The effect of approach direction on damage in MgO due to spherical particle impact

The damage produced by spherical particle impact against {1 0 0} surfaces of MgO has been investigated over a range of impact angles for a fixed particle velocity and over a range of particle velocities for a fixed impact angle. The mass of material removed by each impact was determined gravimetrically, and the crater and surrounding damage were studied by means of surface profilometry and scanning electron microscopy. A numerical computer model of the crater formation process was developed which was able to predict crater geometries in close agreement with those observed experimentally. This same model also provided estimates of the dynamic hardness, the contact time and the energy transmitted to the surface during the impact. The mean dynamic hardness was ~ 25% less than that measured in previous normal impact studies [1] on MgO of similar static hardness. The contact time and energy calculations give some insight into the reasons why the energy balance model, which successfully describes the velocity dependence of mass loss under normal impact conditions, breaks down for oblique impact.     

Fracture due to damage from projectile impact

In the usual application, the methods of fracture mechanics are used to determine growth rates and critical load levels for sharp-tipped flaws such as fatigue cracks. By way of contrast to this classic application, another way of introducing fracture producing flaws is to expose a structure to impacts from penetrating projectiles. This paper presents recent test data and analytical techniques for predicting the fracture response of tensile panels impacted by small caliber bullets. Panels of 7075-T6, 2024-T81 and 6A1-4V were tested in thicknesses ranging from 0.032 to 0.375 in. A bracketing technique was used to establish threshold values of prestress for catastrophic fracture. For comparison, a number of impact damaged panels were statically tested for residual tensile strength. These results are compared with the impact fracture threshold values and with strength predictions using the plane-stress stress intensity factor for center-notched panels.     

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.     

The effect of filler particle size and orientation on the impact fracture toughness of a highly filled plasticized polymeric material

The effect of particle size and orientation on the inherent fracture toughness of a filled plasticized polymeric material has been determined by application of linear elastic fracture mechanics. Testing was caried out in a three-point bend mode under impact conditions. The material was a triple base gun propellant consisting of a matrix of nitrocellulose plasticized with nitroglycerine and filled with particles of nitroguanidine (NQ). The crystalline NQ was used in the “as-received” form of needles and in a “ground” state. The material containing as-received NQ consistently had a higher fracture toughness than the material with ground NQ, and the toughness was a maximum when the fillers were aligned perpendicularly to the fracture surface. The impact fracture toughness was found to be virtually independent of strain rate over the range from 3 to 90 sec⁻¹. Seven-perforated cylindrical grains of the material containing as-received and ground NQ crystals, and the grains were tested in a pneumatic gas gun and a Hopkinson bar apparatus. The grains containing the ground NQ have been shown to be generally less resistant to fracture than the grains containing as-received NQ.     

The effect of particle orientation and/or position on two-dimensional shape measurements

Particle shapes were measured based on the random positioned (R) method, the stable positioned (S) method and the stable positioned/constant orientated (S /C) method. The mean values ϕ and ζ of two shape indices (circularity ϕ and smoothness ζ), previously defined, were calculated. It was found that smoothness ζ was little affected by shape measuring methods, while circularity ϕ differed according to methods. By analyzing the change in circularity using model ellipsoidal particles, it was confirmed that the R method could more effectively evaluate particle shapes than can the S method, based on the results of the S/C method.     

Analytical prediction of damage due to large mass impact on thin ply composites

This article presents analytical models for predicting large mass impact response and damage in thin-ply composite laminates. Existing models for large mass impact (quasi-static) response are presented and extended to account for damage phenomena observed in thin-ply composites. The most important addition is a set of criteria for initiation and growth of bending induced compressive fibre failure, which has been observed to be extensive in thin ply laminates, while it is rarely observed in conventional laminates. The model predictions are compared to results from previous tests on CFRP laminates with a plain weave made from thin spread tow bands. The experiments seem to confirm the model predictions, but also highlight the need to include the effects of widespread bending induced fibre failure into the structural model.     

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     

声压传感器与振速传感器中心不一致带来的定向误差分析

声矢量传感器由声压传感器和质点振速传感器组成,可用在空间某点同步测量声场的声压和质点振速信息。但矢量传感器在实际制作时,可能存在声压传感器和质点振速传感器中心不一致的情况,即矢量声场的非共点测量。这会使得声压与振速通道之间的接收信号存在时延差,从而影响矢量传感器的定向性能。从理论上分析了时延差对平均声强法定向的影响,即降低了信号处理的信噪比及引入π相位的定向误差,并提出采用互相关声能流法进行定向。通过仿真分析了中心不一致、入射角度和信噪比等因素对不同类型的目标定向结果的影响,并验证和讨论了互相关声能流定向算法的有效性和适用性。     

Force propagation speed in a bed of particles due to an incident particle impact

The force propagation speed in granular matter is a very difficult property to be measured. A new technique has been developed to calculate the force propagation speed in granular matter based on measuring experimentally the contact time. The contact time for a particle hitting a bed of particles is estimated as the time taken for a particle to strike a bed of particles till the time of its ejection, and it is calculated using the discrete element method. The speed of force propagation in a bed of particles is estimated by plotting the dependence of the path length of the contact force on the contact time and finding the gradient of such dependence. Such approach leads to accurate results if the impact speed is below the yield velocity, i.e. no plastic deformations. It is found that the force propagation speed in spherical granular matter is proportional to the impact speed of the incident particle, which is different from force propagation in continuum matter. It is also found that the propagation speed is dependent on the material and diameters ratio of the interacting particles, but it is not dependent on the number of bed layers. The propagation speed in granular matter is normalized by dividing it by a reference propagation speed, i.e. the propagation speed at an impact speed of 1 m/s. It is found that the normalized propagation speed is independent of the material and diameter of the interacting particles, but it is logarithmically proportional to the impact speed. The proportionality constant is equal to 0.16, which can be taken as a universal constant for force propagation in spherical granular matter.     

Modelling the damage evolution due to second-phase particle fragmentation and decohesion in an Al alloy

Summary In some Al alloys, the damage observed is often associated with fragmentation and decohesion of hard particles. A non-uniform angular distribution of particles can induce an anisotropic damage evolution with respect to the strain paths. A model is-proposed based on a 3D FE simulation of the growth of cavities associated with fragmentation or decohesion. It is shown that the growth increases approximately linearly with strain, and exponentially with triaxiality. A simple phenomenological model is proposed based on the FE results, and makes use of the initial damage value as well as the initial angular distribution of particles. The predicted results are compared with experimental ones.     

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.     

The effect of particle distribution on damage formation in particulate reinforced metal matrix composites deformed in compression

Image analysis results are reported on the generation of damage in particulate reinforced metal matrix composites during compressive deformation. The technique allows the automated collection of data on the incidence of particle fracture and void formation in the matrix as a function of important microstructural parameters such as local particle volume fraction and particle size. There is a strong relationship between damage and the local volume fraction of the reinforcement proving that damage formation is accentuated in regions of particle clustering. With the SiC reinforced materials examined, there was observed to be a change in dominance of damage mechanism from particle fracture at low local volume fractions to void formation in the matrix within strongly clustered regions. The results are compared with finite element (FE) modelling of the compressive deformation of clustered particles using a simple cluster of equi-spaced particles. The FE results suggest that plastic flow is generally inhibited in clustered regions. In certain highly clustered configurations shielding is such that flow does not occur in the heart of the cluster even at high levels of average plastic strain. The modelling suggests that the change in dominance of damage mechanism is related to the dramatic increase in tensile hydrostatic stresses in the matrix with higher levels of particle clustering.     

Edge fixing effect on the life of a vacuum chamber thin spherical cover subjected to creep damage

Within framework of the numerical studies of creep resource of a thin spherical cover of a vacuum chamber, we present mathematical formulation and the calculation method for the solution of the initial boundary value problems of the creep theory of thin shells with account of the damage accumulation process of the material. The effect of edge fixing along the normal and tangential lines to the median surface, as well as angular edge fixings, on the cover life in creep are studied under atmospheric pressure creep conditions. The calculation data obtained made it possible to determine the dependence between the cover life in creep and its edge fixing conditions: this effect is strong by the latent fracture time and weak by the allowable deflection values.     

突然断线对输电塔线体系的冲击作用研究

突然断线使处于绷紧状态的输电线张力迅速释放,塔线体系将受到动力冲击作用,但设计规范仅按静态荷载来考虑断线荷载。以某输电线路为例,采用有限元方法建立塔线体系模型,进行断一根至六根子导线工况的模拟,并考虑跌落导线与地面的碰撞。分析发现,断子导线的根数越多,或者越接近断线点,直线塔绝缘子的摆动幅度越大,峰值轴力也越大。导线断线将产生两个极大的冲击作用,一个是导线断裂产生的冲击,另一个是跌落导线与地面碰撞产生的冲击。     

Stokes resistance of a porous spherical particle in a spherical cavity

The boundary effect on the asymmetrical motion of a porous spherical particle in an eccentricspherical cavity is investigated in the quasi-steady limit under creeping flow conditions. The porous particletranslates and rotates in the viscous fluid, located within the spherical cavity, normal to the line connectingtheir centers. The fluid inside the porous particle is governed by the Brinkman equation. A tangential stressjump condition at the interface between the fluid and the porous particle is applied. A semi-analytical approachbased on a collocation technique is used. Due to the linearity of the present problem, the flow variables for theclear fluid region are constructed by superposing basic solutions of two problems: the first one is the regularsolution inside the cavity region in the absence of the porous particle where a first system of coordinates has itsorigin at the center of the cavity, while the second problem is the regular solution in the infinite region outsidethe spherical porous particle where a second coordinate system with its origin at the center of the porousparticle is used. Numerical results displaying the resistance coefficients acting on the particle are obtainedwith good convergence for various values of the physical parameters of the problem. The results are tabulatedand represented graphically. The findings demonstrate that the collocation results of the resistance coefficientsare in good agreement with the corresponding results for the impermeable solid particle.     

The effect of stacking sequence on impact damage in a carbon fibre/epoxy composite

The effect of stacking sequence on impact damage in a carbon fibre/toughened epoxy composite was studied. The major form of damage was delamination, which initiated at almost every interface through the panel. During the impact event the force-time response was monitored and the energy absorbed analysed in terms of an initiation and a propagation energy. The energy absorbed in delamination initiation was influenced by the stacking sequence, being increased by placing 45° fibres in the surface plies and by increasing the number of dissimilar interfaces. The residual energy absorbed in delamination propagation was found to increase linearly with increasing total delamination area. The compression-after-impact strength was related to the maximum delamination area.