A crack detection technique using piezoelectric actuator/sensor systems

For new and aging engineering structures in aerospace and marine industries, implementation of an effective health monitoring system can replace the schedule-based inspection/maintenance of structures by condition-based maintenance. This thesis is focused on the systematic investigation of a structural health monitoring technique for quantitatively identifying embedded cracks in structures. A piezoelectric actuator/sensor system is used to generate high-frequency elastic wave propagation and a reverse wave technique is developed to locate the damage's position, shape and dimension using the obtained sensor signals.; A theoretical model of piezoelectric actuators surface-bonded to and/or embedded in structures is adopted and developed to describe their electromechanical behavior, and the outgoing wave propagation in the host structure is analytically obtained to understand the effects of the different parameters of the actuator upon the resulting wave field. For a surface bonded actuator, only the deformation along the longitudinal direction is considered due to the free surface. However, for an embedded actuator, a model involving the deformation in both the transverse and longitudinal directions of the actuators is developed. The single actuator solution is then implemented into the Pseudo-Incident Wave (PsIW) method to study the wave propagation induced by multiple actuators. When the outgoing wave reaches the surface of existing damages, the scattering wave propagation will be generated, which is recorded as sensor signals. A one-dimensional sensor model is then used, from which received strain field can be determined by using voltage output of the sensor.; The second part of this thesis is to develop a new and innovative technique to interpret the obtained sensor signals to quantitatively locate the cracks in the structures. A reverse wave technique is developed to form the image of the cracks by "moving" the recorded sensor signals to their actual spatial locations. To achieve this, the obtained wave signals are used as boundary conditions to induce reversed elastic wave propagation, from which the sizes, shapes and positions of existing cracks can be determined through the developed imaging technique for both the harmonic wave and transient wave cases. The main advantage of this technique is that complicated mode conversion phenomena caused by the crack reflection and wave propagation distortions in the medium are corrected by the back propagation operation; which made this method favorable for detection of multiple cracks of various shapes.