7 Conclusions
In this work, triaxial cyclic loading unloading compression test was
adopted to study the mechanical behavior of granite specimens after
thermal treatment by experiment and numerical simulation. The
stress-strain curves, elastic modulus, Poisson’s ratio, plastic strain,
energy and AE characteristic was analyzed, and the effect of temperature
and confining on the damage evolution was investigated. The following
conclusion are obtained:
(1) On the one hand, the strain energy release gradually after several
cycles under cyclic loading compression, and the damage degree is lesser
than that under monotonic loading. On the other hand, the failure modes
under triaxial cyclic loading are more complicated than that under
triaxial monotonic loading, which make the shear plane rougher.
Therefore, the residual strength is higher than that under monotonic
loading.
(2) After 600°C thermal treatment, the bearing structure of specimen is
destroyed by high temperature, micro-crack continuously increases under
unloading and loading process. Therefore, the peak strength of specimen
under uniaxial cyclic loading is lesser than that under monotonic
loading.
(3) Elastic modulus first increases and enter a stable phase, then
decreases before becoming constant again with cycle number. Applied
confining pressure make the increasing stage shorter, and high
temperature treatment make the elastic modulus more sensitive to
confining pressure. Poisson’s ratio remains near constant in the initial
stage, then increases rapidly, and decreases slightly before entering a
stable phase again with cycle number on the whole. However, the initial
constant stage disappear when T = 600°C.
(4) Axial plastic strain non-linearly increases with the maximum axial
strain on a concave type, whereas the non-linear characteristic
decreases when T = 600°C. It means that damage occurred once
applied compression, and it can be verified by AE characteristic. Radial
strain even appear compressive characteristic in the initial stage,
which indicates that the grain adjust under triaxial cyclic
loading-unloading compression. And, the compressive characteristic is
more obvious with increasing temperature and decreasing confining
pressure.
(5) The radial input and elastic strain energy is relatively small
compared with axial input energy, axial elastic strain energy and
dissipated energy. When T = 25 and 300°C, axial input energy
mainly transfer as axial strain energy at initial loading stage, whereas
axial strain energy and dissipated energy are near equal when T =
600°C. Confining pressure increases the carrying capacity of rock, and
need more dissipated energy to destroy the rock. Therefore, when the
specimen failed, the dissipated energy increases with the increase of
confining pressure. Confining pressure also restrain the crack
initiation and propagation, therefore the variation of dissipated energy
of specimen with confining is irregular before failure.
These observations indicate that changes in mechanical of granite are
impacted significantly by thermal treatment and these effects must be
accommodated in performance assessment.