Detection of bondline delaminations in multilayer structures with lossy components

Abstract

The detection of bondline delaminations in multilayer structures using ultrasonic reflection techniques is a generic problem in adhesively bonded composite structures such as the Space Shuttle's Solid Rocket Motors (SRH). Standard pulse echo ultrasonic techniques do not perform well for a composite resonator composed of a resonant layer combined with attenuating layers. Excessive ringing in the resonant layer tends to mask internal echoes emanating from the attenuating layers. The SRM is made up of a resonant steel layer backed by layers of adhesive, rubber, liner, and fuel, which are ultrasonically attenuating. The structure's response is modeled as a lossy ultrasonic transmission line. The model predicts that the acoustic response of the system is sensitive to delaminations at the interior bondlines in a few narrow frequency bands. These predictions are verified by measurements on a fabricated system. Successful imaging of internal delaminations is sensitive to proper selection of the interrogating frequency. Images of fabricated bondline delaminations are presented based on these studies

Review of the Adhesively Bonded Interface in a Solid Rocket Motor

“…However, some problems still exist due to the acoustic impedance mismatch between the metal case and rubber. The normal beam compression wave is used to address these problems [71,72]. Recently, an oblique ultrasonic pulse echo method with focused probe in immersion [73] was developed to inspect the adhesive quality in multi-layered structure.…”

Properties of an electrostatic transducer

“…[12][13][14] Theoretical models can be used to predict the response of materials, and to search for defects by looking at the effects these have on the vibrational response. 14 One such method is the matrix transfer approach, as detailed by Brekhovskikh 15 and used by Bresse et al 16 This method calculates the transmission coefficient as a function of frequency when the structure is excited by an infinite sinusoidal plane wave in air at an arbitrary angle to the structure's surface.…”

Ultrasonic inverse scattering of multidimensional objects buried in multilayered elastic background structures

“…Thus, we want to solve the so-called inverse problem, which consists in imaging and quantitatively characterizing the object by measuring its impact on the propagation of elastic waves. More specifically, our interest lies in background structures made of layers with markedly dissimilar elastic properties, which is typical of the ultrasonic inspection of pipes or bondlines [2], [3]. The origin of this work stems from applications in the oil well-logging industry, where the goal is to make high resolution, high accuracy images of relatively "soft" material (such as pockets of fluid) that are behind a relatively "hard" resonant layer (such as the steel casing used to prevent an oil well from collapsing).…”