T
Two dimensional and three dimensional thermo elastic
failure criteria, incorporating the effects
of crack/cavity closure, are developed in terms of
imposed thermal and mechanical loads. Corresponding
general forms of the Griffith failure criteria are
obtained and compared. The two dimensional deriva -
tions are applied to predict tensile strengths,
critical stress intensity factors, and failure envelopes
for Pittsburgh coal at elevated temperatures.
The applicability of the general results to other
rock media is indicated.
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The development of thermo-mechanical failure criteria
for rock media is fundamental to the investigation
of several problems related to underground
coal gasification, geothermal energy extraction, and
oil shale retortion techniques. It has been shown
that Griffith''s fundamental work [1] does not predict
the experimentally determined strengths of rocks [2].
To realistically apply Griffith''s failure criterion,
McClintock and Walsh [3] have assumed that it is
possible for the crack to close in compression and
thereby develop normal and shear stresses due to contact
and friction.
Attempts have been made to generalize Griffith''s
bi-axial stress failure criterion to a tri-axial
state of stress. Murrell [4] suggested that the
three dimensional version of Griffith''s parabolic
curve criterion is a parabola of revolution. The
empirical generalization of Griffith''s bi-axial
stress failure criterion is due to Niwa and Kobayashi
[5] who assumed that the maximum and minimum
principal stresses revealed by Griffith''s criterion
and Mohr''s circle stress transformations could be
interpreted in terms of a three dimensional stress
space. Recently, Paul and Mirandy [6] have developed
a three dimensional failure criterion by using
Griffith''s approach on a flat ellipsoidal cavity
under a three dimensional state of applied stress.
To date, most attempts at failure criteria are based
on a critical energy release rate concept [7]. The alternate strain energy density theory advanced by
Sih [8] still remains a subject of considerable controversy.
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