Experimental validation is a precondition for dynamic simulation of high-speed traintrack-bridge interaction system to be applied to railway engineering in the field. The paper first presents an onsite experiment of the train-track-bridge interaction dynamics carried out on the Beijing-Tianjin high-speed railway, and then the experimental results are used to validate the train-track-bridge interaction simulation software (TTBSIM). There is a good correlation between the calculated results and the measured data. As a case study, the Yellow River Bridge in the Beijing-Shanghai highspeed railway is studied, from modelling of the bridge structure to evaluating the dynamic performance of the train-track-bridge interaction system under the CRH 3 EMU passing through at speeds of 250-375 km/h. The calculated and measured results are also compared in the case of such a large steel bridge under high-speed moving train, demonstrating the effectiveness of the TTBSIM simulation for dynamic evaluation of complex bridge structures in high-speed railways.
Coordinated surface chemistry and microstructural studies were carried out, in conjunction with crack growth measurements, to better understand environmentally enhanced creep crack growth in Inconel 718 alloy. Creep crack growth response was determined as a function of stress intensity factor (K) and temperature in air, pure oxygen, moist and dry argon. Crack growth was found to be thermally activated, with apparent activation energies of about 287 f 46 kJ/mol in pure oxygen and 19 1 f 77 kJ/mol in moist argon. The growth rates were independent of oxygen pressure from 2.67 to 100 kPa at 973 K, and were about four orders of magnitude faster than those in high purity argon. Fractographic examinations showed predominantly intergranular cracking and the presence of copious amounts of niobium carbides. X-ray photoelectron spectroscopic (XPS) examinations showed that the oxides on the fracture surfaces were enriched in niobium oxide near the crack tip, and with iron and molybdenum away from the tip. Parallel XPS studies showed considerable segregation of niobium at the grain boundaries of polycrystals, and significant enrichment of single crystal alloy surfaces with niobium after heating for 1 h at temperatures above 775 K (with surface concentration in excess of 25 at% at 975 K). These results and observations suggest that niobium has a strong surface affinity in IN718 alloy and may be responsible for the environmental enhancement of creep crack growth at high temperatures. The oxidation and decomposition of niobium carbides as a source for segregated niobium and as the rate controlling process for crack growth are discussed. Niobium as a source for embrittment of other nickel-base superalloys is considered.
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