1 Subcritical crack growth and stress corrosion cracking

Progressive deformation of rocks, e.g. by creep (Rutter 1972; Itô & Sasajima 1980; Brantut et al. 2012) and fatigue (Attewell & Farmer 1973; Costin & Holcomb 1981; Ko & Kemeny 2013), at low magnitude near constant stresses is facilitated by different subcritical mechanisms causing gradual irreversible damage, commonly termed subcritical crack growth (Atkinson 1984; Atkinson & Meredith 1987). Subcritical crack growth is an effective means of facilitating time-dependent material degradation (Atkinson 1984; Brantut et al. 2013; Nara et al. 2013). These mechanisms include plastic (Atkinson & Meredith 1987), as well as brittle deformations (Atkinson & Meredith 1987; Nara et al. 2013). These mechanisms are enhanced in the presence of chemically active fluids, such as water (Atkinson & Meredith 1981; Lawn 1993).

An interaction of chemical and mechanical processes, as postulated in stress corrosion cracking (SCC), reduces brittle fracture toughness, and therefore increases subcritical fracture propagation velocities. SCC involves a range of range processes, including stress enhanced dissolution (as described by the Charle's law), hydrolysis and embrittlement, and a reduction of surface energies which in turn reduces cohesive forces and promotes granular disintegration. To better understand the coupling of chemical and mechanical processes in stress corrosion cracking and subcritical crack growth, we set up single edge notch bending (SENB) creep tests, mimicking natural conditions.

2 Experimental methods

Six Alta-Quartzite samples (AQ 1–6, 300x30x70 mm) were brought to failure in stepped SENB creep tests. The load was increased over months in steps of 5–10 % of the intact wet flexural strength determined in preliminary tests, starting at 10% for samples AQ1–3 and at 50 % for samples AQ4–6 (Tab. 1). Samples were pre-cracked to ~ 50 % (F= 4000 N) of the intact strength in a standard SENB testing frame with 10 N/s. Straining of the sample in the long-term tests was measured using electrical resistivity strain gages 2 mm below the notch, and four samples were supplied with distilled water in the notch (AQ 1–2, 4–5). To assess the influence of dissolution in the wet samples, this water was dripped in and captured for analysis of silica content by Atomic Absorption Spectrometry (AAS). Scanning Electron Microscopy images were taken of failed dry and wet samples to allow a surface texture analysis and fractography of fracture planes, to better define and discriminate chemo-mechanical mechanisms of stress corrosion and subcritical crack growth.

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