sgr ms thesis - University of Maine

7.4.2. Results and Discussion
COMSOL follows engineering convention, meaning that tensile stress is
positive and compressive stress is negative. The first principal stress relates to
maximum tensile stress, while the second principal stress relates to minimum
tensile stress for a two-dimensional model. Figure 7.7 shows transient
development of the first principal stress distribution in the clast (also see
Animation 2, 3). Local tensile stress buildup begins in the roots of sharp corners
and moves inward as the clast heats up. Two patterns of fracture are evident
from the experiment. The first phase of thermal fracture shows preference for
edge break off. Arrows denote the direction of the second principal stress, or the
direction of minimum tensile stress that a tensile fracture would prefer to follow.
For this clast there is a preferred cuspate-shaped fracture pattern that would
produce more corners. Eventually the number of corners would be reduced by
successive fragmentation and circularity would increase. Next, as the clast
continues to warm, tensile stress in the center of the clast reaches a critical
value, allowing larger fractures to nucleate from the center and break the clast in
half. The clast is slightly elongate, and
increases inward and parallel to
elongation. The first principal stress directions run perpendicular to elongation,
preferring to fracture along the short axis. At this time of thermal exposure,
originally elongate clasts would be preferentially broken up into more equant
fragments, and each new fracture surface provides a fresh face to repeat the
process. The potential for thermal fracture declines as the clast heats up and the
thermal gradient is reduced.
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