Experimental and numerical studies on low velocity impacts detection and damage identification in metaland composite structures using ultrasonic wavespropagation (Lamb waves) di Simone Tancredi (Youcanprint)

In this work we present an efficient thus computationally low cost FEM modeling technique of both low velocity impacts and Lamb waves propagation events, involving Piezoelectric Wafer Active Sensors (PWAS). In the first Chapter both the excitation and sensing of Lamb waves, involving PWAS, are investigated. The low order PWAS modeling techniques used for Lamb waves generation and sensing gave excellent results, as clear from the comparisons with the experimental curves. The great advantage of the developed technique is represented by the low computational cost (order of hours using a 2GHz 8-cores-CPU) with respect to the use of implicit FEM codes, able to include the electro-mechanical coupling of piezoelectric materials yet with very long times for solution (order of days). In the second Chapter the outputs from PWAS, during low velocity impact events, are investigated. The experimental curves registered by the oscilloscope connected to PWAS, during impacts on both aluminum and composite plates, were compared with the curves obtained from FEM, involving only geometrical variables of the piezoelectric sensors; the explicit FEM code LS-Dyna was used in the analyses. As shown in the graphs, all the curves obtained after the post-processing of the FEM solution overlap with a very good agreement the experimental curves. In the third Chapter a test case for damage detection in thin plates with Lamb waves is presented, thus proving the advantage of the PWAS modeling technique for Structural Health Monitoring (SHM) purposes. The substantial improvement that a fast FEM solution can give is the possibility to directly visualize and study the interaction of Lamb waves with the various kinds of defects/damage eventually present in thin structures such as holes, cracks, delaminations, debondings, etc.