Estimating the Strain-Based FLC of a Tube from Straight Tube Hydroforming Experiments and Numerical Models 2008-01-1442

The Extended Stress-Based Forming Limit Curve (XSFLC) failure criterion has been shown to provide good qualitative and quantitative predictions of failure (necking) in straight tube hydro forming when the on the level of end-feed (EF) used during hydro forming, the failure criterion has a tendency to over predict failure pressure at low Keeler-Brazier (K-B) approximation is used to define the XSFLC failure curve. Depending EF and under predict failure pressure for high EF. The over/under predictions suggest that the strain-space εFLC, which the XSFLC is based on, has too high of a plane-strain intercept (FLC_o), when it is obtained using the K-B approximation (developed for sheet metal).

Using the results of DP600 straight tube hydro forming experiments with EF, the hydro forming process was modeled using the finite element code LS-DYNA and a numerical parametric study was conducted to show that the FLC_o, which most accurately predicted failure, was lower than the FLC_o predicted by the K-B approximation for the zero and 67kN EF cases. For the 133kN EF case, the FLCo determined by the K-B approximation under predicted the measured burst pressures. This inconsistency suggests that the shape and FLCo of a tube's strain-based forming limit curve (εFLC) is different than that of sheet metal as shown by Keeler and Brazier. The coefficient of friction (COF) was also varied in the models to show the effect it has on failure prediction using the XSFLC. The COF parametric study showed that the XSFLC method is very sensitive to the COF used in the numerical models. This is most evident at high EF, when a small change in COF resulted in a large change in the predicted failure pressure.