Adhesively bonded aluminum components have been widely used in the aerospace industry for weight-efficient and damage-tolerant structures. Automated squirter jet immersion ultrasonic testing is a common inspection technique to assure the bond integrity of large, contoured assemblies. However, squirter jet inspection presents several limitations in scanning speed, related to water splash noise over protruding stiffeners and splash interference crosstalk in multi-channel inspection systems. Air-coupled ultrasonic testing has been evaluated as an alternative, possibly offering the benefits of increased throughput by enabling higher speeds, and eliminating the contamination concerns and maintenance issues of water couplant systems. Adhesive joints of multi-layer aluminum plates with artificial disbonds were inspected with novel air-coupled ultrasonic probes based on cellular polypropylene. Disbonds of various sizes were engineered in several multi-layer configurations and at various depths. Results were compared with squirter jet immersion and conventional piezoelectric transducer designs in terms of scan contrast, resolution and inspection time.
Airborne ultrasonic inspection is performed in through transmission, where the test piece (e.g. adhesive joint or polymer-based composite plate) is placed between the transmitter and the receiver. However, many structures with difficult shapes allow only one-sided inspection. The strong reflection of the signal from the surface overshadows the signals from the inside, so that broadband pulses are required. Thermoacoustic transmission, where the thermal energy of an electrically heated electrode is transformed into the acoustic energy of an ultrasonic wave, opens the possibility to excite broadband pulses and thus to inspect objects with one-sided access.
We present various thermoacoustic transducers consisting of an electrically conductive film on a solid substrate. The first type of transducer is a transmitter with an indium-tin-oxide electrode on a glass substrate combined with a laser Doppler vibrometer as a receiver. The second type of transducer combines thermoacoustic transmission and piezoelectric reception, having a titanium electrode as a transmitter deposited onto charged cellular polypropylene serving as a piezoelectric receiver.
Using a focusing thermoacoustic transmitter and a separate cellular polypropylene receiver, a through-transmission inspection of a 4 mm thick CFRP test piece with inserts as small as 1 mm was performed. The same emitter and a laser vibrometer as a receiver were used for a one-sided inspection of a Plexiglas block with a cross hole at 15 mm depth. A twin probe consisting of a thermoacoustic transmitter on a cellular polypropylene receiver was applied to a profile measurement on a step wedge with flat bottom holes. The smallest detected diameter of a flat bottom hole was 1 mm. Sound pressure level above 140dB was achieved with each of these transmitters. Thermoacoustic transmitters enable a step towards one-sided air-coupled ultrasonic inspection.
Highly sensitive air-coupled ultrasonic sensors are essential for various applications such as testing of composite materials. One of the major challenges for the development of air-coupled ultrasonic sensors is the impedance matching to air. With a lower acoustic impedance than the usual piezoelectric materials, charged cellular polypropylene film (cPP) offers better matching to air with a similar piezoelectric coefficient. The piezoelectric behaviour demonstrated by cPP comes from polarized air cells that create a permanent internal voltage. The sensitivity of the sensor varies with the application of an additional DC bias voltage. Thus, this work presents a cPP ultrasonic sensor with an improvement of up to 15 ± 1 dB on the signal-to-noise ratio.
Air-coupled ferroelectret ultrasonic transducers applied to testing of fiber-reinforced polymers
(2013)
Ultrasonic testing of adhesively bonded joints using air-coupled cellular polypropylene transducers
(2014)
In air-coupled ultrasonic testing, the impedance mismatch between the transducer and the air is commonly being solved by adding matching layers to composite transducers. To avoid the difficult technological procedure regarding matching layers, some new piezoelectric materials have been proposed. Most promising are ferroelectrets, which are charged cellular polymers, having ferroelectric and consequently piezoelectric properties. In particular, the extreme softness of cellular polypropylene (cPP) leads to a high piezoelectric constant and to a good impedance match with the air, making matching layers redundant. Its elasticity modulus below 1 MPa causes an additional effect not observed with common piezoelectric materials: that is the electrostrictive effect, here defined as the thickness change due to the attractive force between the transducer electrodes. This effect exceeds the piezoelectric effect at excitation voltages over 1 kV. The extreme softness of cPP leads also to high flexibility, enabling easy focusing by bending the transducer.
We have developed air-coupled ultrasonic transducers based on cPP. This includes the electrical matching networks for the transmitter and for the receiver. The transmitter is excited with voltages up to 2.5 kV, so that the electrostrictive effect dominates, leading to sound pressure around 145dB at the transducer surface. These transducers have been applied for testing carbon-fiber-reinforced polymer plates, adhesive joints and other composite structures. Here we report about ultrasonic transmission of two types of adhesive joints. The first one is multi-layer aluminium components with some artificial disbonds, which are common in aerospace industry, and the second one is an aluminium-steel joint with polyurethane adhesive, which is used in automotive industry.
Electromechanical response of polypropylene ferroelectret transducers under application of high-voltage pulses was measured by laser Doppler vibrometry and compared with results of ultrasonic through-air transmission between two ferroelectret transducers. The electromechanical response was completely explained by piezoelectric and electrostrictive effects. The electrostrictive effect dominates at high voltages and provides significant enlargement of the transducer constant, up to factor of 2.5. The induced strain of 1.7% was achieved at – 2000 V. The nonlinear ultrasonic transmission was shown to be well described by the piezoelectric and electrostrictive response of transmitter, except in the range of high negative exciting voltages where some limitation of the transmitted signal was observed. This limitation seems not to be a fundamental one and does not abolish the advantages of high-voltage excitation of polypropylene ferroelectret transducers.
Ferroelectrets are promising materials for air-coupled ultrasonic transducers. A transducer made of polarized cellular polypropylene, including its electronic interface, was developed and compared with conventional air-coupled probes. Test pieces of fiber-reinforced polymer containing impact flaws and flat-bottom holes were inspected in transmission. The ferroelectret transducers achieved a considerably higher signal-to-noise ratio. The impacts were clearly visible with all transducers, but less noisy with ferroelectret transducers. The flat-bottom holes were better detectable than with a conventional probe with about the same focus size.
Air-coupled ultrasound has been applied increasingly as a non-destructive testing method for lightweight construction in recent years. It is particularly appropriate for composite materials being used in automotive and aviation industry. Air-coupled ultrasound transducers mostly consist of piezoelectric materials and matching layers. However, their fabrication is challenging and their signal-to-noise ratio often not sufficient for many testing requirements. To enhance the efficiency, air-coupled ultrasound transducers made of cellular polypropylene have been developed. Because of its small density and sound velocity, this piezoelectric ferroelectret matches the small acoustic impedance of air much better than matching layers applied in conventional transducers. In our contribution, we present two different methods of spherical focusing of ferroelectret transducers for the further enhancement of their performance in NDT applications. Measurements on carbon-fiber-reinforced polymer (CFRP) samples and on metal adhesive joints performed with commercially available focused air-coupled ultrasound transducers are compared to measurements executed with self-developed focused ferroelectret transducers.
Novel air-coupled ultrasonic transducer combining the thermoacoustic with the piezoelectric effect
(2016)
In recent years, there has been an increasing industrial demand for one-sided inspection of various structures by means of air-coupled ultrasonic technique. Lightweight structures based on carbon-fibre-reinforced polymers may have very complex shapes, making air-coupled transmission difficult or even impossible. The inspection of concrete structures is another example where one-sided inspection is required.
To address these challenges a new type of transducer for air-coupled pulse-echo inspection was developed, which unites two principles: thermoacoustic emission and piezoelectric reception. The thermoacoustic emitter is a titanium electrode with a thickness of several tens of nanometer. This electrode was deposited onto charged cellular polypropylene, which serves as a piezoelectric receiver. The thermoacoustic transmission is based on a transformation of the thermal energy of an electrically heated electrode into the acoustic energy of an ultrasonic wave. Thermoacoustic emitters provide resonance-free behaviour and thus extremely broadband pulses. Charged cellular polypropylene is piezoelectric due to the polarization of its cells and it is well matched to air, with a Young modulus in the order of magnitude of MPa. In this contribution we present some pulse-echo measurements with the first prototypes of the combined thermoacoustic-piezoelectric transducer.