·Table of Contents
·Workshop - Guided Wave
Skin to Honeycomb Core delamination Detection with Guided WavesJoseph L. Rose and Thomas Hay
The Pennsylvania State University
Engineering Science & Mechanics Department
University Park, PA 16802
in the aircraft industry .
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 .
|Fig 1: Example of the guided wave inspection setup|
|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).|
|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 25o, 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 . 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.|
|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.
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.
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