Semi‐Brittle Deformation of Carrara Marble: Hardening and Twinning Induced Plasticity
Erik Rybacki, L. L. Niu, Brian Evans
Abstract
Abstract Semi‐brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid‐levels of the Earth's crust. Using a Paterson gas‐deformation apparatus, we performed 67 conventional triaxial deformation experiments on dry samples of Carrara marble in the temperature range of T = 20–800°C; confining pressures ( P C ) were 30, 50,100, 200, and 300 MPa, and strain rates ( ) were 10 −3 , 10 −4 , 10 −5 , and 10 −6 s −1 . Axial strains ( ε ) were ≲0.12. The measured (differential) stress, , changes with strain at most applied conditions. At ε = 0.05, both stress and the hardening coefficient ( h), that is, the rate of increase of stress with strain , increase as T decreases and P C increases. At T ≲ 400°C, h is quite large and the sensitivity of on is low, while both are sensitive to increasing pressure. In this temperature range, the mechanical behavior of the marble is very similar to that exhibited by high‐strength, high‐ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Twinning and dislocation motion are abundant in the samples, as are inter‐ and intracrystalline microfractures. The concurrent activation of these deformation mechanisms leads to complex relationships of and h with the applied T ‐ P C ‐ conditions. This behavior suggests that peak stresses for calcite rocks deforming by semi‐brittle processes will occur at P C ‐ T conditions of the middle crust, and that they might be more strongly influenced by total strain rather than by strain rate.