Abstract
Ultrasonic spectroscopy utilizes the principle of construc- tive and destructive wave interferences to obtain information about ultrasound in the frequency domain. For applications in the nondes- tructive evaluation (NDE) of adhesive-bonded structures, destructive interference of the pulsed sound waves at the boundaries of the adhesive bond layer produces spectroscopic signals characteristic of the bond. The bondline thickness can be determined accurately from the fre- quency minima in the spectrum. The width of the antiresonance dips at half-power points and the amplitude of the dips are related to the acoustical properties at the interfaces of the adhesive layer. The amplitude ratio of the RF signals reflected from the boundary surfaces of the adhesive layer in the time domain is also affected by the acous- tic impedance mismatch at the boundaries. A combination of these measurements in the frequency and time domains characterize the adhesive properties between the adherend and the adhesive. The ex- perimental set-up consists of a wide-band pulser/receiver as an energy source, a highly damped piezoelectric sensor serving as the transmitting and receiving transducer, and a digital computer for waveform digitiza- tion and Fourier transform. Single lap-shear adhesive-bond specimens using three types of adhesive systems were used in a correlation study between bond strengul and NDE parameters from the ultrasonic spectroscopy method. An empirical bond strength/NDE parameters relationship established from the correlation study has been justified by analytical calculation. The experimental procedures, analytical derivations of the sound wave interactions in the layered media, and the statistically valid correlation of the bond strength with NDE parameters is presented. obtained by using the ultrasonic spectroscopy technique wlll be presented. The bondline thickness can be measured ac- curately by means of reflected spectra using a known value of the velocity of sound. An anlytical calculation of the sound wave interference in the bondline is presented. This calcula- tion takes into consideration the attenuation in the adhesive and relates the attenuation coefficients to the antiresonances in the frequency spectra. A brief presentation of the experi- mental setup precedes the results obtained on single lap-shear specimens. Discussions on the potentials of the ultrasonic spectroscopy technique and conclusions on the presentation are included. 11. ANALYTICAL CALCULATION For the normal incidence of compressive ultrasonic waves on an attenuating layer, the wave numbers in the media will, in general, be complex. The complex form of the wave numbers was presented by Brekhovskikh (5) as
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