Analytical and experimental analysis of the permanent deformation after collisions
Type of DegreePhD Dissertation
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This work studied the normal and oblique impact of elastic-plastic metals. The focus of this work was to investigate the relations between the permanent deformations after the collisions and the motion of the impacting objects. The direct solution of the equations of motion during collision using various contact models have been studied. For the first step, to predict the contact force between two elastic-plastic objects, the quasi-static contact of a hemisphere with a half space was modeled with the finite element method. More than 400 different material combinations were modeled. And for the first time in the field, a comprehensive formulation was developed to predict the contact properties of two deformable objects in contact. The results from this study were later used to directly solve the equations of motion of a round end rod normally colliding an elastic plastic flat. The impact was divided into two main phases, compression and restitution. Using this, a new expression for the permanent deformations after collision was presented. It was shown in this work that by using the experimental measurement of the permanent deformations after collision, one can predict the motion of the objects during the collision more accurately. The same approach was taken for the direct solution of oblique impact of a round end rod with a flat. A comprehensive review of the current contact models and their implementation for oblique impact problems was done. The simulation results were compared to the experimental results for the coefficient of restitution and the permanent deformation after the collision for two impact angles. It was shown that the numerical prediction of the oblique impact matches the experimental results in terms of the motion analysis but not for the plastic deformations. The deformation results from the experiments show significantly smaller values, which implies that the normal contact force during the oblique impact should decrease in comparison to the normal collision. To address this phenomena a semi-analytical semi-experimental model was developed that was calculating the impact duration, and the average normal force during the collision. The new model showed that the impact duration during the oblique impact increases significantly while the normal contact force is orders of magnitude larger than the numerical predictions. Finally, we showed that the permanent deformation patterns after the collision can be used to predict the impact parameters much more accurately. The permanent deformation patterns also show interesting insights about the tangential contact forces during collision that need to be studied more in the future.