<p>The lightweight and good strength of oriented thermoplastic tubes, produced through the ram extrusion process, renders this class of material advantageous from an automotive structural part forming perspective. An analytical model, capable of predicting the forming limits for these materials at elevated temperatures, would avoid the need for difficult and time-consuming tube forming experiments.</p> <p> The uniaxial tensile stress-strain properties of the oriented polypropylene (OPP) tubes along the axial and hoop direction wene determined while methods were developed for obtaining the uniaxial compressive properties of the tube. The uniaxial and biaxial testing of the OPP tubes enabled representation of the experimental plastic work contours for the material at a range of temperatures. The use of the pressure-modified Hill criterion, the concept of plastic work contours in stress space, and the stress strain data from uniaxial testing of the tube allowed accurate prediction of the plastic work contours for the OPP tubes as a function of temperature. The analytical prediction was able to capture the anisotropy, pressure dependency and anisotropic hardening exhibited by the material at elevated temperatures. In addition, the use of a localized necking criterion, which involved the point at which the maximum force along the hoop direction of the tube was reached, allowed the temperature dependent forming limits for the material to be predicted. The forming limits increased with temperature for all of the strain paths considered. In addition, at a given temperature, the forming limits decreased with increasing strain ratio. The limiting strains achieved with axial end feeding were markedly higher than those in which no axial end feeding was used. </p> <p> Good agreement was observed between the predicted forming limits and the results from tube forming tests of the material, along a range of strain paths and at various temperatures.</p> / Thesis / Doctor of Philosophy (PhD)

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)