In order to effectively and efficiently hydroform new automotive components, the formability of new tubular steels must be evaluated. Standard forming limit diagrams have been used for decades to evaluate and predict the formability of sheet steel formed along linear strain paths. However, tube hydroforming can present a problem since the pre-bending stage used in many hydroforming operations causes multiple non-linear strain paths. This thesis has modified a formability test method that deforms small-scale sheet steel samples in a single plane. The sample geometries were designed such that the strain paths achieved at the center of the samples were very near the plane strain condition. The four steels chosen for this study were: a deep drawing quality (DDQ), a high strength low alloy (HSLA) and two dual phase steels (DP600 and DP780). The plane strain formability for each of the four steels was tested in both the rolling and transverse directions. Three objective criteria were employed to evaluate and directly compare the formability of the four steels tested: difference in strain, difference in strain rate and local necking. The DDQ steel showed the highest formability followed in order by the HSLA, DP600 and DP780 steels. The repeatability in determining the forming limit strains using the difference in strain, the difference in strain rate and the local necking criteria for a 95% confidence interval was ± 1.5%, ± 1.2% and ± 3.2% engineering strain, respectively. The forming limit data collected for this thesis has been compared to results from full-scale tube hydroforming operations and free expansion tube burst tests carried out by researchers at the University of Waterloo on the same four materials. It was found that local necking results could be used to predict failure of hydroformed HSLA steel tubes with low levels of end-feed. However, this same method could only predict the failure of hydroformed DP600 steel tubes at higher levels of end-feed. The three objective criteria were not found to be suitable for predicting failure of free expansion tube burst tests. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2007-09-27 15:00:35.873

Oriented thermoplastics offer interesting opportunities for making structural automotive components due to their higher strengths. A new process, referred to as the axial feed hot oil tube forming (AF-HOTF) process, has been developed and studied for the forming of oriented thermoplastic tubes. The starting material for AF-HOTF process is an oriented polypropylene (OPP) tube produced by the solid state extrusion process. AF-HOTF was used to study forming and fracture behaviour of OPP tubes at large strains. Mechanical properties and molecular orientation of starting and post-formed materials were investigated to gain a better understanding of structure-property-process relationships during solid state extrusion and subsequent forming of OPP tubes. The development of molecular orientation and other microstructural changes and damage development in extruded and bulged OPP tubes during solid state extrusion and AF-HOTF processes were studied with optical microscopy, wide-angle X-ray diffraction (WAXD) and field emission scanning electron microscope (FE-SEM) techniques. Also, the development of large strains during AF-HOTF of OPP samples were experimentally studied in the form of spatial strain maps, strain/stress state and forming limit strains using an on-line strain mapping method based on digital image correlation (DIC). In addition, tensile tests have been carried out at room temperature on samples machined from the extruded and bulged tubes along the axial and hoop directions. Experimental quantitative relationships amongst molecular orientation parameters and extrusion and AF-HOTF process parameters such as draw ratio, strain and strain state have been obtained. These relationships in the form of White and Spruiell biaxial orientation factors provide a useful insight into molecular reorientation that occurs during extrusion and subsequent forming of OPP tubes. Also, an analytical model for forming limit prediction that takes into account OPP tube properties, tube dimensions and AF-HOTF process parameters was developed based on existing model of tube hydroforming in the literature. In addition, a new biaxial ball stretching test (BBST) system was developed and utilized to subject the thermoplastic tube to biaxial stretching. The design of the test-rig and results were presented for polypropylene (PP) tubes subjected to BBST at various temperatures. The BBST system was combined with an available on-line imaging and strain analysis system (ARAMIS® system from GOM) to observe the development of strains in the biaxial tensile region during the test. BBST samples were studied with wide angle X-ray diffraction (WAXD) pole figures. Three different hot forming processes (Solid-state extrusion, AF-HOTF and BBST) were used in this research. The structure of the extruded samples at draw ratio 5 and higher was completely changed to fibrils structure, and the yield strength and elastic modulus increased by 50%. Also the crystallinity increased from 47% to 68% with an increase in draw ratio. An increase in axial feed during the hot forming process resulted in higher formability (strains values of 0.55 major strain and -0.25 minor strain) and delayed failure. The analytical model prediction of bursting shows good agreement with the experimental results. The results provide an understanding of the orientation development in solid state extrusion of PP tubes as well as an understanding of tube formability, flow localization and fracture characteristics of PP tube from AF-HOTF process and other related processes. / Thesis / Doctor of Philosophy (PhD)