Papers by Keyword: Tensile Creep

Abstract: This paper focuses on the calculation of residual stresses due to shrinkage with a tensile creep effect. Whereas the shrinkage of concrete causes stresses in the material, the tensile creep counteracts the shrinkage as a stress relaxation mechanism. The main objective of this paper is to evaluate the ageing coefficient c (referred to as Trost-Bazant Coefficient) reflecting the load history. The coefficient is used for the residual stress analysis by means of a simplified method called Age-adjusted Effective Modulus Method. The tensile creep effect was evaluated according to the rheological model provided by Eurocode 2. Although the Eurocode predicts the creep for the structural members subjected to compressive stresses, this study proves that it can be used for the tensile creep prediction as well. We tested three types of concrete: reference concrete, high-performance concrete with reduced shrinkage magnitude by means of special admixtures, and fibre concrete with the content of polypropylene fibres. From the obtained results, it can be stated, that the ageing coefficient can be considered to be the value of 0.45 for any shrinkage development. It was also proved, that the tensile creep value essentially affects the magnitude of residual stresses, even in the “early age” concrete. The correctness of the calculated residual stresses was verified by means of a Ring-test.

Abstract: Creep of concrete is a phenomenon that is not only significant in the long term, but also at young concrete. The paper deals with methods considering the tensile creep of concrete due to shrinkage. It was proved, that the tensile creep can be taken into account by simplified methods called Age-adjusted Effective Modulus Method with the ageing coefficient χ value falling between 0.4 and 0.45. Calculated values were experimentally verified by means of Ring-test.

Abstract: A set of concrete tensile creep testing apparatus was constructed. The tensile creep characteristics of concrete under different loading ages (1d, 3d and 7d), different water-binder ratio (0.29, 0.33 and 0.37) and different fly ash proportion (0%, 20% and 40%) were researched. The results show that tensile creep increases with increasing of water-binder ratio obviously as well as with decreasing of loading ages. The tensile creep is inhibited by addition of fly ash, and the inhibition effect increases with the increase of fly ash proportion. Free shrinkage is counteracted 42%~62% by tensile creep. The internal tension of concrete is effectively relieved so that the possibility of cracking of concrete is decreased at early ages.

Abstract: The understanding of stress relaxation and tensile creep behavior is extremely important in accurate stress analysis and crack prediction of early-age concrete. The free shrinkage deformations of concrete with different strength grade were examined. The early-age tensile elastic modulus of concrete was investigated through temperature-stress testing machine. The tensile creep and shrinkage stress were obtained through the modified restrained ring test. The results indicate that the development of free shrinkage coordinates well with the inner strain of steel ring. Tensile creep decreases as water-binder ratio increases. Creep counteracts tensile stress of concrete by 28%~40% , decreases the possibility of cracking of concrete at early ages.

Abstract: Tensile and compressive creep tests of the extruded magnesium alloy AZ91D were carried out in vacuum at 150°C under constant engineering stresses ranged from 60 to 150 MPa. From the test results, the secondary creep rate in tension was found to be significantly higher than that in compression. Moreover, the estimation method of creep curve under a constant true stress was proposed by considering the reduction of cross sectional area during tensile loading where the specimen cross-sectional area and length were measured periodically until the end of creep test. The creep curve under a constant true stress obtained in the present study was still different between tensile and compressive loadings. Therefore, it should be noted that the different creep curves and creep exponents should be used in the creep deformation analysis of structures.

Abstract: This paper presents an experimental investigation on tensile basic creep behavior of HPC at early ages by using a uniaxial tensile creep testing apparatus. Concrete specimens of 100×100×400mm with compressive strength class 60MPa was used, sealed and loaded at different curing temperature. The effects of the curing temperature and the age at loading on creep behavior are discussed. The results show that tensile specific creep and creep rate of HPC at early ages were governed by the age at loading. The specific creep, creep coefficient and creep rate were larger at earlier loading ages, and decreased exponentially with age at loading. The tensile specific creep decreased with curing temperature, but the difference in creep due to different curing temperatures decreased with the age at loading, and could be ignored while concrete specimen being loaded after the age of 7 days.

Abstract: Constant stress tensile creep tests were conducted on an AZ 91–25 vol.% Al₂O₃-SiO₂ short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material is shown to be considerably improved compared with the matrix alloy. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by SEM revealed good fiber–matrix interface bonding during creep exposure. Short fibers have a great function in load bearing and load transfer, and greatly hinder the dislocation movement, thus enhancing the creep resistance of the composite. Damage and multiple rupture of aluminum silicate short fiber, quality of the interface combination between aluminum silicate short fiber reinforcement and the matrix, are two important factors of the creep deformation microstructure process control of Al₂O₃-SiO_2(sf)/AZ91 composite. The creep mechanism of the composite is dislocation and grain boundary sliding control.

Abstract: By means of tensile and compression creep testing and SEM, TEM observation, an investigation has been made into the microstructure evolution of a single crystal nickel base superalloy during tensile / compression creep. Results show that the cubic γ′ phase in the superalloy is transformed into the N-type meshlike structure along the direction vertical to stress axis during tensile creep. The cubic γ′ phase is transformed into the P-type structure along the direction parallel to stress axis during compression creep. An obvious asymmetry strain of the alloy occurs during tensile and compression creep, the formation of the needle-like γ′ rafts during compression creep is a main reason of the alloy displaying a smaller strain. During compressive creep, the deformation feature of the alloy is the <110> and (1/3) <112> super dislocations shearing into the γ′ rafts. The deformation mechanism of the alloy, in the stage state of tensile creep, is dislocation climb over the γ′ rafts.

Abstract: A uniaxial experimental set-up was developed to evaluate the self-induced stresses, visco-elastic strains and cracking potential of concrete from the time of its placement. The totally restrained condition is achieved by keeping the length change within a threshold of 3μm. The temperature rise of the specimens can be controlled within 1.5
, by which the deformation because of temperature change due to cement hydration could be prevented. The tensile force required to compensate the shrinkage could be monitored and elastic strain and tensile creep can be calibrated without an obvious interference of temperature deformation. The preliminary results testified the workability of the system.

Abstract: The tensile creep behavior of NiAl-9Mo eutectic alloy has been investigated over a stress range of 50 to 100MPa at the temperatures ranging from 850 to 950°C. All of the creep curves exhibit the very long steady-state stage. The creep parameters and TEM observations indicates the kinetics of the steady-state creep deformation is governed by dislocation climb in the NiAl matrix phase. The crack origination and development at the colony boundary results in the onset of tertiary creep stage and final fracture of the alloy.