Composites are attractive materials for lightweight structures. In order to inspect these components with ultrasonic imaging techniques optimizations of pulse parameters have been carried out. The know how of the laboratory inspections was successfully employed to field inspections of a specially prepared tail unit of the EH 101. The loss of time for manual scanning is relatively high so that the MUSE-system (Mobile UltraSonic Equipment) based on PC-boards with a motor-driven manipulator for automatic scanning was developed. The water circulation system provides local immersion technique. The non-contact ultrasonic technique with air coupling was investigated using a new system called AirTech 4000. This system is based on PC-boards. The AirTech 4000 delivers for sandwich components much higher resolution than inspections with water coupling. Using Lamb waves a testing from one side is possible.
Composites are high-performance materials especially for lightweight constructions in aerospace,naval and automotive engineering. Their application to primary aircraft structures requires the knowledge of damage incurred after fabrication or in service. Typical damages to be detected are: cracks, delaminations and debondings between skin and stringer, of which only a small part is visible from outside. The ultrasonic technique is principally able to indicate internal defects.
Composites are inhomogeneous and anisotropic materials with an extremely high sound attenuation.Through transmission techniques with separate receiver and transmitter transducers on opposite sides of the component is often used for their testing. This method is much easier in application than the echo technique because the sound has to travel only once in the thickness direction. However, through transmission technique is not practicable for field inspections because the access is limited to one side of the components.
Special developments for the ultrasonic echo technique were necessary in order to obtain a high degree of evidence. Using immersion technique high frequency and focused transducers deliver excellent resolutions if the pulse parameters are well optimized for the different thicknesses and kinds of materials .
2. Mobile Ultrasonic Inspection System "MUSE"
3. Air coupled ultrasonic testing
The air coupled ultrasonic technique avoids the disadvantages of the coupling liquid or coupling paste like time consuming cleaning after the inspection. Therefore, the non-contact ultrasonic technique is very attractive and very interesting for field inspections.
Fig 2: Amplitude curves between transmitter and receiver probe using water and air coupling|
Fig. 2 shows the amplitude differences between water and air coupling using through transmission technique with different transmitter and receiver transducers on opposite sides of a thin CFRP specimen. We set the amplitude on the transmitter to 0 dB. Using water coupling, the sound pressure decreases to -21 dB in the water because of the differences in the acoustic impedance of piezoelectric material (zPiezo =35 MRayl) and water (zwater =1.5 MRayl). The amplitude on the CFRP surface (zCFRP =4.5 MRayl) increases to -18dB and decreases again to -24 dB in the water. At least the amplitude on the receiver becomes -18 dB. This small amplitude difference between transducer and receiver can easily be compensated by amplification. In the case of air coupling, the amplitude difference between transmitter and receiver reaches -156 dB! The extreme high differences between the impedance of air (zair = 0.0004 MRayl) and of the piezoelectric material cause a decrease of -93 dB and also the interface between CFRP and air (-75 dB). In this very simple model only the transmission factors are calculated and no losses of sound attenuation and sound divergences have been regarded.
Therefore, standard flaw detectors and standard transducers cannot be used for air coupling.Special transducers, transmitter and receiver electronics are required. Since 15 years more and more papers describe the air coupled technique . Normally the investigations are carried out in through transmission technique.
4. Ultrasonic system AirTech 4000
Fig. 3 shows the block diagram of the AirTech 4000, an ultrasonic imaging system built on PC-boards for non-contact techniques. The system provides a single-shot-evaluation of the echoes. Two transducer pairs with frequencies of 250 and 450 kHz have been developed by IZfP in Saarbrücken (Germany) . The piezoelectric composite material for the transducers reduce the impedance to 10 to 15 MRayl. The matching layer to air gives a gain of 15 dB. The transducers are focused with a beam diameter less than 5 mm in a distance of 45 mm in air. The length of the transducers is 50 mm, the connectors are Lemo 00, so that they can easily be mounted on scanning systems.
Fig 3: Block diagram of the AirTech 4000 built on PC-bords|
Into the case of the transmitter transducer an electrical impedance matching network was built which increases the amplitude up to 15 dB. The burst transmitter (AT 4100) built on a PC-board generates quartz controlled signals up to 15 cycles with a power of up to 1.2 kW.
A low noise preamplifier with matching network and filter unit both consisting of SMD-devices have been additionally built into the case of the receiver transducer. The gain can be controlled by software from 50 to70 dB. The receiver amplifier and filter unit AT 4030 controls the amplification of the preamplifier. Only a simple RG 175 cable is used to receiver which does not only transmit the echo signal but also the power supply and the gain setting to the preamplifier. The third board of the AirTech system digitizes the echo signal (A/D-converter). The module MUCO is the motor controller for the scanning system. The software provides ultrasonic imaging in A-, C- and D-scans.
5. Comparison between water and air-coupling
6. One-side-testing with air coupling
CFRP components are attractive materials for space and aircraft applications. The high inhomogeneity and anisotropy of these materials cause an extremely high sound attenuation. The ultrasonic imaging technique must be capable of detecting all kinds of defects such as delaminations and debondings. The inspections cannot not be limited to laboratory use but also be practicable for field inspections of "real" structures.
Therefore the MUSE- system (Mobile UltraSonic Equipment) with a motor-driven manipulator and a special water circulation system for coupling has been developed. The MUSE provides ultrasonic imaging of internal defects.
In order to have a non-contact inspection investigations with air coupling were carried out with the new AirTech 4000 system. This system is built on PC-boards and provides ultrasonic imaging in single shot technique. In spite of the large differences between the acoustic impedances of air and solids, the resolution with air coupling is much better than those with water. The echo technique is not possible because of the large pulse length of the narrow band transducers. The application of Lamb waves opens the possibility of a one side testing which is very necessary for field inspections. It was shown that the method can indicate delaminations in CFRP laminates with constant thickness. The Lamb wave amplitude is very sensitive to the
alignment of the transducers. A project for the further development of the transducers and of the electronics has been started for the application of the echo technique.
- Hillger, W.: Ultrasonic imaging of internal defects in CFPRP-Components, 6th European Conference
on Non Destructive Testing. Conference Proc. part 1, (1994) pp. 449-453
- Hillger, W. : Ultrasonic imaging of defects in Sandwich Composites from laboratory research to infield
inspections, 7 th European Conference on Non- Destructive Testing Copenhagen, 26-29 May 1998,
Conf. Proc., pp. 88-94
- Hillger, W., Friederichs, B.: Ultrasonic Inspection of the Helicopter EH 101 Tail Unit, Brite Euram
Project No. BE-5781, Document No. DAMTOS-WP-503-1.1/DLR
- Hillger,W.: Ultrasonic PC- boards for different applications, 7 th ECNDT 1998, Copenhagen, Conf.
- Grandia, W.A.; Fortunko, S.M.:NDE Applications of Air-Coupled Ultrasonic Transducers, 1995
IEEE International Ultrasonic Symposium Seattle, Washington, Conf. Proc.,1995, S. 697-709.
- W. Gebhardt, W. Hillger, P. Kreier: Airborne Ultrasonic Probes: Design, Fabrication, Application, 7.
ECNDT, Kopenhagen, 26-29 Mai, 1998
- Hillger,W.: Optimization of ultrasonic pulse parameters for DAMTOS honeycomb sandwich
structures, Report DAMTOS-WP-504/1.1/DLR,
- M. Castaings and P. Cawley: The generation, propagation, and detection of Lamb waves in plates
using air-coupled ultrasonic tranducers, J. Acoust. Am. 100 (5) Nov. 1996, pp. 3070-3077