MECHANISM OF THICK METAL WALLS PENETRATION BY HIGH-SPEED MICROPARTICLES

Purpose. Analysis and estimation of physical parameters which create conditions for microparticles penetration into metal microstructure to abnormally big depth. Methodology. Quantum mechanical three­site model has been used for studying the regularities of electron motion in the field of two Coulomb centres and numerical solution for the problem of the effect of external electrical charge on stability of the chemical bond. Solution was found for the equation of heat conductivity for estimating the temperature of microparticles heating under compression and acceleration by explosively driven accelerator. Stokes’s law was used for estimating viscosity of hypothetical medium which can be penetrated by microparticle at a great speed and to a great depth. The research was done with the help of X­ray microanalysis, X­ray crystallography, micrographic investigation, mass­spectrometry and electronic spectroscopy. Findings. Solution of the quantum mechanical model testifies that electric charges serve as catalysts responsible for the significant reduction of the energy barrier of chemical reactions. To ensure super deep penetration, it is necessary to achieve acceleration of a great number of microparticles in a special explosively driven accelerator. Heating, intensive stirring and friction result in electrification of the surface of the particles, which is known as triboelectric effect. The hypothesis about physical and chemical mechanism of particles penetration into metals resulting from high­speed impact has been put forward. Originality. The research has established relationship between the sizes of microparticles accelerated by explosion and the density of electric charges on their surfaces, as well as the depth of their penetration into the metal barrier. By experimental research, it was proven that maximum depth of microparticles penetration is directly proportional to the maximum density of surface charges for the particles of the 50…80 µm size. It is assumed that particles penetration into metals to greater depths is conditioned by the reduction of the barrier material viscosity in the zone of particle­barrier contact due to quantum mechanical effects in the solid­state plasma. Practical value. The value of the work includes creating a new generation of metal composites as well as new prospective technologies of reactive materials utilization

Numerical determination of the material properties of porous dust cakes

The formation of planetesimals requires the growth of dust particles through collisions. Micron-sized particles must grow by many orders of magnitude in mass. In order to understand and model the processes during this growth, the mechanical properties, and the interaction cross sections of aggregates with surrounding gas must be well understood. Recent advances in experimental (laboratory) studies now provide the background for pushing numerical aggregate models onto a new level. We present the calibration of a previously tested model of aggregate dynamics. We use plastic deformation of surface asperities as the physical model to bring critical velocities for sticking into accordance with experimental results. The modified code is then used to compute compression strength and the velocity of sound in the aggregate at different densities. We compare these predictions with experimental results and conclude that the new code is capable of studying the properties of small aggregates.Comment: Accepted for publication in A&

Detectability of Small Flaws in Advanced Engine Alloys

As advanced materials are introduced into aircraft engines, operating under higher temperatures at greater stresses for longer lifetimes, the need to detect small, internal defects becomes increasingly important. Ultrasound is often the preferred inspection tool because of its ability to penetrate to the interior of a component. However, sound energy reflected from microstructural features in the component produces a background inspection noise which is seen even when no defects are present. This noise can inhibit the detection of sound energy reflected from critical internal defects such as cracks, pores, or inclusions. The primary objectives of this work are to develop a quantitative understanding of the factors which influence the detectability of small defects in advanced engine alloys, and to lay the foundation for an engineering methodology to predict detection capabilities. To focus the work we concentrate on the specific problem of ultrasonically detecting hard-alpha inclusions in titanium aircraft engine alloys. These inclusions result from excess local concentrations of oxygen or nitrogen which occasionally occur during processing. Such impurities tend to occupy interstitial sites and cause excess brittleness. An engineering understanding of their detectability requires three elements: (1) knowledge of the strength of the backscattered noise signals associated with normal microstructural inhomogeneities such as grain and phase boundaries; (2) knowledge of the strength of the competing ultrasonic signal reflected by the inclusion; and (3) the use of this information to predict quantities which bear on the probability of detection. Major progress was made in each of these three areas. In the course of the project, three models were developed for the prediction of absolute noise levels in normal-incidence, pulse/echo, ultrasonic immersion inspections. These are identified as the Independent Scattering Model for Tone Burst pulse inspections (ISMTB), the Independent Scattering Model for Broad Band pulse inspections (ISMBB), and the Monte-Carlo noise Model (MCM). Each model assumes that the backscattered noise is primarily due to single scattering by the individual grains in the metal specimen, and the models consequently apply to low-noise materials. The observed absolute noise level in a given setting will depend upon the microstructure of the specimen, and upon the details of the measurement system (e.g., the transducer and pulsing unit used, the inspection waterpath, the amplifier gain settings, etc.) Both types of dependencies are incorporated into our models. In the ISMTB and ISMBB, the dependence on microstructure enters primarily through a Figure-of-Merit (FOM) for inherent noise severity, which is a property of the specimen alone and is determined by the density of grains and the average scattering capability of a single grain. These two models relate the FOM and measurement system parameters to average noise characteristics, such as the position-averaged root-mean-square (rms) noise level. Either the ISMTB or ISMBB can be employed in two distinct ways: to deduce the FOM of a specimen from measured noise signals; or to predict average absolute noise levels for various inspection scenarios when the FOM is known. For the MCM, the microstructural inputs are more detailed, but predicted noise properties are more detailed as well: e.g., both peak and average noise levels can be estimated. In our report we document the underlying assumptions and mathematical development for each noise model, and we report on extensive experimental studies carried out to validate the models. For single-phased, equiaxed, randomly-oriented metals it is possible to estimate the FOM from photographs of the microstructure and knowledge of the elastic constants. The FOM value so obtained can be directly compared to that deduced from our model-dependent analysis of backscattered noise. The two values are generally found to agree to within a factor of 2. This level of agreement is considered to be quite good since the noise model contains no adjustable parameters and the predicted average noise level is typically 50-60 dB below a measured front-surface reference signal. For two-phase commercial titanium alloys, it is not yet feasible to determine the FOM from microphotographs and related information. However, it is straightforward to determine the FOM by analyzing backscattered noise data. In such cases the deduced FOM is found to be approximately independent of the measurement system parameters, as expected. However, the FOM has been found to vary significantly from specimen to specimen in a suite of commercial alloys, and with direction within a given specimen. These variations are believed to originate from macrostructural details related to the processing history of the specimen. We describe how particular etching and photographic methods can be used to reveal this macrostructure, and how supporting data can be obtained from x-ray diffraction studies. To estimate the strength of ultrasonic signals reflected from hard-alpha inclusions, one requires a knowledge of how the elastic moduli and density of the inclusions differ from those of the host alloy in which they reside. Based on reviews of the literature plus additional experiments conducted as needed, the influence of interstitial oxygen and nitrogen on those properties has been determined. It is clear that, at solute concentrations of a few percent, there are sufficient changes in the moduli to produce significant ultrasonic signals from hard-alpha inclusions in single-phase microstructures. In two-phase titanium alloys, the hardening of the alpha phase can be accompanied by a conversion of the beta phase to alpha. When it occurs, this conversion may diminish the impedance difference between the inclusion and host metal, and consequently make ultrasonic detection more difficult. For specific, reasonable choices of hard-alpha properties, we have used previously developed models to predict absolute defect signal amplitudes for a range of inclusion diameters. These have been combined with noise model predictions to obtain estimates of signal-to-noise (SIN) ratios for hard-alpha inclusions in representative titanium alloys. Such calculations were performed for hypothetical inspections using both focussed and planar transducers. The calculations indicate that the SIN ratio is approximately inversely proportional to the width of the incident sound beam in the vicinity of the defect. Thus, defect detection can be substantially improved by properly focussing and scanning the beam in the interior of the component being inspected. In addition to performing illustrative SIN calculations, we have developed approximate formulas which allow rapid estimation of relative and absolute SIN ratios. These can be used to estimate the optimal choices of transducer diameter, focal length, and waterpath for inspecting a given region of the component\u27s interior. Much of the formalism developed in this work is fairly general in scope, and is consequently applicable to a wide range of defect-detection problems. Our noise models and associated formulas for SIN ratios can be readily extended to the case of normal-incidence inspection through a curved water/metal interface, and such extensions are currently in progress under different sponsorship. These developments will allow one to estimate SIN ratios for simulated inspections of cylindrical metal billets, and hence to determine the optimum inspection parameters. Straightforward extensions to oblique-incidence inspections are also feasible. The SIN ratio alone cannot be used to fully assess the probability of flaw detection (POD). POD calculations require a complete understanding of both mean noise levels, and the manner in which the noise varies about its mean. Although we have made a good start toward understanding and quantifying the relationship between average and peak noise levels, more work is required on that front. The above accomplishments may be summarized as follows: • Developed experimental techniques and data-acquisition software for measuring noise levels and noise spectra. • Developed three models for predicting absolute backscattered noise levels (and other noise characteristics) seen in a given UT inspection: ISMTB: For toneburst-pulse inspections. Predicts rms average noise level. ISMBB: For broadband-pulse inspections. Predicts rms average spectral components. Monte-Carlo Model: For arbitrary pulse types. Predicts typical noise wave-forms, and hence any average or peak noise characteristic. The first two models relate backscattered ultrasonic noise to a frequency-dependent material figure-of-merit (FOM) and to details of the measurement system. • Performed numerous tests of the models using noise data gathered from specimens with simple microstructures (Cu, Stainless Steel, Alpha-Ti) and from titanium alloy specimens supplied by engine manufacturers. • Demonstrated (in collaboration with J. H. Rose) that the FOM could be predicted from first principles for single-phased, equi-axed, randomly-oriented microstructures. • Measured and analyzed noise data from representative Ti-6246 specimens. Noted strong dependence of noise level on direction of sound propagation in some specimens. • Used x-ray diffraction to investigate the sources of the noise anisotropy. It is now thought to arise from localized texture within the boundaries of prior beta grains . • Prepared specimens containing artificial hard-alpha material (oxygen contaminated case layer, nitrogen contaminated volumes). Measured sound speeds. Analyzed results using theories based on rule of mixtures . • Demonstrated how hard-alpha detectability could be assessed by combining noise models with models which predict echoes from defects. • Predicted signal-to-noise ratios for focussed and planar transducer inspections of embedded hard-alpha inclusions of various diameters. We have thus developed a firn scientific foundation for understanding the interrelationship of material and measurement parameters in determining the detectability of small flaws, particularly hard-alpha inclusions in titanium alloys. The next steps in the application of this knowledge should include: 1.) more extensive validation studies in collaboration with potential users; 2.) their use of the models to optimize the design of inspection systems for billets and other components where normal-incidence inspection is preferred; and 3.) use of the models to improve the quantification of detection reliability based on an analysis of field and laboratory data

Material characterization of nickel-based super alloys through ultrasonic inspection

Nickel-based super alloys are widely used in aircraft engine components, mainly in turbine disks and the high-pressure compressor. Extensive data on the mechanical properties of these alloys is available. However, fundamental data relating micro-structural features to ultrasonic properties, in turn controlling the inspectability, are not available. Knowledge of these relationships would facilitate improvements in current Ni billet inspections. A single scattering model has been found to be very effective in simulating the effects of backscattered noise on the ultrasonic inspection of billets and forgings engine alloy materials. The work on this dissertation is focused on evaluating the effectiveness of the single scattering model in predicting backscattered noise in the nickel-based alloys. Attenuation, grain size and single-crystal elastic constants are important input parameters that need to be evaluated for making the predictions of noise levels. Determination of these quantities on a number of nickel-based super alloy samples is reported and studied how noise levels predicted with the current single scattering model compare with the direct measurements of noise on the same samples;In Chapter 1, experimental investigations of the relationships between ultrasonic properties (velocity, attenuation, and backscattered grain noise FOM) and the microstructure are reported. A close relationship was established between the measured attenuation and the Noise FOM, a result consistent with classical theories for backscattering and attenuation but inconsistent to observations in titanium. Chapter 2 describes a quantitative study of the relationship of the experimental observations reported in Chapter 1 to the predictions of classical theories, which require as input the single-crystal elastic constants of the grains and the grain size. The single-crystal elastic constants for these alloys are unknowns and an effort to infer the same for the nickel-based super alloys from ultrasonic measurements is reported. In Chapter 3, a detailed study of the accuracy of the single scattering model to predict the grain noise generated by the microstructure was carried out. Detailed measurements and model calculations were carried out to quantify the breakdown of the single scattering model the comparison of theory and experiment provided strong evidence for the presence of multiple scattering effects

Investigating the mechanism of deep-focus earthquakes via in-situ acoustic emission experiments on Fe2SiO4 at high temperature and pressure

In subduction zones, earthquakes are thought to be associated with faulting that arises from phase transformations. In order to test the viability of this mechanism experimentally, it was necessary to make microseismic measurements while the mineral under investigation was subjected to the pressure and temperature (P,T) conditions corresponding to their environment at depth. A system has been developed capable of making in situ acoustic emission (AE) measurements on samples under P,T conditions representative of the upper mantle and transition zone. Experiments were performed in a 3000-ton multi-anvil press using an 18/11 octahedral cell with 6 piezoelectric transducers mounted on the rear side of the anvils. AE signals were collected at a sampling rate of 40 MHz using a triggered system and a data buffer for capturing full waveforms of AE events. The use of multiple transducers distributed in a microseismic array allowed for events to be located within the sample through manual or automatic arrival time picking and least squares inversion techniques. Uncertainty in location estimates was ~1mm. The multi-anvil apparatus constitutes an inherently noisy environment both acoustically and electrically, therefore methods of noise reduction were developed. This technique has been used to measure acoustic signals generated from the fracturing of quartz beads during high pressure deformation and to investigate the possibility that the phase transformation from olivine to spinel, known to occur in subduction zones, is associated with deep-focus earthquakes (300 – 690 km depth). The analog material fayalite (Fe2SiO4) was examined. Information about its synthesis and sintering is discussed. Results of AE experiments on samples under high pressure (P = 4-9 GPa) and high temperature (T = 773-1273 K) conditions in the spinel stability field, while experiencing deviatoric stress, showed acoustic events that locate within the sample in multiple experiments defined by the P,T envelope P = 3.8 – 8.4 GPa and T = 650 – 950 K. This is the first time an olivine→spinel structured transition in a silicate mineral has demonstrated macroscopic faulting and associated microstructures, as well acoustic activity, under conditions that would normally promote plastic deformation. The system was also used to detect liquid↔solid phase transformations in Hg by measuring the abrupt change in sound velocity due to the intrinsic change in velocity between phases, and a change in delay between the triggering of an amplitude threshold and the arrival of the waveform

Insights into the short-term tidal variability of multibeam backscatter from field experiments on different seafloor types

Three experiments were conducted in the Belgian part of the North Sea to investigate short-term variation in seafloor backscatter strength (BS) obtained with multibeam echosounders (MBES). Measurements were acquired on predominantly gravelly (offshore) and sandy and muddy (nearshore) areas. Kongsberg EM3002 and EM2040 dual MBES were used to carry out repeated 300-kHz backscatter measurements over tidal cycles (~13 h). Measurements were analysed in complement to an array of ground-truth variables on sediment and current nature and dynamics. Seafloor and water-column sampling was used, as well as benthic landers equipped with different oceanographic sensors. Both angular response (AR) and mosaicked BS were derived. Results point at the high stability of the seafloor BS in the gravelly area (2 dB and 4 dB at 45° respectively. The high-frequency backscatter sensitivity and short-term variability are interpreted and discussed in the light of the available ground-truth data for the three experiments. The envelopes of variability differed considerably between areas and were driven either by external sources (not related to the seafloor sediment), or by intrinsic seafloor properties (typically for dynamic nearshore areas) or by a combination of both. More specifically, within the gravelly areas with a clear water mass, seafloor BS measurements where unambiguous and related directly to the water-sediment interface. Within the sandy nearshore area, the BS was shown to be strongly affected by roughness polarization processes, particularly due to along- and cross-shore current dynamics, which were responsible for the geometric reorganization of the morpho-sedimentary features. In the muddy nearshore area, the BS fluctuation was jointly driven by high-concentrated mud suspension dynamics, together with surficial substrate changes, as well as by water turbidity, increasing the transmission losses. Altogether, this shows that end-users and surveyors need to consider the complexity of the environment since its dynamics may have severe repercussions on the interpretation of BS maps and change-detection applications. Furthermore, the experimental observations revealed the sensitivity of high-frequency BS values to an array of specific configurations of the natural water-sediment interface which are of interest for monitoring applications elsewhere. This encourages the routine acquisition of different and concurrent environmental data together with MBES survey data. In view of promising advances in MBES absolute calibration allowing more straightforward data comparison, further investigations of the drivers of BS variability and sensitivity are required