Study on instability and forming limit of sheet metal under stretch-bending

Under stretch-bending conditions, a significant tensile stress gradient through sheet thickness is induced, especially for a small punch radius. The traditional instability theories were developed assuming a uniform tensile stress / strain distribution through thickness; hence, may lead to unreliable prediction of stretch-bending formability. In this study, the instability behavior of sheet metal under stretch-bending is analyzed via FE-simulation of an Angular Stretch-Bend Test (ASBT). In order to reflect the influence of bending, contact normal stress etc., solid elements are used in the simulation. Three deformation stages are identified: (a). stable deformation; (b). strain localization through sheet thickness; (c). localized necking. Based on the instability characteristics, a localized necking criterion is proposed for predicting forming limits of sheet metal under stretch-bending. By combining the proposed criterion and solid element simulation, good agreement between numerical and experimental results is indicated. This work provides a new approach for predicting stretch-bend formability with sufficient accuracy and convenience.</jats:p

Deformation scenarios of combined stretching and bending in complex shaped deep drawing parts

Bending effects, especially for Advanced High Strength Steels (AHSS), are known to influence the material formability when stretching and bending is combined in sheet forming. Traditional formability measures (e.g. the conventional forming limit curve (FLC)) fail to reliably predict formability when bending is added. Consequently, in order to take full advantage of the available forming potential of AHSS sheets in industrial applications and to ensure a reliable failure assessment at the same time, current research is focusing on the experimental characterization and modeling of the influence of bending effects on the AHSS sheets formability in forming scenarios of combined stretching and bending. It is expected that aside parameters such as bending radius or strain ratio, individual deformation scenarios of combined stretching and bending may affect the material formability too. Due to tool geometry and the resulting material flow in deep drawing various complex scenarios of combined stretching and bending can occur. For example, a material element is subjected to a complex deformation history of in-plane stretching with subsequent stretch-bending over a cylindrical tool contour, followed by unbending under tension. Another material element of the same drawing part presumably starts also with in-plane stretching but is consequently stretch-bent over a doubly curved tool geometry. Consequently, comprehensive knowledge on the stretch-bending deformation scenarios prevailing in deep drawing is crucial for a more reliable formability assessment. This work aims to identify and characterize the stretch-bending deformation scenarios to occur in different complex deep drawing parts (i.e. B-pillar, cross-die test) and small scale tests (i.e. Angular Stretch-Bend Test (ASBT)). For this reason, this investigation uses an innovative approach recently developed by some of the authors and published elsewhere to categorize the stretch-bending scenarios in industrial deep drawing processes. The approach consists of a stretch-bending categorization schema and a procedure to categorize the forming scenarios in deep drawing parts using data of finite element (FE) simulations. Results of the categorization can directly be plotted on the FE mesh of the deep drawing part (i.e. map type plot of deformation scenarios). The categorization approach mentioned uses results of conventional shell-type FE forming simulation and is therefore applicable in an industrial environment. The FE forming simulation results of the parts selected were analyzed using the stretch-bending categorization approach to identify which stretch-bending scenarios occur in deep drawing parts, to quantify which scenarios to prevail and to show that the conventional ASBT does not represent all the relevant deformation scenarios that prevail in typical deep drawing parts. Furthermore, with the use of experimental observations from real part forming, the stretch-bending scenarios which are the most critical (i.e. the scenarios under which failure occurs) in forming the cross-die geometry are identified. Results of these analysis are presented and discussed in detai

Modelling stamped edges in FEM breakage analyses of high-strength steel safety components

Abstract The development process for new safety components includes break load tests, where the component undergoes very large strains. Besides the mechanical properties of the steel, which are changed by the forming process, the break load depends on the quality of the stamped edges. Target of this paper is to investigate whether a hole-tensile test with a stamped hole can demonstrate the effects of stamped-edge quality on the load bearing capacity – and secondary, how such a test procedure can be reproduced with a multi-scale FEM procedure. A series of Hole Tensile Tests (HTT) has been performed with a HSLA steel grade. Here, only minor differences in break load and total elongation were found. The local strain just prior to breakage at the edge of the stamped hole shows remarkably high values, but little difference is found when comparing a machined sample to stamped samples. The multi-scale FEM approach was demonstrated using literature data for dual-phase steel, which is known to be more sensitive to edge cracks. Indeed, a stamped edge, which includes significant hardening and pre-damage, shows earlier fracture in this FEM calculation – however, the crack propagation, which is needed to capture the full breakage of the HTT sample, is not modelled correctly – this limits the application of the method on safety components.</jats:p

Feeding effects of postlarval red abalone, Haliotis rufescens (Mollusca: Gastropoda) on encrusting coralline algae

Volume: 18Start Page: 183End Page: 19

Determination of bending limit curves for aluminium alloy AA6014-T4: an experimental approach

The conventional forming limit curves as proposed by Keeler and Goodwin fail to evaluate formability in case of bending and hemming operations. This is due to the different failure mechanisms involved in biaxial forming and bending/hemming operations. To overcome this difficulty, concept of bending limit curve has been introduced. This work presents an experimental approach to determine the BLC for aluminium alloy AA6014-T4. AA6014-T4 was selected as the workpiece due to its extensive application in outer panels of car bodies. The bending samples were printed with speckle pattern and measurement of bending strain was carried out with the help of GOM Aramis software. The three point bend test was conducted with two punch radii of 0.4 mm and 2 mm to study the effect of punch radius on bending limit strains. The complete formability picture was obtained by plotting combined forming limit and bending limit curves.by Ipsita Madhumita Das, Krishna Kumar Saxena and Jyoti Mukhopadhya

Analytical Approach to Failure Determination of Advanced High-Strength Steel in Stretch-Bending Mode

Experimental and analytical studies on the stretch-bending characteristics of advanced high-strength steel sheets, primarily used in the automotive industry, are conducted. Herein, the stretch-bending test, in which a specimen fixed at both ends is bent at its center with sharp punches until failure occurs, is conducted. Assuming that the failure of the material occurred at the maximum tensile force, a relationship between applied force and forming height is derived. Nine steels including high-strength low-alloy, dual-phase, and transformation-induced plasticity steels are tested, and the calculated results are compared with experimental limit-forming heights. Finally, a failure criterion based on process variables and tensile properties of the material is proposed based on the deformation in the sheet thickness direction.11Nsciescopu

Evaluation of bending limit curves of aluminium alloy AA6014-T4 and dual phase steel DP600 at ambient temperature

Bending/hemming operations are extensively used in automotive industries for assembling the car body panels. Besides the process mechanics, bending operation differs from biaxial sheet metal forming operations in failure mechanism also. Hence, the limit strains that material can sustain are different for both the operations. Thus, the conventional FLC proposed by Keeler and Goodwin fails to predict formability in bending/hemming operations. This necessitates the development of bending limit curves. In this work, bending limit curves are determined experimentally for AA6014-T4 and DP600. Effect of punch radius, nature and level of pre-strain on the bending limits is also studied. Thus, BLC can be used as a post-processing criterion in finite element simulations to assess formability during bending/hemming operations.by Krishna Kumar Saxena, Ipsita Madhumita Das and Jyoti Mukhopadhya