Reverse Stress Research Articles

Reliability Evaluation Procedure of Electromechanical Properties in GdBCO CC Tapes Obtained by Uniaxial Tension and Fatigue Tests at 77 K

In the design of 2G superconducting magnets and coils, both the yield strength and the irreversible stress limit for critical current ( I c ) degradation of coated conductor (CC) tapes under various environmental conditions have been investigated. Equally important and yet often discounted are the reliability tests to evaluate the I c degradation induced by the fatigue loading expected during operation. Because I c degradation at the stress level below the reversible stress limit of the CC tapes is possible (e.g., under a static fatigue load), in this study, the procedures for evaluating the reliability of 2G CC tapes using characteristic limit values obtained through a series of electromechanical properties evaluations are introduced. I c behaviors were obtained at 77 K through specified loading against the elapsed time in a static fatigue test, while electromechanical fatigue limits of CC tapes were determined at a stress ratio (R) of 0.1 in a high-cycle fatigue test. The determination of electromechanical properties through uniaxial tensile and fatigue tests was established, and its long-term reliability was examined.

Nonlinear Dynamics of a Magnetically Supported Flexible Rotor in Auxiliary Bearings

The results of a numerical simulation on the nonlinear dynamics of a flexible rotor mounted on magnetic and auxiliary bearings are presented in this work. The focus of this work was to investigate the effect of shaft flexibility on the rotor response during load sharing operation between the magnetic and auxiliary bearings. For the range of parameters considered herein, results from the numerical simulation showed that the stiffness ratio, which represented the contact stiffness of the auxiliary bearing, was a more effective parameter, as compared to the Coulomb sliding friction coefficient, for the suppression of sub-synchronous and non-synchronous vibrations in the rotor’s response. Sub-synchronous vibrations of period-2, period-4, period-8 and period-16 were observed in the response of the rotor. Non-synchronous vibration which was also seen in the rotor’s response was determined to be chaos. For the case of the rotor response with variation of the stiffness ratio, the route to chaos was found to be due to a sequence of period-doubling bifurcations, where the synchronous or period-1 response bifurcated into period-2, period-4, period-8 and period-16 responses, culminating into chaotic vibration. Vibrations which are of sub-synchronous and non-synchronous nature are best avoided in the rotor’s response during operation, whereby the load is shared between magnetic and auxiliary bearings, as they produce stress reversals. Such stresses can potentially cause fatigue failure of the rotors and their associated structures.

Context and Timing Matter: Language Brokering, Stress, and Physical Health.

Using a bioecological perspective, the current study explored the dynamic relationship between a proximal process (i.e., language brokering [LB]), LB perceptions, environmental stress context, and timing of LB experiences on well-being. College students (N = 559; 19.6% Asian American, 32.0% European American, 33.5% Latino, and 14.9% multiracial/other ethnicity) reported on LB frequency, feelings about LB as a burden or source of role reversal, perceived stress, age of LB onset, and health (i.e., height, weight, somatic symptoms). Among brokers (M = 23.13, SD = 5.66; 78.3% female), younger LB age onset was significantly associated with higher BMI when perceived stress was high but unrelated when perceived stress was low. For individuals who reported high perceived stress or high LB role reversal, but not both, younger LB age onset was associated with greater somatic symptoms. These results highlight the importance of psychosocial context and timing of life events in capturing the effect of immigrant and family experiences on physical health.

Effects of initial static shear stress orientation on cyclic behavior of calcareous sand

The current study investigated the effect of the initial static shear stress orientation on the liquefaction behavior of Hormuz Island calcareous sand. An experimental study was performed using nine monotonic and 43 cyclic undrained bidirectional simple shear tests. The experiments were conducted on loose and dense samples having different cyclic shear ratios and initial static shear ratios (α) at two orientations. It was found that, as the value of α increased, the shear behavior and failure mechanisms changed. The results showed that calcareous sand responded differently to changes in α and in the symmetrical tests (α = 0) showed higher resistance to liquefaction than siliceous sand. Contrary to siliceous sand, as α increased to 0.1, the value of K α increased to greater than 1 for loose Hormuz sand specimens. Moreover, an increase in α from 0.1 to 0.3 caused a slight increase (less than 10%) in the K α value for dense specimens. It was also observed that liquefaction resistance in the perpendicular loading mode was higher than in the parallel mode under the same conditions. It would appear that reversal of shear stress plays an important role in cyclic resistance and behavior in the parallel and perpendicular loading modes. A model is proposed to predict liquefaction resistance for calcareous sand in the presence of α.

Analytical modelling of coal failure in coal seam gas reservoirs in different stress regimes

Coal seam gas (CSG) reservoirs typically have low rock strength. During gas production, pressure depletion and matrix shrinkage may cause the differential stress around the wellbore to exceed the coal mechanical strength and result in rock failure. Coal failure has several detrimental consequences including coal fines production, permeability reduction, wellbore filling and damage to pumps and compressors. The matrix shrinkage causes a unique stress path in CSG reservoirs. However, the details of how matrix shrinkage affects coal failure still have remained uncertain. This paper presents a new workflow to evaluate stress distribution around CSG wells and predicts coal failure by coupling the effects of pressure depletion, matrix shrinkage and wellbore simultaneously. The model calculates Maximum Coal Free Drawdown Pressure (MCFDP) by considering the effects of all contributing parameters and Mogi-Coulomb failure criterion. Data from a vertical well in the San Juan Basin in USA were used to evaluate the validity of the developed model. The developed model was applied to evaluate coal failure under three different stress regimes. The results indicate that pressure depletion and matrix shrinkage have a significant effect on coal failure in all stress regimes. In the case of a normal stress regime, it is found that vertical wellbores are the most stable during the life of a reservoir. However, in the case of strike-slip and reverse stress regimes, pressure depletion and matrix shrinkage could cause the change of stress regime and therefore, the optimum wellbore trajectory could change. Additionally, it is found that in the normal stress regime the depletion and matrix shrinkage reduces the MCFDP of horizontal wellbores more than the vertical wells. However, in the reverse stress regime, depletion and shrinkage cause more reduction of MCFDP in vertical wellbores compared to horizontal wells.

Highly Enhanced Inductive Current Sustaining Capability and Avalanche Ruggedness in GaN p-i-n Diodes With Shallow Bevel Termination

In this work, small-angle beveled-mesa termination technique was developed in GaN p-i-n power diodes for high avalanche performances. With effective alleviation of electric field crowding effect at the junction sidewall, near uniform distribution of avalanche breakdown was realized. Resultantly, state-of-the-art inductive avalanche current density of ~31.5 kA/cm2 and average parallel-plane breakdown electric field of 2.86 MV/cm was achieved in the terminated diodes. Meanwhile, ruggedness of the avalanche breakdown has also been evidently promoted, under both repetitive dc reverse breakdown stresses and unclamped inductive switch conditions.

The role of bending stress on the initiation of reverse transverse defects

This paper investigated the role of longitudinal reverse bending stress on the initiation of reverse transverse defects. The longitudinal reverse bending stress occurs due to the reverse bending of the rail between two-wheel passage leading to the generation of tensile bending stress at the railhead and the lower head areas. The longitudinal bending stress was investigated as part of a parametric study on the rail cant angle, rail stiffness, lateral-to-vertical load ratio, and rail profile. A finite element model was created by using ABAQUS to analyze the extent of reverse bending in rails with respect to the chosen set of parameters. Under different lateral-to-vertical load ratios of 0, 0.3, 0.5, and 0.7, the maximum stress at the rail lower gauge corner was found to vary between 14.57MPa and 15.47MPa under the reverse bending condition. Similarly, low values of tensile stress under the reverse bending scenario were observed with changes in the rail cant angle and axle spacing with respect to different coal and iron ore wagons. The results revealed that the magnitude of the bending stress under different conditions of reverse bending was not significant enough to initiate a crack at the lower gauge corner.

Degradation Behavior and Mechanisms of E-Mode GaN HEMTs With p-GaN Gate Under Reverse Electrostatic Discharge Stress

The degradation behavior and its mechanisms of E-mode GaN high electron mobility transistors (HEMTs) with p-GaN gate under electrostatic discharge (ESD) stress were investigated. Reverse short-pulse stress was generated by a transmission line pulse (TLP) tester in order to simulate the static electricity. The experiment results show that the reverse short-pulse stress leads to the characteristic degradation of the E-mode GaN HEMTs with p-GaN gate. The values of the threshold voltage and ON-resistance increase, and the gate capacitance curve shifts positively. The low-frequency noises (LFNs) were obtained for the E-mode GaN HEMTs with p-GaN gate before and after the reverse short-pulse stress. The concentration of traps was extracted, and it has doubled after 700 cycles. The degradation mechanism could be attributed to the generation of traps at p-GaN/AlGaN heterointerface, AlGaN barrier, and GaN/AlGaN interface. Such an investigation can be a significant reference in the design and application of E-mode GaN power devices.

Changing the critical snap-through loads of post-buckled beams using piezoelectric actuation

Post-buckled and curved structures experience snap-through instabilities when external loads from mechanical, fluid, or thermal environments result in a loss of local stability and a dramatic jump to a remote stable equilibrium. Fatigue caused by snap-through is a concern in many engineered systems because of the large stress reversals involved. This paper studies the extent to which the strategic placement and actuation of piezoelectric materials bonded to clamped-clamped post-buckled beams can influence the loads at which snap-through occurs. The electromechanical system is modeled using elastica theory with extensions to account for the influence of piezoelectric actuation on the structure. Static equilibrium positions and their stability are computed across a large configuration space using numerical integration and a shooting method. The results indicate that the effect of piezoelectric actuation on critical snap-through load depends on the degree to which the beam is buckled, the location of the external load, the placement of the piezoelectric material, and the applied actuation voltage. Experiments are performed to validate the numerical results and provide a physical demonstration of changing snap-through loads with piezoelectric actuation. Experimental results demonstrate that critical snap-through loads can be altered by factors ranging from 0.4 to 2.0, and numerical results indicate that even larger changes to snap-through loads are physically realizable.

Cyber Risk and the U.S. Financial System: A Pre-Mortem Analysis

We model how a cyber attack may be amplified through the U.S. financial system, focusing on the wholesale payments network. We estimate that the impairment of any of the five most active U.S. banks will result in significant spillovers to other banks, with 38 percent of the network affected on average. The impact varies and can be larger on particular days and geographies. When banks respond to uncertainty by liquidity hoarding, the potential impact in forgone payment activity is dramatic, reaching more than 2.5 times daily GDP. In a reverse stress test, interruptions originating from banks with less than $10 billion in assets are sufficient to impair a significant amount of the system. Additional risk emerges from third party providers, which connect otherwise unrelated banks, and from financial market utilities.

Understanding the role of threading dislocations on 4H-SiC MOSFET breakdown under high temperature reverse bias stress

The origin of dielectric breakdown was studied on 4H-SiC MOSFETs that failed after three months of high temperature reverse bias stress. A local inspection of the failed devices demonstrated the presence of a threading dislocation (TD) at the breakdown location. The nanoscale origin of the dielectric breakdown was highlighted with advanced high-spatial-resolution scanning probe microscopy (SPM) techniques. In particular, SPM revealed the conductive nature of the TD and a local increase of the minority carrier concentration close to the defect. Numerical simulations estimated a hole concentration 13 orders of magnitude larger than in the ideal 4H-SiC crystal. The hole injection in specific regions of the device explained the failure of the gate oxide under stress. In this way, the key role of the TD in the dielectric breakdown of 4H-SiC MOSFET was unambiguously demonstrated.

Reversal of age-associated oxidative stress in mice by PFT, a novel kefir product.

Introduction:Oxidative stress is a key contributor to aging and age-related diseases. In the present study, we examine the protective effects of PFT, a novel kefir product, against age-associated oxidative stress using aged (10-month-old) mice.Methods:Mice were treated with PFT orally at a daily dose of 2 mg/kg body weight over 6 weeks, and antioxidant status, protein oxidation, and lipid peroxidation were studied in the brain, liver, and blood.Results:PFT supplementation significantly reduced the oxidative stress biomarkers malondialdehyde (MDA) and nitric oxide; reversed the reductions in glutathione (GSH) levels, total antioxidant capacity (TAC), and anti-hydroxyl radical (AHR) content; enhanced the antioxidant enzyme activities of glutathione peroxidase (GPx), catalase (CAT), and superoxide dismutase (SOD); inhibited the liver enzyme levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT); significantly reduced triglyceride (TG), total cholesterol (TC), and low density lipoprotein (LDL) levels; and significantly elevated high density lipoprotein (HDL) levels. Interestingly, PFT supplementation reversed the oxidative changes associated with aging, thus bringing levels to within the limits of the young control mice in the brain, liver, and blood. We also note that PFT affects the redox homeostasis of young mice and that it is corrected post-treatment with PFT.Conclusion:Our findings show the effectiveness of dietary PFT supplementation in modulating age-associated oxidative stress in mice and motivate further studies of PFT’s effects in reducing age-associated disorders where free radicals and oxidative stress are the major cause.

Exercise alters cerebellar and cortical activity related to working memory in phenotypes of Gulf War Illness.

Gulf War Illness affects 25–32% of veterans from the 1990–91 Persian Gulf War. Post-exertional malaise with cognitive dysfunction, pain and fatigue following physical and/or mental effort is a defining feature of Gulf War Illness. We modelled post-exertional malaise by assessing changes in functional magnetic resonance imaging at 3T during an N-Back working memory task performed prior to a submaximal bicycle stress test and after an identical stress test 24 h later. Serial trends in postural changes in heart rate between supine and standing defined three subgroups of veterans with Gulf War Illness: Postural Orthostatic Tachycardia Syndrome (GWI-POTS, 15%, n = 11), Stress Test Associated Reversible Tachycardia (GWI-START, 31%, n = 23) and Stress Test Originated Phantom Perception (GWI-STOPP, no postural tachycardia, 54%, n = 46). Before exercise, there were no differences in blood oxygenation level-dependent activity during the N-Back task between control (n = 31), GWI-START, GWI-STOPP and GWI-POTS subgroups. Exercise had no effects on blood oxygenation level-dependent activation in controls. GWI-START had post-exertional deactivation of cerebellar dentate nucleus and vermis regions associated with working memory. GWI-STOPP had significant activation of the anterior supplementary motor area that may be a component of the anterior salience network. There was a trend for deactivation of the vermis in GWI-POTS after exercise. These patterns of cognitive dysfunction were apparent in Gulf War Illness only after the exercise stressor. Mechanisms linking the autonomic dysfunction of Stress Test Associated Reversible Tachycardia and Postural Orthostatic Tachycardia Syndrome to cerebellar activation, and Stress Test Originated Phantom Perception to cortical sensorimotor alterations, remain unclear but may open new opportunities for understanding, diagnosing and treating Gulf War Illness.

Implementation of Control Strategies for Optimum Utilization of Solar Photovoltaic System with Energy Storage Systems

Solar Photovoltaic (SPV) system primarily based on autonomous system have advanced as a promising solution to the problem of electrification in areas where the network is now not available. The main challenges in designing such systems are as follows: 1) extraction of maximum power from PV system under fast varying irradiance; 2) extraction of high voltage gain with DC-DC converter; 3) development of efficient power management scheme between SPV and Energy Storage System (ESS). As multiple objectives must be satisfied, existing schemes for autonomous systems require a minimum of three conversion steps, which leads to significant reduction in the reliability and efficiency of the system. The above issues are addressed with different control strategies. For extraction of maximum power a modified non iterative Incremental Conductance MPPT method is developed to generate a fine tuned duty cycle for sudden change in irradiance and regulates that always the intersection point of load line and I-V curve represents Maximum power point (MPP). High voltage gain DC-DC converter generally required for industrial applications and can be achieved by high duty ratio which cause reverse recovery problem and voltage stress on power switches. To overcome this problem a soft switch interleaved boost converter (SSIBC) is proposed which is operate on zero current switching to turn ON active switches and it can achieve high voltage gain for lower duty ratios. Lead acid battery is an energy storage device which is used in SPV systems. Finally to achieve power management, variable DC bus voltage based algorithm is developed with super capacitor (SC) and battery (BAT) as ESS. The capability of the proposed control strategies is tested and validated under various scenarios

How Severe Are the EBA Macroeconomic Scenarios for the Italian Economy? A Joint Probability Approach

Measures of the severity of macroeconomic scenarios have been widely used in the literature, but a consistent methodology for their calculation has not been developed yet. Against this background, we provide a general method for calculating the joint probability of observing a macroeconomic scenario, which can be applied to a wide variety of structural models. By doing so, we can attach probabilities to scenarios produced with multidimensional economic models to compare their severity and plausibility. We apply our methodology to the 2016 and 2018 EBA stress test scenarios and we provide also reverse stress tests applications. Our results show that for the Italian economy both the 2016 and 2018 EBA scenarios are unlikely, especially the 2016 one. The reverse stress tests allow us to identify the key variables that affect our probabilities.

1177 The additive prognostic value of end-systolic pressure-volume relation by CMR in patients with with known or suspected coronary artery disease

Abstract Background The variation between rest and peak stress end-systolic pressure-volume relation is an afterload-independent index of left ventricular contractility. This index is easily obtained during routine stress echocardiography but can be derived also during a stress cardiovascular magnetic resonance (CMR) exam, that is the gold standard for the quantification of biventricular volumes. Purpose The aim of this study was to assess for the first time the prognostic value ofdelta rest-stress ESPVR (DESPVR) by dipyridamole stress-CMR in patients with known or suspected coronary artery disease (CAD). Methods One hundred and sixty-six consecutive patients (37 females, main age 61.96 ± 10.05 years) who underwent dipyridamole stress-CMR in a high volume CMR Laboratory were considered. Abnormal wall motion and perfusion at rest and after dipyridamole were analysed. Macroscopic myocardial fibrosis was detected by the late gadolinium enhancement (LGE) technique. The ESPVR was evaluated at rest and peak stress from raw measurement of systolic arterial pressure by cuff sphygmomanometer and end-systolic volume by biplane Simpson method. Results An abnormal stress CMR was found in 39 (23.5%) patients; 24 patients had a reversible stress perfusion defect in at least one myocardial segment and 15 a reversible stress perfusion defect plus worsening of stress wall motion in comparison with rest. Myocardial fibrosis was detected in 69 patients (41.6%). A DESPVR < 0.009 was detected in 74 patients (44.6%). During a median follow up of 55.51 months (IQ range 33.20 months), 54 patients (32.5%) experienced major cardiac events: 5 deaths, 2 ventricular arrhythmias, 18 coronary syndromes, and 29 heart failure hospitalization.Reversible perfusion deficit, DESPVR < 0.009, diabetes and family history were significant univariate prognosticators. In the multivariate analysisthe independent predictive factors were reversible perfusion deficit (hazard ratio-HR = 2.17, P = 0.010), DESPVR < 0.009 (HR = 1.92, P = 0.028) and diabetes (HR = 2.42, P = 0.004). The Kaplan–Meier curve for DESPVR is shown in Figure 1. The log-rank test revealed a significant difference (P = 0.003). Conclusions DESPRV assessed by CMR provides a prognostic stratification in patients with known or suspected coronary artery disease, in addition to that supplied by reversible perfusion deficit and diabetes. Abstract 1177 Figure 1

Study on liquefaction mechanism of saturated sand considering stress redistribution

Most of the saturated soil in actual condition is anisotropically consolidated, and can liquefy during minor earthquake. Based on the analysis of stress state in actual soil, the present paper considers that the stress redistributed during loading process, which result in the liquefaction. However, in present conventional element tests, the liquefaction in anisotropically consolidated saturated sands (ACS) cannot be induced under dynamic cyclic loading without stress reversal. It contributes to the uncertainty about the liquefaction resistance of soil and the development model of excess pore water pressure (PWP). A brief theoretical interpretation within the theory of limit state of soils is given to explain why the liquefaction in ACS could not be induced using the conventional element test under dynamic cyclic load with low or moderate amplitude, and the validity of the theoretical analysis is verified by experiments. The stress redistribution is realized by confining the lateral deformation based on the conventional test, and the liquefaction in ACS under minor cyclic load is realized. Furthermore, a new concept called “continued liquefaction” is proposed. Apart from “transient liquefaction”, the PWP, the total confining pressure and the total vertical stress are always at the same level, and the mean effective stress of sands is always zero in the “continued liquefaction” stage. Finally, a suggestion that the development model of PWP should be established on the basis of the ratio of the PWP to the initial vertical stress is recommended.

Electrochemical Mechanics of Metal Thin Films: Charge‐Induced Reversible Surface Stress for Actuation

AbstractThe intrinsic charge‐induced surface stress of Ni thin films during electrochemical reactions with an alkaline electrolyte is measured in situ. Surface stresses induced by H absorption/desorption, α‐Ni(OH)2 formation, capacitive double‐layer charging, the α‐ to β‐Ni(OH)2 transformation, and β‐Ni(OH)2/β‐NiOOH redox reactions are identified, and each provided additive contributions to the overall stress state. Surface stresses are magnified in high‐surface‐area nanoporous Ni because local stress‐relaxation mechanisms are restricted when compared to a smooth Ni film. Ni film reversible tensile/compressive surface stresses correlate with anodic/cathodic potential scanning but with an opposite trend to that of a less reactive Au film. Surface stresses in the Ni films are up to 40 times that of Au films and suggest the possibility of using controlled surface‐stress generation for electrochemical actuation.

High strength and superior corrosion resistance of the Ti-Ni-Cu-Zr crystal/glassy alloys with superelasticity

TixNixCu50−xZr50−x (x = 38, 40, 42, 44, at%) crystal-glassy composite alloys with diameter of 2 mm have been prepared by using water-cooled copper mold casting method. The plasticity, strength, and electrochemical corrosion properties of the alloys were investigated. All the Ti-Ni-Cu-Zr alloys showed high strength and excellent plasticity. Specifically, the compressive strength and total strain of the Ti44Ni44Cu6Zr6 alloy were 2750 MPa and 27%, respectively. The superelastic behavior can be attributed to the existence of reversible stress, which induces martensitic transformation. In addition, the Ti-Ni-Cu-Zr amorphous matrix composites exhibited excellent anti-corrosion property in 3.5% NaCl, 0.5 mol/L H2SO4 and 1 mol/L NaOH aqueous solution.

Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles

Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles