Experimental and numerical investigation of localized necking in aluminium alloy tubes during hydroforming

Abstract

Metal forming is an important technological operation in manufacturing, particularly in the automotive industry. Several components are shaped by sheet?metal forming, tube hydroforming, and bending processes. With advancements in computer controls and high?pressure hydraulic systems, tube hydroforming is being increasingly used in automotive applications to achieve weight reduction, reduced part count, and lower cost. Most failures in such operations are due to localized necking caused by extensive bulging of the tube wall. In this context, the forming limit diagram (FLD), which defines the onset of localized necking by relating the critical values of major and minor principal strains in the 2D strain space, can be used as a measure of the maximum formability of the material. The objectives of this thesis are to experimentally investigate failure due to localized necking in aluminium?alloy tubes during free hydroforming and to understand this behaviour through numerical simulations. To this end, hydroforming experiments are carried out by subjecting tubes to different loading histories involving internal pressure and axial compression. A specially designed hydraulic power pack, capable of generating high internal pressure (up to 75 MPa), is used in conjunction with an INSTRON testing machine for this purpose. The circumferential and axial strains experienced by the tubes are continuously recorded using post?yield strain gauges, along with the internal pressure and axial load. It is found that the strain histories (circumferential versus axial) experienced by the tube specimens are strongly non?proportional. After completion of the tests, the limit strains are determined from the analysis of circle grids etched on the tube specimens. Next, numerical simulations are conducted using both 2D axisymmetric and 3D finite?element formulations by applying the experimentally recorded axial?load versus internal?pressure histories. In the 3D simulations, a geometric imperfection is introduced in the form of wall?thickness reduction at the tube mid?length to trigger necking, which occurs after significant bulging and beyond the stage of peak pressure. The strain histories and peak pressures obtained from the simulations agree well with those determined experimentally. Additionally, peak pressures are predicted using an approximate analytical method that incorporates the computed strain paths. These values also match closely with the experimental data. It is found that the peak pressure increases with increasing axial?compression ratio. Furthermore, the forming limit curve (FLC) predicted by the 3D simulations and by a Marciniak朘uczy?ski (M朘) type analysis incorporating the computed strain paths corroborates well with the experimental results. It is shown that the nature of the strain path experienced by the tube specimen has a significant effect on the nature of the FLC when it is represented in terms of principal plastic strains. To eliminate the dependence of the FLC on the strain path, alternative representations using the tangent to the strain?history curve and the principal?stress plane are examined.