Sebastian Suttner

Influence of Stress Relaxation after Uniaxial Pre-Straining on Subsequent Plastic Yielding in the Uniaxial Tensile Test of Sheet Metal

Resource efficiency, design oriented accuracy and lightweight properties are demands on modern sheet metal forming parts in the automotive sector. The use of new materials leads to additional challenges on the numerical design of forming processes. During these forming processes the material undergoes different strain states that cause non-linear strain paths. Since the numerical prediction highly depends on the identified characteristic values of the material, an exact characterisation of the material behaviour is essential. Especially obtuse angles of the stress vector trigger a recovery of the material by returning stress. Besides, a relaxation of the material is investigated during holding a constant strain level. The effect of relaxation lead to an altered material behaviour that appears in a reduction of the beginning of plastic yielding. In addition, a kinematic hardening behaviour as under cyclic loading and load reversal, known as the Bauschinger effect, occurs after the relaxation of the stress and results in a reduced beginning of plastic yielding by loading in the same direction as the introduced pre-strain. Within this research work the effect of relaxation is investigated for two materials, AA5182 and DP600, with an initial sheet thickness of 1.0 mm. These materials are typically used for internal and accordingly functional parts in the automotive sector. The relaxation of the material is analysed with different holding times of a constant pre-strain at different levels of straining. The release of the material is studied by subsequent uniaxial tensile tests after pre-straining with the same load condition. Moreover, the influence of the named effects is shown by comparison of the translation of the yield loci.

Mechanical weld zone characterization for the numerical simulation of forming friction stir welded aluminum blanks

In order to predict the warm formability of friction stir welded aluminum sheets, a new approach for the mechanical characterization at elevated temperature is presented in this paper. Based on warm tensile test and biaxial tensile test local properties of base material and weld nugget material were determined by using specimens with reduced measuring areas. Special characterization specimens are designed according to the weld zone dimensions and numerically optimized to guarantee homogeneous stress distribution. Identified material parameters were used to build up local material models based on approximated flow curves and an anisotropic yield function. The ability of predicting the formability performance of welded sheets was finally evaluated by the validation of numerical simulations with experimental results in dome stretching tests.

Investigation of the Beginning of Plastic Yielding and the Hardening Behaviour under Biaxial Tension

The main focus of the experimental observation deals with the investigation of the plasticyielding of DC06 and DP600 under biaxial tension in comparison to an identified material modelwith an isotropic hardening behaviour. The isotropic hardening law describes the hardeningbehaviour of the material by application and approximation of the flow curve. Prevalent hardeninglaws are proposed by Swift (1952) or Hockett and Sherby (1975) and lead to an expansion of theyield surface in the stress space. By reasons of good accordance in an earlier survey the givenmaterials are modeled by the yield criterion Yld2000-2d and the isotropic hardening law of Swift.In this case the yield loci at different experimental states of plastification are compared with theyield loci given from the isotropic expansion of the Yld2000-2d yield surface. Furthermore apossible approach for modelling the change of the yield criterion’s shape in the stress space duringplastification is shown. With respect to further investigations additional research work is neededincluding extended and complex hardening laws to describe the real material behaviour sufficiently.

An incremental analysis of a deep drawing steel’s material behaviour undergoing the predeformation using drawbeads

Nowadays lightweight design in metal forming processes leads to complex deep drawing geometries, which can cause multiple damages. Therefore, drawbeads are one way to regulate and control material flow during the forming process. Not only in research, but also in industrial practice, it could be determined that material is work hardened passing drawbead geometries. It particularly means when material is pre-deformed with tensile and alternating bending loads. This incident also gives the opportunity to utilize it in a reasonable way if examined properly. To investigate these findings, a process oriented and comprehensive analysis of the material behaviour during these forming operations is needed. In this paper, sheet metal strips are linearly drawn through a drawbead and stopped after passing the drawbead. Within this forming operation, the material undergoes non-linear straining before reaching the in-plane position again. Here, the process will be stopped to investigate a permanent strengthening local along the sheet thickness. Therefore, microhardness measurements are realized before and after passing the drawbead. Because of its common use and its wide known material data, a deep drawing steel DC will be used for these studies. Additionally, the strategy is applied to advanced high strength steel.

Cyclic Tension Test of AZ31 Magnesium Alloy at Elevated Temperature Realized in a Miniaturized Uniaxial Tensile Test Setup

The use of new materials, e.g. aluminum and magnesium alloys, in the automotive and aviation sector is becoming increasingly important to reach the global aim of reduced emissions. Especially magnesium alloys with their low density offer great potential for lightweight design. However, magnesium alloys are almost exclusively formable at elevated temperatures. Therefore, material characterization methods need to be developed for determining the mechanical properties at elevated temperatures. In particular, cyclic tests at elevated temperatures are required to identify the isotropic-kinematic hardening behavior, which is important for numerically modeling the springback behavior. In this contribution, a characterization method for determining the cyclic behavior of the magnesium alloy AZ31B at an elevated temperature of 200 °C is presented. The setup consists of a miniaturized tensile specimen and stabilization plates to prevent buckling under compressive load. The temperature in the relevant area is introduced with the help of conductive heating. Moreover, the complex kinematic model according to Chaboche and Rousselier is identified, to map the transient hardening behavior of AZ31B after load reversal, which cannot be modeled with a single Bauschinger coefficient.

Investigation of the Unloading Yield Effect in Aluminum and Magnesium Sheet Metal Alloys at Room Temperature

The reduction of CO2-emissions and the lowering of fuel consumption are two main objectives in the automotive industry. To reach these targets, conventional materials like deep drawing steels are substituted by new modern lightweight materials such as aluminum and magnesium alloys. During forming of sheet metal parts, the material experiences a plastic deformation, which can affect the part quality regarding the amount of springback or the occurrence of stretcher strain marks. In this context, a time dependent change of the material behavior can emerge after removing the part from the forming tool. Within this contribution, the influence of pre-strain and the unloading yield effect on the subsequent mechanical behavior of the aluminum alloy AA7020 and the magnesium alloy AZ31B are investigated. Additionally, the time dependency of mechanical properties is analyzed for different aging times from 5 seconds to one week after pre-straining. The results show a significant increase of unloading yield effect with increasing pre-strain during uniaxial tension.

Influence of specimen size and sheet thickness on the material behavior of AZ31B under uniaxial tension

Concerning a low specific density, magnesium alloys offer a great potential to reduce the part weight. Due to the conditional formability of magnesium alloys at room temperature, forming processes are mostly realized at elevated temperatures. Within this contribution, uniaxial tensile tests for two different specimen geometries with a gauge length of 50 mm and 2 mm and a cross section of 12.5×1 mm2 and 2×1 mm2 are carried out for the magnesium alloy AZ31B at an elevated temperature of 200 °C. Since the specimen size is reduced, the influence of the sheet thickness needs to be analyzed. The analysis of the sheet thickness results in a change of the determined flow curve above a strain level of 0.1 and a much earlier failure of specimen with an initial sheet thickness of 1.5 mm. The changed material behavior can be explained by a different microstructure, e.g. the influence of the grain size. In summary, a characterization of the magnesium alloy AZ31B at elevated temperatures can be realized with a miniaturized test setup.

Validation of Kinematic Hardening Parameters from Different Stress States and Levels of Plastic Strain with the Use of the Cyclic Bending Test

The recent development of new lightweight sheet metal materials, like advanced high-strength steels or aluminium alloys, in combination with an increasing component complexity provides new challenges to the numerical material modelling in the FEM based process design. An auspicious approach to improve the quality of the numerical results – most notably in springback analysis – is the modelling of the so called Bauschinger effect achieved through implementation of kinematic hardening models. Within this paper the influence of the stress state and the level of pre-strain on the numerical simulation result of the advanced high strength steel DP-K45/78+Z will be analysed. For this purpose, a parameter identification of the kinematic hardening law according to Chaboche and Rousselier is performed at different pre-strains on the basis of experimental data from tension-compression tests as well as cyclic shear tests. Finally, the identified parameters are validated in a comparison between numerical and experimental results of a cyclic bending test.

Experimental Investigation of Ti-6Al-4V with a Biaxial Tensile Test Setup at Elevated Temperature

The requirement for products to reduce weight while maintaining strength is a major challenge to the development of new advanced materials. Especially in the field of human medicine or aviation and aeronautics new materials are needed to satisfy increasing demands. Therefore the titanium alloy Ti-6Al-4V with its high specific strength and an outstanding corrosion resistance is used for high and reliable performance in sheet metal forming processes as well as in medical applications. Due to a meaningful and accurate numerical process design and to improve the prediction accuracy of the numerical model, advanced material characterization methods are required. To expand the formability and to skillfully use the advantage of Ti-6Al-4V, forming processes are performed at elevated temperatures. Thus the investigation of plastic yielding at different stress states and at an elevated temperature of 400°C is presented in this paper. For this reason biaxial tensile tests with a cruciform shaped specimen are realized at 400°C in addition to uniaxial tensile tests. Moreover the beginning of plastic yielding is analyzed in the first quadrant of the stress space with regard to complex material modeling.

Evolution of Yield Loci for Aluminum Alloy AA6016 and Deep Drawing Steel DC06 under the Influence of Non-Linear Strain Paths

The application of modern materials plays an important role directly under the aspect of lightweight potential. To exploit these options effectively a numerical accurate reproduction of the material behavior is indispensable. Especially in the case of large deformations a directional and strain rate dependent hardening behavior can be observed. By disregarding this effect significant failure in the computed stress state can arise, which can conduct to a corruption of the spring-back forecast. Within this contribution a new test method for analyzing the evolution of subsequent yield loci under strain path changes for the aluminum alloy AA6016 and the deep drawing steel DC06 is presented. In the first stage of the experimental investigations, yield loci with linear strain paths were considered to characterize the material behavior for the initial condition. On further experiments with several stress states the strain path dependent hardening behavior of the material is determined. The non-linear strain paths are realized through uniaxial prestrained primary specimens with following extraction of secondary samples for following stress states, e.g. a modified ASTM simple shear test specimen. Subsequent yield loci are investigated and compared to the yield surfaces Hill48 and Barlat 2000 (Yld2000-2d) with an isotropic hardening behavior. With this study the evolution of the yield locus for prestrained specimens is evaluated. The research of the subsequent yield loci for strain path changes serves as basis for further scientific investigations with a view to assess different approaches of isotropic-kinematic hardening models in consideration of the analyzed steel and aluminum sheet metals.