Papers by Keyword: Creep

Abstract: Herein, we compared thermal desorption analysis (TDA) curves obtained by conducting experiments and simulations. In addition, we discussed the validation of our simulations and trapping sites of hydrogen atoms. In as-received F82H, when the samples contained solute atoms, grain boundaries, dislocations, and precipitates, the experimental curve corresponded to the simulated curve. In positron annihilation lifetime (PAL) measurements, di-vacancies were detected in the electron-irradiated F82H. When we changed the growth and the concentration of vacancy-type defects during temperature increase using the rate theory, the simulation results agreed with experiment results. In creep-ruptured Fe, only dislocations were detected by the PAL measurements. However, the existence of a type of defect, which was related to grain boundaries, must be assumed to fit the simulation curve to the experimental one. In the next step, the diffusion of hydrogen atoms on grain boundaries should be added to simulation program.

Abstract: Creep may affect structural behavior by violating service limit states, redistributing stress or losing prestress forces. For that, knowing the significance of factors that influence creep is a must to accurately predict it. This paper uses the Northwestern university (NU) database to investigate the factors that most affect long-term creep of concrete. Factorial ANOVA was applied to identify the significance of factors affecting creep of concrete at 3000 days using R software. The factorial ANOVA results showed that the sustained load and the relative humidity have the major effect on the value of concrete creep at 3000 days.

Abstract: Samples of TiAl-based matrix in-situ composite with the chemical composition Ti-46.4Al-5.1Nb-1C-0.2B (at.%) reinforced with a low volume fraction of primary Ti₂AlC particles were prepared by vacuum induction melting in graphite crucibles and centrifugal casting into graphite moulds. The hot isostatic pressing (HIP) of the as-cast samples and subsequent heat treatments leads to the formation of equiaxed grains with fully lamellar α₂(Ti₃Al) + γ (TiAl) microstructure and uniformly distributed Ti₂AlC and TiB particles. The minimum creep rates of the in-situ composite are significantly lower compared to those measured for the counterpart low carbon benchmark alloy with the chemical composition Ti-47Al-5.2Nb-0.2C-0.2B (at.%) at temperatures ranging from 800 to 900 °C and applied stress of 200 MPa. The studied in-situ composite shows also significantly improved creep resistance compared to that of some TiAl-based alloys with fully lamellar, convoluted and pseudo-duplex microstructures at a temperature of 800 °C and applied stress of 200 MPa.

Abstract: Heat-resistant Ti-Al-Nb-Zr alloys, which don’t contain Sn, have been designed to obtain good oxidation resistance above 600 °C. In addition, to design Ti alloys with best balance of creep and fatigue properties, prior β grain size which affects fatigue properties and lamellar microstructure which affects creep properties were controlled by heat treatment. In the present study, the effect of microstructure on creep properties of one of the alloys, i.e., Ti-7.5Al-4Nb-4Zr alloy, with the bimodal (B), the lamellar structures in small prior β grains (L_S), and the lamellar in large prior β grains (L_L) were investigated at 600 °C. The creep deformation mechanism for each microstructure was a power-law creep. However, the creep life varied depending on the microstructures. The longest creep life was obtained in L_S with prior β grain size of 90 μm and interlamellar spacing of approximately 10 μm, while the shortest creep life was obtained in L_L with prior β grain size of 550 μm and fine interlamellar spacing of less than 2~3 μm. This suggests that creep life is more affected by interlamellar spacing than by prior β grain size.

Abstract: Prolongation of the service life of key components of fossil-fuelled power plants beyond their original design limit must be accompanied by thorough and extensive monitoring of the actual material state and particularly creep damage. The extent of cavitation is nowadays routinely tested in-situ by using replica method and its quantification is based on practices stated in VGB-TW 507, NORDTEST NT TR 302 derived from the original Neubauer's classification of cavitation damage. Evaluation of cavitation damage based on the measurement of the number of cavities and/or creep micro-cracks has become a routine activity, but in cases when steel contains large amount of non-metallic inclusions, they can be wrongly identified as cavities and to invalidate the result of calculation of cavitation damage. Comparative analysis of cavitation damage was performed in a pipe bend and its weldment made of a low-alloy 0.5Cr-0.5Mo-0.3V steel creep exposed at 540 °C for more than 225,000 hours by using OM (SEM) and replica method and revealed the comparative extent of cavitation damage decreasing from the outer to inner surface of the pipe wall.

Abstract: Creep tests using a simple jig have been performed on 63 wt.% tin-37 wt.% lead solder wires of diameters 1 mm and 2 mm at room temperature (23°C). Coils containing 5 or 10 rings were allowed to creep under their own weight for 60 minutes. It was noted that for either of these diameter wires, the coil with 10 rings had significantly large vertical displacements as compared to those with 5 rings. In each particular coil, the highest vertical displacements were in the bottom rings. The overall maximum vertical displacement was 76.5 mm and this was in a bottom ring of the 2 mm diameter wire with 10 rings. However, in all cases, the amount of horizontal displacement was negligible. The bottom ring of the 2 mm diameter wire had the largest initial strain of 0.151 at 5 minutes and final strain of 0.546 at 60 minutes; this was in the coil with 10 rings. Although no consistent pattern in change between the initial and final diameters was noted for the rings in any coil even after 24 hours of creep, it was quite apparent that in a majority of cases a change in diameter occurred. The main factor responsible for the observed creep is attributed to the weight of the rings in the coils rather than capillary flow.

Abstract: The present study investigates the influence of carbon fibers of «Belum» brand on the performance properties of polymer composites based on ultrahigh-molecular weight polyethylene. It was established that the composite complex with the content of carbon fibers in the amount of 5 wt.% has the optimum complex of properties. The rate of mass wear is reduced by 3.3 times while maintaining the physical and mechanical characteristics at the level of the original polymer. It was established that the creep of PCM with the composition of UHMWPE + 5 wt.% Belum is 2 times less than the initial polymer. Also, the properties of the developed material based on UHMWPE were compared with unfilled and modified polytetrafluoroethylene (PTFE). It is shown that the creep of UHMWPE is less than the creep of PTFE by 12.5 times. The creep of the composite based on UHMWPE is less than the creep of the composite based on PTFE 13 times.

Abstract: . A primary current focus in concrete shear design is how to ensure the shear resistance of the concrete structures to withstand the possible adverse creep effects during the service life. All shear-carrying actions are supposed to depreciate due to the enhanced critical shear cracks under sustained loads. However, only a few studies that evaluated the performance of concrete structures failing in shear due to long-term loading. As the longitudinal tensile reinforcement strain influences the shear strength of RC beam, the shear resistance of the beam under sustained load may also be affected by the amount of longitudinal reinforcement. The present study aims at investigating the influence of flexural reinforcement ratio on the shear creep failures of reinforced concrete (RC) beams without stirrups by non-linear finite element analysis. In this study, the numerical model of RC beams with reinforcement ratios varying from 0.4% to 3.2% was evaluated under different loading rates. A loading rate of 1000 and 10000 times slower than the static loading rate were adopted to reveal the creep effects of RC beams.

Abstract: Tensile and compressive creep behavior of SLMed IN718 alloy under 973K (700°C) were investigated. Crept samples were analyzed by SEM and TEM to expose evolution of microstructure, precipitates and dislocation structure during the creep process. Results show that initial creep rate under compression is higher than under tension for the same creep conditions. Minimum creep rates are approximately the same both in tensile and compressive creep tests. The different creep behaviors may be related to the fact that tension stress promotes precipitations of fine needle-like γ′′ phases, while compression stress promotes precipitations of large size δ phases. The tension-compression asymmetry owns to the increment of chemical potential varying with the stress orientation.

Abstract: In this present study, molecular dynamics simulation of creep for ultrafine grain NC Ni specimens with different grain sizes have been carried out under a constant 1 GPa applied load for various creep temperatures to study the dependence of grain growth on creep temperature and grain size during creep process and its influence on creep properties. It is observed that the extent of grain growth in ultrafine grain NC Ni during creep deformation process is more if creep in creep temperature is higher. A noteworthy anomaly, that is NC Ni with smaller grain exhibits better creep property compared to NC Ni with larger grain, is observed in case of higher creep temperatures (i.e. around or greater than 1400K).