Models for parameter assessment in wire drawing

The strain hardening behavior of model polymer glasses is studied with simulations over a wide range of entanglement densities, temperatures, strain rates, and chain lengths. Entangled polymers deform affinely at scales larger than the entanglement length as assumed in entropic network models of strain hardening. The dependence of strain hardening on strain and entanglement density is also consistent with these models, but the temperature dependence has the opposite trend. The dependence on temperature, rate, and interaction strength can instead be understood as reflecting changes in the flow stress. Microscopic analysis of local rearrangements and the primitive paths between entanglements is used to test models of strain hardening. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3487–3500, 2006

The paper presents the results of FEM computer simulations of the drawing process on a cylindrical journal of thin-walled CuSn8 alloy tubes. This study demonstrates through FEM simulations that the drawing angle significantly affects the state of stress, strain and tool wear. Regardless of the geometry of the drawing die, greater wear was noted for the cylindrical plug. Increasing the angle of drawing die 2α from 6° to 38° contributed to a slight 5% increase in wear of the drawing dies and more than 80% increase in plug wear. Accelerated tool wear at high angles is to be associated with higher pipe pressures on the drawing die and plug. Inadequate selection of drawing geometry can cause additional material deformation effort and material fracture in the industrial drawing process of thin-walled tubes. After the drawing process, these tubes may also show non-uniform wall thickness. The optimum drawing angle for thin-walled tubes is 2α = 22°, for which about a 10% decrease in the drawing force was recorded.

Residual stress may influence the mechanical behavior and durability of drawn materials. Thus, this study develops a multiple reduction die (MRD) that can reduce residual stress during the drawing process. The MRD set consists of several die tips, die cases, and lubricating equipment. All the die tips of the MRD were disposed of simultaneously. Finite element analysis of the drawing process was performed according to the reduction ratio of each die tip, and the variables in drawing process with the MRD were optimized using a deep neural network to minimize the residual stress. Experiments on the drawing process with the conventional die and MRD were performed to evaluate the residual stress and verify the effectiveness of the MRD. The results of X-ray diffraction measurements indicated that the axial and hoop residual stresses on the surface were dramatically reduced.

Metastable dual‐phase high‐entropy alloys (HEAs) have become an attractive paradigm as structural materials due to their outstanding mechanical properties compared with single‐phase HEAs, which originate from the multiple strengthening mechanisms. Herein, a theoretical model is established by integrating the effects of lattice distortion, dislocation, grain boundary, and phase transformation, to study the mechanical responses of metastable HEAs during uniaxial tensile deformation. The results show the contribution of various mechanisms to the strength, and strain hardening can be determined quantitatively. In particular, the face‐centered cubic (FCC) to body‐centered cubic (BCC) phase transformation would dominate the flow stress with plastic strain to exceed 6%, due to the strong phase interface strengthening and the formation of hard BCC phase. This work provides a microstructure‐based constitutive model for predicting the flow stress and strain hardening properties of metastable HEAs.

Using the ANSYS program, the stress-strain state of steel wire in the drawing process is investigated by the finite element method. The problem of plastic flow of metal in a conical matrix-fiber is solved. The problem statement is axisymmetric for different positions of the wire relative to the matrix. The fields of radial stresses, axial stresses and equivalent plastic deformations are obtained. The results allow us to proceed to the study of the influence of various drawing parameters on the stress state of the wire. This will make it possible to optimize the drawing process and offer new drawing designs and lubrication methods that reduce wear and increase the service life of this tool.

The best practice in modeling material yield, strain hardening and anisotropic behavior in plastic deformation has been analyzed for an AA 6016 aluminum alloy. The investigation was based on the extensive material property testing, stamping benchmarking and AutoForm simulations. For the material property testing, both the uniaxial tensile test and the hydraulic bulge test were conducted. The elliptic punch test and the cross die test served as the benchmarks to validate the simulation results. In the simulations, the material characteristics was modeled with the combinations of four strain-hardening models and three yield criteria. By comparing the simulation results with the experimental measurements, the influence of material modeling on aluminum stamping simulation accuracy was evaluated. It was concluded from this study that the yield criterion is the key factor in controlling the simulation accuracy. The simulation with the BBC2005 yield model predicts the most accurate results. It was also shown that the combined Swift/Hockett-Sherby strain-hardening model is most suitable to describe aluminum strain hardening behavior.

The work aim is the mathematical modeling of the austenitic steel pipes sink drawing process to establish the effect of the pipe wall thickness on the material stress-strain state and to predict the martensitic transformation development. The drawing process was simulated for thin-walled and thick-walled pipes by the finite element method using the DEFORM-2D software package. There are determined the shape change and deformations distribution, the deformations intensity and the average normal stress for seven variants of pipes with different wall thickness ratios and diameter are determined. Distribution patterns for two extreme variants are shown. The difference is shown in the stress state at the drawing process of thin-walled and thick-walled pipes. The change in the average normal stress in time t of a particle passage through the deformation zone for the outer and the inner surfaces for a thin-walled and extra thick-walled pipe is given. In either case, the compression stresses up to –400 MPa for the inner surface and tensile stresses for the outer surface up to +200 MPa are prevail at the entrance to the deformation zone. Inside the deformation zone, a thin-walled pipe is characterized by the presence of approximately equal compressive stresses both on the outer surface and on the inner one. However, the stresses are close to zero outside and reach –600 MPa inside for a thick-walled pipe. The deformation zone numerical simulation in the sink drawing made of austenitic steel showed that there are differences in the stress-strain state in the case of processing thin-walled and thick-walled pipes. The performed calculations and comparison with the literature data on the stress state influence on the cold martensitic transformation process made it possible to predict the development of this transformation in the drawing process. It was revealed that during thin-walled pipes drawing process are created more favorable conditions for the phase transition development.This work was performed with partial financial support of Resolution No. 211 of the Government of the Russian Federation, contract No. 02.A03.21.

Numerical modelling of stainless steel plates

A Round Robin Programme was established by a working group of Commission X in 1997 in order to evaluate the possibilities of residual stress and distortion prediction (RSDP) in welded structures and to validate and benchmark prediction codes based on finite element simulation of the welding process. Calculations on residual tresses have been carried out on an austenitic steel plate with well-defined material and geometrical data, welding conditions and with the proposal to use the kinematic hardening model. In addition, a second programme for measurements of residual stresses in this plate has been started in 2003. Measurements have been taken on three austenitic steel plates using the X-ray measuring method as well as neutron diffraction methods and different kinds and variations of the hole drilling method. Although a rough agreement between the measured results and the calculated ones was found in principle, distinct differences have been observed in detail — especially for the longitudinal residual stresses. The calculations indicated nearly constant tensile stresses of the order of the initial yield strength in the weld seam and in the adjacent heat affected zone (HAZ). The measurements however revealed stress maxima in the HAZ with values clearly higher than the initial yield strength. Therefore additional calculations have been carried out with different hardening models in order to investigate their influence on the residual stress results. In fact, calculations performed with the isotropic hardening model could approve these stress maxima quite exactly. In further detailed investigations, it could be shown that the reason for the stress maxima is the strain hardening in the HAZ due to thermal stresses during the welding process. Finally basic considerations could explain that the kinematic hardening model, including the Bauschinger effect, cannot account for this hardening effect. Literature indicates that the Bauschinger effect is at least not fully effective at high working temperatures of austenitic steels. Altogether, the calculation programme has shown that a rough estimation of the residual stress state after welding is possible without any special distinction of the hardening model to be used. For a more precise and detailed information, the chosen hardening model is of significant relevance.

A new method is presented for the measurement of axisymmetric residual stresses in a solid rod or hollow cylinder in the region which is in a state of plane strain. The experimental procedure involves, first, making a complete transverse cut, normal to the axis of the part in the region which is initially in plane strain. Next, a number of strain gages are mounted on one of the faces produced by the first cut. A second transverse cut, parallel to the first, is then made to remove a thin slice containing the strain gages. It is shown that the change in strain gage readings due to removal of the slice may be used to deduce the axial stress in plane strain at the location of the first cut. The hoop stresses in the thin slice, which is in a state of plane stress, are readily obtained from the “crack compliance” procedure which we have described in earlier work. From this information and the initial axial stress in plane strain it is shown that the initial hoop and radial stresses in plane strain may be determined. The method is validated by obtaining the stresses in a water quenched rod which are shown to be in excellent agreement with those measured using a different experimental approach. Since the method does not require the measurement to be carried out on the original rod or cylinder, it allows the original residual stresses to be measured from a fractured shaft by making a slice near the plane of the fracture if the fracture is dominated by elastic deformation.

To precisely predict springback, the stress and strain state has to be described correctly during the forming process and after load removal. In this work, different yield locus descriptions are considered for springback prediction of a CR700Y980T-DP steel with a thickness of 1.5 mm. Experiments and Finite Element Analyses (FEA) with LS-DYNA of a U-bending validation tool were performed and springback predictions compared. In the FEA Hill48, Barlat89 and Barlat’s YLD2000 for isotropic hardening, Yoshida-Uemori as a kinematic hardening model and the homogeneous anisotropic hardening (HAH) model are chosen. Monotonic and two-step tensile tests with a directional change of stress are performed and cyclic tension-compression tests are used to determine the material parameters of the investigated hardening models. It is seen that both Yoshida-Uemori and HAH models provide a significant improvement in the springback prediction. The results, however greatly depend on the identification procedure as well as on the modelling of cross-loading contraction effects.

Role of Viscosity in Hydro-forming Process

In this investigation, a design of experiments analysis of distortion for a typical manufacturing process involving pre-straightening, cold drawing, and induction hardening of AISI 1045 cylindrical steel bars was carried out. A careful characterization of the material, including residual stress states and geometrical changes, was done for the different manufacturing steps. In order to identify effects and correlations on distortion behavior, the investigated variables included the batch influence, the combined drawing process itself with two different drawing angles and two different polishing and straightening (P.S.) angles, a stress relief treatment which was applied to a part of the samples, and finally induction hardening with two different surface hardening depths. Main and statistically significant effects on the distortion of the induction-hardened samples were found to be in this order: first, the interaction between the drawing angle and batch, then the interaction between drawing angles, and finally drawing angle and induction hardened layer. It was also found that the distortion potentials are transmitted from the drawing process to further manufacturing steps and, consequently, from one production site to the next.

Consider a long cylindrical bar (of a homogeneous, isotropic, linearly elastic material) that is initially in static equilibrium under the action of uniform radial pressure on its cylindrical surface and axial stress as required to maintain plane strain. At zero time the radial pressure is suddenly removed, but plane-strain conditions are maintained. A finite Hankel transform solution is presented for the ensuing radial displacement and for the radial, circumferential, and axial stresses—all as functions of the radial coordinate and time. Results of computations for Poisson’s ratio equal to 0.3 are presented graphically.

This study clarified the effects of the chosen hardening model, through-thickness normal stress, and non-linear strain path on the precision of forming limit diagram (FLD). The Marciniak-Kuczynski (M-K) instability theory was employed to calculate the FLD of AA5754-O based on the different hardening models. Swift, Voce, modified Voce, Kim-Tuan, and modified Kim-Tuan hardening models were adopted to predict the forming limits, and their constant parameters were presented. Predicted FLDs indicate that modified Kim-Tuan has a better agreement with experimental data than other models. After that the influences of through-thickness normal stress on the forming limit diagram were examined. The three-dimensional stress state was converted to plane-stress condition according to the assumption that hydrostatic stress is ineffective on the plastic deformation. The results show that formability improves when normal stress increases. However, the effect of the normal stress on FLD for various hardening models is different. Due to the complexity of sheet metal forming, the strain path of the material may change during the deformation process. The results indicated that strain path has a significant effect on the FLD, and the path dependence of the forming limit stress diagram (FLSD) will be examined in detail. Finally, the forming limits were computed to analyze the effect of the normal stress on the non-linear loading path deformation. It was observed that by increasing the amount of pre-strain, the effect of the normal stress on the formability will diminish. Moreover, results showed that normal stress will affect the path dependence of the FLSD.