Date of Award

12-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Joseph, Paul

Committee Member

Summers , Joshua D

Committee Member

Blouin , Vincent

Abstract

One of the missions of National Aeronautics Space Administration (NASA) is to develop a vehicle that can travel for a longer distance on the moon and have a greater degree of mobility compared to the currently used Lunar Roving Vehicles (LRV). This led to the development of the All-Terrain Hex-Limed Extra-Terrestrial Explorer (ATHLETE), which requires a significant advance in the type of wheels that must be used on this highly mobile lander. The Michelin Lunar Wheel, which is a non-pneumatic tire invented by Michelin Researche et Technologie has been identified as one of the key designs capable of performing on the lunar environment and satisfying the mobility requirements of the ATHLETE.
One of the critical characteristics of a tire for mobility in sand is to have a low and constant contact pressure throughout the contact patch. Experimental results obtained by the Swiss MICHELIN team for the Michelin Lunar Wheel indicate that the pressure is not uniform and that the pressure is higher than NASA would prefer. Such pressure non-uniformity is inherent to the design of the tire. Since these wheels are very expensive to build, it is desirable to have the modeling capability to predict pressure accurately and to optimize the pressure distribution.
In this thesis, to understand the contact pressure behavior more clearly, the Michelin Lunar Wheel is initially simplified to only a ring that is pressed between two frictionless rigid planes. The analysis is performed using ABAQUS Standard finite element software. It is seen that all the structural elements in the ABAQUS element library face difficulty in predicting accurate contact pressure at the edge of contact for a thin and stiff structural member, such as what is used to design the lunar wheel. Convergence with respect to mesh refinement cannot be achieved. To overcome this problem, a soft tread of reasonable stiffness is added on the outer perimeter of the ring which resolves the convergence problem and unique contact pressure profiles are obtained. The modeling approach developed for the simple ring model was extended to both two-dimensional and three-dimensional wheel models.
Sensitivity analysis was performed on the two dimensional model to determine what design parameters affect the contact pressure. These results show that it is very difficult to define the correct computational model to predict accurately the contact pressure since very small displacements can drastically change the pressure distribution. For example, for the baseline loading the wheel deforms about 14 mm leading to a non-uniform pressure. A non-uniform change in displacement with amplitude less than 0.2 mm can convert this non-uniform pressure into a uniform pressure. In order to predict displacement accurately, it is necessary to precisely model the actual geometry and structural connections between small parts, which are very complex to define. Based on this sensitivity analysis and the approach of introducing a non-uniform displacement by modifying the tread thickness, areas for future work are identified and presented at the end of the thesis.

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