Rupture by damage accumulation in rocks

The deformation of rocks is associated with microcracks nucleation and propagation, i.e. damage. The accumulation of damage and its spatial localization lead to the creation of a macroscale discontinuity, so-called "fault" in geological terms, and to the failure of the material, i.e. a dramatic decrease of the mechanical properties as strength and modulus. The damage process can be studied both statically by direct observation of thin sections and dynamically by recording acoustic waves emitted by crack propagation (acoustic emission). Here we first review such observations concerning geological objects over scales ranging from the laboratory sample scale (dm) to seismically active faults (km), including cliffs and rock masses (Dm, hm). These observations reveal complex patterns in both space (fractal properties of damage structures as roughness and gouge), time (clustering, particular trends when the failure approaches) and energy domains (power-law distributions of energy release bursts). We use a numerical model based on progressive damage within an elastic interaction framework which allows us to simulate these observations. This study shows that the failure in rocks can be the result of damage accumulation

Conductivity/activation energy relationships for cement-based materials undergoing cyclic thermal excursions

The electrical conductivity of a range of concrete mixes, with and without supplementary cementitious materials (SCM), is studied through multiple cycles of heating and cooling over the extended temperature range -30/?70 C. When presented in an Arrhenius format, the experimental results display hysteresis effects at the lowtemperature end of the thermal cycle and, in those concretes containing supplementary cementitious materials at higher water/binder ratios, hysteresis effects were evident over the entire temperature range becoming more discernible with increasing number of thermal cycles. The depression in both the freezing and thawing point could be clearly identified and was used to estimate pore-neck and pore-cavity radii. A simplified approach is presented to evaluate the volumetric ratio of frozen pore water in terms of conductivity measurements. The results also show that the conductivity and activation energy of the concrete specimens were related to the water/binder ratio, type of SCM, physical state of the pore water and the thermal cycling regime

A phenomenological explanation of the pressure-area relationship for the indentation of ice: Two size effects in spherical indentation experiments

Indentation tests provide a simple means to study the inelastic behavior of ice and other materials when loaded under a compressive stress state. Such tests provide force–time plots which are often converted to pressure–area (PA) curves. For ice, PA curves are widely used in the design of ships and offshore structures. Despite their usage, and despite many attempts to relate empirical results to theory, the mechanics underlying PA curves is not clearly understood. In this paper, it is shown that by taking into account the strain-softening behavior of ice when rapidly deformed beyond terminal failure within the regime of brittle behavior, two effects can be explained: the decrease in pressure with increasing area, termed the indentation size effect; and, for a given area, the increase in pressure with increasing radius of indenter, termed the indenter radius effect. The analysis is supported using published data on freshwater, polycrystalline ice that have been obtained using spherically shaped indenters. The indentation size effect for ice reflects a similar effect found in ceramics and rock, but is opposite to the effect found in metals where, owing to strain hardening, indentation pressure or hardness increases with increasing area