Analysis of plastic bending of adhesive-bonded sheet metals taking account of viscoplasticity of adhesive

The present authors proposed a new technique of plastic bending of adhesive-bonded sheet metals. In this process, large transverse shear deformation occurs in the adhesive layer, which in some cases would induce the geometrical imperfection (so-called ‘gull-wing bend’) and the delamination. Since the strength of the adhesive is highly rate-sensitive, the amount of shear deformation of the adhesive layer and, as a result, ‘gull-wing bend’, are strongly influenced by the forming speed. In the present work, the effect of forming speed on the deformation characteristics of adhesively bonded aluminium sheets was investigated by performing V-bending experiments with various punch speeds at room temperature. In order to discuss the effect quantitatively, the numerical simulations for the bending were also conducted using a rate-dependent constitutive model of plasticity for the adhesive. Consequently, it was found that the large shear deformation and ‘gull-wing bend’ are suppressed by high-speed forming since the deformation resistance becomes higher at high strain rate.     

A comprehensive study of plastic deformation mechanism of the metallic materials by ‘X-ray’ computed tomography (‘X-ray’ CT)

The study of microstructure and texture deformation of the metallic materials necessitates detailed information of physical evidence about the plastic deformation mechanism, which involves a direct relationship between mechanical properties and their behaviours under the working conditions. Generally, the mechanical properties of materials are essentially the function of their structure and their compositions. So, the study of texture deformation of mechanical parts with an efficient way in manufacturing engineering is of considerable practical interest. The present paper entails the study of the deformation mechanism in microscopic scale—in situ observation of microstructure and texture deformation by using ‘X-ray’ computed tomography (CT) Medical Scanner installed in the CNAM-Paris, Industrial Materials Laboratory, for evaluating the plastic deformation mechanism. A tomographic inner-health analysis will be presented from 2D slices of the examined parts in the laboratory scale on the as-received and heat-treated aluminium specimens.     

Superplasticity of fine-grained 7475 Al alloy and a proposed new deformation mechanism

A study has been made to investigate the superplastic deformation mechanisms of 7475 Al alloy in relation to the variation of grain size ranging between 5.5 μm and 13 μm. The strain-rate sensitivity (m) was increased with decreasing grain size in the superplastic deformation regime. Microstructural investigation after tension tests revealed that the dispersoid free zones were produced mostly at the grain boundaries normal to the tensile direction. A new model for describing the deformation behavior of the 7475 Al alloy has been proposed based on the assumption that the grain boundary sliding was accommodated by both diffusional flow and slip. This new model well predicts many aspects of experimental results.     

Influence of texture and grain structure on strain localisation and formability for AlMgSi alloys

The influence of texture and grain structure on strain localisation and formability is examined by experiments and numerical simulations for the extruded aluminium alloys AA6063 and AA6082. In the as-extruded condition, the AA6063 alloy has an equiaxed, recrystallised grain structure with strong cube texture, while AA6082 has a fibrous, non-recrystallised grain structure with strong β-fibre texture. By deforming and heat treating the materials after extrusion, a recrystallised equiaxed grain structure with a close to random texture is obtained for both alloys. A comprehensive test programme is conducted to determine the work hardening, plastic anisotropy and formability of the materials. Strain localisation and failure are examined by optical microscopy. An anisotropic plasticity model is calibrated for the materials and used in calculation of forming limit curves by means of the Marciniak–Kuczynski (M–K) analysis for anisotropic materials. It is found that strong cube texture leads to superior formability properties for biaxial stretching while random texture slightly lowers the formability. The strong β-fibre texture of AA6082 in the as-extruded condition leads to reduced formability. The results of the M–K analysis are very conservative compared with the experimental results, and a parametric study is undertaken to investigate the sensitivity of the predicted forming limit curves to some parameters not well defined by the experiments.     

DEFORMATION BEHAVIOR OF AA6063 ALUMINIUM ALLOY AFTER REMOVING FRICTION EFFECT UNDER HOT WORKING CONDITIONS

The hot deformation behavior of AA6063 aluminium alloy has been investigated by means of compression tests at temperatures between 400 and 520℃, and strain rates ranging from 2.5 to 10 s^-1. Owing to the barreling, the theoretical model on the basis of Hills general method is used to calculate the flow stress of a cylindrical specimen under uniaxial simple compression so as to consider the friction effect at the die-specimen interface. A method of evaluating the friction coefficient by combining compression tests with the finite element method is presented. The real flow behavior of AA6063 aluminium alloy can be described with sinh-Arrhenius equation. The hot deformation activation energy Q derived from the corrected stress and strain data is 232. 350 kJ/mol.     

The finite-element deformation modeling of superplastic Al-8090

The deformation behaviors of an Al-Li-Cu alloy (Aluminum 8090) during uniaxial and biaxial stress states were modeled using the nonlinear finite element analysis package ABAQUS 5.8. Two different material models were used for the purpose of comparison and to propose a valid and accurate superplastic material deformation behavior. Using a power law strain hardening model, the material was modeled with either constant or variable properties of strain-rate sensitivity and strain-hardening exponent. Results from the numerical models were compared to experimental results by Chen and Huang, who investigated the uniaxial and biaxial states of stress. Experimental data were primarily used to acquire the needed uniaxial material parameters for the model. The strain-rate sensitivity and strain-hardening exponent were dependent on the strain and strain rates. Comparing the two material models for both stress states indicated that the material model with variable properties better represented the true behavior of the material when compared to the experimental results. For more information, contact Noha M. Hassan, the American University in Cairo, 113 Kasr El Aini St., P.O. Box 2511, Cairo 11511, Egypt; +202-797-5336; fax +202-797-7565; e-mail nhassan@vt.edu.     

An analysis of the tensile elongation to failure of laminated metal composites in the presence of strain-rate hardening

We extend the analysis of Hutchinson and Neale (Hutchinson JW, Neale KW. Acta Metall 1977;25:839) and Ghosh (Ghosh AK. Acta Metall 1977;25:1413) predicting the tensile elongation to failure of strain-rate-dependent plastic materials to two-phase composites deforming quasistatically according to the equistrain rule of mixtures. The analysis incorporates the influence of work hardening and strain-rate hardening in both composite constituent phases. It is shown that the problem can be formulated in a manner that condenses the seven underlying material parameters into four dimensionless numbers for composites of power-law hardening phases, the number of parameters falling to two for linear hardening. It then emerges that the stabilizing influence of both work hardening and strain-rate hardening is, within assumptions of the model, always predominantly exerted by the phase that carries the greater share of the composite stress. It is also shown that the prediction can be simplified so as to enable an approximate but convenient direct graphical deduction of the tensile elongation of ductile laminated metal composites (LMCs), knowing the work hardening and strain-rate hardening characteristics of the two phases making the composite. The utility of this graphical scheme is illustrated with two examples, namely LMCs containing one phase of (moderately ductile) aluminium alloy or of a (low-ductility) nanocrystalline metal.     

Concepts derived from phase diagram studies for the strengthening of Fe–Al-based alloys

Fe–Al-based alloys, i.e. alloys which contain either disordered A2 -(Fe,Al), B2-ordered FeAl or D0₃-ordered Fe₃Al as majority phase, have a considerable potential for developing materials for structural applications, but insufficient strength and creep resistance have been identified as obstacles for the use of Fe–Al-based alloys at high temperatures. At the ‘Discussion Meeting on the Development of Innovative Iron Aluminium Alloys’ held at the Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf on March, 9th 2004 a couple of presentations were made with emphasis on improving these properties at high temperatures. In the current article those strengthening mechanisms which are provided by the phase diagram—solid-solution hardening, strengthening by precipitates, or ordering—are reviewed. Besides results obtained for the binary Fe–Al system special emphasis is put forward to those ternary systems for which results have been presented at the ‘Discussion Meeting’.     

Springback After the Lateral Bending of T-Section Rails of Work-Hardening Materials

This paper studies the springback after the lateral bending of T-section rails, considering the work-hardening materials. A linear-hardening model and an elastic-plastic power-exponent hardening model of the material are adopted and compared with the real experimental stress-strain curve obtained from the uniaxial tension tests. The analytical formulas for the springback and residual curvatures are given. The numerical results indicate that the material hardening directly affects the accuracy of springback prediction compared with the experimental results. Besides, springback prediction is not sensitive to hardening parameters in the beginning of elastic-plastic bending deformation. Although there is an apparent yield stage in the true stress-strain curve, the adopted hardening models can achieve an allowable relative error, if hardening parameters are properly selected.     

Superplastic deformation of SiCp/2024 Al composite fabricated by spray atomization and codeposition

In this paper, data are presented on the microstructure and superplastic deformation mechanics of an aluminum alloy, 2024, containing 10 vol% SiC particles. The material was fabricated by spray atomization and codeposition. The properties were studied after pretreatment by isothermal hot compression and isothermal hot forward extrusion (extrusion ratio 10.0). The experimental results show that the strain-rate sensitivity index (m-value) is 0.48 and the limit elongation (the elongation at fracture) is 345 % during superplastic uniaxial tension. The optimum conditions for superplastic behavior are 753 K of deformation temperature and 1.0 × 10⁻³ s⁻¹ of initial strain rate. Superplasticity may result from the fine grain size and the well-distributed SiC particles during superplastic uniaxial tension. Moreover, the simple and convenient pretreatment used in this paper is easily applied to industrial practice.     

Relaxation mechanisms and microstructure in glass and glass-ceramics

Two LAS (Li₂O–Al₂O₃–SiO₂)-type glass-ceramics and their parent glass have been studied by isothermal mechanical spectroscopy. These materials have the same chemical composition but the two glass-ceramics differ in microstructure: one is a ‘β-quartz’ glass-ceramic whereas the other one is of ‘β-spodumene’ type. The isothermal internal friction measurements performed in a frequency sweep [10⁻⁴–31.6 Hz] with an inverted torsion pendulum, submitted to subresonant forced oscillations, at temperatures between 93 and 820 K, have revealed several mechanical relaxation peaks. A single internal friction peak is observed in the glass sample whereas two peaks occur in the ‘β-quartz’ and ‘β-spodumene’ glass-ceramics. A detailed microstructure analysis (XRD, IRTF, SEM, TEM and DTA) and dielectric loss measurements have allowed to interpret these relaxation phenomena. The mechanical relaxation peak observed in the glass (290 K for 1 Hz) is assigned to the stress-induced movement of lithium ions. In each glass-ceramic the ‘low-temperature’ peak (340 K for 1 Hz) is linked with the ion mobility in the respective main crystalline phase. As for the ‘high-temperature’ peak, its origin is totally different for the two glass-ceramics; in the ‘β-quartz’ glass-ceramic it is due to the Mg²⁺ and Zn²⁺ ion relaxation in the crystalline phase, whereas in the ‘β-spodumene’ glass-ceramic it is linked with a complex entity within the residual vitreous phase.     

拉伸变形应变硬化指数的理论和实验规范

从应力为应变和应变速率函数的状态方程出发,导出了超塑性拉伸变形的微分本构方程,从而解释了方程中应变硬化指数的力学涵义,导出了应变硬化指数的约束方程,定义了典型变形路径的应变硬化指数,导出了它们之间的相互关系,导出了典型变形路径应变硬化指数的测量公式和数值模拟的精确测量方法,以此为依据对应变硬化指数进行了实验测量,对测量方法和测量结果进行了精细分析,研究结果判明：应变硬化指数不仅不是常数,而且与应变的变化规律及变形路径有密切的联系;在一定变形路径下还与所用的测量公式有密切关系．因此,在引用应变硬化指数时,应该标明所对应的变形路径;在测量应变硬化指数时,应采用所对应变形路径的测量公式。     

Twinning and mechanical behavior of titanium aluminides and other intermetallics

Based on the recent studies of deformation twinning in ordered intermetallics, the general background on slip-twinning-microcracking relationships is established. Specific roles of deformation twinning in the generalized plasticity of polycrystals and the crack-tip microplasticity are discussed in terms of the local strain compatibility by invoking both ‘tension’ and ‘compression’ twins. Special examples are given for γ-TiAl, other L1₀ alloys, and Ti₃Al of the D0₁₉ structure. Effects of composition, microstructure and temperature on defect properties that are relevant to the propensity of deformation twinning are discussed.     

Towards understanding anelasticity of the β1′ martensitic phase of Cu–Al–Ni

The present work continues the series of experimental investigations undertaken in order to elucidate the mechanisms controlling elastic and anelastic properties of the β₁′ martensitic phase of Cu-based shape memory alloys. The paper reports an attempt to distinguish between ‘dislocation’ and ‘interface’ mechanisms of the internal friction in the β₁′ martensitic phase of Cu–Al–Ni single crystals. Two types of experiments have been performed. First, the ultrasonic strain amplitude-independent and amplitude-dependent internal friction (ADIF) of a monovariant specimen for temperatures 90–300 K is carefully re-examined. Second, in situ measurements of the ADIF and of the influence of ultrasonic oscillations on the plastic deformation (acoustoplastic effect) were carried out during quasistatic deformation of a quenched polyvariant specimen. Experimental results support a dislocation rather than an interface mechanism of anelasticity, at least at ultrasonic frequencies and moderate strain amplitudes.     

The effect of stress state on high-temperature deformation of fine-grained aluminum–magnesium alloy AA5083 sheet

The effect of stress state on high-temperature deformation of fine-grained aluminum–magnesium alloy AA5083 sheet is investigated over a range of temperatures and strain rates for which the grain-boundary-sliding and solute-drag creep mechanisms govern plastic flow. Experimental data from uniaxial tension and biaxial tension are used in conjunction with finite-element-method simulations to examine the role of stress state. Three different material constitutive models derived from uniaxial tensile data are used to simulate bulge-forming experiments. Comparison of simulation results with bulge-forming data indicates that stress state affects grain-boundary-sliding creep by increasing creep rate as hydrostatic stress increases. Thus, creep deformation is faster under biaxial tension than under uniaxial tension for a constant effective stress. No effect of stress state is observed for solute-drag creep. A new material model that accounts for the effect of stress state on grain-boundary-sliding creep is proposed.     

Deformation Mechanism of AZ31B Magnesium Alloy Under Different Loading Paths

Deformation and texture evolution of AZ31 B magnesium(Mg) alloy sheet under uniaxial tension, compression, and reverse loading after different pre-strain(compression and tension) were investigated experimentally. The results indicate that the pre-compressive strain remarkably affects the reverse tensile yield stress and the width of the detwinning-dominant stage during inverse tension process. Similar to stress–strain curve of the uniaxial compression, the curve of reverse tensile yield value also has ‘S' shape, and its minimum value is only 38 MPa. The relationship between pre-compressive strain and the width of detwinning-dominant stage presents a linear growth, and the greater the precompressive strain is, the smaller the strain hardening rate of the detwinning-slip-dominant stage is. Compared with the reverse tension under pre-compression, the influence of the pre-tension deformation on the deformation mechanism of subsequent compression is relatively simple. With the increase in pre-tension strain, the yield stress of the reverse loading is rising.     

连续胞状结构强化Ta-9.2W-0.5Hf合金材料的拉伸变形特征

采用粉末冶金方法制备了具有连续胞状强化结构的致密Ta-9.2W-O.5Hf合金材料,通过单轴拉伸试验研究了这种材料的形变特征,为其进一步的工程应用提供实验依据.结果表明,连续胞状结构强化Ta-W-Hf材料的变形与传统熔炼加工材的变形有较大不同,变形的整体连续性很好,不产生强烈的局部变形,变形的均匀性、协调性好,其断裂虽呈现晶间断裂的形式,但仍属于韧性断裂.     

Indentation fracture and soft impresser fatigue in sapphire and polycrystalline alumina

The soft impresser technique has many advantages compared with conventional diamond pyramid indentation for studying the deformation and fracture of materials that would normally be considered ‘brittle’. In particular, as utilised in this study, it can be used to study the evolution of deformation and crack initiation and propagation under contact fatigue conditions. Alumina, both in its sintered polycrystalline form and as single crystals (sapphire), has been investigated because of its many uses in engineering load-bearing applications. Intially conical tool steel impressers were used to subject the flat ceramic surfaces to mean pressures of 5.2 ± 2.4 GPa for up to 7.5 × 10⁶ cycles. Plastic deformation in the polycrystalline alumina was observed after only 10³ loadings, followed by cracking at the edge of the contact zone and subsequent crack interlinking and radial extension. The sapphire showed no signs of deformation or cracking under these conditions, but did deform and crack after only one cycle when a harder WC/Co impresser was used.