Temperature and Axial Strain Rate Effects on Micromechanical Behavior in Triaxially Compressed Marbles
Schrodt, Joseph Keith
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https://hdl.handle.net/2142/71129
Description
Title
Temperature and Axial Strain Rate Effects on Micromechanical Behavior in Triaxially Compressed Marbles
Author(s)
Schrodt, Joseph Keith
Issue Date
1983
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Abstract
A series of high resolution, simultaneous stress-axial strain, volumetric strain, and acoustic emission measurements were used to survey the effects of temperature (T) and axial strain rate ((epsilon)) variations on micromechanical behavior in triaxially compressed marbles. Thirty-five tests were run on low porosity, homogeneous marble samples with relatively uniform calcite grain sizes (0.23, 0.50, and 1.90mm), at pressures (P) of 0.75, 1.375, and 2kb, temperatures of 30, 100, and 200(DEGREES)C, and computer-controlled strain rates of 10('-4), 10('-5), and 10('-6)/sec.
A d('- 1/2) grain diameter dependence of the yield stress ((sigma)(,y)) and onset of dilatancy (C') over most of the T/(epsilon)/P conditions and analysis of individual AE signal patterns, was consistent with an association of (sigma)(,y) with an acceleration in twinning and related microcrack nucleation, and of C' with inter- and/or transgranular cracking. At elevated T, decreases in (sigma)(,y) and C' were attributed to thermally induced microcrack activity, and after C', regions of unstable crack growth alternated with periods of crack stability. The amount of dilatancy decreased with T due to plastic activity at crack tips. At high P and/or high T, dilatancy was suppressed and post-yield pore collapse led to accelerated negative volume change.
Crack nucleation and growth in coarser grain marbles and grain boundary sliding in finer grain marbles were enhanced at lower (epsilon). Increases in (epsilon) or T reduced the pressures at which transitions between spontaneous, unstable, and stable crack growth occurred.
The contribution of dilatancy or compaction generated work done on the confining fluid pressure was removed from the observed stress-axial strain curves. A peak stress was observed in the corrected ((sigma)(,w) versus (epsilon)) curves for larger grain size samples deformed at room T and high P. The peak was followed by a brief period of work softening and a subsequent hardening region. The trend toward the occurrence of a peak stress was enhanced with temperature and pressure and the slope of the post-peak hardening region was relatively temperature independent. Similar observations in other systems have been modelled by dislocation multiplication processes.
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