Sudden Stress Release Research Articles

Experimental investigation of tunnel damage and spalling in brittle rock using a true-triaxial cell

Deep underground excavations in brittle rocks are subject to several ground stability hazards such as spalling and rockburst. These hazards are typically associated with brittle failure mechanisms for hard and massive rock mass. In this study, an experimental investigation has been carried out to evaluate the mechanisms underlying these induced hazards in deep underground excavations. The main objective of this study is to investigate the behavior of a freshly excavated circular unsupported tunnel in a brittle synthetic rock with a focus on induced damage and spalling response. The experiment used a miniature tunnel boring machine (TBM) to excavate a tunnel in a cubical specimen placed in a true-triaxial cell. The material selected for this experiment was a reproducible synthetic rock analogous to sandstone with brittle characteristics. In the experiment, the specimen was loaded with increasing confining stress under incremental isotropic conditions in the true-triaxial cell until the tunnel failed. Macro-photography was utilized to verify the excavation damage zones and failure mechanisms around the tunnel boundary. The main observed failure mechanism of the model tunnel was spalling, which occurred due to brittle failure and a sudden stress release at the tunnel excavation boundary. In addition to spalling, three zones of damage were identified by macro-photography in the tunnel due to damage from construction, fracturing, and plastic rock deformation. The outcomes point to a unique and in-depth comprehension of how damage and spalling failure during underground excavation develop and its impact on tunnel stability.

Phase Transition‐Promoted Rapid Photomechanical Motions of Single Crystals of a Triene Coordination Polymer

AbstractMolecular crystals with the ability to transform light energy into macroscopic mechanical motions are a promising class of materials with potential applications in actuating and photonic devices. In regard to such materials, coordination polymers that exhibit dynamic photomechanical motion, associated with a phase transition, are unknown. Herein, we report an intriguing photoactive, one‐dimensional ZnII coordination polymer, 1, derived from 1,3,5‐tri‐4‐pyridyl‐1,2‐ethenylbenzene and 3,5‐difluorobenzoate. Single crystals of 1 under UV light irradiation exhibit rapid shrinking and bending, violent bursting‐jumping, splitting, and cracking behavior. Single‐crystal X‐ray diffraction analysis and 1H NMR spectroscopy reveal an unusual photoinduced phase transition involving a single‐crystal‐to‐single‐crystal [2+2] cycloaddition reaction that results in photomechanical responses. Interestingly, crystals of 1, which are triclinic with space group , are transformed into a higher symmetry, monoclinic cell with space group C2/c. This process represents a rare example of symmetry enhancement upon photoirradiation. The photomechanical activity is likely due to the sudden release of stress associated with strained molecular geometries and significant solid‐state molecular movement arising from cleavage and formation of chemical bonds. A composite membrane fabricated from 1 and polyvinyl alcohol (PVA) also displays interesting photomechanical behavior under UV light illumination, indicating the material's potential as a photoactuator.

Phase Transition-Promoted Rapid Photomechanical Motions of Single Crystals of a Triene Coordination Polymer.

Molecular crystals with the ability to transform light energy into macroscopic mechanical motions are a promising class of materials with potential applications in actuating and photonic devices. In regard to such materials, coordination polymers that exhibit dynamic photomechanical motion, associated with a phase transition, are unknown. Herein, we report an intriguing photoactive, one-dimensional ZnII coordination polymer, 1, derived from 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene and 3,5-difluorobenzoate. Single crystals of 1 under UV light irradiation exhibit rapid shrinking and bending, violent bursting-jumping, splitting, and cracking behavior. Single-crystal X-ray diffraction analysis and 1 H NMR spectroscopy reveal an unusual photoinduced phase transition involving a single-crystal-to-single-crystal [2+2] cycloaddition reaction that results in photomechanical responses. Interestingly, crystals of 1, which are triclinic with space group , are transformed into a higher symmetry, monoclinic cell with space group C2/c. This process represents a rare example of symmetry enhancement upon photoirradiation. The photomechanical activity is likely due to the sudden release of stress associated with strained molecular geometries and significant solid-state molecular movement arising from cleavage and formation of chemical bonds. A composite membrane fabricated from 1 and polyvinyl alcohol (PVA) also displays interesting photomechanical behavior under UV light illumination, indicating the material's potential as a photoactuator.

Photomechanical Motions in Organoboron-Based Phosphorescent Molecular Crystals Driven by a Crystal-State [2 + 2] Cycloaddition Reaction.

Photoluminescent molecular crystals integrated with the ability to transform light energy into macroscopic mechanical motions are a promising choice of materials for both actuating and photonic devices. However, such dynamic photomechanical effects, based on molecular organoboron compounds as well as phosphorescent crystalline materials, are not yet known. Here we present an intriguing example of photomechanical molecular single crystals of a newly synthesized organoboron containing Lewis acid-base molecular adduct (BN1, substituted triphenylboroxine and 1,2-di(4-pyridyl)ethylene) having a capsule shape molecular geometry. The single crystals of BN1 under UV light exhibit controllable rapid bending-shape recovery, delamination, violent splitting-jumping, and expanding features. The detailed structural investigation by single-crystal X-ray diffraction and 1H NMR spectroscopy reveals that the photosalient behavior of the BN1 single crystals is driven by a crystal-to-crystal [2 + 2] cycloaddition reaction, supported by four donor-acceptor type B←N bonds. The instant photomechanical reaction in the BN1 crystals occurs under UV on account of sudden release of stress associated with the strained molecular geometry, significant solid-state molecular movements (supramolecular change), and cleavage of half intermolecular B←N linkages to result in a complete photodimerized single-crystalline product via the existence of two other intermediate photoproducts. In addition, the BN1 crystals display short-lived room temperature phosphorescence, and the photodynamic events are accompanied by the enhancement of their phosphorescence intensity to yield the photoproduct. Interestingly, the molecular crystals of the final photoproduct polymerize at ambient conditions when recrystallized from the solution forming a 2D supramolecular crystalline polymer stabilized by the retention of all B←N coordination modes.

Abrupt elastic-to-plastic transition in pentagonal nanowires under bending.

In this study, pentagonal Ag and Au nanowires (NWs) were bent in cantilever beam configuration inside a scanning electron microscope. We demonstrated an unusual, abrupt elastic-to-plastic transition, observed as a sudden change of the NW profile from smooth arc-shaped to angled knee-like during the bending in the narrow range of bending angles. In contrast to the behavior of NWs in the tensile and three-point bending tests, where extensive elastic deformation was followed by brittle fracture, in our case, after the abrupt plastic event, the NW was still far from fracture and enabled further bending without breaking. A possible explanation is that the five-fold twinned structure prevents propagation of critical defects, leading to dislocation pile up that may lead to sudden stress release, which is observed as an abrupt plastic event. Moreover, we found that if the NWs are coated with alumina, the abrupt plastic event is not observed and the NWs can withstand severe deformation in the elastic regime without fracture. The coating may possibly prevent formation of dislocations. Mechanical durability under high and inhomogeneous strain fields is an important aspect of exploiting Ag and Au NWs in applications like waveguiding or conductive networks in flexible polymer composite materials.

Two complementary stress release processes based on departures from Omori’s law

Three aftershock series in Southern California, associated with the mainshocks of Landers (1992), Northridge (1994) and Hector Mine (1999) are interpreted as the superposition of a long stress relaxation process, in agreement with the Omori’s law, and a number of short episodes with sudden stress release. These short episodes are detected as departures from the expected Omori’s law. Aftershocks following a long stress relaxation process are designed as leading aftershocks (LA) and their generation rate fits well to the modified Omori’s law (MOL). The rest of aftershocks correspond to the different episodes of sudden stress release, every one designed as a cascade (CA). Cascades are found to be characterized by four basic properties. First, although the number of aftershocks NC belonging to every cascade fluctuates greatly along the stress relaxation process, a clear positive trend of NC with the elapsed time since the mainshock is observed in the initial phase of the aftershock sequence. After a critical elapsed time, very short in comparison with the aftershock sequence length, this positive trend diminishes significantly or even becomes negative. Second, the aftershock generation rate, GR, for every cascade, is almost constant. Third, GR for the successive CAs, represented as a function of its starting time measured since the mainshock, decreases according to a power-law. Four, the validity of the Gutenberg-Richter law is preserved for the sequence of all aftershocks belonging to cascades, with values of the b-parameter quite similar to those deduced for the complete aftershock series. Additionally, some statistical and fractal (self-affine) properties of CAs are analyzed. Given that the number of aftershocks with minimum magnitudes assuring catalogue completeness ranges, for the three seismic crises, from 6,000 up to 20,000, properties concerning LA and CA can be established with a high degree of confidence. As a conclusion, a single stress relaxation process following the Omori’s law should be discarded. A physical explanation, based on the complex spatial patterns of the stress field and tectonic fractures, as well as on the two proposed relaxation processes, is qualitatively discussed.

Three-Dimensional Simulation of Balloon Dynamics by the Immersed Boundary Method Coupled to the Multiple-Relaxation-Time Lattice Boltzmann Method

AbstractThe immersed boundary method (IBM) has been popular in simulating fluid structure interaction (FSI) problems involving flexible structures, and the recent introduction of the lattice Boltzmann method (LBM) into the IBM makes the method more versatile. In order to test the coupling characteristics of the IBM with the multiple-relaxation-time LBM (MRT-LBM), the three-dimensional (3D) balloon dynamics, including inflation, release and breach processes, are simulated. In this paper, some key issues in the coupling scheme, including the discretization of 3D boundary surfaces, the calculation of boundary force density, and the introduction of external force into the LBM, are described. The good volume conservation and pressure retention properties are verified by two 3D cases. Finally, the three FSI processes of a 3D balloon dynamics are simulated. The large boundary deformation and oscillation, obvious elastic wave propagation, sudden stress release at free edge, and recoil phenomena are all observed. It is evident that the coupling scheme of the IBM and MRT-LBM can handle complicated 3D FSI problems involving large deformation and large pressure gradients with very good accuracy and stability.

Aftershock sequences of three seismic crises at southern California, USA, simulated by a cellular automata model based on self-organized criticality

Several properties of aftershock series related to the main shocks of Landers, Northridge and Hector Mine (southern California, USA) are reproduced by the Dynamic Fiber Bundle Model, DFBM. Optimum values for the three parameters governing DFBM are determined by searching for the best agreement of real aftershock series properties and those of synthetic sequences generated by this model. The analysis of the model parameter values provides details on the underlying physical mechanism of the aftershock sequence generation. First, the ratio of seismic energy radiated as seismic waves and transferred as stress-strain to adjacent faults; second, the degree of stress heterogeneity reproducing the complex behavior of real aftershock series. Additionally, the results of simulations support the coexistence of two types of relaxation processes. One of them is associated with the well-known modified Omori’s (MO) law, which involves elapsed times between consecutive aftershocks monotonically increasing; the other is manifested by episodes of sudden stress release, with inter-event times much shorter than those predicted by MO law. These episodes are assumed to be a consequence of the complex distribution of tectonic stresses and fault geometry. The first process is associated to events designed as leading aftershocks, LA. The second process generates series of events which are designed as cascades, CA. It is worth of mention that several properties concerning CAs can be reasonably related to critical changes on stress field along the simulation process.

Teaching Geophysics with a Vertical-Component Seismometer

Earthquakes are some of the more dramatic expressions of the dynamics of our planet. The sudden release of stress built up slowly by tectonic or volcanic processes often has far-reaching consequences, and can be measured (in classrooms) around the world. This is one reason why designing and building seismometers has been a popular activity,1, 2 and why different versions of “Seismometer in Schools” projects thrive in the United States, Australia, and Europe. We present a cheap, robust, and easy-to-build seismometer—called the TC1 —to measure seismic displacements in the vertical direction. Its components are easy to obtain and assemble, yet the resulting instrument is accurate enough to record earthquakes from around the globe. The parts list and building instructions of the TC1 seismometer are freely available online. Alternatively, a complete kit can be purchased for around US$300. Assembling the system naturally introduces students to a number of concepts in physics and engineering, while upon completion seismic recordings trigger discussions about the dynamics and internal structure of the Earth. The discussions are fostered by service learning and shared in the network of TC1s called the Z-NET.

Microseismicity: Beyond dots in a box — Introduction

This special section of Geophysics is a consequence of a very successful SEG workshop titled Microseismicity: Beyond dots in a box. The workshop was held 22 October 2010, following the SEG Annual Meeting that was held in Denver. More than 120 people attended the meeting, and nine oral papers and 22 posters were presented. Small microseismic events, or acoustic emissions, occur naturally and a result of anthropogenic influences in reservoirs. Sudden stress release leads to elastic rock failure, which serves as an effective seismic source. These microearthquakes may be the result of production or hydraulic stimulation, but they also may be a consequence of natural tectonic activity. They are usually detectable using only sensitive sensors and after careful data processing. Such passive seismic monitoring has been used in mining settings for more than 100 years, but its application in petroleum setting is relatively new. As such, it is a rapidly advancing field of technology, where the challenges are manifold and span issues associated with data acquisition, processing, and interpretation. Much can be learned from methods and case studies developed for microseismic monitoring in volcanological, geothermal, and mining settings, but one of the key advantages in oilfield monitoring is that a great deal is ordinarily already known about the reservoir. Comparatively good velocity models exist, and production or injection information is available — the same is obviously not true for a volcano. The microearthquake or microseismic event location is useful in itself. Clusters of events delineate faults and fracturing, highlighting reactivation or the generation of new zones of failure. This has been very effective in helping assess the efficacy of hydraulic stimulation (i.e., frac monitoring). Longer term monitoring will no doubt provide an early warning system for detecting top seal leakage and fault reactivation in CO 2 sequestration projects, for example. Until …

Exploring the fundamental effects of deposition time on the microstructure of graphene nanoflakes by Raman scattering and X-ray diffraction

A systematic study is reported of the growth of vertically aligned few layered graphene (FLG) nanoflakes on Si (100) substrates by microwave plasma enhanced chemical vapour deposition (MPECVD) method. Asymmetric grazing incident angle X-ray diffraction (GIAXRD) studies revealed a structural transformation, from nanocrystalline graphite layers to FLG, with the increase of growth time. As the growth time increased we observed a preferred vertical orientation of FLGs accompanied by a sharp decrease in the d002 spacing. Transmission electron microscopy shows these structures have highly graphitized edge planes which terminate in a few layers (1–3) of graphene sheets. Detailed Raman studies not only support the structural transformation but also confirm that the process occurs via the sudden release of stress in nanocrystalline turbostratic graphite films. Graphical plot of all major Raman parameters (such as G peak position, ID/IG value, FWHM of D, G, and G′ peaks) vs.growth time shows a well defined trend. Using the graphical plots a tentative trajectory of the Raman parameters is proposed, which can be very useful in understanding structural transformation during growth process. Finally, a possible growth mechanism of FLGs is presented.

Damage identification from flexural strain histories

Damage identification of a thin disk struck by a penetrating projectile is analyzed. The disk is segmented into annular regions each with different properties. The footprint and damage radii form two of several segments in the discretization. Modal response of the segmented disk is determined by combining transfer matrices of successive annular segments and dynamic impedance of the central segment. Transient response of the intact disk proceeds by modal analysis until damage occurs. Following damage, transient response continues for the segmented disk excluding the damaged zone with initial conditions equal to those of the final state of the intact disk at the instant of damage. Peak amplitude of the damaged disk is smaller than that of the intact disk. This is caused by cessation of the forcing pulse and sudden stress release along the damaged perimeter. Non-linear coupling with extensional motions reduces peak flexural strain while extensional strain remains comparatively small.

Pseudotachylytes in the southern border fault of the Cenozoic intracontinental Teletsk basin (Altai, Russia)

The Cenozoic intracontinental Teletsk basin in the Central Asian Altai Mountains is composed of a complexly structured northern and a more simple southern sub-basin. These sub-basins formed in two distinct kinematic stages when first the NNW-striking Teletsk- and then the NE-striking West-Sayan shear zones became reactivated in the Cenozoic under dominant NS-oriented horizontal compression. Although the entire Teletsk basin strikes roughly NS, the southern sub-basin is parallel to the NNW-trending, amphibolite facies Teletsk ductile shear zone, while the northern sub-basin is NS-striking and flanked by differently structured, greenschist facies basement. Basement reactivation closely controlled the southern sub-basin formation, but this is less clear for the northern sub-basin. Contrasts between northern and southern basement and the exclusive occurrence of pseudotachylytes along the margins of the southern basin are explored for their contribution to the formation of the Teletsk basin with two distinct sub-basins. In the ductile shear fabric of the basement flanking the southern sub-basin, concordantly interleaved pseudotachylytes and isolated breccia lenses reflect local brittle deformation along the ductile fabric. The genetic link between breccia lenses and pseudotachylyte occurrences was defined by microstructural investigation. It allows to explore their possible development in a dextral strike–slip zone. These rocks occur in a large fault-bounded segment of the basement. The geometry of the structures in the segment is comparable with a dextral strike–slip sidewall-ripout structure along the Teletsk shear zone. Seismic slip related to pseudotachylytes is attributed to the sudden stress release on the NNW-striking Teletsk shear zone, when the latter became unconstrained by reactivation of the NE-trending West-Sayan fault zone at its northern boundary. The boundary of the sidewall-ripout structure was reactivated as a large listric fault in a later stage. The northern sub-basins roughly develop along an NS strike and are assumed to reflect reactivation of the ductile shear zone underneath the variably structured greenschist facies basement outcropping along the flanks of the sub-basin.

3-D Physical Modelling of Stress Accumulation Processes at Transcurrent Plate Boundaries

We constructed a 3-D physical model of tectonic loading at transcurrent plate boundaries by considering viscoelastic stress relaxation in the asthenosphere and spatial variation in frictional properties (peak strength and critical weakening displacement) of faults. With this model we simulated the process of stress accumulation and release at a seismogenic region with relatively high strength on the plate interface. In low strength regions surrounding the seismogenic region, quasi-static fault slip gradually proceeds with the progress of relative plate motion. The increase of slip deficits in the seismogenic region brings about stress concentration at its margin. The stress accumulation rate is roughly proportional to the inverse of the effective fault length. The accumulated stress is released by unstable dynamic rupture if the critical weakening displacement D c is small, and by stable fault slip if D c is very large. When a fault system consists of two adjacent seismogenic regions, sudden stress release in one region accelerates the stress accumulation process in another region through transient viscoelastic stress transfer as well as instantaneous elastic stress transfer. This indicates the importance of elastic and viscoelastic interaction between adjacent seismic faults even in stress accumulation processes.

Self‐organized Criticality from Separator Reconnection in Solar Flares

A new cellular automaton model for solar flares is presented in which a complex coronal magnetic field is stressed by photospheric shear. The minimum current corona model is used to describe the slow buildup and sudden release of stress in the field. Stress takes the form of currents flowing along the field's network of magnetic separators; it is released by magnetic reconnection. By this model we show how magnetic reconnection can occur as an avalanche, releasing stress along an extended region of the separator network. The model exhibits self-organized criticality; thus the sizes of reconnection avalanches are distributed according to a power law. Flare durations and peak emission levels are also distributed according to power laws, with different exponents. Because the model is derived from the application of magnetohydrodynamics to a coronal field, it is possible to assign physical sizes to its avalanches. These agree well with the sizes and frequencies of observed flares. The total power from flares of all sizes agrees with observation and is therefore well below the inferred coronal heating power.

Point process analysis for Italian seismic activity

Several point process models are considered for describing the occurrence times of earthquakes in different seismic regions in Italy. Clustering effects due to aftershocks, the influence of magnitude and of sudden stress release are investigated. Parameters are estimated by means of likelihood methods, while the Akaike information criterion is used to compare models. The goodness of the estimated models is then tested through analysis of the residual processes.

Point process analysis for Italian seismic activity

Several point process models are considered for describing the occurrence times of earthquakes in different seismic regions in Italy. Clustering effects due to aftershocks, the influence of magnitude and of sudden stress release are investigated. Parameters are estimated by means of likelihood methods, while the Akaike information criterion is used to compare models. The goodness of the estimated models is then tested through analysis of the residual processes.

Structural phase transitions in donor doped BaTiO3 and effects on PTCR behavior

In this study, in situ transmission electron microscopy studies were used to examine and characterize domain formation and phase transformation mechanisms in donor doped, PTCR BaTiO3 materials. A significant and abrupt change in resistivity occurs as a result of a non-uniform boundary structure, the result of variability in strain state and non-uniform potential energy barrier along the grain boundary. In the highly stressed grain boundary regions, the ferroelectric phase transformation is very rapid with martensitic type character. During the phase transformation process, the sudden release of stress and change in structure, in the limited number of conducting regions along the grain boundary, were found to be the dominant factors behind the driving force for a sudden and abrupt change in resistance.

Methane venting as a possible mechanism for glacial plucking and fragmentation of Precambrian crystalline bedrock

Abstract A number of processes have been suggested by glaciologists and Quaternary geologists to explain the type of glacial erosion named plucking. These presume that hydraulic jacking, frost-induced wedging or temperature variations in one way or another have loosened joint blocks from the bedrock surface and evacuated and entrained these in the ice. Large mounds of huge angular boulders and caves therein, have been ascribed to splitting by sudden stress release related to neotectonic activity in postglacial time. This paper presents a hypothesis which relates glacial plucking and split bedrock surfaces to violent venting of methane released during isostatic rebound from methane hydrate formed under permafrost and continental ice.

ON THE VARIANCE PROPERTIES OF STRESS RELEASE MODELS

summaryA class of piece‐wise linear stochastic processes is described, which can be used to model the gradual accumulation and sudden release of stress in earthquake models. Interest centres on the variance properties of the sequence of time points at which the sudden release (earthquakes) occur. Some general conditions are derived for the existence of moments, and under these conditions the variance properties are examined. It is shown that the point process of such events can have a bounded variance function, as is known to happen in the case of constant stress releases, only in this degenerate case. In the final section an example is examined which is closely related to the classical collective risk model.