Entire Stress Research Articles

English

The effects of calcium on respiratory and nitrogen metabolism of apple roots (Malus baccata Borkh.) exposed to temperature stress (5°C ~ 20°C ~ 0°C) were investigated. Seedlings were treated with distilled water (control), calcium chloride (CaCl2) or calmodulin antagonist trifluoperazine (TFP) before temperature stress. Temperature was increased from 5°C to 20°C (1°C h -1 ) and then decreased to 0°C (1°C h -1 ). Temperature stress decreased root vitality and increased root malondialdehyde (MDA) concentration, the effect of which was exacerbated by TFP treatment. Treatment with CaCl2 improved root vitality and decreased root MDA concentration. At 20°C, exogenous CaCl2 alleviated the negative effects of temperature stress on the total respiration rate by enhancing the activity of tricarboxylic acid cycle (TCA). Activities of key enzyme in nitrogen metabolism were strongly inhibited by temperature stress. Exogenous CaCl2 significantly increased key enzyme activities of nitrogen metabolism compared to the control. However, the TFP treatment markedly reduced the activity of glutamate synthase (GOGAT) at 20°C and noticeably inhibited glutamate dehydrogenase (GDH) activity during the entire temperature stress period. The data showed that the Ca 2+ -calmodulin (Ca 2+ -CaM) signal system was involved in increase of GOGAT and GDH activity that occurred with an increase in temperature, and played a role in the increase in the total respiration rate and GDH activity which occurred with a decrease in temperature. Cultural practices that improve plant calcium (Ca) status in the early spring may mitigate damage induced by temperature stress. © 2016 Friends Science Publishers

Comparison of the current reasons for undergoing pharmacologic stress during echocardiographic and radionuclide stress testing

BackgroundSymptom-limited exercise is the preferred method of cardiac stress testing, but pharmacologic testing has been increasing over time. The exact reasons for pharmacologic stress testing have not been rigorously categorized. Thus, we systematically explored the reasons for pharmacologic stress testing in patients referred for cardiac stress imaging. MethodsWe studied consecutive patients referred for stress imaging [stress echocardiography or radionuclide myocardial perfusion imaging (MPI)] at Mount Sinai St Luke’s hospital between August 2013 and April 2014. Baseline information was obtained using a standardized questionnaire and a trained physician triaged the patient for symptom-limited exercise stress testing or pharmacologic stress testing. ResultsIn total, 551(48%) of our entire stress cohort underwent cardiac imaging following initial exercise testing and 589 (52%) underwent imaging with initial pharmacologic stress testing. Deconditioning and inability to walk (primarily due to musculoskeletal conditions) constituted the top two reasons for performing pharmacologic stress, followed by frailty, left bundle branch block (for MPI), resting wall motion abnormality (for echocardiography), and failed exercise attempts. The reasons for performing pharmacologic stress testing were similar in the MPI and echocardiography patients, despite a much higher level of disease acuity in the MPI group. ConclusionsWe have applied a systematic approach for categorizing the reasons for pharmacologic stress. These reasons are heterogeneous, but similar across MPI and echo stress laboratories.

Dynamic Gate Stress-Induced $V_{\text {TH}}$ Shift and Its Impact on Dynamic $R_{\mathrm {ON}}$ in GaN MIS-HEMTs

Very fast transients of $V_{ {\text {TH}}}$ shift and their impact on $R_{\mathrm{\scriptscriptstyle ON}}$ under dynamic AC (1 k–1 MHz) positive gate stress in depletion-mode (D-mode) metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) are revealed. We achieve data acquisition within 120 ns right after each stress pulse throughout the entire stress time range from $10^{ { {-7}}}$ up to $10^{ { {3}}}$ s, by virtue of a short stress-to-sense delay of $\sim 100$ ns and high sampling rate up to 50 MSa/s. Despite the considerable $V_{\mathrm {\mathbf {TH}}}$ shift, its impact on $R_{\mathrm{\scriptscriptstyle ON}}$ in D-mode MIS-HEMT is modest, if the device is under sufficient gate overdrive. Furthermore, $V_{\mathrm {\mathbf {TH}}}$ shift and the consequent $R_{\mathrm{\scriptscriptstyle ON}}$ increase under dynamic stress, which are more relevant to high-frequency switching operation, exhibits frequency dependence within 1 k–1 MHz and is always smaller than that under conventionally used static (constant) stress.

High-cycle fatigue of notched plain concrete

This paper investigates the accuracy of the so-called Theory of Critical Distances (TCD) in modelling, in the high-cycle fatigue regime, the behaviour of notched plain concrete. The TCD postulates that the fatigue damage extent has to be estimated by directly post-processing the entire linear-elastic stress field damaging the material in the vicinity of the crack initiation locations. According to the TCD’s modus operandi, the high-cycle fatigue assessment is performed by using a scale length parameter which is treated as a material property. The accuracy of this method was checked against a number of experimental results generated by testing, under four-point bending, notched specimens of plain concrete. This validation exercise allowed us to prove that the TCD is successful in estimating the high-cycle fatigue strength of notched concrete beams, resulting in predictions falling within an error interval of about ±15%.

The Theory of Critical Distances to estimate static and dynamic strength of notched plain concrete

The Theory of Critical Distances (TCD) is a well-known design method allowing the strength of notched/cracked components to be estimated accurately by directly post-processing the entire linear-elastic stress fields damaging the material in the vicinity of the stress concentrators being designed. By taking full advantage of the TCD’s unique features, in the present study this powerful theory was reformulated to make it suitable for designing notched plain concrete against static and dynamic loading. The accuracy and reliability of the proposed reformulation of the TCD was checked against a set of experimental results generated by testing, under different displacement rates, square section beams of plain concrete containing notches of different sharpness. This validation exercise has demonstrated that the proposed reformulation of the TCD is capable of accurately assessing the static and dynamic strength of notched unreinforced concrete, with the estimates falling within an error interval of ±20%. The level of accuracy that was obtained is certainly satisfactory, especially in light of the fact that static and dynamic strength was estimated without explicitly modelling the stress vs. strain dynamic behaviour of the concrete being tested.

Experimental determination of fatigue behavior of lead free solder joints in microelectronic packaging subjected to isothermal aging

The effects of aging on the cyclic shear stress–strain and fatigue behavior of lead-free solders have been explored experimentally and have been presented in this paper. An experimental procedure has been developed for preparing Iosipescu shear specimens of SAC105 (Sn–1.0Ag–0.5Cu) lead-free solder, and the resulting solder joint specimens have been subjected to cyclic shear stress/strain loading at different aging conditions. A combination of four-parameter hyperbolic tangent empirical models has been used for the empirical fit of the entire cyclic stress strain curve. The fatigue life data were then fit using popular empirical failure criteria such as the strain-based Coffin–Manson model and the energy-based Morrow model. Evolution of shear hysteresis loop of SAC 105 with aging has been studied. Degradation of isothermal fatigue life due to aging has also been studied in this paper. A comparison between uniaxial fatigue data and shear fatigue data is shown and a good qualitative agreement has been found. Subsequent microstructure analysis has also been presented in the paper in support of isothermal aging effects.

Identification of post-necking stress–strain curve for sheet metals by inverse method

Simulation of sheet metal forming requires the flow curve (i.e. the true stress–strain relation) over large strains. This paper presents a method for obtaining the flow curve of sheet metals over a large range of strain through the combination of simple tensile test and finite element analyses. The appropriate finite element model for accurate simulation of the anisotropic plastic deformation during diffuse necking was determined. Different hardening functions were evaluated for their capabilities in approximating the entire flow stress curves up to localized necking. A modified Hockett–Sherby function was proposed and its performance was demonstrated.

Determination of plastic constitutive properties of microparticles through single particle compression

There is a need for information on plastic properties of microparticles for application in powder technology. This includes powder compaction processes, in which densification involves particle deformation. The present paper reports on the evaluation of yield strength, ultimate tensile strength and the entire stress–strain relationship of single metallic particles, by combining mathematical modelling and compression tests using a modified nanoindenter. Development and application of the method is initially demonstrated for the case of copper particles of 13–87μm in diameter. The examined copper particles exhibited similar plastic properties to one another, but different to bulk copper. The method is further employed to work out plastic properties of a CoNiCrAlY powder, for which no bulk data exists. The findings are discussed in view of implications for cold spraying, where the deposition and coating characteristics are strongly influenced by plastic constitutive properties of particles.

Facilitation consequences for reproduction of the benefactor cushion plant Laretia acaulis along an elevational gradient: costs or benefits?

Environmental stress may favour facilitative interactions among plants but whether these interactions are positive for the benefactor and how this depends on stress factors, remains to be determined. We studied the effect of beneficiary cover and biomass on reproduction of the benefactor cushion plant Laretia acaulis (Apiaceae) in the central Chilean Andes during three years. Study sites were situated along an elevational gradient at 2600, 2800, 3000 and 3150 m a.s.l. This range comprises a cold‐ and a drought‐stress gradient, with moisture increasing and temperature decreasing with elevation. We studied the effect of natural gradients in beneficiary cover and of experimental cover removal on cushion flower and fruit production. Beneficiary cover had a negative effect on flower production but not on infructescence and fruit densities or fruit weights. A positive effect of beneficiaries on the fraction of flowers converted into fruits was detected for hermaphrodite cushions. The effect of beneficiary cover on flowering was independent of elevation or cushion gender, although these latter factors explained most of the variation. Removing the aboveground parts of the beneficiaries positively affected flowering at 2800 m a.s.l. but not at the other elevations. Our results suggest negative effects of facilitation on L. acaulis flowering, but these are neutralized in fruit production. Surprisingly, this conclusion holds along the entire elevational or stress gradient. This suggests that this system of facilitation is evolutionarily stable and not very sensitive to environmental change. It remains to be tested, however, whether facilitation affects fitness via growth and long‐term survival in these slow‐growing alpine cushions.

Comparison of analytic distribution function models for hot-carrier degradation modeling in nLDMOSFETs

We analyze the applicability of different analytic models for the carrier distribution function (DF), namely the heated Maxwellian, the Cassi model, the Hasnat approach, the Reggiani model, and our own concept, to describe hot-carrier degradation (HCD) in nLDMOS devices. As a reference, we also obtain the carrier distribution function as a direct solution of the Boltzmann transport equation using the spherical harmonics expansion method. The DFs evaluated with these models are used to simulate the interface state generation rates, the interface state density profiles, and changes of the linear and saturation drain currents as well as the threshold voltage shift. We show that the heated Maxwellian approach leads to an underestimated HCD at long stress times. This trend is also typical for the Cassi and Hasnat models but in these models HCD is underestimated in the entire stress time window. While the Reggiani model gives good results in the channel and drift regions, it cannot properly represent the high-energy tails of the DF near the drain, and thus leads to a weaker curvature of the degradation traces. We show finally that our model is capable of capturing DFs with very good accuracy and, as a result, the change of the device characteristics with stress time.

Modal acoustic emission of damage accumulation in C/C–SiC composites with different fiber architectures

The stress-dependent material behavior of carbon fiber reinforced ceramic matrix composites (C/C–SiC) was investigated with modal acoustic emission (AE) technique. AE observation is a promising health monitoring method which provides accurate location and identification of the recorded damage-related AE events. For a better understanding and identification of different damage mechanisms C/C–SiC composites with different fiber orientations were studied. Load/unload/reload tensile tests were performed and measurements were made over the entire stress range in order to determine the stress-dependence of acoustic activity for increasing damage states. In general it was found that in C/C–SiC composites a significant damage-related increase in AE energy was observed close to the ultimate tensile stress. Also, fiber architecture dependent differences during damage accumulation in terms of total number of AE events and their typical AE energy could be determined and correlated to different damage mechanisms.

Comparative Analysis of Overburden Strata Movement in Fully Mechanized Top Coal Caving Mining in Entire (Sub) Layer of Ultra Thick Seam

Reasonable determination of entire layer or sublayer mining method is the key factor of safe and efficient mechanized caving mining of 14.0～28.0m ultra thick seam , numerical simulation on the entire (sub) layer coal caving mining by the FLAC3D , studying the stress and displacement variation of overburden strata , combining the underground pressure theory , comparative analysis on the calculation results . The results show that entire (sub) layer mining roof stress , movement contours are presented in a semi-elliptical arch morphology , indicating the existence of a high arch structure overburden formations , mining process appears roof tensile destruction , but the entire layer caving tensile stress intensity is lower than sublayer caving . When advancing the same distance , entire layer caving puts a greater impact on the overburden strata than the sublayer caving , has some features such as big range of influence and extent , small roof pressure , lead abutment pressure forward from grand , short time of achieving full extraction , etc.

Shear behavior of basalt fiber reinforced polymer (FRP) and hybrid FRP rods as shear resistance members

This paper investigates the shear properties of basalt fiber reinforced polymer (BFRP) and the hybrid FRP rods as shear resistance members. The shear properties of FRP rods and the corresponding resin casting rods were studied separately, including variables representing rod diameters of 4–16mm, resin types of epoxy and vinyl ester resins and fiber types comprising basalt, carbon and their hybridization. The ratio between shear stress and shear deformation was first described according to experimental results, and the shear contribution mechanism was clarified for different stages. An equation for predicting the shear strength of BFRP and hybrid FRP rods was proposed by discussing the influence of different variables. The results show that the shear strength and shear deformation ratio of different FRP rods is maintained almost constantly regardless of the variation in diameters, resin types and fiber types. The major shear resistance in FRP rods is contributed by the internal fibers, whereas the resin contributes only 8% of the overall strength and acts only in the first stage. A three-stage model is proposed to describe the entire stress to deformation ratio. A derived equation based on the shear contribution analysis is also used to predict the shear strength of the BFRP and hybrid FRP rods with high accuracy.

An evolutionary approach to modelling the thermomechanical behaviour of unsaturated soils

SummaryA new data‐mining approach is presented for modelling of the stress–strain and volume change behaviour of unsaturated soils considering temperature effects. The proposed approach is based on the evolutionary polynomial regression (EPR), which unlike some other data‐mining techniques, generates a transparent and structured representation of the behaviour of systems directly from raw experimental (or field) data. The proposed methodology can operate on large quantities of data in order to capture nonlinear and complex relationships between contributing variables. The developed models allow the user to gain a clear insight into the behaviour of the system. Unsaturated triaxial test data from the literature were used for development and verification of EPR models. The developed models were also used (in a coupled manner) to produce the entire stress path of triaxial tests. Comparison of the EPR model predictions with the experimental data revealed the robustness and capability of the proposed methodology in capturing and reproducing the constitutive thermomechanical behaviour of unsaturated soils. More importantly, the capability of the developed models in accurately generalizing the predictions to unseen data cases was illustrated. The results of a sensitivity analysis showed that the models developed from data are able to capture and represent the physical aspects of the unsaturated soil behaviour accurately. The merits and advantages of the proposed methodology are also discussed. Copyright © 2014 John Wiley & Sons, Ltd.

A new approach to the determination of plastic flow stress and failure initiation strain for aluminum alloys cutting process

Abstracts Material flow stress in cutting is difficult to measure experimentally due to the complex deformation behavior involved. In this work, a finite element (FE) simulation of cutting and a theoretical flow stress model including plastic and failure criteria were developed to determine the flow stress during aluminum alloy (7075-T6) cutting process. In a stable plastic stage, the Johnson–Cook model considering the adiabatic temperature rise (ATR) was proposed to calculate the plastic flow stress, and the relative errors of the calculated and FE simulated stresses were less than 4%. In the ductile failure stage, an improved failure initiation criterion in terms of temperature, pressure and strain rate was proposed to determine the failure initiation strain in cutting. In addition, an energy-density based ductile failure criterion was used to predict the flow stress in the failure stage. By combining the proposed theoretical model results and the FE simulation results, the entire flow stress during cutting was obtained. The influence of the failure initiation strain of the chip layer on cutting performance was also discussed. In addition, a set of aluminum alloy (A2024-T351) orthogonal cutting simulations were performed to validate the effect of the failure initiation strain on the cutting process. The predicted variation trends of the failure initiation strain for the two types of aluminum alloys were similar, and the predicted forces and tool–chip contact lengths were compared with the measured values in Atlati et al. [9]. Finally, the prediction of the failure initiation strain and its influence on tool–chip contact behavior were validated indirectly.

On the use of the Theory of Critical Distances to estimate the dynamic strength of notched 6063-T5 aluminium alloy

In this paper the so-called Theory of Critical Distances is reformulated to make it suitable for estimating the strength of notched metals subjected to dynamic loading. The TCD takes as its starting point the assumption that engineering materials’ strength can accurately be predicted by directly post-processing the entire linear-elastic stress field acting on the material in the vicinity of the stress concentrator being assessed. In order to extend the used of the TCD to situations involving dynamic loading, the hypothesis is formed that the required critical distance (which is treated as a material property) varies as the loading rate increases. The accuracy and reliability of this novel reformulation of the TCD was checked against a number of experimental results generated by testing notched cylindrical bars of Al6063-T5. This validation exercise allowed us to prove that the TCD (applied in the form of the Point, Line, and Area Method) is capable of estimates falling within an error interval of ±20%. This result is very promising especially in light of the fact that such a design method can be used in situations of practical interest without the need for explicitly modelling the non-linear stress vs. strain dynamic behaviour of metals.

Measurement of Residual Stresses in Work and Backup Rolls for Strip and Plate Mills and its Effect on the Final Load Situation

Internal stresses or residual stresses remain in almost every part after manufacturing and/or further processing. Even if the entire stress state inside a system is in an equilibrium, single stresses due to their direction and strength may have positive or negative influences to the properties of a body. Especially in big parts, the residual stress state is relatively unknown, because it can only be determined by destructive methods as sectioning or slitting. The possibility of the use of non destructive measuring methods is only given for surface near regions or thin parts and not useful for the specification of the entire residual stress state inside a large compound work roll. This paper outlines an approach for the determination of residual stresses in centrifugal casted work rolls with an indefinite chill double poured or high speed steel shell. In several steps, different measurement techniques are tested and the results are to be presented. Beside the residual stress state, which is caused by manufacturing or heat treatment, these work rolls with different shell and core material differ in their thermophysical and elastic properties. These parameters in combination with the residual stress state and the thermal and load stresses, which arise during the hot rolling process, are causing a complex stress field that is presented by a combined model for work and backup rolls in operation.

이탈리안 라이그라스 품종별 NaCl 처리가 발아 및 초기생육에 미치는 영향

이탈리안 라이그라스의 발아 및 초기 생육단계에서 내염성 수준 정도를 조사하기 위하여 in vitro 조건에서 다양한 염농도 조건에서 그린팜, 플로리다80, 코원어리 및 화산101호 품종을 이용하여 염해에 대한 반응을 발아율과 산화 스트레스 관련 효소들의 활성을 통해 조사하였다. 저농도의 NaCl 처리구에서는 모든 품종에서 발아율의 차이가 1~3% 정도 감소하는 경향을 보였으며 250 mM 이상의 NaCl 처리구에는 화산101호 품종을 제외한 모든 품종에서 50% 이상의 발아율 감소를 보였다. 또한, NaCl 농도가 높아질수록 발아에 걸리는 시간이 지연되었으며, 350 mM NaCl이 첨가된 처리구에는 화산101호 품종만이 치상 10일후에 발아가 시작하여 12% 정도 발아하였다. NaCl 농도에 따른 shoot과 root의 길이와 생초와 건조 후 무게의 변화도 만생종으로 갈수록 더 생육이 우수한 것으로 타나났다. 품종별간의 잎 조직에서 항산화효소 활성을 분석 결과 고염 조건 (150 mM NaCl)에서 POD 활성은 대조군과 유사한 수준으로 확인 되었다. 반면 APX 활성은 플로리다80과 코윈어리 품종에서는 증가하는 경향을, 화산101 품종에서는 감소하는 경향이 나타났으나 유의적인 수준에서 차이는 확인되지 않았다. 반면 그린팜 품종에서는 유의적인 수준에서 고염 처리에 의한 APX 활성이 감소되는 경향이 나타났다. 지속적으로 in vitro screening을 통해 선발된 계통들은 향후 특성평가와 인공교배를 통해 내염성 이탈리안 라이그라스 개발에 필요한 유용한 육종모재로 활용될 수 있을 것으로 판단된다. This study was conducted to evaluate the performance of Italian ryegrass cultivars for salt tolerance under in vitro condition. Italian ryegrass cultivars such as Greenfarm, Florida80, Kowinearly, and Hwasan101 were tested for their tolerance to various sodium chloride levels (0, 50, 150, 250, and 350 mM). The seed germination, growth, and activities of antioxidant enzymes were investigated under salt treatment. Physiological traits such as seed germination percentage, germination period, shoot and root length, and dry weight were suppressed under entire salt stress conditions. The results indicated that the highest germination percentage and shoot and root length were recorded at normal conditions. Increased sodium chloride levels caused a significant reduction in the seed germination and growth rate. Among the four tested cultivars, Italian ryegrass 'Hwasan101' could be considered as salt tolerant owing to its higher germination percentage, better seedling growth and antioxidant activities under salinity stress, whereas Greenfarm cultivar was more sensitive. The selection of Italian ryegrass cultivars for greater tolerance to saline environment would allow greater productivity from large saline lands.

A constitutive model of the deformation behaviour of twinning induced plasticity (TWIP) steel at different temperatures

The mechanical behaviour of Fe–18Mn–0.6C–1Al (wt%) TWIP steel was modelled in the temperature range from room temperature to 400°C. The proposed constitutive model was based on the Kocks–Mecking–Estrin (KME) model. The model parameters were determined using extensive experimental measurements of the physical parameters such as the dislocation mean free path and the volume fraction of twinned grains. More than 100 grains with a total area of ~300μm2 were examined at different strain levels over the entire stress–strain curve. Uniaxial tensile deformation of the TWIP steel was modelled for different deformation temperatures using a modelling approach which considers two distinct populations of grains: twinned and twin-free ones. A key point of the work was a meticulous experimental determination of the evolution of the volume fraction of twinned grains during uniaxial tensile deformation. This information was implemented in a phase-mixture model that yielded a very good agreement with the experimental tensile behaviour for the tested range of deformation temperatures.

Laboratory investigation of asphalt concrete dynamic modulus testing on the criteria of meeting linear viscoelastic requirements

In this study, asphalt concrete (AC) dynamic modulus testing is evaluated for meeting linear viscoelasticity (LVE) requirements. The evaluation is performed by performing stress sweep dynamic modulus testing at selected test frequencies. Three well-known linearity principles, namely superposition principle, proportionality principle and 5% stiffness reduction principle, are implemented to classify the AC response as a LVE. Analysis of the stress sweep dynamic modulus test results using these principles consistently showed that the stress and strain linearity limits for the studied mix are 155, 414, 506, 772, 470 and 462 kPa and 55, 69, 84, 99, 36 and 50 micro-strain each for loading frequency of 0.1, 0.5, 1, 5, 10 and 25 Hz, respectively. The current dynamic modulus testing standard, American Association of State Highway and Transportation Officials (AASHTO) TP-62, recommends maintaining test strains and stresses below 150 micro-strain and 700 kPa to make sure AC dynamic modulus testing is preformed within LVE range. However, these recommended limiting stress and strain values are considerably higher than the results found in this study. Moreover, the softening rate at which the dynamic modulus decreases is found to be approximately the same over the entire stress sweep range, which indicates the dependency of dynamic modulus on stress. This indicates that there is a significant chance of conducting the dynamic modulus test at stress and stain levels beyond the LVE region.