Experimental Studies Pertaining to the Interaction of Ultrasound with Metal-Metal Bonds

Abstract

INTRODUCITONThe 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 (A.) of the interrogating sound is large compared to the characteristic defect size, the quasi-static distributed-spring (DS) model applies [1]. 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 [2]. 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-vsfrequency (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.In the second section, we consider optimized oblique-incidence reflection measurements in two classes of diffusion-bonded metal specimens supplied to us by aircraft engine manufacturers. Experiments on four bonded Ti-6Al-4V specimens were carried out to ascertain the ability of low-frequency reflection measurements to distinguish contaminated and uncontaminated regions of diffusion bonds. Experiments on four IN100 specimens were performed to similarly gauge the ability of such measurements to distinguish bonds created using different temperature-pressure-upset recipes. In the final section we conclude with a brief summary of our fmdings.

50673 Metal-metal bondline NDE methods

“…Thus, this method can be used to determine the ultrasonic reflection coefficient whenever the scattering amplitude is known, and so can be used, in principle, for any type of flaw and for both longitudinal and shear wave modes. Some experimental tests of this approach were reported by Margetan, et al (7).…”

Nondestructive Testing System to Assess Lack-Of-Bond in Brazed Generator Coils by Ultrasonic Retro-Reflection

“…Margetan et al investigated the problem of assessing the integrity of diffusion bonds using reflected ultrasound at oblique incidence [1,2]. They presented a quasi-static distributed spring model to derive the ultrasonic reflectivity of an imperfectly-bonded interface as a function of frequency and angle of incidence.…”