Effects of strain rate and deformation heating in tensile testing

The effects of strain, strain rate and deformation heating in tensile testing of sheet steel are numerically analyzed. An experimentally determined strain-hardening formula is used as a basis of the calculation. The effects of heat conduction and free or forced con-vection during the test are taken into account by using the approximate solution method originally given by Bishop for metal extrusion. The calculated values agree well with the experimental data. It is shown that deformation heating considerably affects the uni-form strain. Consequently the stretchability of sheet metals can be improved by using very low forming speeds or more economically by using an effective cooling system.     

An analysis of flow localization during torsion testing

A general analysis of the occurrence of flow localization during the torsion testing of solid round bars is presented. The formulation is based on equations which enforce torque equilibrium during straining and which account for the thermal effects resulting from heat generation and transfer via conduction and convection. These equations are solved in conjunction with the appropriate boundary conditions and a torque relation in the form of either an “evolutionary” differential equation or an expression in terms of the twist, twist rate, temperature, and specimen radius. To illustrate the use of the present formalism, several predictions are made for hypothetical and real materials. A qualitative analysis of flow localization in the special cases of

• 1.(a) isothermal and
• 2.(b) adiabatic conditions demonstrates that, irrespective of the material flow law, unbounded strain concentrations can only occur under the latter conditions.

The analysis is applied to the problem of flow localization during the room temperature torsion of 304 stainless steel bars containing an initial geometric defect. To this end, finite difference forms of the governing equations are derived which were utilized in computer simulations. The simulations also employed analytic expressions for the material properties and a torque relationship obtained from nominally isothermal tests on samples without defects. A comparison of measured and predicted torque-twist curves and strain histories within and exterior to the defect shows excellent agreement when the effects of length changes during testing are taken into account.     

On the origin of strain softening during deformation of aluminum in torsion to large strains

An investigation of the evolution of the stress-strain curve during deformation in torsion has been carried out. The stress first increases to a narrow plateau, followed by a decrease in flow stress of about 20 pct until steady state is reached. In order to determine the factors that are responsible for the fall in flow stress, investigations of the microstructure and texture were made. The subgrain size was found to increase by about 20 pct corresponding to a fall in the flow stress of about 10 pct. The texture was found to change from a close to random texture at the peak of the flow stress to a texture mainly consisting of the components {001}〈110〉 and {112}〈110〉 at the strain where the stress-strain curve reaches the second steady state. A fall in the flow stress of about 5 pct could be ascribed to changes in texture occurring during deformation.     

Effect of changing strain rate on stress-strain behaviour during high temperature deformation

Plane strain compression tests have been carried out on a ferritic stainless steel at a nominally constant temperature of 917°C and at strain rates of 0.15–5 s⁻¹, and on commercially pure aluminium and an Al-1% Mg alloy at 450°C and strain rates in the range 0.4–12 s⁻¹. Tests have been conducted both at constant strain rate and with strain rate increasing or decreasing from one constant rate to another in a controlled manner after steady state conditions had been established. A wide range of rates of change of strain rate have been studied, with initial and final strain rates differing by up to one and a half orders of magnitude.The ferritic stainless steel and the Al-1% Mg alloy follow a mechanical equation of state under all test conditions of changing strain rate in the sense that the flow stress is dependent only on the instantaneous strain rate and not on the way this strain rate is reached. On the other hand the commercial purity aluminium shows deviations from a mechanical equation of state, which increase systematically with the rate of change of strain rate but are the same magnitude for increasing and decreasing strain rate.     

Texture evolution during plane strain deformation of aluminum

Plane strain forming in commercial processes commonly includes elevated temperature multi-step reductions. An understanding of microstructural development during this process is critical because it dictates the properties of the material subsequent to hot forming. Earing behavior, formability, and mechanical response of the final product are all dependent upon the processing parameters which dictate microstructural evolution. The present work focuses upon the development of hot deformation textures in aluminum during this type of processing. Commercial purity aluminum specimens exhibiting two different starting textures were deformed in channel die compression experiments to simulate the plane strain deformation conditions imposed by the rolling mill. Initial structures consisted of a randomly textured material and a preferentiallycube oriented texture to investigate the effects of starting texture on hot deformation processing and the resulting microstructures. Rate and temperature dependence of texture evolution was experimentally and theoretically investigated in conjunction with this. The relative stability of cube orientations within polycrystals deformed in plane strain is demonstrated for certain deformation conditions. Finally, experimental observations of the evolution of orientation flow from the alpha to the beta fiber are discussed.     

Effect of temperature and strain rate on the compressive flow of aluminum composites containing submicron alumina particles

Uniaxial compression tests were conducted on aluminum composites containing 34 and 37 vol pct submicron alumina particles, to study the effect of temperature and strain rate on their flow stress. For temperatures between 25 °C and 600 °C and strain rates between 10⁻³ and 1 s⁻¹, the flow stress of the composites is significantly higher than that of unreinforced aluminum tested under similar conditions. This can be attributed to direct strengthening of the composites due to load sharing between the particles and the matrix, and an enhanced in-situ matrix flow stress resulting from a particle-induced increase in dislocation density. The composites, however, exhibit the same stress dependence on temperature and strain rate as unreinforced aluminum, indicating a common hardening mechanism, i.e., forest dislocation interactions. The forest hardening present under the explored testing conditions masks the effects of direct dispersion strengthening operative at lower deformation rates in these materials.     

Influence of changing strain rate on microstructure during hot deformation

Hot plane strain compression tests have been carried out on a ferritic stainless steel, commercial purity aluminium and an aluminium 1% magnesium alloy under conditions of constant strain rate and with strain rate changed in a controlled manner from one constant value to another. At constant strain rate, subgrain size and dislocation density inside subgrains are related to flow stress in the manner observed previously on other materials. During and after changes in strain rate, subgrain size and dislocation density are not uniquely related to flow stress. Significant strain intervals after a strain rate change are required to establish the new equilibrium subgrain size. For the alloys, these strain intervals are larger when strain rate is increased than when it is decreased. The opposite is found for the commercial aluminium. This difference in behaviour is explained In terms of the relative rates of glide and climb of dislocations.     

回归加热速率对7050铝合金组织及抗腐蚀性能的影响

采用电导率、抗腐蚀性能测试及透射电镜观察等手段,研究了回归再时效处理过程中回归加热速率(340,57及4.3℃·min^-1)对7050铝合金组织与抗晶间腐蚀和应力腐蚀性能的影响.研究发现,回归加热速率对7050铝合金的抗腐蚀性能有显著的影响,在顾及合金的综合力学性能的情况下,中等回归加热速率能使合金具有较好的抗腐蚀性能.7050铝合金在中速(57℃·min^-1)回归加热条件下,经适当地回归再时效处理后,晶间腐蚀最大深度为50μm,等级为3级,比在340℃·min^-1和4.3℃·min^-1回归加热速率条件下具有更好的抗腐蚀性能,其晶界析出相为较粗大的非连续颗粒,并有较宽的无沉淀析出带.     

Effect of grain size on high strain rate deformation of copper

Experiments were performed to observe the deformation characteristics of oxygen-free high-conductivity (OFHC) copper at high strain rates (up to 40,000 s⁻¹) and to relate differenc in grain size with differences in deformation behavior. The rod impact and torsional Hopkinson bar test methods were used in these experiments. Results show that grain size reductions substantially reduce surface irregularities that develop during deformation. The effect of grain size on the yield stress and on the strain-hardening behavior of copper is small and is similar to the effect of grain size in copper at quasistatic strain rates. The observation that grain size has a substantial effect on surface irregularities may have important implications for applications in which stable deformation of thin sections is of concern.     

Hall-petch relation and the localization of plastic deformation in aluminum

The plastic deformation macrolocalization parameters are compared to the Hall-Petch relation parameters for the flow stress in polycrystalline aluminum specimens with a grain size of 8 × 10⁻³−5 mm. Two branches in the dependence of the localized plastic deformation autowave length on the grain size and two versions of Hall-Petch hardening are found in this grain size range. The boundary between the versions of the dependences corresponds to d _b = 0.1 mm for both cases. The correspondence between localized plastic flow patterns and the Hall-Petch relation is studied.     

Martensite formation,strain rate sensitivity,and deformation behavior of type 304 stainless steel sheet

The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed. Formerly Visiting Scientist, Department of Metallurgical Engineering, Colorado School of Mines.     

Effect of friction,strain hardening,and strain rate hardening on the metal flow in rod extrusion

The metal flow in a rod extrusion is theoretically discussed in relation to a frictional condition, strain hardening and strain rate hardening. An existing rigid plastic FEM-program was modified in order to take a rate sensitivity of a material into consideration through a constitutive equation of the form. An interfatial friction was incorporated by a constant Coulomb-frictional coefficient. When the frictional coefficient is smaller than 0.1, a large effect of the friction does not appear. The larger the n-value is, the stronger the distortion of the grid in a billet becomes even under small frictional resistance. The effect of the m-value is similar qualitatively to that of the n-value. When the rate sensitive material is extruded at very low punch speed under large frictional resistance, the material is strained very irregularly all over the billet at an early stage.     

Strain hardening at high strain in aluminum alloys and its effect on strain localization

The modes of strain localization in the tensile testing of a sheet sample are diffuse necking, localized necking and, in some materials, localization in an unstable shear band. In a tensile test of a rate insensitive material, the normalized strain hardening parameter,H = (1/σ)(dσ/dε) has the values ofH = 1 for diffuse necking andH = 0.5 for localized necking. Curves ofH vs strain were obtained up to large values of plastic strain using the hydraulic bulge test. The materials selected were commercially important sheet alloys in the condition normally used for forming. It is shown that the materials have similarH vs strain curves in the range of uniform tensile straining, but the curves diverge widely at higher strains whereH falls below 1. This has important consequences on strain localization behavior. The limit strains of the alloys in simple tension and punch stretching show reasonable correlation with their values ofH and those alloys which are susceptible to catastrophic shear failure have low values ofH at high strains. Strain rate sensitivity adds to or subtracts from theH values obtained in this study and has an additional influence on strain localization. Formerly with Alcoa Laboratories, Alcoa Center, PA 15069, U.S.A     

The effects of superimposed hydrostatic pressure on deformation and fracture: Part I. Monolithic 6061 aluminum

The effects of various levels of superimposed hydrostatic pressure on the tensile ductility and fracture micromechanisms were determined for 6061 specimens heat-treated to underaged and overaged conditions of equivalent yield strength. Superimposed pressures of 0.1, 150, and 300 MPa were selected; the ductility increased between 0.1 and 150 MPa and remained constant between 150 and 300 MPa. It is shown that the levels of pressure chosen inhibit void growth and coalescence. Void nucleation occurred at nonmetallic inclusions, and neither the ductility nor pressure response were significantly affected by the heat treatments chosen. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.