Papers by Keyword: Bulge Test

Abstract: This paper presents a finite element analysis of a pre-cracked freestanding gold thin film subjected to bulge test. These tests were conducted in order to determine the elasto-plastic properties and fracture toughness of the gold films. For the experimental tests, a pre-crack was introduced in the center of the film by focused ion beam (FIB) milling with a length of 10 and a width of 100nm. For the numerical fracture analysis, the problem was divided into two stages; the first stage was the development of the numerical model on the whole film without pre-crack (elasto-plastic analysis) and the second one was performed on a film portion that included the pre-crack (sub-modeling stage). Three different notches (rounded, sharp and V-sharp) were applied to calculate the stress intensity factor around the crack tip using path independent J-integral. The obtained results show that the load-deflection curves for non-cracked and pre-cracked film reproduced the experiments using the calculated elasto-plastic properties. This indicates that the proposed models presented a good correlation and robustness. Additionally, fracture toughness values were calculated between 0.288 and 0.303with J-integral values 1.037 J/m² (elastic) and 1.136 J/m² (elasto-plastic) which correspond with other calculations available in the literature.

Abstract: Production of thin packaging sheets has gone through significant changes in recent years. Thickness of thin packaging sheets has decreased from 0.24mm down to 0.14 – 0.18mm due to the material saving, and the thickness of tin coating layer has decreased as well. Thin packaging sheets with the thickness lower than 0.18mm are produced with so called double reduction. Materials produced with this method possess higher strength properties and lower plastic properties. There exist several tests for examination of strength and plastic properties. Requirements of the processors of these sheets are that these tests are simple, without a complicated production of specimens, and easily repeatable. Exactly because of these reasons, the springback test is used for evaluation of double reduced packaging sheets by many processors from various countries. This contribution compares properties of double reduced thin packaging sheets obtained with the springback test, uniaxial tensile test and biaxial tensile test. Goal of this comparison is to objectify mechanical and plastic properties of packaging sheets obtained with the individual tests.

Abstract: Optical measuring systems enable a very accurate determination of the flow stress for the hydraulic bulge test. The flow stress is strain rate and temperature dependent and for the description of work hardening an approximation of the temperature during the test is required. Measuring the temperature during the test usually interferes with the optical strain measurement. A model for the temperature distribution on the bulged surface is developed based on heat generated by plastic work, convection to air on the outer surface, conduction to the tools at the die diameter and conduction to oil on the inside. The plastic work is derived from an approximation of the shape of the bulged surface and an approximation for the thickness distribution, starting from the initial thickness at the die ring to the established thickness at the pole, making use of volume conservation for the bulged sheet. The parameters of the model are tuned to bulge test temperature measurements of four different steel grades using a thermo couple at the pole. The results of the analytical temperature model are in good agreement with the measurements.

Abstract: The following paper explores the development the bulge test technique combined with the micro-Raman analysis and a refined load-deflection model for high quality 3C-SiC squared-membranes. By the minimization of the total elastic energy, starting from the isotropic relation between the stress tensor and the strain tensor, it is possible to calculate the relationship between the maximum deflection and the applied pressure, in both regime of small and large deflection. From the measured breaking pressure through the refined model it is possible to evaluate the breaking strain of the membrane. Furthermore, the relationship between the measured shift of Raman Transverse Optical (TO) phonon modes and the total residual strain (Δa/a) within the epitaxial 3C-SiC layer was found.

Abstract: In recent years, hot stamping of sheet metal parts has emerged to satisfy the contrary demands of the automotive industry for components with increased strength at reduced weight. To analyse the material behaviour during these processes, a hot gas bulge test at high temperatures and high strain rates is promising, since the bulge test at room temperature has already proven itself as a useful test for the material characterization of sheet metals up to high strains. Therefore, a hot gas bulge test at elevated temperatures and high strain rates is being developed at the Institute of Metal Forming (IBF) in cooperation with the Institute for Fluid Power Drives and Controls (IFAS) at the RWTH Aachen. To verify if the concepts of the membrane theory, which are used for the evaluation of bulge tests at room temperature, are adaptable to such a hot gas bulge test, a simulation study using finite element calculations was conducted. The purpose of this simulation study is is to estimate the errors which occur if the equivalent stress at the bulge pole is calculated by using the membrane theory. In addition to this study several approaches were examined to obtain the sheet thickness at the bulge pole by measuring the bulge height. The study showed that a hot gas bulge test can be described very well by the membrane theory if the sheet thickness, the curvature at the bulge pole and the pressure inside the bulge are exactly known. However, substantial errors can occur if the sheet thickness at the bulge pole is determined by measuring the height of the bulge pole.

Abstract: Due to new material concepts (e.g. boron-manganese steels), hot stamping of sheet metal parts has emerged in order to produce high strength components. Thereby, the design of hot stamping processes by means of finite element simulations requires information about the thermo-mechanical material behaviour up to high strain levels at various temperatures as simulation input. It is known that hot tensile tests are only evaluable until low strain levels. Therefore, a hot gas bulge test for temperatures in the range of 600 °C to 900 °C and strain rates up to 1/s is being developed. In order to design such a hot gas bulge test, the requirements (e.g. forming pressure) are estimated by finite element simulations. The result is a test bench, which already enables a pneumatic forming of specimens at room temperature and pressures up to 200 bar without any unexpected side effects.

Abstract: The mechanical behaviour of a superplastic material is often modelled by the power law relationship between the equivalent flow stress, the equivalent strain and the equivalent strain-rate at least over a limited range of strain rates. This paper introduces an original mathematical modelling to determine the superplastic material constants m, n and K by means of experimental tests carried out using a standard forming die geometry.

Abstract: This paper shows a numerical-experimental comparison to validate a mathematical model which is able to determine the superplastic material constants by means of experimental tests carried out using a standard forming die geometry. In particular, the constants m, n and K for the lead-tin alloy PbSn60, for the alloy AZ31 at different forming temperatures and for the alloy AA5083 are evaluated.

Abstract: The aim of this work is to develop the bulge test technique combined with the micro-Raman analysis and a refined load-deflection model for high quality 3C-SiC squared-membranes. By the minimization of the total elastic energy, starting from the isotropic relation between the stress tensor and the strain tensor, it is possible to calculate the relationship between the maximum deflection and the applied pressure, in both regime of small and large deflection. Furthermore, the relationship between the measured shift of Raman Transverse Optical (TO) phonon modes and the total residual strain (Δa/a) within the epitaxial 3C-SiC layer was found and in order to understand the stress distribution within the membrane, TO Raman shift maps were performed along the corner and the border of the membrane.

Abstract: Magnetorheological (MR) fluid is a kind of smart material. In this paper, magnetorheological (MR) fluid is applied as a flexible-die, the rheological properties of which can be changed rapidly under a magnetic field, in sheet metal forming process. Bulge tests using a kind of MR fluid, MRF-J01T, are carried out on self-designed experimental device. Stainless steel 1Cr18Ni9Ti combined with different process parameters are adopted to investigate the effect of property of MR fluid on the formability of material tested. The results showed that MR fluid can be utilized successfully as a pressure-carrying medium in sheet forming process. The rheological properties of MR fluid can greatly influence the dome height and thickness strain distribution of specimens.