Indentation of a laminated composite plate with an interlayer rectangular void

We employ the Eshelby–Stroh formalism to study generalized plane strain infinitesimal deformations caused due to the indentation by a rigid circular cylinder of an elastic laminated plate with a throughthe-width rectangular void between two adjoining layers. Assuming that the void does not close during the indentation process, we find the indentation modulus (i.e., the slope of the indentation load vs. the indentation depth curve) as a function of the void size, the void position, elastic moduli of the layers, and boundary conditions at the edges. The change in the indentation modulus caused by an interlayer void parallel to the major surfaces of an anisotropic plate can potentially be used to estimate the void size and location. Due to their high specific stiffness and specific strength, composite are being increasingly used in numerous engineering applications. However, mechanical properties of composites may degrade severely with repeated impact and cyclic loading. Failures of structures, particularly aircraft structures, often have tragic consequences. Applications of composites tend to be limited or inhibited by the lack of long-term service experience and the difficulty to accurately quantify damage and thus determine the remaining useful life of the structure. Damage detection techniques such as thermal deplying and optical microscopy require either the partial or the total destruction of the structural component [2]. Most conventional nondestructive evaluation techniques such as ultrasonic C-scan, X-ray, thermography and eddy current have limited applications as they require a structural component to be taken out of service for a substantial length of time for damage inspection and assessment. Global damage detection methods [3], based on the assumption that a change in physical properties of a structure alters its modal characteristics, have also been developed. These methods either depend on analytical models or prior test data for the detection and the location of damage or on the output from several sensors bonded to the structure. Technical issues that need to be considered during in situ monitoring of structures include the following: surface-bonded resistive strain gauges are susceptible to electromagnetic and electrical interference in addition to physical damage, and the acoustic emission suffers from low signal-tonoise ratio. Sometimes it is advisable to simultaneously use two or