Tube Bulging Test Research Articles

Determination of material properties for hot hydroforming

For the process design of hydroforming in the “hot” temperature range, reliable data are necessary to describe the material behaviour at elevated temperatures under the occurring loads of hot hydroforming processes. State-of-the-art technologies for the investigation of material behaviour, like uniaxial tensile tests or hydraulic bulge tests, do not provide enough similarity with the process of hot hydroforming. This paper describes a new testing technique, capable of realizing high process temperatures and constant strain rates. It represents a further development of the established technology of tube bulge tests. The hardware is described, its functionality is proven and mathematical approaches for the calculation of stress/strain-curves from experimental data are presented.

Steel and copper flow stress determination for THF applications

Tube hydroforming (THF) process is nowadays a developed and successful way for forming complex shapes with less operations required compared to conventional tube forming processes. Since THF is a recently developed technology, there is a lack of knowledge on the process which can be overcome by using FE simulations. In fact, simulation allows a remarkable saving in time and money when developing a feasibility study or a prototype for new THF operations. The reliability of FE simulations depends on several factors such as interface friction and material properties. The classical method used for evaluating the material flow stress is the tensile test. This is a simple test which stresses the material in only one direction, differently from the actual tube stress state in THF. A biaxial stress state can be achieved by using the tube bulge test, which, consequently gives more reliable data for determining the material flow stress. The present paper describes the experiments and the analytical model of tube bulge test for the identification of the flow stress under a biaxial stress state. The innovative aspect of the proposed approach is related to the fact that the tube ends are blocked and, in the analytical model, the stress state is derived from the flow rule and the volume constancy. The experimental data of bulge curvature and tube thickness are used by the analytical model to calculate the material stress-strain relation. Results of tests conducted on different tube materials show that bulge test allows to obtain material properties for high strain thus avoiding possible errors in extrapolating flow stress for FE simulations.

Determination of tube material hardening law using bulging tests

The paper is focused on the determination of mechanical characteristics for metallic materials shaped like tubes as used in tube hydroforming process. First, a procedure is described to carry out experimental tube bulging tests. It permits to identify a law linking the internal pressure to the bulge height. Second, based on this law and an original geometric modelling, strain and stress can be evaluated all over the free bulge zone of the tube. Third, numerical results obtained by finite element simulation are treated to validate the experimental behaviour law of the tube. It is shown that: measuring experimentally internal pressure and bulge height during tube bulging allows to determine tube hardening law and thickness distribution along the bulging area; a deviation of 2% is noticed between numerical and experimental results.

Tube bulging test: Theoretical analysis and numerical validation

To perform pertinent numerical simulations of tube hydroforming process, it has been demonstrated that material data obtained from tube bulging test were more suitable than the classical tensile test. Theoretical developments based on a novel geometrical modeling have led to an analytical evaluation of the strain and stress states all over the bulged part of the tube. Errors due to imprecision of the measure location and geometrical imperfection of tube can be linked to errors on stress and strain evaluation by a differential analysis of the analytical model. At last, to validate the model, numerical results obtained from FE simulation of tube bulging test have been used as experimental data. In that way, it is shown that: - hardening curves can be obtained through the measurement of internal pressure and bulge height; - thickness variation along the bulged part of the tube can be calculated; - scattering on the hardening curve can be evaluated by using inaccuracy of the measurement point location during experiments and manufacturing tolerances on tube thickness.

The determination of flow stress of tubular material for hydroforming applications

Tube hydroforming (THF) process is nowadays a developed and successful way for forming of complex shapes with less operations required compared to conventional tube forming processes. FE simulations are powerful tools which allow a remarkable saving in time and money when developing a feasibility study or a prototype phase for a new THF operation. However the successful use of FE simulations depends on several factors such as interface friction or material properties. While in the past flow stress used as input in FE simulations was generally obtained from tensile test conducted on the sheet prior to rolling and welding operations in case of rolled and welded tubes, or tensile test conducted on the whole tube for extruded or seamless tubes, in the last years tube bulge test has been widely used. In this research test experiments, coupled with suitable analytical model of the process, are used to identify the flow stress of tubes (under biaxial stress state) by measuring geometrical features of the tube under study as the fluid pressure increases. In the present paper a new approach to tube bulge test is described. The innovative aspect is related to the fact that the tube ends are blocked and so the equilibrium expression in axial direction normally used to calculate stresses is no more valid. The stress state is therefore derived from the flow rule and the volume constancy. The new proposed analytical model was validated by means of FE simulations. Results of test conducted on seamless tubes show that bulge test allows to obtain material properties for high strain thus avoiding possible errors in extrapolating flow stress for FE simulations.

하이드로 포밍용 튜브의 성형 한계선도 측정

The forming limit diagram of tube is required for the part design and the formability analysis of tube hydroforming. The finite element analyses of simple bulge test were done to obtain the various strain combinations on FLC. The finite element analysis results were shown that the bursting at various strain combinations could be induced by simple bulge test. The experiment oi tube bulge test was carried out according to the test condition that obtained from finite element analysis and the left hand side of forming limit diagram was built.

Analytical and numerical approach to prediction of forming limit in tube hydroforming

Analytical and numerical analyses of forming limit in tube hydroforming under combined internal pressure and independent axial feeding are discussed in this paper. To predict the initiation of necking, Swift's criterion for diffuse plastic instability is adopted based on Hill's general theory for the uniqueness to the boundary value problem. In addition, in order to predict fracture initiation, Oyane's ductile fracture criterion is introduced and evaluated from the histories of stress and strain calculated by means of finite element analysis. From the comparison with a series of tube bulge tests, the prediction of the bursting failure based on the plastic instability and the ductile fracture criterion demonstrates to be reasonable so that these approaches can be extended to a wide range of practical tube hydroforming processes.