A model of hydrolytic weakening in quartz

Abstract

Hobbs et al. (1972) suggest that the Haasen theory for diamond structure crystals may explain the observed behavior of hydrolytically weakened single crystals of quartz. There is no provision for the effect of varying OH content in the theory, of course, nor does it explain the great work hardening observed in quartz at low temperature or high strain rate. This paper is concerned with extensions to the Haasen theory required to account for the varying shapes of the observed stress-strain curves, the effect of varying water content, and the behavior observed in stress relaxation experiments. The rate-controlled process in hydrolytically facilitated slip is believed to be dislocation growth, which requires either ‘radial’ diffusion of HOH to the newly growing segment or ‘core’ diffusion, in which HOH diffuses parallel to the dislocation line to permit creation of a new segment. Radial diffusion is limited by exhaustion of free HOH as the dislocation density increases. Dislocation velocity proportional to HOH concentration is assumed, as suggested by Blacic. A Haasen-type model incorporating these processes plus recovery fits the observations of Hobbs et al., Balderman and Blacic, and Griggs on crystals of the O+ orientation. The Hobbs et al. observations on ⊥r crystals of varying water content are in conflict with this model. The model predicts the existence of a ‘fundamental strength’ in a temperature-strain rate regime that is easily accessible for experimental verification or disproof.