Short Standoff Distance Research Articles

Effect of water jet geometric parameters on rock fracturing

Fluid-driven rock fracturing has been attempted in the fields of mining and civil engineering. This type of rock fracturing is initiated by applying high-pressure water to target rock. In this study, the fracturing characteristics of hard granite specimens were explored through experimental tests using a high-pressure water jet system. Various tests were performed to evaluate the effects of several geometric parameters: standoff distance, indentation depth, and free surface condition. The fracture depth, radius, volume, and angle were measured, and then analyzed to investigate the fracturing characteristics. In addition, we prepared a theoretical estimate of maximum shear stress via shear stress analysis using a crack-tip stress field model. In these experimental and theoretical studies, efficient rock fracturing was demonstrated with a combination of short standoff distance, deep indentation, and nearby free surfaces. Meanwhile, the fracture angle for infinite free surface boundary specimens was identified as a unique value regardless of the standoff distance and indentation depth.

Phase composition and microstructure of yttrium aluminum garnet (YAG) coatings prepared by suspension plasma spraying of Y2O3–Al2O3 powders

Yttrium aluminum garnet (Y3Al5O12, YAG) is a promising high-temperature ceramic. YAG coating was deposited using the suspension plasma spraying process. The phase composition, microstructure, and hardness properties of the as-deposited coatings were investigated. X-ray diffraction analysis confirmed that YAG is the major phase with a small amount of YAP and YAM phase. SEM characterize results indicate that high power and short standoff distance will result in a compact coating with high hardness and low porosity, and with the Vickers hardness of 739HV. On the contrary, increasing standoff distance and decreasing torch power will lead to form more re-solidify spherical particles and unmelted powder, respectively.

Quantification and Taxonomy of Pores in Thermal Spray Coatings by Image Analysis and Stereology Approach

Porosity is one of the most important microstructural features in thermal spray coatings and has been actively studied and measured by many methods. Image analysis techniques have become popular techniques in determining porosity in coatings because of simplicity, accessibility, and an ability to measure both open and closed porosities as well as pore characteristics such as size, shape, orientation, and spatial distribution. In the current study, an image analysis technique has been complemented by several stereology procedures to determine the porosity level and characteristics of pores within coatings. Stereology protocols such as Delesse, DeHoff, and Cruz-Orive analyses were used to derive the porosity level, pore size, and shape distributions, and the effectiveness of each stereology protocol was compared. Standoff distance (SOD) and annealing process did not alter the distribution trend of number of pores but influenced the distribution of pore volume fractions significantly. The bivariate size–shape distribution of the pores was used to predict the dominant pore type and fractions of pores that arose from different formation mechanisms. It was found that nearly spherical pores that originated from gas bubbles and entrapped gas pockets dominate at shorter SOD, while the different types of pores become more evenly distributed when the SOD was increased.

Effect of standoff distance on the reconstruction of in-duct velocity field and regeneration of pressure field

Identification of in-duct acoustic source characteristics is essential in the design of fluid machinery system for reducing and predicting the flow-generated noise. To this end, the inverse estimation method can be employed by using the measured sound field and matrix formulation for wave propagation within a duct. In this paper, the effect of the distance between source and measurement plane is investigated. At each standoff distance, pressures are measured at three planes with two different spacings to widen the estimation frequency range, and measurements are conducted with three different standoff distances. Modal decomposition is applied to estimate modal amplitudes, and the result is used to reconstruct the velocity field at the source plane and to obtain the regenerated pressure field at the measurement planes. It is shown that the modal amplitude identified by measured pressure field at the short standoff distance, i.e., at nearfield, can yield an accurate reconstructed velocity field of the source and regenerate the pressure field with smaller error, which is similar to the other inverse techniques such as equivalent source method and nearfield acoustical holography. A field reduction example by suppressing some parts of source velocity field is shown for demonstrating the effectiveness of the method.

セラミック溶射皮膜構造評価法としてのキャビテーション・エロージョン試験の有効性

The microstructure of plasma sprayed ceramic coatings is characterized by the existence of splats along with other morphological features such as interlamellar and globular pores, intrasplat microcracks and splat boundaries. These various process-dependant micro- defects extremely influence physical and mechanical properties such as thermal conductivity and elastic modulus and then influence coating behavior. In this approach, the process involved in the cavitation erosion of plasma sprayed ceramic coatings have been studied using the ultrasonic vibratory technique. The weight loss occurring during fifteen minute intervals of exposure to cavitational erosion was determined, the surface of specimens and the wear debris were examined by Scanning electron microscopy (SEM). The results suggest that the improvement in coating performance could be achieved by controlling the effective contact between lamellae of which the coating is made up using the plasma spray conditions, such as sufficient preheat, short standoff distance and slow gun traversing movement.

Verification of Nondimensional Energy Spectrum-Based Blast Design for Reinforced Concrete Members through Actual Blast Tests

A method has been developed for the design of reinforced concrete (RC) members when subjected to blast loadings based on the nondimensional energy spectrum. Numerical analyzes showed good agreement between the approximate responses of RC members designed to their respective levels of target response. This paper further verifies this method through the actual response of a RC column and beam subjected an actual denotation of explosives at a short standoff. When subjected to blast loading from a short standoff distance, the cantilever column behaved predominantly in direct shear. Thus, ways to mitigate the damage of a cantilever column should draw focus on the robustness. The developed design procedure provides a feasible method for the blast resistant design of members, subjected to blast loads at a short standoff distance.

Effect of standoff distance on coating deposition characteristics in cold spraying

In this study, the effect of standoff distance on coating deposition characteristics in cold spraying in cold spraying was investigated by the experiment and numerical simulation of particle acceleration. Al, Ti and Cu powders of different sizes were used as feedstocks. It was found that the deposition efficiency was decreased with the increase of standoff distance from 10 mm to 110 mm for both Al and Ti powders used in this study. However, for Cu powders, the maximum deposition efficiency was obtained at the standoff distance of 30 mm, and then the deposition efficiency decreased with further increasing the standoff distance to 110 mm. The standoff distance had a little effect on coating microstructure and microhardness for these three powders. Both the stain-hardening effect of the deposited particles and the shot-peening effect of the rebounded particles take the roles in coating hardness. It was also found that the surface of substrate or previously deposited coating could be exposed to a relatively high gas temperature at a short standoff distance.

Analysis of water ice and water ice/soil mixtures using laser-induced breakdown spectroscopy: application to Mars polar exploration.

Recently, laser-induced breakdown spectroscopy (LIBS) has been developed for the elemental analysis of geological samples for application to space exploration. There is also interest in using the technique for the analysis of water ice and ice/dust mixtures located at the Mars polar regions. The application is a compact instrument for a lander or rover to the Martian poles to interrogate stratified layers of ice and dusts that contain a record of past geologic history, believed to date back several million years. Here we present results of a study of the use of LIBS for the analysis of water ice and ice/dust mixtures in situ and at short stand-off distances (< 6.5 m) using experimental parameters appropriate for a compact instrument. Characteristics of LIBS spectra of water ice, ice/soil mixtures, element detection limits, and the ability to ablate through ice samples to monitor subsurface dust deposits are discussed.

Composite Reinforcement to Strengthen Existing Concrete Structures against Air Blast

The Wright Laboratory Air Base Survivability Section has been studying the development, application, and effects of externally applied composite reinforcing materials. The strengthened facilities would be capable of surviving an air-blast load at relatively short stand-off distances (11–15 m). An agreement was reached with the Israeli officials to conduct full-scale explosive tests in Israel using 860 kg of TNT on structures that had been reinforced externally with composite reinforcing materials. The strengthening procedure employed in this study involved two types of material: (1) an autoclave-cured, three-ply, carbon fiber-epoxy laminate; and (2) a knitted biaxial E-glass fabric. The Air Base Survivability Section applied the composite materials in Israel after the facilities had been constructed using an epoxy adhesive to bond the composite materials to the concrete substrate. This provided a simple, effective, and quick method of retrofitting an existing structure. The free-field and reflected pressures and accelerations on the walls were measured. The results of these tests were considered successful, considering the fact that the externally reinforced walls suffered high displacements, yet did not fail. The pressure and impulse data indicate that both structures would have failed catastrophically without the externally applied composite reinforcing materials.

INVESTIGATION OF FACTORS AFFECTING THE SHAPED CHARGE PERFORMANCE

An analytical model has been developed to investigate the shaped charge phenomena. In this paper, the analytical model is only used to predict the influence of each shaped charge factor on jet penetration process through a metallic target. The main factors are: (i) liner thickness, (ii) liner material, (iii) cone angle, (iv) type of explosive, and (v) stand-off distance between the charge base and target surface. An experimental program has been conducted to determine the effects of some shaped charge factors on its performance. These factors are: (a) liner material, (b) type of explosive, and (c) distance between charge base and target surface. Six shaped charges have been prepared and exploded at different distances from a steel target surface. For each tested shaped charge, the experimental measurements are concerned with the determination of the total depth of penetration and crater radius at the target surface. For each shaped charge factor, both the measured depth of penetration and crater radius at the steel target surface are compared with the corresponding predicted results of the model. Moreover, representative samples of the model predictions concerning with the effect of each factor on the shaped charge performance, using the data of some tested shaped charges, are presented and discussed. The present results show that the depth of penetration increases with the increase of: (i) liner thickness up to a value of 0.05 the charge base diameter at short standoff distances, (ii) density of liner material at short standoff distances, (iii) power of used explosive, and (iv) standoff distance. In addition, the crater radius at the target surface increases with the increase of power of used explosives, and decreases with the increase of density of liner material at short standoff distances

Chamber-Protection Issues for Near-Target Experiments in NIF

The National Ignition Facility (NIF) will support multiple user groups who will use NIF's unique capability to generate intense pulses of x-ray, neutron and gamma radiation from non-ignition and ignition targets. Contamination of the final-optics debris shields by target, near-target, and remobilized first-wall debris will determine the types of experiments that can be fielded. Some of these experimental packages will require target-facing surfaces that nearly enclose the target. Due to the short stand-off distances of these surfaces, x-ray ablation inside these confined spaces will generate conditions in NIF quite similar to those envisioned for future Inertial Fusion Energy (IFE) target chambers like HYLIFE. The design of these NIF experiments provides an excellent opportunity to apply the analytical target-chamber design tools the IFE research community has created, and in a synergistic way, these experiments will in turn provide a rich source of experimental data for IFE target-chamber research.

A Study of the Erosive Mechanism on Metals by a High Pressure Water Jet.

In this paper, the erosive mechanism of metals caused by a high-speed water jet is discussed through several experiments that check closely connected factors. The influence of dynamic jet velocity fluctuation on material erosion was investigated first. Next, the rising trend of temperature in the target materials by jet pressure and the noise spectrum near the target surface were also investigated, connecting with ability to cause erosion. Finally, the relation between the mass loss of erosion and shearing strength of materials is discussed by mass loss curve arranged (eroded volume) × (shearing strength). Thereby, in the short standoff distances, shearing strength of materials was the most important factor to estimate the mass loss.

Single-beam diode-laser technique for optical path-length measurements

A simple single-beam technique employing radio-frequency modulation of a tunable diode laser with homodyne demodulation is demonstrated as a means of measuring optical path lengths. This technique offers a straightforward method for determining path lengths traversed through optical multipass cells or performing optical range-finding over short (i.e., tens of meters) standoff distances. The radio-frequency phase-sensitive nature of the technique permits narrow-band detection and high signal-to-noise ratios, even when range-finding measurements are made with range resolutions of «1 m. This compares favorably with traditional short-pulse, wide-bandwidth optical range finders.