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Non-Contact Ultrasonic Testing of Aircraft Lap Joints D. Cerniglia, Department of Mechanics and Aeronautics, University of Palermo, 90128 Palermo, Italy K. Y. Jhang, School of Mechanical Engineering, Hanyang University, 133-791 Seoul, Korea B. B. Djordjevic Center for Nondestructive Evaluation, The Johns Hopkins University, Baltimore, MD 21218 Contact

SUMMARY:

Bond quality of adhesively bonded and riveted aluminum lap splice joints is investigated using non-contact remote ultrasonic nondestructive evaluation (NDE). Non-contact ultrasonic tests are performed using laser generation and air-coupled transducer detection. A Q-switched Nd:YAG laser and a periodic transmission mask are used to generate a selected Lamb mode. The Lamb wave is generated on one side of the lap splice joint, propagates along the plate, interacts with the joint and is detected on the other side by a micromachined air-coupled capacitance transducer. Analysis of recorded signals allows to evaluate the condition of the bond. This ultrasonic inspection system overcomes the drawbacks of conventional contact ultrasonic methods. The experimental results demonstrate potential to employee this approach for a reliable rapid inspection of aircraft panels.

KEYWORDS: laser ultrasound, lamb waves, air-coupled transducer, lap joints.

INTRODUCTION

Due to the aging, adhesively bonded and riveted aircraft lap joints can contain disbonds, cracks emanating from rivets, flaws, voids and corrosion. A reliable nondestructive testing technique for periodic inspections during the service life of aircraft is essential to detect hidden damage and NDE is even more critical because aircraft are being used longer than their design life. Such testing techniques are desirable to perform the inspection from only one side of the structure.
Conventional contact ultrasonic NDE techniques have been applied to inspect aircraft lap splice joints [1]. However, for large aircraft area inspection contact ultrasonic testing is time consuming and as a result it is important to develop a rapid and more efficient ultrasonic testing technique.
Non-contact air-coupled generation and detection of ultrasound shows the potential for such application. In fact, because there is no requirement for a coupling medium to transmit the ultrasound the structure can be inspected remotely and at higher test speed then conventional setups.
Several non-contact ultrasonic techniques have been investigated using electromagnetic acoustic transducers (EMAT's), laser ultrasonics and air-coupled transducers in same possible combinations [2-6]. However, EMAT's must be placed very close to the sample surface whereas a laser-based reception system shows low sensitivity. An air-coupled transducer generation/reception system can be used but due to the large acoustic impedance mismatch between air and solid materials (e.g., aluminum) it is more convenient to combine laser generation and air-coupled transducer detection [7]. Using laser generation, ultrasound is produced directly on the material surface. By eliminating one of the air/aluminum interfaces a higher amount of acoustic energy is transmitted into the material, thus improving the detection by air-coupled transducer.
In this work the bond quality of an adhesively bonded and riveted lap joint is assessed using a pulsed Nd:YAG laser and a micromachined air-coupled capacitance transducer [8-9] to generate and detect, respectively, ultrasonic Lamb waves.
Lamb wave modes are very efficient for global inspection of lap joints [6,10-11] because they propagate through the whole joint area. Lamb waves are multimode and dispersive and it is desirable to generate a single Lamb mode [12] so that any change in the received wave signals may be attributed to the bond condition.
In order to select a single Lamb wave mode we utilize a spatial modulation technique that employs a periodic transmission mask [13]. Such mask, illuminated by the expanded laser beam, projects on the sample surface line sources whose spatial frequency represents the wavelength of the generated Lamb mode. By spreading the laser energy on a large area, this technique allows ultrasound generation in thermoelastic regime that is essential to avoid any damage to the surface of the object under inspection.
The main purpose of this study is to develop an efficient non-contact ultrasonic testing technique suitable for inspection of aircraft bonded joints in a reasonable time and with a high degree of accuracy.

EXPERIMENTAL PROCEDURES

A lap splice joint similar to an aircraft panel (see Fig. 1) has been fabricated using two layers of 1.6 mm thick aluminum alloy 7075-T6 alclad. The two plates are bonded with a 30 mm overlap by a FM300 adhesive film, cured at 350° F for one hour using heat-up rate of 5° F/min.
Before applying the adhesive, bond area is wiped with acetone, rinsed with water and is etched with a phosphoric acid. Bond quality is critically dependent on the interaction at the adhesive/adherent surface. Thus, sample preparation followed aerospace bonding practices.
Lap joint sample bondline incorporated disbond areas that had close mechanical contact but poor adhesion at the adhesive/metal interface. Disbond areas shown in Fig. 1 were filled with aged adhesive that did not bond to the aluminum. The joint was reinforced by a row of 4 mm diameter aluminum rivets.

Fig 1: Illustration of the lap joint sample. 30 mm

Fig 2: Diagram of the experimental setup consisting of ultrasonic laser generation and air-coupled detection.


To inspect the joint the sample is shifted vertically under the hybrid ultrasonic test system shown schematically in Fig. 2. The generation laser is a Nd:YAG laser which produces 10 ns long pulses at a wavelength of 1.06 mm with a pulse repetition rate of 20 Hz and a pulse energy of about 0.3 J. The laser beam is expanded by a plano-concave lens over an 11 mm wide by 14 mm long area of a mask and projects on the sample surface six lines 1 mm wide with a spatial frequency l = 2.1 mm.
The line sources, generated on one side of lap splice joint, produce directional waves propagating along the plate perpendicularly to the lines [14]. After interacting with the joint, Lamb waves are received by a wideband micromachined air-coupled capacitance transducer located in pitch-catch configuration on the other side of the joint. The aperture area on the air-coupled transducer has a diameter of 3 mm and is aligned to the center of the line sources.
Orientation angle is determined experimentally and adjusted for maximum through-transmitted energy at 8° with respect to the sample surface normal. Source/receiver distance, from the closest line source to the intersection of the transducer normal to the surface, is 92 mm whereas stand-off distance between the transducer and the sample surface, along the 8° orientation, is 18 mm. Experimental data shows a signal amplitude attenuation as a function of stand-off distance of approximately 2.5 dB/cm.
Signals detected by the air-coupled transducer are amplified by a Cooknell charge amplifier, which also provides a 100 V DC bias to the detector. Output signals from the charge amplifier are high-pass filtered at 0.3 MHz to eliminate the low frequency noise, displayed and averaged in a digital oscilloscope, that is triggered from the pulsed laser, and stored for subsequent analysis.

RESULTS

The typical signal waveform acquired from a 1.6 mm thick aluminum plate, in an area outside the joint, using the described setup is shown in Fig. 3. The recorded wave has a frequency of 1.2 MHz and from the dispersion curves phase velocity vs. frequency ´ plate thickness (c_ph vs. f´d) for aluminum, for given product f´d and in correspondence of the oblique line with slope c_ph/(f´d)= l/d (l=2.1 mm), it is possible to identify the generated mode with A₀.

The typical signal waveforms for a good-bonded region, for a transition region to the disbond and for a disbonded area, recorded on the lap splice joint, are shown respectively in Figs. 4(a), 4(b) and 4(c).

Fig 3: Signal waveform from an aluminum plate without joint	Fig 4a: Signal waveforms from a good-bonded region
Fig 4b: Signal waveforms from a transition region to the disbond	Fig 4c: Signal waveforms from a disbonded area.


Peak-to-peak signal amplitude was measured for different lap joint locations.
As can be seen, A₀ Lamb mode waveform going through the lap joint changes when compared to that propagating along joint free plate (see Fig. 3). However, the measured signals remain at frequency of 1.2 MHz.
As expected, A₀ wave mode signal shows a low transmitted signal amplitude going through a transition region to the disbond, while a well bonded region is associated with a high transmitted signal amplitude. Ultrasonic signals are reduced below instrumentation noise for disbonded area as shown in Fig. 4(c).
In Fig. 5 normalized signal amplitude is shown for test positions along the lap joint sample. Signals are normalized using as a reference the maximum amplitude of the wave from a well-bonded region.

Fig 5: Normalized ultrasonic signal amplitudes positioned relatively to the tested regions in the sample.


Results show an attenuation of the signal due to the presence of the rivets of approximately 7 dB. Regions of the sample with disbonds severely attenuate signal. In current setup the signal level across disbonds is reduced below signal noise of the detection system. Experimental results using non-contact system on the above sample indicate that measurements of A₀ Lamb mode signal amplitude reflect the condition of the bond.
Repeated experiments from lap joint test panel show reproducibility of the measurements.

CONCLUSIONS

It has been shown that adhesively bonded and riveted lap splice joints can be successfully inspected using a non-contact remote ultrasonic inspection system, overcoming the disadvantages of conventional contact ultrasonic NDE.
Non-contact ultrasonic tests on a manufactured lap joint sample with disbonded area have been performed using laser generation and air-coupled transducer detection. A Q-switched Nd:YAG laser and a periodic transmission mask are used to generate a single Lamb wave mode that is detected by a micromachined air-coupled capacitance transducer in pitch-catch configuration.
Experimental results acquired from the sample show signal amplitude attenuation related to the bond condition.
Lamb waves test configuration using hybrid non-contact ultrasonic system has potential for rapid and reliable inspection of lap splice joint. The tests can be performed with a single scan along the bondline joint by monitoring A0 Lamb wave amplitude. Lap joint structure with mechanical integrity exhibits uniform and intense Lamb wave coupling. Loss of the signal is associated with the loss of the bondline integrity.
As a result the inspection of lap splice joint can be carried out quickly, slowing down the scan of the sample only where a change in attenuation of the signal is noted.
Additional work will be carried out to detect cracks and corrosion in lap splice joints using the hybrid laser/air-transducer ultrasonic system. Our goal is to employ such technique to inspect real aircraft lap splice joints.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Larry Rouch for manufacturing the sample. This work was performed at CNDE facilities at Johns Hopkins University. K. Y. Jhang is visiting professor and D. Cerniglia is visiting scholar.

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