Comparison of dynamic softening in 301, 304, 316 and 317 stainless steels

The mechanical torsion data in the form of flow curves and strain hardening rates from both as-cast and worked 300 series austenitic stainless steels, tested in the range 1200-900°C and 0.1 to 5.0 s-1, have been analysed to deepen understanding of dynamic softening mechanisms. The critical strain for dynamic recrystallization (DRX) is determined from the downward inflection of the strain hardening rate-stress curves, and completion of DRX is taken from the start of the steady-state regime. The rate of softening can be described by means of the Avrami equation with a mean k value of 1.27. These conclusions, based upon mechanical data, have been confirmed by optical metallographic methods. The peak strain (e _p) at which there is about 30% DRX is shown to be a function of the Zener-Hollomon parameter (Z) and the original grain size (D₀). The transition from multiple-peak grain coarsening to single-peak grain refinement behaviour has been determined. While the DRX grain size is a linear function of the steady-state flow stress with a power of -1.23, the subgrain diameter function has a power of -1. The stress and strain for subgrain formation were determined from changes in slope of the strain hardening-stress curves.     

Effects of elastic unloading on multiple necking in tensile bars

The deformation of round tensile bars subject to dynamic loading is studied numerically using two different plasticity theories: J₂-flow theory and J₂-deformation theory. Time development of neck localization is visualized and a comparison of the necking patterns obtained using the two different constitutive models is presented. The possibility of using J₂-deformation theory as an alternative to J₂-flow theory to model the dynamic behaviour of tensile bars subject to high rate deformation is investigated and the effects of elastic unloading are discussed. Influence of specimen geometry and initial thickness imperfections is also considered. The effects of elastic unloading on multiple necking are of particular interest, since elastic unloading may serve as a restricting factor regarding the growth of multiple neck localizations. In the present analysis, multiple necking is found to appear using both J₂-flow theory and J₂-deformation theory. The use of stubby bars leads to similar necking patterns when comparing the results obtained with J₂-flow theory and J₂-deformation theory. For slender bars subject to certain loading conditions a larger number of necks is obtained when J₂-deformation theory is applied rather than J₂-flow theory. In addition, results of simulations near the ideal-plastic limit are discussed as well as results where the yield stress is varied.     

Evaluation on thermal stability of embankments with different strikes in permafrost regions

The former studies on embankment in permafrost regions indicate that the maximum thawed depth on the embankment central line is regarded as the evaluation criterion, through which the roadbed thermal stability is evaluated. In fact, thermal effect problems associated with slope orientations result in the maximum thawed depth position being deviated in the roadbed transverse direction rather than thawed on the embankment central line. So the thermal stability of the permafrost embankment will highly have been changed. In this paper, the thermal stability of permafrost embankment is modeled with the assumption that the air temperature will be warmed up by 0.022 °C/a when the running time is 20 a and 50 a, respectively. Different roadbed surfaces, different embankment heights and different roadbed strikes are all analyzed. Surface material includes gravelly and asphalt. Embankment height ranges from 0 m to 5.0 m with a step of 0.5 m. For the embankment strike, east to west (EW), southwest to northeast (SW–NE), south to north (SN) and symmetric route are taken into consideration. There are several significant findings summarized from the calculated results. Firstly, for the embankments with the shady–sunny slope that there are hardly linear relations between the changes of natural permafrost table and artificial permafrost table and the heights of embankment. Secondly, there are three results when the running time is 20 a: a) with the completely symmetric thermal boundary conditions and under both the gravelly and asphalt surfaces, there are almost linear relations between the changes of permafrost table and the embankment height; b) under the gravelly surface, for embankments with sunny–shady slopes, there is no linear relation between the changes of permafrost table and the embankment height; c) under the asphalt surface, for embankments with sunny–shady slopes, if the embankment is not high enough there is a linear relation between the changes of permafrost table and the embankment height, but the thermal stability cannot be reached. Thirdly, when the running time is 50 a, the thermal stability of all embankments cannot be reached other than the symmetric strike under the gravelly surface. Finally, when designing an embankment, besides considering the minimum embankment height and the maximum embankment height, the Yin-Yang Slope effect should be taken into account.     

Ligand effects on electronic and optoelectronic properties of two-dimensional PbS necking percolative superlattices

The inter-nanocrystal (NC) distance,necking degree,ordering level,and NC surface ligands all affect the electronic and optoelectronic properties of NC solids.Herein,we introduce a unique PbS structure of necking percolative superlattices to exclude the morphological factors and study the effect of ligands on the NC properties.X-ray photoelectron spectroscopy data indicate that 1,2-ethanedithiol (EDT),oxalic acid,mercaptopropionic acid,and NH4SCN (SCN)ligands were attached to the surface of NCs by substrate-supported ligand exchange.Field-effect transistors were tested and photodetector measurements were performed to compare these NC solids.An SCN-treated film had the highest mobility and responsivity under high-power intensity irradiation owing to its high carrier density,whereas an EDT-treated film had the lowest mobility,photocurrent,and dark current.These findings introduce new avenues for choosing suitable ligands for NC applications.     

Complementarity framework for non‐linear dynamic analysis of skeletal structures with softening plastic hinges

In this paper, we describe an algorithm for the incremental state update of elasto‐plastic systems with softening. The algorithm uses a complementary pivoting technique and is based on casting the incremental state update as a complementarity problem. In developing the algorithm, we take advantage of the special features of solid and structural mechanics problems to achieve good computational performance, and hence the ability to compute numerical solutions to practical size problems. For example, the notion of a tangent stiffness matrix arises. Numerical examples using models of skeletal structures are presented to demonstrate the practicability of the algorithm. The numerical examples also raise some interesting questions about multiplicity of the solutions. Copyright © 2010 John Wiley & Sons, Ltd.     

An inverse analysis of warpage for trilayer thin-plate under thermal cycles

Thermal–mechanical behavior of a trilayer plate with micro-scale layer thicknesses is investigated using the hybrid experimental analytical inverse method (HEAIM). This study adopts and modifies an existing analytical solution by Suhir to evaluate the warpage of trilayer structures consisting of a mid-layer comparable in thickness and mechanical properties to other layers under thermal cycles. A phase-shifted shadow moiré is employed to obtain accurate measurements of the plate thermal warpage during a thermal cycle. The measurements are then inversely analyzed following a numerical approach that finally determines those uncertain material parameters of the thermal and mechanical properties for the layered plates. If all the material parameters are known, the behavior of the trilayer structure can be further evaluated analytically with enhanced certainty. HEAIM was employed to characterize and evaluate the thermal and constitutive behavior of substrate trilayer structure and its constituent materials under a thermal cycle. The proposed method can provide pertinent guidance to the modeling and design of complex trilayer structures.     

Morphological,thermal and dynamic mechanical properties of Cathay poplar/organoclay composites prepared by in situ process

In order to improve the thermal stability and dynamic mechanical properties of Cathay poplar (Populus cathayana Rehd.) wood, a kind of organoclay, that is, organo-montmorillonite (OMMT), was introduced into its structure via an in situ process by sequentially impregnating poplar wood with sodium-montmorillonite (Na-MMT, in concentrations of 1.0%, 2.0%, and 4.0%) and didecyldimethylammonium chloride (DDAC, in a concentration of 2.0%). Consequently, the wood/organoclay composites were prepared. The X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersed X-ray analysis (SEM-EDXA) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the morphological and chemical alterations of the composites. Also the effects of clay type and concentrations on the thermal stability and dynamic mechanical properties of the composites were studied. The results showed that didecyldimethylammonium ions were intercalated into the galleries of Na-MMT through cation exchange, partially separating the silicate layers. Thereafter, the inorganic Na-MMT transformed to OMMT during the in situ synthesis process, and the latter was successfully intercalated into the wood cell wall. The thermal degradation was alleviated in the wood/clay composites, among which the wood/OMMT composites exhibited the best thermal stability. According to dynamic mechanical analysis (DMA) results, the wood/OMMT composites showed an enhancement in energy storage and a diminution in energy dissipation compared to other groups. The improvements in the thermal stability and dynamic mechanical properties of the composites became more significant with the increasing clay content.     

Numerical simulation of the thermal history of droplets during multi-stage atomization

Analyses are made on the characteristics of cooling and undercooled solidification of multi-stage atomized aluminum droplets. Models are established for Newtonian cooling, highly undercooled heterogeneous nucleation and steady-state continuous growth. They are used for numerical simulation of the thermal history of the droplets and the effects of the main atomization process parameters such as the extent of superheat and the speed of the rotating disk. Results show that large undercooling occurs mainly in the later stage of multi-stage atomization while recalescence and rapid quenching take place mainly in the forced cooling stage. The undercooling and cooling rates of the droplets are primarily dependent on the droplet diameter, and the main atomization parameters have little effect on the nucleation and solidification behavior of the droplets.     

薄壁筒形件粘性介质外压缩径失稳起皱分析

缩径成形受筒坯稳定性的影响,容易产生失稳起皱,很难成形出缩径量较大的零件.提出了采用粘性介质外压缩径工艺成形直径变化量较大的薄壁异形曲面筒形零件新方法.针对坯料端部约束和自由条件下的失稳起皱问题进行了试验和有限元模拟,分析了试件失稳起皱过程中几何形状和应力的变化规律.研究表明,约束坯料端部可以改善试件失稳起皱后的应力分布规律,有利于消除褶皱,在较高粘性介质压力条件下可以直接成形出缩径量为15%的试件.     

Estimates on thermal effects of the China-Russia crude oil pipeline in permafrost regions

Using the finite element method, thermal effects of the Chinese-Russian crude oil pipeline in permafrost regions were estimated under two construction modes (conventional burial and aboveground embankment), two thermal control techniques (bare and insulated pipe), and three climate conditions along the route (marked under the natural mean ground surface temperatures of − 0.5, − 1.0, − 1.5 °C, respectively). The results show that with the silt embankment on gravel berm and the insulated pipe, the thaw of permafrost under the pipeline could be prevented during its service life. However, thawing of the permafrost under the conventionally buried pipeline cannot be prevented regardless of climate conditions or thermal insulation around the pipe. So it is strongly recommended that over-excavation and/or thermosyphons be applied for the conventional burial construction mode, especially in the warm and ice-rich permafrost sections to mitigate frost heave and thaw settlement in the subsoil.     

Numerical and experimental analysis of superplastic-like uniaxial tensile necking of coarse-grained LY-12

In this work, numerical and experimental studies of superplastic-like uniaxial tensile behavior of coarse-grained LY-12 have been performed. Larger tensile elongation to fracture is observed and several necks are exhibited at 10⁻¹ and 10⁻⁴ s⁻¹ respectively although not very clearly. Chaboche viscoplastic constitutive equations are used and implemented in a finite element code to simulate the process of necks formation and development before fracture during uniaxial tension. The simulated characteristics of more than one necks along the specimens help to obtain large elongation, which is in agreement with experimental observations. Local strain rate distribution as the function of strain can explain how and when microscopic necks take place.     

动态规划在运动图像分析中的应用

对运动图像序列中的目标进行运动跟踪和估计,可以分别采用变形模型和弹性配准的方法,两问题的求解均可归纳为成本函数的最优化问题。提出了应用动态规划算法求解离散成本函数最优化的方法,把问题分解成多个子问题分别求解,并存储子问题的解以避免重复计算。该方法不仅可保证解的全局最优性,而且计算量小,有利于实现实时处理。在对临床X射线冠状动脉造影图像序列的实验中得到了满意的结果,匹配大约2000个点的血管骨架只需要一秒钟左右的时间。对运动场已知的模拟图像的实验证明运动估计误差小于1个像素（1像素=0．3mm）。     

A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part I: Stationary conditions

This paper describes numerical simulations, using thermal networks, of shield temperatures and radiative and conductive heat losses of a super-insulated cryogenic storage tank operating at 77 K. Interactions between radiation and conductive heat transfer modes in the shields are investigated, by calculation of local shield temperatures. As a new method, fluid networks are introduced for calculation of stationary residual gas pressure distribution in the evacuated multilayer super-insulation. Output from the fluid network is coupled to the iterative thermal network calculations. Parameter tests concern thickness and emissivity of shields, degree of perforation, residual gas sources like desorption from radiation shields, spacers and container walls, and permeation from the inner container to the evacuated insulation space. Variations of either a conductive (thickness of Al-film on Mylar) or a radiative parameter (thermal emissivity) exert crosswise influences on the radiative or conductive heat losses of the tank, respectively.     

基于CONWEP动态加载的建筑物爆破拆除数值模拟

建(构)筑物爆破拆除数值模拟通常采取单元删除法模拟爆破切口的形成,而忽略了在爆炸载荷下形成爆破切口时动态冲击作用对结构失稳倒塌的影响。基于Abaqus/Explicit,将内置的CONWEP爆炸加载方式和经典JwL状态方程加载方式进行对比。对CONWEP方式加载进行当量转化,将工程装药量转化为模拟装药量,实现了爆破加载模拟。最后将CONwEP动态加载方式用于模拟爆破切口的形成,分析爆炸载荷下建(构)筑物的连续倒塌过程,采取更接近拆除实际的模拟方法,并获得有益的进展。     

基于CONWEP动态加载的建筑物爆破拆除数值模拟

建(构)筑物爆破拆除数值模拟通常采取单元删除法模拟爆破切口的形成,而忽略了在爆炸载荷下形成爆破切口时动态冲击作用对结构失稳倒塌的影响。基于Abaqus/Explicit,将内置的CONWEP爆炸加载方式和经典JwL状态方程加载方式进行对比。对CONWEP方式加载进行当量转化,将工程装药量转化为模拟装药量,实现了爆破加载模拟。最后将CONwEP动态加载方式用于模拟爆破切口的形成,分析爆炸载荷下建(构)筑物的连续倒塌过程,采取更接近拆除实际的模拟方法,并获得有益的进展。     

Investigation of thermal stability, phase formation, electrical, and microstructural properties of sputter-deposited titanium aluminide thin films

Titanium aluminide thin films are being considered as coating materials for high temperature applications due to their high melting points and high oxidation resistance. In this study, Ti₃₇Al₆₃ and Ti₅₃Al₄₇ thin films are deposited onto SiO₂ substrates by RF magnetron sputtering using compound targets and then annealed in vacuum to investigate the properties of the films. Rutherford backscattering spectrometry, X-ray diffractometry, transmission electron microscopy, and four-point probe measurements are used to analyze the characteristics of Ti₃₇Al₆₃ and Ti₅₃Al₄₇ thin films for high temperature electronics applications. The films show good thermal stability up to 700 °C for 1 h in vacuum. Reasonable resistivity is obtained when appropriate compositions and anneal conditions are used.     

Effect of fire engulfment on thermal response of LPG tanks

A model has been developed to predict the thermal response of liquefied-pressure gases (LPG) tanks under fire, and three-dimensional numerical simulations were carried out on a horizontal LPG tank which was 60% filled. Comparison between numerical predictions and published experimental data shows close agreement. The attention is focused on the influence of different fire conditions (different fire scenarios, various engulfing degrees and flame temperatures) on thermal response of LPG tanks. Potential hazard probabilities under different fire conditions were discussed by analyzing the maximum wall temperature and media energy after the internal pressure rose to the same value. It is found that the less severe fire scenario and lower engulfing case may lead to a greater probability of burst hazard because of the higher maximum wall temperature and media energy before the pressure relief valve (PRV) opens.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

基体局部软化对纤维/环氧树脂复合材料力学行为影响的数值分析

聚合物基体的变形局部化在复合材料破坏过程中起着重要作用。采用有限元分析方法, 借助用户材料子程序(UMAT), 描述了具有应变软化特点的高聚物弹塑性的本构关系, 研究了纤维/环氧树脂复合材料在拉伸破坏过程中基体局部变形的演化规律, 分析了基体的局部应变软化对纤维/环氧树脂复合材料应力的影响。结果表明: 纤维的应力分布及基体的塑性变形具有不均匀性; 基体局部变形降低了邻近断点的完好纤维的应力集中程度; 随着纤维间距的增加, 邻近断点的完好纤维的应力集中区域变宽, 而且应力集中程度降低。