Directional Transduction for Guided Wave Structural Health Monitoring.

Abstract

The principal objectives of structural health monitoring (SHM) are the detection, location, and classification of structural defects that may adversely affect the performance of engineering systems. Ultrasonic testing based on guided waves (GW) is one of the most promising solutions for SHM. These waves are capable of inspecting large structural areas, and can be made sensitive to specific defect types by controlling the testing parameters. A key challenge in the development of GW SHM systems is the lack of robust transduction devices for efficient structural interrogation. This dissertation presents the design, fabrication, and testing of the Composite Long-range Variable-length Emitting Radar (CLoVER) transducer. This device is composed of independent piezocomposite sectors capable of efficiently exciting highly directional GW for structural inspection. The first step in the development of the new device consists of formulating a theoretical model based on 3-D elasticity to characterize its GW excitation properties. In contrast to reduced structural theories, the developed model captures the multi-modal nature of GW at high frequencies (MHz-range). After a thorough numerical verification, the model is used to determine the efficiency of the transducer relative to conventional configurations under similar electric inputs. The in-house fabrication and characterization procedures for CLoVER transducers are described and applied to more conventional piezocomposite transducer geometries. The free strain performance of these conventional in-house actuators is shown to be similar to that of commercially available piezocomposite ones. An extensive experimental investigation is subsequently presented to assess the CLoVER GW excitation characteristics in isotropic and composite materials. The radiation patterns excited by these devices are spatially characterized using laser vibrometry, and the results confirm the ability of the devices to induce highly directional GW fields. The performance of the proposed interrogation approach is experimentally assessed using the pulse-echo method with simulated defects, and the ability of CLoVER transducers to detect and localize damage is verified. The final development presented in this dissertation is the design, fabrication, and characterization of a variable-length piezocomposite transducer. This novel device enables significant modal selectivity (up to 6 dB mode attenuation in the designed prototype), and is able to compensate for environmental effects.