Brittle creep and subcritical crack propagation in glass submitted to triaxial conditions

Abstract

An experimental work is presented that aimed at improving our understanding of the mechanical evolution of cracks under brittle creep conditions. Brittle creep may be an important slow deformation process in the Earth's crust. Synthetic glass samples have been used to observe and document brittle creep due to slow crack‐propagation. A crack density of 0.05 was introduced in intact synthetic glass samples by thermal shock. Creep tests were performed at constant confining pressure (15 MPa) for water saturated conditions. Data were obtained by maintaining the differential‐stress constant in steps of 24 h duration. A set of sensors allowed us to record strains and acoustic emissions during creep. The effect of temperature on creep was investigated from ambient temperature to 70°C. The activation energy for crack growth was found to be 32 kJ/mol. In secondary creep, a large dilatancy was observed that did not occur in constant strain rate tests. This is correlated to acoustic emission activity associated with crack growth. As a consequence, slow crack growth has been evidenced in glass. Beyond secondary creep, failure in tertiary creep was found to be a progressive process. The data are interpreted through a previously developed micromechanical damage model that describes crack propagation. This model allows one to predict the secondary brittle creep phase and also to give an analytical expression for the time to rupture. Comparison between glass and crystalline rock indicates that the brittle creep behavior is probably controlled by the same process even if stress sensitivity for glass is lower than for rocks.

Experimental study of Young’s modulus dispersion and attenuation in fully saturated sandstones

“…Three standard samples were chosen to test the experimental setup and procedure: (1) a synthetic glass sample made of amorphous silica (Mallet et al, 2013(Mallet et al, , 2015, (2) a pure gypsum sample (Brantut et al, 2012), and (3) a Plexiglas (PMMA) sample (e.g., Batzle et al, 2006). These samples were chosen because (1) they are homogeneous and isotropic media at the sample scale, (2) their static and dynamic elastic properties are known, and they show a large range in elastic moduli, (3) these samples have no porosity, and their elastic properties are not expected to change with confining pressure or added axial stress, and (4) although glass and gypsum elastic properties are independent of frequency, Plexiglas is a viscoelastic material whose elastic properties are frequency dependent.…”

Brittle and semibrittle creep of Tavel limestone deformed at room temperature

“…As in other materials, cracks in calcite can propagate subcritically in mode I [ Henry et al , ; Dunning et al , ; Røyne et al , ; Rostom et al , ], which leads to possible time dependence of brittle failure [ Johnson and Paris , ; Scholz , ; Kranz , ; Atkinson , ; Heap et al , ; Mallet et al , ] reviewed in Brantut et al []. Thus, limestones deformed under constant stress conditions can undergo brittle creep, as recently shown by Brantut et al [].…”

“…Ashby and Sammis [], Deshpande and Evans [], Bhat et al [], Brantut et al [], and Mallet et al [] among others showed that brittle creep can be modeled adequately by the propagation of wing cracks from initial monosized flaws, a geometry in good agreement with direct observations on limestone by Olsson []. Since subcritical crack growth is thermally activated, the growth of crack length ( l ) can be described using the law proposed by Darot and Guéguen [] and Mallet et al []: where is a characteristic crack speed dependent on the interatomic distance and atomic vibration frequency, E a is an activation energy, s is an elementary surface, k is Boltzmann's constant, T is temperature, K I is the stress intensity factor, and γ is the surface energy. Mallet et al [] showed that this crack growth law leads to an exponential relation between differential stress and strain rates, as described by equation .…”

“…In the latter case, failure occurs by static fatigue because subcritical crack growth (see review by Atkinson [1984]) leads to a localization of damage (see review by Brantut et al [2013]). Subcritical crack growth was observed in brittle media such as glasses [Orowan, 1944;Charles, 1958;Wiederhorn and Bolz, 1970;Swanson, 1984;Mallet et al, 2014Mallet et al, , 2015a, sandstones [Holder et al, 2001;Heap et al, 2009a], granites [Kranz, 1979;Swanson, 1984;Lockner, 1993;Wang et al, 2015a], shales [Swanson, 1984], basalts [Swanson, 1984;Heap et al, 2011], gabbros [Meredith and Atkinson, 1985], or limestones [Rutter, 1972;Eslami et al, 2012;Brantut et al, 2013Brantut et al, , 2014, among other rock types [Atkinson and Meredith, 1987], and modeled micromechanically [e.g., Amitrano and Helmstetter, 2006;Brantut et al, 2012;Mallet et al, 2015a;Wang et al, 2015b].…”

Investigation of Macroscopic Brittle Creep Failure Caused by Microcrack Growth Under Step Loading and Unloading in Rocks

“…1 (Ashby and Sammis 1990). This classical model is widely applied to study the micromechanical properties of brittle material (Deshpande and Evans 2008;Bhat et al 2011;Brantut et al 2012;Mallet et al 2015). The rock is assumed to be an isotropic elastic body that contains initial penny-shaped microcracks with radius a, and the angle between each initial microcrack and maximum principal stress r 1 is assumed to be u.…”