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.