Papers by Keyword: Creep Behaviour

Abstract: Inorganic Matrix Composite (IMC) systems have been extensively studied in recent years as an effective alternative to the most known organic Fiber Reinforced Polymer (FRP) systems, especially for strengthening applications on masonry. Despite intensive research has been conducted to study mechanical properties and bond mechanisms of such systems, also under different environmental conditions, little information is available on their long-term behaviour under sustained load. In the framework of a large assessment plan for the analysis of fabric reinforced inorganic matrix composites, long-term tensile tests have been performed on different systems in a newly developed test setup. The paper will aim at describing the proposed setup and instrumentation, as well as the results in terms of measured displacements for different combination of fabrics and matrices. Results in terms of retained tensile strength will also be presented, showing the little influence of the long-term loading on the performance of the studied composite systems.

Abstract: Creep is defined as a time dependent component of plastic deformation. Creep tests can be performed either at constant load or at constant applied stress. Engineering creep tests carried out at constant load are aimed at determination of the creep strength or creep fracture strength, i.e. the data needed for design. The constant stress tests are important as a data source for fundamental investigations of creep deformation and fracture mechanisms and for finite element modelling of more complex stress situations. For some materials, the difference between the two type of testing can be very small, while for other materials is large, depending on the creep plasticity of the material under testing. The paper aims to compare the creep results of two different creep-resistant materials: the advanced 9%Cr martensitic steel (ASME Grade P91) and a Zr1%Nb alloy obtained by both testing methods and to clarify the decisive factors causing observed differences in their creep behaviour.

Abstract: The Abaqus software was used to simulate the creep behavior of a cylindrical tube of Sanicro-25 austenitic steel under an internal pressure of 11.3 MPa at a uniform temperature at 750 °C. The data used for the simulation input were obtained from the experimental data of a previous work. The hidden information of material parameters was estimated from the shape of creep strain versus time plots obtained from the experimental data. The validated results between the simulation and the experimental data produced the material parameter of the creep power law which were set at 2.6 x 10^-22 for the power law constant and 9 for the stress exponent. The parameters were further used to explore the stress and strain inside and outside of the tube wall and the thickness changes of the tube wall.

Abstract: The tensile and compressive creep behaviour of aluminium alloy 2050 with T34 initial temper (AA2050-T34) during creep-ageing process has been experimentally investigated and analysed in detail. Both tensile and compressive creep-ageing tests under various stress levels (ranging from 100 MPa to 187.5 MPa) have been carried out at a temperature of 155 °C for 18 hours. The results show that creep strains under tensile stresses are much larger than those under the same levels of compressive stresses and a new “double primary creep feature” with five-stage creep behaviour has been observed in the alloy during the creep-ageing tests. The conventional power-law creep equation was applied to analyse the new creep behaviour of the alloy at the steady-state creep stage. Furthermore, the power-law relationship between the applied stress and the corresponding creep strain rate was found to be effective in all creep-ageing stages of the alloy and was used for further analysis. These analyses indicate that the dislocation and diffusion mechanisms may both contribute to this new creep behaviour and may play different roles in different creep-ageing stages. Moreover, the evolution of the creep resistance or threshold creep stress of the alloy during the creep-ageing process was quantitatively investigated by the proposed relationship. These results help to not only understand the new creep behaviour of AA2050-T34 during the creep-ageing process, but also facilitate further constitutive modelling of this new creep behaviour for its creep age forming applications.

Abstract: Experiments were conducted to investigate deformation-induced processes during in-situ tensile test at elevated temperature. Consequently the microstructure after creep loading was examined by 3D Electron Back Scatter Diffraction (EBSD) technique. The billets of coarse-grained copper were processed by equal-channel angular pressing (ECAP) at room temperature using a die that had an internal angle of 90° between the two parts of the channel and an outer arc of curvature of ~ 20°, where these two parts intersect. The pressing speed was 10 mm/min. To obtain an ultrafine-grained (UFG) material, the billets were subsequently pressed by route Bc by 8 ECAP passes to give the mean grain size ~ 0.7 μm. The constant strain-rate test in tension was performed at 473 K using testing GATAN stage Microtest 2000EW with EH 2000 heated grips which is configured for in-situ electron back scatter diffraction (EBSD) observations. Microstructure was examined by FEG-SEM TESCAN MIRA 3 XM equipped by EBSD detector HKL NordlysMax from OXFORD INSTRUMENT. The tensile test was interrupted by fast stress reductions after different deformation step and observation of microstructure changes was performed. Despite of a considerable interest in ECAP processing method, there are not many works documenting microstructure evolution and changes during creep testing and determining creep mechanisms of ultrafine-grained materials processed by ECAP. It was found that creep resistance of UFG pure Al and Cu is considerably improved after one ECAP pass in comparison with coarse grained material, however, further repetitive pressing leads to a noticeable deterioration in creep properties of ECAP material. Recently it was observed the coarsening of the grains in microstructure of ECAP copper during creep at elevated temperature. It was suggested that creep behaviour is controlled by storage and dynamic recovery of dislocations at high-angle boundaries. In the present work was found that ultrafine-grained microstructure is instable and significant grain growth has already occurred during heating to the testing temperature. Static recrystallization during heating led to the formation of high fraction of special boundaries Σ3 and Σ9. The tensile deformation at 473 K led to the additional grain growth and formation of new grains. Microstructure was investigated also by 3D EBSD.

Abstract: Microstructures and creep property of the Mg-4Al based alloy with addition of 2% Sr and 1%Ca were investigated. The as-cast microstructures of the present alloy consist of dendritic α-Mg and two major intermetallics: lamellar eutectic C14-Mg₂Ca and bulky type Mg-Al-Sr ternary phase. These intermetallics mainly distribute along grain or cell boundaries and form an almost continuous network. The alloy studied shows an excellent creep resistance under the experimental conditions. This is primarily attributed to formation of the thermostable intermetallics with addition of Sr and Ca to Mg-Al based alloy. The values of stress exponent, n, and creep activation energy, Q, imply that both dislocaiton motion and grain boundary sliding contribute to the creep deformation.

Abstract: The nickel base alloy IN 738 LC is in use for gas turbine blades since more than 25 years. In high temperature creep testing the conventionally cast alloy exhibits a comparably large property scatter which requires high safety margins in design and dimensioning and subsequently causes an incomplete exploitation of the materials potential. The reasons for this property scatter were investigated and traced back to different influencing factors. Parallel to investigations on the microstructure of post-exposure material and conventional scatter band analysis, artificial neural networks were successfully applied to discover relations between the chemical composition of the individual melt and the position of the corresponding test results within the global scatter band. Recommendations for a lifting of the lower scatter band boundary and the mean curve are derived.

Abstract: By the measurement of creep curves and microstructure observation, an investigation has been made into the creep behaviors and microstructure evolution of a single crystal nickel-based superalloy containing 4.2%Re. Results show that the superalloy displays an obvious sensibility on the applied temperatures and stresses in the range of the applied temperatures and stresses. During the initial creep, the cubical g¢ phase in the alloy is transformed into an N-type rafted structure along the direction vertical to the applied stress axis. After crept up to fracture, the rafted g¢ phase in the region near fracture is transformed into a twisted configuration. The dislocation climbing over the rafted g¢ phase is considered to be the main deformation mechanism of the alloy during the steady creep state, and dislocations shear into the rafted g¢ phase is the main deformation mechanism of the alloy in the later stage of creep.

Abstract: The objective of this study is to investigate and evaluate the effect of fine aggregates (aggregate size smaller than or equal to 2.36 mm) on the compressive strength and creep behavior of asphalt mixtures. The variables that are considered in the study include the sizes and gradations of fine aggregate. A kind of standant aggregate gradation and four kinds of reduced aggregate gradation mixture specimens are used. Uniaxial compression and static creep tests were realized at different loading conditions. The test results showed that the different fine aggregate sizes do not result in significant differences in compressive strength and creep values using the same percentage of fine aggregates (38.4%). Only the different gradations showed a little differences for mixtures made with different gradations but same aggregate size (between 2.36 and 1.18 mm).

Abstract: The creep experiments are performed under the various stresses at 595°C, 610°C, 640°C and 670°C for P92 steel which broadly used in USC plant pipeline. The specimens are taken from the extrados part of a new manufactured main steam elbow. A series of integrated creep curves are obtained, and the maximal test time last over 13000h. The relationship between the minimum creep rate and stress at different temperatures is explained by Norton formula. Both the 105 h creep rupture strength and stress of 1% creep strain in 105h of P92 steel at the four above temperatures are extrapolated with a list by using the Monkman-Grant formula, which are consistent with the reference experiment data at 600°C and 650°C. Meanwhile, damage during the creep process is discussed applying Norton creep damage formula and θ function method.