Behavior Of Sheet Metals Research Articles

Novel Technique for Static and Dynamic Shear Testing of Ti6Al4V Sheet

Few shear test techniques exist that cover the range of strain rates from static to dynamic. In this work, a novel specimen geometry is presented that can be used for the characterisation of the shear behaviour of sheet metals over a wide range of strain rates using traditional tensile test devices. The main objectives during the development of the shear specimen have been 1) obtaining a homogeneous stress state with low stress triaxiality in the zone of the specimen subjected to shear and 2) appropriateness for dynamic testing. Additionally, avoiding premature specimen failure due to edge effects was aimed at. Most dimensional and practical constraints arose from the dynamic test in which the specimen is loaded by mechanical waves in a split Hopkinson tensile bar device. Design of the specimen geometry is based on finite element simulations using ABAQUS/Explicit. The behaviour of the specimen is compared with the more commonly used simple shear specimen with clamped grips. Advantages of the new technique are shown. The technique is applied to Ti6Al4V sheet. During the high strain rate experiments high speed photography and digital image correlation are used to obtain the local shear strain in the specimen. Comparison of experimental and numerical results shows good correspondence.

Auto-Body Panel Springback Analysis Using Yoshida-Uemori Model

The forming process simulation and springback prediction of an automobile body panel is implemented by using JSTAMP/LS-DYNA. Yoshida-Uemori model is selected to characterize the anisotropic material behavior of sheet metal during forming. Simulation predictions on spingback are compared with experiment measurements along with numerical results from other material models to demonstrate the effectiveness and accuracy of the Yoshida-Uemori model.

판재성형 해석시 금형내의 공기거동 모델링

During stamping processes, the air trapped between sheet metal and the die cavity can be highly compressed and ultimately reduce the shape accuracy of formed panels. To prevent this problem, vent holes and passages are sometimes drilled into the based on expert experience and know-how. CAE can be also used for analyzing the air behavior in die cavity during stamping process, incorporating both elasto-plastic behavior of sheet metal and the fluid dynamic behavior of air. This study presents sheet metal forming simulation combined simultaneously with simulation of air behavior in the die cavity. There are three approaches in modeling of air behavior. One is a simple assumption of the bulk modulus having a constant pressure depending on volume change. The next is the use of the ideal gas law having uniform pressure and temperature in air domain. The third is FPM (Finite point method) having non-uniform pressure in air domain. This approach enables direct coupling of mechanical behavior of solid sheet metal and the fluid behavior of air in sheet metal forming simulation, and its result provides the first-hand idea for the location, size and number of the vent holes. In this study, commercial software, PAM- and PAM-, were used.

평면 응력 조건에서 정의된 비이차 비등방 변형률 속도 포텐셜

본 연구를 통하여 비이차 비등방 항복 응력 포텐셜들의 근접 짝되는 변형률 속도 포텐셜들에 대해서 정리하고 Yld2000-2d의 근접 짝되는 Srp2003-2d에 대해서 상세 설명하였다. 제안된 비이차 비등방 변형률 속도 포텐셜 Srp2003-2d 식 형태가 소개 되었고, 볼록성이 증명되었다. 아울러 이방성 상수를 구하는 방법이 제시되었다. Srp2003-2d의 소성 거동을 살펴보기 위하여 자동차 용 알루미늄 합금 판재 AA6022-T4와 항공재료용 알루미늄 합금 AA2090-T3에 응용되었다. Srp2003-2d는 항복 응력 포텐셜 Yld2000-2d와 거의 흡사한 짝됨을 보여 주었으며, 알루미늄 판재의 비등방성을 적절히 묘사하였다. Srp2003-2d는 알루미늄 판재의 성형 공정의 모사를 위하여 이상 공정 이론을 비롯한 강소성체 재료에 대한 정식화에 적절히 응용될 수 있을 것이다. A non-quadratic anisotropic strain rate potential was introduced as a conjugate potential of the yield stress potential Yld2000-2d to describe anisotropic behavior of sheet metals, in particular, aluminum alloy sheets under plane stress state. This strain-rate potential takes into account the anisotropic yield stresses and R-values measured along the directions measured at 0, 45 and 90 degrees from the rolling direction, as well as the balanced biaxial yield stress and strain-rate ratio. The convexity of the strain-rate potential was completely proven. The strain-rate potential was applied for two anisotropic aluminum alloy sheets, AA6022-T4 and AA2090-T3. The results verified that the strain rate potential properly described the anisotropic behavior of aluminum alloy sheets and was closely conjugate of Yld2000-2d under the plane stress state.

Geometry and grain size effects on the fracture behavior of sheet metal in micro-scale plastic deformation

The demand on micro-parts is significantly increasing in the last decade due to the trend of product miniaturization. When the part size is scaled down to micro-scale, the billet material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. The size effect phenomena occur in micro-scale plastic deformation or micro-forming and there are still many unknown phenomena related to size effect, including geometry and grain size effects. It is thus critical to investigate the size effect on deformation behavior, especially for the fracture behavior in micro-scale plastic deformation. In this research, tensile test was conducted with annealed pure copper foils with different thicknesses and grain sizes to study the size effects on fracture behavior. It is found that flow stress, fracture stress and strain, and the number of micro-voids on the fracture surface decrease with the decreasing ratio of specimen size to grain size. Based on the experimental results, dislocation density based models which consider the interactive effect of specimen and grain sizes on fracture stress and strain are developed and their accuracies are further verified and validated with the experimental results obtained from this research and prior arts.

An incremental-update formulation to simulate earing behaviour with crystal plasticity model

To gain a better understanding about the influence of initial texture on deformation behaviour of sheet metals, an incremental-update formulation is adopted for implementing rate-dependent crystal plasticity model into the explicit finite element package, ABAQUS/Explicit. In this formulation, the configuration in the last increment serves as a reference frame, and the stress, grain orientation and hardening behaviour of crystal are updated at the end of each increment. Two user subroutines VUMAT are developed, respectively, for face-centred cubic and hexagonal close packed alloys due to different deformation mechanisms. Earing behaviours of aluminium sheet and AZ31 alloy sheet are simulated for model validation.

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 strain rate on the forming behaviour of sheet metals

The strain rate dependence of plastic yield and failure properties displayed by most metals affects energies, forces and forming limits involved in high speed forming processes. This paper investigates the influence of the strain rate on the forming properties of one laboratory made and three commercial steel grades: a CMnAl TRIP steel, the ferritic structural steel S235JR, the drawing steel DC04 and the ferritic stainless steel AISI 409. First, split Hopkinson tensile bar (SHTB) experiments are carried out to assess the influence of the strain rate on the materials’ stress–strain curves. Subsequently, the obtained SHTB results, together with static tensile test results, are used to model the constitutive behaviour of the investigated steels using the phenomenological Johnson–Cook (JC) model and the Voce model, thus allowing dynamic modelling of forming processes. Finally, forming limit diagrams (FLDs) are calculated using the Marciniak–Kuczynski method. The results clearly show that the effect of the strain rate on forces and energies involved in a forming process, and the forming limits is non-negligible and strongly material dependent.

Identification of Post-Necking Hardening Behaviour of Sheet Metal: a Practical Application to Clinch Forming

Clinching is a mechanical joining technique which involves severe local plastic deformation of two or more sheet metal parts resulting in a permanent mechanical interlock or joint. The required forming load and energy can be determined with the aid of the finite element method. However, a good knowledge of the elasto-plastic properties is of utmost importance to perform a sufficiently accurate simulation. This paper presents two alternative material tests to identify the hardening behaviour of sheet metal beyond the point of maximum uniform elongation. In addition, the material tests were applied to DC05 and the identified material behaviour is evaluated through the prediction of the forming load during clinching.

An Investigation on Effect of Residual Stress on Springback Behavior of High Strength Steel in U-Bending Process

Bending process is an important process in the metal sheet forming in many industries. The main problem of the bending process is the spring-back phenomenon after removing the punch. This research aims the investigation on effect of residual stress on springback behavior of sheet metal in U bending process. The corner setting technique and bottoming process were designed for experiments. The corner setting technique and bottoming has reduced the thickness in bending area to 5, 10, 15 and 20 percent of the original sheet thickness. Clearance between punch and die of both processes was equal to same the thickness. The residual stress value and springback phenomenal were investigated by commercial program code DEFORM 2D which was able to analyze the effect stress and force in bending area. Electrolytic zinc coated carbon steel grade JIS; SECC, JIS; 440 and JIS; 590 which having the thickness of 1 mm were employed as the workpiece material for all experiments. The result of three materials in conventional U bending die shows larger spring back than the corner setting technique and bottoming process. Moreover, the corner setting technique reduces spring back value in bending process but requires high bending force. Corner setting technique and bottoming process at 15% and 20% shows that the spring go of all parts. The FEM simulation results explained the effect of residual stress to springback phenomenal. Comparisons between experimental and finite element method results were also performed.

Identification of the post-necking hardening behaviour of sheet metal by comparison of the internal and external work in the necking zone

Abstract The most common way to evaluate the stress–strain relation of sheet metal is by performing standard tensile tests. However, those tests only allow the identification of the hardening behaviour up to the point of maximum uniform elongation, at least when using standard measuring equipment and simple analytical formulas to derive the relation between force and stress, and, elongation and strain. Usually the hardening behaviour beyond this point is estimated by extrapolation of the hardening behaviour before the point of maximum uniform elongation. Such procedure may yield very different results, depending on the hardening law which is fitted to the available experimental data. Other methods which deal with the problem of extended yield curve identification have been proposed and the most applied among them are finite-element based inverse approaches. This paper presents an alternative method to identify the post-necking hardening behaviour of sheet metal without using a finite element model. The key point in the presented method is the minimization of the discrepancy between the internal and external work in the necking zone during a tensile test. The main focus of this paper is on the presentation of the method, the underlying assumptions and the experimental validation. In addition, the method was applied to identify two common hardening laws for DC05, which is a mild deep drawing steel, using experimental data from the pre- and post-necking region.

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.

768 Reproducibility of gene expression measurements in microarray studies relies on filtering of expressed genes: implications for the understanding of childhood papillary thyroid cancer transcriptome

In this paper, a new experimental technique is reported to study behavior of sheet metals in the range of low and high strain rates. The main purpose of this work was to analyse visco-plastic behavior of sheet metals for an industrial application. It may be mentioned that a variety of industrial processes, such as drawing or even crashworthiness, involve a wide range of strain rates. The test method which is based on previous work, (Klepaczko, J.R., 1994. An experimental technique for shear testing at high and very high strain rates. The case of mild steel. Int. J. Impact Engng. 15, 25–39), is a combination of two experimental techniques, that enables one to obtain the range of strain rates from 10−4 to 103 s−1, (Rusinek, A., Klepaczko, J.R., 1998. Etude experimentale du double cisaillement des tôles, modélisation de l'écoulement viscoplastique. Rapport interne, LPMM). The double-shear specimen is loaded by a fast hydraulic machine, and in the second case a specimen of the same geometry can be loaded by the direct impact of a projectile. The combination of direct impact with the double shear specimen, which is based on the Hopkinson tube technique of force measurement, permits to reduce the rise time of loading in comparison to the bar-tube configuration (Campbell, J.D., Ferguson, W.G., 1970. The temperature and strain rate dependence of shear strength of mild steel. Phil. Mag. 21, 63–82). The combination of two schemes of loading assures a complete determination of the shear stress τ(t), the shear strain Γ(t) and shear strain rate Γ̇(t) as a function of time at wide range of strain rates. The test method has been applied to determine the visco-plastic behavior of cold rolled, Al-calmed sheet steel used in the automotive industry. The experimental results obtained with those two experimental techniques are in agreement with other reported data. An original visco-plastic constitutive relation has been proposed. It permits to model the viscoplastic behavior of the sheet metal tested in the whole range of strain rates considered. A general procedure for the identification of the material constants is proposed (Rusinek, A., Klepaczko, J.R., 1998. Etude experimentale du double cisaillement des tôles, modélisation de l'ecoulement viscoplatique. Rapport interne, LPMM.). It offers the advantage of subdividing the total number of constants into groups, this procedure simplifies the problem of identification. The constitutive relation proposed takes into account the strain hardening, the strain rate sensitivity and temperature effects. A comparison with experimental results is presented in the form of stress–strain curves at different strain rates, up to 103 s−1.

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.

Determination of the flow stress and contact friction of sheet metal in a multi-layered upsetting test

Abstract This paper describes the identification of hardening parameters of DC05 sheet metal and contact friction coefficients using a multi-layered upsetting test (MLUT), the modified two specimen method (MTSM) and a finite element based inverse method. The MTSM is adopted in order to identify the friction coefficient between the tools and the stacked circular specimens which are cut from the base material by spark erosion. Next, the hardening behaviour is identified inversely by combining the results of a MLUT and finite element simulations of the test setup. Finally, the results are compared with standard tensile tests and it is shown that the MLUT is a viable alternative for the identification of the local hardening behaviour of sheet metal where standard test specimens cannot be prepared due to size limitations of the specimen.

Analysis of numerically simulated true strain on high stampability sheets

The sheet metal forming is a manufacturing process widely used industrially. The quality and efficiency of the process is dependent on the correct choice of the process parameters and the correct understanding of the sheet metal behavior. The identification of the regions where stretching and/or deep drawing occurred during the forming process can aid the process through the tools design optimization. This work consisted of evaluating three punch models with varied geometries. The purpose of this work was to simulate different punch geometries using the ANSYS/LS_DYNA program and to compare the results with those reported in the literature. An evaluation was also made of the chosen program's response quality (validation). The material model chosen was the Barlat's model. The friction coefficient influence was also investigated. A comparison of the major true strain (e1) obtained by simulation compared with the experimental results indicated that this material model is able to satisfactorily reproduce the major true strain found in practice for the DC 06 steel.

Experimental characterization and model identification of directional hardening effects in metals for complex strain path changes

The purpose of the current work is the development and application of a new experimental technique and testing device for investigating the complex behavior of sheet metals during non-proportional loading. The method is based on plain strain pure bending, enabling the investigation of large deformation cyclic reversed loading, orthogonal pure bending, as well as springback. The key feature of the pure bending experiment is the absence of contact forces, material slip and friction. Furthermore, during the pure bending test, the strain gradient through the thickness is kinematically prescribed because the specimen is subjected to a plane strain condition in de direction parallel to the rotational axis ( Tan et al., 1995), which allows for a straightforward comparison of the pure bending experiments and parallel simulations. The latter is used here via the identification of a recent model for directional hardening effects and arbitrary strain path changes, ( Wang et al., 2006, 2008). The current method facilitates experimental investigation of hardening stagnation after reverse loading and cross hardening going well beyond that which is possible with existing methods based on the cyclic shear or tension-shear of sheet metal strips ( Bouvier et al., 2005, 2006a,b; Flores et al., 2007.), or pure and three-point bending ( Omerspahic et al., 2006; Antonelli et al., 2007; Carbonnière et al., 2009; Yoshida et al., 1998; Weinmann et al., 1988).

Fem study on the penetration of projectiles into steel sheets

The impact resistance and the fracture behaviour of sheet metals under ballistic impact conditions which depend significantly on some material characteristics like cold work hardening and ductility are important for the development of passive protection systems for passenger cars. To establish the impact of a projectile during penetration of a sheet metal, a FEM model was generated using the commercial FEM-code Abaqus/Explicit. The model was evaluated with real shooting experiments, where the impact behaviour of sheets made of different steels was tested at a shooting stand and the penetration was analysed metallographically. The evaluation showed that reasonable results from the simulation can only be achieved using a damage model concept and when the deformation of the bullet is considered in interaction with the deformation of the sheet metal. The variation of sheet thickness in the simulation shows good agreement with empirically developed formulations to predict the penetration power of a full metal jacket in steel sheets. With the evaluated model, parameter studies were performed to investigate the energy loss during the impact of a projectile into steel sheets with different hardening rates. The studies show that FEM simulation can give good basic knowledge about the impact behaviour of a projectile into steel sheets and can help to achieve higher protection in conjunction with weight reduction in protected passenger cars.

About the sheet metal testing by hydraulic bulging

At the present time a number of experimental methods are widely used for both qualitative and quantitative estimation of the sheet metal behaviour under processing by forming. There exist some special features of sheet metal deformation and, therefore, it is not possible yet to apply one and only testing procedure for all types of forming operations. The paper gives a brief description of the bulge testing of planar isotropic sheet metals and also presents some preliminary results about the applicability of this method for work hardening curves evaluation using data for specimen areas out of pole and for strength estimation under hydrostatic or hydrodynamic loading.

Experimental and numerical investigation of combined isotropic-kinematic hardening behavior of sheet metals

To prevent a sheet specimen from buckling subjected to a tension-compression cyclic loading, a new fixture has been developed to use with a regular tensile-compression machine. The novelty of this device lies in 4-block wedge design with pre-loaded springs. This design allows blocks to freely move in the vertical direction while providing the normal support to the entire length of the specimen during the tension-compression cycle. The entire test is easy to setup, which is another advantage of this design. In order to measure the strain accurately, the transmission type laser extensometer was utilized together with the implementation of double-side fins in the specimen. Experimental results of tension-compression tests are presented followed by a review of existing testing methods. In order to describe the accurate cyclic tension-compression behavior, the combined isotropic-kinematic hardening law based on the modified Chaboche model and the practical two-surface model based on Dafalias-Popov and Krieg models have been modified in this work, considering the permanent softening behavior during reverse loading and the non-symmetric behavior during reloading. Through tension-compression tests, the material characterization has been performed for three base materials, BH180, DP600 steels and AA6111-T4 sheets.