Guided Wave Interpretation for Integrated Vehicle Health Managemet Sensors

Integrated Vehicle Health Management (IVHM) combines the use of onboard sensors with artificial intelligence algorithms to automatically identify and monitor structural health issues. A fully integrated approach to IVHM systems demands an understanding of the sensor output relative to the structure, along with sophisticated prognostic systems that automatically draw conclusions about structural integrity issues. Ultrasonic guided wave methods allow us to examine the interaction of the signals within key structural components. Since they propagate relatively long distances within plateand shell-like structures, guided waves allow inspection of greater areas with fewer sensors. In order to interpret the signals that we receive from transducers we look not only at the wave mechanics but also the signal processing using the dynamic wavelet fingerprinting technique to deliver the information in a form that does not require extensive knowledge of the guided wave physics. Introduction With a national fleet of aging aircraft and infrastructure, safety will become an increasing priority. IVHM promises low-cost, real-time sensing/inspection methods to detect damage before catastrophic failure. Ultrasonic guided waves, Lamb waves, allow for large areas of plate-like structures, such as airframes, storage tanks and pipes to be inspected with fewer sensors than conventional point by point measurements since they interrogate the entire region between sensor pairs. By understanding the waveguide physics we can develop systems that are tailored to the application at hand. However due to the complicated nature of guided wave propagation, we also develop algorithms that automatically analyze the waveforms and present the critical information in a form that doesn’t require users to have extensive knowledge of the physics. The propagation of the Lamb wave modes depends on the vibrational frequency along with the thickness and material properties of the structure. So variations in the waveforms can be used to assess the integrity of the structure for flaws such as disbands, corrosion and cracks that represent changes in effective thickness and/or local material properties. The research presented in this paper deals with our preliminary efforts to understand the wave propagation in airframe stringers and their interaction with corrosion and thickness loss flaws. We work our way through a propagation distance study, an incremental thickness loss experiment and finally an accelerated corrosion test. In each case we employ the dynamic wavelet fingerprinting technique (DWFT) to extract mode arrivals. Experimental Details This set of studies was conducted on sample aluminum airframe stringers that are 1m in length and have a “T” cross-section. The original flange thickness was 1.6mm. We use piezoelectric contact transducers in a pitch-catch arrangement to inspect the samples. The transmitting transducer excites Lamb wave modes that are then recorded by the receiving transducers. Since the Lamb