Elastic moduli evolution and accompanying stress changes with increasing crack damage: implications for stress changes around fault zones and volcanoes during deformation

Abstract

S U M M A R YThe elastic moduli of rock in areas susceptible to crack damage, such as within fault zones or volcanic edifices, can be subject to large modifications. Knowledge of how elastic moduli may vary in such situations is important for both the reliable modelling of volcano deformation and stability and for linear and non-linear elastic crack models for earthquake rupture. Furthermore, it has previously been shown that changes in elastic moduli can induce changes in the stress field surrounding faults. Here we report both uniaxial experimental measurements of changes in elastic moduli during increasing-amplitude cyclic stressing experiments on a range of different rock types (basalts, sandstones and granite) and the results of modelled stress modifications. The trend in elastic moduli evolution with increasing damage was remarkably similar for each rock type, with the exception of essentially crack-free intrusive basalt that exhibited very minor changes. In general, Young's modulus decreased by between 11 and 32 per cent and Poisson's ratio increased by between 72 and 600 per cent over the total sequence of loading cycles. These changes are attributed to an increasing level of anisotropic crack damage within the samples. Our results also show that acoustic emission (AE) output during any loading cycle only commenced when new crack damage was generated. This corresponded to the level of stress where AE ceased during the unloading portion of the previous cycle. Using the multilayer elastic model of Faulkner et al. we demonstrate that the damage-induced changes in elastic moduli also result in significant decreases in differential stress, increases in mean stress and rotation of the applied greatest principal stress relative to the orientation of the mechanical layering. The similar trend in the evolution of the elastic moduli of all the rocks tested suggests that stress modification in the damage zone of faults might take the same form, regardless of the lithology through which the fault runs. These observations are discussed in terms of their applicability to both fault zones and deformation at volcanoes.

Cyclic and Fatigue Behaviour of Rock Materials: Review, Interpretation and Research Perspectives

“…Strain accumulates slowly with the number of cycles before peak (Heap et al 2010;Schaefer et al 2015) but in a much lower amount than previously described for constant amplitude cycles. However, cycles show a hysteresis, increasing as the maximum stress applied rises up (Martin and Chandler 1994;Heap et al 2010;Yang et al 2015). Poisson's ratio Young modulus Young modulus [GPa] Poisson's ratio [-] Cycle number [-] Fig.…”

“…The increasing stress from one cycle to the other approaches a monotonic loading, although some differences are noted, especially a slight decrease in strength (Yang et al 2015). Figures 5, 6 and 7 from Heap et al (2009Heap et al ( , 2010 are representative of many damage-controlled tests, and the main observations are summarised below.…”

“…The number of cycles applied is mostly low and varies between 5 and 50 (Eberhardt et al 1999;Heap et al 2010;Trippetta et al 2013;Schaefer et al 2015;Yang et al 2015). The increasing stress from one cycle to the other approaches a monotonic loading, although some differences are noted, especially a slight decrease in strength (Yang et al 2015).…”

“…In these studies, such kind of low-cycle test is used to correlate damage with variations in cohesion and mobilised friction. Some other authors investigate the evolution of the Young modulus and Poisson's ratio as representative of damage (Heap et al 2010;Trippetta et al 2013;Schaefer et al 2015;Yang et al 2015) or assess the stress history or a rock sample (Lavrov 2001). Finally, ramp signals are defined by a constant amplitude but an increasing mean stress, as represented in Fig.…”

“…Poisson's ratio Young modulus Young modulus [GPa] Poisson's ratio [-] Cycle number [-] Fig. 6 Evolution of static elastic modulus and Poisson's ratio with increasing cycle number for damage-controlled test of Etna Basalt, from Heap et al (2009Heap et al ( , 2010 …”