Table of Contents ECNDT '98
Ultrasonic Imaging of Defects in Sandwich Composites from Laboratory Research to In-Field InspectionDr.-Ing. W. Hillger
DLR Braunschweig, German Aerospace Center, Institute of Structure Mechanics
Lilienthalplatz 7, D-38108 Braunschweig, Germany
Telephone +49 531 295-2306, Telefax +49 531 295-2875
Email: Wolfgang.Hillger@t-online.de , URL: NDTnet Exhibition - Hilger
|TABLE OF CONTENTS|
Sandwich components with CFRP skins and Nomex cores are attractive materials for lightweight structures. In order to inspect these components with ultrasonic imaging techniques optimizations of pulse parameters have been carried out. System components such as transducer, transmitter (pulser) and receiver had to be optimized for the application of the echo technique not only for the skin inspection but also for the whole sandwich thickness. C- and B- scans show conclusively that the defects caused by impacts are mostly situated in the core.
The know how of the laboratory inspections was successfully employed to in-field inspections of a specially prepared tail unit of the EH 101, the largest European helicopter produced by E.H.Industries Ltd. For B- and C- scan recordings a manual scanner was used. 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 excellent coupling.
High-performance materials such as sandwich components are attractive materials for lightweight constructions in aerospace, naval and automotive engineering. Their application to primary aircraft structures requires the knowledge of damage incurred after fa-brication or in service. Typical damages to be detected are: cracks and delaminations in the skin, debondings between skin and core and defects in the core (crushing), of which only a small part is visible from the outside. The ultrasonic technique is principally able to indicate internal defects. Honeycomb sandwich components are inhomogeneous and anisotropic materials with an extremely high sound damping. 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 than the echo technique because the sound has to travel only once in the thickness direction. However, through transmission technique is not practicable for in-field inspections because of 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. This report describes the optimizations of pulse parameters for the ultrasonic imaging of internal defects in sandwich specimens with Nomex cores, and the results of laboratory and in-field inspections.
Different test frequency spectra have to be used for the skin inspections and for testing of the whole sandwich thickness. The skin thickness is only 0.5 to 1.0 mm, so that high frequencies in the range of 15 to 35 MHz are required to separate interface and backwall echoes. The high frequency inspection of thin laminates are state of art at DLR . The skin inspections provide no information about core defects. Therefore the investigations concentrated to the inspection of the whole sandwich thickness by the echo technique .
Best results were obtained with a broadband transducer 0.8-3 MHz with an aluminium oxide protective layer. Water split coupling was used.
Different types of ultrasonic pulsers were investigated for sandwich components. Best results delivered a rectangular pulser. In comparison with the normally used broadband spike pulser (useful for high frequency ultrasonic testing of the skin) the frequency spectrum is more concentrated in a smaller range which is adjustable by the pulse width. A special pulser/receiver module for the HFUS 2000 system was developed. The adaptations to the different DAMTOS types A, D and F were carried out with different pulse widths. On the receiver side a broadband amplifier with a 0.3 MHz high pass filter and a 2.0 MHz low pass filter is used. The application of the echo technique opens the possibility to a powerful ultrasonic imaging of defects in sandwich components.
Fig. 1: C-scans and echo- dynamics of specimen 39/4 with impact energy levels of 4J and 8J. left: Only skin inspection; middle: Through transmission technique (whole thickness); right: Echo - technique (whole thickness)
The identified defect sizes are quite different: the 18J- impact damage identified by skin inspection shows an area of 644 mm², the inspection of the whole thickness delivers areas of 2574 mm² (through-transmission technique) and 2435 mm² (echo-technique). The C-scan of the skin only shows portions of the defect areas which are equal to the sizes visible from outside. A comparison of the horizontal echo dynamic curves (whole thickness) of the 18 Joule impact indicates a -20dB drop of the amplitude (through transmission technique), and a -10 dB drop in echo technique.
The results show that the echo technique with optimized pulse parameters provides a precise identification of defects in the skin and in the core. Additional imaging is possible with B-scans which clearly show the defects in different depths of the specimens. An example is given in Fig. 2 in the form of two B-scans. The 18 Joule impact causes many echoes in the core region which are impressively displayed in the horizontal B-scan.
|Fig 2a: B-scans of 4 joule impact in horizontal direction||Fig 2b: B-scans 18 joule impact in horizontal direction|
Fig 3: Tail-Unit of the EH 101 and ultrasonic equipment
Fig 4: C-scan of defect 8E
Fig. 3 shows the tail unit together with the DLR ultrasonic equipment, composed of an ultrasonic flaw detector HFUS 2000 and a manual 'Sinus' scanning system which was used for the C-scan recordings . The Bt-scan (B-scan over time without coordinate recording) was carried out with an ultrasonic system based on a PC-slot card (HILL-SCAN 3000 ) installed in a docking station of a notebook-PC.
A typical C-scan recorded with backwall echo evaluation (of the lower skin) is shown in Fig. 4. The defect causes an amplitude decrease of about -24dB shown in the echo-dynamic curve in Fig. 4.
The Bt-scans of the skin displayed in Fig. 6 and 7 show echoes coming from different depths caused by
impact damages. For this purpose, a 15 MHz transducer with delay line was used.
Fig 5: Echo-dynamic curve of defect (horizontal)
Fig 6: Bt -scan of the skin (vertical)
Fig 7: Bt- scan of the skin (horizontal)
The figures demonstrate that the optimizations of test parameters executed in the laboratory can also successfully applied to in-field inspections on real components.
|Fig 8: MUSE-manipu- lator with adapter for the transducer|
Honeycomb sandwich components are inhomogeneous and anisotropic materials with an extremely high sound damping. The ultrasonic imaging technique for these materials must be capable of detecting all kinds of defects such as delaminations in the skin, debondings between skin and core and defects in the core (crushing). The inspections should not be limited to laboratory use, but also be practicable for field-inspections of "real" structures. In-field inspections require the application of the echo technique because the components can only accessed from one side.
In order to solve this problem, optimizations of pulse parameters were carried out. The test frequencies for a single skin which only show minor damages have to be higher than 10 MHz in order to separate the interface and the backwall echo (0.5 to 1.0 mm thickness).
The attenuation of the whole sandwich rises with increasingly higher frequencies. Therefore only frequencies below 1 MHz can pass through the whole thickness. For different types of skins (CFRP, GFRP, fabrics, and prepregs) and varying thicknesses of the Nomex cores different frequency spectra are necessary. System components such as transducer, transmitter (pulser) and receiver had to be optimized.
A water-split based coupling between transducer and component was used. Inspections of sandwich components with impact damages (energy levels from 4 to 18 Joule) clearly displays defect areas in C- scans. B-scans delivered more detailed informations about the defect depths.
The experiences gained by laboratory inspections were successfully employed to in-field inspections of a specially prepared tail unit of a helicopter. A manual scanner was used for the recording of C- and B- scans. Defects caused by impacts have been clearly identified with amplitude decreases of more than 15 dB. Using a manual scanning system a stable coupling of the transducer is difficult, and the scanning is time consuming as well. 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 components with optimized pulse parameters.
The author is grateful to the AGUSTA S.p.A. Company, Italy, for the opportunity to inspect the EH 101 tail unit. We also thank the European Community and all partners of the BE-DAMTOS project for the support.