The interaction of an ultrasonic guided Lamb wave mode with delamination type defects in a quasi-isotropic laminated composite plate has been studied, using both simulations and experiments. In a laminated composite plate with a symmetric delamination, when the primary anti-symmetric mode, Ao, is incident at the entrance and exit of a delamination, it generates a new mode, So, that is confined only to sub-laminates and undergoes multiple reflections in the delaminated region. It was observed that only the incident and mode-converted Ao modes propagate in the main laminate. The two modes reverberate between the two ends of the delaminations while undergoing multiple mode conversions, leading to a trail of signals that is captured by the finite element model. The numerical observations were validated using experiments conducted using air coupled ultrasonic transducers.

Propagation of the primary anti-symmetric Lamb mode (A o ) in an asymmetrically delaminated cross-ply laminate has been studied through both numerical simulations and experiments employing the air coupled ultrasonic technique. When the A o mode interacts with the entrance and the exit of an asymmetrically located delamination, in addition to A o , a mode converted to the primary symmetric Lamb mode (S o ) also propagates in each of the two sub-laminates as well as the main laminate. These Lamb modes propagate independently in each of the sub-laminates. In addition, turning modes (i.e. the mode propagating in one sub-laminate interacts with delamination edge and starts propagating in the other sub-laminate) and a mode converted turning mode (a new mode is generated during the interaction of turning mode with delamination edge) were also observed in the numerical simulation. The presence of the 'mode converted turning modes' was also validated through the experiments in this study.

The reconstruction of all nine unknown elastic moduli of orthotropic plate structures has been achieved using a single-transmitter-multiple-receiver (STMR) compact structural health monitoring (SHM) array. This method uses the velocity measurement of the fundamental guided Lamb wave modes (S0 and A0), generated from a central transmitter, and received by a sparse array of receivers that encircle the transmitter. The measured velocities are then used in an inversion algorithm based on genetic algorithms. A prototype compact STMR array was developed and used in the measurement. Simulated data were used to demonstrate the feasibility of the technique. Experiments were conducted on 3.15 mm graphite–epoxy composite plate using a PZT based STMR array as well as laser vibrometer based displacement measurement. Experimental Lamb wave velocity data were used to validate the present technique. This technique finds application in the areas of material characterization and SHM of anisotropic plate-like structures used in aerospace and automobile components made using fiber reinforced composites.

Lamb wave tomography offers a new dimension to the challenging field of in situ health monitoring of structures. The possibility of constructing tomograms from a network of sensors generating and sensing Lamb waves in thin, multi-layered, anisotropic composite plates is explored in the present study. It is shown that improved tomograms result when the anisotropic and attenuative characteristics of composite plates are accounted for by (a) using the newly identified energy of the earliest Lamb wave signals as the reconstruction parameter, (b) modifying the sensor configuration from conventional geometries (and thereby also optimizing the number of sensors), and (c) normalizing the Lamb wave energy data of the defective sample with respect to that of the defect-free sample.

No abstract