Papers by Keyword: Creep

Abstract: In order to study the similarities and dissimilarities between creep and stress relaxation behavior of age formed aluminum alloys, both creep ageing and stress relaxation ageing experiments have been conducted with plate shaped 7055 aluminum alloy specimens on the 100 KN tensile testing machine performed at 120 °C for 20 h, under different stress levels from 190.0 to 357.8 MPa. The experimental results show that similar variation trends for creep and stress relaxation behavior were observed. Both creep and stress relaxation curves can be divided into two stages. During the first stage, higher creep rate and stress relaxation rate occur, which increase with stress levels but decrease with ageing time. While during the second stage, both the creep rate and the stress relaxation rate reach its lowest value and keep constant. A set of unified creep ageing constitutive equations has been developed and calibrated from creep experimental data, which can be used to predict the creep strain under age forming conditions perfectly. But the experimental results from stress relaxation ageing tests cannot be predicted with the established creep ageing constitutive equations, which shows that there is not a one-to-one correspondence between creep and stress relaxation, creep deformation is the most important but not the only reason for stress relaxation under age forming condition.

Abstract: Microstructures, mechanical and creep properties of as-cast and as-annealed Mg-4Al-2Sr-1Ca (AJX421) alloy were investigated. The as cast microstructures of the alloy consists of the α-Mg, lamellar eutectic Mg2Ca and bulky Mg-Al-Sr phase. After annealing at 400°C, lamellar eutectic tended to be spheroidised and the continuous interphase network breaks up gradually, meanwhile, C14-Mg₂Ca completely transforms to C15-Al₂Ca. These results in obvious decrease of creep property. It is proposed that the continuous network distribution of compounds paly a major role in restricting the creep deformation of Mg-Al alloy at elevated temperatures, and the grain boundary sliding is an important creep mechanism for the alloy studied.

Abstract: In this paper, three kinds of 12% Cr ferritic steels without Co and Cu, with 3%Co and with 3%Cu are produced. The addition Co and Cu lead to an evident increase in creep-resistant of the ferritic steel, and furthermore also markedly affect the fracture behavior. Microstructures were studied by comparing three kinds of the 12% ferritic steels to better understand different in those creep behaviors and fracture behaviors. It was found that the addition of Co and Cu not only inhibit the formation δ-ferrite but also are benefit for the growth of prior austenite grains. Additionally, the part of the added Cu precipitates in the matrix of the ferritic steel. These changes in microstructure and the solution of Co and Cu strengthen the ferritic steel.

Abstract: The research on the conductive asphalt concrete(CAC) is to melt snow and ice on the pavement and improve the property of skid resistance. However, research at present mainly focuses on the electrical property not on the fatigue behavior which is also important to the application of CAC in the future. The specific rheological model is developed and the relation between creep and fatigue is investigated in this paper. By the single axial static creep test, the existed rheological model (Burgers model) and method of regression, the creep deformation of asphalt concrete is measured with time and parameters of the model are obtained. The results indicate that strain increases quickly with increasing time at initial loading phase, and then the change of strain tend to a stable value in a constant load. At unloading phase, strain decreases immediately and then keep another stable value which was greater than zero. The addition of conductive fillers improves the elastic properties and has no significant influence to the viscosity of asphalt concrete. In addition, the fatigue life can be predicted by the parameters of the Burgers model. The addition of conductive fillers improves energy dissipated, which leads to shorter fatigue life of conductive asphalt concrete compared to the control at levels of longer fatigue life.

Abstract: Dislocation configurations in two single-crystal superalloys during high-temperature low-stress creep (1100°C, 137 MP) were illustrated schematically with the use of transmission electron microscope (TEM). For an alloy with a small lattice misfit, the dislocations move in the combination of climbing and gliding processes. In the primary stage, the dislocations first move by slip in the g-matrix channels. When they reach the g¢ cuboids, they move by climb along the g¢ cuboid surfaces. In the secondary creep stage, dislocation reorientation in the (001) interfacial planes happens slowly, deviating from the deposition orientation of <110> to the misfit orientation of <100>. For an alloy with a large lattice misfit, the dislocations are able to move smoothly by cross slip in the horizontal g channels. The dislocation reorientation from the deposition orientation of <110> to the misfit orientation of <100> in the (001) interfacial planes can be completed in the primary creep stage.

Abstract: Influences of aging on the creep rupture properties of super-clean 9%CrMoV steel and 1%CrMoV steel, the heat resistant steels for steam turbine rotors of thermal power plants, are investigated. Using the as-received and the aging-treated materials of the two steels, creep rupture tests are carried out at 566°C. Creep rupture lives, creep fracture modes as well as the microstructural changes of the specimens are examined. It is made clear that the creep strength and the microstructural stability of super-clean 9%CrMoV steel are superior to those of 1%CrMoV steel in long-term services.

Abstract: In the present work, Zn-1.0Cu-0.2Ti alloy was prepared by melt casting and extruding processes. High temperature creep property of the alloy was determined using electronic creep relaxation testing machine. Microstructures of the alloy before and after creep test were observed and its high temperature creep mechanism was discussed. The results show that the steady-state creep rate of the alloy increases with temperature and stress. The logarithm of steady-state creep rate (ln) shows a linearity relationship with the logarithm of the stress (lnσ) and reciprocal of temperature (1/T). The stress exponent and apparent activation energy for creep have been determined to be 5.10 and 83.7 kJ/mol, separately. The predominant mechanism is mainly self-diffusional creep. The second phases on the grain boundary can block the slip of grain boundary and dislocation motion which can improve creep resistance of the alloy.

Abstract: Tensile behaviors of extruded and rolled AZ80 Mg alloy were investigated with elongation-to-failure tensile tests at constant temperatures of 300 °C, 350 °C, 400 °C, and 450 °C, and constant strain rates of 10^-2 s^-1 and 10^-3 s^-1. Experimental data show that the material exhibits tensile ductilities of over 100% at 400 °C and 450 °C, featured by long steady state deformation. Microstructure studies show that annealed coarse grains were remained in the gauge region during the tensile tests, and the enhanced tensile ductilities resulted from dislocation creep, other than dynamic recrystallization or grain boundary sliding. Cavity evolution and recrystallized coarse grains near fracture end caused premature failure of the material.

Abstract: The surface modification of carbon nanotubes（CNTs）has been recently observed to influence the distribution of CNTs in epoxy resin and the mechanical properties and electrical conductivities of these CNTs. Accordingly, the treatment of CNTs to with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties and electrical conductivities of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and electrical conductivities of the composites thus formed is investigated, and the strength of the material is tested at different temperatures. Moreover, the creep behavior of carbon fiber（CF）/epoxy resin thermosetting composites was tested analyzed at different stresses, orientations of fiber, temperatures and humidities. The creep exhibits only two stages- primary creep and steady-state creep. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures. However, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain even at a high temperature of 55°C. Possible room temperature creep mechanisms have been proposed and discussed. Creep strain is believed to increase with applied stress, creep time, humidity, temperature and degree of the angle θ between the orientation of fiber and the direction of the applied stress. The test results also indicate that mechanical strength and electrical conductivity increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it. An SEM image of the fracture surface reveals debonding and the pulling out of longitudinal fibers because of poor interfacial bonding between fiber and matrix, which reduce overall strength.