be used in non-electrified areas. As we all known, solar energy is cost effective, renewable and environmentally friendly.

Micromechanical modeling of load transfer in fibrous composites

Abstract A unified analytical treatment is presented for the study of micromechanical stress distribution in unidirectional fibrous composites loaded with various thermal and mechanical loads. Two models are considered to represent the composite. Both use a concentric cylindrical system with the difference that one requires laterally free while the other requires laterally constrained outer boundaries, broadly describing situations of plane stress and plane strain, respectively. The present work has been motivated by the recent work of McCartney (McCartney, Proc. Roy. Soc. London, Ser. A 425 (1989) 215–244) who analyzed the laterally free system, and by our previous work (Nayfeh, Fibre Sci. Technol. 10 (1977)) in which we analyzed the laterally constrained one. For axisymmetric loading, and upon adopting some appropriate restrictions on the radial behavior of some field quantities, an elasticity-based procedure reduces the two-dimensional field equations, which hold in both the fiber and matrix components, together with the appropriate interface and boundary conditions, to a quasi-one-dimensional system. The resulting system is capable of identifying the stress distribution in each component as influenced by the other component via the readily identifiable interaction (transfer) terms. The model is general and applicable to a large variety of situations. These include situations of matrix cracking, fiber break and even regions of slip at the fiber–matrix interface. As a by-product, the model was capable of obtaining the classical Lame solutions (the iso-strain case) as a degenerate case. Confidence in the modeling was gained when it identically reproduced all of the numerical examples presented by McCartney. Numerical results that parallel some of the ones presented by McCartney are included in the form of comparisons between results obtained based upon the laterally constrained and the laterally free systems.

Stress transfer and stiffness reduction in orthogonally cracked laminates

A micromechanical continuum mixture model is constructed for the stress transfer and residual stiffness in orthogonally cracked laminates. According to this technique, the discrete composite behavior is replaced by that of a higher order continuum. A rational construction of an alternative set of coupled field equations that automatically satisfy all interface conditions leads to simple coupled governing equations for the total composite. This construction procedure is based on some through-thickness approximate distributions for some of the field variables. Once the stress free crack surface boundary conditions are imposed and the stress field components are obtained, we proceed to derive expressions for the residual stiffness of the damaged composite. This is presented in the form of Youngs and shear moduli. Treating three-dimensional problems and then identifying the results for the two-dimensional case, it will be shown that much of the earlier results in the literature are obtained as special cases of our work. Confidence in the modeling is further enhanced by showing good agreement with experimental measurements available in the literature.

Analyses of axisymmetric waves in layered piezoelectric rods and their composites

An exact treatment of the propagation of axisymmetric waves in coaxial anisotropic assembly of piezoelectric rod systems is presented. The rod system consists of an arbitrary number of coaxial layers, each possessing transversely isotropic symmetry properties. The treatment, which is based on the transfer matrix technique, is capable of deriving the dispersion relations for a variety of situations. These include the case of a single rod system that is either embedded in an infinitely extended solid or fluid host or kept free. The procedure is also adapted to derive approximate solutions for the cases of a periodic fiber distribution in a matrix material, which model unidirectional fiber-reinforced composites. The results are numerically illustrated for a widely used piezoelectric-polymer composite. It is seen that piezoelectric coupling can significantly change the morphology of the dispersive behavior of the composite.

AN APPROXIMATE MODEL FOR WAVE PROPAGATION IN PIEZOELECTRIC MATERIALS. I. LAMINATED COMPOSITES

A continuum mixture theory with microstructure is developed for guided wave propagation in bilaminated periodic composites of piezoelectric materials. The theory leads to the simple governing coupled equations for the actual composites which retain the integrity of the propagation process in each constituent but allow them to coexist under analytically derived interaction parameters. As a consequence of the analysis, effective mixture properties of the composite are obtained in the zero-frequency (static) limit. The accuracy of the approximation is demonstrated by direct comparison with the exact solution for the propagation of harmonic waves in the system.

An Approximate Model for Wave Propagation in Rectangular Rods and Their Geometric Limits

An approximate model for wave propagation in rods of rectangular cross section was developed that is based on neglecting the dependence of the shear stresses and the longitudinal displacement on one, or both, of the in-cross-sectional coordinates. The resulting approximate system of equations, together with the appropriate boundary conditions, are then solved exactly, leading to a simple, but general, characteristic dispersion equation. The degenerate geometric limiting cases of infinite media and flat plates are obtained as special cases. The model also predicts all of the general features exhibited in Morses experimental data (1948). These include the correct low- and high-frequency limits of the wave speeds, the cut-off frequencies, and the common point of crossing of the higher group of modes.

Graphical Techniques for Managing Field Failures of Aircraft Systems and Components

This paper presents an application of graphical techniques based on cumulative and mean cumulative function plots for extracting management information from field failures of aircraft systems. Monitoring field failures is very important for aircraft operators who carry out in-house maintenance as well as for companies who provide maintenance services to the operators on a contractual basis. Very valuable information can be obtained from field failure analysis which will aid in tailoring the maintenance services in accordance with the operators own local deployment, environmental and operating conditions. Analysis based on field failures is also very desirable for manufacturers, because the information received from the field gives a true measure of product performance and points out the areas of improvement to refine the product by design changes. Reliability practitioners usually attempt to analyze failures of aircraft systems with sophisticated statistical methods. Management and engineers who maintain and support the systems are easily intimidated by such intricate methods. Monitoring the failure of aircraft systems does not necessarily require complicated methods. This paper indicates a few simple but very powerful plots that help in tracking field failures of aircraft systems with an example of an air cooling system application. The plots based on mean cumulative function allow for measuring and monitoring system failures and maintaining statistical rigor without resorting to complex stochastic techniques. It is easily understandable by management and engineers, and enables them to quickly identify failure trends and unusual behaviors, compare different subsets of systems, reveal hidden information and gain insight.

Dynamic stress transfer in fibrous composites with damage

Abstract A micromechanical model is developed in order to study the dynamically induced stress distribution in fibrous composites with damage. Damage is taken in the form of either a broken fiber or a matrix crack normal to the fiber direction. The unidirectionally reinforced periodic composite, when loaded in the axial (fiber) direction is modeled as a concentric cylindrical system subjected at its outer boundaries to vanishing radial displacement and shear stress. Guided by the symmetry and the fiber–matrix interface continuity conditions, we first assume an approximate radial dependence of some of the field variables. Therefore, we reduce the two dimensional field equations that hold in both the fiber and the matrix together with their interface conditions to a quasi-one-dimensional system, which automatically satisfies all interface and radial boundary conditions. The resulting simple system retains the integrity of the distribution in the fiber and the matrix, individually, with their interaction reflected in well-defined transfer terms. The system is suitable for treating a variety of situations. Besides the case of damage free composites, the cases of broken fibers and cracked matrix are treated by invoking appropriate boundary conditions at the crack faces. Also simple analytical expressions are derived for the crack width opening for both the fiber break and the matrix crack. For our numerical illustration, we subject the composite slab to an axial cyclic loading with varying frequency. We compare results obtained for broken fibers and cracked matrix with the damage free composite system.

Micromechanical modeling of stress distribution in undulated composites under axial loading

The previously developed micromechanical model to predict the thermoelastic response of unidirectionally reinforced fibrous composites (Nayfeh and Abdelrahman, 1997) is extended for the case of undulated fiber reinforcement under axial loading. For the present purposes, the composite is modeled as a concentric cylindrical system laterally constrained such that it satisfies the periodicity condition of the unidirectionally reinforced composites. The field equations, combined with the interface, boundary and symmetry conditions, are first solved for the stress distribution in the system for cases involving straight fibers. Based upon local directions (slopes) of the undulated fibers, the linear transformation is used to obtain local stress distributions along the undulated fibers. The total stress field is found to be combinations of these local stresses and the inherent contributions obtained from the transformations of the normal loads along the undulated directions in the absence of reinforcement. The results of this modeling are compared with finite element computations for a variety of situations.

An improved continuum mixture model for wave propagation in fibrous composites

A new improved continuum mixture model is developed for the propagation of axisymmetric longitudinal waves in fibrous composites. The major improvement on the original model of Hegemier, Gurtman, and Nayfeh [Int. J. Solids Struct. 9, 395 (1973)] is achieved by the inclusion of the axial rate of change of the radial displacement in the shear constitutive relations which was neglected previously. This model has also been extended to treat situations in which the fiber and the matrix are anisotropic. The improved model is found superior to the original one, when compared with the recently acquired experimental data and exact solutions.