Showing papers by "Christoph Schaal published in 2022"

Guided Waves in Pipes Versus Lamb Waves in Plates - A Convergence Analysis

Abstract: This work is concerned the with analysis of the convergence of guided waves in pipes to Lamb waves in plates for isotropic materials. The main goal is to be able to define a threshold frequency above which it is reasonable to approximate waves propagating in a pipe with a certain wall thickness to radius ratio as Lamb waves. The study involves a detailed comparison of velocity differences for symmetric and antisymmetric waves in plates versus longitudinal and flexural modes in pipes. Phase and group velocities of pipes with various wall thickness-to-radius ratios are compared to a plate of corresponding thickness. An empirical convergence criterion is defined to determine the frequency above which a pipe will have a plate-like response. It is shown that “convergence” may already be reached at frequencies commonly used for nondestructive testing purposes. Analytical considerations are supported and validated by experimental results, showing good agreement of predicted and measured wave velocities.

Recent advancements on autonomous Lamb wave-based nondestructive inspection

Abstract: In order to support the continued trend of increased use of composite materials especially in aviation, efficient testing systems need to be developed. The anisotropic material properties of composites allow for high specific stiffnesses and strengths. However, some failure modes in composite structures cannot be identified through visual inspection and to ensure the health of the structures, a time-consuming and costly inspection approach must be taken. Often, this approach includes disassembly and premature part replacements. Thus, complete nondestructive inspection (NDI) and monitoring of composite structures in aviation is virtually nonexistent. Hence, there is a need to introduce an autonomous inspection method to reduce time and cost, while increasing aircraft reliability. To this end, several recent advancements of a mobile robotic platform and related algorithms for Lamb wave-based inspection of aircraft surfaces are presented here. The robots are envisioned to be operated in a low cardinality swarm, where each robot employs guided ultrasound technology to collaboratively inspect plate-like components. For the purpose of implementing a fully autonomous platform, simultaneous localization and mapping (SLAM) methods are combined with Lamb wave-based NDI techniques. Specifically, it is demonstrated that a novel Lamb wave-based edge seeking and tracing methodology can contribute to increasing testing efficiency, with the overarching goal of creating a full map of the tested structure including all potential flaws.

Visualization and analysis of the scattering of high-intensity focused ultrasound at bone phantoms

Abstract: High-intensity focused ultrasound (HIFU) has been studied for the purpose of developing a variety of medical therapies. Numerical and laboratory work has led to many clinical trials as well as first approved therapies, such as in the case of prostate cancer. However, little research has been performed to validate numerical simulations and in-vivo HIFU treatments in the presence of bones. To this end, recent advancements on visualization and optical measurements using schlieren techniques are presented in this work. In laboratory experiments, HIFU is induced in a tank filled with distilled water, and the incident waves are scattered at a bone phantom plate. Advanced filtering and computer vision techniques are adopted and their general feasibility is demonstrated for unobstructed and partially obstructed HIFU wave fields. In particular, it is shown that low-amplitude reflected wave peaks can be tracked despite their superposition with high-amplitude incident waves.