Alexander Bardelcik

Prediction of Necking in Tubular Hydroforming Using an Extended Stress-Based Forming Limit Curve

This paper presents an extended stress-based forming limit curve (XSFLC) that can be used to predict the onset of necking in sheet metal loaded under non-proportional load paths, as well as under three-dimensional stress states. The conventional strain-based eFLC is transformed into the stress-based FLC advanced by Stoughton (1999, Int. J. Mech. Sci., 42, pp. 1-27). This, in turn, is converted into the XSFLC, which is characterized by the two invariants, mean stress and equivalent stress. Assuming that the stress states at the onset of necking under plane stress loading are equivalent to those under three-dimensional loading, the XSFLC is used in conjunction with finite element computations to predict the onset of necking during tubular hydroforming. Hydroforming of straight and pre-bent tubes of EN-AW 5018 aluminum alloy and DP 600 steel are considered. Experiments carried out with these geometries and alloys are described and modeled using finite element computations. These computations, in conjunction with the XSFLC, allow quantitative predictions of necking pressures; and these predictions are found to agree to within 10% of the experimentally obtained necking pressures. The computations also provide a prediction of final failure location with remarkable accuracy. In some cases, the predictions using the XSFLC show some discrepancies when compared with the experimental results, and this paper addresses potential causes for these discrepancies. Potential improvements to the framework of the XSFLC are also discussed.

The Effect of Element Formulation on the Prediction of Boost Effects in Numerical Tube Bending

This paper presents advanced FE models of the pre‐bending process to investigate the effect of element formulation on the prediction of boost effects in tube bending. Tube bending experiments are conducted with 3″ (OD) IF (Interstitial‐Free) steel tube on a fully instrumented Eagle EPT‐75 servo‐hydraulic mandrel‐rotary draw tube bender. Experiments were performed in which the bending boost was varied at three levels and resulted in consistent trends in the strain and thickness distribution within the pre‐bent tubes. A numerical model of the rotary draw tube bender was used to simulate pre‐bending of the IF tube with the three levels of boost from the experiments. To examine the effect of element formulation on the prediction of boost, the tube was modeled with shell and solid elements. Both models predicted the overall strain and thickness results well, but showed different trends in each of the models.

Application of an Extended Stress‐Based Flow Limit Curve to Predict Necking in Tubular Hydroforming

This paper reports a novel extension of the stress‐based flow limit curve approach. A conventional strain‐based FLC is converted into a stress‐based FLC using the method proposed by Stoughton. This stress‐based FLC is then transformed into the XSFLC by casting the stresses in terms of the invariants — effective stress and mean stress that are the variables that constitute the XSFLC. This curve is used in conjunction with LSDYNA finite element simulations that use eight‐noded solid elements to predict the onset of necking in tubular hydroforming. The particular case of straight tube hydroforming of ENAW 5018 aluminum alloy tubes is considered. The XSFLC is able to predict the internal pressure at the onset of the neck and the final failure location in the tube with remarkable accuracy.

Three Point Bend Performance of Solutionized, Die Quenched and Heat Treated AA7075 Beam Members

To overcome the low room temperature formability of AA7075-T6 aluminum sheet, without sacrificing the high strength properties of this alloy, a hat section beam member was formed and quenched within a cold die immediately after a 20 minute solutionizing treatment. Natural aging for 24 hours followed the forming process which was then followed by various heat treatments that included a typical precipitation hardening (PH) and industrial paint bake (PB) temperature-time treatment. Tensile specimens were extracted from the beams to evaluate their mechanical properties. When compared to the as-received AA7075-T6 mechanical properties, the beams heat treated with the PH, PHPB and PB treatment resulted in a 5%, 13% and 20% reduction in ultimate tensile strength respectively. A similar trend was shown for the yield strength measurements. There was little effect of the heat treatments on the total elongation, with the PH condition showing a slight improvement. A backing plate was riveted to the beams and a quasi-static 3 point bend test was conducted to evaluate the crush performance. The peak load for the PH, PHPB and PB beams was 9.2, 8.5 and 7.3 kN respectively, but the calculated energy-displacement (or energy absorption) curves were similar for the PH and PHPB parts due to a more ductile fracture behavior for the PHPB material condition.

Straight Tube Hydroforming of Dual Phase (DP780) Steel Tubes With End‐Feed

Dual phase (DP780) 76.2 mm (3”) diameter steel tubes were hydroformed with various levels of end‐feed (EF). The results of this study showed that when zero EF was applied during hydroforming, the average burst pressure was 70 MPa (10,075 psi) and the corner‐fill expansion (CFE), which is a measure of formability, was 6.4 mm. When an EF force of approximately 50% of the material yield strength was applied during hydroforming, the tube was able to support an internal pressure of 151.7 MPa (22,000 psi) without failure and achieved a CFE of 11.5 mm. Finite element (FE) models of the hydroforming process accurately predicted the CFE of the tubes for the various EF cases tested. Upper and lower bound strain‐based forming limit curves (eFLC) were determined from free‐expansion burst test data. These curves were then used to derive the upper and lower bound extended stress‐based forming limit curves (XSFLC), which were in turn used to predict the necking (failure) pressure in the FE models. For the two cases where burst was achieved in the experiments, the upper and lower XSFLC failure criteria curves bound the measured burst pressures. Also, two different friction coefficients were used in the FE models to evaluate their effect on predicting the failure pressure, CFE and end‐feed displacement.