The high strain rate compression of 1023 and 4130 steels

Steel specimens of type 1023 and heat treated 4130 steel of hardnesses, between 257 and 450 HV10, were compressed at strain rates between 3 × 10³ and 3 × 10⁴ s⁻¹ using a modified Hopkinson bar. Both steels showed viscous type flow behaviour with a linear increase in flow stress with strain rate. For the 1023 steel there were two linear regions, one from 3 to 12 × 10³ s⁻¹ with a macroscopic viscosity of 4·7 kPa s and two from 14 to 27 × 10³ s⁻¹ with a viscosity of 0·7 kPa s. These results indicate some change in the controlling mechanism. For the 4130 steel the macroscopic viscosity decreased linearly with increasing hardness from 7·4 kPa s at 257 HV 10 to 2·2 kPa s at 450 HV 10. This was probably due to a decrease in the density of mobile dislocations with increased tempering temperatures. At lower hardness values (257 and 300 HV 10) and higher strain rates, a levelling off of the flow stress occurred but the data were too scattered to give a definite trend. The heat generated due to deformation of the 1023 steel was sufficient to produce steady state flow at strains above about 0·3. In the 4130 steel the thermo-mechanical properties were such that linear work softening was observed at a rate inversely proportional to the square root of the strain rate and independent of the hardness. This suggests that the work softening was a function of properties of the common ferrite matrix. The strain at which the work softening commenced decreased with increasing hardness, because of the more rapid heat generation at higher flow stresses. At higher hardness and higher strain rates 4130 steel failed by shear. Shear failure occurred at lower strain rates with increasing hardness and, for a given hardness, at lower strains with increasing strain rate. This behaviour was consistent with Recht's analysis of catastrophic shear failure.     

Measurement of strain rate sensitivity of aluminium foams for energy dissipation

In this paper the structural performance of aluminium alloy foams have been investigated under both static and dynamic compression loads. Three foam typologies (M-PORE, CYMAT, SCHUNK) in a wide range of density (from 0.14 to 0.75 g/cm³), made by means of different process-routes (melt gas injection, powder metallurgy, investment casting) have been analysed. Foams microstructural characterization has been carried out through morphometric measurements by means of Scanning Electron Microscopy (SEM) and Computed Tomography (CT) and subsequent digital image processing in order to determine average cells size and cell distributions on different section planes. The experimental study aims to assess the strain rate sensitivity and energy absorption capability of commercially available metal foams and to point out the correlation between the mechanical behaviour and the physical and geometrical properties of the foam. It has been found that the specific energy dissipation of foams with similar density can be quite different: for the same volume of foam, average values of 1770, 1780 and 5590 J/kg at 50% nominal compression have been measured on M-PORE (0.19 g/cm³), CYMAT (0.28 g/cm³) and SCHUNK (0.28 g/cm³) foams, respectively. Impact tests showed that the dependence of the plateau stress on strain rate could be considered negligible for M-PORE and CYMAT foams while it is quite remarkable for SCHUNK foams. Moreover, it was found that the peak stress of CYMAT foams has a quite large sensitivity on the loading rate.     

Modelling the plane strain compression test to obtain constitutive equations of aluminium alloys

Hot plane strain compression tests on 1050, 1198, 3003 and 3004 aluminium alloys have been conducted. Based on these experiments and on a set of internal type constitutive equations for hot working, the values of the parameters in the constitutive functions are determined. The constitutive equations proposed here, with the constitutive functions and material parameters associated, accurately reproduce the basic tests. The procedure used to fit the material parameters is improved, in comparison with classical slip line analysis, by using a finite element modelling of the plane strain compression test. It is demonstrated that accurate plane strain or three-dimensional large strain finite element analysis can be used to correct the friction and lateral spread effects. Furthermore, it is demonstrated from comparison with the experimental observations that microstructural parameters can be accurately determined from numerical modelling. The constitutive equations and finite element procedure proposed here can be useful for obtaining an improved analysis of hot rolling of aluminium alloys.     

High strain rate compression of titanium and some titanium alloys

A modified Hopkinson bar was used to compress specimens of commercially-pure titanium, IMI 125 and titanium alloys, AMS 4911B and AMS 4916B, at natural strain rates of between 3 × 10³ s⁻¹ and 3 × 10⁴ s⁻¹. All three materials deformed in a viscous manner with a linear increase of flow stress with strain rate and a macroscopic viscosity of 4·5 kPa s, 5·7 kPa s and 9·2 kPa s, respectively. At strain rates above about 1 × 10⁴ s⁻¹ there was a decrease in macroscopic viscosities to 0·9 kPa s and 2·5 kPa s for the titanium and 4911B alloy respectively, which is attributed to changes in the dislocation drag mechanisms. The results for the 4916B alloy at the higher strain rates were too scattered to give a definite trend. It is suggested that the increase in the macroscopic viscosity with alloying is due to an increase in the dislocation drag coefficient by solution hardening and to a reduction in the density of mobile dislocations by dislocation pinning.

It is also proposed that the susceptibility of these materials to catastrophic shear failure which occurs without prior linear work-softening is due to the higher propensity for titanium and titanium alloys to shear failure as a consequence of their thermo-mechanical properties. The decrease of strain at which shear occurred with increasing strength is in accord with this suggestion.     

Viscoplastic behavior of AA7075 aluminum alloy at high strain rate

Journal of Mechanical Science and Technology - The flow stress increases with the increase in strain rate. This phenomenon is the strain rate effect of plastic deformation. Hopkinson experiment...     

Analysis of fragment size and ejection velocity at high strain rate

On the basis of the strain energy coupled with damage and within the framework of thermodynamics, a model for predicting the properties of fragmentation in brittle solids under high-rate loading is formulated in this paper. The model provides a direct, explicit and quantitative method to determine the average fragment size generated by crack coalescence and the fragment ejection velocity in the dynamic fragmentation process. The theoretical predictions of the fragment size are compared with the experimental data, and it is found that the predicted values agree reasonably well with the test results. The dependence of the ejection velocity on the strain rate is discussed. It is concluded that the value of the ejection velocity increases with the increment of the strain rate.     

Quasi-static axial compression of thin-walled circular aluminium tubes

This paper presents further experimental investigations into axial compression of thin-walled circular tubes, a classical problem studied for several decades. A total of 70 quasi-static tests were conducted on circular 6060 aluminium tubes in the T5, as-received condition. The range of D/t considered was expanded over previous studies to D/t=10–450. Collapse modes were observed for L/D10 and a mode classification chart developed. The average crush force, F_AV, was non-dimensionalised and an empirical formula established as F_AV/M_P=72.3(D/t)^0.32. It was found that test results for both axi-symmetric and non-symmetric modes lie on a single curve. Comprehensive comparisons have been made between existing theories and our test results for F_AV. This has revealed some shortcomings, suggesting that further theoretical work may be required. It was found that the ratio of F_MAX/F_AV increased substantially with an increase in the D/t ratio. The effect of filling aluminium tubes with different density polyurethane foam was also briefly examined.     

Dynamic deformation behavior of aluminum alloys under high strain rate compressive/tensile loading

Mechanical properties of the materials used for transportations and industrial machinery under high strain rate loading conditions such as seismic loading are required to provide appropriate safety assessment to these mechanical structures. The Split Hopkinson Pressure Bar (SHPB) technique with a special experimental apparatus can be used to obtain the material behavior under high strain rate loading conditions. In this paper, dynamic deformation behaviors of the aluminum alloys such as A12024-T4, A16061T-6 and A17075-T6 under both high strain rate compressive and tensile loading conditions are determined using the SHPB technique.     

高温应变片在高温下的性能特性

近十几年来,高温称重传感器在冶金工业部门的应用越来越普遍。但是,由于高温称重传感器所用的高温应变片和高温胶粘剂等的特殊性,在制作过程中必须经繁杂的处理和测试工序,使产品生产效率降低、成本提高。人们试图改变这种状态,但是由于有关高温应变片在高温下性能的系统资料甚少,给工作带来不少困难。为了增加人们对传感器用高温应变片的了...     

采用高速高精度磨削的优点

分析高速高精度磨削工艺,提出其可以提高工件表面质量和加工精度及磨削效率,且操作简便,成本低.     

Modes of deformation in aluminium rings subjected to static axial compression

Aluminium rings of varying height/outside diameter/inside diameter ratios machined from extruded solid bars, were subjected to static axial compression. Etched diametral planes of deformed cylinders revealed the existence of shear bands, the configuration of which were found to change with initial specimen geometry and deformation.     

7039铝合金的热压缩变形本构方程

采用圆柱试样在Gleeble-1500材料热模拟试验机上对7039铝合金进行等温压缩变形试验,研究了该合金在变形温度为300~500℃、应变速率为0.01~1 s-1条件下的流变行为。结果表明:变形温度和应变速率对7039铝合金的流变应力有显著影响,流变应力随变形温度的升高而降低,随应变速率的增加而升高;7039铝合金的高温流变行为可用包含Arrhenius项的Zener-Hollomon参数来描述,其热变形本构方程为.ε=5.30×1012[sinh(0.011σ)]5.28×exp[-173.68×103/(RT)]。     

Finite-element simulation of the lateral compression of aluminium tube between rigid plates

The lateral compression of aluminium and clad tubes owing to a large deformation is examined by an incremental elasto-plastic finite-element method based on an updated Lagrangian formulation in which a sliding-sticking friction mode is specially considered. It is mainly expected to predict the buckling process and load–deflection curves for energy dissipation capacity during the design stage, before trials. The high non-linearity of the process due to geometric changes, the inelastic constitutive behavior, and the deformation-dependent boundary conditions are taken into account in an incremental manner. A static explicit approach to the solution is applied, tangent stiffness matrix equation is solved without iteration and the r_min technique is employed to limit the step size to linear relation. The simulated load–deflection curve agrees with a published experimental result. The predicted geometries of the compressed tube clearly demonstrate the processes of the formation of buckling until unloading. The effects of various parameters of the process, such as elastic modulus, strain hardening exponent, tube thickness, friction coefficient and configurations of the clad tube, on the occurrence of buckling of tube are discussed and interpreted in simulation. The present work may be expected to improve the understanding of the buckling mechanism of lateral compression.     

Poroelastic behavior of trabecular bone: The effect of strain rate

A one dimensional poroelastic model of trabecular bone was developed to investigate the pore pressure effect on mechanical behavior. The poroclastic properties were determined based upon the assumed drained Poisson's ratio of 0.3 and the experimental results reported in the literature. Even though the free escape of the fluid through the loading end was allowed during deformation, model predictions showed that the pore pressure generation within the trabecular bone would cause significant variations in total stress. The total stress increase resulted in a stiffening of the trabecular bone, which supports the concept of hydraulic stiffening that has been advocated by several investigators. Model predictions showed a good agreement to the mechanical behaviors of trabecular bone specimens with marrow in a uniaxial strain condition observed in a previous study. These results support the hypothesis that the trabecular bone is poroelastic and the pore pressure effect on the mechanical behavior at the continuum level is significant. Thus, the incorporation of the fluid effect in future studies is recommended to improve our understanding of trabecular bone mechanics.     

高精度铝带材纵剪机组研制

阐述了高精度铝带材纵剪机组最新技术和设备组成。论述了影响剪切质量的关键因素、张力控制形式;详细讨论了圆盘刀剪切工艺、张力控制设备等影响剪切精度的因素,对高精度铝带材纵剪机组研制及提高带材剪切质量具有借鉴意义。     

The effects of strain rate and anisotropy upon the sheet-stretching process

A rate-dependent elasto-plastic finite element formulation has been developed for the large-deformation sheet-stretching process. The formulation used was based on the power-form constitutive equation applied for the stress-strain-strain rate relation of the material and Hill's anisotropic yield criterion in the plastic range. A principle of virtual work rate applied towards a Lagrangian reference system was adopted. The resulting nonlinear equilibrium equations have been solved through usage of an incremental method with the R_min technique. Additionally, the location of fracture initiation in the material and the total punch stroke were predicted by the strain energy density criterion.Experimental tests were performed and have exhibited good agreement with numerical simulations. The method proposed here has been proven to be capable of evaluating the effects of various parameters on metal flow and could be modified for solving problems related to other manufacturing processes.     

Plane strain compression of pre-shaped material between wedge-shaped dies

The plane strain compression of taper-shaped stock between wedge-shaped dies of small angles (≤5°) was investigated for several different conditions of interfacial friction, die angles and width/thickness ratios of the stock. The experiments were performed on specimens of pure lead, half hard aluminium and high conductivity copper. Vertical mean die pressure was measured and compared with that evaluated theoretically using slip-line field theory. A comparison was also made of the end velocities of the stock observed experimentally and calculated analytically.     

Experimental studies of the strain distributions in bends from aluminium sheet

The use of microhardness measurements to follow the development of the plastic-strain distributions during bending is described. The microhardness values are calibrated in terms of true, equivalent plastic strain in subsidiary experiments. The technique is shown to be sufficiently sensitive to enable the determination of the strains present in shear bands that form during bending.