Showing papers by "Ajit Mal published in 2006"

Measurement of thin film interfacial properties using nanosecond laser source

Abstract: A high energy Nd:YAG laser source is used to determine the intrinsic adhesion strength of thin films deposited on substrates. The specimen is designed to convert the thermal energy of the short duration laser pulse to a strong compressive stress on the back face of the substrate. The compressive stress propagates through the layered structure, and upon reflection from the free surface of the film, generates a tensile wave which produces tensile failure of the interface. The stress associated with the interface failure is calculated from a theoretical model of wave propagation through the layered medium. The compressive stress produced by the laser source is determined from a second experiment involving the homogeneous substrate by removing the film. Examples of the applications of the technique in cell biology are presented.

Autonomous health monitoring of a stiffened composite plate

Abstract: The paper presents a unified computer assisted automatic damage identification technique based on a damage index, associated with changes in the vibrational and wave propagation characteristics in damaged structures. An improved ultrasonic and vibration test setup consisting of distributed, high fidelity, intelligent, surface mounted sensor arrays is used to examine the change in the dynamical properties of realistic composite structural components with the appearance of damage. The sensors are assumed to provide both the low frequency global response (i.e., modal frequencies, mode shapes) of the structure to external loads and the (local) high frequency signals due to wave propagation effects in either passive or active mode of the ultrasonic array. Using the initial measurements performed on an undamaged structure as baseline, the damage indices are evaluated from the comparison of the frequency response of the monitored structure with an unknown damage. The technique is applied to identify impact damage in a woven stiffened composite plate that presents practical difficulties in transmitting waves across it due to scattering and other energy dissipation effects present in the material and the geometry of the structure. Moreover, a sensitivity analysis has been carried out in order to estimate a threshold value of the index below which no reliable information about the state of health of the structure can be achieved. The feasibility of developing a practical Intelligent Structural Health Monitoring (ISHM) System, based on the concept of "a structure requesting service when needed," is discussed.