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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 ferroelectret ultrasonic transducers applied to testing of fiber-reinforced polymers
(2013)
There are many different methods to characterize air-coupled ultrasonic transducers for non-destructive testing. Data sheets of various manufacturers contain information about some parameters important for the performance of transducers, but this information is not standardized, so that a comparison between probes of different manufacturers is difficult. Therefore, the German Society for Non-Destructive Testing (DGZfP) is working on a guideline to characterization of air-coupled probes.
One of the topics in this guideline is the application of thermoacoustic transducers for the characterization of receivers and another topic is the application of microphones for the characterization of transmitters. In this presentation we compare various characterization methods with the particular focus on the characterization of thermoacoustic transducers using an optical microphone. Both thermoacoustic transmitters and optical microphones have a very large bandwidth compared to conventional air-coupled transducers, but their spectrum is not entirely linear, which needs to be taken into account if they are applied as reference transducers.
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.
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.
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.
Air-coupled ultrasonic testing (ACUT) has experienced rapid growth within the last years. It is especially well suited to inspection of lightweight structures consisting of composite materials and adhesive joints. Uniform coupling and easy maintenance are its advantages compared to contact technique. However, the impedance mismatch between the transducer and air poses a major challenge to the development of ACUT transducers. Commercially available air-coupled transducers consist of a piezocomposite material and matching layers. Their fabrication is difficult in handling and their signal-to-noise ratio sometimes not sufficient for various testing requirements. However, there are several innovative approaches using other materials and other physical principles to transmit and receive an ultrasonic pulse. We present a review of the latest advances in research on air-coupled transducers for non-destructive testing, including previously unpublished results. We recognize two major directions as most promising: ferroelectrets and thermoacoustic transducers. Ferroelectrets are charged cellular polymers exhibiting piezoelectric properties. Their small acoustic impedance is matched to air better than matching layers applied in conventional air-coupled transducers. Applying bias voltage to a ferroelectret receiver is the latest development in this field, which increased the received signal by 12 to 15 dB. Thermoacoustic transducers use heat to initiate an ultrasonic wave, acting as transmitters. The working principle is known from nature as thunder and lightning: thermal energy of an electrically heated material, which can also be air, is converted into acoustic energy. Some thermoacoustic transmitters consist of a conductive layer with a thickness in the nanometer range deposited on a solid substrate. Another possibility is to use an electric spark. For the first time, measurements of the sound field of an electric spark up to 500 kHz were performed. Thermoacoustic transducers enable excitation of extremely broadband pulses while producing high pressure levels, which opens new possibilities for advanced signal processing.
Air-coupled ultrasonic testing (ACUT) has experienced rapid growth within the last years. It is especially well suited to inspection of lightweight structures consisting of composite materials and adhesive joints. Uniform coupling and easy maintenance are its advantages compared to contact technique. However, the impedance mismatch between the transducer and air poses a major challenge to the development of ACUT transducers. Commercially available air-coupled transducers consist of a piezocomposite material and matching layers. Their fabrication is difficult in handling and their signal-to-noise ratio sometimes not sufficient for various testing requirements. However, there are several innovative approaches using other materials and other physical principles to transmit and receive an ultrasonic pulse. We present a review of the latest advances in research on air-coupled transducers for non-destructive testing, including previously unpublished results. We recognize two major directions as most promising: ferroelectrets and thermoacoustic transducers. Ferroelectrets are charged cellular polymers exhibiting piezoelectric properties. Their small acoustic impedance is matched to air better than matching layers applied in conventional air-coupled transducers. Applying bias voltage to a ferroelectret receiver is the latest development in this field, which increased the received signal by 12 to 15 dB. Thermoacoustic transducers use heat to initiate an ultrasonic wave, acting as transmitters. The working principle is known from nature as thunder and lightning: thermal energy of an electrically heated material, which can also be air, is converted into acoustic energy. Some thermoacoustic transmitters consist of a conductive layer with a thickness in the nanometer range deposited on a solid substrate. Another possibility is to use an electric spark. For the first time, measurements of the sound field of an electric spark up to 500 kHz were performed. Thermoacoustic transducers enable excitation of extremely broadband pulses while producing high pressure levels, which opens new possibilities for advanced signal processing.
Air-coupled ultrasound (ACU) is increasingly used for automated and contactless inspection of large-scale composite structures as well as for non-destructive testing (NDT) of water-sensitive or porous materials. The major challenge to overcome using ACU in NDT is the enormous loss of ultrasonic energy at each solid-air interface caused by the high acoustic impedance mismatch. Resonant low-frequency piezoceramic transducers are specially designed to achieve high sound pressure levels. For an expanded use of this technique, however, the spatial resolution needs to be increased.
Recent studies of our collaborative research group demonstrated the successful application of a resonance-free, highly sensitive receiver that uses a Fabry-Pérot etalon instead of piezoceramic materials or membranes. However, to reach the full potential of this broadband small-aperture optical microphone, novel transmitter concepts have to be developed and evaluated for advanced NDT applications.
Different types of transmitter were tested in combination with the optical microphone acting as receiver and they were compared to conventional piezoceramic transducers in through-transmission mode. Monolithic carbon fiber-reinforced plastics (CFRP) and CFRP sandwich structures containing different defect types were inspected. Presented results are processed as C-scan images and further evaluated for spatial resolution, signal-to-noise ratio and sensitivity of each measurement setup. Novel transmitter concepts, such as ferroelectret and thermoacoustic emitters, show promising findings with a considerably improved time and spatial resolution for ACU-NDT.