FORMABILITY OF ADVANCED HIGH STRENGTH STEEL TUBES IN TUBE BENDING AND HYDROFORMING
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An investigation of the tube hydroforming process is conducted in order to understand the effect of pre-bending operation on formability in tube hydroforming and assess the application of the newly developed Extended Stress-Based Forming Limit Curve (XSFLC) method to the prediction of failure in tube hydroforming. Two sets of experiments on straight tube hydroforming and pre-bent tube hydroforming were conducted on tubes manufactured from three steel grades, namely DDQ, HSLA350 and DP600, which represent mild steel, high strength steel and advanced high strength steel, respectively. All tubes had the same outer diameter of 76.2 mm and the same nominal wall thickness of 1.8 mm, which enabled direct assessment of the effect of material strength on formability in tube hydroforming. For pre-bent tube hydroforming the tubes were bent to 90 degrees before hydroforming. The effect of the increased axial compressive load, termed the end-feed load, on tube formability in hydroforming was investigated. All experiments were simulated using the explicit dynamic finite element code LS-DYNA in order to investigate the accuracy of numerical predictions in the tube hydroforming process. The numerical simulations, validated using the experimental data, were then utilized to investigate the prediction of necking in straight and pre-bent tube hydroforming using the XSFLC method. The formability, burst pressure and corner-fill expansion in hydroforming of the pre-bent tubes was considerably less than that exhibited in hydroforming of the straight tubes. In both straight and pre-bent tube hydroforming, the application of the end-feed load postponed failure and significantly increased internal pressure and corner-fill expansion at burst. The finite element models accurately predicted the results of the tube bending and tube hydroforming experiments. The straight tube hydroforming simulations, validated using the experimental results, enabled accurate prediction of the failure location and tube internal pressure at the onset of necking using the XSFLC method. In order to obtain the XSFLC for each alloy, strain-based FLCs were calibrated using the results of tube free expansion tests. The results of the tube free expansion tests and corresponding numerical simulations also served to validate the tube material properties for the FE models. Straight tube hydroforming simulations were utilized to investigate the effect of friction between the tube and the die on the hydroforming process parameters and necking predictions using the XSFLC method. The validated pre-bent tube hydroforming simulations captured the trends in the increase of tube internal pressure at the onset of necking with the increase of end-feed load using the XSFLC method.