Deformation Behavior Of Sheet Metals Research Articles

A Novel High-Speed Bulge Test to Identify the Large Deformation Behavior of Sheet Metals

A Novel High-Speed Bulge Test to Identify the Large Deformation Behavior of Sheet Metals

Subregional flexible-die control in magnetorheological pressure forming

Magnetorheological pressure forming (MRPF) has shown great flexibility in process control by adjusting the mechanical property of MR fluid. However, the regulation of flexible-die property at present is holistic. In this paper, MRPF based on subregional flexible-die control was proposed, which makes the MR fluid show different force transfer characteristics to meet the requirements of flexible-die performance in different areas. Two kinds of non-uniform magnetic fields were established using the combination of coil and iron core groups based on the measurement and finite element simulations. The MRPF experiments of the Al1060-O sheet using the holistic and subregional flexible-die control methods were conducted. The effects of subregional flexible-die control on loading curve, strain distribution, configuration, wall thickness thinning rate, and fracture characteristics were analyzed. Compared with the case that the specimen ellipticity changes little under the central-dominated magnetic field (CMF), the ellipticity gradually decreases under the fringing-dominated magnetic field (FMF) with the increase of the current. The fracture modes can be divided into three types, and the fracture position transfers from the center of the specimen to the circle of R10mm and then to the die fillet area. The influence mechanism of subregional flexible-die control on sheet deformation behavior and fracture mode is further clarified. The non-uniform distribution of cavity pressure and friction conditions caused by the magnetorheological effect is the main factor affecting the stress states and final deformation behavior of sheet metal. This study provides a new path for intelligent control in sheet metal forming of complex thin-walled parts.

Mechanism of magnetic field condition on deformation behavior of sheet metal using a property-adjustable flexible-die

To provide intelligent control options during sheet metal forming process, magnetorheological (MR) fluid has been applied as a property-adjustable flexible-die. However, the mechanism of the magnetic field effect on the sheet deformation behavior is not clear yet. In this paper, to clarify the deformation mechanism, the bulging behavior of thin-walled Al1060-O sheet was systematically investigated through experiment and numerical simulation. Special attention was paid to the effect of magnetic field on loading curve, bulge dome height, and thickness strain distribution. Results showed that the maximum bulging force increased obviously with the increase of magnetic flux density. The bulge dome height and thickness strain were influenced by magnetic field condition and magnetic particle content synthetically. Appropriate magnetic field condition and magnetic particle content can improve formability. However, excessive magnetic flux intensity and magnetic particle volume fraction, for example, B = 0.20 T and φ = 46%, would lead to premature rupture. Finite element analysis was adopted to assist in explaining the bulging behavior. Detailed analysis indicated that the flow stress variations caused by magnetic field affect the stress states, velocity distribution of MR fluid, and sheet/MR fluid contact condition, further leading to different stress and strain distribution of sheet metal. Finally it causes the great difference in specimen geometry and wall thickness distribution.

Towards neural network models for describing the large deformation behavior of sheet metal

Neural networks provide a potentially viable alternative to a differential equation based constitutive models. Here, a neural network model is developed to describe the large deformation response of a Levy-von Mises sheet material with isotropic strain hardening. Using a conventional return-mapping scheme, virtual experiments are performed to generate stress-strain data for random monotonic biaxial loading paths (up to strains of 0.2). Subsequently, a basic feedforward neural network model is trained and validated using the results from virtual experiments. The results for a shallow network with only two hidden layers show remarkably good agreement with all experimental data. The identified neural network model is implemented into a user material subroutine and used in basic structural simulations such as uniaxial tensile and notched tension experiments. In addition to demonstrating the potential of neural networks for modeling the rate-independent plasticity of metals, their application to more complex problems involving strain-rate and temperature effects is discussed.

Analytical Model and Testing Method for Equivalent Stress-Strain Relation of Anisotropic Thin-Walled Steel Tube

Abstract The deformation behavior of sheet metal is currently presented by the stress-strain curve, which gives the variation of flow stress as strain increases. The correlation between stress and strain is also used to characterize the material hardening under different stress states. For anisotropic materials, the deformation behavior depends on stress state, and it is important to realize similar deformation conditions to obtain the stress-strain relationship. In this article, the profile of the free bulging zone was described by a tangent circle model during tube bulging test with fixed ends. In conjunction with the newly suggested linear-model for instantaneous thickness at the middle point, an analytical model and testing method is developed to establish the equivalent stress–equivalent strain relation of tubes. Experiments of STKM11A steel tube have been carried out, and different yield criteria are adopted to define the equivalent stress and equivalent strain. The effect of the anisotropic parameters on the mechanical property is analyzed. Results show that the profile of the free bulging zone can be well reproduced by the tangent circle model for practical application, and the analytical model and data measuring method for the bulging test can be simplified. The equivalent stress–equivalent strain curve by bulging test is rather different from that by uniaxial tension test, with a lower equivalent stress in the initial yielding stage and a greater rate of the equivalent stress increasing with the increase of the equivalent strain. In addition, the shorter the bulging zone length is, the greater the maximum equivalent strain is, because equal-biaxial stress state is approached. As the axial stress is significantly smaller than the hoop stress in bulging test, the anisotropic parameter along the hoop direction has a stronger effect on the determined curve and should be determined precisely.

Simulation and numerical analysis of warm stamping (AA6061)

Abstract Stamping process is an important sheet metal forming process used in domestic and automotive components. In this process, blank material is converted into the desired complicated shape by punching process.Stamping includes a process such as punching using a punch or ram, blanking, embossing, bending, flanging, and coining.Cost minimisation ,less time consumption, increase in efficiency are very important in the recent manufacturing industry. Today simulation techniques are used to analyse the performing of dies, processes and billets prior to real timeimplementation. Simulations enable us to find out the common defects such as tears, wrinkles, and material thinning. Finite element analysis (FEA) is the method of simulating the process of sheet metal forming to determine various behaviour of the sheet metal and helps us to find out whether it will produce parts free of defects such as fracture or wrinkling. Sheet metal forming is a popularly used metal working process. In this paper stamping process is analysed numerically using a simulation software package LSTC – LSDYNA. A three dimensional thermo-mechanically coupled stamping model is analysed and its validations are presented. The analysis data from the numerical simulation helps in understanding the real-time mechanism of sheet metal forming process.There areonly few works were reported on numerical analysis of warm stamping of AA6061 such as load requirement, blank holder pressure, stress-strain analysis.This work is based on study the deformation behaviour of sheet metal, flow stress analysis, contact pressure and punch velocity.

Size effect affected springback in micro/meso scale bending process: Experiments and numerical modeling

The springback behavior has been revealed to affect the dimensional accuracy of sheet metal products significantly in the forming process. As the forming scale decreases to micro/meso level, the deformation behavior of sheet metals has been reported to be different from that at the macro level due to the so-called size effect. This study thus aims to investigate and characterize the springback behavior of sheet metals affected by the size effect at micro/meso scale. The copper specimens with different thicknesses and grain sizes were employed in the uniaxial tensile, nano-indentation and micro hardness tests to analyze the size effect on the material elastic/plastic deformation behavior at first. The flow stress, yield strength and hardness are revealed to decrease with the increase of grain size. Following that the bending experiments with three different punch angles were performed. The effects of both feature and grain sizes on the springback angle were investigated. The springback angle is found to decrease with the punch angle, the grain size, the ratio of thickness to grain size (t/d) and the reduction of thickness. To dig the fundamental mechanism behind the size-effect affected springback behavior, a composite model based on the surface layer theory was employed to describe the constitutive plastic deformation of material with different grain size and thicknesses. The model was further applied in the finite element (FE) simulations of bending process. Based on the experimental and FE results of sheet metals at micro/meso scale bending process, the interactive effect of feature and grain sizes is revealed to be critical in microforming processes. As a preliminary work, the flow stress reduction described by the composite model is found to be able to characterize the global springback behavior during the micro bending test.

Experimental validation of numerical model for asymmetric deep drawing of DP780 steel sheet using digital image correlation

A validated numerical model for in-plane stress/strain prediction is essential in understanding the deformation behaviour of sheet metal forming process, in particular, asymmetric deep drawing of advanced high strength steel sheet. In this work, the Yld2000-2d anisotropic yield criterion integrated with the initial homogeneous anisotropic hardening model was employed to describe the complex material behaviours of DP780 steel as well as the adoption of Yoshida chord model for elastic modulus degradation. Digital image correlation technique was utilized to measure the in-plane strain and shape deviation of the developed P- channel. The validity of the numerical model was assessed by comparing the predicted strain distribution and twist springback with the measured results. It indicates that the developed numerical model based on the selected constitutive models is acceptable for the deformation analysis, although the predicted discrepancies still exist.

Material parameters identification procedure for BBC2003 yield criterion and earing prediction in deep drawing

Formation of wavy boundaries at the open end of a drawn cup is referred to as earing. Plastic anisotropy is the main parameter contributing to formation of ears. Therefore, in order to eliminate the undesirable ears in sheet metal parts, it is essential to predict the deformation behavior of sheet metals using advanced anisotropic yield criteria. Accordingly, the objective of this article is that to use BBC2003 yield criterion, to predict earing in deep drawing of cylindrical cups from AA3105 aluminum alloy. First, a new and simple approach is introduced to calculate the eight constants of this yield function. In this approach, only the uniaxial and plane strain tension tests results, were used. Nonlinear governing equations of these constants were derived and solved by means of well-known steepest descend method. In order to investigate performance and accuracy of this method, the calculated constants were utilized to predict normalized uniaxial yield stress, anisotropy parameters and plane stress yield surface. Then, based on this yield criterion and the associated flow rule, a VUMAT subroutine was developed and employed in the commercial finite element software ABAQUS/Explicit dynamics. The simulation of drawing process was conducted for anisotropic AA3105 aluminum alloy sheet. Cup height (earing profile) and thickness distribution were obtained from experiments and corresponding numerical simulations. Comparison of these results demonstrates great potential of BBC2003 yield criterion in accurate prediction of earing.

MULTIAXIAL STRESS TESTS FOR METAL SHEETS AND TUBES FOR ACCURATE MATERIAL MODELING AND FORMING SIMULATIONS

This paper is a review of the mechanical testing methods developed by the author's research group: multiaxial stress test methods using a cruciform test piece and a tubular test piece. The former is useful for small strain ranges under several percent while the latter is useful for larger strain ranges (from yielding to fracture). These test methods are useful to determine appropriate materials model for performing accurate metal forming simulations. Special attention is given to the measurement and modeling of the anisotropic plastic deformation behavior of sheet metals commonly used in industry and to the validation of the material models based on phenomenological yield functions for large plastic strain ranges. The effects of material models used in metal forming simulations on the improvement of the predictive accuracy for forming defects are also discussed.

Deformation behavior and fracture morphology in the uniaxial tension process of stainless steel sheets under the fluid pressure

The sheet metal hydroforming technique is easy to achieve lightweight and is widely applied in the aerospace and automotive fields. In order to realize the reliability test for uniaxial tensile forming properties of sheet metals under the condition of three-dimensional stress, experimental apparatus and dedicated fixtures for uniaxial tensile tests of sheet metals under fluid pressure are designed in this article. The stainless steel 304 is employed for this experiment, and commercial finite element software ABAQUS is also adopted to study the deformation behavior of sheet metals under fluid pressure. The results indicate that when the fluid pressure of gauge length reaches a certain value, the specific region will present “double necking” and the peak values of equivalent stress and reduction rate are much lower than that of the conventional ones. When the strain rate is 0.01 s−1, the elongation of the 0.95-mm thickness sheet increases more obviously, reaching 24.1%. With fluid pressure, the fracture morphologies of stainless steel thin sheets and thick sheets are all ridge distributed from both sides to the center of symmetry, and tiny dimples distribute mainly in the rest parts. Based on the results of the experiment and finite element method, the uniaxial tensile forming properties of sheet metals under fluid pressure have been evaluated successfully and easily by this experimental apparatus.

二軸バルジ試験法による高強度鋼板の加工硬化特性の測定と材料モデリング

The deformation behavior of high strength steel sheets with a tensile strength of 590 MPa under biaxial tension was investigated for a strain range from initial yielding to fracture. The test material was bent and laser welded to fabricate a tubular specimen with an inner diameter of 44.6mm, a wall thickness of 1.2mm, and an axial length of 200mm. Multiaxial tube expansion tests were performed using a servo-controlled tension-internal pressure testing machine. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve (FLC) and forming limit stress curve (FLSC) of the as-received sheet material, in addition to the contours of plastic work and the directions of plastic strain rates from initial yielding to fracture. Results calculated using the Yld2000-2d yield function with exponents of 6 to 8 provided the closest agreement with the measured work contours and directions of plastic strain rates for a reference plastic strain range of 0.002 ＜ε0p＜ 0.20. It was concluded that the multiaxial tube expansion test is effective for measuring the multiaxial deformation behavior of sheet metals for a wide range of plastic strains.

Multiaxial tube expansion test method for measurement of sheet metal deformation behavior under biaxial tension for a large strain range

A servo-controlled multiaxial tube expansion testing machine was developed to measure the multiaxial plastic deformation behavior of sheet metals for the range of strain from initial yield to fracture. The testing machine is capable of applying arbitrary principal stress or strain paths to a tubular specimen using an electrical, closed-loop servo-control system for axial force and internal pressure, in addition to having a novel strain measurement apparatus for tubular specimens. Tubular specimens with an inner diameter of 44.6mm were fabricated from cold rolled interstitial-free steel sheet with a thickness of 0.7mm by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve (FLC) and forming limit stress curve (FLSC) of the as-received sheet material, in addition to contours of plastic work and directions of plastic strain rates. Results calculated using the Yld2000-2d yield function with an exponent of 6 provided the closest agreement with the measured work contours and directions of plastic strain rates for an equivalent plastic strain range of 0.005–0.36. Moreover, a Marciniak–Kuczyński-type forming limit analysis was performed using the most appropriate yield function with the assumption of isotropic hardening; the calculated and measured FLC and FLSC were in fair agreement. Therefore, the multiaxial tube expansion test is effective to measure the multiaxial deformation behavior of sheet metals for a large range of plastic strain.

Material modeling of 6000 series aluminum alloy sheets with different density cube textures and effect on the accuracy of finite element simulation

Biaxial tensile tests of 6000 series aluminum alloy sheet with different density cube textures were conducted using cruciform specimens. The specimens were loaded under linear stress paths in a servo-controlled biaxial tensile testing machine. The plastic orthotropy remained coaxial with the principal stresses throughout every experiment. Successive contours of plastic work in stress space and the directions of plastic strain rates were precisely measured and compared with those calculated using selected yield functions. The Yld2000-2d yield functions with exponents of 12 and 6 were capable of reproducing the general work contour trends and the directions of plastic strain rates observed for the test materials with high and low density cube textures, respectively. Hydraulic bulge tests were also conducted and the variation of thickness strain along the meridian directions of the bulged specimen was compared with that calculated using finite element analysis (FEA) based on selected yield functions. Differences in the cube texture density caused significant differences in the strain distribution of the bulged specimens, and were in good agreement with the FEA results obtained using the Yld2000-2d yield functions. It is thus concluded that the biaxial tensile testing method with a cruciform specimen is an effective material testing method for accurately detecting and modeling the deformation behavior of sheet metals under biaxial tension.

Anisotropic Plastic Behavior of TRIP 780 Steel Sheet in Hole Expansion Test

Plastic behavior of advanced high strength steel sheet of grade TRIP780 (Transformation Induced Plasticity) was investigated using three different yield functions, namely, the von Mises’s isotropic, Hill’s anisotropic (Hill’48), and Barlat’s anisotropic (Yld2000-2d) criterion. Uniaxial tensile and balanced biaxial test were conducted for the examined steel in order to characterize flow behavior and plastic anisotropy in different stress states. Additionally, disk compression test was performed for obtaining the balanced r-value. According to the different yield criteria, yield stresses and r-values were calculated for different directions and then compared with experimental data. To verify the modeling accuracy, a hole expansion test was carried out experimentally and numerically by FE simulation. Stress-strain curve from the biaxial test was described using voce and swift hardening models. Punch load and stroke, final hole radius, and strain distribution on specimen surface along the hole circumference and the specimen diameter in rolling and transverse directions were determined and compared with the experimental results. It was found that the simulations applying Yld2000-2d yield function provided an acceptable agreement. Consequently, it is noted that the anisotropic yield potential significantly affects the accuracy of the predicted deformation behavior of sheet metal subjected to hole expanding load.

Electro-hydraulic forming of sheet metals: Free-forming vs. conical-die forming

This work builds upon our recent advances in quantifying high-rate deformation behavior of sheet metals, during electro-hydraulic forming (EHF), using high-speed imaging and digital image correlation techniques. Aluminum alloy AA5182-O and DP600 steel sheets (1mm thick, ∼152mm diameter) were EHF deformed by high-energy (up to ∼34kJ) pressure-pulse in an open die (free-forming) and inside a conical die. The deformation history (velocity, strain, strain-rate, and strain-path) at the apex of the formed domes was quantified and analyzed. The data shows that the use of a die in the EHF process resulted in an amplification, relative to free-forming conditions, of the out-of-plane normal velocity and in-plane strain-rate at the dome apex. This amplification is attributed to the focusing action of the die on account of its conical geometry. Further, while the strain-path at the dome apex was generally linear and proportional, the use of a die resulted in greater strain at the apex relative to the strain during free-forming. The sheet deformation profile in the EHF process was found to be different from that previously observed in electromagnetic forming (EMF) and, thus, the two processes are expected to result in different strain-paths and formability. It is anticipated that quantitative information of the sheet deformation history, made possible by the experimental technique developed in this work, will improve our understanding of the roles of strain-rate and sheet–die interactions in enhancing the sheet metal formability during high-rate forming.

FEM Simulation of Deformation Field during the Sheet Metal Forming

In this paper, computer aided engineering of some innovative sheet metal forming processes is reviewed. Because of the complexity of this process, numerical simulation becomes necessary for its analysis. The deformation behavior of sheet metal in forming process is numerically simulated using quasi-static implicit and explicit finite element codes. The main advantages of the finite element method are: the capability of obtaining detailed solutions of the mechanics in a deforming body, namely, stresses, shapes, strains or contact pressure distributions; and the computer codes, can be used for a large variety of problems by simply changing the input data.

Effect of Loading Location of Viscous Medium on Sheet Deformation in Viscous Pressure Bulge (VPB) Test

Viscous pressure forming (VPF) uses a highly viscous but flowable material as pressure-carrying medium (PCM). Due to the relative low flowability of viscous medium compared with fluid, nonuniform pressure distribution in viscous medium can be used to control and regulate the deformation sequence of the workpiece through controlling the loading mode of viscous medium. In the present study, viscous pressure bulge (VPB) tests with three kinds of loading location of viscous medium (central zone, corner zone and the whole deformation zone) are conducted and the influences of loading location of viscous medium on sheet deformation behavior are investigated via numerical simulations and experiments. It is found that changing the loading location of viscous medium can greatly affect the deformation behavior of sheet metal. When the viscous medium is injected from the die corner zone, a local high pressure formed at the corner zone of sheet metal and a higher limiting dome height and strains are obtained.

Investigation on deformation behavior of sheet metals in viscous pressure bulging based on ESPI

In order to analyze the effect of viscous medium on the deformation behavior of sheet metals in viscous pressure bulging (VPB), the entire deformation process including instability and fracture was investigated real-timely by the aid of electronic speckle pattern interferometry (ESPI). Images of speckle patterns were captured continuously to obtain fringe patterns representing the full field strain rate. Values of strain rates were calculated based on the fringe patterns. The evolution of the weak region from the initial defect to the groove until crack was also observed through the fringe patterns. The onset of diffuse and localized necking were determined qualitatively and quantitatively. Experimental results show that the deformation of sheet metals in VPB passed through five states, namely, uniform deformation, strain localization, diffuse necking, localized necking and fracture. A defect emerged in strain localization. The growth of the defect caused the diffuse necking and generated a groove. The groove expanded mainly in length direction until the localized necking occurred. Finally the specimen fractured as a result of groove deepening. The tangential adhesive stress provided by viscous medium in VPB restricted the locally larger strain of the specimen. The diffuse necking was postponed greatly. Theoretical prediction of the limit strains of sheet metals in VPB would be made based on the experimental results in further work.

EXPERIMENTAL STUDY ON EFFECT FACTORS OF VISCOUS ADHESIVE STRESS IN VISCOUS PRESSURE FORMING

Because of the coupled deformation characteristic between viscous medium and sheet metal, however, it is difficult to analyze the deformation behavior of sheet metal under the interaction of viscous adhesive stress. Based on the above situation, viscous medium tensile test is carried out by using a kind of polymethylvinylpolymer as viscous pressure-carrying medium. Using the viscous adhesive stress as tensile stress, sheet metal deformation can be measured directly. The influence of viscous adhesive stress on the deformation of sheet specimen is investigated and some effect factors of viscous adhesive stress are analyzed preliminarily. Experimental results show that the viscous adhesive stress increases with an increasing of pressure difference on the surface of sheet metal, flowing velocity and hydrostatic pressure of viscous medium. Research results provide a good foundation for utilizing viscous adhesive stress to improve the formability of sheet metal in viscous pressure forming.