Shigen-to-sozai, Vol.118, No.10,11, pp.650-658.
Effects of In-situ Stress on Thickness of Disc in Core Discing
Noriyuki KAGAa, Koji MATSUKIb and Kiyotoshi SAKAGUCHIc
| a. Graduate student, Tohoku University, Sendai
980-8579, Japan b. Professor, Department of Geosicence and Technology, Graduate School of Engineering, Tohoku University c. Assistant Professor, Department of Geoscience and Technology, Graduate school of Engineering, Tohoku University |
| Core discing occurs due to tensile stress
induced by boring within or below a core
stub. To determine effects of the length
of core on the magnitude and direction of
tensile principal stress, a finite element
analysis was carried out for an HQ core with
different lengths for 77 in-situ stress conditions.
We analyzed the semi-axial tensile principal
stress, which is inclined from core axis
by less than 45°, and the maximum semi-axial
tensile stresses were determined with respect
to the core axis for each position of the
cross-section of the core. The minimum value
and the mean inclination relative to the
core axis of the maximum semi-axial tensile
stresses were analyzed to determine the stress
condition under which core discing is likely
to occur. As a result, 30 in-situ stress
conditions were identified as the stress
condition under which core discing is likely
to occur and the necessary condition for
the in-situ stresses was proposed. The critical
tensile stress, which is the maximum tensile
stress that can produce a tensile fracture
propagated throughout a cross-section, was
analyzed for these stress conditions and
a new criterion for core discing, which can
be applied to any length of core, was proposed.
Stress conditions estimated by the criterion
were consistent with previous experimental
results for a long core and for discs with
small thickness. According to the criterion, the relationship between the length of core and the in-situ stress necessary for core discing was discussed, which showed that the stress field can be divided into three regions and that core discing with small length mostly occurs at great depth. The relationship between the length of core and the thickness of disc was determined by assuming that the position of a fracture is given by the mean position of the maximum semi-axial tensile stresses. Theoretical estimation reproduced previous experimental restilts on the effects of stress magnitudes on the thickness of disc. |
| KEY WORDS: Core Discing, Three-dimensional Stresses, Tensile Principal Stress, Length of Core, Thickness of Disc |