Showing papers by "Ajit Mal published in 1996"

Wave theory of acoustic emission in composite laminates

Abstract: This paper is concerned with the development of acoustic emission (AE) waveform analysis in advanced structural composites. The relationship between the surface response and microfracture modes in composite laminates is studied to establish the theoretical background for waveform analysis of AE signals. Lamb waves produced by arbitrary internal sources in unidirectional and cross-ply composite laminates are investigated. Laboratory experiments are performed to validate the theoretical models. The results of this research should be useful in developing practical nondestructive testing tools to monitor damage initiation and evolution in composite structures.

Response of multilayered composite laminates to dynamic surface loads

Abstract: A theoretical investigation of the response of multilayered composite laminates to concentrated and distributed dynamic surface loads is carried out. Each layer of the laminate is assumed to be transversely isotropic and dissipative with arbitrarily oriented symmetry axis. The dissipative property of the material is modeled approximately through the introduction of a frequency-dependent damping function. A multiple transform technique is used to calculate the spectra and time histories of the displacements and stresses produced by a variety of dynamic loads, and the quantitative features of the waves produced in the laminate are determined. The methodology developed in this work is expected to be useful in the prediction of the response of composite laminates to impact loads and also in the characterization of acoustic emission (AE) sources in these materials under static and dynamic loads.

Lap-joint inspection using plate waves

Abstract: The problems of guided waves propagating across a lap joint as well as reflected by the free edge of a semi-infinite plate with defects are investigated both theoretically and experimentally. The theoretical analysis is accomplished by using a hybrid method called the global local finite element method. A bounded region enclosing the lap joint, or the free edge, is described and analyzed by the finite element method, and the Lamb wave modal expansion is used to represent the wave field outside this region. The experiments are performed by using the fracture wave detector. The theoretical and experimental results are compared and show good agreement.

Lamb Wave Propagation Across a Lap Joint

Abstract: Lap joints are common elements of aircraft and other engineered structures. They are often subject to hidden defects which are caused by corrosion and fatigue, and are very difficult to detect. Development of accurate and efficient methods for the early detection of corrosion and fatigue cracks in lap joints is of considerable current interest. Ultrasonic techniques using guided waves offer the possibility of improving the technology of detecting and characterizing flaws within lap joints. It is well known that the characteristics of guided waves can be used to detect defects in plates [1]. However, the geometry of the lap joint makes it difficult to extend these techniques to lap joints. In this paper we consider the theoretical problem of the propagation of guided waves across a simple model of the lap joint in an effort to understand the interaction of the guided waves with the geometrical features of the lap joint. The geometry of the lap joint, including the vertical stress free boundaries, the rectangular corners, and the change in thickness, makes it impossible to derive a closed form solution to the problem of wave propagation across it. The problem can only be attacked by numerical methods. Conventional finite element methods fail when dealing with problems with infinite domains. In order to handle problems with local inhomogeneities or irregular shapes in an infinite domain, hybrid methods must be used.

Measurement of Thickness and Elastic Properties of Electroactive Polymer Films Using Plate Wave Dispersion Data

Abstract: Electroactive thin-film polymers are increasingly being used as sensors and actuators in aerospace structures [1,2]. They also have significant potential for applications in muscle mechanisms and micro-electro-mechanical systems (MEMS). In these applications, polymer films of thickness varying between 20 and 300 μm are utilized. Actuation of these polymers is attributed to piezoelectric, electrostrictive or electrostatic effects. Recent investigations suggest that polymers may produce striction which can be stronger than that delivered by electroactive ceramics. Such response may be produced by polymers with isotacticity or syndiotacticity in their molecular structure, where tacticity is the position of a pendant polymer group with a strong dipole moment that is mounted on a backbone polymeric chain.

Low Frequency Guided Plate Wave Propagation in Fiber Reinforced Composites

Abstract: The use of composite materials has increased steadily during the past two decades, particularly for aerospace, underwater and automotive structures. This is largely because many composite materials exhibit high strength-to-weight and stifihess-to-weight ratios, which make them ideally suited for use in weight-sensitive structures. The elastic properties of composite materials may be significantly different in specimens manufactured under the same general specifications and the bulk material properties may be different from those of the lamina. The elastic properties degrade as a result of aging, environmental and other effects (e.g., matrix cracking) resulting in overstress and eventual failure of the material. The elastic properties determine the performance of the material and it is necessary to assure the conformance of these properties with design requirements. Conventional destructive techniques for determining the elastic stiffness constants can be costly and often inaccurate. This is particularly true for the through-the-thickness properties. Nondestructive determination of these properties offers a better alternative for material characterization and for assuring structural performance.