International Journal of Mechanical Sciences 179 (2020) 105654 Contents lists available at ScienceDirect International Journal of Mechanical Sciences journal homepage: www.elsevier.com/locate/ijmecsci Experimental investigation and micromechanical modeling of the brittle-ductile transition behaviors in low-porosity sandstone Si-Li Liu a,b , Huan-Ran Chen c , Shuang-Shuang Yuan a,b , Qi-Zhi Zhu a,b,∗ a Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China b College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, China c College of Civil Engineering, Nanjing Institute of Technology, Nanjing 211167, China a r t i c l e i n f o Keywords: Brittle-ductile transition Low-porosity sandstone Micromechanical model Microcracking Plasticity-damage coupling a b s t r a c t This paper presents a unified experimental and numerical investigation on mechanical behaviors of a sandstone with brittle-ductile transition. A series of triaxial compression tests are performed at room temperature under a wide range of confining pressure, from which some critical states of damage are originally identified. It is shown that shear-induced dilation occurs in brittle faulting while macroscopic dilatant cataclastic flow takes place in the rock under elevated pressures without occurrence of shear bands by strain localization. There is a clear trend for peak strength to increase nonlinearly with confining pressure and that microcracking-related local friction is the dominant mechanism of inelastic deformation. In order to describe the nonlinear mechanical behaviors of the sandstone, a micromechanics-based isotropic plasticity-damage coupling model is formulated, in which two critical states of damage at rock failure are taken into account to establish the relations between the model’s parameters and experimental data. Theoretical predictions of the strength envelope and nonlinear mechanical responses of the sandstone show a quantitative agreement with the test results including the main features of stress-strain curves with different confining pressures. 1. Introduction The brittle-ductile transition in rock mechanical responses is an im- portant topic in many geological applications, including the tectonic deformation [2,39], the coupling between strain localization and fluid flow [45,51,52], reservoir compaction and subsidence [30], borehole instability and well failure [15], seismic activity [1], etc.. In view of the limitation of in-situ observations, laboratory investigations under spe- cific loading conditions are important means to provide complementary insights and deep understanding of the mechanisms behind the brittle- ductile transition phenomena. In literatures, a large number of labora- tory tests on various rocks have been reported. Limestone and marble have often been worked on as they can undergo the brittle-ductile tran- sition at room temperature under confining pressure easily attained at laboratory [3,31,34]. The transition in failure mode from brittle faulting to ductile flow has been reported for a relatively porous silicate rock at room temperature just by increasing confining pressure [27,36]. Byerlee [7] observed that Serpentine-bearing dunite and gabbro can also expe- rience a transition from brittle to cataclastic flow at room temperature with the increase of pressure. Existing studies demonstrated that the brittle-ductile transition be- havior is closely related to rock porosity and its loading path. [43] stated ∗ Corresponding author at: College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, China. E-mail address: qzhu@hhu.edu.cn (Q.-Z. Zhu). that the pressure at brittle-ductile transition decreases with the in- crease in initial porosity and grain size. Limestone and chalk with ini- tial porosities between 3% and 45% were observed to undergo com- pactive cataclastic flow [3,40]. For low-porosity rocks, dilatant type of cataclastic flow was recorded, while for high-porosity rocks, com- pactive cataclastic flow was generally reported [43,51,52]. Fredrich et al. [16] observed significant dilatant behavior in Carrara marble (with initial porosity of 0.7%) that undergoes cataclastic flow. Brace [6] sug- gested that rocks could be considered ”porous” when the porosity ex- ceeds 5%, in which case the cataclastic flow would be compactant. However, Baud et al. [3] found that Solnhofen limestone with an ini- tial porosity as low as 3% can still experience compactant cataclastic flow. Earlier experimental studies on the brittle-ductile transition behav- iors argued that the major deformation mechanisms were primarily cat- aclastic flow [7,19,27,31]. Zhang et al. [44] found that the operative micromechanical process in relatively compact sandstones was brittle grain crushing. The micromechanisms of both brittle faulting and cat- aclastic flow associated with dilatancy in low-porosity rocks were at- tributed to pervasive microfracturing [28,45,53]. It was found from the microstructural observations and acoustic emission (AE) measure- ments that the micromechanical processes of ductile behavior involves https://doi.org/10.1016/j.ijmecsci.2020.105654 Received 16 November 2019; Received in revised form 28 February 2020; Accepted 27 March 2020 Available online 4 April 2020 0020-7403/© 2020 Elsevier Ltd. All rights reserved.