Fluidic ultrasound generation for nondestructive testing

Ultrasonic testing has become an indispensable method for verifying integrity, dimensional accuracy and material properties in numerous technical domains. Air-coupled ultrasound offers advantages in terms of measurement flexibility and reduced measurement time, but also presents challenges due to limitations in transmitted sound pressure amplitude and sensor positioning. Due to the high impedance mismatch at the interfaces between the transducer, air, and specimen, only a fraction of the generated sound pressure interacts with the specimen and is received by a sensor. To reduce these losses, a novel fluidic ultrasonic transducer is presented. This device produces a transient triggered ultrasound pulse based on aeroacoustic sound generation mechanisms in a bistable fluidic amplifier. Little is known about its performance characteristics and suitability for nondestructive testing (NDT). The publications included in this thesis are the first to present a transient aeroacoustic ultrasonic generator using a bistable fluidic amplifier. This dissertation demonstrates that such a fluidic ultrasonic transducer is applicable for common measurement tasks in NDT. By disseminating the acoustic and flow characteristics of the device, the resulting challenges and opportunities concerning its applicability to NDT tasks are identified and addressed. It is found that the generated pulse contains frequency components below 60 kHz, fluctuating in amplitude and phase delay, and is accompanied by a high velocity free jet that partially interacts with the sound pulses. In order to prevent interaction between the jet and the specimen surface and to increase transducer directivity, the attachment of sonic crystals and an exponential horn were successfully tested. The distinct spectral characteristics of each ultrasonic pulse were exploited to develop a signal processing approach that allows better differentiation between two pulses received in quick succession. This improved the usability of the fluidic transducer in multiple-input multiple-output (MIMO) setups. To address the stochastic ultrasound generation behavior, a novel, fully non-contact through-transmission measurement setup is presented that allows time-of-flight measurements without prior knowledge of trigger time, pulse shape, or distance between the transducer and the specimen. Successful measurement of longitudinal propagation velocity in various materials demonstrates that the fluidic transducer is capable of nondestructively measuring a variety of geometric and material properties. Thus, a new type of ultrasonic transducer has been established and its applicability to common NDT tasks has been demonstrated. The usability of these novel procedures extends beyond fluidic ultrasonic testing and can also be employed to improve conventional air-coupled ultrasonic measurements. The results presented not only offer the amplifier-based fluidic transducer as a robust alternative ultrasound source for NDT, especially in civil engineering, but also raise a number of research questions related to the use of aeroacoustic transducers as an alternative to conventional air-coupled ultrasonic devices.

Author dc:creator

• Bühling, Benjamin

Advisors dc:contributor.advisor

• Sarradj, Ennes
• Strangfeld, Christoph
• Maack, Stefan

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-20006

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/21206