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Ultrasonic Testing of Composites - From laboratory Research to In-field Inspections W. Hillger DLR Braunschweig, Ing. Büro Dr. Hillger, Germany E-mail: Wolfgang.Hillger@t-online.de Contact

Abstract

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.

1. Introduction

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 [1].

2. Mobile Ultrasonic Inspection System "MUSE"

A specially prepared tail unit of the EH 101 helicopter with impact damages and inserted teflon foils loaded in fatigue equal to 10,000 flight hours was inspected at AGUSTA S.p.A. in Italy. The inspections were carried out in echo technique with a manual scanning system and imaging in B- and C- scans. The investigation shows that the laboratory research (development of the echo technique for sandwich components) can be transmitted to field inspections [2]. But the manual scanning was very time-consuming. Reasons were the incorrect coupling in small areas so that those parts had to be inspected again and the large number of points in the C- scans to which the transducer had to be manually positioned [3].

MUSE-manipulator with adapter for the transduer	Water circulation system	Flaw detector with built-in motor conroller
Fig 1: MUSE-manipulator with water circulation system and flaw-detector


From these experiences the mobile ultrasonic inspection system "MUSE" (Mobile UltraSonic Equipment) was developed. This system shown in Fig. 1 consists of a motor-driven manipulation system, a water circulation system for the coupling and the PC-board based ultrasonic system USPC built into a portable computer [4]. MUSE-manipulator with adapter for the transducer Water circulation system Flaw detector with built-in motor controller
The manipulator is attached to the component with sucking devices and accommodates an inspection area of 170mm to 210 mm, a resolution of 0.020 mm and a maximum speed of 100mm/s. The test frequency range for the MUSE is 0,05 to 35 MHz. Ultrasonic imaging is possible in A-, B-, C- and D- scans.
The coupling technique developed for the MUSE (" local immersion" technique) provides the application of focused transducers without problems with air bubbles so that the high resolution is also available for field inspections. This local immersion technique is also useful for laboratory investigations because no water tanks are required and the components stay dry.

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 (z_Piezo =35 MRayl) and water (z_water =1.5 MRayl). The amplitude on the CFRP surface (z_CFRP =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 (z_air = 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 [5]. Normally the investigations are carried out in through transmission technique.

4. Ultrasonic system AirTech 4000

Fig 3: Block diagram of the AirTech 4000 built on PC-bords

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) [6]. 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.
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

Sandwich components with CFRP skins and Nomex cores in this case with a total thickness of 14 mm cause a very high sound attenuation so that the test frequencies have to be in a frequency range from 0.4 to 0.8 MHz [7] (with water coupling). Normally those components are tested in through transmission technique (squirter technique).
Fig. 4 displays two C- scans of the same sandwich component, on the left hand side with water coupling, on the right hand side with air-coupling. In both cases the 10 J impact is clearly indicated. The air-coupled C-scan demonstrates the high resolution of the AirTech 4000 and gives more details such as the indication of cores. The wave length in water is about five times larger than in air, so that (using the same test frequency) the sound can be focused in air much better than in water and also the resolution of materials with high sound attenuation is much better with air coupling. This result is also impressively indicated by echo dynamic curves below the C- scans in Fig. 4.

Fig 4: Comparision between water and air-coupling using a CFRP-sandwich component with impact damage

6. One-side-testing with air coupling

For field inspections, however, the component can be only tested from one side, so that the through transmission technique is not practicable. The application of the echo technique is not possible because the narrow band transducers generate pulses which are not able to separate the echoes of the interface and of the backwall. The generation and detection of Lamb waves in components with a constant thickness opens the possibility for a single sided inspection [8].
Fig. 5 shows the two 450 kHz transducers (AT 4410 and AT 4420) in one-side inspection of a 4.2 mm thick CFRP laminate with an impact damage. The C- scan with Lamb waves clearly indicates the defect. The echo-dynamic curve shows a -16 dB amplitude drop in the defect area. The amplitude of the Lamb waves is very sensitive to the alignment of the transducers. In spite of this it is possible to have a non-contact one-side-inspection of CFRP-components.

Inspection from one side with air coupling
	C-scan and echo -dynamic curve
Fig 5: Testing a 4.2 mm thick CFRI-laminate with an impact damage from oneside wih air coupling,C-scan and ech-dynamic curve

6. Summary

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.

7. References

1. 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
2. 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
3. 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
4. Hillger,W.: Ultrasonic PC- boards for different applications, 7 th ECNDT 1998, Copenhagen, Conf. Proc.
5. 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.
6. W. Gebhardt, W. Hillger, P. Kreier: Airborne Ultrasonic Probes: Design, Fabrication, Application, 7. ECNDT, Kopenhagen, 26-29 Mai, 1998
7. Hillger,W.: Optimization of ultrasonic pulse parameters for DAMTOS honeycomb sandwich structures, Report DAMTOS-WP-504/1.1/DLR,
8. 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

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