Rapid cold hardening in young hoppers of the migratory locust Locusta migratoria L. (Orthoptera: Acridiidae)

This paper describes a rapid cold hardening process for first instar hoppers of the migratory locust Locusta migratoria L. First instar hoppers of this species are often subjected to subzero temperatures or frosts in early April or May after their emergence from the soil. The mean supercooling point of hoppers is -13.0 +/- 1.4 degrees C; the fact that none could survive this temperature suggests they are freezing intolerant. When young hoppers were transferred directly from 30 degrees C to -7 degrees C for 2 h, there was only 35.8% survival. However, exposure to 0 degrees C for 2 h prior to transfer to -7 degrees C increased the apparent survival to 75%. A similar rapid cold hardening response can also be induced by gradual cooling at rates of between 0.05 and 0.1 degreess C min(-1). Rapid cold hardening also elevates the Ltime50 of first instar hoppers at -7 degrees C by approximately 3 fold, and reduces the lethal temperature from -10 degrees C to -12 degrees C. However, the protection from cold shock gained through rapid cold hardening was transient and easily lost within 2 h of hoppers being returned to 30 degrees C. The rapid cold hardening response is possibly advantageous to first instar hoppers that are often exposed to large temperature fluctuations in spring or early summer.     

Insect cold hardiness: a Canadian perspective

The cold climates and diverse environments of Canada have allowed key studies of insect cold hardiness that developed and widened the understanding of this subject. For example, freezing tolerance, chilling tolerance, freezing resistance, supercooling, cryoprotectants and other features can be combined in many different ways, reflecting a wide range of adaptations. Many other factors interact with and influence cold hardiness, such as habitats and their selection, and water and energy balances. These findings suggest several topics that would be especially fruitful for further study in northern Canada.     

Influence of dispersoids on microstructure evolution and work hardening of aluminium alloys during tension and cold rolling

The influence of dispersoids on work hardening of aluminium during tension and cold rolling has been studied by comparing Al–Mn alloys containing similar amounts of solutes but various dispersoid densities. The microstructure evolution with deformation strain was examined in transmission and scanning electron microscopy. It is found that a high density of fine dispersoids strengthens the materials significantly, but their strengthening effect diminishes as the strain increases. From a series of Bauschinger tests, it is found that the internal stress, due to particles, increases rapidly at the initial stage of deformation, but saturates at strains larger than 5%. It is concluded that the internal stress makes a small contribution to the work hardening and contributes to less than 10% of the total flow stress during monotonic loading at strains larger than 5%. The work-hardening behaviour has been correlated to the corresponding microstructure, and the strengthening mechanisms are discussed.     

Instantaneous work hardening behaviour of two-phase tungsten heavy alloys: a phenomenological approach

ABSTRACT

The present work describes a phenomenological approach to explain the instantaneous behaviour of tungsten heavy alloys (WHAs) in heat-treated and swaged conditions. The strengths and elongation values of heat-treated materials are lower and higher than those of the swaged samples respectively. The heat-treated materials exhibit two slopes in true stress–true plastic strain curves and follow the Ludwigson constitutive equation. On the other hand, swaged materials display a single slope and adhere to typical Swift constitutive equation. The latter reflect the presence of pre-strain in the materials due to swaging deformation. The fracture surfaces in heat-treated materials consist of W-W decohesion along with matrix rupture and W-cleavage, while swaged samples consist of mainly W-cleavage. Both the materials display three typical stages (I, II and III) of work hardening. The second derivatives of true stress–true plastic strain curves of these alloys exhibit a perfect parabola although the nature of true stress–true strain as well as true stress–true plastic strain curves is quite different in heat-treated and swaged materials. This has been observed for the first time in WHAs consisting of matrix and W-grains. The shape of the parabola is simple and easy to fit. The fitting parameters of parabolas have been successfully employed to explain the flow behaviour of a large number of tungsten heavy alloys having two-phase microstructure in different processing conditions.     

On the injection of a cold relativistic electron beam into a cold plasma

In a one-dimensional model the initial phase of the injection of a relativistic electron beam into a plasma is studied, both analytically and computationally. We find that in a time of a few beam plasma periods the beam obtains a “temperature” which is much higher than expected from the analytic treatment.     

Single cell mechanics: stress stiffening and kinematic hardening

Cell mechanical properties are fundamental to the organism but remain poorly understood. We report a comprehensive phenomenological framework for the complex rheology of single fibroblast cells: a superposition of elastic stiffening and viscoplastic kinematic hardening. Despite the complexity of the living cell, its mechanical properties can be cast into simple, well-defined rules. Our results reveal the key role of crosslink slippage in determining mechanical cell strength and robustness.     

Spin-charge separation in cold fermi gases: a real time analysis

Using the adaptive time-dependent density-matrix renormalization group method for the 1D Hubbard model, the splitting of local perturbations into separate wave packets carrying charge and spin is observed in real time. We show the robustness of this separation beyond the low-energy Luttinger liquid theory by studying the time evolution of single particle excitations and density wave packets. A striking signature of spin-charge separation is found in 1D cold Fermi gases in a harmonic trap at the boundary between liquid and Mott-insulating phases. We give quantitative estimates for an experimental observation of spin-charge separation in an array of atomic wires.     

Strain hardening in polymer glasses: limitations of network models

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While traditional entropic network models can be fit to the total stress, their underlying assumptions are inconsistent with simulation results. There is a substantial energetic contribution to the stress that rises rapidly as segments between entanglements are pulled taut. The thermal component of stress is less sensitive to entanglements, mostly irreversible, and directly related to the rate of local plastic rearrangements. Entangled and unentangled chains show the same strain hardening when plotted against the microscopic chain orientation rather than the macroscopic strain.     

Theoretical and numerical analysis of stresses in a laser hardening model

Kuibyshev Branch of the Lebedev Physics Institute. Translated from Preprint No. 167 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1990.     

Electrostatic surface trap for cold polar molecules on a chipElectrostatic surface trap for cold polar molecules on a chipElectrostatic surface trap for cold polar molecules on a chip

We propose a simple scheme for trapping cold polar molecules in low-field seeking states on the surface of a chip by using a grounded metal plate and two finite-length charged wires that half embanked in an insulating suhstrate, calculate the electric field distributions generated by our charged-wire layout in free space and the corresponding Stark potentials for ND3 molecules, and analyze the dependence of the trapping center position on the geometric parameters. Moreover, the loading and trapping processes of cold ND3 molecules are studied by using the Monte Carlo method. Our study shows that the loading efficiency of the trap scheme can reach 11.5%, and the corresponding temperature of the trapped cold molecules is about 26.4 inK.     

采用Damman光栅实现冷原子或冷分子囚禁的光阱阵列

提出了一种采用Damman光栅和球面透镜组合光学系统产生二维光阱阵列的新方案. 在使用红失谐高斯激光束照射的条件下，推导了计算光阱阵列的周期、光强分布、光强梯度和光阱几何参数的经验公式，讨论了此光阱阵列的特点以及在原子光学和分子光学中的应用. 研究结果表明，这种光阱阵列方案比已有的光阱阵列方案更为简单可行、操作方便，非常适用于冷原子或冷分子的阵列囚禁，以及制备新颖的光学晶格. 关键词： 冷原子或冷分子 光阱阵列 Damman光栅 光偶极势     

Deformation field interaction in sequential circular indentation of a strain hardening material

An experimental study was made to characterise and model the deformation field in sequential circular indentation of a model strain hardening material. Digital image correlation was used to measure the evolving subsurface deformation field in terms of displacement, strain rate and strain as a function of indentation spacing and depth. These measurements were used to validate a finite element model for complementary simulations. The results identify relationships between sequential indentation parameters and overlap of subsurface strain distributions, maximum subsurface strains and indentation loads. Maximum strain and the degree of strain field overlap in the deformed subsurface were maximised when the ratio of indentation spacing (S) to projected indentation contact length (L) was approximately S/L?=?[1.1, 1.2]. Also discussed are the implications for understanding process-scale considerations for indentation-based mechanical surface treatments, including energy dissipation and relationship of surface coverage measures to subsurface strain overlap. Relative differences in energy expended were found for conditions that produce similar levels of subsurface plastic strain and strain field overlap. Finally, the role of sequential indentation parameters on strain path changes and path reversals in the deformed subsurface is investigated and discussed in the context of heterogeneous mechanics and corresponding effects on subsurface microstructure evolution.     

Intense two-cycle laser pulses induce time-dependent bond hardening in a polyatomic molecule

A time-dependent bond-hardening process is discovered in a polyatomic molecule (tetramethyl silane, TMS) using few-cycle pulses of intense 800 nm light. In conventional mass spectrometry, symmetrical molecules such as TMS do not exhibit a prominent molecular ion (TMS(+)) as unimolecular dissociation into [Si(CH(3))(3)](+) proceeds very fast. Under a strong field and few-cycle conditions, this dissociation channel is defeated by time-dependent bond hardening: a field-induced potential well is created in the TMS(+) potential energy curve that effectively traps a wave packet. The time dependence of this bond-hardening process is verified using longer-duration (≥100 fs) pulses; the relatively slower falloff of optical field in such pulses allows the initially trapped wave packet to leak out, thereby rendering TMS(+) unstable once again.     

Axion as a cold dark matter candidate

Here we generally prove that the axion as a coherently oscillating scalar field acts as a cold dark matter in nearly all cosmologically relevant scales. The proof is made in the linear perturbation order. Compared with our previous proof based on solutions, here we compare the equations in the axion with the ones in the cold dark matter, thus expanding the valid range of the proof. Deviation from purely pressureless medium appears in very small scale where axion reveals a peculiar equation of state. Our analysis is made in the presence of the cosmological constant, and our conclusions are valid in the presence of other fluid and field components.     

Radiation trapping in a cold atomic gas

We experimentally study radiation trapping of near-resonant light in a cloud of laser-cooled rubidium atoms. Unlike in most previous studies, dealing with hot vapors, collisional broadening is here negligible and Doppler broadening due to the residual atomic velocity is narrower than the homogeneous broadening. This is an interesting new regime, at the boundary between coherent and incoherent radiation transport. We analyze in detail our low-temperature data (quasi-elastic regime) and then provide some experimental evidence for Doppler-based frequency redistribution. The data are compared with an analytical model valid for coherent transport and a Monte Carlo simulation including the Doppler effect. PACS 42.25.Dd; 32.80.Pj     

硬化伪影的新型表现形式及其校正

X射线硬化是导致工业CT重建图像质量下降的物理原因之一。硬化伪影通常表现为两种形式,即杯状伪影和带状伪影。描述并证实了硬化伪影的一种新型表现形式,这种伪影与真实结构相关,且分布规则,容易造成伪影与真实结构的混淆。采用线性化校正方法对该伪影进行抑制,提高了重建图像质量,改善了通过CT重建图像进行几何测量的精度。     

Case hardening of steel by a CO2 laser beam

Case hardening of a carbon steel, Armco Iron, an alloyed steel and a high-speed steel was carried out with a CO₂ laser beam. The indispensibility of an absorptive coating in the laser heat treatment was established. Four coatings capable of absorbing CO₂ laser radiation of a 10.6 μm wave length were examined. The influence of spot size, power and scanning speed on the depth of hardening were studied. Processing at optimal conditions yielded a hardening depth of up to 0.7 mm and a surface hardness of up to 220 H_v.