Showing papers by "Ajit Mal published in 2004"

Analysis of transient Lamb waves generated by dynamic surface sources in thin composite plates

Abstract: A theoretical analysis is carried out in an effort to understand certain unusual properties of transient guided waves produced in a thin unidirectional graphite/epoxy composite plate by a localized dynamic surface load. The surface motion is calculated using an approximate plate theory, called the shear deformation plate theory (SDPT), as well as a recently developed finite element analysis (FEA), for their mutual verification. The results obtained by the two methods are shown to have excellent agreement. An interesting, nearly periodic "phase reversal" of the signal with propagation distance is observed for each propagation direction relative to the fiber direction. For clarification, a closed form analytical expression for the vertical surface displacement in an aluminum plate to an impulsive point force is obtained using the steepest descent method. It is found that the strong dispersion of the first antisymmetric waves at low frequencies is the main reason behind the phase reversal. This is verified further by measuring the surface response of a relatively thick aluminum plate to a pencil lead break source. The understanding developed in the paper is expected to

Automated structural health monitoring system using acoustic emission and modal data

Abstract: This paper is concerned with the detection and characterization of hidden defects in advanced structures before they grow to a critical size. A methodology for automatic damage identification and localization is developed using a combination of vibration and wave propagation data. The structure is assumed to be instrumented with an array of actuators and sensors to excite and record its dynamic response, including vibration and wave propagation effects. A damage index, calculated from the measured dynamical response of the structure in a reference state and the current state, is introduced as a determinant of structural damage. The indices are used to identify low velocity impact damages in composite as well as aluminium plates for different arrangements of the source and the receivers. The potential applications of the approach in developing health monitoring systems in defects-critical structures is discussed.

Guided acoustic emission waves in a thick composite plate

Abstract: In this paper, an efficient and accurate semi-analytical method based on the wavenumber integral representation of the elastodynamic field is described to calculate the surface responses produced by localized dynamic loads in a relatively thick composite plate. Two types of loads are considered: a pencil lead break source located on the surface and a localized shear delamination within the interior of the plate. In the case of the pencil lead break source, the calculated results for the surface motion are compared with those obtained in laboratory experiments on a 4.4 mm thick 32 layered cross-ply graphite/epoxy using high-fidelity broad band transducers. The waveforms consist of both flexural and extensional modes; the amplitude variations of these modes are found to be strongly dependent on their propagation direction. For the delamination source at the mid plane, the results from the exact calculation are compared with those from an approximate laminate theory with shear correction factor and moment tensor representation of the source. The results obtained by the two methods are shown to have excellent agreement in the low frequency ranges. Although, the motion due to the delamination is dominated by flexural waves of lower frequency in both thin and thick plates, the presence of extensional waves are observed in thicker laminates. The acoustic emission waveforms from the initiation of a shear delamintaion source at various interfaces are also calculated. It is found that the amplitude of the flexural modes decreases and that of the extensional modes increases as the source moves farther away from the mid plane.

Impact Damage Monitoring in Composite Plates

Abstract: This paper is concerned with the real-time detection of internal damage in composite structural components during impact using the far-field surface motion generated by these events. Impact tests are carried out on graphite epoxy composite plates using an instrumented impact testing system. Contact force and surface motion are measured at several locations on the plate surface. The far-field surface motions, both flexural and extensional waves in the composite plate, are modeled using both approximate and exact solution methods. Postimpact test were performed to determine the extent of internal damage caused by the impact load. Further research on the detection method can lead to the development of a viable impact monitoring system for composite aerospace structures using distributed sensors.Copyright © 2004 by ASME