Failure Regime Research Articles

True triaxial tests – using permeability and extensional stress parameters to simulate geological history in rocks

In most underground facilities, the existence of water and anisotropic stress can create serious problems when excavating and maintaining the underground cavities. In this article, we performed laboratory experiment of true triaxial tests on Shirahama sandstone to investigate the effect of the intermediate principal stress on rock permeability. In the brittle failure regime (low minimum principal stress), the specimen showed a rapid increase in permeability after brittle fracture. At higher minimum principal stress, the permeability decreased with increasing axial strain, but after yield stress, the permeability recovered gradually small. As a result, the final permeability in the specimen after failure was less than the initial value. We focus on the importance of the intermediate principal stress and introduce the observations related to the mechanical characteristics on a wide range of stress states from confined triaxial compression to confined triaxial extension. The true triaxial test is superior in obtaining continuous deformation behaviors from ductile under a compression regime to brittle under an extension regime. The maximum principal stress increased by a finite amount in changing from a compression state to an extension state.

In-plane column response of metallic corrugated core sandwich panels

The in-plane compressive response of corrugated core sandwich columns is investigated analytically, numerically, and experimentally. Failure mechanisms have been identified and include macro buckling, shear buckling, and face wrinkling. Analytical formulae are developed for these mechanisms and used to create failure mode maps as a function of column geometry and material properties. Failure maps are created using measured material properties of 304 stainless steel, from which corrugated core columns are designed to experimentally probe each failure regime. The results are compared to the predictions. The results demonstrate that the predictions accurately capture both the critical failure load and failure mechanism. They also highlight the influence of both local and global boundary conditions on the column response. Lastly, optimal corrugated core column designs that minimize mass for a given load capacity are calculated using the failure mode predictions. The results show that corrugated core columns compare favorably with competing pyramidal core and hat-stiffened panel designs and are a viable alternative for in-plane load bearing applications.

Rheology of Discrete Failure Regimes of Anisotropic Sea Ice

ABSTRACT A rheological model of sea ice is presented that incorporates the orientational distribution of ice thickness in leads embedded in isotropic floe ice. Sea ice internal stress is determined by coulombic, ridging and tensile failure at orientations where corresponding failure criteria are satisfied at minimum stresses. Because sea ice traction increases in thinner leads and cohesion is finite, such failure line angles are determined by the orientational distribution of sea ice thickness relative to the imposed stresses. In contrast to the isotropic case, sea ice thickness anisotropy results in these failure lines becoming dependent on the stress magnitude. Although generally a given failure criteria type can be satisfied at many directions, only two at most are considered. The strain rate is determined by shearing along slip lines accompanied by dilatancy and closing or opening across orientations affected by ridging or tensile failure. The rheology is illustrated by a yield curve determined by combining coulombic and ridging failure for the case of two pairs of isotropically formed leads of different thicknesses rotated with regard to each other, which models two events of coulombic failure followed by dilatancy and refreezing. The yield curve consists of linear segments describing coulombic and ridging yield as failure switches from one lead to another as the stress grows. Because sliding along slip lines is accompanied by dilatancy, at typical Arctic sea ice deformation rates a one-day-long deformation event produces enough open water that these freshly formed slip lines are preferential places of ridging failure.

The Clash of Visualizations: Counterinsurgency and Climate Change

In the decade since the 9-11 attacks on the United States, the rhetoric of the of civilizations promoted by Samuel Huntington has been the dominant means of imagining global culture (1993). Concomitant with this purported clash between the West and Islam has been a war of images, in which each side appeared to use images as weapons against the other and against internal dissent. In this essay, I will suggest that the image is deployed within a regime of visualization, whose success or failure accounts for its reception. I suggest that the U.S. concentration on counterinsurgency has not only supplanted and displaced climate change as a central issue but actively contests the possibility of visualizing it as such. The of visualizations between counterinsurgency and climate change is the engagement by which counterinsurgency seeks to (re)legitimize itself, without becoming beholden to the very different claims to social ordering that a prioritization of climate change issues would entail. Looking at this clash of visualization at the U.S. national level in New Orleans and at the level of the global imaginary via the Pacific Ocean and its island states, I stress that from both the formal and political point of view, such clashes center precisely on the definition of the real, the realistic and their attendant realisms.

Artificial neural networks as a basis for new generation of rock failure criteria

Conventional methods for prediction of rock strength are based on using classical failure criteria. In this study, artificial neural networks were regarded as new tools for considering the strength of intact rock in a wide range of loading condition from uniaxial tension to triaxial compression. A comprehensive data set of the values of major and minor principal stresses at failure from 1638 laboratory tests on seven rock types was collected. For each rock type, data were randomly divided into two subsets, training and test sets. Neural networks were trained using training sets to predict the value of major principal stress at failure from uniaxial compressive stress and minor principal stress. Small architecture and regularization method were adopted to avoid over-fitting problems. The same training sets were used in determining the constants of two popular empirical failure criteria, namely Bieniawski–Yudhbir and Hoek–Brown. Then, the test sets were used to examine the accuracy and generalization of the models in predicting the strength in new situations. Comparison of the results of the neural network models with those of the empirical criteria showed that the former approach always lead to less root mean squared error and higher coefficient of determination. On average, for different rock types, using ANN models led to about 30% decrease in prediction error relative to best empirical models. These models also showed better flexibility in the prediction of major principal stress at failure in both brittle and ductile failure regimes.

Effect of shear rupture on aggregate scale formation in sea ice

A discrete element model is used to study shear rupture of sea ice under convergent wind stresses. The model includes compressive, tensile, and shear rupture of viscous elastic joints connecting floes that move under the action of the wind stresses. The adopted shear rupture is governed by Coulomb’s criterion. The ice pack is a 400 km long square domain consisting of 4 km size floes. In the standard case with tensile strength 10 times smaller than the compressive strength, under uniaxial compression the failure regime is mainly shear rupture with the most probable scenario corresponding to that with the minimum failure work. The orientation of cracks delineating formed aggregates is bimodal with the peaks around the angles given by the wing crack theory determining diamond‐shaped blocks. The ice block (floe aggregate) size decreases as the wind stress gradient increases since the elastic strain energy grows faster leading to a higher speed of crack propagation. As the tensile strength grows, shear rupture becomes harder to attain and compressive failure becomes equally important leading to elongation of blocks perpendicular to the compression direction and the blocks grow larger. In the standard case, as the wind stress confinement ratio increases the failure mode changes at a confinement ratio within 0.2–0.4, which corresponds to the analytical critical confinement ratio of 0.32. Below this value, the cracks are bimodal delineating diamond shape aggregates, while above this value failure becomes isotropic and is determined by small‐scale stress anomalies due to irregularities in floe shape.

Computational post failure analysis with Cosserat continuum

ABSTRACT We present computational analysis of geomechanics problems in the post failure regime based on a Cosserat material modelThe introduction of internal length (related to grain size) in the Cosserat model improves the computational stability and allows for robust progressive localization modelling. The model is capable of predicting advanced deformation modes such as surface buckling and localization of deformation in shear bands that may cause failure in a structure. The obtained results show clearly a progressive failure mechanism and the computed modes are in a good qualitative agreement with laboratory and field observations. As illustration we present results of two applications from petroleum engineering: the modelling of failure in thick walled cylinder test and the stability of elliptical shape perforations.

“Electrochemical Shock” of Intercalation Electrodes: A Fracture Mechanics Analysis

Fracture of electrode particles due to diffusion-induced stress has been implicated as a possible mechanism for capacity fade and impedance growth in lithium-ion batteries. In brittle materials, including many lithium intercalation materials, knowledge of the stress profile is necessary but insufficient to predict fracture events. We derive a fracture mechanics failure criterion for individual electrode particles and demonstrate its utility with a model system, galvanostatic charging of . Fracture mechanics predicts a critical C-rate above which active particles fracture; this critical C-rate decreases with increasing particle size. We produce an electrochemical shock map, a graphical tool that shows regimes of failure depending on C-rate, particle size, and the material’s inherent fracture toughness . Fracture dynamics are sensitive to the gradient of diffusion-induced stresses at the crack tip; as a consequence, small initial flaws grow unstably and are therefore potentially more damaging than larger initial flaws, which grow stably.

Investigation of high cycle and Very-High-Cycle Fatigue behaviors for a structural steel with smooth and notched specimens

The high cycle and Very-High-Cycle Fatigue (VHCF) properties of a structural steel with smooth and notched specimens were studied by employing a rotary bending machine with frequency of 52.5 Hz. For smooth specimens, VHCF failure did occur at fatigue cycles of 7.1 × 10 8 with the related S– N curve of stepwise tendency. Scanning Electron Microscopy (SEM) was used for the observations of the fracture surfaces. It shows that for smooth specimens the crack origination is surface mode in the failure regime of less than 10 7 cycles. While at VHCF regime, the material failed from the nonmetallic inclusion lies in the interior of material, leading to the formation of fisheye pattern. The dimensions of crack initiation region were measured and discussed with respect to the number of cycles to failure. The mechanism analysis by means of low temperature fracture technique shows that the nonmetallic inclusion in the interior of specimen tends to debond from surrounding matrix and form a crack. The crack propagates and results to the final failure. The stress intensity factor and fatigue strength were calculated to investigate the crack initiation properties. VHCF study on the notched specimens shows that the obtained S– N curve decreases continuously. SEM analysis reveals that multiple crack origins are dominant on specimen surface and that fatigue crack tends to initiate from the surface of the specimen. Based on the fatigue tests and observations, a model of crack initiation was used to describe the transition of fatigue initiation site from subsurface to surface for smooth and notched specimens. The model reveals the influences of load, grain size, inclusion size and surface notch on the crack initiation transition.

Energy-based approach for the evaluation of low cycle fatigue behaviour of 2.25Cr–1Mo steel at elevated temperature

The energy-based approach for the evaluation of low cycle fatigue behaviour of 2.25Cr–1Mo steel at elevated temperature has been investigated and detailed analyses discussed. Plastic strain energy was determined per cycle and found to characterise both crack initiation and propagation to failure regimes. At cyclic stabilisation, average plastic strain energy may be used as a suitable damage parameter and correlations between experimental and predicted data determined. The fatigue toughness to failure of the material was established and the development of a fatigue toughness to crack propagation analysis is presented.

Towards a Non Empirical Kinetic Model for the Lifetime Prediction of Polyethylene Pipes Transporting Drinking Water

AbstractA kinetic model has been elaborated to predict the lifetime of polyethylene pipes transporting slightly pressurized (3–12 bars) drinking water disinfected by free radical reagents. This model is composed of three levels: i) A system of differential equations, derived from a mechanistic scheme for radical chain oxidation in the presence of free radical reagents, giving access to the spatial distribution (in the pipe wall) of oxidation products and stabilizer concentration; ii) Equations allowing to predict the profiles of average molar masses from the spatial distribution of chain scissions and crosslinking events; iii) An empirical creep equation and a failure criterion derived from regression curves obtained in pure water (without disinfectant). It is assumed that the chemical degradation modifies only the time to transition between ductile and brittle regimes of failure, and that this time is linked to the weight average molar mass according to the classical power law. By superimposing these three levels, it is possible to predict the time to failure under the coupled effects of pressure and chemical degradation. This model is successfully applied to the case of chlorine dioxide.

Prediction of solid block masonry prism compressive strength using FE model

Masonry strength is dependent upon characteristics of the masonry unit, the mortar and the bond between them. Empirical formulae as well as analytical and finite element (FE) models have been developed to predict structural behaviour of masonry. This paper is focused on developing a three dimensional non-linear FE model based on micro-modelling approach to predict masonry prism compressive strength and crack pattern. The proposed FE model uses multi-linear stress–strain relationships to model the non-linear behaviour of solid masonry unit and the mortar. Willam–Warnke’s five parameter failure theory developed for modelling the tri-axial behaviour of concrete has been adopted to model the failure of masonry materials. The post failure regime has been modelled by applying orthotropic constitutive equations based on the smeared crack approach. Compressive strength of the masonry prism predicted by the proposed FE model has been compared with experimental values as well as the values predicted by other failure theories and Eurocode formula. The crack pattern predicted by the FE model shows vertical splitting cracks in the prism. The FE model predicts the ultimate failure compressive stress close to 85% of the mean experimental compressive strength value.

The brittle deformation regime of water-saturated siliceous sandstones

SUMMARY We present here new experimental data on the mechanical behaviour of water-saturated Bentheim and Fontainebleau sandstones deformed in the brittle failure regime. Bentheim sandstone samples were stressed at room temperature and subjected to confining pressures PC ranging from 12 to 120 MPa and pore pressures PP ranging from 1 to 70 MPa. For all samples the evolution of the volumetric strain first shows compaction eventually reversing to dilation of the pore volume when approaching the peak stress. All samples failed localized on a single shear zone. Critical stresses that is onset of dilatancy and peak stress, can be uniquely defined as a function of the effective pressure Peff = PC – PP. The failure curve parameters, when fitted with the Hoek–Brown criterion or a parabolic envelope, are consistent with previous values obtained on similar material. When compared to data obtained on dry samples of the same rock, the present data show no notable effect of the presence of water on the critical stress levels. The same conclusion holds for the stress–strain curves when dry and water-saturated experiments under equivalent effective confining pressures are compared. When compared to previously published data obtained on various quartzose rocks, which show a quite variable water-weakening effect, our results lead to the conclusion that the quasi-exclusive presence of quartz grains bonded together by a quartzose cement combined with the absence of clayey minerals may explain the absence of this effect in Bentheim sandstone. Moreover, this conclusion is supported by complementary results obtained by Fontainebleau sandstone with similar microstructural and compositional characteristics. A previously developed micromechanical model based on the interaction of pore cracks has been used and modified to take into account the presence of water. When compared to the experimental data, the results of the model show good agreement for low to intermediate effective confining pressures. Critical stresses as well as deformation curves are well reproduced as was the case for the dry rock provided that effective pressure is introduced. At higher effective pressures, a mix of mechanisms including shear-enhanced compaction may explain the progressive discrepancy between the modelled and the experimental curves when approaching the apex of the failure envelope.

The out-of-phase thermomechanical fatigue behavior of Ultra SCS-6/Ti–24Al–17Nb– xMo (at.%) metal matrix composites

The out-of-phase thermomechanical fatigue behavior of Ultra SCS-6/Ti–24Al–17Nb–2.3Mo (at.%), Ultra SCS-6/Ti–24Al–17Nb–1.1Mo (at.%), and Ultra SCS-6/Ti–24Al–17Nb–0.6Mo (at.%) continuously-reinforced metal matrix composites (MMCs) was investigated in order to understand the effect of Mo on the microstructure and fatigue behavior of titanium–alloy–matrix composites. From the recorded stress versus cycle behavior, two failure regimes were noted. For the highest stresses examined, fatigue-dominated failure occurred. For the lowest stresses examined, a creep-dominated failure occurred. The Ultra SCS-6/Ti–24Al–17Nb–1.1Mo MMC exhibited the shortest fatigue lives for all the applied stress levels examined. The Ultra SCS-6/Ti–24Al–17Nb–2.3Mo MMC exhibited the longest fatigue lives for the lowest applied stress levels, and the Ultra SCS-6/Ti–24Al–17Nb–0.66Mo MMC exhibited the longest fatigue lives for most of the higher applied stress levels.

UK National Health Service (NHS) failure regime: up to 92 Trusts may be culled

UK National Health Service (NHS) failure regime: up to 92 Trusts may be culled

Sub Prime Crisis in US: Emergence, Impact and Lessons

The sub prime crisis in US is the result of excessive amounts of loans made to people who could not afford them and excessive amounts of money thrown into the mortgage arena by investors who were very eager for high return. The crisis represents the other side of a phase when a low rate of interest, rising home prices and mortgage securitization brought huge gains. A number of factors like legislations like Community Reinvestment Act, low rate of interest, mortgage brokers and lenders, rating agencies played their role in generating crisis. Three important dimensions of the sub prime saga relate to poor regulation of investment banks, relaxation in lending standards led by greed in a regime of unbridled competition and failure of the asset market to realize the dues from the defaulter. It once again brings home the fact that financial sector is distinctive in nature and can be exposed to unbridled and unregulated competition only at the cost of a complete peril.

Collapse of towers as applied to September 11 events

The subject of the paper is the collapse of towers and highscrapers, particularly, the collapse of the World Trade Center towers in New York on September 11, 2001. The deduced equations of progressive collapse are used to refute the generally accepted opinion of experts about progressive collapse of the WTC towers in the free-fall regime, which is the official version of the US government. It is proved that progressive collapse is much slower than free fall. The critical floors where collapses started from are estimated using the well-established fact of the free-fall time of all WTC collapses. To this end, the most comprehensive “hybrid” analysis is advanced taking into account that collapses could start on several floors simultaneously, not on one floor as suggested before. According to this “hybrid” model, at the first stage, several floors collapsed simultaneously as a result of fracture waves causing a dust cloud and, at the second stage, the lower part of tower being intact in the first stage collapsed in the regime of progressive failure. Five different collapse types are studied, including the fastest and slowest collapses, and then the hybrid mode is examined with initial collapse of several floors followed by the “domino-effect” of the remaining floors. It is established that the floors where the WTC collapses started from were located significantly lower than the floors hit by the terrorists and subjected to fire. This conclusion confirms the same former result obtained by using the simple official theory of pure progressive collapse.

Effects of shear velocity oscillations on stick‐slip behavior in laboratory experiments

We report on laboratory friction experiments in which simulated faults are exposed to shear velocity oscillations at different amplitudes and frequencies. Granular layers are sheared in a servo‐controlled biaxial apparatus at constant normal stress and background shear velocity, with a shear velocity sinusoid superimposed. Correlation between oscillations and stick‐slip events is determined by the timing of dynamic failure with respect to the oscillation phase. Schuster's test is used to calculate the statistical likelihood of phase recurrence. We find that correlation of failure with the shear stress oscillation depends on oscillation frequency and amplitude at low frequencies and solely on oscillation amplitude at high frequencies. The frequency boundary between these two regimes is proportional to the inverse of the time needed to displace the frictional critical slip distance. We evaluate changes in failure characteristics, including failure strength, recurrence time between events, creep, and phase of the oscillation at failure, to assess the effects of stressing rate oscillations. Failure occurs at maximum shear stressing rate at low frequencies and lags peak stressing rate in the high‐frequency regime. Friction at the onset of dynamic failure decreases with increasing frequency. The distribution of events through time depends on the frequency of the shear oscillation; low‐frequency oscillations produce bimodal distributions and high‐frequency oscillations produce unimodal distributions. If the transition between the failure regimes depends on the critical displacement length as our experiments imply, the critical frequency will vary for faults with different gouge layer thicknesses and total displacement.

Modelling surface orientation and stress at failure of concrete and geological materials

AbstractIn the numerical simulation of failure for structures, crack initiation is normally based on a criterion that provides both the stress and the orientation of the material failure plane. Most constitutive models provide reasonably good criteria for stress, but the predictions of orientation are often at variance with experimental data. If a crack initiates in the wrong direction, it is likely that the direction of propagation will also be in error.The object of this paper is to describe an anisotropic decohesion function that provides predictions of both stress and failure orientation which are in good agreement with basic features of experimental data. The essential new feature is the addition to the decohesion function of components of stress that are not part of the traction vector on the material failure surface. The result is that a given decohesion surface describes two general regimes of failure. For the first, the normal to the material failure surface is in a direction of principal stress, not necessarily the maximum principal stress. The second involves shear for which there exists two or more solutions for the normal. Typically, the normal vector forms positive and negative angles of equal magnitude with a principal direction of stress and the angle varies continuously with stress. Experimental data for concrete and ice are used as representative examples for isotropic and anisotropic materials, respectively. Copyright © 2006 John Wiley & Sons, Ltd.

Mechanics of the WTC collapse

Two theories of the WTC collapse are examined. The first one is the theory of progressive failure, and the other one is the theory of fracture waves. The collapse in the regime of progressive failure is shown to occur at an acceleration, which is several times less than the gravitational acceleration and, hence, this theory contradicts to the observed free fall. Evidently, the WTC towers were disintegrated at the very beginning of each collapse. To explain this fact an alternative model based on the theory of failure waves is proposed.