Abstract

This paper addresses ultrasonic investigations of the pore content in CFPR laminates as part of the German research project MaTech. Project partners are Airbus, Bremen, Ingenieurbüro Dr. Hillger, Braunschweig, and TuTech, Hamburg-Harburg, with DLR, Braunschweig, as a subcontractor. As backwall echo evaluations are not possible in the skins of sandwich components, investigations in echo-technique had to be carried out with the aim of optimising the test parameters. Based on these results, a portable ultrasonic system with C-scanning for pore content evaluation has been developed (Demonstrator). This paper also presents results of quantitative pore measurements.

Introduction

Carbon fibre reinforced plastics (CFRP) are attractive materials for aerospace structures because of their high specific strength and stiffness. However, a porosity of more than 2.5 volume-% can significantly reduce strength. Therefore, after production the control of the porosity content is very important.

The ultrasonic imaging technique is able to detect defects like delaminations and debondings with a high degree of resolution. The indication of porosity by ultrasonic echo technique, however, is difficult. Pores are very small voids statistically distributed in the component. The pores do not cause reflections like delaminations but only sound scattering. The quantitative determination by NDT is possible by evaluation of the backwall echo amplitude, or better, of the evaluation of changes of the frequency spectrum of the backwall echo [1, 2, 3]. This kind of evaluation, however, is not possible for CFRP skins bonded to honeycomb or foam because the backwall echo is attenuated by the bonding in an undefined way.

Therefore investigations on test specimens had to be carried out with the aim of optimising the test parameters. The results were used for the development of a portable ultrasonic imaging system (“demonstrator I”) for the measurement of the pore content.

Basic investigations for porosity detection

Two types of CFRP fabric specimens produced by the Airbus company have been investigated: one type („D“) contains respectively circular plate reflectors with diameters of 3; 4 and 5 mm in different depths in the form of Teflon foils, the other type (“ M”) contains artificially inserted Freon capsules (in one or in different depth ranges) which represent porosities of different concentrations. According to the guide lines of the project, the thickness of the specimens ranges between 1.4 mm and 9.1 mm.

The test specimens with circular plate reflectors served for the calibration of the amplification and for the optimisation of constant sensitivity in the thickness direction.

Fig 1: A-scan of a 9.1 mm thick test specimen, scale: 1µs/div. and 0,2 V/div.

The investigations were carried out in immersion technique in order to obtain a reproducible coupling. Fig. 1 shows an A- scan recorded from a defect-free region of a 9.1 mm thick CFRP-specimen. The gate for the evaluation is situated between the interface- and the backwall-echo. The scales are 1 µs/div. and 0.2 V/div.

Fig 2: C-scan and echo-dynamic curve of a 9.1 mm thick test specimen with circular plate reflectors

Fig. 2 presents a C-scan recorded from a D-type specimen with a thickness of 9.1 mm. High amplitudes are plotted in red, low ones blue. The look-up table is calibrated in 2 dB- steps, which provides a constant resolution in a dynamic range of 32 dB. The echo- dynamic curve clearly indicates that all reflectors deliver the same amplitude (-4 dB ±1 dB). This constant sensitivity was reached by a focussed broadband transducer (frequency range 2-6 MHz) with an active area of 12 mm, a 5 MHz filter in the receiver and a DAC with a gradient of 2,1 dB/µs. An avalanche pulser was used for excitation. For the thickness range of 1 to
< 3mm (thin components), a focussed broadband transducer ( 6-12 MHz) with an active diameter of 6 mm and a broadband amplifier (0.5 to 20 MHz) delivered best results. Only an avalanche pulser provides optimum results for thin CFRP- components.

Development of the demonstrator

The demonstrator consists of the components shown in Fig. 3: scanner with transducer adapter, water box with pressure and suction pumps as well as the ultrasonic system USPC with built-in motor controller a portable PC.

The transducer scans the component to be examined. The acoustic coupling is carried out via a water split technique with water circulation.

The ultrasonic amplitude between the interface- and backwall- echo of the component is recorded during scanning and a C-scan is displayed.

If the inspector accepts the C-scan, an evaluation of the amplitude histogram and a calculation of the pore content is carried out.

The water box contains a vacuum pump which is required for the vacuum pads at the manipulator in order to keep it attached to the component. Two different transducers built in adapters are required for two thickness ranges of <3 mm and 3 to 10 mm. The delay lines of the adapters are produced of materials which allow transmission of ultra sonic frequencies even above 15 MHz.

The developed ultrasonic system was attached to a PC-board. The transmitter (pulser) contains an avalanche transistor which provides a broad frequency spectrum with high acoustic power above 10 MHz because of the extremely fast switching.

The preamplifier consists of discrete, low-noise transistors in order to get a high signal-to- noise ratio. Two different frequency filters were developed for two thickness ranges of the CFRP-components.

The main amplifier contains a DAC (distance amplitude control), which provides a constant sensitivity over the component thickness. A commercially available ADC (analogue to digital converter) with a sample rate of
100 Msamples/s digitises the amplified and filtered ultrasonic signal.

The Windows^ä - software for the demonstrator provides control of the manipulator and of the ultrasonic system, adjustment of the gate for the amplitude measuring, the data recording including a software peak detector as well as the C-scan presentation and the pore evaluation. In addition, a go-to function permits to store A-scans from a reference C- scan.

Evaluation of porosity content

Fig. 4 shows a C-scan of a 4.2 mm thick CFRP-specimen type “M” with artificially inserted areas of pores. The volume porosities are 1.3; 1.6 and 2.3 % distributed over the thickness range. The areas are clearly indicated in yellow. The echo-dynamic curve indicated the amplitude range caused by the pores which is situated between -19 and -8 dB. The amplitude range of the „natural“ scattering is below -19 dB, the amplitudes of the circular plate reflectors are higher than -8 dB (see Fig. 2). These defined amplitude ranges for defect-free areas, delaminated ones and areas with pores were found at all 26 test specimens in a thickness range of 1.4 to 9.1 mm.

Fig 3: The three easily transportable components of the Demonstrators for pore examinations in CFRP components.

Fig4: Indication of pores in a test specimen with a thickness of 4.2 mm

Fig 5: Detection of the porosity content

Fig. 5 illustrates the detection of pores using histogram evaluations. The four amplitude histograms are extracted from the C-scan in Fig. 4. The relative frequency of the amplitudes can be found below the histograms under the colours of the different amplitudes. Histogram a) in Fig. 5 was obtained from an area without artificial porosity. There are only a few amplitudes displayed in the range of -25 to -15 dB, which indicate a high degree of quality. Histograms b), c) and d) from areas with artificially inserted porosities show a much larger amplitude range up to -7dB. These results show that the porosity content can be extracted from the amplitude histogram. An evaluation procedure is currently in development.

Summary

This report deals with ultrasonic investigations of the porosity content in carbon fibre reinforced (CFRP) aircraft structures.

Basic investigations on specimens with different thicknesses (1.4 to 9.1 mm) and with foil delaminations of different sizes in different depths were carried out. The results were used to derive optimal acoustical parameters and to create an improved ultrasound set-up (“demonstrator”) for the pulse-echo technique:

• focussed and extreme broadband transducer in a C-scanning system;
• high depth independent resolution up to 10 mm thick CFRP-components;
• optimised components of the demonstrator are: avalanche pulser with high power above 10 MHz, different filters, low-noise preamplifier, main amplifier with DAC, ADC with 100 Msamples/s;
• Windows^ä -based software for ultrasound settings, scanner control, data acquisition, C-scan display, and histogram evaluation for calculating the porosity in volume-%.

Tests of specimens with artificially inserted pore contents resulted in:

• C-scans with obvious resolution improvement in comparison to former results;
• histogram analyse allowing a quantitative assessment of volume porosity values in the range of 1 to 2 %.

Work is continuing to complete software algorithms, including results from signal analysis investigations and possibly including corrections, which may become necessary for other types of porosities in real serial production.

References

1. Gundtoft, Hans Erik: Quantitative material characterisation of composites by ultrasonic scanning, 15th World Conference on Nondestructive Testing, Roma (Italy) 15-21 October 2000, Conference Proc. on CD
2. L.-K. Shark , C. Yu , J. P. Smith: Automatic estimation of ultrasonic attenuation for porosity evaluation in composite materials, 15th World Conference on Nondestructive Testing, Roma (Italy) 15-21 October 2000, Conference Proc. on CD
3. A. Demma, B.B. Djordjevic: Effects of porosity on the Mechanical Strength and Ultrasonic Attenuation of CF-Peek fibre Placed Composites, 15th World Conference on Nondestructive Testing, Roma (Italy) 15-21 October 2000, Conference Proc. on CD

Acknowledgement

The work is sponsored by the bmbf+f of the German Government under the Material Technologies Programme MaTech.

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