Acoustic Nonlinearities in Adhesive Joints

Abstract

Ultrasonic techniques have been used successfully to measure important bond parameters and to detect various defects in adhesive joints for about twenty years. Recent reviews of nondestructive testing of adhesively bonded structures can be found in the literature [1–3]. For direct strength assessment, the reliability of these techniques leaves much to be desired. Linear acoustic parameters are only indirectly correlated to material and bond strength, therefore we must rely on dubious empirical relations between the measured parameter (e.g., velocity or attenuation) and the sought strength parameter on a case-to-case basis. On the other hand, it is well known that failure of most materials and bonds is usually preceded by some kind of nonlinear mechanical behavior, well before appreciable plastic deformation occurs, i.e. within the range of nondestructive testing. This macroscopic nonlinearity is due to a number of different causes such as weakening of covalent bonds with increased atomic spacing, reduction in the number of these bonds, etc. It seems to be reasonable to assume that nonlinear parameters measured at approximately 10–20% of the ultimate stress level are more directly correlated to mechanical strength than linear ones measured at negligibly low ultrasonic amplitudes: