Experimental Studies Pertaining to the Interaction of Ultrasound with Metal-Metal Bonds
Thompson, R. Bruce
The problem of deducing the integrity of a diffusion bond using reflected ultrasound is a difficult one, since many defect structures can be imagined which reflect sound in essentially similar fashions but have different failure consequences When an imperfect bond consists of a near-planar distribution of small defects, and the wavelength (λ) of the interrogating sound is large compared to the characteristic defect size, the quasi-static distributed-spring (DS) model applies . This model can serve as a useful tool for interpreting signals reflected from imperfect bonds, and for choosing inspection parameters to optimize the inspection of a given bonded structure . The DS model predicts that the low-frequency specular reflection properties of an imperfect bond are solely determined by a small number of inertia and stiffness constants which characterize the defect layer. Since many defect distributions can give rise to the same mass and stiffness constants, low-frequency specular reflection measurements can provide only incomplete knowledge of the distribution, even when the defect type is known. For example, if the bond plane contains a sparse distribution of identical flat circular cracks, such reflectivity measurements can determine only the product of the crack area fraction and the crack diameter [2,3]. Recent theoretical work employing an independent scattering (IS) model [4,5] predicts that resonance peaks in reflectivity-vs-frequency (R-vs-f) curves, which occur when A. and the average defect diameter are similar, can be used to size the defects. Resonance information could then be used to complement the low-frequency specular reflection data, thus permitting the determination of both average defect diameter and total defect area fraction. In the present work we report on several experiments carried out to examine the feasibility of this procedure. Specifically, we report on measurements of R-vs-f for longitudinal sound waves at normal incidence carried out in three model systems containing near-planar distributions of defects: cylindrical inclusions in water; cylindrical voids in iron; and spherical inclusions in plastic. The measurements are compared with the predictions of the DS and IS models. It is found that the IS model accurately predicts the frequency of the first peak in the R-vs-f curve, which is directly releated to the size of the defects.