Doctoral Dissertations

Date of Award

8-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Dayakar Penumadu

Committee Members

Eric C. Drumm, Richard M. Bennett, Hahn Choo

Abstract

Advanced nondestructive neutron technology has been utilized to study fundamental issues in the Lost Foam Casting (LFC) process and in the mechanical behavior of infrastructure materials.

Time lapsed neutron radiography combined with digital image processing was used to investigate the real-time LFC process. Behavior and characteristics of the pyrolysis front in the LFC processes were discussed. Evidence shows that neutron radiography offers new insights into the pyrolysis front and the dynamics of the processes involved with the casting. Behavior and characteristics of the pyrolysis front and the molten metal interface in the LFC processes were revealed. The proposed approach will prove to be a powerful tool to characterize the degradation behavior of the expanded polystyrene foam during the LFC process and the interactions of liquid metal.

The stress-strain relationship of particulate materials is complex, and depends on the initial state of packing, past stress history, and the applied stress path. A novel in-situ study methodology has been developed using neutron scattering technique to obtain strains both globally and locally. The significant differences between the global deformation and the local lattice strain for silica sand have been found and discussed. The measured lattice strain was at least one order of magnitude smaller than the measured related global strain. However, the actual stress within the particles could be much higher than the applied global stress. Research results from this study will be useful for developing suitable elasto-plastic constitutive models of frictional granular materials.

Residual stress has a significant impact on the mechanical behavior materials. It is difficult to be measured or predicted using analytical methods, and can lead to premature failure of materials if not appropriately considered in design. Residual strains of identical steel tubular specimens after being subjected to either torsion or tension corresponding to a target equivalent strain invariant were probed using both reactor and spallation neutron sources. The lattice strains based on the hkl reflections that are reported to be both weakly and strongly affected by intergranular strain for tension stress path were investigated. The results indicate the essential difference between tension and torsion from the perspective of yield and failure criteria for materials.

An innovative approach has been developed to study the complete 3-D strain tensor using the 2nd Generation Neutron Residual Stress Facility at Oak Ridge National Laboratory. A procedure was also established to understand the mechanism and to analyze the errors of the calculated strain tensor. This newly developed approach makes it possible to study the strain/stress state in materials under complex conditions.

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