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Cold pilgering is a challenging tube forming process in terms of modeling due to the complexity in kinematic of tools, friction condition and material behavior. The process development has mostly been based on simple formulas and costly full-scale tryouts. The aim in this study is to develop validated Finite element models of cold pilgering to increase the understanding of influence of the process parameters on the produced tubes. In the course of this thesis, three-dimensional mechanical and thermo-mechanical Finite element models of cold pilgering were developed. The commercial code MSC.Marc was used in the simulations. General 3D models are needed to be able to capture asymmetric deformation in cold pilgering. It was found that tool deflections together with elastic deformation of roll dies have considerable influence on the rolling force. Furthermore, the strain rate and temperature effects on the response of the material and thereby on the rolling force were evaluated.

In human pregnancy, the feto-placental vessels are crucial for efficient materno-fetal transfer; hence they play a pivotal role in the pathogenesis of fetal growth restriction (FGR). We, as well as other research groups, have observed abnormalities in the FGR feto-placental vasculature, which, though inconclusive, were suggestive of a state of panhypovascularity. The goal of the work presented in this thesis was to investigate this. We hypothesised that the placenta may be panhypovascular in FGR due to failed angiogenesis; and enhancing angiogenesis in the placenta may improve fetal growth. Custom-designed techniques including advanced imaging, computer-aided analyses and tube-forming experiments were employed to compare feto-placental vessels and endothelial cells in placentas from normal and FGR-complicated pregnancies while aiming to answer two main research questions: (i) is the FGR placenta panhypovascular? (ii) can angiogenesis be induced or enhanced to improve placental vascularity?Findings include: (i) shorter arterial [p = 0.03 and 0.009 when data adjusted for placental surface area (PA) and weight (PW) respectively] and longer venous path [p = 0.05 and 0.03, adjusted for PA and PW respectively] in FGR placentas though no difference in the total number of arterial or venous branches, diameter, and tortuosity of the vessels compared to normal; (ii) altered angiogenic behaviour/response of FGR placental endothelial cells following in vitro pharmacological manipulation of WNT signalling; (iii) human placental endothelial cells are capable of regaining their angiogenic potential following withdrawal of WNT inhibition. These findings discount the hypothesis of panhypovascularity in FGR placentas, but identify additional, previously unreported, feto-placental vascular abnormalities associated with FGR. Also, the findings provide evidence that impairment of WNT signalling may play a role in defective angiogenesis and consequent dysvascularity in the FGR placenta. The evidence suggests the WNT pathway should be explored as a potential new target for therapeutic interventions to correct placental dysvascularity in FGR.

Cold pilgering is a challenging tube forming process in terms of modeling due to the complexity in kinematic of tools, friction condition and material behavior. The process development has mostly been based on simple formulas and costly full-scale tryouts. The aim in this study is to develop validated Finite element models of cold pilgering to increase the understanding of influence of the process parameters on the produced tubes. In the course of this thesis, three-dimensional mechanical and thermo-mechanical Finite element models of cold pilgering were developed. The commercial code MSC.Marc was used in the simulations. General 3D models are needed to be able to capture asymmetric deformation in cold pilgering. It was found that tool deflections together with elastic deformation of roll dies have considerable influence on the rolling force. Furthermore, the strain rate and temperature effects on the response of the material and thereby on the rolling force were evaluated.

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)