Air-Ultrasound – Device Technology
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Air-ultrasound – Device technology
General
Material testing using ultrasound is a traditional non-destructive testing method that has been introduced technologically. It is mainly used in ultrasonic weld inspection, defectoscopy and wall thickness characterisation, as well as in quality assurance for components in the automotive and aerospace industries, in addition to investigating the mechanical and elastic properties of materials. Contact coupling is used, especially when mobile device technologies are employed, but it is not suitable for every material [1].
In laboratories, the ultrasonic immersion bath technique is commonly used to significantly improve the resolution of the objects being examined and to perform surface scans, which can be used to create an overall acoustic image of the test specimen or component (see: imaging ultrasonic testing). There are also other mobile methods for defectoscopy of components, such as the ultrasonic squirter technique or the immersion bath technique (see: ultrasonic immersion bath sensors), which can be used to examine even complex component geometries for defects.
A significant advantage over these conventional ultrasound measurement methods, which require the coupling of ultrasound via a coupling agent, water or steam, is the coupling of the test object via an air gap. This test method, known as the air-ultrasonic measurement method, therefore works without a coupling agent. This means that there is no direct contact, which is an advantage for test specimens or components that are sensitive to coupling agents, i.e. hydrophilic materials. However, the high sound attenuation in the air gap poses a problem for ultrasonic technology.
While this is not a problem for low resolutions, such as distance measurements in car parking aids, higher requirements exist in non-destructive material testing. This is solved by using adapted ultrasonic composite sensors with a low transmission frequency and a special matching layer, suitable excitation of the transmission pulse (burst pulse) and broadband amplifier technology. Of course, this means that at low frequencies, the local resolution for defects or inhomogeneities in the material under investigation decreases.
Coupling
In principle, analogous to the direct coupling of ultrasound to test pieces, there are also various types of air coupling for determining, for example, the elasto-mechanical properties or inhomogeneities (voids, inclusions or cracks) in the material. These are, of course, the well-known transmission method and the pulse-echo testing method, which is preferred for metallic materials.
Due to the high sound losses, the transmission method is usually used for testing with air-ultrasound, so that the sound path in the test piece is only traversed once and thus contributes only minimally to the overall sound attenuation. In contrast, the use of pulse-echo ultrasonic technique places significantly higher demands on the sensor and amplifier technology. This poses a major problem, especially with geometrically complex plastic components, even when using suitable robot technology. For this reason, the transmission method (Fig. 1) has been state of the kind for years and is also used successfully with plastics and composite materials.
| Fig. 1: | Schematic representation of the acoustic transmission arrangement with coupling via air |
Another technical implementation variant of air coupling is the transmitter-receiver (S/E) arrangement, which, similar to the internal structure of the S/E sensors, are aligned with each other at a roof angle and can thus form a focus in the test specimen, enabling a higher resolution of the received signal. Figure 2 shows this measurement arrangement, with the focus here on the rear wall.
| Fig. 2: | Schematic representation of the transmitter (S)-receiver (E) arrangement for coupling via air with focus on the rear wall of the test piece |
Features of air-ultrasound measuring systems
Air-ultrasound testing systems generally operate at comparatively low frequencies of less than 500 kHz, which are subject to less attenuation in air. The difference in measurement frequencies compared to contact or immersion bath sensors is approximately one order of magnitude. Depending on the frequency and wavelength, this naturally results in lower resolution in defectoscopy and in the assessment of surfaces. To counteract this effect, there are various approaches that are usually applied in combination:
- high incident sound power,
- excitation with burst signals,
- combination of several piezo ceramics in one test specimen (focusing) and
- narrowband designed output amplifiers.
A higher incident power increases the intensity of the ultrasonic signal. To ensure that as much intensity as possible is transmitted into the test material, burst or square pulses are usually used for excitation. However, this limits the frequency band, which is why the burst signals must be matched to the measurement and amplifier technology. The output amplifier is therefore designed to be relatively narrow-band, which allows a relatively high signal amplitude to be generated.
Due to the specific requirements and the complex measurement technology, air-ultrasound measurement is currently only useful in the laboratory or in online quality assurance, but is not yet possible on the basis of a mobile device.
Examples of air-ultrasound measuring systems
Merseburg University of Applied Sciences has an air-ultrasound measuring system (Fig. 3) that was developed in collaboration with Polymer Service GmbH Merseburg, the Ultrasound Research Centre (FZU) gGmbH and Sonotec Ultraschallsensorik GmbH Halle as part of a BMBF joint project and is currently being further developed [3, 4].
| Fig. 3: | Demonstrator measuring system from Merseburg University of Applied Sciences testing a CFRP structure using robot technology |
This measuring system, which was developed as a demonstrator, operates using the ultrasonic transmission technique with dual-channel 400 kHz transducers mounted on an ISEL manipulator (robot). Various ultrasonic images (A-scan, B-scan, C-scan and F-scan) can be generated with the system to detect defects and interpret the measurement results (Fig. 4).
| Fig. 4: | C-scan representation of a knot hole in pine wood (a) as integration of the amplitudes over the entire scanned volume and (b) over a selected depth range |
Sonotec Ultraschalltechnik GmbH, Halle, has launched the SONOAIR air-ultrasound testing system, which is primarily designed for materials with high sound attenuation]. [https://www.hillger-ndt.de/de/ Hillger NDT GmbH was one of the first manufacturers of commercial airborne ultrasound measuring systems and offers corresponding systems from its AIRTECH product family on the market.