T
We have studied experimentally the formation of
thermally-induced microcracks and their effect on the
physical properties of Westerly granite and Charcoal
granite subjected to slow (<l.5°C/min), uniform tem
perature
changes. More specifically, we have measured
(at room temperature) the longitudinal-wave velocity
(VL), permanent linear and volumetric strain (?L/L and
?V/V, respectively), seismic quality factor (Q),
brazilian tensile strength( sT,) compressive strength
(sc) and tangent modulus (Et) , and permeability (k) as
a function of maximum thermal-cycle Temperature (Tmax).
Permeability, compressive strength and tangent modulus
also are measured as function of confining pressure.
Thermally-induced and microcracks are characterized direct
ly
using optical and scanning electron microscopy and
indirectly through their effect on the various physical
properties.
In the Westerly and Charcoal granites, differen
tial
Expansion of neighboring quartz and feldspar
grains plays the dominant role in thermal crack development
and widening at temperatures below the a to ß
transition of quartz (T ). At higher temperatures,
differential thermal expansions of either neighboring
feldspar grains or feldspar-mafic grains appears to be
the primary cause for continued "loosening" of the
aggregate. Microcracks develop both along grain boun
daries and as intragranular cracks and result largely
from the action of tensile intergranular stresses.
The effects of uniform temperature changes are
similar for both rocks, differing only in detail. As
indicated by acoustic emission, thermal cracking init
iates in both rocks upon exceeding threshold temper
ature(Tc) of 75°C. With increasing temperature, VL,
?L/L, ?V/V, sT, and k change dramatically, whereas sc,
Q and Et are affected significantly only at the higher
temperatures. Tensile strengths do not exhibit a sta
tistically
significant decrease until Tmax> 150°C to 200°C.
At temperatures below T, the decrease of velocity
and tensile strength and the increase of permanent
strain (i.e., crack porosity) closely mirror the differential
thermal strain of quartz and feldspar, which
becomes markedly nonlinear with T > 450°C.
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