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1.
The formation and distribution of dioxin-like polychlorinated biphenyls (dl-PCBs) during cooking was investigated. Concentrations of dl-PCBs in liquid residues, cooked beef, and oil fumes generated during heating were determined by isotope dilution HRGC/HRMS. Although the levels of dl-PCBs in well-done beef were lower compared with those of raw beef, relatively high concentrations of dl-PCBs were detected in the oil fumes produced during heating. This suggests that dl-PCBs in raw beef may have volatilized into the oil fumes during the cooking process. Sucralose and chloropropanols contained in raw materials may have resulted in increased dl-PCB concentrations and the level of toxic equivalents (TEQ) in the oil fumes produced under high temperature during cooking. Concentrations of dl-PCBs did not vary greatly in cooked beef, except when higher levels of chloropropanols were presented in the uncooked raw materials. Results indicate that sucralose and chloropropanols may promote the formation of dl-PCBs during the cooking process. The newly produced dl-PCBs from raw beef cooking were mainly present in oil fumes, which gave rise to high levels of TEQ in oil fumes.  相似文献   
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Low velocity impacts on energetic materials induce plastic deformations and sliding friction which can lead to ignition. If some ignition criteria have been proposed, the remaining difficulty is to characterize the mechanical behavior of the material when submitted to the corresponding solicitations (high pressure and high strain rate). Thus, a technique based on the Split Hopkinson Pressure Bars system is proposed to carry out a triaxial compression test. A cylindrical specimen is placed into a confining ring and is compressed by the system of bars. The ring prevents the radial extension of the specimen and creates a lateral confining pressure. The material and dimensions chosen for the ring maintain a constant radial pressure during the test. Some tests were carried out on an inert aggregate material and proved the validity of this experimental device. The experimental data processing shows the influence of both the pressure and the strain rate. The shear stresses, which contribute to thermal dissipation and then to the ignition threshold, increase according to the pressure.  相似文献   
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In spite of the many advantages offered by micro-tunnelling, significant problems of a technical nature can be encountered when the excavation machine tends to sink under its weight in poor sandy–silty soils and there is deviation from the theoretical advancement direction. This work presents a new calculation procedure that is able to evaluate the settlements and rotations of a Microtunnel Boring Machine (MTBM) and, therefore, to signal the danger of sinking of the excavation machine. The procedure involves the combined use of the hyperstatic reaction method (with a Winkler approach) and of a finite difference numerical method. A parametric study developed with the proposed procedure for poor grounds, from the geotechnical point of view, has led to useful indications on the courses of action to take (integrative structural works or ground reinforcement) when it is possible that the phenomenon of machine sinking could occur.  相似文献   
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《Fire Safety Journal》2001,36(5):477-489
A two-dimensional model to simulate the flow and temperature fields in underground coal fires has been studied. The model is based on the theory of natural convection in porous media. A numerical solution procedure is used. Its predictions are compared with the limited field temperature data available. It is found from the model predictions that fractures or higher permeability are necessary to enhance natural convection. The air penetrates in the area behind the fire where many fractures of higher permeability occur due to subsidence. Air flows out from the hot area or cold area in front of the fire. In a shallow depth fire convection plays an important role, whereas in a deep fire convection velocities are smaller. Moreover, secondary combustion of volatiles produced from coal seam in the fracture or crack can markedly increase the convection.  相似文献   
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The development of shear bands at the stretched surface of a bent plate is analysed numerically, based on an approximate continuum model of a ductile porous material. This material model accounts for the nucleation and growth of voids as well as the effect of the yield surface curvature, which is represented by a combination of kinematic hardening and isotropic hardening. An imperfection in the form of an initial surface waviness is assumed, which triggers shear bands at the wave bottoms. The corresponding periodic pattern of shear bands is considered, and the growth of the bands is followed, until shear cracks develop from the void-sheets inside the bands. The delay of localization due to the nonuniform strain field is studied for different versions of the material model. Furthermore, the stability of the uniform growth of several adjacent shear bands is investigated.  相似文献   
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