Vibrothermography nondestructive evaluation, also known as Sonic IR and thermosonics, finds cracks by vibration-induced crack heating. Some kind of vibration source is required. In the laboratory sometimes tunable broadband vibration sources such as piezoelectric stack actuators are used for precision measurements, but these generally lack the power demanded for industrial applications. The most common vibration source in the industrial practice of vibrothermography is the ultrasonic welder. Ultrasonic welders are tuned resonators that convert electrical energy into ultrasonic vibration at very high 
efficiencies. Because they are tuned, they are narrowband and generate vibration at only a single frequency, which is determined during manufacturing. Commercially manufactured ultrasonic welders are available at a few standard frequencies such as 20 kHz and 40 kHz.Because vibrothermographic heating is usually dominated by resonance effects, in order for the welder to be effective some mechanism must be provided to transduce the narrowband vibration generated by the welder into a broader range of frequencies that can excite resonances in the specimen. The most common method for this is a contact nonlinearity. Sometimes termed "acoustic chaos", a dry contact between welder tip and specimen supports compression but not tension, so that as the welder tip vibrates 
there is a repeated series of impacts as welder and specimen move.Modeling the welder excitation is difficult because it involves a highly nonlinear process -- the impact of welder tip against the specimen. It is also difficult because accurate vibration prediction is notoriously difficult, largely because it requires meaningful models of the dynamic behavior of mounting boundary conditions. We address these difficulties by breaking the problem down into pieces, so to the largest extent possible any problems in each piece can be addressed independently.The ultrasonic welder excitation used in vibrothermography can be modeled as a system of three components: The welder, the specimen, and the nonlinear contact. Breaking the system into those components drastically simplifies the model and makes it possible to validate each component against experiment as an intermediate step, leading to a model that helps explain the phenomena observed experimentally.

Ultrasonic Welder Modeling for Vibrothermography

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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