A micromechanical damage model for concrete

The present paper proposes a constitutive theory for concrete based on the geometry of its mesostructure and the actual kinetics of the microcrack evolution. The only material parameters used within the context of the proposed formulation are those associated with the elastic, macroscopic response and the critical stress intensity factors of the constituent phases. The model allows for a rational determination of the response in function of the volume fraction of the coarse aggregate, sieving curve, distribution and size of initial defects, relative strengths of the cement and aggregate, etc.     

Fluid-solid interaction of reactor core components, a preliminary analysis

A simple analytical model is used to examine the mechanical interaction of LMFBR core components with the surrounding coolant. Approximating a subassembly duct by a single degree of freedom oscillator and the coolant by a thin layer of viscous fluid, the problem was reduced to a nonlinear differential equation governing the motion of the oscillator. Approximate analytical and numerical solutions are presented which enable parametric studies useful in LMFBR core design.     

A Comparative Study of Reactive Wetting of Lead and Lead-Free Solders on Cu and (Cu6Sn5/Cu3Sn)/Cu Substrates

Reactive wetting of solders on Cu and Cu₆Sn₅/Cu₃Sn/Cu substrates was investigated using both (1) the wetting balance, and (2) the hot-stage real time, in situ visualization of the triple-line movement. To understand the phenomenology of the spreading behavior better, comprehensive real-time in situ observations were performed. It was found that the wetting time during the wetting balance tests for both the lead solder (63SnPb) and lead-free solder systems (Sn0.7Cu and Sn3.5Ag) is shorter on Cu substrates than it is on Cu₆Sn₅/Cu₃Sn/Cu substrates. The wetting force was not remarkably different on these two substrates for the same solder system. The hot-stage tests indicate a more pronounced spreading of 63Sn-Pb on Cu₆Sn₅/Cu₃Sn/Cu substrates, along with a much larger spreading area. Spreading of lead-free solders in terms of the triple-line kinetics studied by using the hot-stage visualization shows no significant difference in the spreading evolution either over Cu or over Cu₆Sn₅/Cu₃Sn/Cu substrates.     

Is crime in Mexico a disamenity? Evidence from a hedonic valuation approach

Since Roback’s seminal work (J Polit Econ 90(6):1257–1278, 1982), the literature on hedonic prices has evaluated the role of amenities in equilibrating regional differentials in nominal wages and prices. While these studies generally find evidence for traditional amenities and disamenities in developed countries, there remains little research on how characteristics such as violence affect the equilibrium in less developed countries. This article explores violence and other local characteristics as an amenity or disamenity for Mexico and employs the hedonic wage and rent theory proposed by Roback. This research uses a multilevel estimation technique using data from the Mexican Household Income and Expenditure Survey, along with other information from the municipal and state levels. This article finds evidence to suggest that illegal earning opportunities outweigh crime disamenity by inhabitants of some traditional drug-trafficking regions, because such crime appears to be the modus vivendi in those regions in a way that does not reduce economic performance.     

Surface modification of nanoporous alumina membranes by plasma polymerization

The deposition of plasma polymer coatings onto porous alumina (PA) membranes was investigated with the aim of adjusting the surface chemistry and the pore size of the membranes. PA membranes from commercial sources with a range of pore diameters (20, 100 and 200?nm) were used and modified by plasma polymerization using n-heptylamine (HA) monomer, which resulted in a chemically reactive polymer surface with amino groups. Heptylamine plasma polymer (HAPP) layers with a thickness less than the pore diameter do not span the pores but reduce their diameter. Accordingly, by adjusting the deposition time and thus the thickness of the plasma polymer coating, it is feasible to produce any desired pore diameter. The structural and chemical properties of modified membranes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray electron spectroscopy (XPS). The resultant PA membranes with specific surface chemistry and controlled pore size are applicable for molecular separation, cell culture, bioreactors, biosensing, drug delivery, and engineering complex composite membranes.     

Novel methods for the characterization of water-soluble polymers

The liquid chromatography of four water-soluble polymers [pullulan, polyacrylamide, poly(ethylene glycol), and poly(ethylene oxide)] have been investigated and used to categorize separation processes which couple enthalpic and entropic mechanisms. Experiments were carried out with a binary mobile phase which combined a thermodynamically good solvent (water or aqueous 0.02M Na₂SO₄) and nonsolvent (methanol). The polymer solute was injected in a good solvent. By varying the solvent–nonsolvent ratio in the eluent, conditions could be obtained where the free energies of exclusion and solvation were balanced. This has been given the nomenclature “liquid chromatography under limiting conditions of solubility” (LC-LCS) since the polymer elutes just in front of the system peak at the “limit” of its solubility. Conditions can also be identified where exclusion is balanced with adsorption (“liquid chromatography under limiting conditions of adsorption,” or LC-LCA). To our knowledge, these are the first experimental reports of LC-LCS for any polymer and the first LC-LCA observation on water-soluble macromolecules. All measurements were carried out over a poly(hydroxymethylacrylate) sorbent. Cloud point curves were found to generally distinguish the regions where LC-LCA or LC-LCS dominate. The data illustrate the need to consider the polymer when analyzing LC-LCA. Conversely, polymer adsorption may play an important role in LC-LCS. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2549–2557, 1998     

控制气氛下的AgCuTi/TiAl润湿铺展动力学

采用热台原位加热法系统研究了不同保护气体流量对润湿铺展过程、润湿铺展最大半径、润湿铺展动力学以及界面组织结构的影响.保护气体为氩气,保护气体流量分别为5,10,15 mL/s;加热曲线设定最高温度为1273 K,保温时间120 s;采用扫描电镜、能谱分析、光学显微镜等分析了界面组织结构.结果表明,保护气体流量为15 mL/s时,润湿铺展等效半径最大,原始半径0.9 mm的钎料铺展半径达到约1.4 mm.各种保护条件下,AgCuTi/TiAl体系的润湿铺展动力学过程相似,铺展半径与时间之间呈n次幂关系,即rn~kt;润湿铺展中钎料熔敷/TiAl的界面结构为:残余钎料的富银相/AlCuTi三元相层(AlCu₂Ti, AlCuTi, Ti₃Al)/TiAl母材,保护条件对其影响不大.     

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB₂ composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50 vol% of TiB₂ created by in situ reaction between TiO₂, B₄C and carbon. Al₂O₃ and Y₂O₃ were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940 °C. The sintered samples were subsequently heat treated at 1970 °C. It was found that the presence of TiB₂ serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB₂ composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24 vol% TiB₂. The maximum flexural strength of ∼600 MPa was obtained in samples with 12 vol% TiB₂ whereas the maximum fracture toughness of 6.6 MPa m^1/2 was obtained in samples with 24 vol% TiB₂. Typical microstructures of samples with the mentioned volume fractions of TiB₂ consist of TiB₂ particles (<5 μm) uniformly dispersed in a matrix of elongated SiC plates.     

Nanopore gradients on porous aluminum oxide generated by nonuniform anodization of aluminum

A method for surface engineering of structural gradients with nanopore topography using the self-ordering process based on electrochemical anodization of aluminum is described. A distinct anodization condition with an asymmetrically distributed electric field at the electrolyte/aluminum interface is created by nonparallel arrangement between electrodes (tilted by 45°) in an electrochemical cell. The anodic aluminum oxide (AAO) porous surfaces with ordered nanopore structures with gradual and continuous change of pore diameters from 80 to 300 nm across an area of 0.5-1 cm were fabricated by this anodization using two common electrolytes, oxalic acid (0.3 M) and phosphoric acid (0.3 M). The formation of pore gradients of AAO is explained by asymmetric and gradual distribution of the current density and temperature variation generated on the surface of Al during the anodization process. Optical and wetting gradients of prepared pore structures were confirmed by reflective interferometric spectroscopy and contact angle measurements showing the ability of this method to generate porous surfaces with multifunctional gradients (structural, optical, wetting). The study of influence of pore structures on cell growth using the culture of neuroblastoma cells reveals biological relevance of nanopore gradients and the potential to be applied as the platform for spatially controllable cell growth and cell differentiation.