Research On The Effects Of Stamping Process About The Fatigue Property Of Auto Body Components

Automobile's body structure and engineering goods are all subject to fatigue rupture, which is a usual phenomenon of damage .Automobile has to bear continuously changing random vibration and impact when travelling on rugged road surface, and after certain miles of distance have been travelled, fatigue rupture will take place. A component's fatigue properties are closely related with many other factors, such as concentration stress, dimensional effect, how rough its surface is, etc. Many researches have been done on these factors, unlike the Stamping technology, a factor haven't been studied so much. Since the Stamping technology have been widely adopted in manufacturing surface covering and other modern auto body components, especially truck body, now there is a tendency to study its influence on auto body components both inside and outside China. This article studies punching stress technology's influence on auto body component's fatigue property in terms of the deformation of material, cold-work hardening, variation of thickness and predeformation created by it.The research content of this article:The first chapter, introduction, is mostly about the basis and significance of the chosen subject The research on fatigue problem at current stage will be introduced from three aspects: the classification of fatigue, the classification of tests on material fatigue, and the significance of fatigue analysis at current stage, including the course development of research on fatigue problem both inside and outside China. In the research history of fatigue problem, many researches have been done on thickness changes caused by Stamping technology; while there have not been many researches on deformation and the simultaneous predeformation. Supported by"Stamping Steel Sheet S—N curves Test", which is subordinate to"National High Technology Research and Development Project"(863 project) :"High Quality Commercial Automobile Development Technology", this article deals with the influences brought on auto body component's fatigue property by thickness change and predeformation resulted from Stamping technology.The second chapter handles fatigue theory, including fatigue rupture and the characteristics of fatigue rupture, mainly focusing on the study of S—N curves and factors that affect S—N curves. This article starts from the research on S—N curves to study the influences on auto body component's fatigue properties. From the research on S—N curves, we can get the material's limit of yielding and endurance limit, which can reflect the material's mechanical properties, and from which the material's fatigue life can be further deduced, then finally the material's fatigue properties. Next, measures can be offered on how to reinforce auto body component's fatigue strength.The third chapter presents tests and handling of data. In order to study Stamping technology's influence on auto body component's fatigue properties, I select primitive test-piece and prestress of 5%, 10%, 15%, 20% after predeformation, two stamping steel sheet test-pieces of different standards of thickness: 1.2 mm and 2.0mm respectively, and two different materials: one ST13 and the other ST14, to do the S—N curves test. From the introduction of testing methods, testing equipments, test-pieces, check on test-pieces, testing conditions, to selection of parameters and comment on failures, they have all been given detailed explanations. When choosing R=-1 symmetrical-cyclical test-piece, instability has been created at early testing stage, and as a result the test cannot be finished with equipments now available at automobile material department, so R=-1 unsymmetrical-cyclical test-piece is adopted. Through a reversible inference with the help of average stress correction formula, I discover that when R=-1, the material's endurance limit is subject to the same law. Further, in the process of simplifying and fitting S—N curves, I also find that, with the same material and thickness, S—N curves has different density when predeformation are different. Thus the conclusion can be drawn that different predeformaion have different influences on S-N curves, when predeformation is small, the influence is big; when predeformation is big, the influence is small. When predeformation is over 20%, its influence is negligible. This is due to different predeformation influences on limit of yielding, reflecting predeformation influence on fatigue strength in one aspect.On the basis of the above tests, chapter four studies two typical models of auto body components'changes in quiet strength and fatigue life before and after introducing into nodal thickness and predeformation. Considering different components have different geometries, deformations occurring in different places are different, and the following influences on fatigue properties are also different. During the drawing of A pillar inner and beam outer,some parts get different degrees of thickness deduction, so nodal points'changes of thickness have different influences on them, but they have the same tendency of fatigue quiet strength after introducing into nodal point thickness, that is, when the thickness of nodal points decreases, the components under the same loading will produce the most enormous stress increase. On fatigue properties, after introducing into nodal point thickness, the components'life will be shorter than that of components with original thickness, thus it is clear that the changes of nodal point thickness weaken fatigue properties. When introducing into predeformation alone, the above tested different predeformation S—N simplified curves should be given according to different nodal point thickness, and when calculating, the thickness should remain the same, so the comparative analysis of fatigue life can be carried out. After the predeformation being introduced, the model's fatigue life is longer than that of original component model, so as we can see, predeformation influences on component and material's fatigue life are the same: strengthening fatigue properties. And when introducing into nodal point thickness and predeformation simultaneously, through calculating fatigue life, we find that, under comprehensive influences, the two components'lowest fatigue life increases, compared with that of original components. Their degrees of increase are not the same, but we can still see the tendency of punching stress technology's influences on components'fatigue life. Take our tests and test-pieces for example, even if the nodal point thickness deduction somehow weakens the component's fatigue properties, still we can not neglect the simultaneous predeformation that strengthens component's fatigue properties. Under comprehensive influences, punching stress technology strengthens the fatigue properties of the auto body component we choose.