Uncertainty Quantification of Thickness Estimation Via Full-Field Ultrasonic Inspection

Abstract

Acoustic steady-state excitation spatial spectroscopy (ASSESS) is a full-field ultrasonic inspection technique that can be utilized for structural health monitoring applications. ASSESS can rapidly identify damage or defects over a 360o field of view of a structure using geometry mapping and a scanning laser Doppler vibrometer (LDV) – enabling inspection and characterization across multiple large surfaces in a single measurement. However, variables like the incidence angle between the surface and LDV beam change when inspecting large or geometrically complex structures; greatly affecting the signal-to-noise ratio (SNR) of the measurement and the accuracy of damage estimation to an extent that has not previously been quantified. This work experimentally quantifies the uncertainty of thickness estimation derived from full-field steady-state wave fields over various incidence angles to provide trusted operating bounds. A rotation stage and hexapod assembly were used to orient an aluminum plate with manufactured thickness losses to up to 80o both horizontally and vertically in increments of 5o. The plate specimen was designed with an assortment of manufactured damage, including discrete regions with both continuous and discontinuous material thickness loss. An affixed transducer excited the plate with a 199 kHz tone and an LDV recorded local surface response velocities. A LiDAR was used to measure the inspection surface geometry and random sample consensus (RANSAC) planar extraction was utilized to determine the orientation of the specimen with respect to the LDV and correct perspective distortions in the wavefield. This work lays an essential foundation for identifying physical limitations of the measurement system for scanning large and complex structures and providing quantified uncertainties for detected damage within a trusted operating range – an imperative step for adoption of this inspection technique within highly standardized industries.