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81.
82.
A. Moropoulou N. Kouloumbi G. Haralampopoulos A. Konstanti P. Michailidis 《Progress in Organic Coatings》2003,48(2-4):259-270
Susceptibility of stone to salt decay is directly related to its microstructural and mechanical characteristics. In the present work, a porous stone from a quarry in Cyprus was examined. Samples of this stone were consolidated and protected with certain conservation materials. The modification of microstructural characteristics of the stone was evaluated by mercury intrusion porosimetry, while its mechanical characteristics were measured by compressive and bending strength tests. From these data, the susceptibility of Cyprus stone to salt (NaCl) decay was estimated. The values of crystallization pressure of NaCl were calculated and compared to the experimentally measured compressive and tensile strength of the stone, in order to examine the probability of stone disruption due to salt crystallization. The development of the phenomenon after coarse pores’ filling with crystals, i.e. crystallization of salts in capillaries or mechanical failure of the stone, was determined by thermodynamic analysis. Additionally, artificial weathering tests of marine salt spray were performed for the evaluation of stone’s durability after the conservation. The treated stone proved to be resistant to salt decay, as it presented ameliorated microstructural and mechanical characteristics, concerning decay due to soluble salt crystallization. The most possible thermodynamic scenario was small pores repletion with crystals, avoiding mechanical failure of the stone. 相似文献
83.
Isothermal short-term creep of poly (vinylidene fluoride) (PVDF) monofilament sutures was determined at several temperatures between 10 and 90 °C under the stress of 10 MPa. Long term service performance was predicted for 10 decades of time. The compliance master curve as a function of time fits a hyperbolic sine equation. The temperature shift factor as a function of the temperature aT (T) is accurately represented by a general equation based on free volume. A simple relationship between the two parameters of the equation is explored. The viscoelasticity of PVDF is also seen in dynamic mechanical analysis performed at the frequency of 1 Hz. The origin of the viscoelastic character well present in the deformability of the PVDF in service is due to the occurrence of the αc relaxation that is active at ∼50 °C (E″ peak at 1 Hz). 相似文献
84.
MoSi2-based intermetallics containing different volume fractions of MoB or Mo5Si3 were fabricated by hot-pressing MoSi2, MoB, and Mo5Si3 powders in vacuum. Both classes of alloys contained approximately 5 vol.% of dispersed silica phase. Additions of MoB or Mo5Si3 caused the average grain size to decrease. The decrease in the grain size was typically accompanied by an increase in flexure strength, a decrease in the room temperature fracture toughness, and a decrease in the hot strength (compressive creep strength) measured around 1200 °C, except when the Mo5Si3 effectively became the major phase. Oxidation measurements on the two classes of alloys were carried out in air. Both classes of alloys were protected from oxidation by an in-situ adherent scale that formed on exposure to high temperature. The scale, although not analyzed in detail, is commonly recognized in MoSi2 containing materials as consisting mostly of SiO2. The MoB containing materials showed an increase in the scale thickness and the cyclic oxidation rate at 1400 °C when compared with pure MoSi2. However, in contrast with the pure MoSi2 material, oxidation at 1400 °C began with a weight loss followed by a weight gain and the formation of the protective silica layer. The Mo5Si3 containing materials experienced substantial initial weight losses followed by regions of small weight changes. Overall, the MoB and Mo5Si3 additions to MoSi2 tended to be detrimental for the mechanical and oxidative properties. 相似文献
85.
86.
H.P. Stüwe 《Advanced Engineering Materials》2003,5(5):291-295
The concept of equivalent plastic strain is discussed. Definitions may be based on kinematic arguments (like change of geometry) or on equivalence of plastic work. Complications arise when strain is accumulated in increments with variable strain path. 相似文献
87.
88.
Backlash is one of the most important non-linearities that limit the performance of speed and position control in industrial, robotics, automotive, automation and other applications. The control of systems with backlash has been the subject of study since the 1940s. This survey reveals that surprisingly few control innovations have been presented since the early path breaking papers that introduced the describing function analysis of systems with backlash. Promising developments are however taking place using adaptive and non-linear control strategies. 相似文献
89.
自韧Si3N4陶瓷的显微结构及其性能研究 总被引:4,自引:0,他引:4
利用热压的方法制得室温断裂韧性和抗弯强度分别为11.2MPa·m ̄(1/2)、823MPa,高温(1350℃)断裂韧性和抗弯强度分别为23.9MPa·m ̄(1/2)、630MPa的自韧Si_3N_4陶瓷。研究了显微结构和力学性能之间的关系。结果表明:玻璃相的含量、β-Si_3N_4的长径比等对性能有重要影响。分析了自韧Si_3N_4陶瓷的增韧机理,通过SEM明显观察到Si_3N_4中存在裂纹偏转、分支和β-Si_3N_4拔出现象。 相似文献
90.
Nitrogen addition to iron powder by mechanical alloying 总被引:3,自引:0,他引:3
Nitrogen was alloyed into iron (a) by mechanical processing in a nitrogen gas environment, and (b) by mechanically alloying with iron-nitride powders to characterize resulting nano-structure and nitrogen distribution. Although the infused nitrogen concentration was significantly greater than the thermodynamic equilibrium solubility of iron, no nitrides formed, even for nitrogen concentrations as high as 4.1 wt.% However, a bctFe phase did form. Lattice expansion calculations indicate that the sum of the interstitial bcc-Fe and bctFe nitrogen concentrations was significantly less than the total measured nitrogen concentration. A considerable portion of the mechanically infused nitrogen was determined to be associated with nanograin boundaries. 相似文献