The identification of strain-stress curve for 5049 aluminum based on tube hydraulic bulging test

Tube hydraulic bulging tests with fixed-end conditions are carried out to explore tubular material characteristics for 5049 aluminium. Tube diameter at the center of specimen and pole thickness under different internal pressures are recorded during forming process. Based on experimental data, two types of theoretical models using membrane mechanics and total strain theory are applied to determine the flow stress curve of tubular specimens. A tension specimen is cut from the same tube along longitudinal direction and strain-stress curve is fitted by a universal tensile test. In order to test their accuracy, obtained material parameters from three methods are imported into a finite element model (FEM) and its predicted results are compared with bugle height measured from experiments. The comparison shows that the flow stress curve of 5049 aluminium tube can be identified by these three methods and simulated results from total strain model has a better agreement with experimental measures compared with the other two methods.

Methods for measuring friction-independent flow stress curve to large strains using hyperbolic shaped compression specimen

The accurate measurement of flow stress curve to large strains using cylindrical compression specimen is always a great challenge due to the influence of friction. Recently, the present authors designed a hyperbolic shaped compression (HSC) specimen which can yield an average true stress- strain curve independent of friction and proposed a stress correction function for fast estimation of flow stress curve to large strains. The aim of this paper is threefold. Firstly, to investigate whether the analytical method for stress correction of tensile necking can, or cannot, be extended to HSC specimen for correcting average true stress into flow stress. Secondly, to develop an inverse method based on Kriging surrogate model for identifying the optimal parameters of modified Voce model using HSC specimen. Lastly, the advantages and disadvantages of these three methods were compared and the recommendations for application were also discussed. The results show that the analytical method is more suitable to the stress correction for material with higher n-value but shows worse capability for correcting flow stress related to large strains for material with lower n-value. For Q420 steel, the maximum strain achieved by HSC specimen (0.8) is far higher than that achieved by cylindrical tension specimen (0.55). The analytical method can correct the flow stress in the strain range of 0–0.5 effectively but underestimating the flow stress in the strain range of 0.5–0.8 due to its low n-value. Both inverse method and stress correction function can determine the flow stress in the strain range of 0–0.8 successfully. Thus, for isotropic material with tension–compression yield symmetry, it is recommended to use the HSC specimen instead of conventional tension and compression tests of cylindrical specimens to determine the flow stress curve to large strains.

Study on the Creep Properties and Microstructures of EW75 Alloy

The creep properties and microstructures of the aged (220°C, 6h) EW75 magnesium alloy at different temperature and of stress conditions were studied. The experimental results show that, the deformation temperature and deformation rate have visible influence on flow stress curve of EW75 alloy. When the deformation temperature was constant, the stress rheological increases with the strain rate increasing. When the deformation rate was constant, the flow stress decreases with the deformation temperature increasing. When the deformation temperature was low or deformation rate was quickly, the shear fracture was along 45° direction. At low temperature and high strain rate conditions, the recrystallization of EW75 alloy was not obvious. When raising the temperature and decreasing deformation rate, the dynamic recrystallization ratio was improved markedly. The generating and moving of dislocations during deformation were main factor to determine the final microstructures. At 100MPa, 200°C conditions, the creep rate control mechanism of EW75 alloy was dislocation slip by solute drag. Precipitation phases coarsening was the main reason that the creep properties of EW75 alloy decreased.

A Flow Stress Model for AZ31B Considering Recrystallization Kinetics

According to the different softening mechanism, a flow stress model for magnesium alloy AZ31B is established. At the stage of dynamic recovery (DRV), the effect of work hardening (WH) and DRV on flow stress is described by dislocation evolution model. At the stage of dynamic recrystallization (DRX), the flow stress curve is obtained from Avrami equation denoting the recryatallization kinetics. Model parameter and its dependence on deformation condition are identified by the measured flow stress curve. The calculated curves agree well with the measured ones, which demonstrate the availability of the method.

A New Constitutive Equation to Predict Single Peak Flow Stress Curves

In this study a new constitutive equation, using the extrapolation of dynamic recovery (DRV) flow stress curve and kinetic equation for dynamic recrystallization (DRX), has been developed. This model is expressed in terms of characteristic points such as critical stress, critical strain, DRX steady state stress, and DRV saturation stress. Moreover, this analysis was done for the stress-strain curves under hot working condition for Ti-IF steel, but it is not dependent on the type of material and can be extended for any condition that a single peak dynamic recrystallization occurs. The results indicate that the stress-strain curves predicted by this model are in a good agreement with experimentally measured ones at all deformation conditions.

Study on the Flow Stress Curve of the Warm Deformation of Medium Carbon Steel

The warm compression experiment of medium carbon steel was conducted using the Gleeble-3500 thermal/mechanical simulator system. By the experiment, the warm deformation of medium carbon steel was studied within the temperature (500~700°C) and the strain rate (0.001~10s-1). The results indicate that the flow stress was increasing with the lowering temperature and the higher strain rate. And the stress-strain curves could be divided into four parts, including four stage of the Strain-Hardening, the First Softening, the Strong Softening, and the Steady Deformation. Dynamic recovery softening has little effect on the flow stress. The peak stress was caused by kink and fracture of the lamellar cementite. Strong softening stage was longer than other one, while its softening influence was stronger compared with hot deformation.

Determination of Uniaxial Flow Stress Curve Using Aero-Bulge Test for Very Thin Copper Sheet

Determination of the flow stress curve is an important step for precisely describing material behavior in Finite Element simulations. The flow stress curve is generally determined by taking a uniaxial tensile test as a standard. In the case of very thin sheet, since the fracture is generated at a low strain, there is not enough uniaxial data obtained to be applied in the FE simulation. The reason for this is that charactering plastic deformation at a large strain values by extrapolating a flow stress curve which is based on insufficient measurement data is highly susceptible to error. Bulge test is useful method for determining the equivalent biaxial flow stress curve up to a large strain. In this paper, the biaxial flow stresses curve for very thin copper sheet with thickness 35 and 50 μm were determined using the aero-bulge test. A new empirical model was derived for the estimation of the sheet thickness at the pole. After the compatibility between uniaxial and biaxial flow stresses was verified, the uniaxial flow stress curve was determined from the aero-bulge test using reverse engineering. The methodology of extrapolation of the flow stress curve at a large strain was finally proposed for application in FE simulations.