Ultrasonic Bessel Beams for Inspection of Multilayered Structures and Attenuative Material Systems

NASA is developing new spacecraft for human spaceflight. These spacecraft will utilize a variety of material systems, and many of the components are targeted for reusability and long-term use. This offers many challenges, not only for design and manufacturing, but also for inspection and maintenance. For example, inspection of thermal protection systems (TPS), bondlines, or multilayered structures using traditional nondestructive evaluation (NDE) is time consuming, expensive, and often not possible when access is limited (e.g. covered by TPS). All conventional NDE methods have significant limitations in evaluating conditions beneath attenuative TPS material, making inspection of the bondline very challenging. To inspect the bondline between the TPS and the composite substrate, a mechanical wave such as ultrasound is necessary. However, TPS materials are highly attenuative to conventional ultrasound waves. An opportunity exists to meet this challenge by utilizing non-diffracting ultrasonic Bessel Beams that resist attenuation.nbsp;In this project, it is proposed to develop an NDE technique to inspect carbon fiber composite substrates and bondlines beneath attenuative TPS material. A comparatively new approach using a Restorative Ultrasonic Bessel Beam Yielding (RUBY) method is proposed to perform NDE using an ad-hoc metastructure with multi-stage acoustic axicons in-front of the transducer to create self-healing ultrasonic Bessel Beams that focus the wave energy without being affected by the attenuative layer. Bessel Beams are a special class of beams that resist diffraction and, in some cases, attenuation up to a certain distance (depth of field). The objective of the project is to develop a system capable of generating nondiffracting beams that resist the loss effects and maintain amplitude and spot size of their central core for long distances, suitable for inspection beneath thick attenuative layers.