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Skin to Honeycomb Core delamination Detection with Guided Waves Joseph L. Rose and Thomas Hay The Pennsylvania State University Engineering Science & Mechanics Department University Park, PA 16802 Contact

ABSTRACT

Detection of skin delamination from an aluminum honeycomb core using guided waves is presented in this paper. Mode optimization through frequency tuning is illustrated on an aluminum skin - aluminum core and composite skin - aluminum core specimens with simulated skin-core delamination. It is shown that mode optimization is related to wave structure and some examples are provided to demonstrate how the distribution of particle displacement and power apply to this inspection problem. Some preliminary guided wave based results are included for a Navy F-18 aircraft rudder.

INTRODUCTION

Ultrasonic guided waves are currently being considered for the inspection of
• fuselage wall thinning,
• lap splices,
• tear straps,
• helicopter blade transmission beams,
• landing gear, and
• cracks in the second layer of multi-layer structures

in the aircraft industry [1].
These structures are examples of natural waveguides with appropriate boundaries to accommodate guided wave modes. Likewise, the skin-honeycomb core structure is also a natural waveguide. In general, for the skin-core delamination problem, it is desirable to find a mode with a wave structure that has strong particle displacement components close to the skin-core interface. In principle, these modes should be absorbed by the core if the skin is adequately bonded to core. When the skin delaminates from core due to water damage or other environmental degradation factors the interface bond deteriorates and transmission of energy through the interface is less efficient. Therefore, the mode travelling in the skin experiences an increase in amplitude in the delaminated areas [2].

Fig 1: Example of the guided wave inspection setup

MODELING ANALYSIS

Sample phase and group velocity dispersion curves for an aluminum plate and a 2-layer aluminum-epoxy structure are shown in Figure 2. For dispersion curve generation of single and n-layer structures see Rose [3]. The dispersion curves show the possible wave propagation states. The phase velocity - frequency space can be scanned by varying wedge angle in oblique incidence and by sweeping frequency with a tone burst function generator. Although, many wave propagation states are possible on the dispersion curves, only a few will have the wave structure and frequency required for performance of a reliable inspection.

Fig 2: Phase and Group Velocity Dispersion Curves

Figure 3 shows wave structures from two points selected on the aluminum-epoxy dispersion curves. Considering the power and in and out-of-plane displacement, across the thickness of the structure, we note in Figure 3a, from a power point of view, most of the energy is in the skin. Hence, leakage into the adhesive is minimal making this a poor point to carry out an inspection. In Figure 3b, we see that considerable energy is propagating in the adhesive layer and is therefore a potentially a good point to carry out the inspection. The best points can be found by sweeping the frequency - phase velocity space on a structure with known bonding states.

Fig 3: Sample wave structure profiles for two different points on the dispersion curves in (Ux is in plane displacement and Wx out of plane).

SKIN-CORE DELAMINATION DETECTION STUDY

The RF waveforms shown in Figure 4 were obtained from the aluminum skin - aluminum core specimen shown in Figure 1 at frequency of 1 MHz and an incident angle of 45^o. Delamination was simulated by machining out the honeycomb core.

Fig 4: Sample RF waveforms from aluminum skin - aluminum core.

When the bond is good, the ultrasonic energy leaks into the core, resulting in a low amplitude signal as shown in Figure 4a. When the bond is bad, the guided wave travels with a high concentration of energy along the skin, resulting in a signal of significantly higher amplitude as shown in Figure 4b.

Figure 5 shows a potential calibration specimen for composite skin on top of an aluminum honeycomb core. At an angle of 25^o, frequency was swept from 0.4 to 1 MHz and the RF waveforms shown above were recorded. Due to the anisotropic nature of the composite skin, the transducer direction is critical since a new dispersion curve is necessary for each direction. See [3]. By tuning through angle and frequency while monitoring frequency, a mode with substantial energy at the skin-core interface was found. From 0.4 to 1.0 MHz, the majority of this mode is absorbed by the core as shown by the low amplitude signals. At 0.7 MHz a strong mode propagates in the skin but is significantly absorbed by the core. Note that as frequency is tuned up and down, the amplitude of the mode in the skin decreases until its amplitude is comparable to that in the non-delaminated zone. A change in wave velocity is also observable, via arrival time, in the good and bad regions. A significant increase in wave velocity occurs in the delaminated region since the core and adhesive are completely removed. This feature of wave velocity should always be checked but it may not always useful depending on the system being analyzed.

Fig 5: Composite skin - aluminum core specimen and sample RF waveforms.

Preliminary F-18 Rudder Delamination Detection Results

Guided wave based results shown below and are in good agreement with conventional C-scan data [2].

Fig 6: Sample RF waveforms and amplitude frequency spectra from good and bad zones on the F-18 rudder.

The RF waveform from the good zone in Figure 6 has a relatively low amplitude compared to that from the bad zone since most of the energy is absorbed by the rudder's core. In the bad zones considerably more energy propagates in the skin resulting in the reception of a higher amplitude mode. The same principle is illustrated in the frequency spectra showing that mode(s) in the 1.8 MHz range seep into the core when the skin is adequately bonded to the core and propagate with relatively high amplitude in the skin when the bond is poor.

Concluding Remarks

Guided wave based results show that detection of core - honeycomb structure delamination is possible in aluminum skin - aluminum core and composite skin - aluminum core structures. Theoretical modelling analysis of an aluminum/adhesive layer structure shows the effect that frequency and phase velocity tuning has on wave structure and group velocity. Amplitude and wave velocity are two potential signal features that could be considered for classifying good and bad areas.

Guided wave inspection for skin to honeycomb core weakness and delamination detection, compared to conventional methods, is easy to carry out, fast, inexpensive, reliable, and can be used on the actual aircraft itself without any disassembly being required. A guided wave system can be housed in a portable lunchbox PC data acquisition and analysis system and may be used for random location inspection. Software development for data acquisition control and analysis should focus on automated phase velocity and frequency tuning.

Acknowledgements

Thanks are given to Vinod Agarwala of the Naval Air Warfare Center and John Sedricks and the Office of Naval Research for their support of this project. Thanks are also given to Robert Mathers of the Naval Aviation Depot, North Island, San Diego, for his technical support and supplying the F-18 rudder and C-scan results.

REFERENCES

1. Rose, J.L., L. E. Soley, T. Hay, V.S. Agarwala, "Ultrasonic guided waves for hidden corrosion detection in Naval aircraft", CORROSION NACExpo 2000, 55th Annual Conference & Exposition, March 26-31, 2000 in Orlando, FL.
2. Rose, J. L., T. Hay, V.S. Agarwala, "Skin to honeycomb core delamination detection with guided waves", Fourth Joint DoD/FAA/NASA Conference on Aging Aircraft, May 15-18,200 in St. Louis, MO
3. Rose, J.L., Ultrasonics waves in solid media, Cambridge University Press, 1999.

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