In ultrasonic structural health monitoring (SHM) and nondestructive evaluation (NDE), the scattered waves caused by damage sites and defects are the key to damage diagnosis. However, structural components and boundaries also interact with traveling waves, creating events that can bury damage scattered waves. The baseline subtraction method, which directly subtracts the waveform of a damage signal from that of a pristine baseline signal, is a common processing technique to separate these damage scattered waves. However, baseline subtraction is less effective when a component is measured in different environmental/loading conditions from when its baseline was recorded. For instance, baseline subtraction can be ineffective in aerospace structural parts because such parts expect significant and routine changes in ambient temperature, pressure, and humidity. To overcome the limitations of baseline subtraction, this paper proposes to develop an spex-shifted Radon transform (ASRT)-based damage scattered event extraction technique without baseline subtraction. Our proposed ASRT method converts the original time-space [Formula: see text] domain signals to a time delay–curvature–apex offset [Formula: see text] domain, which targets specific geometries of scattered/reflected waves in the [Formula: see text] domain and compresses them into a single point-like region in the new domain. In this domain, identifying and isolating targeted events becomes significantly easier. To benchmark the performance of this baseline-subtraction-free (BSF) method, ASRT is applied for signals acquired from (1) spectral finite element-based simulations to scan a water-immersed steel specimen (2) 3-D wave simulations in a bent aluminum plate, and (3) a full matrix capture for experimentally scanning a high-density polyethylene (HDPE) with multiple holes. The goal in each case is to target and separate events of interest from the original signals using the proposed algorithm. The results suggest the ASRT method is effective as a damage scattered wave extraction tool for ultrasonic SHM and NDE.
Complex structures, such as composite laminates with irregular shapes or with structural reinforcements, pose unique challenges for ultrasonic inspection. These material and geometric complexities interact with the ultrasonic waves and can bury damage-scattered waves in the time-space domain. An Apex-shifted Radon Transform is proposed in this research to convert these signals to an apex time-curvature-apex offset domain (τ − q − y). Through this process, the energy of damage-scattered events can be compressed to localized regions at their apex location for easy extraction. This method shows potential in complex signal extraction and is promising as a tool for damage detection in complex structures without using baseline subtraction.
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