Key Engineering Materials Vols. 504-506

Abstract: Sheet metal formability has been studied for a half century. The sheet formability is mostly described by the Forming Limit Diagram (FLD). A prediction of this FLD is a source of interest for industrial companies. Indeed knowing the FLD of a material allows optimization of the production processes which leads to money saving. Nevertheless, the formability tests (tensile, bulge and Nakazima tests) which give the experimental FLD do not really represent the process that the sheet will undergo in industrial conditions. The paper therefore focuses on a cross stamping test. The material of interest is a Zinc sheet. FLD prediction is reported for a wide variety of metals [1] but literature about Zinc is nearly non existent. The studied Zinc sheets exhibit a highly anisotropic mechanical behaviour due to the hcp lattice structure and the inherited rolling texture. This anisotropic behaviour results in an anisotropic formability. The Zinc sheet FLD is influenced by the orientation of the rolling direction during the process. Experimental cross stamping of this material allows describing the studied material behaviour in a large range of mechanical solicitations from tensile to biaxial tension. The experimental results are compared with the finite element simulation and permit to understand where and why failures appear, which leads to a better understanding of Zinc anisotropic formability.

Abstract: In order to evaluate the formability of sheet materials forming limit diagrams (FLD) are recorded which represent the values of major and minor strain when necking occurs. FLDs are recorded based on the assumption that exclusively linear strain paths occur. In real forming parts, however, particularly in those with complex shapes, predominantly non-linear strain paths occur which reduce the accuracy of the failure prediction according to a conventional FLD. For this reason forming limits after loading with non-linear strain paths have to be investigated. In this contribution a systematic analysis of the forming limits of a conventional AA6014 alloy after loading with non-linear strain paths is presented. This material is pre-stretched in uniaxial, plane strain and biaxial direction up to several levels before performing Nakajima experiments in order to determine FLDs. During the pre-stretching process as well as during the Nakajima experiment the strain distribution can be measured online very precisely with the optical deformation measurement systems GOM Aramis or VIALUX. The gained curves are compared to the FLD of the as-received material. The results prove a significant influence of the pre-stretching condition on the forming limits of the used aluminum alloy. For a low pre-stretching in uniaxial as well as in biaxial direction the FLDs show a slightly reduced formability while after higher pre-stretching levels the forming limit can be improved such as for biaxial loading after uniaxial pre-stretching. The formability after pre-stretching in plane strain direction was changed. Also, a shift of the FLD depending on the direction of pre-stretching can be observed.

Abstract: The accuracy of the forming limit curves predicted by the Marciniak-Kuczynski model depends on the type and flexibility of the constitutive equations used to describe the mechanical response of the sheet metal. From this point of view, the yield criterion has the most significant influence. The paper presents a comparative analysis referring to the quality of the forming limit curves predicted by the Marciniak-Kuczynski model for the case when the plastic anisotropy of a DC04 sheet metal is described by the BBC2005 yield criterion. The coefficients included in the expression of the BBC2005 equivalent stress are evaluated using different identification strategies (with 4, 6, 7, and 8 mechanical parameters). The forming limit curves predicted by the Marciniak-Kuczynski model in each of the cases previously mentioned are compared with experimental data.

Abstract: This study aims to assess the evolution of surface roughness and waviness parameters with plastic strain in Interstitial Free – IF steel sheet. For the achievement of this study, it was considered various roughness and waviness profile parameters such as: arithmetic average roughness (Ra), maximum distance peak-valley (Rt), average waviness (Wa) and waviness of the total height peak-valley (Wt). Tensile test specimens of IF steel at 0º, 45º and 90º to the direction of rolling were fabricated. After preparing the sheet proof specimens, it was performed simple tensile tests and measurements of roughness and waviness of the specimen surface at various strain stages resulting in a large quantity of data. During the tensile test, it has been measured the following plastic strain to indicate the incremental step: (e1) longitudinal strain and (e2) transverse strain. From these data, it was possible to obtain points needed to plot the curves of roughness and waviness parameters versus strain and to determine the material behavior using equations of the equivalent strain. From the curves drawn it was possible to see how the material roughness and waviness behaves during the straining in the uniaxial tensile state, with the possibility to predict the influence of plastic strain on roughness and waviness parameters and the onset of local necking of IF steel sheet. The waviness parameters Wt is the best for characterizing the onset of local necking.

Abstract: 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.

Abstract: Tube hydroforming (THF) is an advanced metal forming process that is used widely in automotive industry, but the application of the THF process in aerospace field is comparatively new with many challenges due to high strength and limited formability of aerospace materials. The success of THF process largely depends on many factors, such as mechanical properties of the material, loading path during the process, tool geometry and friction condition. Due to complexity of this process, finite element modeling (FEM) can largely reduce the production cost. One of the important input in FEM is the material behavior during hydroforming process. The true stress-strain curve before necking can be easily determined, using either tensile testing or bulge testing, but for an accurate failure prediction in a large deformation, such as hydroforming, the study of true stress-strain curve after necking is important because it improves the quality of the analysis due to utilizing a real extended stress-strain curve. Hence, the objective of this research was to establish a methodology to determine the true stress-strain curve after necking in order to predict burst pressure in the THF of aerospace materials. Uniaxial tensile tests were performed on standard tensile samples (ASME E8M-04) to determine the true stress-strain before and after necking, using an analytical method presented in this study. To validate the approach, burst pressure in the THF process was predicted using the extended stress-strain curve in conjunction with Brozzo's decoupled fracture model. The approach was evaluated using data obtained from the free expansion (tube bulging) tests performed on stainless steel 321 tubes with 2 inches diameter and two different thicknesses, 0.9 mm and 1.2 mm. The comparison of the predicted and measured burst pressures was promising, indicating that the approach has the potential to be extended to predict formability limits in THF of complex shapes.

Abstract: The automotive industry nowadays, uses numerical simulation systems to determine process safety of car body parts. Forming simulations are usually used to predict local necking and cracks during the deep-drawing operation or to calculate the spring-back behaviour. Furthermore, FEA is also used for optimizing the hemming process. In this contribution, further development and the use of an enhanced failure criterion for the evaluation of flanging and hemming processes are shown. This criterion describes material failure caused by incipient surface cracks on the bending edge keeping the predominant bending load conditions in consideration. The investigations of the bending conditions in this criterion include loads from previous forming operations and geometrical aspects, such as bending radii. The approach presented in this contribution can deliver a more reliable prediction regarding the expected material failure.

Abstract: The paper describes a new experimental procedure for the determination of the curves relating the equivalent stress and equivalent strain of sheet metals by means of the hydraulic bulge tests through elliptical dies. The procedure is based on an analytical model of the bulging process and involves the measurement of only two parameters (pressure acting on the surface of the specimen and polar deflection).

Abstract: Compared to steel, aluminum has a reduced formability. The consequence is that the drawability of aluminum needs to be extended. This can be achieved by a material recovery that takes place near the zones in which a material failure is initiated during deep drawing. In the considered process, first the aluminum component will be preformed to a specific stress state. In the second step, it will be partial heat treated, before the component is getting finished. Based on the selective intermediate introduction of heat, the material flow of the pre-drawn part is influenced in such a manner that the most highly stressed zones are subjected to further reduction in sheet thickness. This is possible by sacrificing material out of zones near the crack. These areas are referred to below as “sacrificial zones”. They depend on the position of the critical region as a result of the material pre-strain. In these regions, the temperature can be varied. This paper focuses on the development of a methodology to determine a layout of intermediate heat treatment of preformed aluminum sheet metal components. In order to determine such a layout, a principal part must be designed on which the methodology can be reviewed.

Abstract: The increasing requirements of car body manufacturers concerning formability and load of sheet metal structures necessitate new approaches in forming technology. Especially the requirements as regards local loads and local forming behaviour of the components vary significantly. In the field of welding technology there is a verified effect of emerging higher strengths in the welding seam. Using a laser, similar effects can be applied to change the material properties locally. In the current investigation the use of a laser beam allows for creating very small heat-affected areas of a width from 0.5 mm up to 5 mm. These areas are characterised by a significantly higher hardness and strength. Depending on the sheet metal material, areas of higher and lower hardness can be produced. Using laser heat-affected zones in sheet metal makes it possible to increase the drawing depth without necking or cracks. The heat-affected zones increased the stiffness in the critical necking area and hence moved the critical strains to the surrounding area. The required formability was guaranteed by the adjacent areas and the draw depth can be increased.