جستجوی مقالات مرتبط با کلیدواژه « creep » در نشریات گروه « فنی و مهندسی »

In many construction projects, it is necessary to excavate the land so that its walls are vertical or close to vertical. Lateral pressure is exerted on these walls due to the movement of the soil behind it. In order to prevent the collapse of the walls of the excavated site and its possible consequences, temporary or permanent structures are implemented, which is called stabilization. Excavations are secured for various reasons. They may be stabilized to protect personnel entering and working in the excavation or to protect buildings or municipal services adjacent to excavation. Over the past years, there have been a large number of excavations in large cities, including Tehran, which were abandoned due to some problems. Furthermore, while some excavations need to be designed for a long time, long-term design basics are not observed during such processes. Stabilization of most of these excavations has been done using anchors. In this method, after placing and implementing the anchors, they are prestressed by applying force. Observations and long-term surveys of these excavations show that in some cases, locking force of anchors decreases, leading to dangers. Therefore, it is very important to know the long-term behavior of anchors in excavations to investigate the stability of these excavations that are prolonged or abandoned for a long time. In this study, field data of the long-term behavior of a excavation in Tehran is used and numerical modeling is done based on this case study. Verification and calibration of the numerical model has been done based on field measurements. Also in this article, relationships based on the results of the numerical model have been proposed to predict the anchor load in the long term in cohesive coarse-grained soil. The proposed relationships predict well the anchors load at one year after the end of excavation. These relationships are separated to three categories of five strands anchors, six strands anchors and six strands anchors with short length. In the following, the variables affecting the long-term behavior of the anchor loads embedded in the excavation have been studied. These variables include soil properties, depth of the excavation and neighbor surcharge of the excavation. The results of this article, in addition to presenting the relationship for anchor load prediction, include the introduction of variables that affecting long-term behavior. The parametric study shows that with the increase of the angle of internal friction or the increase of soil cohesion, the amount of anchors load decreases over the time. Also, by increasing the depth of the excavation or increasing the neighbor surcharge of the excavation, the load of the anchors decreases less over time. Of course, the impact of excavation depth occurs mostly in the lower anchors and does not affect the anchors of the first row and close to the ground. Studies showed that anchors close to the earth surface have a greater rate of load reduction over time among anchors with the same length and number of strands. Also, among the same anchors, anchors with shorter length experience more load reduction over time.

In this paper, by using the Chaboche kinematic hardening model with the isotropic hardening law, the effect of temperature and bending moments is investigated on the strain accumulation behavior of carbon steel piping branch. Carbon steel branch junctions under internal pressure and temperature with dynamic bending moment are tested at five temperatures of 20, 50, 100, 150 and 200 ° C. The results obtained by numerical analysis show that the highest amount of ratcheting occurred near the branch junctions in the circumferential direction. The strain ratcheting occurred mainly because of dynamic moments and high temperatures. The results show that in all three samples, the amount of strain ratcheting increases with increasing of dynamic moment level and temperature. With increasing of the ratio of diameter to thickness in branch junctions, the onset of strain accumulation occurs at low moment levels. It can be concluded that initially, the rate of strain ratcheting is high and with the increase of loading cycles, this rate decreased due to the strain hardening phenomenon. The increase of strain ratcheting at high temperatures is due creep strain because of high temperature and mainly accumulated plastic strain under dynamic bending moments because of cyclic plasticity.

The aim of this study was to investigate the creep behaviour of GTD111 nickel base superalloy produced by directional solidification method. First, a cluster of cylinders of this alloy were cast by Bridgman method. Then, by preparing the tensile specimens, they were subjected to creep test at 870˚C and different stresses. Creep behaviour and microstructural changes of this alloy were measured and evaluated by various tests. Considering that the aim of this project is to identify the creep behaviour in the condition that the grains deviate from the ideal angle during directional solidification, first the grain structure was determined in casting samples, then their mechanical behaviour was investigated by determining the grain deviation. Also, numerical simulation was used to model the grain size of the cast samples. By three-dimensional modelling of grains in creep specimens and then changing the growth angle of dendrites in each grain, numerical analysis of the creep of the specimen using Norton equation and changes in creep life by changing the deflection angle of the grains was obtained. The results showed that increasing the angle of deviation during the growth in a grain from zero to 3 degrees caused a 0.4% increase in mean stress and increasing the growth angle in the same grain from 3 to 10 degrees, caused a 5% increase and an increase from 10 to 20 degrees caused an 11% increase in mean stress in this grain in the middle section of the sample.

Today, due to the importance of reducing the time between design and production, the use of methods such as fused deposition modeling (FDM), is considered as a good alternative compared to reduction production methods. Poly lactic acid (PLA) is used as a biocompatible polymer in many engineering and medical fields. Improving the long-term load bearing capacity of such products is one of the essential criteria to ensure the proper performance of these components. In this study, in order to improve the fatigue life and creep resistance of samples made of poly lactic acid polymer during the fused deposition modeling, to evaluate the effect of layer height, layer thickness, infill percentage and infill pattern on fatigue behavior. And creep resistance of the samples was investigated. Also, in order to benefit from statistical analysis, the response surface methodology has been used. Analysis of variance (ANOVA) showed that the percentage of filling and layer thickness had the greatest effect on increasing the fatigue life and reducing the creep rate of the samples. The results showed that layering based on layer height of 0.1 mm, layer thickness of 0.3 mm, infill percentage of 75% using honeycomb pattern had the highest fatigue life and creep resistance of the samples.

The time-dependent behavior of sands is one of the main issues that can influence many projects. In this study nine sand models with different particles shape and sizes and different grain distribution have been used to investigate creep behavior. The samples were prepared at 60% relative density and the constant stresses of 45 kPa for 30 days and 26 to 416 kPa for seven days (with the double increase in stress increment) under dry conditions in the Oedometer apparatus (one-dimensional consolidation) were tested. The results showed that the rotation, slipping, and crushing of the sand grains over time under constant load was the cause of creep deformation in the dry samples. Sand samples exhibit 1.5 to 4.2% creep strains in 30 days, and 0.61 to 2.5% creep strains in seven days’ time interval. Finally, two linear equations are presented to predict the deformation of sands over time. The results were in good agreement with previous studies.

In this paper, the creep strength of hybrid composites used in the new generation of power transmission line conductors is predicted. The hybrid composite rods consist of carbon fiber/epoxy composite core surrounded by a glass fiber/epoxy composite shell. The hybrid rods were fabricated by using the pultrusion process. Dynamic mechanical analysis was carried out at various frequencies on specimens cut from the carbon/epoxy fiber composite core. In addition, the hybrid composite rods were subjected to three-point bending experiments at constant loading rate and different temperatures. The master curve of the storage modulus corresponding to carbon/epoxy composite core was derived at the desired reference temperature based on the time-temperature superposition principle. Consequently, the master curve of the constant strain rate flexural strength was constructed using the time-temperature shift factors and the monotonic flexural strengths of hybrid composites at different temperatures. Based on these data, the creep strength master curve was developed at the operating temperature. The prediction of creep life based on the constructed master curve shows a proper response of these conductors at service condition.

HP austenitic steels are widely used to produce cracking tubes due to their oxidation resistance, thermal stability and high-temperature creep resistance. Operating life of these cracking tubes is usually less than their design life, which is one of the most important problems of industrial units. Aluminizing has also been used in recent years to protect superalloys and steels in oxidizing and corrosive environments. Among the various aluminizing methods, out-of-pack cementation is ideal due to its uniformity of thickness and low cost. In this study, creep samples were first prepared according to ASTM E8/E8M standard, coated for 4 hours at 1050°C. Then, creep tests were performed at 850°C, 900°C and 950°C. Finally, microstructure of the coating and substrate was studied and by examining the creep test results and comparing with the uncoated samples, it was concluded that factors such as heat treatment applied to the samples during coating process or creating an interdiffusion zone in the structure can lead to reduce the high-temperature creep life of the samples.