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In this article, general procedures for vulnerability assessment and retrofitting of a generic seismically designed bridge are outlined and the bridge’s damage criteria for blast resistance are explained. The generic concrete bridge is modeled and analyzed with the finite element technique implemented in ANSYS LS-DYNA environment and explosion threats are categorized into three main levels. Uncoupled dynamic technique is adopted to apply the blast loads on the bridge structure; damage and performance levels are resulted based on quantitatively verified damage mechanisms for the bridge members. The results show that, among different loading scenarios, the explosions that happen under deck are more critical compared to blasts initiating from over deck sources. Furthermore, two retrofitting methods: (1) concrete filled steel tube (CFST) and (2) concrete jacket are applied on the bridge columns. The program AUTODYN is used with coupled dynamic analysis of a column to compare the effectiveness of each method. Afterward, more efficient method for a column is applied to the whole bridge and its efficiency is revaluated. It is shown that CFST can decrease concrete spall, scabbing, rotation, displacements and shear forces more than the concrete jacket. Considering the proposed damage and performance levels, the bridge retrofitted with CFST reacts with lower damage level and higher performance level to blast loads.

AbstractContact between hot objects and liquids occurs in many industries, such as nuclear reactors, metal casting industries and paper production. In such cases growth of vapour film leads to steam explosion that may cause human and financial damages. It is obvious that possibility of these phenomena can be judged by comparing vapour film radius growth and pressure inside vapour film. In this paper vapour film growth and pressure inside the vapour film formed, on hot sphere interaction with water, are investigated. The numerical simulation of problem is obtained and then validated using experimental test and other available results. The effects of the variations of different parameters such as hot sphere diameter, temperature, immersion depth into water and bulk water temperature are investigated on the vapour film radius, vapour pressure inside vapour film and the saturation temperature of phase interface surface. Finally, the overall results show that the effect of hot sphere interaction with water would be the same pressure inside vapour film suddenly increases up to 5 times more than initial pressure which would lead to hazard and put the safety of the system at risk.

Based on results from microscopic analysis of steel samples, flow field calculations thermodynamic and kinetic considerations, a model has been derived that describes the evolution of the concentration, composition and size of the inclusions in the liquid steel during the injection of calcium. The mathematical solution of the model requires the incorporation of Population Balance Equations. The modelling results are compared to the actual inclusion composition in the steel from IJmuiden BOS2.

The crystallography of aluminium nitride precipitating on nitriding of an Fe-2at.% AI alloy has been investigated by electron microscopy and by X-ray diffraction. The precipitation of AIN can be described as a classical nucleation and growth process; no evidence has been obtained for clustering of alloying element atoms or other pre-precipitation phenomena. Two modes of AIN precipitation have been observed depending on the initial deformation state of the specimens: (i) in cold-rolled specimens, precipitation of cubic (rock salt) AIN particles with platelet-type morphology exhibiting a Bain orientation relationship with the α-Fe matrix, and (ii) in recrystallized specimens, precipitation of hexagonal (wurtzite) AIN particles without specific morphology and, for a significant part exhibiting a Pitsch-Schrader orientation relationship with the α-Fe matrix. A small fraction of cubic (rock salt) AIN also occurs in the recrystallized specimens. Although the hexagonal (wurtzite) crystal structure of AIN is thermodynamically the most stable one, the cubic (rock salt) AIN precipitates in the cold-rolled specimens have not been observed to convert to hexagonal (wurtzite) ones even after long-time nitriding (60 h at 833 K).

Nitrogen absorption isotherms of cold-rolled Fe-2 at.%. Al have been determined using thermogravimetric techniques to study the uptake of so-called excess nitrogen. The total mass increase by nitrogen absorption has been analysed assuming that nitrogen can be located at four different types of sites: contained in the AIN precipitates, at the precipitate-matrix interfaces, at dislocations and at the octahedral interstices of the ferrite matrix. The nitrogen solubility of the ferrite matrix is a few times larger than the equilibrium solubility as a result of lattice dilatation induced by the misfitting AIN particles. The distribution of nitrogen over these sites can be determined semi-quantitatively from the absorption isotherm data. This distribution depends on the size of the AIN precipitates. The dimensions of the AIN precipitates were varied by means of an annealing treatment at temperatures higher than those used in the absorption isotherm determination. Interpretation of the enhanced nitrogen solubility of the iron matrix indicated that a partial transformation from cubic AIN into hexagonal AIN occurred on annealing at a relatively high temperature (1123 K). This partial transformation was confirmed by X-ray diffraction.

Abstract: The kinetics of the precipitation of aluminum nitride on internal nitriding the Fe-2 at. pct Al alloy was investigated for cold-rolled and recrystallized specimens exhibiting “ideally weak” interaction behavior of the solutes Al and N. The kinetic analysis was performed using mass-increase data obtained for thin foils (thickness ⪯0.1 mm) upon nitriding in a NH3/H2 gas mixture at temperatures in the range 803 to 853 K. Activation-energy analysis revealed that precipitation of AlN in the recrystallized specimens is associated with a Gibbs free energy barrier for the formation of a precipitate of critical size; the precipitation rate is controlled by both nucleation and growth. On the other hand, precipitation of AlN in the cold-rolled specimens occurs without a Gibbs free energy barrier for formation of a precipitate of critical size; the precipitation rate is controlled by growth with kinetics governed by volume diffusion of alu-minum. Analysis of the total Gibbs free energy of formation of AlN in the α-Fe matrix showed that in the case of the recrystallized specimens, the formation of incoherent AlN precipitates with a hexagonal crystal structure is favored. In the case of the cold-rolled specimens, containing a high dislocation density, the formation of coherent AlN precipitates with cubic crystal structure is favored, at least in the beginning of precipitation.