Formation Of Tensile Cracks Research Articles

Water-tight stratum rupture under large-scale mining. Part II

The mathematical modeling of variations in the stress-strain state of a water-tight stratum under large-scale mining has been carried out in the frame of the calibrated geomechanical model of the said stratum. The authors assessed the mining impact using the movement trough parameters, found the mechanism for the shear and tensile crack formation inside a water-tight-stratum and worked out the criteria to assess its disturbed continuity.

Structure, stresses and stress relaxation of TiN/Cu multilayer and nanocomposite coatings

AbstractStructural evolution, residual stresses, and stress relaxation at elevated temperatures were investigated for sputtered TiN/Cu multilayer films with a bilayer thickness between 100 and 800 nm, as well as nanocomposite films with a Cu content up to 30 at.%, to illuminate the effect of adding a soft metal to a hard nitride. In the nanocomposites the Cu domain size increases with rising Cu content from 7 to 22 nm at the expense of the TiN grain size, while it is strongly related to the bilayer thickness in the multilayers at constant TiN domain size. Films were thermally cycled in vacuum from room temperature up to 650 °C. Stresses during heating are governed by thermo-elastic behaviour and at temperatures above 300 °C by plastic deformation of the Cu phase and stress relaxation of both Cu and TiN, whereas the cooling phase is determined by thermo-elasticity, plastic deformation of the Cu phase and eventually tensile crack formation.

The role of the 1999 Chamoli earthquake in the formation of ground cracks

The moderate magnitude Chamoli earthquake that occurred in the Garhwal Higher Himalaya, in the early hours of March 29, 1999, caused intense damage to the ground and mountain slopes of the Alaknanda–Mandakini river valley and adjoining region. A systematic survey of this induced damage was conducted immediately after the earthquake occurred. Prominent shallow cracks of significant length, negligible width and indeterminate vertical extent, conspicuously tensile in nature, with little or no slip across the crack planes, were observed in the ground at several places along the surveyed route. These cracks had formed in the dynamic phase of the Chamoli earthquake process that is in the period of time during which the earthquake-generated seismic waves were passing through the geographic region of interest. However, we use the theory of earthquake-induced static (or long time) stress changes to visualize such cracks at some selected sites where ground damage was relatively more intense and varied to suggest lower bound estimates of the dynamic stress contributions of the main shock for their formation.Based on the results of our analysis we conclude that, just prior to the earthquake occurrence, under the influence of the local ambient stress field, the ground at these sites was already near failure in tension. To this, in its dynamic phase, the Chamoli earthquake induced stress perturbations, having, across the planes of the cracks, (i) shear components which were nearly equal and opposite to similar components of the ambient stress field and (ii) normal (tensile) components, necessary for triggering tensile failure of the ground. The σ3 (or minimum principal stress) component of the resultant perturbed failure stress field thus became sufficiently tensile while the transverse stresses became sufficiently insignificant. This facilitated formation of major tensile cracks in the ground there. Our static estimates of the tensile stress changes at the different sites are, in essence, estimates of the minimal triggering stress perturbations that was provided by the Chamoli earthquake in the dynamic state for the formation of the tensile cracks there.

Fracturing in saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements: experimental study

Frequency dependent complex electrical conductivity measurements have been made on sandstones saturated with distilled water during triaxial deformation in both drained and undrained regimes. The resulting electrical and mechanical data show how the rock undergoes compaction, followed by dilatancy due to new crack formation, crack growth, interlinkage and failure as axial strain is increased. Electrical data are particularly good at indicating how early the formation of new cracks begins, showing that the quasi-linear portion of the stress–strain curve for triaxial deformation of saturated rocks does not represent truly elastic behaviour, but the combined effects of (i) crack closure perpendicular to the strain axis and (ii) the formation of tensile cracks parallel to the strain axis. A difference in the stress–strain behaviour between the drained and undrained samples was also observed, with the undrained samples developing a pronounced strain-softening phase before failure. The experimental data have also been used to derive the volumetric porosity, electrical porosity, cementation exponent and electrical tortuosity of the pore/crack network during deformation. The relative importance of crack closure and dilatation (a) during the progress of deformation and (b) between crack populations, controls these parameters and the electrical data over a wide range of frequencies. However, the frequency dependence of the micro-structural parameters and the electrical data was found to be not affected significantly by the hydrostatic pressurisation or the triaxial deformation. The development of large scale crack connectivity is observed to be confined to just prior to failure, and is controlled by the loss of cracks perpendicular to the axis of current flow and deformation.

On the orientation and patterns of wing cracks and solution surfaces at the tips of a sliding flaw or fault

Sliding along a preexisting flaw can result in the formation of tensile cracks where stresses concentrate near the flaw tips. These tensile cracks are referred to as wing cracks and are generally oriented oblique to the preexisting flaw. Previous studies based on linear elastic fracture mechanics (LEFM) showed that the kink angle depends on the ratio of normal to shear loading on the flaw. We present analytical solutions for cohesive end zone (CEZ) flaw models and find that the relationship between kink angle and load differs significantly from that for LEFM flaws. Furthermore, the remote flaw‐parallel normal stress may significantly reduce or increase the kink angle, especially for CEZ flaws with large end zones. These results suggest that multiple interpretations are possible for some measured kink angles. In some materials, solution surfaces may form at the tip of the sliding flaw. By considering the angle between wing cracks and solution surfaces it is possible to determine whether the LEFM or CEZ model is more appropriate and thus to provide a better constrained interpretation of the boundary conditions that accompanied sliding. For some CEZ flaws the stress state in the cohesive end zone is nearly homogeneous, possibly promoting formation of arrays of opening mode cracks and solution surfaces that together form a shear zone. The CEZ flaw model can explain some orientations and patterns of cracks and solution surfaces commonly observed along natural faults that cannot be accounted for with the LEFM model.

Damage of saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements: Experimental results

The frequency dependent complex electrical conductivity of brine saturated rocks is extremely sensitive to changes in the volume, connectivity, orientation, and surface topography of pores and cracks. We have made triaxial deformation experiments on sandstone specimens saturated with distilled water. Experiments were carried out for several values of confining pressure, and in both drained and undrained regimes. During the deformation the full complex (in-phase and out-of-phase) electrical parameter set was measured (i.e. conductivity, resistivity, permittivity etc.) for 50 frequencies from 20 Hz to 1 MHz. Only the data at 1 kHz will be discussed here. This data tracks how the rock undergoes crack closure, followed by dilatancy, crack linking, and finally failure, as axial strain is increased. The data indicates well how early the formation of new cracks begins, showing that the quasilinear portion of the stress-strain curve for triaxial deformation of saturated rocks does not represent truly elastic behaviour, but represents the combined effects of crack closure perpendicular to the strain axis and the formation of tensile cracks parallel to the strain axis. The electrical data has also been used to derive an electrical-equivalent change in porosity, and to examine the way that the cementation exponent and the tortuosity of the pore and crack network change during deformation.

Micromechanical properties of amorphous carbon coatings deposited by different deposition techniques

Amorphous carbon coatings about 20 nm thick are commonly used as an overcoat on magnetic thin-film rigid disks and tape and disk head surfaces to improve their wear performance. In this study, we deposited amorphous carbon coatings with thicknesses ranging from 20 to 400 nm on single-crystal silicon substrates by four deposition processes: cathodic arc, ion beam deposition, r.f.-plasma-enhanced chemical vapor deposition, and r.f. sputtering. R.f.-sputtered SiC coatings were also deposited for comparison. The hardness, elastic modulus, and scratch resistance of these coatings were measured by nanoindentation and microscratching using a nanoindenter. The cathodic arc carbon coatings followed by sputtered SiC coatings exhibited the highest hardness, elastic modulus, scratch resistance/adhesion, and residual compressive stresses. The critical load, a measure of the scratch resistance/adhesion of the coating, increases with thickness. The cathodic arc coatings of lower thicknesses (~ 30 nm) exhibited instant damage when the normal load exceeded the critical load, whereas thick coatings (greater than or equal to 100 nm) exhibited gradual damage through the formation of tensile cracks. The sputtered carbon coatings exhibited damage to the coating at very low loads and ploughing of the tip into the coating occurred right from the beginning of the scratch.

HENGILL TRIPLE JUNCTION, SW ICELAND 2. ANOMALOUS EARTHQUAKE FOCAL MECHANISMS AND IMPLICATIONS FOR PROCESS WITHIN THE GEOTHERMAL RESERVOIR AND AT ACCRETIONARY PLATE BOUNDARIES

The Hengill ridge‐ridge‐transform triple junction is a complex tectonic unit containing two volcanic systems, one active and one extinct, and widespread geothermal resources. The area exhibits continuous, small‐magnitude earthquake activity. Good constraint was obtained for the radiation patterns of 178 small earthquakes within the area, of which 50% are non‐double couple and are interpreted as resulting from tensile crack formation. These events occurred in the depth range 2–7 km and indicate that at these depths in the area extensional stresses must be prevalent, that outweigh the compressive effect of the overburden. The tensile crack events were mingled with double couple events, and were confined to the high‐temperature geothermal area. Outside of this, seismic faulting was almost entirely shear. During a 3‐month recording period, tensile crack type events were of low magnitude and accounted for only 2% of the seismic stress release. The fault plane of the shear events was deduced from the orientation of the tensile crack strikes. The continuous small‐magnitude seismicity is attributed to the action of circulating groundwater fluids on hot rock at depth which remove heat and cause contraction of the rock, thereby reducing the effective confining stress. This process induces cooling contraction cracks that release thermal and regional stress in a mode that is consistent with the regional stress regime. In close proximity to the accretionary plate boundary, this stress regime is extensional, and some of the fractures formed are tensile cracks. The spatial distribution of the continuous earthquake activity represents a map of the heat source of the geothermal area. In the Hengill area the earthquake locations indicate that the high‐temperature geothermal area is fueled by two main distinct heat sources. These are associated with the two volcanoes, and separate geothermal fields that exhibit contrasting reservoir characteristics. Continuous small‐magnitude earthquake activity is commonly associated with geothermal areas worldwide, and the results from the Hengill area indicate that they may be used elsewhere as a prospecting tool to gain unique information about the heat source.

Anomalous focal mechanisms: tensile crack formation on an accreting plate boundary

Conventional double-couple solutions do not satisfactorily account for many small magnitude earthquakes in the Hengill geothermal area in the Neovolcanic Zone of Iceland. The far-field radiation pattern is predominantly compressional but a few dilatational arrivals have been recorded. We prefer the interpretation that these events are due to tensile crack formation at a shallow (1–7 km) depth within a cooling intrusive body. Volume considerations indicate that initial fracturing of the rock is seismic but that subsequent widening proceeds aseismically. Other reports of anomalous radiation patterns from earthquakes on the Mid-Atlantic Ridge describe events of a similar type, that is, exhibiting predominantly compressional radiation patterns and a reduced dilatational field1–4.

Fracture energy of stick‐slip events in a large scale biaxial experiment

The concept of apparent fracture energy for the shear failure process is employed by many authors in modeling earthquake sources as dynamically extending shear cracks. Using records of shear strain and relative displacement from stick‐slip events generated along a simulated, prepared fault surface in a large (1.5m × 1.5m × 0.4m) granite block and a slip‐weakening model for the fault, direct estimates of the apparent shear fracture energy of the stick‐slip events have been obtained. For events generated on a finely ground fault surface, apparent fracture energy ranges from 0.06 J/m² at a normal stress of 1.1 MPa to 0.8 J/m² at a normal stress of 4.6 MPa. In contrast to estimates for tensile crack formation, we find that the apparent fracture energy of stick‐slip events increases linearly with normal stress. The results for the slip‐weakening model for the stick‐slip events are generally consistent with constitutive fault models suggested by observations of stable sliding in smaller scale experiments.

Seismic structure and seismicity of the cooling lava lake of Kilauea Iki, Hawaii

The use of multiple methods is indispensable for the determination of the seismic properties of a complex body such as a partially solidified lava lake. The combined results from various active and passive experiments conducted on Kilauea Iki in March 1976 suggest a rather thin (less than 10 m) zone of residual melt, the lateral extent of which is delineated by a sharp drop in the activity of seismic events occurring within the crust of the lake. The heavily fractured crust is characterized by P velocities ranging from 0.8 to 2.2 km/s and the P velocity could be even lower in the melt possibly because of the presence of gas bubbles. The present seismicity of the lake is compatible with the formation of tensile cracks caused by contraction due to cooling.