Critical Stress Level Research Articles

Permeability Evolution and Rock Brittle Failure

This paper reports an experimental study of the evolution of permeability during rock brittle failure and a theoretical analysis of rock critical stress level. It is assumed that the rock is a strain-softening medium whose strength can be described by Weibull’s distribution. Based on the two-dimensional renormalization group theory, it is found that the stress level λc (the ratio of the stress at the critical point to the peak stress) depends mainly on the homogeneity index or shape parameter m in the Weibull’s distribution for the rock. Experimental results show that the evolution of permeability is closely related to rock deformation stages: the permeability has a rapid increase with the growth of cracks and their surface areas (i.e., onset of fracture coalescence point), and reaches the maximum at rock failure. Both the experimental and analytical results show that this point of rapid increase in permeability on the permeability-pressure curve corresponds to the critical point on the stress-strain curve; for rock compression, the stress at this point is approximately 80% of the peak strength. Thus, monitoring the evolution of permeability may provide a new means of identifying the critical point of rock brittle fracture.

Bone Response to Static Compressive Stress at Bone-Implant Interface: A Pilot Study of Critical Static Compressive Stress.

Mechanical imbalance caused by mechanical overload or poor bone quality around a dental implant can result in osseointegration failure. To avoid that, it is important to identify an appropriate safety stress margin (critical stress level). For this study, a novel device was developed to generate a quantitative amount of static compressive stress under an aseptic closed condition. The aim was to clarify the amount of critical stress produced on the cortical bone when static compression is applied to the osseointegrated bone-implant interface. Small parts for bone sustaining, load generation, and load transmittance were developed to generate quantitative static compressive stress at the bone-implant interface and implanted inside the tibial cortical bone in adult beagle dogs. Each tibia in two dogs received bone-sustaining parts, then after 2 months, the load-transmitting parts were placed into the bone-sustaining parts. After another 2 months, various magnitudes of static compressive stress (0-180 MPa) were generated by tightening the load-generating part to the osseointegrated bone-implant interface. After 7 days, the animals were euthanized, and dissected blocks were prepared for histomorphometric analyses. There were no obvious signs of bone resorption or loss of osseointegration in any of the dogs. The change in shape of osteon was not influenced by the amount of static compressive stress. However, periosteal reactions were observed under the cortical bone on the opposite side. These results indicate that osseointegrated bone-implant interfaces show minimal response based on the magnitude of static compressive stress, even when such stress is greater than 120 MPa.

Ionic switch controls the DNA state in phage λ.

We have recently found that DNA packaged in phage λ undergoes a disordering transition triggered by temperature, which results in increased genome mobility. This solid-to-fluid like DNA transition markedly increases the number of infectious λ particles facilitating infection. However, the structural transition strongly depends on temperature and ionic conditions in the surrounding medium. Using titration microcalorimetry combined with solution X-ray scattering, we mapped both energetic and structural changes associated with transition of the encapsidated λ-DNA. Packaged DNA needs to reach a critical stress level in order for transition to occur. We varied the stress on DNA in the capsid by changing the temperature, packaged DNA length and ionic conditions. We found striking evidence that the intracapsid DNA transition is ‘switched on’ at the ionic conditions mimicking those in vivo and also at the physiologic temperature of infection at 37°C. This ion regulated on-off switch of packaged DNA mobility in turn affects viral replication. These results suggest a remarkable adaptation of phage λ to the environment of its host bacteria in the human gut. The metastable DNA state in the capsid provides a new paradigm for the physical evolution of viruses.

Shape Memory Effect in Cast Versus Deformation-Processed NiTiNb Alloys

The shape memory effect (SME) response of a deformation-processed NiTiNb shape memory alloy is benchmarked against the response of a cast alloy. The insoluble Nb element ternary addition is known to widen the hysteresis with respect to the binary NiTi alloy. Cast microstructures naturally consist of a cellular arrangement of characteristic eutectic microconstituents surrounding primary matrix regions. Deformation processing typically aligns the microconstituents such that the microstructure resembles discontinuous fiber-reinforced composites. Processed alloys are typically characterized for heat-to-recover applications and thus deformed at constant temperature and subsequently heated for SME recovery, and the critical stress levels are expected to facilitate plastic deformation of the microconstituents. The current work employs thermal cycling under constant bias stresses below those critical levels. This comparative study of cast versus deformation-processed NiTiNb alloys contrasts the strain–temperature responses in terms of forward ΔT F = M s − M f and reverse ΔT R = A f − A s temperature intervals, the thermal hysteresis, and the recovery ratio. The results underscore that the deformation-processed microstructure inherently promotes irreversibility and differential forward and reverse transformation pathways.

Mechanical response of low stacking fault energy Co–Ni alloys – Continuum, mesoscopic and atomic level treatments

Low stacking fault energy fcc alloys deform via either mechanical twinning or dislocation slip. A single crystal poses an excellent avenue to studying inherent material preference for plastic flow mechanism. This paper examines the physical process underlying the flow behavior of Co-33%Ni single crystals at continuum, mesoscale and atomic levels. Digital image correlation, electron backscatter diffraction and transmission electron microscopy permitted an in-depth analysis of the deformed microstructure. Emphasis is placed on rationalizing the experimentally discovered competing modes of plasticity from an atomistic perspective. Experimentally, predisposition for slip or twinning is observed to be specific to a given crystal orientation and loading direction (tension/compression). Particularly, flow in 〈111〉tension and 〈100〉compression proceeds with a single twinning system and ensuing twin-slip interactions. By contrast, dislocation slip nucleation followed by multi-system forest hardening prevails in the post-yield behavior of 〈123〉tension and 〈111〉compression cases. The stress-strain curves reveal that the twinning-mediated flow induces superior hardening characterized by pronouncedly serrated flow behavior. A first principle based mechanics model is proffered to account for the relative material propensity for either slip or twinning on the basis of atomic level generalized stacking/planar fault energies. The model considers the energy balance between the applied work and the total energy expenses as contributed by both continuum and atomistic effects. Upon establishing the γ surfaces from density functional theory, critical stresses for the initiation of slip, twin and post-yield twin-slip interactions are computed within a Peierls-Nabarro modeling framework. Predicted critical stress levels demonstrate reasonably good agreement with the experimentally determined critical resolved shear stresses. Concurrence of the theoretical inferences with the experimental observations is discussed in the context of the related literature.

Cantilever and pull-out tests and corresponding FEM models of various dowel bars in airport concrete pavement

Typically, one side of a dowel bar is coated to allow a dowel bar to move freely in a horizontal direction. However, the coating may induce a frictional force at the dowel–concrete interface and, therefore, a critical level of stress can develop in concrete surrounding the dowel bar at the joint that would result in premature cracking. The main objective of this study is to develop a method to reduce the friction at the concrete–dowel interface while maintaining the adequate level of load transfer efficiency. This study investigates the bonding forces between different types of dowel bar and concrete and explores a new type of dowel bar that would provide a sufficient separation between dowel bar and concrete while maintaining an acceptable level of load transfer efficiency. Both laboratory cantilever and pull-out tests were performed on various types of dowel bar embedded in concrete blocks. Finite element modeling of these tests was then performed to verify the laboratory test results. Based on the cantilever and pull-out test results and the finite element analyses of various types of dowel bar, it can be concluded that a carbon steel dowel bar wrapped with a polyethylene sheet would be effective in providing a sufficient level of load transfer efficiency while minimizing the stress at the joint of concrete pavements.

The origin of strain avalanches in sub-micron plasticity of fcc metals

Numerous experimental observations have recently demonstrated two aspects of sub-micron plasticity in nano pillars: (1) the statistical nature of the yield strength, and (2) the existence of dynamic temporal oscillations in the hardening regime. The present work is focused on the second aspect, where we study strain avalanches in nickel nano pillars via a series of 3-D dislocation dynamics (DD) simulations. The computer simulations reveal the existence of a key mechanism that explains the origin of oscillations in post-yield plastic flow in fcc metals. Small dipolar-loops easily form through several cross-slip mechanisms, and then act as internal glide dislocation sources. When dislocations exit pillar boundaries, they leave many dipolar-loops behind. At this stage, the pillar is starved from dislocations and an increase in stress is necessary to keep up with the applied strain rate (hardening stage). At some critical stress level, dipolar-loops become effective Frank–Read sources of new glide dislocations that collectively move in a burst mode, producing strain avalanches (softening stage). The time delay between these two states, which is determined by the average dislocation velocity and pillar diameter, leads to plastic strain oscillations and intermittent dislocation avalanches, consistent with experimental observations.

Modelling of martensite slip and twinning in NiTiHf shape memory alloys

High-temperature shape memory alloy NiTiHf holds considerable promise for structural applications. An important consideration for these advanced alloys is the determination of the magnitude of the twinning stress. Theoretical stresses for twinning and dislocation slip in NiTiHf martensites are determined. The slip and twinning planes are (0 0 1) and (0 1 1) for monoclinic and orthorhombic crystals, respectively. The determination of the slip and twinning stress is achieved with a proposed Peierls–Nabarro-based formulation informed with atomistic simulations. In the case of the twin, multiple dislocations comprising the twin nucleus are considered. The overall energy expression is minimized to obtain the twinning and slip stresses. The magnitude of the predicted twinning stresses is lower than slip stresses which explains why the NiTiHf alloys can undergo reversibility without plastic deformation. In fact, the predicted critical resolved shear stress levels of 433 MPa for slip and 236 MPa for twinning in the case of 12.5% Hf agree very well with the experimental measurements. The high slip resistance confirms that these materials can be very attractive in load-bearing applications.

Modeling Group Size and Scalar Stress by Logistic Regression from an Archaeological Perspective

Johnson’s scalar stress theory, describing the mechanics of (and the remedies to) the increase in in-group conflictuality that parallels the increase in groups’ size, provides scholars with a useful theoretical framework for the understanding of different aspects of the material culture of past communities (i.e., social organization, communal food consumption, ceramic style, architecture and settlement layout). Due to its relevance in archaeology and anthropology, the article aims at proposing a predictive model of critical level of scalar stress on the basis of community size. Drawing upon Johnson’s theory and on Dunbar’s findings on the cognitive constrains to human group size, a model is built by means of Logistic Regression on the basis of the data on colony fissioning among the Hutterites of North America. On the grounds of the theoretical framework sketched in the first part of the article, the absence or presence of colony fissioning is considered expression of not critical vs. critical level of scalar stress for the sake of the model building. The model, which is also tested against a sample of archaeological and ethnographic cases: a) confirms the existence of a significant relationship between critical scalar stress and group size, setting the issue on firmer statistical grounds; b) allows calculating the intercept and slope of the logistic regression model, which can be used in any time to estimate the probability that a community experienced a critical level of scalar stress; c) allows locating a critical scalar stress threshold at community size 127 (95% CI: 122–132), while the maximum probability of critical scale stress is predicted at size 158 (95% CI: 147–170). The model ultimately provides grounds to assess, for the sake of any further archaeological/anthropological interpretation, the probability that a group reached a hot spot of size development critical for its internal cohesion.

Fracture Resistance of a Zirconium Alloy with Reoriented Hydrides

Zirconium alloy cladding materials typically contain circumferential hydrides that may be reoriented to align along the radial direction when the cladding tubes are heated above and then cooled below the solvus temperature. The objectives of this study were to investigate the critical stress levels required to cause hydride reorientation (HRT) and to characterize the fracture resistance of Zircaloy-2 after hydride reorientation. HRT heat-treatment was performed on hydrogen-charged Zircaloy-2 specimens at 593 K (320 °C) or 623 K (350 °C) for 1 to 2 hours, followed by cooling to 473 K (200 °C). Fracture testing was conducted on hydride-reoriented three-point bend specimens at 473 K (200 °C) using an in situ loading stage inside a scanning electron microscope. Direct observations indicated that the reoriented hydrides, which ranged from ≈1 to 22 μm in lengths, were more prone to fracture at larger sizes (>10 μm) compared to smaller sizes (<0.5 μm). The reoriented hydrides reduced fracture resistance through a void nucleation, growth, and coalescence process at the crack tip. The resulting crack-resistance curves for Zircaloy-2 with reoriented hydrides decrease from 38 to 21 MPa(m)1/2 with increasing hydrogen contents from 51 to 1265 wt ppm hydrogen.

Malaysian Lesbian’s Stress Level

ABSTRACT This research focuses on Pengkid. It is a term or a slang used in Malaysia to refer to lesbians in the butch category. They are a group of individuals who are having non-traditional sexual orientation,in which they are sexually attracted to biologically-defined women. This research was conducted in order to focus on their sources and level of stress and a semi-structured interview schedule was administered with 25 pengkid participants from Pulau Pinang, Kuala Lumpur and Selangor. Participants were recruited through various methods including from Malaysian Pengkid official website and snowballing technique.Various challenges related to day-to-day life were reported by participants. However, financial problems, conflicts in work place or study place and conflictswith girlfriends have been identified as their main sources of stress. Since they are supporting families, girlfriends and themselves, they face financial problems almost every month. At the same time, having more expenses compared to their income also caused this problem. Findings show that Malaysian Pengkid does not face any kind of problems or stress because of their identity. Malaysia’s Pengkid seems to face the same problem that is faced by people with traditional sexual orientation and. They also do not have critical stress level and it is still manageable. Keywords: pengkid, lesbian, stress, life challenges

Development and Evaluation for Microdamage of Concrete under Uniaxial Compressive Load

Development of microcracks and microstructures under uniaxial compressive load were studied by the measurement of axial residual strain and the calculation of specific crack area. Two opposite mechanisms could be distinguished for describing the development of microstructures, i.e. densification and micro-crack formation. Based on the experimental results, consistent tendency was found for the development of inside microcracks and overall deformation. Higher load level induced larger residual strain and residual specific crack area after unloading. Significant difference in the development of cracks was found for concrete specimens of different proportions. Specimen with higher water to binder ratio and lower strength developed earlier corresponds to lower initial stress-strength ratio for the beginning of crack propagation, and lower critical stress level for the beginning of unstable development. Effect of uniaxial compressive load on the durability of concrete specimen depends on the relative relationship between the two mechanisms.

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ∼70nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [100] stock was subjected to shock compression generated by high-energy laser (∼350–850J), creating pressure pulses with initial duration of ∼3ns and amplitudes of up to ∼145GPa. The laser beam, with a spot radius of ∼1mm, created a crater of significant depth (∼135μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ∼24GPa for the monocrystalline to ∼150GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684J.

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

The majority of crustal faults host earthquakes when the ratio of average background shear stress τb to effective normal stress σeff is τb/σeff≈0.6. In contrast, mature plate‐boundary faults like the San Andreas Fault (SAF) operate at τb/σeff≈0.2. Dynamic weakening, the dramatic reduction in frictional resistance at coseismic slip velocities that is commonly observed in laboratory experiments, provides a leading explanation for low stress levels on mature faults. Strongly velocity‐weakening friction laws permit rupture propagation on flat faults above a critical stress level τpulse/σeff≈0.25. Provided that dynamic weakening is not restricted to mature faults, the higher stress levels on most faults are puzzling. In this work, we present a self‐consistent explanation for the relatively high stress levels on immature faults that is compatible with low coseismic frictional resistance, from dynamic weakening, for all faults. We appeal to differences in structural complexity with the premise that geometric irregularities introduce resistance to slip in addition to frictional resistance. This general idea is quantified for the special case of self‐similar fractal roughness of the fault surface. Natural faults have roughness characterized by amplitude‐to‐wavelength ratios α between 10−3 and 10−2. Through a second‐order boundary perturbation analysis of quasi‐static frictionless sliding across a band‐limited self‐similar interface in an ideally elastic solid, we demonstrate that roughness induces an additional shear resistance to slip, or roughness drag, given by τdrag=8π3α2G∗Δ/λmin, for G∗=G/(1−ν) with shear modulus Gand Poisson's ratio ν, slip Δ, and minimum roughness wavelength λmin. The influence of roughness drag on fault mechanics is verified through an extensive set of dynamic rupture simulations of earthquakes on strongly rate‐weakening fractal faults with elastic‐plastic off‐fault response. The simulations suggest that fault rupture, in the form of self‐healing slip pulses, becomes probable above a background stress level τb≈τpulse+τdrag. For the smoothest faults (α∼10−3), τdrag is negligible compared to frictional resistance, so that τb≈τpulse≈0.25σeff. However, on rougher faults (α∼10−2), roughness drag can exceed frictional resistance. We expect that τdrag ultimately departs from the predicted scaling when roughness‐induced stress perturbations activate pervasive off‐fault inelastic deformation, such that background stress saturates at a limit (τb≈0.6σeff) determined by the finite strength of the off‐fault material. We speculate that this strength, and not the much smaller dynamically weakened frictional strength, determines the stress levels at which the majority of faults operate.

The effect of ageing of the [011]-oriented Co35Ni35Al30 single crystals in free state and under loading on their functional properties

Using Co35Ni35Al30 (at.%)single crystals oriented along the [011]-axis, the influence of ageing at 673 K for 0.5 h in free state and under loading in compression on the temperature dependence of critical stresses of formation of L10-martensite, yield stress of the В2-phase, temperature interval of superelasticity, and stress hysteresis is investigated. Ageing of these single crystals in free state and under loading is found to result in strengthening of the high-temperature В2-phase, widening of the temperature interval within which superelasticity is observed, and a nearly threefold decrease in the width of the stress hysteresis compared to the quenched state. It is shown for the first time that stress-assisted aged [011]-oriented Co35Ni35Al30 single crystals possess lower critical stress levels of stress-induced martensitic transformation at T < 473 K, a higher yield stress of the austenite phase and a higher starting temperature of the B2–L10 martensitic transformations upon cooling by ∆Ms ~ 40 K than those aged in free state.

Relationship between Crack Extension Behavior and Fatigue Life of C/C Composites

The fatigue and fracture mechanism of C/C composites material was investigated in notched and smooth specimen. Also, the initiation and growth behaviors of cracks were continuously observed. Specially, the phenomenon of fracture constraint in notched specimen was discussed. This phenomenon was related to the crack growth behavior and shear damage in the matrix. In the present study, fatigue limit was defined by the critical stress level that the specimen was not broken even after applied 1x106 stress cycles. When the fatigue life was longer than 1x106 stress cycles, the initiated crack was stopped to growth after reaching some critical length. Also, the density of crack initiation was low. When the fatigue life became shorter than that, the cracks extended rapidly and coalescences of cracks were observed. The crack growth behavior was strongly related to the fatigue limit and unstable fracture conditions. Also it is found that the compliance or elastic modulus of the specimen was related to residual fatigue life. Now, an observation of crack in C/C composites was performed effectively in the present study.

Surficial sediment stability on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals

Surficial sediment stability was estimated on Georges Bank, in the Great South Channel and on eastern Nantucket Shoals (36,699km2) by determining where benthic shear stresses derived from an ocean model matched or exceeded the critical shear stress of the observed surficial sediments. The shear stress resulting from M2 and S2 semi-diurnal tides was estimated with the Finite-Volume Community Ocean Model. Mixed-sediment critical shear stress levels were calculated for sediment compositions ranging from sand to boulder-dominated using 67,400 underwater video quadrats sampled from 1999 to 2010. Stresses matched or exceeded the sediment critical levels in 16,926km2 (46%) of the study area, and were inversely related to water depth (r2=69.1%). In depths >50m (10,953km2) all sediments were stable due to weak flow (≤0.4Nm−2). In the shallower higher flow areas (>0.4Nm−2, 25,716km2) only sediments containing gravel remained stable. The largest stresses occurred on Nantucket Shoals and central and northeastern Georges Bank (≥2Nm−2); in these areas only sand with cobbles or sediments dominated by gravel remained stable. Outcrops of these stable sediments were surrounded by highly unstable areas with stresses 2 to 9 times higher than the sediment critical levels. This analysis identifies the locations which likely remain stable even under the high shear stresses typical of Georges Bank, the Great South Channel and eastern Nantucket Shoals. Further, we provide the map products needed to begin investigating the influences of natural sediment disturbance on the spatial and temporal patterns of the benthos including the resilience of stable versus unstable areas to anthropogenic disturbances.

A MLS-BASED LATTICE SPRING MODEL FOR SIMULATING ELASTICITY OF MATERIALS

A MLS-based lattice spring model is presented for numerical modeling of elasticity of materials. In the model, shear springs between particles are introduced in addition to normal springs. However, the unknowns contain only particle displacements but no particle rotations. The novelty of the model lies in that the deformations of shear springs are computed by using the local strain obtained by the moving least squares (MLS) approximation rather than using the particle displacements directly. By doing so, the proposed lattice spring model can represent the diversity of Poisson's ratio without violating the requirement of rotational invariance. Relationships between micro spring parameters and macro material constants are derived from the Cauchy-born rules and the hyperelastic theory. Numerical examples show that the proposed model is able to reproduce elastic solutions obtained by finite element methods for problems without fractures. Therefore, it is capable of simulating solid materials which are initially continuous, but eventually fracture when critical stress and/or displacement levels are reached. A demonstrating example is presented.

Numerical-Experimental Comparison in the Simulation of Rotor/Stator Interaction Through Blade-Tip/Abradable Coating Contact

Higher aircraft energy efficiency may be achieved by minimizing the clearance between the rotating blade tips and respective surrounding casing. A common technical solution consists in the implementation of an abradable liner which improves both the operational safety and the efficiency of modern turbomachines. However, unexpected abradable wear removal mechanisms were recently observed in experimental set-ups as well as during maintenance procedures. Based on a numerical strategy previously developed, the present study introduces a numerical-experimental comparison of such occurrence. Attention is first paid to the review and analysis of existing experimental results. Good agreement with numerical predictions is then illustrated in terms of critical stress levels within the blade as well as final wear profiles of the abradable liner. Numerical results suggest an alteration of the abradable mechanical properties in order to explain the outbreak of a divergent interaction. New blade designs are also explored in this respect and it is found that the interaction phenomenon is highly sensitive to (1) the blade geometry, (2) the abradable material properties, and (3) the distortion of the casing.

The design and testing of subscale smart aircraft wing bolts

Presently costly periodic inspection is vital in guaranteeing the structural integrity of aircraft. This investigation assesses the potential for significantly reducing aircraft maintenance costs without modification of aircraft structures by implementing smart wing bolts, manufactured from TRIP steel, which can be monitored for damage insitu. TRIP steels undergo a transformation from paramagnetic austenite to ferromagnetic martensite during deformation. Subscale smart aircraft wing bolts were manufactured from hot rolled TRIP steel. These wing bolts were used to demonstrate that washers incorporating embedded inductance coils can be utilized to measure the martensitic transformation occurring in the TRIP steel during bolt deformation. Early in situ warning of a critical bolt stress level was thereby facilitated, potentially reducing the costly requirement for periodic wing bolt removal and inspection. The hot rolled TRIP steels that were utilized in these subscale bolts do not however exhibit the mechanical properties required of wing bolt material. Thus warm rolled TRIP steel alloys were also investigated. The mechanical properties of the best warm rolled TRIP steel alloy tested almost matched those of AISI 4340. The warm rolled alloys were also shown to exhibit transformation before yield, allowing for earlier warning when overload occurs. Further work will be required relating to fatigue crack detection, environmental temperature fluctuation and more thorough material characterization. However, present results show that in situ early detection of wing bolt overload is feasible via the use of high alloy warm rolled TRIP steel wing bolts in combination with inductive sensor embedded washers.