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.).     

Multiscale modelling of particle debonding in reinforced elastomers subjected to finite deformations

Interfacial damage nucleation and evolution in reinforced elastomers subjected to finite strains is modelled using the mathematical theory of homogenization based on the asymptotic expansion of unknown variables. The microscale is characterized by a periodic unit cell, which contains particles dispersed in a blend and the particle matrix interface is characterized by a cohesive law. A novel numerical framework based on the perturbed Petrov–Galerkin method for the treatment of nearly incompressible behaviour is employed to solve the resulting boundary value problem on the microscale and the deformation path of a macroscale particle is predefined as in the micro‐history recovery procedure. A fully implicit and efficient finite element formulation, including consistent linearization, is presented. The proposed multiscale framework is capable of predicting the non‐homogeneous micro‐fields and damage nucleation and propagation along the particle matrix interface, as well as the macroscopic response and mechanical properties of the damaged continuum. Examples are considered involving simple unit cells in order to illustrate the multiscale algorithm and demonstrate the complexity of the underlying physical processes. Copyright © 2005 John Wiley & Sons, Ltd.     

Fracture of elastomers by cavitation

Cavitation phenomenon is studied in rubber-like materials by combining experimental, theoretical and numerical approaches. Specific tests are carried out on a Styrene Butadiene Rubber to point out main characteristics of cavitation phenomenon. Hydrostatic depression is numerically modelled using finite element method. Numerical results are compared to Ball’s and Hou & Abeyaratne’s models with regard to cavity nucleation in the material. Both models well fit experimental observations suggesting that the cavitation nucleation in elastomers depends on the confinement degree of the specimen. Finally, critical hydrostatic pressure and critical global deformation are proved to govern cavitation nucleation in the studied material. Critical loadings are identified by comparing experimental and numerical load–displacement curves.     

The influence of particle content on the equi-biaxial fatigue behaviour of magnetorheological elastomers

The equi-biaxial fatigue behaviour of silicone based magnetorheological elastomers (MREs) with various volume fractions of carbonyl iron particles ranging between 15% and 35% was studied. Wöhler curves for each material were derived by cycling test samples to failure over a range of stress amplitudes. Changes in complex modulus (E¹) and dynamic stored energy during the fatigue process were observed. As for other elastic solids, fatigue resistance of MREs with different particle contents was shown to be dependent on the stress amplitudes applied. MREs with low particle content showed the highest fatigue life at high stress amplitudes while MREs with high particle content exhibited the highest fatigue resistance at low stress amplitudes. E¹ fell with the accumulation of cycles for each material, but the change was dependent on the particle content and stress amplitude applied. However, each material failed in a range suggesting a limiting value of E¹ for the material between 1.22 MPa and 1.38 MPa regardless of the particle content and the magnitude of the stress amplitude. In keeping with results from previous testing, it was shown that dynamic stored energy can be used to predict the fatigue life of MREs having a wide variation in particle content.     

Simulation of particle bed breakage by slow compression and impact using a DEM particle replacement model

The present work introduces a particle replacement model implemented in the commercial software EDEM to describe breakage of particles. Several model parameters were initially estimated on the basis of single-particle breakage tests on iron ore pellets. The model was then used to simulate breakage of particle beds by both slow compression and impact. Model predictions were compared to experiments in terms of compressive force versus packing density, breakage probability of the particles versus compressive force applied to the bed, and the product size distribution in compression and impact. The model showed the expected trends as well as some agreement with the measured product size distributions both from confined and unconfined stressing conditions of the bed of particles, being a simple and effective approach to describe breakage in systems where particles are stressed as assemblies.     

基于颗粒动力学演化的磁弹体力磁耦合数值模型

基于颗粒动力学演化的磁致微观结构建立了横观各向同性磁弹体（MREs）三维几何模型,在考虑了磁场和变形耦合作用的基础上,依据当前MREs研究较热的两种磁颗粒作用模型构建了颗粒的控制方程,从而建立MREs多颗粒的力磁耦合数值模型,从细观角度研究MREs的力磁耦合性能。数值模型和剪切实验对比表明,点偶极子作用力模拟的MREs磁流变效应远低于实验数据,而多极作用力在量级上更接近实验数据。基于构建的数值模型,还详细探究了磁感应强度和颗粒浓度对磁致剪切模量的影响,模拟结果和实验趋势吻合较好,颗粒体积分数在20%附近时,相对磁流变效应达到最大。     

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.     

Erosion mechanisms of aluminium nitride ceramics at different impact angles

In this paper, erosion wear behaviour of aluminium nitride (AlN) ceramics is studied. The influence of particle hardness and shape on erosion of the AlN surface is examined. The effect of varying the impingement angle on the weight loss and the roughness parameters of AlN ceramics testing sample is also determined. Therefore, erosive wear behaviour of AlN ceramics was investigated using SiC and SiO₂ particles as erodents, at following impact angles: 30°, 45°, 60°, 75° and 90°. Scanning electron microscopy (SEM) was used to analyze the eroded surfaces in order to determine erosion mechanisms. The roughness parameters (R_a, R_z and R_max), before and after erosion with SiO₂ and SiC particles at 30° and 90° angles of impingement, respectively, were determined using a profilometer. It was found that the impact angle is influencing the erosion wear of the AlN ceramics and maximum erosion takes place at impact angle of 90°. The results indicate that hard, angular SiC particles cause more damage than softer, more rounded SiO₂ particles.     

Review of ultrafine particle generation by laser ablation from solid targets in gas flows

A comparative review is given of laser ablation generation of ultrafine particles (UFPs: size range about 1–100 nm) in gas flows carried out by different research groups. Common features of the laser ablation deposition (LAD) method obtained for different materials (mainly metals or oxides) and lasers are discussed. Special emphasis is given to CO₂-LAD of oxide UFPs (see (1)) for elucidation of basic mechanisms of particle formation and the LAD process.     

Impact dynamics with applications to solid particle erosion

A reformulation and solution is presented of the equations of impulse and momentum for an arbitrarily shaped rigid body striking a flat massive surface. The model makes use of the classical coefficient of restitution, e, and tangential coefficient, μ. The latter is defined as the ratio of the tangential to normal impulse components. It is shown that a distinction between μ and friction coefficients, f, is necessary for a proper interpretation of impact data.The expression of impact energy loss is placed into a particularly convenient form. Comparisons are made of this impact model to others from the field of solid particle erosion and wear.