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Investigation of Forming Limit Curves of Various Sheet Materials Using Hydraulic Bulge Testing With Analytical, Experimental and FEA Techniques
Abstract
In sheet hydroforming, variation in incoming sheet coil properties is a common problem for forming process, especially with materials for automotive applications. Even though incoming sheet coil may meet tensile test specifications, high rejection rate is often observed in production due to inconsistent material behavior. Thus there is a strong need for a discriminating method for testing incoming sheet material formability. The hydraulic sheet bulge test emulates biaxial deformation conditions commonly seen in production operations. This test is increasingly being applied by the European automotive industry, especially for obtaining reliable sheet material flow stress data that is essential for accurate process simulation. This paper presents determination of Forming Limit curves (FLCs) of materials Aluminium, Mild steel and Brass. Theoretical analysis is carried out by deriving governing equations for determining of Equivalent stress and Equivalent strain based on the bulging to be spherical and Tresca's yield criterion with the associated flow rule. For experimentation Circular Grid Analysis is used. Validation of Experimental results is carried out with explicit solver ANSYS LS-DYNA using inverse analysis method.
... Numerous studies reported that there has been a trend toward the use of lightweight metals and alloys, in the automotive industry, markedly aluminium sheets, to address environmental concerns, the problem of fuel consumption and weight reduction in automotive parts, during the design and the construction phases of automotive bodies [3,4,10,11,12]. This can be deduced from Figure Furthermore, Miller et al. [10] testified that aluminium usage in automotive applications has grown more than 80% within a period of 5 years, see Hirofumi and Takayuki [11] equally reported that in recent years, a trend of weight reduction in automobiles is rising from the viewpoint of reducing fuel consumption and exhaust gas emissions. ...
... Likewise, Santosh et al. [12] recounted that an increase in demand for weight reduction of automotive parts becomes a driving factor in research and design work for both supplier industry and end users. Hence, a large number of studies have been carried out to optimize design and process, to ensure that the production of parts with minimum thickness, weight reduction and uniform thickness is attained. ...
... Several studies pointed out that deep drawing is the most common forming process for sheet metals used in modern industry [2,4,12,17]. Mohammad et al. [17] pointed out that, nowadays, the deep drawing process is used in the modern industry extensively for forming automotive inner and outer parts. However, sheet metal deep drawing technology is one of the most challenging forming processes in the manufacturing industry. ...
Little information is available on formability of aluminium alloy AA 6082-O sheet. The deep drawability of AA 6082-O was experimentally investigated by carrying out tensile tests and Erichsen cupping tests. From the XRF analysis done on the tested specimens, the alloy used for the present study contained Silicon (0.95 %wt) and Magnesium (1.16 %wt). The tensile strength and modulus of elasticity were highest in the rolling direction (0º) and increased with sheet thickness. The range of r values of the alloy for the two thicknesses in the three rolling directions were 0.23 ≤ r1.0mm ≤ 0.75 and 1.23 ≤ r2.0mm ≤ 2.40, the upper bound and lower bound values being for 0º and 90º, respectively, in both cases. The range of n values in the three rolling directions for the two thicknesses were 1.422 ≤ n1.0mm ≤ 1.690 and 1.424 ≤ n2.0mm ≤ 1.674 , both being higher than unity (n > 1.0). The obtained FLC level of the FLDs from the Erichsen Cupping test was found to be higher for 2.0 mm thickness than 1.0 mm thickness. Based on the results of the two testing methods it was observed that the deep drawability of AA6082 – O sheet is largely affected by the sheet thickness and the rolling direction. It increases with increasing sheet thickness, but the alloy exhibits planar anisotropy (Δr < 0) as evidenced by some test samples earing. The alloy fractures with little or no observable necking, but the general stress-strain behaviour is typical of that of the aluminium 6000 alloy series.
... In processing of metal sheets, it is essential to know the mechanical properties of the material and the factors that can affect the precision and the efficiency of the process. Researches in recent years have shown that, in addition to the alloy composition, temperature and strain rate are the most important parameters [1,2,3,4]. ...
... Electromagnetic forming or pulse magnetic forming (EMF), due to principle of the process, involves high strain rates and, as a consequence, modifies the path of a stress-strain curve of the material in comparison to the same curve at conventional strain rates. Some recent works show that depending of the material type, the formability of sheets increases, more or less, with increasing strain rate [2,4,7]. In addition, in some papers was studied the influence of the temperature on the formability of metal sheets [1,5,6]. ...
Based on finite element analysis, this paper investigates a possible new technology for electromagnetic processing of thin metal sheets, in order to improve the productivity, especially on automated manufacturing lines. This technology consists of induction heating process followed by magnetoforming process, both applied to metal sheet, using the same tool coil for both processes.
... Figure 1.12 shows the multi-step process used to form seamless cavities with hydroforming. The term bulging is used to define free-forming blanks with hydroforming and the biaxial strain in the sheet makes it a suitable process to determine the forming limit diagram of a material [44]. An advantage of this technique and electro-hydraulic forming for SRF cavities is the reduced risk of contamination on the RF surface from the inclusion of metallic particles caused by the contact with the punch in deep drawing or the mandrel in spinning, since the inner surface is only in contact with water [3]. ...
Manufacturing of superconducting radiofrequency (SRF) cavities with high performances is paramount to increase the collision energy in new particle accelerators. The use of high-speed sheet forming techniques, such as electro-hydraulic forming, can be beneficial, but requires a detailed understanding of the mechanical properties of the materials being deformed and the consequence on their microstructure. This thesis focuses on the characterization of high-purity niobium single crystals, polycrystalline niobium sheets, and polycrystalline OFE copper sheets. The results from this study are separated in two parts. In Part I, the characterization of niobium single crystals focused on the mechanical properties in tension and compression at strain rates of 10-4 to 103 s-1 and on the microstructure (analyzed using SEM, EBSD, TEM, and nanoindentation) of the deformed specimens. The effect of crystal orientation, strain rate, and loading direction on the mechanical properties, the crystal rotation, and the dislocation substructures are presented. In Part II, the forming limit diagram (FLD) of polycrystalline niobium sheets and OFE copper sheets were measured at a quasi-static strain rate. The FLDs of those materials should provide important data for manufacturers using conventional techniques, such as deep-drawing and spinning. The second part also presents the mechanical properties of electron beam (EB) welded polycrystalline niobium sheets and OFE copper sheets deformed in tension and compression at strain rates of 10-3 to 103 s-1.
... The onset-of the failure line (Forming Limit Curve -FLC) divides all possible strain combinations into two zones: the safe zone (in which failure during forming is not expected) and the failure zone (in which failure during forming is expected). Numerous studies [14,[23][24][25][26][27][28][29][30][31][32][33][34] indicate that Forming Limit Diagrams are effective tools for measuring the formability of sheet metals. However, the forming limits are affected by punch arc radii, condition of the sheet metal, material properties (e.g. ...
AA6082 is a relatively new structural alloy in the 6000 aluminium alloy series. This study evaluated the deep drawability of AA6082-O sheet metal. Uniaxial Tensile tests were conducted on specimens prepared according to DIN 50125-E standard, for three angular orienta-tions (0⁰, 45⁰, and 90⁰) relative to the rolling direction. Erichsen Cupping tests were conducted on 60 mm × 60 mm blanks of two gauge thicknesses (1.0 mm and 2.0 mm) and also on segmented blanks. A WP 300 Universal Material Tester, with a loading capacity of 20 kN, was used for all the tests. The Tensile Strength was higher in the rolling direction (0⁰) than in the transverse orientations (45⁰ and 90⁰). The resultant Forming Limit Curve (FLC) level of the established Forming Limit Diagrams (FLDs) was higher for the 2.0 mm thick blanks than the 1.0 mm thick blanks. Thus the alloy's formability is affected by the sheet thickness and orientation. It increases with sheet thickness, but the alloy exhibits planar anisotropy (∆r<0). AA6082 sheet fractures with no observable necking under uniaxial tension conditions, and exhibits non-uniform yielding characteristics. However, the general stress-strain behaviour is typical of that of the aluminium 6000 alloy series.
... However, there are, unfortunately, some drawbacks in processing aluminum alloys using conventional technologies, in relatively slow forming processes, such as large springback and low formability. Previous years researches [4,5,6,7,8] prove that these shortcomings can be overcome when high-speed forming processes, like hydroforming or magnetoforming, are used. As a consequence, a detailed analysis of the high-speed forming process and an optimal design of the device are compulsory and an accurate numerical model is an essential tool in this purpose. ...
The paper proposes an approach in numerical modeling of electromagnetic forming of aluminum tubes. The investigated technology refers to a free expansion/bulging, without a die, of a thin wall aluminum tube. The aim is to build an accurate model that can take into account the particularities of the process, which is useful for numerical investigations regarding an optimal design of the device.
This article describes an investigation of Veerman’s interpolation method and its applicability for determining sheet metal formability. The theoretical foundation is established and its mathematical assumptions are clarified. An exact Lagrangian interpolation scheme is also established for comparison. Bulge testing and tensile testing of aluminium sheets containing electro-chemically etched circle grids are performed to experimentally determine the forming limit of the sheet material. The forming limit is determined using (a) Veerman’s interpolation method, (b) exact Lagrangian interpolation and (c) FE-simulations. A comparison of the determined forming limits yields insignificant differences in the limit strain obtained with Veerman’s method or exact Lagrangian interpolation for the two sheet metal forming processes investigated. The agreement with the FE-simulations is reasonable.
Considerable changes have occurred in metal forming in the last decade. A record of these changes can be found in keynote papers presented by the members of the Scientific Technical Committee—Forming, at the CIRP Annual General Meeting each year. The keynote papers are excellent references on important developments in metal forming and are used as a reference, globally. Not only is this paper a compendium of most of the keynotes presented, but from 2001 onward, it has updates on new information on five keynote subject areas. The authors of each keynote have written an update with new information that has developed since the writing of the keynote. The authors of each section are shown in order of presentation.
Purpose: Optimization of the operating conditions is one of the most significant studies in the hydroforming process, which affect the forming of successful components.Design/methodology/approach: Finite Element simulations have been carried out using the commercial finite element package ANSYS in order to predict the most efficient and acceptable operating condition for certain material properties of multilayered blank and initial blank geometry.Findings: This paper studies the hydroforming process involving combined (axial feed and internal pressure) and multi stage non-linear loading action theoretically and experimentally for a multi layered tubular blank placed in a pre-shaped die block.Research limitations/implications: Finite Element simulation of a multi layered tube hydroforming and optimisation of the best forming conditions based on a number of simulations were carried out to avoid wrinkling or bursting of the tubular blanks.Originality/value: Experimental validation for a certain simulation has been presented.
Usually, flexible die forming technology and conventional stamping technology have their own separate and limited application range, as well as their own particular shortcomings. So a more advanced forming method is needed to meet the requirements of increasing component complexity. Viscous pressure forming (VPF), which uses a semi-solid viscous material as the pressure-carrying medium, is suitable for forming parts requiring large sheet deformation using a sequential forming method. In this paper, a sheet metal bulging experiment with three kinds of pressure-carrying media, viscous medium, polyurethane and steel, was studied. The results show that the specimens formed with VPF, compared with those formed with polyurethane and steel, have less wall-thinning and consequently a more uniform wall thickness distribution, and the shape of part is closer to the desired hemisphere. When bulged with back-pressure, the formability of sheet metal can be improved significantly. Thus VPF is a promising sheet metal forming method.
Tresca's yield criterion and the associated flow rule have been used to develop a solution for the plastic bulging of circular diaphragms by lateral fluid pressure. The strain distribution in the neighbourhood of the pole is derived in explicit form and a new formula is given for the polar strain at instability. The theory is found to be in good agreement with experimental results for the bulge test. A numerical method of solving the problem for Mises materials is also proposed.
Room temperature uniaxial tensile and biaxial Viscous Pressure Bulge (VPB) tests were conducted for five Advanced High Strength Steels (AHSS) sheet materials, and the resulting flow stress curves were compared. Strain ratios (R-values) were also determined in the tensile test and used to correct the biaxial flow stress curves for anisotropy. The pressure vs. dome height raw data in the VPB test was extrapolated to the burst pressure to obtain the flow stress curve until fracture. Results of this work show that the flow stress data can be obtained to higher strain values under biaxial state of stress. Moreover, it was observed that some materials behave differently if subjected to different state of stress. These two conclusions, and the fact that the state of stress in actual stamping processes is almost always biaxial, suggest that the bulge test is a more suitable test for obtaining the flow stress of AHSS sheet materials for use as an input to Finite Element (FE) simulation models.
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