Mircea Calomfirescu Lamb Waves for Structural Health Monitoring in Viscoelastic Composite Materials

Structural Health Monitoring (SHM) is a novel philosophy for an autonomous, built-in nondestructive evaluation of structural “health” on demand to reduce life-cycle costs, increase safety and reduce structural weight. This dissertation investigates ultrasonic guided waves, particularly Lamb waves, and their propagation properties as a method to perform Health Monitoring of viscoelastic composite structures. One of the objectives of this work lies in the analytical description of Lamb wave propagation in anisotropic, viscoelastic composite materials by a higher order plate theory taking into account traversal shear and rotational inertia. To take account of material damping the theory of complex moduli was adopted. For the prediction of dispersion and attenuation behavior of guided waves, the developed models based on an analytical, higher order plate theory were implemented into a software enabling the calculation of S0, A0 and SH0 modes, which are the most commonly used guided wave types for SHM applications. The results were verified successfully experimentally. For experimental verification, an experimental test set-up was built up in which Lamb waves could be excited and their propagation was measured by small and lightweight, surface-bonded piezoelectric wave active sensors (PWAS). The frequency range was 10 – 500 kHz. Moreover, the dispersion and attenuation behavior of Lamb wave propagation was experimentally and analytically studied for six carbon fiber reinforced plastic (CFRP) laminates, which are typical in the aerospace industry. Based on these fundamental studies, different SHM applications were analyzed in detail: on the one hand, a method for impact detection, localization and quantification; on the other hand, a novel method for the non-destructive, quantitative viscoelastic material characterization was proposed and developed. In order to detect and quantify impact events, an existing method was extended and further developed for application to anisotropic, viscoelastic composites. The flexural waves excited by the impact were measured by surface bonded PWAS and the signals were analyzed by the Wavelet transform (WT), which increased significantly the accuracy of the arrival time detection. The impact was applied with an instrumented, hand-held impuls hammer. For the reconstruction of the impact force history, an analytical structural model was adopted, which calculates the propagation of flexural waves due to the transverse impact. The analytical model was extended to account for material damping. A verification of the proposed methodology showed good agreement with the real impact position and force-time history. Another important issue in this thesis is the development of a novel non-destructive, quantitative viscoelastic material characterization method based on Lamb wave dispersion and attenuation measurements using PWAS. The inversion of the viscoelastic material stiffness coefficients was realized by a non-linear linear simplex optimization method. The results showed good agreement with the material properties measured by conventional (destructive) tensile tests and by another state-of-the art, nondestructive technique. The method developed in this dissertation is appropriate for insitu applications.