Failure Wave Research Articles

The plastic shear channeling effect and the nonlinear waves of localized plastic deformation and fracture

Approaches adopted in physical mesomechanics and nonequilibrium thermodynamics are combined in an integrated framework to examine the mechanisms involved in generation of nonlinear waves of localized plastic deformation and fracture in a loaded solid treated as a multiscale system. It has been found that in a loaded solid, local structural transformation fluxes develop at the nanoscale level and undergo channeling at the mesoscale level, whereas accommodation kink bands form at the macroscale level. Generation of the nonlinear waves of localized plastic flow is shown to be due to self-organization of the three processes in question. In the case where work hardening takes place at deformation, traveling waves of localized plastic flow are eventually transformed into steady-state phase failure waves localized at the macroscale level. In a hierarchy of mesoscale levels, propagation of channeled local structural transformations in a heterogeneous medium gives rise to strain-induced defects, such as dislocations, mesoscale shear bands, etc. These deformation mechanisms define the dissipative components of the overall plastic flow process developing in solid bodies under loading. A theory of nonlinear waves of localized plastic flow in a deformed material is proposed. The theoretical predictions made conform with experimental data known from the available literature.

Strength and failure of LK7 borosilicate glass under shock compression

The Hugoniot elastic limit (HEL), the spall strength, the failure threshold, and the failure wave velocity in LK7 glass during shock compression are measured. The HEL estimated from the profiles of compression waves of various intensities is 7.1 ± 0.1 GPa. The spall strength exceeds 7 GPa during shock compression in the elastic range and remains high when passing through the HEL. Failure waves form in the stress range from 5.7 to 10.3 ± 0.5 GPa.

Household Reaction to the Financial Crisis: Scared or Scarred?

The current financial and economic crisis has been characterized as the worst since the Great Depression. While the crisis may be bottoming out, its effects may linger for quite some time. After all, the Great Depression left an entire generation of Americans scarred by their experiences. Times were good, the stock market was booming, and then suddenly it all fell apart. The stock market collapsed, firms went bankrupt, jobs disappeared, large numbers of banks failed and confidence in the economy withered. Many Americans faced poverty and a desperation they never expected.For many Americans today, the similarities are frightening. Household net worth has plummeted, the unemployment rate has soared, foreclosures and bankruptcies have risen sharply and banks are failing. In response, the personal saving rate has risen significantly as many households have cut back spending. However, it is too early to tell how persistent these responses will be. It wasn’t just the depth of the Great Depression that affected subsequent behavior, it was its persistence. The economy didn’t bounce back. Had the downturn been sharp, but relatively short, people would have been scared and suffered economic and emotional pain. They would have had to try to rebuild their wealth and their lives, but this would have been made easier by a recovering economy. Thus, it is less likely that they would have been scarred so deeply that many of the survivors retained a lifelong fear of experiencing another Great Depression, causing them to become more financially conservative, wary of taking on debt and distrustful of financial markets.How will Americans respond to the current crisis? It is hard to gauge the outcome because we do not have a similar recent experience that can be used as a benchmark. Certainly, we have experienced recessions, and even a wave of bank failures in the late 1980s and early 1990s, but they didn’t persist. On the other hand, Japan experienced a malaise that rivaled the Great Depression in length, but not in depth, yet still created its own “Lost Generation.” Thus, the present financial crisis is an unprecedented experience for the current generation. Free-spending and optimistic Americans accumulated massive amounts of debt. Of course, they had growing incomes and wealth they thought (or at least hoped) would enable them to meet their obligations and still have a comfortable retirement. However, as the values of the assets they had used as collateral collapsed, and many lost the jobs that generated the income from which they were making their debt service payments, defaults and foreclosures became widespread. Confidence in financial markets, and the economy more generally, evaporated for many, only to be replaced with uncertainty about future prospects, both for their families and the economy overall. Many Americans will cut their spending sharply out of necessity, others out of fear of what the future holds. Saving rates have already risen substantially. Thus, we are facing the “paradox of thrift” as households try to rebuild their net worth, with the reduced spending likely to delay and weaken recovery from the “Great Recession.” With consumer expenditures accounting for about two-thirds of GDP, severe cutbacks in household spending could prolong our economic malaise, increasing the risk of a more permanent retrenchment of consumer expenditures. But the current uncertainty also will hold back business investment spending, further restraining recovery. With widespread projections that high unemployment and low house prices will be with us for an extended period, as well as the rise in problem loans at banks that will restrain their willingness and ability to provide credit, it is unlikely that the dramatic rise in loan delinquencies, home foreclosures and bankruptcies will soon abate by a meaningful amount.Unfortunately, we face the possibility of being caught in a vicious circle. The cutbacks in consumer and business spending are likely to contribute to a more anemic recovery. A slower recovery, in turn, will likely deepen and prolong the weakness in consumer and business spending, further undermining the recovery. The longer the malaise in economic activity continues, the more likely that diminished spending persists, adversely affecting future economic growth and the standard of living.

Failure Waves and their Possible Roles in Determining Penetration Resistance of Glass

1 High‐velocity impact can produce failure waves in glass. Failure waves limit the speed at which a glass target can become comminuted. The resulting time dependence of strength is likely to influence resistance to ballistic penetration.

Experimental Research on Failure Waves in Soda-Lime Glass

A series of plate impact experiment with soda-lime glass specimens in different thicknesses are conducted on a 57 mm diameter one-stage gas gun in order to further investigate the so-called failure wave phenomena under dynamic compressive loads. With the aid of the VISAR technique, the failure wave trajectory is explored, which shows that, apart from a constant failure wave velocity, an initial delay time for the failure wave to initiate at the impact surface of the specimen should be taken into consideration. Comparing our experimental results with the available data presented in the previous open literature shows that, with the increasing magnitude of the impact loads, the failure wave velocity increases and the initial delay time decreases. Moreover, the derived initial delay time τ = 0.694 μs for the soda-lime glass specimens under the impact stress of 4.7 GPa is the same order of magnitude as that of the incubation time proposed by Morozov and Petrov (2000), which shows that the incubation time plays a dominant role in the total initial delay time, and it also provides an reasonable explanation to the fundamental question pointed out by Clifton (Appl Mech Rev 46: 540-546, 1993).

Intracranial MR Dynamics in Clinically Diagnosed Alzheimers Disease: The Emerging Concept of “Pulse Wave Encephalopathy”

As increasingly shown in neuropathological and predementia clinical studies, cognitive decline with altered intracranial dynamics can fulfill current clinical criteria of dementia of the Alzheimer's type (DAT) and there is a marked pathogenic complexity of this epidemic syndrome. Whereas structural studies only suggest the unexpected frequency of cerebrovascular changes in late life DAT, flow quantification MR sequences now offer a great opportunity of in vivo accurate analysis of cerebrovascular function. Their first applications have allowed development of a modern concept of the intracranial dynamics; a complex windkessel system allows two processes that are crucial to insure brain oxygenation and nutrition, a periodic systolic marked expansion of the intracranial blood compartment within the rigid cranial cavity on the one hand, a marked dampening of the arterial pulse wave before it reaches capillary level on the other hand. This modern concept has allowed better understanding of two archetypes of windkessel failure or so-called pulse wave encephalopathy, normal pressure hydrocephalus and subcortical arteriosclerotic encephalopathy. Dynamic MRI may now help to detect and classify distinct patterns of cerebrovascular dysfunction in DAT. This dynamic approach helps to understand the major association between aging and DAT as well as the increasingly recognized overlap between Alzheimer's pathology, normal pressure hydrocephalus and arteriosclerotic neurodegeneration. Evidence of such a great variety of disturbances in intracranial dynamics behind a single clinical syndrome of DAT can greatly impact therapeutic research on this devastating disorder.

Search for conditions of compressive fracture of hard brittle ceramics at impact loading

In this paper we discuss three different experimental configurations to diagnosing the modes of inelastic deformation and to evaluating the failure thresholds at shock compression of hard brittle solids. One of the manifestations of brittle material response is the failure wave phenomenon, which has been previously observed in shock-compressed glasses. However, based on the measurements from our “theory critical” experiments, both alumina and boron carbide did not exhibit this phenomenon. In experiments with free and pre-stressed ceramics, while the Hugoniot elastic limit (HEL) in high-density B 4C ceramic was found to be very sensitive to the transverse stress, it was found relatively less sensitive in Al 2O 3, implying brittle response of the boron carbide and ductile behavior of alumina. To further investigate the effects of stress states on the shock response of brittle materials, a “divergent flow or spherical shock wave” based plate impact experimental technique was employed to vary the ratio of longitudinal and transversal stresses and to probe conditions for compressive fracture thresholds. Two different experimental approaches were considered to generate both longitudinal and shear waves in the target through the impact of convex flyer plates. In the ceramic target plates, the shear wave separates a region of highly divergent flow behind the decaying spherical longitudinal shock wave and a region of low-divergent flow. Experiments with divergent shock loading of alumina and boron carbide ceramic plates coupled with computer simulations demonstrated the validity of these experimental approaches to develop a better understanding of fracture phenomena.

To the mechanisms of failure wave

Results of shock-wave experiments with free and prestressed samples of K8 and K14 crown glasses are presented. Controlled confinement pressure on the specimen of around 200 MPa was provided by installing a shrink-fit metal sleeve on the lateral surface of the sample disk. Results of measurements of the free surface velocity histories show that prestressing leads to earlier stopping of the failure wave. Thus, the compressive transverse stress increases the failure threshold that is in agreement with existing criteria of compressive fracture. However, sensitivity of the failure threshold to the confinement stress value is not as high as it could be expected. Measurements of the refractive indexes for the particles from the failure wave zone of recovered glass samples have shown their small increase whereas no index change for particles from the residual zone was found. It has been concluded that irreversible densification makes essential contribution to the mechanism of failure of glasses.

Dynamics and quenching of non-premixed edge-flames in oscillatory counterflows

The dynamics of non-premixed edge-flames, including the generation of cellular structures, in an unsteady, symmetric counterflow are examined for positive rates of strain. A one-step reaction is assumed, ν Y F + ν X O → ν p P , in which the oxidizer Lewis number is 1. For a variety of Damköhler numbers, we examine the edge-flame evolution for two values of the fuel Lewis number Le Y , 0.3 and 1, and two values of the initial mixture fraction γ, 0.36 and 1, representing fuel lean and stoichiometric supply conditions. For Le Y = 0.3 and γ = 0.36 , unsteady forcing can convert non-cellular edge-flames into ones containing various characteristics of near- or sub-limit cellular structures, including drifting, splitting and stationary flame strings. The transition regimes between the different edge-flame structures are examined as a function of the amplitude and frequency of the strain rate variations in the unsteady counterflow and also as a function of the instantaneous and equivalent strain rate functions. For Le Y = 0.3 and γ = 1 , while no cellular edge-flames can be generated for steady counterflows, we show that cellular structures can be observed in the presence of unsteady forcing. For Le Y = 1 and γ = 1 , it is shown that unsteady forcing can significantly modify the mean propagation speeds of both ignition and failure waves. Finally, the quenching boundaries of two-dimensional edge-flames induced by the unsteady counterflow are examined for Le Y = 0.3 , γ = 0.36 and Le Y = 1 , γ = 1 .

Numerical assessment for impact strength measurements in concrete materials

Abstract Measurement of dynamic strength of concrete at impact relevant strain rates and pressures is the purpose of the described study. Therefore, an experimental design of direct planar impact experiments with longitudinal and transverse strain gauges is analyzed in predictive hydrocode simulations using an elastic–plastic damage model for concrete. The calculations and first experimental results on mortar show decreasing phase velocities of stress waves both in longitudinal and lateral gauges. The model clearly associates it with the onset of damage, possibly interpreted as a failure wave. Numerical analysis is furthermore used to compare a monolithic target block to a thoroughly assembled concrete sample in order to include flat gauges in the material. The planned experimental procedure to derive wave speeds, particle velocities and strain rates from stress measurements is anticipated and validated on the basis of simulated gauge signals. The most important finding is the prediction and first experimental confirmation that concrete ultimate strength and damaged yield stress can be derived at strain rates in the order of 10 4 /s from the proposed type of experiments. This technique promises new insight into the strength and failure processes of concrete in the challenging loading region around the characteristic minimum of the shock particle velocity relationship.

Collective modes in the microshear ensemble as a mechanism of the failure wave

Results of theoretical and experimental study of failure wave phenomena are presented. A description of the failure wave phenomenon was proposed in terms of a self-similar solution for the microshear density. The mechanisms of failure wave generation and propagation were classified as a delayed failure with the delay time corresponding to the time of excitation of self-similar blow-up collective modes in a microshear ensemble. Experimental study of the mechanism of the failure wave generation and propagation was carried out using a fused quartz rod and included the Taylor test with high-speed framing. The results obtained confirmed the “delayed” mechanism of the failure wave generation and propagation.

Bank insolvency and the problem of nonperforming loans

Bank insolvency has been a significant problem in many parts of the world in the last 30 years. There have been waves of bank failures in developed and developing countries, and also in countries with transitional economies. Between 1997 and 2002, banks had to be closed in more than 50 countries. This paper focuses on one particularly important aspect of the cause of bank insolvency: the relationship between nonperforming loans (NPLs) and bank failure. It will consider both the prevention and control of NPLs and in so doing will examine regulatory and supervisory issues as well as initiatives for dealing with impaired assets. The need for an effective bank insolvency law is a crucial factor which is also addressed. The main aim of the paper is to provide some guidance on a framework for developing and transitional economies, many of which are still suffering severe problems with NPLs.

Constitutive Model for Damaged Borosilicate Glass Under Confinement

Penetration experiments on glass found in the literature show that the target fails ahead of the projectile (the “failure wave” is faster than the penetration velocity). Thus, during penetration, the projectile is in contact with failed glass ahead; consequently, the properties of failed material are required for accurate numerical simulations. This paper shows, using a confined compression experiment of a cylindrical specimen combined with an analytical model for interpretation of the data, how it is possible to determine the elastic and the Drucker–Prager constants for damaged borosilicate glass (or, in principle, any other brittle material). An important and unexpected result from these characterization tests is that the elastic properties of damaged glass do not change dramatically even if the glass is pulverized (in situ comminuted). This is probably because the tests are highly confined and the glass has no space to move, and thus the elastic properties remain essentially unchanged. Estimates for the Drucker–Prager constitutive constants for in situ damaged borosilicate glass are also obtained.

Deformation model for brittle materials and the structure of failure waves

Constitutive equations that describe the experimentally observed failure waves are proposed to model inelastic strains of brittle materials. The complete system of equations is hyperbolic, each equation of this system has divergent form. The model is based on the assumption that continual failure is the process of transition from an intact state to a “fully damaged” state described by the kinetics of the order parameter. The structure of stationary traveling compressive waves is analyzed using a simplified model. It is shown that in a certain range of amplitudes, the wave splits into an elastic precursor and a failure wave.

Dynamic Failure Behavior of Polycrystalline Alumina Under Impact Loading

Plate impact experiments and impact recovery experiments were performed on 92.93wt.% aluminas using a 100mmdia compressed-gas gun. Free surface velocity histories were traced by a velocity interferometry system for any reflector (VISAR) velocity interferometer. There is a recompression signal in free surface velocity, which shows evidence of a failure wave in impacted alumina. The failure wave velocities are 1.27km∕s and 1.46km∕s at stresses of 7.54GPa and 8.56GPa, respectively. It drops to 0.21km∕s after the material released. SEM analysis of recovered samples showed the transit of intergranular microcracks to transgranular microcracks with increasing shock loading. A failure wave in impacted ceramics is a continuous fracture zone, which may be associated with the damage accumulation process during the propagation of shock waves. Then a progressive fracture model was proposed to describe the failure wave formation and propagation in shocked ceramics. The governing equation of the failure wave is characterized by inelastic bulk strain with material damage and fracture. Numerical simulation of the free surface velocity was performed in good agreement with the plate impact experiments. And the longitudinal, lateral, and shear stress histories upon the arrival of the failure wave were predicted, which present the diminished shear strength and lost spall strength in the failed layer.

A delayed failure of inhomogenous brittle material under shock wave compression

The failure property of inhomogenous brittle materials under plate impact experiment has been studied by using AOW (amphibolized olivine websterite) rock as a representative sample. A small recompression signal was observed in the VISAR record of the rear free surface velocity at shock stress much below σHEL, indicating the generation of the failure wave. By using samples with different thickness but impacted at approximately the same shock stress of 3.9 GPa, the trajectory of the propagation of the failure wave through the stressed sample has been determined. The failure wave is generated with a delay time of about 0.5 μs behind the shock wave front, and propagates into the stressed material with a velocity comparable to that of the shock wave front. A physical mechanism involving the “in situ activation and growth” of the originally existing micro-cracks due to the local shear stress under shock wave compression is suggested for these observations.

Fall of “organ bank” relationship over bank failure and consolidation wave: Experience in pre-war Japan

This paper examines how the close ties between banks and non-bank firms – so called “organ bank” relationship in Japanese banking literature – declined through bank failures and banking consolidations in pre-war Japan. With a unique dataset compiled for 1,007 Japanese banks from 1926 to 1936, we measure the degree of “organ bank” relationship by the number of persons who worked as directors or auditors for a bank and a non-bank firm at the same time. We observe that this number of “director interlocking” declined along our sample period, when there were lots of bank failure, bank merger and acquisition events. Our findings suggest that banking consolidation and selection thorough failures may help to eliminate the detrimental connections between banks and non-bank firms, based on Japan’s experience

Re-examination of the evidence for a failure wave in SiC penetration experiments

Previous work suggested the possibility that the effects of a failure wave, evidenced through a change in the slope of the penetration velocity vs. impact velocity ( u– v p) curve resulting from an increase in target penetration resistance, could be observed in penetration experiments of SiC. However, the previous work had to combine two different sets of experimental data, one using long tungsten rods and the other copper shaped-charge jets. A new set of experiments was conducted to address the uncertainties associated with combining the two disparate data sets. Analysis of the new experiments showed no evidence of a distinct change in the slope of the u– v p response of SiC, up to an impact velocity of 6.2 km/s. We re-examine the original data and analysis in light of the new experiments to understand the origins of the original misinterpretation.

Hypervelocity penetration of gold rods into SiC-N for impact velocities from 2.0 to 6.2 km/s

This paper presents the experimental design and results of an advanced set of reverse ballistic experiments with long gold rods, impacting SiC-N ceramics at impact velocities from 2.0 to 6.2 km/s. Important issues for these experiments were the high accuracy and position requirements necessary to detect a possible failure wave or failure kinetics in SiC-ceramics as might be evidenced by a change in the slope of the penetration velocity–impact velocity curve. New and sophisticated evaluation methods were developed for this purpose and produced very reliable results. Analyses of the experimental results show clearly that there is no change in the slope of the penetration velocity–impact velocity curve, contrary to that inferred from previous data and analysis.

Flames in narrow circular tubes

We examine an axi-symmetric deflagration located in a tube with thermally active walls. It is noted that the flame-in-tube configuration defines a classical edge-flame, a flame in a shear flow for which there is a water-shed solution for a critical value of the Damköhler number ( D), ignition front solutions for larger values of D, and failure wave solutions for smaller values. We examine semi-infinite tubes with a heat flux imposed at the tube wall ends, to simulate experiments reported in the 30th Symposium. We identify parameters for which stable solutions are obtained at certain flow rates, but unstable solutions are generated at higher flow rates, followed by stable solutions at still higher flow rates. These solutions are consistent with the experimental record. Instability leads either to regular oscillations or to a violent process characterized by quenching and re-ignition.