Multi-scale fatigue damage model for steel structures working under high temperature

In order to better understand the fatigue mechanisms of steel structures working under high temperature, a multi-scale fatigue damage model at high temperature is developed. In the developed model, the macroscopic fatigue damage of metallic materials due to the collective behavior of micro-cracks is quantified by using the generalized self-consistent method. The influence of temperature on fatigue damage of steel structures is quantified by using the previous creep damage model. In addition, the fatigue damage at room temperature and creep damage is coupled in the multi-scale fatigue damage model. The validity of the developed multi-scale damage model is verified by comparing the predicted damage evolution curve with the experimental data. It shows that the developed model is effectiveness. Finally, the fatigue analysis on steel crane runway girders (CRGs) of industrial steel melt shop is performed based on the developed model.

Layer Structure Analysis of Low-Carbon Steel Containing Rare Earth by High-Temperature Carburizing of Liquid Cast-Iron

The layer structure of low-carbon steel containing RE by high-temperature (T>1200 ℃) carburizing of liquid cast-iron was studied and the diffusion activation energy of carbon was calculated by metallographic microscpe, chemical analysis etc. The result shows that the technology of carburizing in liquid cast-iron can expedite caburization distinctly and changes the carburizing layer structure. The carburizing rate is 60～80 times of that of the traditional technology, and there is about 43% decrease in the activation energy compared with gas-carburization. In outer structure layer, cementite is formed simultaneously both on the crystal boundary reticularly and inside the crystal grains stripedly. In inner carburizing layer, there is undissolved blocky ferrite in reticular cementite. Besides, rare earth element can expedite carburization process.

Design and evaluation of UHPP steel bridge deck pavement for high-temperature and rainy regions

To enhance the serviceability of steel bridge deck pavement(SBDP)in high-temperature and rainy regions,a concept of rigid bottom and flexible top was summarized using engineering practices,which led to the proposal of a three-layer ultra-high-performance pavement(UHPP).The high-temperature rutting resistance and wet-weather skid resistance of UHPP were evaluated through composite structure tests.The internal temperature distribution within the pavement under typical high-temperature conditions was analyzed using a temperature field model.Additionally,a temperature-stress coupling model was employed to investigate the key load positions and stress response characteristics of the UHPP.The results indicate that compared with the traditional guss asphalt+stone mastic asphalt structure,the dynamic stability of the UHPP composite structure can be improved by up to 20.4%.Even under cyclic loading,UHPP still exhibits superior surface skid resistance compared to two traditional SBDPs.The thickness composition of UHPP significantly impacts its rutting resistance and skid resistance.UHPP exhibits relatively low tensile stress but higher shear stress levels,with the highest shear stress occurring between the UHPP and the steel plate.This suggests that the potential risk of damage for UHPP primarily lies within the interlayer of the pavement.Based on engineering examples,introducing interlayer gravel and optimizing the amount of bonding layer are advised to ensure that UHPP possesses sufficient interlayer shear resistance.

Effects of low temperature on properties of structural steels

The experiments were carried out to measure the mechanical properties ofthree grades of structural steels (Q235A, 16Mn and Q390E steel) at low temperature. It was shownthat the strength of the steels increases while the plasticity and toughness decrease as temperaturedrops. In the transitional area the toughness drops rapidly with temperature. Among the threestructural steels, Q390E steel has the best toughness and the lowest sensitivity.

Temperature control and cracking prevention in coastal thin-wall concrete structures

A three-dimensional finite element program for thermal analysis of hydration heat in concrete structures with a plastic pipe cooling system is introduced in this paper. The program was applied to simulation of the temperature and stress field of the Cao＇e Sluice during the construction period. From the calculated results, we can find that the temperaiure and stress of concrete cooled with plastic pipes are much lower than those of concrete without pipes. Moreover, plastic pipes could not be corroded by seawater. That is to say, a good effect of temperature control and cracking prevention can be achieved, which provides a useful reference for other similar nearshore concrete projects.

Experimental study on stress-strain-temperature models for structural steel

For the research on steel structure in fire,it is very important to determine the properties of structural steel at elevated temperature.Up to now,the high-temperature properties of material is believed to be related to only temperature state,which is not precise enough to simulate the behavior of steel structures under different combinations of heating,cooling,loading,and unloading.To analyze the influence of the temperature-load history on the steel properties,a series of tests were carried out under different temperature-load paths about steel Q235,which is widely used in steel structures in China.In this paper,the method to set the temperature-load paths was introduced;the variety regulation of steel properties changing with temperature was analyzed under different paths;according to experimental results,the formulas of elastic modulus and yield strength at elevated temperature were fitted,and the stress-strain-temperature 3D relationships of structural steel under different paths were presented.

Ice Induced Low Temperature Fatigue Crack Propagation in Offshore Structural Steel A131

To investigate the low temperature fatigue crack propagation behavior of offshore structural steel A131 under random ice loading, three ice failure modes that are commonly present in the Bohai Gulf are simulated according to the vibration stress responses induced by real ice loading. The test data are processed by a universal software FCPUSL developed on the basis of the theory of fatigue crack propagation and statistics. The fundamental parameter controlling the fatigue crack propagation induced by random ice loading is determined to be the amplitude root mean square stress intensity factor K-arm. The test results are presented on the crack propagation diagram where the crack growth rate da/dN is described as the function of K-arm. It is evident that the ice failure modes have great influence on the fatigue crack propagation behavior of the steel in ice-induced vibration. However, some of the experimental phenomena and test results are hard to be physically explained at present. The work in this paper is an initial attempt to investigate the cause of collapse of offshore structures due to ice loading.

Estimating extreme temperature differences in steel box girder using long-term measurement data

The extreme temperature differences in fiat steel box girder of a cable-stayed bridge were studied.Firstly,by using the long-term measurement data collected by the structural health monitoring system installed on the Runyang Cable-stayed Bridge,the daily variations as well as seasonal ones of measured temperature differences in the box girder cross-section area were summarized.The probability distribution models of temperature differences were further established and the extreme temperature differences were estimated with a return period of 100 years.Finally,the temperature difference models in cross-section area were proposed for bridge thermal design.The results show that horizontal temperature differences in top plate and vertical temperature differences between top plate and bottom plate are considerable.All the positive and negative temperature differences can be described by the weighted sum of two Weibull distributions.The maximum positive and negative horizontal temperature differences in top plate are 10.30 ℃ and -13.80 ℃,respectively.And the maximum positive and negative vertical temperature differences between top plate and bottom plate are 17.30 ℃ and-3.70 ℃,respectively.For bridge thermal design,there are two vertical temperature difference models between top plate and bottom plate,and six horizontal temperature difference models in top plate.

Temperature Stress Analysis of Prestressed Concrete Box Girder with Corrugated Steel Webs

To figure out the distribution of temperature gradient along the girder height of steel-concrete composite box girder, combined with the mechanical characteristics of prestressed concrete composed box girder with corrugated steel webs, the calculation formulas of cross-sectional temperature stress along the span in a simply-supported beam bridge with composite section were derived under the conditions of static equilibrium and deformation compatibility of the beam element. The methods of calculating the maximum temperature stress value were discussed when the connectors are assumed rigid or flexible. Theoretical and numerical results indicate that the method proposed shows better precision for the calculation of temperature self-stress in both the top and the bottom surfaces of the box girder. Moreover, the regularity of temperature stress distribution at different locations along the girder span is that the largest axial force of the top or the bottom plate of the box girder is located in the midspan and spreads decreasingly until zero at both supported ends, and that the greatest longitudinal shear density in steel-concrete interface appears at both supported ends and then reduces gradually to zero in the midspan.

Double⁃Shear Experiments of Cold⁃Formed Steel Stud⁃to⁃Sheathing Connections at Elevated Temperatures

The behavior of cold⁃formed steel(CFS)stud⁃to⁃sheathing connections at elevated temperatures is an important parameter for the fire resistance design and modeling of mid⁃rise CFS structures.In this paper,three kinds of sheathings,namely,medium⁃and low⁃density calcium⁃silicate boards and oriented strand board,were selected for double⁃shear experiments on the mechanical properties of 253 screw connections at ambient and elevated temperatures.The effects of the shear direction,screw edge distance and the number of screws on the behavior of the connections were studied.The results showed that the shear direction and the screw edge distance more significantly influenced the peak deformation,while their impacts on the peak load varied with the type of sheathings.Compared with the single⁃screw connections,the peak loads of the specimens with double⁃screw connections obviously increased but did not double.Finally,a simplified load⁃displacement curve model of stud⁃to⁃sheathing connections at elevated temperature was generated first by establishing the prediction formula for characteristic parameters,such as the peak load,the peak deformation and the elastic stiffness,and then by considering whether the curves corresponded to stiffness increase phenomena.The present investigation provides basic data for future studies on the numerical modeling of CFS structures under fire conditions.

Regularity of Structural Superplasticity of Hypoeutectoid Steel

The curve of the relation between fracture elongation and temperature for unltrafine grained low alloy hypoeutectoid steels generally shows that three peaks occur at temperatures just below A_ c1, between A_ c1 and A_ c3, and just above A_ c3, When these steels are subjected to superplastic tensile test at different temperatures, the value of the last one is the lowest and the other two peaks are different one from another depending on components, microstructures and deformation conditions of the steels.

超长钢结构屋盖施工及服役过程健康监测与分析

武汉江夏大花山户外运动中心钢结构屋盖为超长空间网格结构,为研究其在施工卸载过程和服役状态的受力和变形情况,在结构关键部位布置传感器,并进行了为期两年的应力、变形和温度健康监测。施工监测结果表明结构整体的应力和变形变化平稳,应力和竖向位移变化最大值出现在馆外屋盖上,且均满足设计和规范要求。服役健康监测结果表明温度作用是导致结构成型后受力和变形变化的主要因素,在不均匀温度场的影响下馆外边桁架对温度变化最为敏感,其应力与温度的线性系数最大为-4.64,同时温度升高会使得屋盖发生整体膨胀变形,导致边桁架跨中竖向位移减小。最后针对钢结构屋盖提出了优化的弹性支座分布方案,该方案相较于支座固接方案的温度应力平均减少了74.07%。

不同奥氏体温度下22MnB5热成形钢连续冷却转变规律研究

为探讨22MnB5热成形过程中的组织演变规律,对其热成形过程组织与性能进行调控处理,采用热膨胀法研究了22MnB5热成形钢在不同奥氏体化温度(850℃,900℃,950℃)和不同冷却速率下的连续冷却相变行为,利用光学显微镜、扫描电镜与维氏硬度计对其组织与显微硬度进行了表征分析。结果表明,奥氏体化温度对22MnB5相变规律与性能影响较大,低温奥氏体化有利于中高温扩散型相变发生,铁素体与珠光体转变覆盖冷速范围广;随奥氏体化温度的升高,实验钢各相变向“右”移动,铁素体与珠光体转变区域缩小,发生贝氏体与马氏体相变临界冷却速率减小。低冷却速率条件下,奥氏体化温度对实验钢的硬度影响不大,硬度值为170~185 HV_(0.5);高冷却速率条件下,奥氏体化温度对实验钢的硬度影响显著,20℃/s条件下,硬度值由850℃奥氏体化的299.4 HV_(0.5)增加至950℃奥氏体化的437.7 HV_(0.5)。在900℃奥氏体化且以较高冷却速率进行冷却,获得细小马氏体(或含极少量细小铁素体)组织,此时强度与塑性(冷弯角)匹配最好。

大跨度钢网壳卸载过程温度响应模拟与实测研究

对于常出现大温差、骤降温度、太阳辐射等特殊环境的沙漠地区,强温度作用对处于施工阶段的空间结构产生较大影响。以民勤文体中心体育馆网壳卸载过程为例,运用支座位移法模拟分析双层网壳卸载施工过程,并对卸载过程中结构不同边界条件及不同温度作用进行参数化分析,同时结合健康监测的温度、应力及变形数据,分析温度效应对结构卸载阶段的影响。结果表明,太阳辐射作用和边界约束条件的改变会直接影响卸载过程中应力变化规律,固定约束下仅考虑自重的卸载应力仅为温度应力的10%~30%;在实际施工过程中,大跨度钢结构考虑温度影响的模拟值更趋近实测值,需考虑特殊温度环境对结构力学性能的影响。

600 MW超超临界机组S30432钢内壁氧化膜变化规律及水蒸气氧化动力学

通过SEM对某600 MW超超临界机组运行80000 h后的S30432钢高温再热器管内壁氧化膜进行氧化膜形貌和膜层结构分析,再用EDS、XRD对氧化物中成分测定,并分析氧化膜形成.依据氧化膜厚度评估其脱落风险.测定在640、670、700℃下S30432钢水蒸气的氧化膜厚度和时间的动力学曲线,确定炉管的运行温度和对氧化的影响.结果表明:氧化膜在形成和变化中,存在晶粒变化和成分变化.晶粒变化主要是由融合状的纳米级晶粒长大为颗粒状或碗状微米级晶粒,由表及里,晶粒越来越细小.依氧化膜厚度推算的运行温度为670℃,处于超温运行状态;S30432钢产生氧化膜脱落的厚度为39.81μm,初生氧化膜厚度远大于39.81μm,处于随时脱落状态,再运行8552 h后次生氧化膜会达到脱落阈值厚度,管内存在氧化膜大量脱落并堵塞弯管引起爆管的巨大风险.

Predicting the strut forces of the steel supporting structure of deep excavation considering various factors by machine learning methods

The application of steel strut force servo systems in deep excavation engineering is not widespread,and there is a notable scarcity of in-situ measured datasets.This presents a significant research gap in the field.Addressing this,our study introduces a valuable dataset and application scenarios,serving as a reference point for future research.The main objective of this study is to use machine learning(ML)methods for accurately predicting strut forces in steel supporting structures,a crucial aspect for the safety and stability of deep excavation projects.We employed five different ML methods:radial basis function neural network(RBFNN),back propagation neural network(BPNN),K-Nearest Neighbor(KNN),support vector machine(SVM),and random forest(RF),utilizing a dataset of 2208 measured points.These points included one output parameter(strut forces)and seven input parameters(vertical position of strut,plane position of strut,time,temperature,unit weight,cohesion,and internal frictional angle).The effectiveness of these methods was assessed using root mean square error(RMSE),correlation coefficient(R),and mean absolute error(MAE).Our findings indicate that the BPNN method outperforms others,with RMSE,R,and MAE values of 72.1 kN,0.9931,and 57.4 kN,respectively,on the testing dataset.This study underscores the potential of ML methods in precisely predicting strut forces in deep excavation engineering,contributing to enhanced safety measures and project planning.

环境变化对三跨连续钢箱梁桥振动特性的影响

随着基础设施的老化和交通荷载的增加,桥梁的结构健康监测变得尤为重要。针对一座三跨连续钢箱梁桥,通过在不同温度条件和交通载荷下进行实地监测,分析了环境因素对桥梁振动特性的影响。研究在桥梁的不同位置安装了多个加速度计和热敏电阻,通过长时间的数据采集,探讨了桥梁振动频率随温度和交通振动的变化规律。结果表明,温度升高会导致振动频率降低,且高次振动模式对温度的敏感性更强,而重型车辆的通过则会引起桥梁振动频率的轻微降低。

Effect of Morphology for Novel Bainite/Martensite Dual-Phase High Strength Steel on Its Hydrogen Embrittlement Susceptibility

With TEM、SEM, various high temperature deformed structures in W9Mo3Cr4V steel were investigated. The sub structures,recrystallized nuclei, as well as the dynamic precipitation were also studied and analyzed. The relationship between recrystallized structures and dynamic precipitation was discussed. The results showed that the deformed structures in W9Mo3Cr4V steel are more complicated than those in low alloy steels. Because W9Mo3Cr4V steel is a high speed steel, there are a large number of residual carbides on the matrix. Also, much dynamic precipitating carbides will precipitate during deformation at high temperature.