Failure in internally pressurized bent tubes
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2/12/04
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Failure in Internally Pressurized Bent Tubes ROBIN STEVENSON, BOON-CHAI NG, and PETER POLIDORO The analysis and modeling of tube-hydroformed components is more complicated than that employed for sheet-metal panels, due to the lengthier process sequence and variable strain path—from flat-rolled sheet to tube; from straight tube to bent tube; and from bent tube to hydroformed component. These additional process steps make it difficult to determine whether post mortem analyses of tube failure during hydroforming can, and should, be conducted with the same tools and databases as used for simple stampings. To provide a partial answer, the properties of commercially fabricated welded straight tubes were evaluated using a free-expansion internal pressure test and compared with those of free-expansion internal pressure tests on bent tubes. The results demonstrated that the behavior of the bent tube was consistent with the mechanical properties of the as-received tube, provided due notice was accorded to the complex strain history of the bent tube. However, due to the strain-path changes occurring at the failure location, conventional approaches for monitoring strain history would yield (apparently) anomalous results.
I. INTRODUCTION
TUBE hydroforming using seamless welded tube has become a major process for fabricating automotive structural components. The process is deceptively simple: a tube is bent to the approximate shape of the desired component, placed in a die of the appropriate shape, and internally pressurized (typically, using a water-based fluid) so that it deforms to fill the die cavity and adopts the desired shape. However, a multiplicity of deformation events are experienced by the material of the tube wall.[1] Even before the hydroforming process itself, the tube is fabricated by deforming flat-rolled product by bending it into an “O” shape, welding the edges together, and then further deforming it to develop the desired size and ovality. Thus, the tube-wall properties differ from those of the flat sheet from which it was fabricated and also vary through the thickness. The tube is then bent (typically, locally) to conform to the die geometry, thereby introducing strain gradients both along the tube (corresponding to the bent and unbent regions) and “across” the tube (corresponding to the inner and outer bend radii). Finally, the tube is expanded by internal pressurization and is subjected to a strain state largely corresponding to plane-strain expansion. Typically, varying amounts of expansion are imposed along the length of the tube, since one of the production advantages of tube hydroforming is its ability to generate varying cross sections along the tube. Thus, any element of the tube may have been subjected to a series of deformations of varying strain paths during the overall fabrication process.[1] There is continuing interest in modeling this process and in performing post mortem analyses of failed processes to understand whether these failures are more related to materi
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