W. Hwang

Finite Element Modeling of Piezoelectric Sensors and Actuators

A finite element formulation for vibration control of a laminated plate with piezoelectric sensors/actuators is presented. Classical laminate theory with the induced strain actuation and Hamiltons principle are used to formulate the equations of motion. The total charge developed on the sensor layer is calculated from the direct piezoelectric equation. The equations of motion and the total charge are discretized with four-node, 12-degreeof-freedom quadrilateral plate bending elements with one electrical degree of freedom. The piezoelectric sensor is distributed, but is also integrated since the output voltage is dependent on the integrated strain rates over the sensor area. Also, the piezoelectric actuator induces the control moments at the ends of the actuator. Therefore, the number, size, and locations of the sensors/actuators are very important in the control system design. By selective assembling of the element matrices for each electrode, responses with various sensor/actuator geometries can be investigated. The static responses of a piezoelectric bimorph beam are calculated. For a laminated plate under the negative velocity feedback control, the direct time responses are calculated by the Newmark-/? method, and the damped frequencies and modal damping ratios are derived by modal state space analysis.

Stacking sequence design of composite laminates for maximum strength using genetic algorithms

Abstract This paper uses genetic algorithms (GAs) for the optimal design of symmetric composite laminates subject to various loading and boundary conditions. To analyze these laminates, the finite element method based on shear deformation theory is used. The Tsai–Hill failure criterion is taken as the fitness function, and the ply orientation angles are the design variables. In the GA, tournament selection and the uniform crossover method are used. The elitist model is also used for an effective evolution strategy and the creeping random search method is adopted in order to approach the solution with high accuracy. Optimization results are given for various loading and boundary conditions. The results show that optimization via a GA can find the global optimal solution leading to a substantial decrease in the failure index.

Vibration Control of a Laminated Plate with Piezoelectric Sensor/Actuator: Finite Element Formulation and Modal Analysis

The combined effects of passive and active control on the vibration control of a com posite laminated plate with piezoelectric sensors/actuators are investigated. Finite element formula tion and modal analysis are presented. Classical laminated plate theory with the induced strain ac tuation and Hamiltons principle are used to formulate the equation of motion of the system. The total charge developed on the sensor layer is calculated from the direct piezoelectric equation. The equa tions of motion and the total charge are discretized with 4-node, 12-degree of freedom quadrilateral plate bending elements. The stiffness and damping property changes of composite structures by varying the layer angles are used as a passive control method. Piezoelectric sensors/actuators with negative velocity feedback control are used as an active control method. By numerical simulation, the effects of stiffness and damping property changes of composite structures and the effects of sen sor/actuator division on the response of the structure and the performance of the vibration control are investigated. Since active control and passive control affect each other, the active control and the passive control should be considered simultaneously in designing the efficient adaptive structures.

Failure of carbon/epoxy composite tubes under combined axial and torsional loading 1. Experimental results and prediction of biaxial strength by the use of neural networks

Abstract Biaxial tests have been conducted on cross-ply carbon/epoxy composite tube under combined torsion and axial tension/compression up to failure. Strength properties and distributions were evaluated with reference to the biaxial loading ratio. The scatter of the biaxial strength data was analyzed by using a Weibull distribution function. Artificial neural networks were introduced to predict failure strength by means of the error back-propagation algorithm for learning, providing a different and new approach to the representation of complicated behavior of composite materials. Further prediction is made from experimental data by the use of Tsai–Wu theory and a combined optimized tensor polynomial theory. Comparison shows that the artificial neural network has the smallest root-mean-square error of the three prediction methods.

Stacking sequence optimization of laminated plates

Optimum fiber orientations of laminated composite plates for the maximum strength are found under multiple inplane loading conditions. Tsai-Wu failure criterion is taken as objective function. Based on the state space method, effective optimal design formulation is developed and solution procedure is described with the emphasis on the method of calculations of the design sensitivities. Numerical results are presented for the several test problems.

Impact damage and antenna performance of conformal load-bearing antenna structures

Experimental investigations into the impact response and damage of conformal load-bearing antenna structures composed of a face-skin, a core and a ground plate have been carried out. The damage was examined using non-destructive and destructive methods. The performance of the damaged antenna structure was investigated by measuring the return loss and the radiation pattern in a laboratory compact range. We found that the antenna performance was related to the impact damage, and that there exists a threshold in the impact energy above which the antenna fails to function. For the structure that we studied, the threshold of the impact energy for correct antenna function was between 1.5 and 1.75 J.

Fatigue Characteristics of a Surface Antenna Structure Designed for Satellite Communication

Our objective in this work was to design a Surface Antenna Structure (SAS) for satellite communication and investigate the fatigue behavior of an asymmetric sandwich structure (SAS). The term, SAS indicates that a structural surface itself acts as an antenna. Constituent materials were selected for their electrical properties, dielectric constant, and tangent loss as well as mechanical properties. Antenna elements were inserted into structural layers to create a multilayer microstrip array antenna. Electrical measurements showed that antenna performance was in good agreement with the design specifications. Under cyclic four-point bending, the flexure behavior was investigated by static and fatigue testing. The fatigue life curve of the SAS was obtained. These experimental results were then compared with several single load level fatigue life prediction equations (SFLPE), and good agreement was found. The SAS concept is the first serious attempt at integration of antenna and composite engineering and promises to be an innovative future communication technology.

Failure of carbon/epoxy composite tubes under combined axial and torsional loading. 2. Fracture morphology and failure mechanism

Failure mechanisms of cross-ply composite tubes made by the lapped moulding technique were investigated following biaxial testing, as reported in an earlier study (Part 1). Before mechanical testing the undamaged specimens were inspected to characterize their microstructure, and the source of first material damage was also inspected. From phenomenological failure analysis three types of failure mode were exhibited, depending on the biaxial ratio, and the corresponding failure mechanisms are suggested. By means of fractographic observations of the fracture surface, microscopic failure was investigated as a function of biaxial ratio, and it is suggested how the performance of fiber reinforced composite materials tube for engineering applications might be improved. The main factors involved at low biaxial ratio are matrix strength, the bond strength of the seam, and uniform distribution of fiber and matrix, while at a high biaxial ratio the fiber strength is the main factor.

Fatigue life prediction of circular notched CFRP laminates

Abstract Fatigue life prediction and fatigue behavior of circular notched carbon fiber reinforced plastic laminates are presented. Point and average stress criteria proposed by Whitney and Nuismer are generalized to fatigue fracture criteria for notched laminates. Residual strength degradation model and the assumptions on the stress redistribution are introduced during the derivation of prediction equations. S-N curve, Basquins relation, and Hwang and Hans fatigue life prediction equation (FLPE) 1 are chosen for evaluation of residual strength of unnotched laminates and six prediction equations are derived. Experiments are performed using carbon fiber reinforced plastic laminates. Presented prediction equations are reasonably close to experimental data and proposed approach is found to be suitable to predict fatigue life of notched composite laminates.

Stacking Sequence Design of Fiber-Metal Laminate for Maximum Strength

FML (Fiber-Metal Laminate) is a new material combining thinmetal laminate with adhesive fiber prepreg. It has nearly all the advantages of metallic and composite materials, including good plasticity, impact resistance, processibility, light weight and excellent fatigue properties. However, in most FML the fiber prepreg is staked in only one direction, although FML can be designed with a varying stacking sequence angle of fiber prepreg. No work has been published on the optimum design of FML. This paper uses genetic algorithms to study the optimal design of FML under various loading conditions. To analyze FML the finite element method is used based on shear deformation theory. The Tsai-Hill failure criterion and theMiser yield criterion are used as the objective functions of the fiber prepreg and the metal laminate, and the ply orientation angles are the design variables. In the genetic algorithm, tournament selection and the uniform crossovermethod are employed. The elitist model is also used for an effective evolution strategy, and the creeping random search method is adopted so as to approach the solutionwith high accuracy. Optimization results are given for various loading conditions and are compared with CFRP (Carbon Fiber Reinforced Plastic). The results show that FML is better than CFRP in most loading conditions. In particular, FML shows good mechanical performance in point and uniform loading conditions and is more stable to unexpected loading.