一种基于熵增理论的疲劳-蠕变交互作用损伤模型及试验验证

根据经典热力学理论,材料疲劳-蠕变交互作用下的损伤过程可视为系统熵增的累积,当熵增积累到临界值时,材料发生失效断裂。按此理论,并基于连续损伤力学和能量守恒定律,以系统熵增的变化来描述材料损伤,建立了一种疲劳-蠕变交互作用的损伤模型。为验证该模型,进行了540℃和520℃环境下1.25Cr-0.5Mo钢应力控制的梯形波加载试验,以材料的残余应变反映熵增积累,选取残余应变的变化作为损伤变量,用上述损伤模型进行了材料疲劳-蠕变交互作用的损伤演化描述,结果表明实测损伤点数据与该模型的损伤演化规律符合较好。     

Damage Behavior of SnAgCu Solder under Thermal Cycling

电子封装焊点的热循环失效是焊点材料损伤逐步发展的结果,本工作旨在对SnAgCu钎料的热循环损伤失效行为进行研究.以连续损伤力学理论为基础,提出了一种适用于热循环条件下SnAgCu钎料蠕变-疲劳交互作用的损伤模型.据此,设计了热力循环实验和热循环实验用以标定损伤模型相关参量.自行设计了双金属剪切加载装置并结合温度循环实验,对SnAgCu钎料的热力耦合损伤行为进行了深入研究.以电阻变化率作为损伤变量,并在热循环的不同周次测量试样的损伤值从而验证损伤模型.结果表明:所提出的幂函数形式的损伤模型能较好的描述SnAgCu钎料的热循环损伤演变.最后,对热循环条件下SnAgCu钎料试样的微观组织演变进行了SEM分析,从而揭示其损伤演变机理.     

Experimental study on the effect of nanosized carbon particles on fatigue resistance of asphalt binders

The present research study investigates the fatigue resistance of asphalt binders modified with carbon nanoparticles. Three different modified bitumens containing three different dosages of carbon nanoparticles were tested to capture fatigue properties under the concept of viscoelastic continuum damage mechanics through frequency sweep and linear amplitude sweep tests. Results show that adding carbon nanoparticles can enhance the fatigue life of neat asphalt binders.     

Generalized Thermoelastic Models for Linear Elastic Materials with Micro-Structure Part I: Enhanced Green–Lindsay Model

In this article, we derive constitutive thermoelastic models for linear elastic materials with micro-structure. The elastic behavior is assumed to be consistent with Mindlins’ Form II gradient elasticity theory, whereas for the thermal behavior the generalization of Clausius-Duhem inequality, proposed by Green and Laws, is adopted. The resulting model is actually a generalization of the thermoelastic theory of Green and Lindsay for linear elastic materials with micro-structure, taking into account micro-inertia effects, as well. It is demonstrated that classical thermoelasticity models are retrieved from the present general formulation, when some of the model constants are set to zero. Finally, the uniqueness of solution for the general case of anisotropic materials is proved.     

Longitudinal changes in macroinvertebrate assemblages in the Meuse River: anthropogenic effects versus natural change

A collaborative study among three nations (France, Belgium, Netherlands) along the Meuse River developed a consistent approach for collecting and interpreting macroinvertebrate data. Specific mesohabitats were sampled in 16 locations along an 800‐km stretch of this lowland regulated river. The objective was to assess the ‘river health’ using macroinvertebrate communities as indicators of biological and ecological variation in space. The main changes in assemblages were investigated using multimetric and multivariate approaches. The authors examined relationships between faunal variations and both physico‐chemical gradients and man‐made disturbances. We related species traits to faunal changes and habitat characteristics. Both a gradual shift from a macroinvertebrate assemblage dominated by insects to a community dominated by crustaceans and molluscs and a drastic decrease in biotic index values were observed along the longitudinal gradient. Taxa were distributed according to oxygen, nitrate and ammonium concentrations, pH, conductivity and summer hydraulic conditions. But major faunal differences among sites could not be explained simply by physico‐chemical variables. The trait analysis underlined the role of temporary habitats in structuring the summer macroinvertebrate community of sites of the uppermost French sector, which supported the most diverse community in terms of trait combination. Downstream the macroinvertebrate community exhibited a more simple and less stable functional organization. We concluded that the Meuse River exhibited both a high biodiversity and a ‘reasonably good’ water quality in the upper reaches. Two transition zones highlighted the influence of a high degree of human impact on stream integrity. Regulation for navigation, ship traffic and heavily polluted effluent discharges influenced instream conditions via multiple processes determining a decline of both habitat stability and diversity. However, the rare occurrence of habitat‐sensitive species in the lower reaches indicated that a partial recovery of communities may be predicted if restoration and protection of disturbed (especially riparian) habitats are fulfilled. Copyright © 2002 John Wiley & Sons, Ltd.     

常见炸药分子的溶剂化效应

用密度泛函理论（ DFT）和极化连续介质模型（ PCM）研究了7种溶剂对22种常见炸药分子的特征键键长、硝基电荷、偶极矩、溶剂化能及键离解能的影响。结果表明,溶剂能极化炸药分子,改变其电子结构,影响其几何结构和能量。对介电常数为0～10．36的溶剂,随溶剂极性增大,炸药分子的特征键长及溶剂化能减少,而硝基负电荷及偶极矩增加。     

Strain energy distribution in a crack-tip region in random fiber networks

A closed form relation for the strain energy density in the vicinity of a macroscopic mode I crack in a random fiber network is derived using an implicit gradient nonlocal continuum field theory. An expression for the characteristic length, used in the nonlocal formulations, in terms of microstructural properties is derived and it is found that the characteristic length is proportional to the average fiber segment length to the power of two. It is illustrated that the crack-tip singularity vanishes for a characteristic length greater than zero. An open fiber structure exhibits a distributed strain energy field in the crack tip vicinity. As the network becomes relatively denser, the characteristic length decreases and the networks mechanical behavior approaches the behavior of a classic elastic continuum. Only for an infinitely dense network is the r ⁻¹-singularity in strain energy field achieved. The theory explains why open network structures have difficulties in localizing failure to macroscopic cracks. It is found that there is a one-to-one relation between characteristic length and size of the smallest crack that can initiate macroscopic failure.     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

Modelling Temperature,Body Size,Prey Density,and Stream Gradient Impacts on Longitudinal Patterns of Potential Production of Drift‐Feeding Trout

In this study, we modelled idealized stream reaches using empirical hydrodynamic and bioenergetic parameters to predict how rainbow trout production depends on physical and biological variations across a downstream gradient, and we compared these downstream effects in a low and high‐gradient stream reach. We found that longitudinal production potential (i.e. net rate of energetic intake per 100 m of stream length) generally increased with increasing stream size when stream gradient was low. This was not the case, however, for high‐gradient streams, wherein maximum longitudinal production potential was associated with middle or low stream size (Q_MAD = 2.5 to 25 m³ s^?1). Areal production potential (net rate of energetic intake per m² of wetted stream bed) reached a maximum at low stream size (Q_MAD = 2.5 m³ s^?1) with both high and low gradients. We also showed that high stream temperature and low drift density could potentially cause adult rainbow trout to be excluded from stream reaches with high flow. The models presented here have a stronger mechanistic basis for predicting fish production across heterogeneous stream environments and provide more nuanced predictions in response to variation in environmental features than their physical habitat‐based predecessors. Copyright © 2016 John Wiley & Sons, Ltd.     

改进混合律方法研究梯度材料力学性能

利用连续介质力学方法对复合材料的有效弹性模量进行了分析,以复合材料混合律模型,建立了一种改进的混合律方法,并给出了对有效弹性模量进行预测的改进公式.实例计算出的图例表明,以混合律公式比Voight模型、修正的混合率模型以及指数函数变化规律建立弹性常数的变化规律而得出的公式,更逼近梯度材料有效弹性模量的实验值,且实用有效.