Introduction

The functional advantages of laminated composites are often compromised due to the presence of hidden defects. Damages such as delaminations, ply failures, cracks in the matrix or debonding may lead to severe reduction in the load bearing capacity of a composite. Therefore, it is important to develop a technique for monitoring the severity, type and location of damage in such composites. Experimental damage monitoring techniques mainly involve non-destructive sensing of damage in the structures such as using ultrasonics, magnetic field or x-ray based scanning etc. Many of these methods of identification involve experimental techniques which are quite expensive and also difficult for in-situ applications.

Chung has developed an electrical resistance based method for structural health monitoring of composite materials. It is limited to composite materials which are electrically conductive such as composites with carbon fibers. Pandey et al [1991] have developed a damage identification technique based on curvature of mode shapes. They have shown that the absolute difference in the curvature of mode shapes between the healthy and the damaged ply may be used as a parameter for predicting the damage and its location. This concept has been applied to a vibrating flat plate with the assumption that the modulus of elasticity in the damaged area becomes equal to zero. Liu et al [2003] have shown that embedded piezoelectric sensors can act as wave transmitters as well as sensors. The evaluation of the structural status can be monitored using information carried by waves propagating in the structure and interacting with any internal damage. Coverley and Staszewski [2003] have shown that using a classical sensor triangulation scheme and a genetic algorithm procedure the impact location can be accurately identified. This procedure substantially alleviates the complexity in learning and matching and thus becomes computationally efficient.