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Title: Finite deflections during impact of hollow balls
Author: Ashcroft, A. D. C.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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The current work begins with an examination of the sources of stiffness in hollow shells. The findings are used to accurately predict the impact behaviour of hollow spherical shells. Velocity distribution in balls and losses in kinetic energy caused by finite deflections during impact are also investigated. A thorough development of the concept of momentum flux is presented as it applies to the normal impact of hollow spherical shells, while the sources of loss during impact are identified and their magnitudes quantified by means of measurement and analysis. An accurate method for predicting losses during normal impact of hollow balls based on simple compression experiments is presented. Results are shown to be remarkably accurate for the case of both pressurized and pressureless tennis balls. Finally, investigations into the effects of finite deflections and the coefficient of friction during oblique impacts of hollow shells are described. It is shown that the relation between friction predictions for cases with cross sliding provide an accurate description of changes in the moment of inertia of a hollow ball. For the first time, the concept of tangential compliance is applied to the case of oblique impact in hollow shells. The related impact model accurately predicts the phenomenon of overspin, in which the final rotational velocity of the ball is greater than the final tangential velocity of the centre of mass. New understandings of the concept of momentum flux, have led to the development of an oblique impact model with rotation that incorporates momentum flux forces acting around the perimeter of contact into the equations of motion and dynamic stiffness. Results for impact of a well inflated thin-walled shell (basketball) show that effects of finite deflections can not be neglected.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available