Lamb Wave Propagation and Material Characterization of Metallic and Composite Aerospace Structures for Improved Structural Health Monitoring (SHM)

Abstract

The most commonly used methods for solving the Lamb wave equations for composite laminates consist of using laminated plate theory or 3D linear elasticity by assuming an orthotropic and/or higher symmetry. This assumption may not be true, if the actuators and sensors in an orthotropic or transversely isotropic laminates are installed in a non-principal direction. Therefore, this dissertation presents a full derivation and experimental validation of Lamb wave equations for n-layered monoclinic composite laminates. The derivation is based on linear 3D elasticity by considering the displacement fields in all three directions and by using the partial wave technique in combination with the Global Matrix (GM) approach. A robust method for numerically solving the Lamb wave equations is also presented. The presented method is verified experimentally by analyzing the propagation of Lamb waves in two different composite laminates constructed out of unidirectional carbon-fibre epoxy prepreg (Cycom G40-800/5276-1) and fibre-metal laminate (GLARE 3-3/4). The experimentally verified carbon-fibre epoxy laminate is further analyzed to study the effects of changes in the material properties such as E11, E22, G12, and density on the Lamb waves’ propagation characteristics. The analysis is performed by using the experimentally verified Lamb wave equations to generate the phase velocity dispersion and slowness curves by reducing E11, E22, G12, and density with the intent of representing defects. In order to accurately generate the Lamb wave dispersion curves, proper material properties such as stiffness and Poisson’s ratio are required. Therefore, this dissertation also presents a one-sided in-situ method based on the ultrasonic wave velocity measurements to determine the stiffness properties of isotropic and transversely isotropic material. The technique consists of generating and receiving quasi-longitudinal and quasi-transverse waves at different propagation angles and using a non-linear least square technique to inversely calculate the stiffness constants. The method is experimentally verified on an isotropic aluminum Al 7050-T7451 with two different thicknesses and a transversely isotropic (Cycom 977-2-12k-HTA) composite sample fabricated using 24 plies of unidirectional prepregs.