Bond Strength Measurements by Ultrasonic Spectroscopy
The goal of this project has been to discover techniques for predicting the strength of a metal-to-metal adhesive bond from nondestructive measurements on the completed structure. Both the cohesive strength of the adhesive material itself and the adhesive strength of the metal-to-adhesive interface must be determined separately. In previous phases of the program, it was demonstrated that the Fourier transform of the ultrasonic echo returned from a metal-adhesive-metal sandwich structure immersed in a water bath contained sufficient information to obtain a prediction of the cohesive strength of the joint. Furthermore, certain features of the Fourier transform were shifted by thin layers of "different" materials at the metal-to-adhesive interfaces so that detecting poor adhesion was also a possibility. During the current phase of the program, more reliable mechanical tests and more accurate measurement techniques were developed. As a result, measurements of the wave velocity in FM-400 adhesive joints subjected to different degrees of cure correlated with the cohesive shear strength of the joints. Quantitative measurements of the standing wave resonant frequencies in Chemlok 304 adhesive joints showed a correlation with the strength of adhesion at the metal-to-adhesive interface.