Free vibration analysis of bridges

Most bridges are designed using a static analysis, adjusted by a dynamic amplification factor which is a function of the first flexural frequency. This method is imprecise but continues to be used, partly because of the apparently complex processes required to estimate accurately and to provide for the levels of vibration in a bridge. This paper investigates the natural frequencies and associated mode shapes of bridge superstructures. It compares field observations with theoretical idealizations and finds that, while a single beam idealization is accurate for straight, nonskewed bridges and for some continuous superstructures, many other bridges require an eigenvalue analysis of a finite beam element grillage. A simplified method for estimating the natural frequency of vibration is developed. An application of the Rayleigh method to a grillage model of the bridge, it is quick to apply and accurate to within 10%. The paper also compares the effects of using the static and dynamic moduli of elasticity of concrete in estimating the natural frequency of vibration, and concludes that the dynamic modulus is more appropriate. Finally, it debates the significance of certain types of support stiffness in estimating the fundamental frequency, and finds that their effect is negligible.     

PTT形态记忆织物的染整工艺

文章对PTT纤维及形态记忆面料的性能作了介绍.对PTT纤维记忆面料的染整工艺作了探讨.     

Electron spin resonance study of a cured epoxy resin exposed to high-energy radiation

Tetraglycidyl-4,4′-diaminodiphenyl methane cured with 4,4′-diaminodiphenyl sulphone was irradiated with varying dosages of 0.5 MeV electrons and 1.17 and 1.33 MeV gamma rays and was studied using electron spin resonance. The radical concentration increases with increasing radiation dose and decays, at ambient temperature, in a way consistent with a model that assumes two simultaneous second-order reactions occurring in different zones.     

Effect of 1.33 MeV γ radiation and 0.5 MeV electrons on the mechanical properties of graphite fiber composites

Epoxy/graphite fiber, polyimide/graphite fiber, and polysulfone/graphite fiber composites were exposed to 1.33 MeV γ irradiation and 0.5 MeV electron bombardment for varying periods of time. The effects of the irradiation treatments on the breaking stress and Young's modulus were studied by a three point bending test. Effects were small; indeed, both electron radiation up to 5000 Mrad and γ radiation up to 350 Mrad resulted in slight increases in both stress and modulus.     

Effects of water sorption at different temperatures on permanent changes in an epoxy

Crosslinked epoxy resins, tetraglycidyl 4,4′-diamino diphenyl methane cured with 4,4′-diamino diphenyl sulfone, were soaked in water at either 25°C or 70°C for varying lengths of time. The infrared spectra and DSC thermograms were obtained for samples that were soaked, or soaked and dried. There was a monotonic decrease in exothermic reaction energy with water content. The glass transition was also lowered, although samples soaked at 70°C showed a leveling in the T_g around 115°C. When the soaked samples were dried, the exothermic reaction energy showed near reversibility for samples soaked at 25°C while the 70°C samples were highly irreversible. IR of the latter samples showed that the 70°C water soaking resulted in reaction of some of the unreacted epoxide groups that remained after the initial cure.     

光盘记录介质研究进展

随着材料科学、光存储技术的发展,国内外对光存储介质的研究不断深入.文章对全息、光谱烧孔、电子俘获、双光子、光致变色材料等光盘记录介质及其存储机理研究进展进行了综述.