Global material characterization of composite structures using lamb wave STMR array technique

The Single Transmitter Multiple Receiver (STMR) Arrays have been shown before to have the ability to locally characterize the stiffness properties of a composite material with anisotropy. The STMR arrays have also been demonstrated for SHM applications using phase reconstruction techniques. Full-ring STMR array comprises of a transmitter in the centre of a circular ring and 30 equally spaced receivers at radius 60 mm. The guided ultrasonic Lamb waves are used where the central piezoelectric wafer-active sensor (PWAS) emits the guided waves, and the other PWAS sensors receive the Lamb wave signals. In the current work, this technique has been extended to the determination of global elastic moduli using the Lamb wave SO and AO mode signals that are reflected from features in the structure such as edge of the plate, bolt holes, etc. that are known apriori. The reconstruction of elastic moduli is accomplished using a Genetic Algorithm (GA) based inversion algorithm that optimizes an objective function for a particular configuration of the STMR array and the elastic moduli of the component. The solution to this inversion is the global elastic moduli of the composite which is then used to determine the unknown defects in the test component. Simulated velocity data of SO and AO modes was generated for several composite iayups such as unidirectional, cross-ply, and quasi-isotropic graphite-epoxy composite Iayups. The inversion algorithm was tested using the simulated velocity data and found to agree well with the expected values. Experimental validation has been performed using the velocity data measured on 3.15 mm quasi-isotropic graphite-epoxy composite.