12 A High-Speed Dual-Stage Ultrasonic Guided Wave System for Localization and Characterization of Defects 125 12.3 Experimental Procedures 12.3.1 Equipment and Preparation Considerations The DSSHM system includes a National Instruments Data Acquisition system for data capture and signal generation. The NI system includes an 8 16 matrix switch to make connections between any pair of transducers. Other hardware includes a set of ultrasonic transducers and a Scanning Laser Doppler Vibrometer (LDV). All of the control and data processing are performed using Matlab. In order to successfully perform a DSSHM test, several preparatory steps must be taken. A test structure is first identified and instrumented with the piezoelectric transducers, each of which will be used both as an actuator and a sensor at different times in the testing process. 12.3.1.1 Transducer Attachment The manner in which the transducers are attached to the test surface can cause significant variation in the shape and mode of the waveforms that are produced and sensed. For example, when a transducer is attached with accelerometer wax, sensor actuation produces far more pronounced A0 waves than S0. This is because the transducer is well coupled only in the direction normal to the plane. The lack of shear coupling prevents the transducer from inducing longitudinal (in-plane) strain in the test medium. Thus, the S0 wave mode (where particle motion occurs in the same direction as wave propagation) is not nearly as pronounced as the anti-symmetric mode (where particle movement is perpendicular to the plane of the surface). On the other hand, attaching transducers with superglue or an epoxy imparts far better shear coupling to the transducer-surface interface, meaning that both the S0 andA0 wave modes are generated by actuation. 12.3.1.2 Frequency Selection Regardless of what method is used to attach the transducers, it is important to pick a test frequency that minimizes (to the extent possible) the amplitude of the S0 mode produced. As mentioned before, the S0 mode travels faster than the A0 mode. This property can be useful in seeking out an ideal test frequency for a structure of given material properties. Pulses of varying frequency are generated by an actuator and sensed by a nearby transducer, and the relative amplitudes of the first arrival (S0) and second arrival (A0) waves can be visually inspected to see which frequency minimizes (to the extent possible) the S0 mode and maximizes the A0 mode. After taking a measurement of the material’s thickness the corresponding lamb wave velocity can be estimated. 12.3.2 Baseline Acquisition DSSHM takes a supervised learning approach to damage detection and localization. Following the detection, in order to locate damage in a structure, the DSSHM system compares data corresponding to the damaged state against data corresponding to the undamaged state. It is in comparing the pre- and post-damage data that the system is able to extract the information necessary to determine the approximate location of the defect. Thus, the stage-one localization and characterization can only be performed after appropriate “baseline” time history data has been obtained. The baseline time history corresponding to the undamaged state of a structure is obtained in the following manner: Assume that a structure to be tested is instrumented with P piezoelectric transducers. One by one, the transducers take turns acting as the actuator node in a pitch-catch setup. The actuator emits a narrowband high-frequency Gaussian-envelope pulse. Every other transducer acts as a sensor while the pulse propagates radially through the structure and reverberates from structural boundaries and features. Thus, we can say that there are M unique actuator-sensor pairs, where MDP .P 1/. The data acquisition system records a time-history of the voltage across the sensor transducer for each of the M actuatorsensor pairs. Each time-history consists of N samples. Several example baseline time-histories are shown in Fig. 12.2. The pulse which appears on the left end of each time-history plot is the electromagnetic interference from the actuation signal. Recall that the baseline waveforms reflect the condition of the test structure in its undamaged state. That said, it is important to recognize that the acoustic properties of a structure tend to vary significantly according to environmental
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