Ultrasonic vibration potential in structural health monitoring for plate-like structures

Ultrasonic guided waves and low frequency vibration modal analysis are two useful Nondestructive Evaluation (NDT) and Structural Health Monitoring (SHM) techniques. The guided wave method yields good sensitivity to certain defects by optimally selecting different guided wave loading functions. However, actuators and receivers at some distance apart must be placed in different positions in order to get reasonable coverage in a field inspection, since only the defect close to the wave propagation path can be detected. Therefore, the testing process is not always efficient. In contrast, as a global monitoring method, the vibration-based technique can achieve much faster testing speeds. But this technique has very low sensitivity to small defects due to the low operating frequencies. Taking advantage of both the ultrasonic guided wave and vibration techniques, a novel SHM method ultrasonic vibration is developed in this thesis. Ultrasonic vibration is capable of achieving defect detection sensitivity almost as good as ultrasonic guided waves, while maintaining the efficiency of the traditional low frequency vibration by fixing the actuator location and using only a few receivers to cover the whole structure. In this new method, continuous guided wave energy will be impinged into the structure to make the structure vibrate steadily. The steady state vibration depends on the initial ultrasonic guided wave loading function, and the boundary conditions as well as the structural geometry.;It is observed that the steady state vibration can be achieved by employing long cycles of guided wave inputs, say 1500 cycles or so. The structural vibration response relates to the guided wave loading. In order to use ultrasonic vibration for defect detection, it is necessary to study the guided wave sensitivity for defect inspection. Guided wave detection for delaminations in composites is studied in a sample problem. The sample problem demonstrates the significance of selecting the right guided wave loading function in defect detection in order to achieve good results.;In ultrasonic vibration, before the structure reaches a steady state, there is a transition process, during which the wave energy is bouncing among the edges. The edge reflection of guided waves was also studied here based on the semi-analytical finite element (SAFE) method and the mode expansion technique. The mode conversions are solved quantitatively when a guided wave hits the boundary at an angle.;The relation between steady state vibration and transient guided waves is revealed by the guided-wave-based vibration analysis on a 2-D plane strain model. Applying the partial wave technique, a full field solution was constructed, which includes all the guided wave modes traveling forward and backward. Among those guided wave modes, only a combination set satisfying the boundary conditions can exist in the structures. This set composed of guided waves with relative amplitudes and phases is identified as the natural vibration mode. The forced response of the structure is highly related to the excitation source. For vibration modes that are associated with certain guided wave modes, they have a high chance to be generated by exciting those guided wave modes.;A phased annular array transducer was designed for mode and frequency selection of the guided waves. By applying different phase delays, the annular array transducer is able to excite any guided wave mode in the dispersion curve space. Therefore, the structural response can be easily controlled by the frequency and phase delay of the annular array transducer, providing a method of finding optimal points for inspection.;Based on theoretical studies, an ultrasonic vibration testbed was developed. In the ultrasonic vibration experiments, an annual array transducer was used as the actuator. The loading functions were tuned by the frequencies and phase delays among the transducer elements. Experiments demonstrate that ultrasonic vibration has a good capability of detecting defects and boundary condition changes in a variety of different structures.