Erosion of MgO by solid particle impingement at normal incidence

A study has been made of the erosion of almost fully dense, fine-grained MgO by millimeter-scale WC-6 wt% Co spheres impinging at normal incidence with velocities between 10 and 90 m sec^–1. Scanning electron microscopy showed that the damage consisted of a central crater surrounded by an array of intergranular radial and/or median and/or lateral cracks characteristic of an elastic-plastic impact. The crater had a thin lining of plastically deformed material, but appeared to have been formed primarily by localized transgranular and integranular fracture processes, suggesting that any mode of irreversible deformation in the contact region will suffice to produce the changeover from Hertzian cracking to radial, median and lateral cracking. The accompanying gravimetric studies showed that mass loss, which was caused primarily by intersection of lateral cracks with the free surface and with radial and/or median cracks, increased threefold during the short incubation period in which the as-received surface evolved into its steady-state eroded condition. During this period the exponent relating erosion to impact velocity decreased asymptotically towards a value smaller than that predicted by any current theory of erosion in the elastic or the elastic-plastic impact regime. Nor are these theories any more successful at explaining the observed particle-size dependence of the erosion of polycrystalline MgO.     

Role of surface oxides in modifying solid particle impact damage

Abstract

In this paper the interaction between impacting particles and a growing surface oxide scale is examined for conditions pertinent to high temperature erosion. The impact condition is analysed to predict the impact damage morphology and oxide fracture and hence high temperature erosion behaviour. The importance of oxide scale thickness relative to the impacting particle size is highlighted as the critical parameter in determining scale fracture and the onset of plastic damage to the target surface.

MST/1177     

High-speed testing and material modeling of unfilled styrene butadiene vulcanizates at impact rates

High-speed tensile tests were performed on unfilled SBR strip and sheet specimens at room temperature. Uniaxial dynamic stress-extension ratio curves indicated three distinct regions of rate-dependent behavior when strain rates were below 180 s^–1, between 180–280 s^–1and above 280 s^–1. With increasing strain rate, the toughness increased in the first region, remained roughly constant in the second region, and decreased in the third region. Time-temperature shift on SBR near the glass transition temperature used to obtain high strain rate tensile strength at room temperature did not give the same results as those found in the impact tensile test. The dynamic toughness was used to predict failure of rubber sheets under impact loads using ABAQUS Explicit. Predicted values of the sheet extension at the onset of failure were within 10% of experimental values.     

Granule attrition by coupled particle impact and shearing

A novel device has been developed for continuous shearing and repeated impact of granules in order to simulate granule attrition and dust formation under realistic plant conditions of mechanical stresses, shear strains and strain rates. The device subjects the granules to multiple impacts at a range of velocities prevailing in typical process plants, and to shear deformations using two rollers with an adjustable gap to simulate the level of shear stresses and strains experienced during bulk motion, e.g. discharge from silos onto conveyor belts, etc. In this paper, the device operation and tests carried out to determine the settings required for attaining a desired impact velocity and shear strain rate are described. Subsequently, the extent of breakage of the granules is determined for the specified settings and the results are compared with data obtained by more established methods, e.g. annular shear cell and single particle impact tests.     

Effect of solid particle impact on light transmission of transparent ceramics: Role of the microstructure

Sand erosion was done on soda lime glass and transparent ceramics such as alumina and magnesium-aluminate spinel with different microstructures. Surface roughness and optical transmission were measured before and after erosion. The increase of surface roughness depends on both the hardness and grain size of the material. Nearly no surface degradation occurs on polycrystalline samples with HV3 > 15 GPa. The decrease of the real in-line transmittance (RIT) after sand blasting is linked to the increase of surface roughness. We have found that this RIT decrease is correlated to three parameters: incident light wavelength, nature of the material (mechanical properties like hardness) and material microstructure. The influence of these will be discussed. Finally, for all polycrystalline ceramics and single crystals, the RIT is only slightly or not altered after sand blasting either at IR or visible wavelengths.     

Damping of dynamic effects with elastomers in instrumented impact testing

The efficiency of four silicon elastomers as damping materials was studied in high rate (2.9 m/s) instrumented impact testing. The measurements were done on injection molded PP specimens. Dynamic effects could be efficiently reduced by all four silicon rubbers. Mechanical damping leads to smooth force versus deflection correlations, which considerably facilitates the determination of valid fracture mechanics characteristics. Damping does not influence the maximum force measured during fracture, K_Ic is independent of rubber type and thickness. Since the damper consumes considerable energy, G_Ic is significantly modified by damping, the effect depends both on the viscoelastic properties and the thickness of the damper. The approach proposed earlier for the correction of energy could be applied in all cases where a load versus deflection trace void of oscillations was registered. Similarly to K_Ic, corrected G_Ic values proved to be completely independent of the conditions of damping, i.e. the type and thickness of the damper. The parameters of the non-linear constitutive equation which was used to describe the deformation behavior of the damper could not be related to properties determined by simple measurements (hardness, modulus, rebound elasticity, etc.).     

A simple model for debris clouds produced by hypervelocity particle impact

A simple debris cloud model is developed by considering the one dimensional shock wave motion in the material together with the catastrophic fragmentation theory by Grady. The model provides a simple method for calculating the velocities at the outer perimeter of the cloud and the average particle size in the cloud.     

The impact of microelectronics on particle detection

Progress in microelectronics has an impact on silicon detector manufacturing technology and on detector system design. Noise performance, miniaturization and lower cost of hybrid or integrated front-end electronics enable thinner, segmented silicon detectors to be used for a number of new applications.     

Single particle impact damage of fused silica

Single-impact damage of fused silica by spherical and conical tungsten carbide (WC) projectiles of velocities up to 200 m sec⁻¹ has been investigated with a high-speed framing camera photographing at a rate of up to 1.7×10⁶ frames per second. For spherical particles the Hertzian cone cracks, which for higher impact velocities are accompanied by median and radial cracks, form during the loading part of the impact; some growth of all these cracks also occurs during unloading. With the conical particles the Hertzian cone cracks do not form; only radial and median cracks form during the loading; in this case both radial and median cracks grow during the unloading. In both cases “lateral” cracks form during unloading. From these experiments values of the static equivalent of the dynamic stress-intensity factor for high-velocity cracks are also obtained; these are found to be considerably lower than those obtained from quasi-static indentation experiments. Finally, the extent of the damage produced by a single impact has been discussed.     

Characterisation of elastomers fracture behavior by energetic parameters

The knowledge of fracture behavior of elastomers necessitates the comprehension of crack initiation and propagation phenomena which pose difficulties related to the deformation of elastomers. The reliability of elastomer materials is linked to their resistance to rupture. This resistance can be evaluated using the global approach of fracture mechanics. The objective of this work is to numerically analyze by finite element method the characterization of rupture behavior of these materials on the basis of energetic parameters. Consideration is given to the evolution of the deformation energy density to quantify the energy of tear of a real material identified by hyper-elastic material models.     

Fractography of unfilled and particulate-filled epoxy resins

The objective of this work was to analyse and understand the types of fracture surface morphology found in unfilled and particulate-filled epoxy resins in the light of the thermomechanical history of the specimen (loading rate or duration of loading, temperature, strain at break). Short-term tensile tests and long-term creep tests were conducted at four different temperatures. The fracture surface features were analysed using the scanning electron and optical microscopes and, where suitable, an image analyser. In order to correlate these morphologies with certain regimes of crack velocity, fracture mechanics tests were also conducted, varying the crack speed between 10^–7 and 10² m sec^–1. In the case of the filled resin, the lifetime under static loading is governed by a phase of slow, sub-critical crack growth which is manifested by resin-particle debonding. Thereafter, the crack accelerates and finally may reach terminal velocities depending on the amount of stored elastic energy available at the moment of fracture.     

Single particle fragmentation in ultrasound assisted impact comminution

Impact fragmentation is the underlying principle of comminution milling of dry, bulk solids. Unfortunately the outcome of the fragmentation process is more or less determined by the dimensionality of the impactor and its impact velocity. Since fragmentation is dominated by interfering shock waves, manipulating traveling shock waves and adding energy to the system during its fragmentation could be a promising approach to manipulate fragment mass distributions and energy input. In a former study we explored mechanisms in impact fragmentation of spheres, using a three-dimensional Discrete Element Model (DEM) Carmona et al. (Phys Rev E 77:051302, 2008). This work is focused on studying how single spheres fragment when impacted on a planar vibrating target.     

Motion of a solid particle in a rotating potential flow

It is shown that the process of stabilizing the motion of a particle in a rotating potential flow takes the form of aperiodic damped oscillations.     

Response of space structures to orbital debris particle impact

All long-duration space and aerospace and transportation systems, such as the Space Station Freedom and the Space Shuttle, are susceptible to impacts by pieces of orbital debris. These impacts occur at high speeds and can damage the flight-critical systems of such spacecraft. Therefore, the design of a structure that will be exposed to a hazardous orbital debris environment must address the possibility of such hypervelocity impacts and their effect on the integrity of the entire structural system. A technique is developed for analyzing the response of dual-wall structures to oblique Hypervelocity projectile impact. Ballistic limit curves that predict the potential of an impacting projectiles to perform the main wall of a dual-wall strucutral system are obtained using the techniques and are compated against experimentally derived curves. Comparisons are performed for a variety of impact velocities, trajectory obliquities and projectile masses. It is shown that the results obtained using the technique developmed herein compare very well with experimetanl results.     

The effects of flow rate and particle shape on the convective diffusion to a solid catalytic particle

Summary The flow of a fluid, containing a reactant, past a solid catalytic particle on which a reaction takes place is considered for large Péclet number. The concentration of the reactant is given by the diffusion boundary-layer equation, and this is solved in the case when the rate of reaction on the particle surface and the rate of diffusion of reactant onto the surface are of the same order of magnitude.For a spherical particle, a series solution for the concentration is found for the case of Stokes flow, and numerical solutions are found for Stokes flow and for flow at higher Reynolds numbers (up to Re=10). To examine the effect of particle shape, numerical solutions are found for prolate and oblate spheroids in Stokes flow.     

Multiscale modeling of solid propellants: From particle packing to failure

We present a theoretical and computational framework for modeling the multiscale constitutive behavior of highly filled elastomers, such as solid propellants and other energetic materials. Special emphasis is placed on the effect of the particle debonding or dewetting process taking place at the microscale and on the macroscopic constitutive response. The microscale is characterized by a periodic unit cell, which contains a set of hard particles (such as ammonium perchlorate for AP-based propellants) dispersed in an elastomeric binder. The unit cell is created using a packing algorithm that treats the particles as spheres or discs, enabling us to generate packs which match the size distribution and volume fraction of actual propellants. A novel technique is introduced to characterize the pack geometry in a way suitable for meshing, allowing for the creation of high-quality periodic meshes with refinement zones in the regions of interest. The proposed numerical multiscale framework, based on the mathematical theory of homogenization, is capable of predicting the complex, heterogeneous stress and strain fields associated, at the microscale, with the nucleation and propagation of damage along the particle–matrix interface, as well as the macroscopic response and mechanical properties of the damaged continuum. Examples involving simple unit cells are presented to illustrate the multiscale algorithm and demonstrate the complexity of the underlying physical processes.     

Velocity recovery factors of a particle repelled from a solid surface

On the basis of the published experimental data, recovery factors of velocity components of microparticles impacting a target heuristic expression, which are suitable for a wider class of pairs of materials of colliding bodies, have been proposed. The considered range of values of impact velocities is bounded on the side of both low velocities (since the adhesive forces are neglected) and higher velocities (at which irreversible deformation of the particle and/or the target occurs). __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 38–44, September–October, 2007.