Inline Monitoring of continous Ultrasonic Welding Processes of Thermoplastic Composites via a custom polyCMUT based Ultrasound Array
Authors:   Deutsches Zentrum für Luft- und Raumfahrt (DLR), German Aerospace Center, Görick Dominik ,   University of British Columbia, Welsch Jonas ,   University of British Columbia, Gerardo Carlos D. ,   Deutsches Zentrum für Luft- und Raumfahrt (DLR), German Aerospace Center, Kupke Michael ,   University of British Columbia, Cretu Edmond ,   University of British Columbia, Rohling Robert 
Published: May 2024

Ultrasonic welding (UW) of thermoplastic composites (TCs) is an emerging technology in the field of composite
joining techniques in the aerospace sector. Through a mechanical oscillator, ultrasound at a frequency of 20kHz is
induced into the material via a welding horn, where microscopic friction and damping effects melt the
thermoplastic. Under further pressure the weld area cools down, permanently joining both parts together. Like all
joining processes in the aerospace industry the resulting joints need to be tested for their quality and structural
integrity. The traditional testing method using water-coupled ultrasound includes extra steps. This process could
be considerably improved by assessing the quality of the weld directly after or even during the welding process,
allowing for immediate rework or discard of the parts in question. Ultrasound is still the best solution for this
quality assessment, being inexpensive, well understood, and able to create B-Mode images, allowing a look into
the cross-section of the weld. However, there are several major problems:
To increase the system complexity as little as possible it is necessary to attach the ultrasound unit next to the
welding equipment, and as close to the welding horn and compactor as possible to save space and keep the end-
effector manoeuvrable. This brings problems for classic piezoelectric ultrasonic arrays: The low welding frequency
and its resonance modes reach into the lower resonance modes of the piezoelectric sensors leading to immense
noise, hiding any potential echo from the welding zone. Classic piezoelectric crystals are also very brittle and can
suffer damage from sustained exposure to this violent environment.
The authors present a novel solution: a custom-made polymer-based capacitive micromachined ultrasonic
transducer array (polyCMUT). polyCMUTs are tiny drums with two electrodes. One on the bottom and the other
suspended over a cavity sandwiched between two layers of polymers. By applying a DC-bias an electrical field is
created and the membrane is set under tension. If then an AC voltage is applied, the strength of the electric field
decreases, allowing the membrane to snap back into its original position. If done at the resonance frequency of the
membrane, a strong ultrasonic signal is created. To receive this signal the polyCMUT is charged with a DC-bias,
allowing it to receive the echo of the transmitted signal by measuring the changing capacitance. Not only is the
polymer robust and inexpensive to fabricate, the general architecture of CMUTs also allows a design where the
first mode of resonance is the actual mode the CMUT is operating in. By designing for a resonance frequency over
5 MHz all noise from the initial welding process is ignored, leading to a working pulse echo imaging system. The
array is then mounted onto a PEEK block attached to the compactor unit of the welding end-effector. This
publication is intended to present initial results, the design process of the custom array and the tests leading there.