Modeling for Ultrasonic Health Monitoring of Foams with Embedded Sensors

In this report analytical and numerical methods are proposed to estimate the effective elastic properties of regular and random open-cell foams. The methods are based on the principle of minimum energy and on structural beam models. The analytical solutions are obtained using symbolic processing software. The microstructure of the random foam is simulated using Voronoi tessellation together with a rate-dependent random close-packing algorithm. The statistics of the geometrical properties of random foams corresponding to different packing fractions have been studied. The effects of the packing fraction on elastic properties of the foams have been investigated by decomposing the compliance into bending and axial compliance components. It is shown that the bending compliance increases and the axial compliance decreases when the packing fraction increases. Keywords: Foam; Elastic properties; Finite element; Randomnes

Magnetic states of granular layered CoFe-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

The granular layered magnetic system Co80Fe20(t)/Al2 O3 (3 nm), where the Co80Fe20 layers of nominal thickness t form separate, almost spherical magnetic granules of typical diameter 2-3 nm between the Al2O3 spacers, was studied. We discuss measurements of the dc and ac magnetic susceptibility χ for 1 n

An Ultrasonic Method for Simultaneous Determination of Elastic Moduli, Density, Attenuation and Thickness of a Polymer Coating on a Foil

Ultrasonic spectroscopy has for a long time been thought promising for characterization of thin layers immersed in water or embedded between two known materials (similar or dissimilar) [1]. Significant effort has been put forth by many authors [2]–[14]. An ultrasonic spectroscopy method for determination of the complete set of acoustical and geometrical properties of an isotropic layer embedded between two known materials (similar or dissimilar) has been developed recently in [15]. The uniqueness of the method is its ability to determine all the layer properties - thickness, density, elastic moduli and attenuation (longitudinal and shear) - at two incident angles: one normal and one oblique.</p

On the Relationship Between Ultrasonic and Micro-Structural Properties of Imperfect Interfaces in Layered Solids

The interaction of ultrasonic waves with interfaces formed by two non-conforming, rough surfaces in contact has been the subject of numerous investigations [1–10]. The motivations behind these studies have been various: from the assessment of the real area of contact between two rough surfaces [1], to the modeling of crack closure near the tip of a fatigue crack [4]; from the identification of the nature of interfacial imperfections in kissing and partial bonds [6], to the generation of ultrasonic waves [8]. In most of these studies, the characterization of the interfacial properties has been attempted by studying the reflection of longitudinal and shears waves at normal incidence. Only recently, the problem concerning the interaction of ultrasonic waves with realistic complex systems such as that formed by two neighboring imperfect interfaces has been addressed. Lavrentyev and Rokhlin [9, 10] used ultrasonic spectroscopy to evaluate the interfacial conditions from the spectra of longitudinal and shear waves reflected normally from the interfaces.</p

Nondestructive materials characterization: with applications to aerospace materials

With an emphasis on aircraft materials, this book describes techniques for the material characterization to detect and quantify degradation processes such as corrosion and fatigue. It introduces readers to these techniques based on x-ray, ultrasonic, optical and thermal principles and demonstrates the potential of the techniques for a wide variety of applications concerning aircraft materials, especially aluminum and titanium alloys. The advantages and disadvantages of various techniques are evaluated. An introductory chapter describes the typical degradation mechanisms that must be considered and the microstructure features that have to be detected by NDE methods. Finally, some approaches for making lifetime predictions are discussed. It is suitable as a textbook in special training courses in advanced NDE and aircraft materials characterization

Time Resolved Line Focus Acoustic Microscopy of Composites

Acoustic microscopy has been used to measure material properties since the 1980s [1–4]. The velocity of the leaky surface wave can be accurately determined from the V(z) curve which is formed by the interference between the leaky surface wave and specular reflection. By fitting the leaky wave velocity or the V(z) curve itself, Kim et al. [4] reconstructed the material properties (elastic constants and mass density). Another approach is time-resolved acoustic microscopy [5–8]. In this method, the leaky surface wave and the specular reflection are separated in the time domain and the velocity is determined from the time of flight. For a graphite/epoxy composite, due to the complexity of the reflected signal and the absence of Rayleigh wave excitation, it is impractical to determine material properties from the V(z) curve. In time-resolved acoustic microscopy, the different reflection signals are separated in the time domain and the velocity measurement is simplified. For graphite epoxy composite materials, due to their low density and significant fluid loading, the acoustic microscopy response is significantly different from that for higher density materials. To model the time domain acoustic microscopy response for a mutilayered composite, we applied the global matrix method in the form similar to that of Mal [9], thus avoiding the numerical instability at high frequency.</p

Nanoscale Viscoelastic Characterization Using Tapping Mode AFM

The tapping mode atomic force microscopy (AFM) has been widely used as a tool to image sample surfaces [1–3]. It has been modeled as a single degree-of-freedom nonlinear oscillator [4–11], In this model, the tip-sample interactions are described by contact theory with adhesion (Johnson-Kendall-Roberts (JKR) theory) [12–14]. The viscoelasticity is considered as a friction force by adding a damping constant. Magonov and Elings [15] presented experimental results which show different phase sensitivity for stiff and soft samples. Anczykowski et al.[16] presented results on amplitude vs. tip-sample separation and showed the existence of hysteresis due to nonlinearity and the transition between attractive and repulsive forces. Kuhle et al [17] demonstrated experimentally the frequency response hysteresis and pointed out the effect of attractive force on this hysteresis using a linear interaction force approximation.</p

Microradiographic Characterization of Pitting Corrosion Damage and Fatigue Life

Due to the increasing number of aging aircraft, the inspection and assessment of corrosion and fatigue damage on airframe structures is of major concern [1]. Among the different types of corrosion damage, pitting corrosion is an important factor in triggering widespread fatigue crack initiation and reduced fatigue life. Therefore, the ability to predict surface crack origination from pitting corrosion is necessary for timely maintenance of aging aircraft.</p

In-Situ Surface Acoustic Wave Monitoring of Fatigue Crack Initiation and Propagation

Material degradation due to pitting corrosion and fatigue crack initiation from pits may trigger widespread fatigue damage in aging aircraft structures. Since corrosion damage is often hidden, the corrosion pits and initiated cracks can remain undetected by surface inspection techniques for long service periods. Therefore, the ability to predict the initiation and propagation of surface damage originating from pitting corrosion is of great importance for the timely maintenance of aging airplanes [1]. Recently, we reported a microradiographic method quantifying pitting corrosion [2] and fatigue life prediction model [3] based on the measured pit depth. These have been applied successfully to A1-2024-T3 alloy.</p