Effects of Grazing Management on Grassland Production and Animal Performance

Five grazing treatments with a set of design for evaluation of seasonal grazing influencing grassland conditions and animal performance have been conducted across northern and western China, in order to identify the key solutions for degradation of grazing grasslands. Here, the effects of seasonal grazing within two systems-one based on current ‘survival’ practices and the other taking more of a ‘production’ focus in Bashang grasslands were reported. The experiment involved a factorial combination of alternative practices (survival vs. production systems) in spring, summer and autumn with a layout of 15 plots and 1.5 ha per plot. Results of consecutive two years studies showed that the vegetation composition changed significantly across grazing treatments, and spring rest treatments can significantly improve grassland production but with low LWG across all grazing seasons. Animal lost live weight on all grazing treatments in autumn indicating that the quality of grassland was lower and need supplementary feed in this season. In the meantime, the average LWG is lowest across the whole grazing season in the continuous survival grazing treatments, but highest in the continuous production treatment. In conclusion, spring rest is most important for grassland quality maintenance, and the continuous production grazing is more appropriate for animal production

Experimental studies and constitutive modelling of anelastic creep recovery during creep age forming

This paper presents a study of anelastic creep recovery during creep-ageing of an aluminium alloy AA7050-TAF. Uniaxial Creep-Ageing and Recovery Test (CART) was used to characterise the influence of anelastic creep strain on total creep deformation for determining the actual amount of springback in creep age forming (CAF) process. CART was performed on aluminium alloy AA7050-TAF at 174°C between the stress levels of 137.5 to 162.5 MPa. A constitutive model was developed for the prediction of the creep-ageing and recovery response of material in creep age forming. A 'back stress' variable was used to represent the net effect of the internal stresses of the material which causes anelastic creep recovery. Other microstructural variables were introduced to model complex micro-mechanisms and hardening effects including solid solution hardening, dislocation hardening, and age hardening. It has been found that the permanent strain after creep-ageing depends not only on total creep strain but also on anelastic strain. Simulation results from the constitutive model developed in this study show a good agreement with experimental data

High Dimensional Apollonian Networks

We propose a simple algorithm which produces high dimensional Apollonian networks with both small-world and scale-free characteristics. We derive analytical expressions for the degree distribution, the clustering coefficient and the diameter of the networks, which are determined by their dimension

A unified constitutive model for asymmetric tension and compression creep-ageing behaviour of naturally aged Al-Cu-Li alloy

A set of unified constitutive equations is presented that predict the asymmetric tension and compression creep behaviour and recently observed double primary creep of pre-stretched/naturally aged aluminium-cooper-lithium alloy AA2050-T34. The evolution of the primary micro- and macro-variables related to the precipitation hardening and creep deformation of the alloy during creep age forming (CAF) are analysed and modelled. Equations for the yield strength evolution of the alloy, including an initial reversion and subsequent strengthening, are proposed based on a theory of concurrent dissolution, re-nucleation and growth of precipitates during artificial ageing. We present new observations of so-called double primary creep during the CAF process. This phenomenon is then predicted by introducing effects of interacting microstructures, including evolving precipitates, diffusing solutes and dislocations, into the sinh-law creep model. In addition, concepts of threshold creep stress σth and a microstructure-dependant creep variable H, which behave differently under different external stress directions, are proposed and incorporated into the creep model. This enables prediction of the asymmetric tension and compression creep-ageing behaviour of the alloy. Quantitative transmission electron microscopy (TEM) and related small-angle X-ray scattering (SAXS) analysis have been carried out for selected creep-aged samples to assist the development and calibration of the constitutive model. A good agreement has been achieved between the experimental results and the model. The model has the potential to be applied to creep age forming of other heat-treatable aluminium alloys

Effects of asymmetric creep-ageing behaviour on springback of AA2050-T34 after creep age forming

This study investigates the effects of asymmetric creep-ageing behaviour on springback of aluminium alloys during creep age forming (CAF) process. An Al-Cu-Li alloy, AA2050, which shows an apparent asymmetric tension and compression creep-ageing behaviour is used for investigation. Several CAF trial tests to form singly-curved AA2050 plates with different thicknesses (3, 5 and 8 mm) are carried out with a four point bending setup. Meanwhile, two sets of finite element (FE) models of corresponding processes have been developed, in which either the conventional symmetric or the new asymmetric creep-ageing behaviour of the alloy was used. The asymmetric models can well predict the shape of formed plates for thicker materials (5 and 8 mm), while symmetric models provide over-prediction of final deflections. The results from asymmetric models indicate that more creep strain is generated in the tension stressed part of the forming plate than that in the other part with compressive stresses and therefore, resulting in an asymmetric distribution of the relaxed stresses through the thickness of the forming plates after creep-ageing. After springback, both top and bottom surfaces of the formed plates show significant compressive stresses while tensile residual stresses exist in the centre of the formed plates. The work in this study helps to understand the particular springback behaviour of AA2050 with asymmetric creep-ageing behaviour in CAF, and can be used to guide future industrial applications of the alloy in the CAF process

Influence of Fermion Velocity Renormalization on Dynamical Mass Generation in QED3_3

We study dynamical fermion mass generation in (2+1)-dimensional quantum electrodynamics with a gauge field coupling to massless Dirac fermions and non-relativistic scalar bosons. We calculate the fermion velocity renormalization and then examine its influence on dynamical mass generation by using the Dyson-Schwinger equation. It is found that dynamical mass generation takes place even after including the scalar bosons as long as the bosonic compressibility parameter $\xi$ is sufficiently small. In addition, the fermion velocity renormalization enhances the dynamically generated mass.Comment: 6 pages, 3 figures, Chinese Physics Letter, Vol 29, page 057401(2012

Plasma Sheet Pressure Variations in the Near‐Earth Magnetotail During Substorm Growth Phase: THEMIS Observations

We investigate the plasma sheet pressure variations in the near‐Earth magnetotail (radius distance, R, from 7.5 RE to 12 RE and magnetic local time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure (Peq) decrease. The occurrence rates of Peq decrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (> 50%) than in the midnight region (20:00 to 04:00,  −16%). The mean value of Peq increase percentages at the end of substorm growth phase is the highest (~ 40%) in the premidnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% of Peq decrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (< 10%), and ~ 59.0% events have a more than 10% increase. This study also reveals the importance of electron pressure (Pe) in the variation of Peq in the substorm growth phase. The Pe variations often account for more than 50% of the Peq changes, and the ratios of Pe to ion pressure often display large variations (~ 50%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally toward the dayside magnetosphere with velocity of ~ 20 km/s. It is proposed that the Peq decreases in the near‐Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux toward the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates for Peq decrease events and the Peq increase percentages in the investigated region.Key PointsAbout 40% of the selected events in the near‐tail region display a phenomenon of equatorial plasma pressure decreaseAn enhanced equatorial convection with speed of ~ 20 km/s is observed in our cases during the substorm growth phaseStatistical studies for the distributions of Peq properties and electron pressure variations are performedPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141851/1/jgra53963.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141851/2/jgra53963_am.pd

An efficient closed-form solution for springback prediction and compensation in elastic–plastic creep age forming

Accurately predicting the amount of springback has always been a prior focus in metal forming industry, particularly for creep age forming (CAF), for its significant effect on tool cost and forming accuracy. In this study, a closed-form solution for CAF springback prediction covering deformation from elastic to plastic loadings was developed by combining the beam theory and Winkler’s theory, based on which an efficient springback compensation method for CAF was proposed. This developed solution extends the application area beyond the traditional beam theory-based springback prediction methods, maintaining its validity with large loading deflection in plastic range. Finite element (FE) simulation and four-point bending CAF tests adopting a 3rd generation Al-Li alloy were conducted in both elastic and plastic forming regions and the results showed close agreement with the closed-form springback predictions. For the proposed compensation method, an adjustment factor was introduced for complex flexible tool CAF to consider its deviation from the uniform stress loading and can be obtained using the closed-form solution. The flexible tool CAF tests using the Al-Li alloy demonstrated the applicability of the proposed compensation method to obtain the target shape within reasonable iterations, which can be further reduced by combining FE simulation

Improved creep behaviour for a high strength Al-Li alloy in creep age forming: Experimental studies and constitutive modelling

The creep deformation of 3rd generation 2xxx Al-Li alloys in creep age forming (CAF) is unsatisfactorily low due to its high strength and required low ageing temperature, especially for the commonly used T8 temper. Promoting creep deformation in such process will significantly reduce springback in CAF, making easier the springback compensation in tool design. In this study, two possible process routes of inducing increased creep deformation were explored, one by employing high applied stress to introduce small plastic strain in loading stage; and the other by conducting pre-deformation prior to creep-ageing. The creep behaviour with and without pre-deformation was investigated through creep-ageing tests under stresses ranging from 300 to 430 MPa at the ageing temperature of 143°C. Due to the decrease of threshold stress with introduced plastic strain in the loading stage and/or pre-stretching, a faster increase of creep strain with applied stress was observed for both methods when the applied stress surpassed 400 MPa. The strain promotion of the former was higher than the latter when above 415 MPa. A mechanism-based constitutive model was proposed, with additional work hardening equations to describe the relationship between threshold stress and dislocation density induced by plastic strain in the loading stage and pre-stretching, to model the microstructural evolution and reflect the nonlinear increase of creep strain with stress in both methods. The adequacy of this proposed unified model was demonstrated with good agreement with experimental data utilising both methods. The adaptability of the model in multiaxial case was verified using four-point bending CAF tests with the same Al-Li alloy

Plasmapause surface wave oscillates the magnetosphere and diffuse aurora

Energy circulation in geospace lies at the heart of space weather research. In the inner magnetosphere, the steep plasmapause boundary separates the cold dense plasmasphere, which corotates with the planet, from the hot ring current/plasma sheet outside. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations, but direct evidence of the waves and their role in magnetospheric dynamics have not yet been detected. Here, we show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multisatellite and ground-based measurements. The wave oscillates the plasmapause in the afternoon-dusk sector, triggers sawtooth auroral displays, and drives outward-propagating ultra-low frequency waves. We also show that the surface-wave-driven sawtooth auroras occurred in more than 90% of geomagnetic storms during 2014–2018, indicating that they are a systematic and crucial process in driving space energy dissipation