Plastic macrodeformation of polycrystalline and submicrocrystalline titanium for biomedical applications

The deformation behavior of commercially pure submicrocrystalline and coarse-grained titanium was studied at the macroscopic level. Stress-strain curves of the materials were analyzed. Time-space distributions of local strains were studied at all stages of strain hardening using speckle interferometry. The life time of test specimens of the materials and the coordinates of the fracture region were calculated theoretically and confirmed experimentally. The motion of the zones of localized plasticity was studied. The prefracture stage was shown to involve “condensation” of the zones of localized plasticity and migration of deformation to the fracture neck. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 3, pp. 164–172, May–June, 2008.     

Burning of a powder under harmonically varying pressure

A powder model is examined in which the surface temperature of the powder T_s is a function of the pressure and the initial temperature T₀. All the processes taking place in the reactive layer of the condensed phase and in the gas phase are assumed to be inertialess. It is shown that the equation of heat conduction with a convection term, the boundary conditions for which relate the burning rate, the surface temperature, and the temperature gradient at the burning rate can be treated in the same terms as oscillations described by ordinary differential equations. It is meaningful to introduce such concepts as natural frequency, damping constant, and resonance.The author is indebted to O. I. Leipunskii for constructive discussion and many useful comments.     

纵向冲击压缩下LY12铝合金圆环的塑性失稳

通过对铝合金圆环的纵向冲击压缩研究发现,一定条件下在试件的宏观塑性硬化阶段会出现明显的应力降过程。为揭示此应力降的发生机制,对润滑、细磨、粗磨3种端面粗糙条件下,外径、内径和高度比值为6:3:2的LY12铝合金圆环进行系统的Hopkinson压杆纵向冲击实验。结果表明:应力降主要发生在较大的应变和较高的应变率条件。进一步对实验样品的金相观察发现:应力降产生的内在机制为绝热剪切带的形成和发展,此现象是一种动态塑性失稳的过程。以上结果为金属材料在冲击条件下绝热剪切带产生的研究提供了参考。     

Nonlinear deformation and stability of reinforced elliptical cylindrical shells loaded by internal pressure under twisting and bending

The problem of stability of cylindrical shells with an elliptical cross-sectional contour reinforced by a set of stringers under combined loading by bending and twisting moments, transverse force, and internal pressure is studied with the use of the variational method of finite elements in displacements. The subcritical stress-strain state of the shells is assumed to be moment and nonlinear. The effect of nonlinearity of deformation of the shells and their ellipticity on the critical loads and buckling type is determined.     

Crystal plasticity based analysis of localized necking in aluminum tube under internal pressure

The finite element method is used to numerically simulate localized necking in aluminum alloy tube under internal pressure. The measured electron backscatter diffraction (EBSD) data are directly incorporated into the finite element model and the constitutive response at an integration point is described by the single crystal plasticity theory. The tube is assumed sufficiently long, so that length changes as well as the end effects can be ignored and a plane strain analysis can be performed. Localized necking is assumed to be associated with surface instability, the onset of unstable thinning. It is demonstrated that such a surface instability/necking is the natural outcome of the present approach, and an artificial initial imperfection required by other approaches is not necessary in the present analysis. The effects of spatial grain orientation distribution, material strain rate sensitivity, work hardening, and initial surface topography on necking are discussed. It is found that localized necking depends strongly on both the initial texture and its spatial orientation distribution, while the initial surface topography has a negligible effect on necking.     

Plastic flow for non-monotonic loading conditions of an aluminum alloy sheet sample

Non-linear deformation paths obtained using uniaxial tension followed by simple shear tests were performed for a 1050-O aluminum alloy sheet sample in different specimen orientations with respect to the material symmetry axes. In order to eliminate the time influence, the time interval between the first and second loading steps was kept constant for all the tests. Monotonic uniaxial tension tests interrupted during loading were used to assess the recovery that takes place during this time. In order to eliminate the influence of the initial plastic anisotropy and to compare the results as if the material hardening was isotropic, the flow stress was represented as a function of the plastic work. The behavior of the material after reloading was analyzed in terms of dislocation microstructure and crystallographic texture evolutions. For more quantitative assessment, the full constraints [Int. J. Plasticity 13 (1997) 75] and visco-plastic self-consistent [Acta Metall. Mater. 41 (1993) 2611] polycrystal models were used to simulate the material behavior in the non-linear deformation paths. Based on experimental and simulation results, the relative contributions of the crystallographic texture and dislocation microstructure evolution to the anisotropic hardening behavior of the material were discussed.     

Anisotropic plastic deformation of extruded aluminum alloy tube under axial forces and internal pressure

The anisotropic plastic deformation behavior of extruded 5000 series aluminum alloy tubes, A5154-H112, of 76 mm outer diameter and 3.9 mm wall thickness is investigated, using a servo-controlled tension-internal pressure testing machine. This machine is capable of applying arbitrary stress or strain paths to a tubular specimen using an electrical, closed-loop control system. Detailed measurements were made of the initial yield locus, contours of plastic work for different levels of work-hardening, and the directions of the incremental plastic strain vectors for both linear and combined stress paths. It is found that the measured work contours constructed in the principal stress space are similar in shape, and that the directions of the incremental plastic strain vectors remain almost constant at constant stress ratios. The work-hardening behavior predicted using Hosford's or the Yld2000-2d yield functions under the assumption of isotropic hardening agrees closely with the observations for both linear and combined stress paths. The material is thus found to work-harden almost isotropically. Both yield functions are effective phenomenological plasticity models for predicting the anisotropic plastic deformation behavior of the material.     

Behavior of graphite/epoxy plates with holes under biaxial loading

An experimental investigation was conducted to study the behavior under biaxial tensile loading of quasiisotropic graphite/epoxy plates with circular holes and to determine the influence of hole diameter on failure. The specimens were 40 cm×40 cm (16 in.×16 in.) laminates of [0/±45/90] _s layup. Four hole diameters, 2.54 cm (1.00 in.), 1.91 cm (0.75 in.), 1.27 cm (0.50 in.) and 0.64 cm (0.25 in.), were investigated. Deformations and strains were measured using strain gages and birefringent coatings. Equal biaxial loading was introduced by means fo four whiffle-tree grip linkages and controlled with a servohyraulic system. Initially, the circumferential strain is uniform around the boundary of the hole. Subsequently, with increasing load, regions of high strain concentration with nonlinear response develop at eight characteristic locations 22.5 deg off the fiber axes. Failure in the form of cracking and delamination initiates at these points. Maximum strains at failure on the hole boundary reach values up to twice the ultimate strain of the unnotched laminate. The effect of hole diameter on strength was described satisfactorily using an average biaxial-stress criterion. Good correlation was also obtained with theoretical predictions based on a tensor-polynomial failure criterion for the lamina and a progressive degradation model.     

基于晶体塑性理论研究铝材料高压高应变率下的强度特性

为了了解金属材料在极端加载下复杂动态响应过程中的多种机制和效应,重点针对Al材料在高压、高应变率加载下的塑性变形机制,在经典晶体塑性模型的基础上,对其中的非线性弹性、位错动力学和硬化形式进行改进,建立适用于高压、高应变率加载下的热弹-黏塑性晶体塑性模型。该模型可以较好地描述单晶铝和多晶铝材料屈服强度随压力的变化过程,相比宏观模型,用该模型还获得了多晶Al材料在冲击加载下的织构演化规律,揭示了织构择优取向行为和压力的关系。     

铝粉含量和粒度对CL-20含铝炸药水中爆炸反应特性的影响

为了研究CL-20基含铝炸药的爆炸反应机理,利用水中爆炸实验,测量了不同铝粉含量和粒度的CL-20炸药水中爆炸的冲击波参数、二次压力波参数,计算了冲击波能和气泡能。结果表明,水中爆炸的冲击波能和气泡能表征了爆轰和二次反应两个阶段的炸药爆炸能量分配,CL-20炸药中的铝粉主要在二次反应阶段发生反应,只有少部分的铝粉参与了早期的爆轰反应。气泡脉动形成的二次压力波能描述铝粉含量和粒度对二次反应过程的影响,铝粉含量对炸药的二次反应有显著的影响;铝粉粒度对炸药的水下爆炸的初始冲击波参数、冲击波能和气泡能的影响很小,对铝粉与爆轰产物的二次反应速率影响较大。     

Preparation and characterization of core-shell Al@PFHP with improving the combustion and ignition properties of aluminum powder

The dense alumina shell on the surface of aluminum powder will hinder the combustion of aluminum powder and increase its ignition temperature. In this study, the aluminum oxide shell layer on the surface of aluminum powder was removed with hydrofluoric acid by one-pot method, and 3-Perfluorohexyl-1, 2-epoxypropane (PFHP) (F₃C(CF₂)₅CH₂C₂H₃O) was coated to form a uniform and controllable core-shell Al@PFHP. The core-shell Al@PFHP showed better thermal reaction and ignition performance. The exothermic enthalpy of Al@0.15 PFHP was increased by about 1.9 times, with lower ignition temperature (reduced by about 140 °C) and longer burning duration (increased by about 1.5 times) after coating with PFHP, compared with raw aluminum powder. In addition, the formation of PFHP coating shell can effectively improve the hydrophobicity and corrosion resistance of aluminum powder.     

Plastic strip model of circular crack under cyclic loading with damage accumulation

A cylindrical solid containing a penny-shaped crack in its mid plane is subjected to a remote tensile stress field. In the plastic region near the crack border, damage due to continuous deterioration of the material accumulates, and may lead to crack instability and crack growth. A damage model relating the crack opening displacement in the plastic zone to the fraction of the cross-sectional area occupied by voids is used to describe the conditions necessary for the onset of crack instability, fatigue crack propagation by cyclic loading, and rates of crack growth for time-dependent environmental effects.     

Plastic flow for axisymmetric-tension specimens with necking

Plastic deformation of a tension specimen bounded by a curved surface of revolution is considered. Such a configuration may occur, for example, as a result of necking. The specimen material satisfies the Tresca yield condition and the associated flow rule. Approximate solutions for the stress distribution in the neck were examined in [6]. The extension of notched bars was investigated by numerical and graphic methods in [2, 4]. Below, the problem is solved analytically for a sufficiently smooth neck; a small degree of nonuniformity of the plastic properties is taken into account.     

Photoelasticity for stress analysis under high hydrostatic pressure

Experimental research relative to pressure effects on the mechanical behavior of materials is frequently handicapped by the difficulties associated with making load and deformation measurements in a hostile environment. The application of photoelasticity in high-pressure experiments provides a means for studying the effect of hydrostatic stress on varying stress fields. The purpose of this paper is to examine the feasibility of using the photoelastic method of stress analysis in a high-pressure environment. The unusual feature of this application is the finite elastic deformations suffered by the photoelastic model under high pressure. As a result, the mechanical and stress-optical properties of the model materials are functions of pressure. Another important feature in this study is the selection of a suitable model material. Since the model must come into contact with the liquid pressure media, chemical and absorption resistance are essential considerations. Although it was found that photoelastic investigations can be carried out at high pressure, limitations are imposed by the presence of the optical vessel and pressurized fluid.     

爆炸载荷作用下铝蜂窝夹芯板动力响应研究

采用自行设计的冲击摆实验系统对铝蜂窝夹芯板在爆炸载荷作用下的动力响应进行了系统实验研究,给出了面板和铝蜂窝不同区域的不同变形模态,得到了不同炸药当量对铝蜂窝夹芯板动态响应的影响规律,证实了铝蜂窝夹芯板产生较大塑性变形时,比一般的结构具有更好的能量吸收特性.并利用LS-DYNA对其动力响应进行了数值仿真,考察了炸药起爆、接触界面及上表面接触力对夹芯板变形影响的全过程,得到了板中心的最终变形和芯层的变形模式,与实验结果吻合较好.     

Instrumentation for measuring load and deformation under high pressure

This paper describes some internal instruments designed to measure the load and the deformation of a specimen under high hydrostatic pressure. The design consideration, calibration and experimental results for the internal instruments are given.     

A new apparatus for studying mechanical behavior under pressure

Using a servohydraulic intensifier, a servohydraulic universal test machine and a novel test chamber, pressures of 750 MPa and axial stresses of almost any magnitude are simultaneously generated and applied to the gage section of a solid, cylindrical specimen. Under combined axial-stress/external-pressure loadings, a solid specimen experiences a truly three-dimensional, homogeneous stress state in which the axial stress equals σ₁ and the radial and circumferential stresses equal the negative of the pressure, —P. Through independent computer control of the pressure and axial stress, amterial behavior under monotonically and cyclically applied multiaxial stresses are investigated. This paper describes the experimental details of the test chamber design, high-pressure fluid production and sealing, load and strain measurement under high pressure, computer control and data acquisition. Experimental results obtained from triaxial monotonic tests on low alloy steel and triaxial cyclic tests on a Ni-Ti alloy using this unique apparatus are presented.     

在径向载荷和轴向冲击联合作用下的圆柱壳塑性稳定性分析

对有径向载荷的圆柱壳，受轴向冲击时塑性失稳的临界速度进行理论分析。分析结果与实验值相吻合，并给出一般的规律，它也适用于无径向载荷的情况。