·Table of Contents
·Aeronautics and Aerospace
A new approach to the inspection of Bonding Quality of Composites of Aircraft Radome and EmpennageRong S Geng
(Beijing Aeronautical Technology Research Centre, Beijing 100076, China)
Keywords: Composite, Plate Wave, Radome, Non-destructive Testing(NDT)
Apparatus based on sound impedance change was used to test bonding quality. It is noted that although the apparatus was effective in finding unbond or disbond for aluminum skin composite, it was not effective for composites with skins of glass fibre and/or carbon fibre reinforced plastics. Also, the instrument is viable to external circumstance change because it uses a single parameter of amplitude to diagnose the bond quality, and it hence has very low diagnostic reliability. In order to solve the problem, new approach using multiple parameters to inspect composite fault is needed.
|Fig 1: cross section of composite plate|
Supposing that a pulse stress of F0f(t) is applied to the surface of a plate of thickness 2d (see figure 1), then the velocity response at a distance z from the source can be expressed as;
Where, m is the shear modules, cl and ct are longitudinal and shear wave velocities of the solid plate material, respectively. For particle velocity at surface, z in equation (1) can be replaced by 2d or by a parameter closely related with 2d, the thickness of the plate. It can be assumed that response at surface is dependent on plate thickness 2d, the amplitude of applied force and shear module of the plate. The effect of thickness on signal amplitude and phase for longitudinal wave is different from that for shear wave. It can be seen from (1) that thickness d has a much larger effect on shear wave than on longitudinal one. The flexual wave (mainly shear wave) is, therefore, always preferable in using phase-amplitude diagram approach. In general however, large plate thickness will lead to weak response for both waves, and vise verse.
For a thin plate and steady input of sinusoidal signal, instead of longitudinal and shear wave, the main waves propagating in it is the lowest symmetric (S0) and asymmetric(A0) modes respectively, depending on the initial state of excitation. Wave velocities for the two waves are[2,3]
respectively, where E is Young's module, n Poisson ratio, v angular frequency, r density and d half plate thickness. For the purpose of practice, perpendicular excitation is the most likely one to be employed, and hence the dominant mode of existing wave in the plate is the asymmetric one (the flexual wave), see fig.2. Possible modes which can exist in the plate are dependant on the multiply of plate thickness and frequency. For fixed frequency, when thickness is increased, possible modes are also increased, resulting in further decrement of the amplitude of the lowest asymmetric mode. Based on this, an instrument was designed to use planar phase-amplitude diagram of the received response signal to give an intuitive picture of the bonding conditions, details of which will be described later in this paper.
|Fig 2: possible modes inside an aluminum plate（S for symmetric, A for asymmetric） x: frequency×d; y: phase velocity|
|Fig 3: schematic diagram showing working principle of low-frequency ultrasound bond flaw detector (A0 mode dominant excitation)|
For certain plate material, plate wave velocity is a function of frequency and plate thickness. If the excitation amplitude is kept fixed and thickness is varied, it can be made that the lowest asymmetric mode is dominant through whole frequency range. In fact, due to different propagation velocities, the extension wave (symmetric mode) can be filtered out through adding a gate of delay trigger in the receiving circuit.
In reality, T- and R- probes are enclosed in a rigid compartment and hence the distance between the two probes are fixed. Due to perpendicular excitation being applied to radome surface, flexual wave is always predominant inside the plate. Instead of pulse force excitation, a swept frequency burst of sinusoidal wave of 8-12 complete cycles was applied to T-probe, with its frequency being varied from fl to fh, say from 2kHz to 70 kHz. The change was made in 256 steps in a fixed time of less than 1 second. For each fixed frequency, a vector (x,y), where x stands for amplitude and y for phase, was obtained from the output signal of R-probe, which was displayed as a point on the phase-amplitude plane of the monitor screen. For swept frequency input, a phase-amplitude diagram can be obtained from the 256 sweeping points, which was closely related to bonding conditions. While disbond or unbond occurs, the "equivalent" plate becomes thinner and the energy of received signal becomes larger. Hence, a large phase-amplitude diagram will be displayed on the screen. The other way to achieve the same results is to use frequency-modulated sinusoidal burst signal to excite the T-probe instead of present swept frequency one. Comparisons of the two different approaches were omitted here.
The schematic electronic diagram of the set-up was shown in fig.4, where the burst signal of sinusoidal wave applied to T-probe was supplied by frequency synthesizer, and the received signal, after being properly conditioned, was digitized by A/D converter and further processed by CPU. The entire system was controlled by the logic controller and the CPU.
|Fig 4: Schematic electronic diagram of the apparatus|
System software flow chart is shown in Fuig.5, and all menu is written in Chinese for the convenience of field application.
|Fig 5: main flow chart of the detector|
Fig.6 is the photography of thus developed inspector. It is a fully menu driven and smart instrument. Most operation programs and procedures available for existing aircraft can be stored in it.
|Fig 6: Photography of bond quality inspector|
|Fig 7: phase-amplitude diagrams for carbon fibre composite (a) good bonding (b) disbonding |
Test was also carried out for horizontal empennage of an airplane. Two separate disbonding areas were found. These faults are not fatal but much care is needed for the aircraft empennage to prevent any possible accident in the future. These tests show that so developed inspector is capable for field inspection of aircraft composite conditions.
The instrument also can easily be modified to become an impedance measuring system which is useful in many cases for evaluating internal faults of composite, such as corrosion, crushed honeycomb core etc. For brevity, detailed descriptions on these will not be given here.
|Fig 8: evaluate bonding structure by using separated probes|
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