Design of notched bars for creep-rupture testing under tri-axial stresses

A previously developed finite element procedure for the solution of axi-symmetric creep deformation problems has been used to study the design of circumferentially notched bars for use in the creep-rupture testing of materials. Three bar geometries are studied in an attempt to pin-point the design features which enable a reasonably large volume of material to be subjected to a uniform state of tri-axial tension stress and strain. Bar geometries are identified which satisfy these requirements.     

Fatigue growth simulation for cracks in notched and unnotched round bars

Fatigue crack growth for various cracks in both unnotched and notched round bars is directly modelled by using an automated numerical technique, which calculates the stress intensity factors at a set of points on the crack front through the three-dimensional finite element method and then applies an appropriate fatigue crack growth law to this set of points to obtain a new crack front. This technique also has the capability of automatic remeshing so that the crack propagation can conveniently be followed. The crack geometries modelled in the present paper include a small internal crack near the free boundary and several initially part-elliptical or irregular surface cracks in a smooth round bar under tension, a surface crack in different semi-circularly notched bars under both tension and bending, a surface crack initiated from the root of a V-notched bar and an initially twin crack configuration within a smooth tension bar. Some fatigue growth characteristics relevant to each type of cracks are also revealed. It is demonstrated that the fatigue growth analyses of various cracks commonly occurring in bars can reliably be made by using the automated finite element technique proposed.     

Fracture of polymeric materials under tensile loads

The frictional mechanism in a journal bearing lubricated with grease containing powdered PTFE was examined by infrared spectroscopy and by scanning electron microscopy. An explanation of the modifying action of grease containing powdered PTFE is presented in terms of the mechanically induced physico-chemical interactions between PTFE, grease, environment and the metallic substrate of the bearing.     

Structure and properties of hard-alloy tool produced by impact molding

Results are derived regarding the influence of hard-alloy tool production by impact molding and high-temperature sintering on the dislocation density in tungsten-carbide and titanium-carbide powder.     

Wave dispersion and attenuation in viscoelastic polymeric bars: Analysing the effect of lateral inertia

Elementary one-dimensional wave theory is often used to describe the propagation of longitudinal stress waves in polymer rods. More accurate solutions are available, but they are mathematically difficult. A new wave equation is derived for long polymeric rods in this paper. The material properties are modelled as a Maxwell viscoelastic material acting in parallel with an elastic material. Lateral motions of the rod that result from the Poisson effect are accounted for using a new concept called the “effective density”. The effects of both the material properties and the diameter of the bar on dispersion and attenuation coefficients are highlighted. The new wave theory simplifies to the one-dimensional solution for waves in polymer rods if the Poisson ratio is set to zero. The predictions simplify to Love's equation for stress waves in elastic bars when rate dependency is removed from the material model.     

Dynamic response of bars subjected to longitudinal impact—An experimental approach

The response of structure to shortduration impact is studied experimentally by employing explosively induced stress pulses in long aluminum bars containing central circular holes. The amplitude of the strain is measured in all cases. The effect of the direction of loading on the existing strain is also studied.It is concluded that the addition of more holes will decrease the dynamic stress concentration, especially if the direction of loading is from the smaller to the larger diameter holes.     

A study of crater volumes produced by single particles at low impact velocities

Targets of lead, aluminium, cold-worked copper, 60-40 brass and steel were struck by steel spheres of various masses at low velocities in the range 6–10 m s^?1. Impacts were both normal and oblique. The crater volumes were calculated and in some cases evaluated by using plasticine. Crater volumes do not indicate wear. However, microscopic observations show severe radial deformation of a narrow zone of the target around the crater. The striking spheres skid as they escape. Splinters of very small amounts of metal are removed from the crater during the first strike. Surface melting appears to take place even at such low velocities.     

Fracture behaviour of adhesively-bonded composite materials under impact loading

In this paper, a polymeric unidirectional carbon-fibre epoxy-resin composite is both experimentally and numerically investigated to study the nonlinear material behavior of impacted DCB (Double Cantilever Beam) specimens. For the impact analysis, the load and the displacement applied from pin onto end block as well as the crack energy release rate are measured and compared with the finite element analysis results. The energy release rate is a critical measure of the resistance to crack propagation, which can be estimated by the force and displacement at the crack tip. It is found that the energy release rate measured from impact tests on the specimens is well predicted by the suggested finite element model in this study.     

Effect of process parameters on impact strength of Al-7% Si alloy castings produced by VAEPC process

The castings produced by the evaporative pattern casting (EPC) process have blow holes. The blow holes in EPC castings are because of the non-escape of the gas produced as a result of burning of polystyrene pattern in the sand mold. To overcome the problem of blow holes, the EPC process is combined with the vacuum (V)- process. The vacuum applied to EPC mold draws the decomposed gases and improves the casting quality produced by the EPC process. The developed hybrid process has been termed as the vacuum assisted evaporative pattern casting (VAEPC) process. The objective of this paper to investigates the effect of process parameters, i.e, degree of vacuum, pouring temperature, grainfineness number, amplitude of vibration and time of vibration on the impact strength of Al-7% Si alloy castings in VAEPC process. In order to evaluate the effect of selected process parameters, the response surface methodology (RSM) is used to formulate a mathematical model which correlates the independent process parameters with the desired impact strength. The central composite rotatable design has been used to conduct the experiments. The results indicate that the impact strength decreases with increases in the grainfineness number and pouring temperature. Whereas, it has an inverse relationship with amplitude of vibration, time of vibration and degree of vacuum. The best value of impact strength (2.34 N/mm²) has been obtained at 400 mm Hg degree of vacuum imposed, 650°C as pouring temperature, 60 as sand grainfineness number, 460 μm as amplitude of vibration, and 70 s as time of vibration.     

Erosion damage of polymeric material by slurry

An attempt was made to reveal the erosion mechanism of a plastic material in slurry containing glass beads of approximately 176 μm in diameter. The experimental observations were interpreted in terms of parameters such as the impact velocity V_p, the impact angle α of the beads and the striking efficiency η. A theoretical flow analysis for a solid-liquid two-phase flow was made and it was found that the trajectories of the particles curved as they approached the specimen, indicating that the actual surface onto which the particles were impinging was much narrower than that which would be expected for sand erosion in an air stream. In the slurry erosion the impact velocity and impact angle differed greatly at different positions of the specimen. This is possibly due to the differences in the density and viscosity of liquid and air. Two distinctly different types of erosion were observed: a typical brittle behaviour occurred near a stagnant point, after an initial incubation period, whereas at a distance from the stagnant point only a slight surface roughness was produced. These two different types of erosion produced a clear boundary. Erosion rates depended on the positions of the specimen and were found to be proportional to (V_p sin α)^2.6. This suggested that the normal component of the impact velocity of the particle determined the erosion rate of the plastic material due to slurry.     

Use of notched samples in technological testing of materials by method of hot torsion

Features of experimental determination of plasticity and stiffness parameters of deformed metallic materials by the method of mechanical shear test using cylindrical samples with small V-shaped notches are considered.     

Micropattern forming of polymeric plate by hydrostatic pressing

In this paper, we have proposed a novel hydrostatic forming process for fabricating large micropatterned polymeric plates; this process is suitable for mass production. The key idea of the process is to hydrostatically form a number of vacuum-packed stacks, each composed of a workpiece and a polymer die in a large water-containing vessel, by controlling the temperature and pressure of water as the pressure-transmitting medium. A prototype hydrostatic press system has been developed to simultaneously form 25-??m microprismatic structures on 30 pieces of PMMA plates (dimensions: 265?×?172?×?0.7?mm) within a cycle time of 35?min. The formed parts are examined at both microscopic and macroscopic levels, and the results are discussed.     

On the problem of a longitudinal impact on a bar supported with a plate

The wave processes that evolve in a plate-supported bar hit with a hard striker with a yielding spherical impact surface are considered. The linearized Hertz contact characteristics are used to derive analytical solutions describing the wave processes that may be conveniently applied for engineering calculations. The reliability of the results obtained with the proposed models is confirmed by experimental data.     

Measuring the mechanical recoil impulse of a polymeric target upon impact of a high power electron beam

A method for measuring the mechanical recoil impulse of a target produced by the relativistic electron beam of the Calamary accelerator is described. A detector based on a piezoelectric sensor is used in measurements. Results of measurements are presented for the mechanical recoil impulse produced by the relativistic electron beam with an energy as high as 300 keV, a current of up to 30 kA, and a duration of ~100 ns that is incident on an epoxy target. The energy flux density on the target surface is varied in the range of 1–10 GW/cm². The maximum measured impulse value is 0.32 N · s at an energy flux density of 10 GW/cm² (an energy fluence of 810 J/cm2).     

Stress analysis of an elastic half-space perforated by twin circular holes and under gravitational loading

The plane elastic problem which simulates gravitational loading, with an accompanying horizontal pressure, of an identical pair of circular tunnels at a common depth below a stress-free horizontal surface is treated. The analysis employs a sequence of Airy stress functions to construct an analytical solution which is then evaluated for a full range of geometries and Poisson's ratios, and used to gauge the effect of the coefficient of horizontal pressure, k₀. Results for hoop stress concentrations at the tunnel surfaces quantify the significant interactions that can occur between the tunnels and the free surface and also define the marked dependence on k₀.     

Survey of plain stress condition of a perforated plate by the acoustoelasticity method

During stretching of a steel plate with elliptical holes by an ultrasonic nondestructive method the stress state of the part material was surveyed in the elastic area. The results of experimental determination of biaxial stresses were checked by the reference method of analytical calculation. The check showed that the difference between the experimental and theoretical data does not exceed several per cent of the material yield limit, which permits us to rely on wider practical application of the acoustoelasticity method for surveying the effects of stress concentrations near fabrication holes in the parts of machines and structures. The IN-5101 A portable device developed by “INKOTES” Engineering Company was used in the tests.     

Extracting local mechanical properties of steel bars by means of instrumented flat indentation

The evaluation of local mechanical properties of metal parts can be used for the prediction of forming and cutting forces during the following working stages. In order to show the feasibility of using indentation tests for work-piece characterization, C40 steel disks were cut from an extruded bar and machined by facing operation under different conditions. Indentation tests were carried out with 1 mm and 2 mm diameter indenters made of tungsten carbide; yield strength and indentation pressure values were extracted along the diameter of each disk. For the first time, indentation results were used to extract local mechanical properties of steel bars as a function of their process conditions: it was observed that small pins gave information on the effect of the machining process whereas large pins were more suitable to evaluate work-piece bulk properties.     

Transient temperatures produced by abrasion of aragonite

The transformation of aragonite to calcite has been used to provide an indication of the thermal history of debris and the surface of aragonite during abrasion on 320 grade silicon carbide papers. The temperature rise of debris was estimated to be 500°–600°C whereas the specimen surface temperature was considerably lower. This supports the view that the mechanism of material removal during the abrasion of non metallic materials is essentially similar to that occurring in metals.     

穿孔和非穿孔消声器压力损失研究

本文利用三维CFD方法,研究了抗性消声器内部流体动力学仿真和压力损失计算的过程,分析了四种穿孔和非穿孔单腔消声器的流场流速和压力分布,研究了相应的压力损失随入口流速的变化趋势.得出结论:,穿空管消声器的流体动力学性能要好于非穿孔类消声器,对膨胀腔冲击比较小;在相同边界条件下,穿孔管消声器的压力损失大于非穿孔类消声器.对复杂抗性消声器的设计有比较好的参考价值.