A 1 MHz ultrasonic wave is used which corresponds to 
. The
experimental configuration for the generation and detection of the ultrasonic
guided waves is illustrated in Fig. 3. A piezoelectric transducer is used for
Figure 3: Principle of the generation and detection of guided waves.
R -- Rayleigh wave, B -- Bulk wave, G-Guided wave,
B.S. -- Optical beam splitter.
the generation of the surface wave. The presence of the edge of the adhesive layer splits the ultrasonic wave into several different types of waves, each having a different propagation velocity. A part of the energy in the Rayleigh wave is transformed into the desired guided wave. It propagates through the adhesive layer till it reaches its second edge. Part of the guided wave is now continuing as a surface wave which may be detected.
Remote detection of ultrasonic waves is most commonly achieved by means of an optical interferometer. For the present purpose one of the most common designs, the Michelson interferometer, is used. The probing beam is tightly focused on the surface of the specimen. In this two beam instrument a laser beam strikes a half-silvered mirror (beam splitter), which splits the beam into two paths. One beam reflects off the specimen, the other off the reference mirror. When the beams recombine, a single fringe occurs across the interference image in the case of parallel beams. Displacements on the surface of the specimen due to ultrasonic vibrations, change the length of the path traveled by the first beam, altering the relation between the two beams which in turn change the intensity of the light incident on the detector. The materials used in the experiments are identical to the materials described in the numerical calculations. Several surface treatments were implemented and two different thicknesses were examined for each type of surface treatment. Nine samples from each adhesion type and thickness were measured and the results were averaged. The various types of surface preparation of the adherends and their notation symbol, are summarized in Table 1.
| Type | Comments |
| Chromic acid anodization (A) | Unsealed chromic acid anodization in accordance with US MIL-B-8625 specification without primer |
| Chromic acid anodization | Unsealed chromic acid anodization in accordance with US MIL-B-8625 specification with primer BR-127, manufactured by the Bloomingdale department of American Cyanamid. The primer was applied by brushing to a thickness of approx. 2.5µm. |
| Tmura (T) | Commercial chromate conversion coating using solution no. 720 manufactured by Chemotas under license from Metallgesellschaft in accordance with US MIL-C-5541 specification. |
| Sand blasting (H) | . |
| Acetone cleaning (AZ) | Wiping the adherend with acetone. |
Two different adhesive thickness were fabricated for each surface treatment by choosing both 0 mm spacer thickness and a 0.1 mm spacer thickness. For each specimen, two signals were monitored and stored in memory. The first signal is the one immediately exiting the piezoelectric transducer, before the wave is arriving to the adhesive layer, called "Reference", and the second one is registered after the wave passed through the adhesive layer, called "After the bond". The notation symbol for every specimen is composed of three parts. The first part is the symbol of the surface treatment type, the second part denotes the spacer thickness, the third part refers to the counting index of nine similar bonds for every type of bond. The last part is constructed by a letter and a number like ij, where i=A,B,C and j=1,2,3. For example, the specimen denoted by A0A1 refers to the first (A1) sample of the anodizing bond type (A) with 0 mm spacer thickness. A typical ultrasonic wave received by the optical interferometer is presented in Fig. 4. The signal, after passing through the adhesive layer, has a smaller
Figure 4: Recorded ultrasonic signals.
amplitude due to scattering processes and attenuation associated with the
adhesive layer. The phase velocity of the wave can be directly measured from
Fig. 4, since there is only one mode in the system ( 
) and therefore
the group velocity is equal to the phase velocity.
The velocity is measured between the two recorded signals, the reference and the signal after the bond. There is some distance which the wave is passing as a Rayleigh wave on the Aluminum material. This time is taken into consideration after a calibration is made in which the exact velocity of the Rayleigh wave is measured. Two parameters are measured and calculated in this case, the relevant distance that the wave is propagating through, the length of the adhesive layer x, and the time of propagation through the adhesive layer t.
The ultrasonic signals that were received were also transformed by discrete Fourier transform (DFT) to the frequency plane. Before applying the DFT, the energy of the ultrasonic wave after the bond is equated to that of the reference. An example of the Fourier transform of the two signals is presented in Fig. 5. The frequency of the piezoelectric transducer is 1 MHz. The
Figure 5: Recorded ultrasonic signals DFT's.
Signal after passing through the adhesive layer is shifted towards the low frequencies. The adhesive layer acts as an low-pass filter.
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