Similarity and scaling in creep and load relaxation of single‐crystal halite (NaCl)

Abstract

[1] This work explores the physical basis for Hart's mechanical equation of state in hightemperature plasticity. The experiments seek to identify a possible microstructural basis for the ''hardness'' parameters associated with load relaxation curves. The experiments also seek to examine the microstructural basis for scaling in load relaxation data and to explore the relationship between creep and load relaxation. Constant stress creep and load relaxation tests were conducted on [100] oriented single crystals of halite at 700°C and stresses between 0.6 and 3 MPa. Load relaxation tests were performed at 400°C up to a stress level of 13 MPa. After testing, specimens were sectioned, and dislocation densities and subgrain size distributions were measured. Results at 700°C reveal that distributions of subgrain size in crystals crept at different stress levels are similar to each other; that is, they have the same shape but different average subgrain sizes depending on stress level. Hardness curves obtained from load relaxation experiments at different levels of work hardening were found to correspond to different average subgrain size. Load relaxation data from 700°C and 400°C belong to a single-parameter family of curves, with hardness curves translating onto each other with a scaling slope m = 0.33 ± 0.05. Subgrain size distributions generated in creep are statistically identical to those from load relaxation. The hardness parameter, s*, specified as the (apparent) high-strain rate limit of stress in the load relaxation data, is approximately 50Gb/D I , where G is the shear modulus, b is the Burgers vector, and D I is the mean intercept subgrain diameter. During creep under constant stress the subgrain size evolves until a steady value is approached. The experimental data lend credence to Hart's interpretation that load relaxation data represent (nearly) constant ''structure'' with subgrain size playing the role of the structural variable.

The constant‐hardness creep compliance of polycrystalline ice

“…Fits of the lambda law to the data were determined by evaluating ln( σ ) versus via least squares linear regression, from which and σ * were determined from the slope and intercept. In each regression plotted in Figure , λ was held constant at 0.15 (the theoretical value [ Hart , ; Stone et al ., ]), and all data for a steady‐state‐stress group were included. The data are well fit by the lambda law at relatively high stresses, but at low stresses they exhibit higher strain rates than predicted by the lambda law.…”

The microstructural and rheological evolution of shear zones

“…These internal state variables need to be included in the appropriate flow laws, along with the relevant material properties and external thermodynamic conditions (Covey-Crump 1994Stouffer & Dame 1996;Stone et al 2004;Evans 2005).…”

“…Internal state variables may be described using either implicit or explicit state variables (Covey-Crump 1994, 1997aStouffer & Dame 1996;Stone et al 2004;Evans 2005). Implicit internal state variables describe, empirically, some macroscopic aspect of a rock's structure.…”

“…Implicit internal state variables describe, empirically, some macroscopic aspect of a rock's structure. For example, the hardness parameter (Hart 1976) has been used to describe the mechanical evolution of calcite (Covey-Crump 1994, 1997a and halite (Stone et al 2004). Stone et al (2004) further argued that the hardness parameter depends on the mean subgrain size.…”

“…The most commonly investigated explicit state variables include: subgrain structure (e.g. Poirier & Nicolas 1975;White 1979;Karato et al 1980;Stone et al 2004); dislocation density/structure (e.g. White 1973;Goetze & Kohlstedt 1977;de Bresser 1996); CPO (e.g.…”