Steady-State Flow in Marble at 500° to 800°C
The stress-strain behavior of Yule marble, both parallel and normal to the foliation, has been determined in a series of constant strain-rate extension tests at 5000 bars confining pressure, at temperatures of 500° to 800° C and at strain rates ranging from 10−3 to 10−7/sec. These data correlate well with earlier results at 25° to 500° C and l0−1 to 10−8/sec. Steady-state flow is dominant at 400° C and rates <10−7/sec, at 500° C < 10−4/sec, and at all strain rates at higher temperatures. Transient flow (work hardening) is prevalent at lower temperatures and higher rates.

Transmission and reflection microscopy as well as etch-pit studies reveal that translation gliding on r, f and twinning on e are dominant in the transient region, becoming less prominent in the steady-state region at high temperatures. Polygonization by dislocation climb becomes characteristic in the steady-state flow region, along with intergranular and intra-granular recrystallization. Grain boundary sliding is present, but of minor importance.

The effect of temperature and strain rate on the mechanical behavior, as well as on the correlation of subgrain formation and recrystallization with steady-state flow, is consistent with a diffusion-controlled process. Comparison of the data with several models for diffusion-controlled flow show poor correlation with the Eyring equation, e = A exp ( −E/RT) sinh (Bσ), but excellent correlation with the Weertman equation, e = C exp ( −E/RT) <σN.

The activation energy E and the stress exponent N are nearly identical for both orientations of this highly anisotropic marble, and thus this material becomes nearly isotropic (mechanically) for steady-state flow. On the basis of the empirical-flow equation, extrapolations to representative natural strain rates of 3 × 10−14/sec predict stresses of 103 bars at 250° C, ranging to about 30 bars at 700° C. Work-hardening can be expected at these rates at temperatures <250° C; in this region, this marble is expected to be mechanically anisotropic. Calculated equivalent viscosities at natural strain rates range from 3 × 1022 poises at 25° to 4 × 1020 poises at 700° C.