Damage Detection Sensitivity, Specificity and Classification Data Analysis for SHM Systems Design, Verification and Validation

In this paper, we consider applying derived knowledge base regarding the sensitivity and specificity of damage(s) to be detected by an SHM system being designed and qualified. These efforts are necessary toward developing capabilities in SHM system to classify reliably various probable damages through sequence of monitoring, i.e., damage precursor identification, detection of damage and monitoring its progression. We consider the particular problem of visual and ultrasonic NDE based SHM system design requirements, where the damage detection sensitivity and specificity data definitions for a class of structural components are established. Methodologies for SHM system specification creation are discussed in details. Examples are shown to illustrate how the physics of damage detection scheme limits particular damage detection sensitivity and specificity and further how these information can be used in algorithms to combine various different NDE schemes in an SHM system to enhance efficiency and effectiveness. Statistical and data driven models to determine the sensitivity and probability of damage detection (POD) has been demonstrated for plate with varying one-sided line crack using optical and ultrasonic based inspection techniques.

A Method for Delineation of Bone Surfaces in Photoacoustic Computed Tomography of the Finger

Photoacoustic (PA) imaging of interphalangeal peripheral joints is of interest in the context of using the synovial membrane as a surrogate marker of rheumatoid arthritis. Previous work has shown that ultrasound (US) produced by absorption of light at the epidermis reflects on the bone surfaces within the finger. When the reflected signals are backprojected in the region of interest, artifacts are produced, confounding interpretation of the images. In this work, we present an approach where the PA signals known to originate from the epidermis are treated as virtual US transmitters, and a separate reconstruction is performed as in US reflection imaging. This allows us to identify the bone surfaces. Furthermore, the identification of the joint space is important as this provides a landmark to localize a region-of-interest in seeking the inflamed synovial membrane. The ability to delineate bone surfaces allows us to identify not only the artifacts but also the interphalangeal joint space without recourse to new US hardware or a new measurement. We test the approach on phantoms and on a healthy human finger.

Effect of W addition on the electrical switching of VO2 thin films

Vanadium Oxide has been a frontrunner in the field of oxide electronics because of its metal-insulator transition (MIT). The interplay of different structures of VO2 has played a crucial role in deciding the magnitude of the first order MIT. Substitution doping has been found to introduce different polymorphs of VO2. Hence the role of substitution doping in stabilizing the competing phases of VO2 in the thin film form remains underexplored. Consequently there have been reports both discounting and approving such a stabilization of competing phases in VO2. It is reported in the literature that the bandwidth of the hysteresis and transition temperature of VO2 can be tuned by substitutional doping of VO2 with W. In this work, we have adopted a novel technique called, Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) to deposit VO2 and W- doped VO2 as thin films. XRD and Raman spectroscopy were used to investigate the role of tungsten on the structure of VO2 thin films. Morphology of the thin films was found to be consisting of globular and porous nanoparticles of size similar to 20nm. Transition temperature decreased with the addition of W. We found that for 2.0 at % W doping in VO2, the transition temperature has reduced from 68 degrees C to 25 degrees C. It is noted that W-doping in the process of reducing the transition temperature, alters the local structure and also increases room temperature carrier concentration. (c) 2016 Author(s).

Structural Evolution and Property Changes in Nd_{60}Al_{10}Fe_{20}Co_{10} Bulk Metallic Glass During Crystallization

The structural evolution and property changes in Nd60Al10Fe20Co10 bulk metallic glass (BMG) upon crystallization are investigated by the ultrasonic method, x-ray diffraction, density measurement, and differential scanning calorimetry. The elastic constants and Debye temperature of the BMG are obtained as a function of annealing temperature. Anomalous changes in ultrasonic velocities, elastic constants, and density are observed between 600–750 K, corresponding to the formation of metastable phases as an intermediate product in the crystallization process. The changes in acoustic velocities, elastic constants, density, and Debye temperature of the BMG relative to its fully crystallized state are much smaller, compared with those of other known BMGs, the differences being attributed to the microstructural feature of the BMG.

Detection and Characterization of Long-Pulse Low-Velocity Impact Damage in Plastic Bonded Explosives

Damage not only degrades the mechanical properties of explosives, but also influences the shock sensitivity, combustion and even detonation behavior of explosives. The study of impact damage is crucial in the vulnerability evaluation of explosives. A long-pulse low-velocity gas gun with a gas buffer was developed and used to induce impact damage in a hot pressed plastic bonded explosive. Various methods were used to detect and characterize the impact damage of the explosive. The microstructure was examined by use of polarized light microscopy. Fractal analysis of the micrographs was conducted by use of box counting method. The correlation between the fractal dimensions and microstructures was analyzed. Ultrasonic testing was conducted using a pulse through-transmission method to obtain the ultrasonic velocity and ultrasonic attenuation. Spectra analyses were carried out for recorded ultrasonic signals using fast Fourier transform. The correlations between the impact damage and ultrasonic parameters including ultrasonic velocities and attenuation coefficients were also analyzed. To quantitatively assess the impact induced explosive crystal fractures, particle size distribution analyses of explosive crystals were conducted by using a thorough etching technique, in which the explosives samples were soaked in a solution for enough time that the binder was totally removed. Impact induces a large extent of explosive crystal fractures and a large number of microcracks. The ultrasonic velocity decreases and attenuation coefficients increase with the presence of impact damage. Both ultrasonic parameters and fractal dimension can be used to quantitatively assess the impact damage of plastic bonded explosives.

Microstructure and Evolution of Mechanically-Induced Ultrafine Grain in Surface Layer of Al-Alloy Subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Two critical crack propagating velocities for PMMA fracture surface

Ultrasonic fractography and scanning electronic microscopy (SEM) are used to determine the direct relationship between the fracture surface morphology and the main crack velocity during the rapid rupture of polymethylmethacrylate (PMMA). Two critical crack velocities are found for the fracture. Quasi-parabolic markings will appear when the crack speed exceeds the first critical speed. Crack propagating at speed above the second critical speed leaves a thicket of small branches penetrating the surface behind them. Both critical speeds are functions of the thickness of the specimens.

Polymers with aligned carbon nanotubes: Active composite materials

We review the current state of the polymer-carbon nanotube composites field. The article first covers key points in dispersion and stabilization of nanotubes in a polymer matrix, with particular attention paid to ultrasonic cavitation and shear mixing. We then focus on the emerging trends in nanocomposite actuators, in particular, photo-stimulated mechanical response. The magnitude and even the direction of this actuation critically depend on the degree of tube alignment in the matrix; in this context, we discuss the affine model predicting the upper bound of orientational order of nanotubes, induced by an imposed strain. We review how photo-actuation in nanocomposites depend on nanotube concentration, alignment and entanglement, and examine possible mechanisms that could lead to this effect. Finally, we discuss properties of pure carbon nanotube networks, in form of mats or fibers. These systems have no polymer matrix, yet demonstrate pronounced viscoelasticity and also the same photomechanical actuation as seen in polymer-based composites. © 2008 Elsevier Ltd. All rights reserved.

Damage Evolution In Cement Mortar Due To Erosion Of Sulphate

Ultrasonic technique is used to detect the velocity change of stress wave propagated in the cement mortar immersed in the solution of sodium sulfate for 425 days. Also the density change of specimens at different erosion time is measured. By curve fitting, the effect of solutions' concentration and water/cement ratio on the damage evolution is analyzed. The SEM observation on the growth of delayed ettringite is also performed. It shows that the damage evolution of specimens attacked by sulphate solution is dominantly induced by the nucleation and growth of delayed ettringite, and the average size of microvoids in cement mortar affects the damage evolution significantly. (c) 2008 Elsevier Ltd. All rights reserved.

Microstructure, deformation and failure of polymer bonded explosives

Polymer bonded explosives (PBXs) are highly particle filled composite materials comprised of explosive crystals and a polymeric binder (ca. 5-10% by weight). The microstructure and mechanical properties of two pressed PBXs with different binder systems were studied in this paper. The initial microstructure of the pressed PBXs and its evolution under different mechanical aggressions were studied, including quasi-static tension and compression, ultrasonic wave stressing and long-pulse low-velocity impact. Real-time microscopic observation of the PBXs under tension was conducted by using a scanning electron microscope equipped with a loading stage. The mechanical properties under tensile creep, quasi-static tension and compression were studied. The Brazilian test, or diametrical compression, was used to study the tensile properties. The influences of pressing pressures and temperatures, and strain rates on the mechanical properties of PBXs were analyzed. The mesoscale damage modes in initial pressed samples and the samples insulted by different mechanical aggressions, and the corresponding failure mechanisms of the PBXs under different loading conditions were analyzed.