Interface waves traveling along the boundary between two solids have been studied for nearly a century. However, little attention has been given to the case where interface waves travel at the boundary between a soft and stiff solid and when the soft material is relatively light and viscoelastic. In this paper, the characteristics of interface waves that propagate along a soft-stiff boundary are described. These waves are similar to a leaky Rayleigh-like wave on the stiff solid in terms of the wave velocity and displacement wave structure. Analytical and finite element models are used to model and simulate wave propagation. An example problem of bond evaluation for coatings on metal structures is considered. Experiments on 2.5 cm thick steel plate with 2.5 cm viscoelastic coatings show good agreement to models. Additionally, the results of models and experiments show several promising features that may be used to evaluate bonds in a non-destructive evaluation approach.
The interfacial shear behavior in near-equiatomic NiA1 reinforced by sapphire filaments has been examined at room temperature using a fiber pushout test technique. The loaddisplacement data indicate a large variability in the initial interface failure stress, although reverse push behavior indicates a comparatively constant interfacial sliding friction stress. The observed behavior suggests that the presence of asperities on the fiber surfaces and nonuniformities in fiber diameter require constrained plastic flow within the NiAl matrix in order for interfacial shear to occur. The location, shape, severity, and distribution of fiber asperities as well as the uniformity of fiber diameter are critical to the interfacial shear process.
The interface between an anisotropic composite material and a metallic material is inspected for disbonded regions using ultrasonic guided waves. The material properties of the composite and metal have been tailored to demonstrate their effect on inspectability. The material properties have been designed to be either favorable or unfavorable to the existence of propagating Stoneley waves. Stoneley waves can exist because the layer thicknesses are large enough compared to the wavelength to be considered half-spaces. The existence of Stoneley waves between generally anisotropic materials depends on the elastic constants and densities in a complicated way. The range of material properties that allow Stoneley waves is small; however, when the vertically polarized shear wave speeds are similar in the two materials, the existence of Stoneley waves is generally possible. If the conditions do not strictly allow Stoneley waves, other interface waves can still exist such as leaky waves. Disbonds are inspected using interface waves in a finite element simulation. Sensitivity to disbonds is determined and thus inspectability is demonstrated for cases that are favorable and unfavorable to Stoneley waves.
The C-130 Hercules has been a workhorse for the U.S. Air Force for decades and is projected to continue to accrue flight hours for years to come as a highly capable platform utilized for numerous mission profiles. In the past few years, many C-130 aircraft have developed fatigue cracks in the center wing box rainbow fittings. These cracks can be a significant flight safety risk, when the damage is extensive. In order to detect the existence of cracks in the rainbow fittings so that they can be repaired, implementation of labor intensive eddy current nondestructive inspection technology has been used to assess the condition of each aircraft on a scheduled basis. The Air Force is interested in the development and application of alternative fault detection techniques that are less time demanding to implement by the maintainers, while providing a high damage detection probability. Additionally, the future capability to embed effective rainbow fitting fault detection capability into the aircraft as an integrated systems health management (ISHM) solution is also desired.ARL Penn State has evaluated acoustic emission (AE) technology for detection, fault localization and fault severity indication of cracks in the rainbow fittings of C-130 aircraft. The AE technology involves monitoring for the emission of high frequency vibration (> 100,000 Hz) as an existing structural defect (crack) is stressed from the static loading of the wing. Testing has been conducted on undamaged rainbow fittings at the component level in the laboratory and on a damaged rainbow fitting at the platform level on aircraft. The preliminary analysis results indicate that this technology could provide an effective indication for crack detection, and localization, that is less time and labor intensive to implement than other conventional technologies. The purpose of this paper is to provide the project background information, describe the capability of the Eddy Current and AE technologies and show the analysis results from AE testing. The presentation will also discuss how the technology could be implemented as an onboard ISHM solution for aging C-130 aircraft.
The ability to predict the remaining lifetime of a mechanical system is an area that, although it is not presently realised, commands considerable attention from both commercial and military organisations. Effective machinery prognostic systems hold the promise of being able to reduce maintenance costs, improve safety margins, increase mission readiness and reduce waste through implementation of retirement for cause maintenance programmes. Critical to this objective is the ability to estimate the time required for fault initiation and to track the progression of these failure precursors. Since even modern manufacturing methods cannot create truly flawless materials, the notion of flaw initiation is one of semantics. Flaws are lnherent in the material, thus the objectives should be to detect, identify and track the progression of flaws with increased accuracy. Therefore, it is generally accepted' that improvements in the prediction of remaining
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