大峡工程低微水泥混凝土徐变变形研究

本文结合大峡水电工程混凝土徐变变形资料,研究了补偿收缩型低热微膨胀水泥混凝土的自生体积变形、徐变变形及应办松驰系数等,并和其它工程资料进行了分析比较,为工程坝体温控计算提供了依据.     

非线性徐变模型在泄洪洞衬砌混凝土温控仿真分析中的应用

针对地下工程衬砌混凝土温控仿真分析中徐变变形和温度应力的关系,基于有限元分析软件ANSYS,以溪洛渡泄洪洞无压段衬砌混凝土温控仿真分析为例,比较分析了线性和非线性徐变模型计算的温度应力的差异。结果表明,当温度应力与相应龄期抗拉强度的比值小于0.5时,差异很小;但当温度应力与相应龄期抗拉强度的比值大于0.5时,差异则较显著,为地下工程衬砌混凝土温控分析提供了参考。     

黑河引水工程粉煤灰混凝土试验研究

结合黑河引水工程粉煤灰混凝土的试验，得到粉煤灰掺量与粉煤灰混凝土强度及抗渗性的关系，为黑河引水工程提供了合理的配合比，取得了显著的经济效益。另外，该结论为粉煤灰混凝土在今后水利水电工程上的应用提供了参考资料。     

超大拱式变截面渡槽收缩徐变影响研究

以跨径200m的超大跨径变截面拱式渡槽的结构设计为例,通过建立有限元模型,对渡槽主拱圈的分段分环施工全过程进行了收缩徐变分析。结果表明,徐变收缩可引起较大的后期变形,对渡槽竖向位移影响显著。建议在超大跨拱式渡槽预拱度设计时,应充分考虑混凝土的收缩徐变对变形的影响,采取分段分环施工并在拱脚处使用微膨胀混凝土以减小收缩徐变的影响,并采取相应措施控制渡槽线形。     

杨家坝大跨度简支预应力渡槽徐变效应分析

针对杨家坝大跨度简支预应力渡槽在长期荷载作用下单跨结构稳定、挠度是关系到渡槽安全与正常运行的问题,采用有效模量法借助有限元软件ANSYS建模,计算了渡槽槽身在假定荷载保持不变条件下由徐变引起的应力和挠度,分析了渡槽的安全稳定性.结果表明,该渡槽徐变应力和挠度均满足现行规范要求,为工程设计提供了重要参考.     

确定混凝土受拉应力—应变全曲线的一种新型试验方法

针对目前混凝土受拉应力—应变全曲线下降段获取方法不足的问题,从试件构制改进出发,提出了阻断钢筋的概念及一种新的试验方法,并通过试验获得了混凝土受拉应力应变全曲线,验证了该方法的可行性。     

组合试验法推求混凝土拉伸软化曲线的研究

针对混凝土拉伸软化曲线测定难度大、成功率低的问题,提出了一种新的确定混凝土拉伸软化曲线的方法——组合试验法,即通过两种简易的混凝土基本试验混凝土轴心抗拉强度试验和三点弯曲断裂试验,结合简单的数值计算,确定混凝土拉伸软化曲线。为验证组合试验法的准确性,通过算例获得了混凝土拉伸软化曲线,并与混凝土直接拉伸试验获得的拉伸软化曲线进行了对比分析。结果表明,组合试验法获得的拉伸软化曲线的精度满足工程要求,且该方法简便,具有较广的应用前景。     

对混凝土蜗壳断面计算方法的研究

本文提出了计算混凝土蜗壳断面尺寸的解析法,并进行了推证.最后对同一实例,用不同方法计算的结果表明,解析法简单易行,结果正确可靠.混凝土蜗壳在实际工程中运用颇多.现有的蜗壳外形尺寸计算方法有图解法、图解分析法及半图解法,这些方法不仅繁琐,而且不便于应用计算机进行程序设计,因此有必要提出解析法.     

蒸压加气混凝土砌块单轴拉伸力学性能研究

为了研究蒸压加气混凝土砌块(AACB)在静力状态下单轴拉伸的力学行为,通过对不同密度的AACB进行单轴拉伸试验,分析了AACB抗拉破坏特征、破坏机理,并探讨了其抗拉强度、弹性模量在不同密度下的变化规律及单轴拉伸强度与孔隙率、抗压和劈裂拉伸强度关系的相关公式。试验结果表明,随着AACB的密度增大,其拉伸屈服强度、弹性模量逐渐提高并呈幂函数增加,然而拉伸屈服应变却逐渐减小;AACB材料在单轴拉力作用下的宏观破坏主要由材料受拉损伤机制控制,并表现出明显的脆性破坏特征。基于多孔材料理想预测模型及试验结果回归分析,拟合得到试验条件下AACB单轴拉伸强度与孔隙率半经验公式及修正系数,并提出AACB抗拉强度与其抗压强度、劈裂拉伸强度的换算关系。     

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.     

混凝土类材料的动力损伤特性研究

本文根据混凝土试样在模拟爆振台上的爆振损伤试验，利用声波作为无扰动损伤测试主要方法，推导了用波速表述的爆振损伤本构方程及动力损伤演化律，理论模型的计算结果与试验结果吻合较好。     

Creep damage in small punch creep specimens of Type 304 stainless steel

Small punch creep tests on Type 304 stainless steel have been performed at 650 °C. Based on these tests, a finite element model, with modified Kachanov–Rabotnov creep damage constitutive equations, was established. The variation of central deflection and creep strain with time and the evolution of creep damage under constant loads were analysed by using the finite element model. The central creep deflection curves in the specimens were obtained at different loads in both tests and simulations and have three different stages, similar to conventional creep tests. There is good agreement between experimental results and simulation data. The creep damage at the central part is high, and localization of damage is obvious. Initial failure occurs at the bottom surface, about 0.8 mm away from the centre which agrees well with the finite element mode observation.     

Creep crack growth data and prediction for a P91 weld at 650 °C

Creep crack growth tests have been carried out on compact tension (CT) specimens machined from a P91 weldment. Four of these specimens were cut from the parent material side of the weld and another seven specimens were cut across the weld. For the cross-weld specimens, starter cracks were positioned into (or close to) the Type IV region. The creep tests were carried out under constant loads, at 650 °C. The results obtained showed that, the creep crack growth rates for parent material specimens are about ten times lower than those for the cross-weld specimens and that the scatter in the data is relatively high. In this respect, the accuracy of the crack tip location, in the cross-weld CT specimens, plays an important role. Finite Element (FE) analyses were carried out, on notched bar and CT models, using damage mechanics material behaviour models. These analyses were used to estimate the triaxial stress factor, α, for the parent material (PM), the weld metal (WM) and the heat affected zone (HAZ). FE analyses were then used to predict the creep crack growth in the CT specimens. Results from the FE analyses for both the PM and the cross-weld CT specimens were in good agreement with the corresponding experimental results. The effect of the potential drop versus crack length calibration on the calculated C* values was also investigated.     

Creep damage assessment of 10Cr-1Mo-1W-VNbN steel forging through EBSD observation

10Cr-1Mo-1W-VNbN steel forging was observed through TEM (Transmission Electron Microscope), SEM (Scanning Electron Microscope) with EBSD (Electron BackScattering Diffraction) pattern method and tested through nano-indentation tester to investigate the microstructural change during creep damage process. Long-term creep rupture and interrupted creep test samples were investigated and effective damage parameters were selected. Dislocation substructure through TEM thin foil method showed increasing block/lath width especially near grain boundary according to creep damage accumulation and the same feature was observed through EBSD IPF mapping more clearly and easily. Area averaged KAM (Kernal Average Misorientation) KAM_ave was shown to be effective for evaluating dislocation microstructural changes during creep. Nano-indentation tests were conducted at the same position in EBSD measurement, which showed a good correlation between hardness value and the square root of KAM_ave. The differential equation of dislocation density with creep time was applied to estimate the relationship between averaged KAM_ave and time through the relationship between hardness and dislocation density. The creep damage estimation curves were obtained by the integrated form of the equation. As the KAM_ave showed an apparent drop against time fraction in the primary creep stage near grain boundary followed by almost constant trend for later stage. The statistical distribution of KAM_ave during creep damage process suggested the localized recovery of dislocation substructure near grain boundary.     

Effect of frame material on the creep of solid oxide fuel cell

During long term operation at high temperature, creep is inevitable and can cause damages and cracks, which should be decreased to ensure the stack integrity. A strain based creep damage model is used to predict the creep damage behavior of a planar solid oxide fuel cell (SOFC). It demonstrates the maximum creep damage locates at the corner of glass-ceramic (GC) facing the frame after 50 000 h creep. The effect of frame material on the creep is studied. By increasing the creep parameter B of frame, the creep and damage in the cell and GC are decreased. This indicates the frame with a larger creep parameter can alleviate the interaction between components and decrease the deformation of the system. It recommends to use frame materials which have creep parameters larger than 1.3752 × 10⁻¹⁵ MPa^-nh⁻¹ besides CTEs closed to the cell to compensate and mitigate the creep and damage of SOFC system.     

Creep deformation characterization of heat resistant steel by stress change test

To evaluate the creep deformation mechanism of heat resistant steel, stress change tests were conducted during creep tests for Modified 9Cr–1Mo Steel and 2.25Cr–1Mo Steel. In this study it was confirmed that the dislocation behavior during the creep tests was in viscous manner because of no instantaneous plastic strain observed at stress increments. Transient backward creep behavior was observed after stress reduction for these steels in this work. Mobilities of dislocation were evaluated by observed backward creep behavior after stress reductions. Internal stresses were evaluated by the changes of creep rate in stress increments. And mobile dislocation densities were evaluated with the estimated mobilities of dislocation and the changes of creep rate in stress increments. It was found that the variation of evaluated mobile dislocation densities during creep deformation showed the same tendency as the variation of creep rate. Therefore mobile dislocation density is the dominant factor that influences the creep rate variation in creep deformation of these steels in this work. The mobilities of dislocation showed a good correlation with 1/T and related with solute amount of Mo that is a solution hardening element.     

Creep crack growth in service-exposed weld metal of 2.25Cr1Mo

Creep crack growth (CCG) has been studied for ex-service weld metal of 2.25Cr1Mo (P22). The testing was conducted at a temperature of 550°C and prior to testing, the material had been exposed to high temperature service for 110 000 h at 530°C. The results show a marked effect of the service exposure on the CCG properties of the material when compared to similar testing performed on a new material. The CCG rate was higher by a factor 3.1 in the service-exposed material, which should be compared to the model value of 3.2 based on the relations between the elongation values. The consumed deformation capacity was also estimated with the omega model for tertiary creep. In this case, an enhanced growth rate of 2.4 was obtained. Within a distance of about 10 mm in front of the propagating cracks, the number of creep cavities was significantly higher than in the surrounding material. The variation of the density of cavities as a function of distance from the crack tip was successfully modelled.     

Creep property of carbon and nitrogen free high strength new alloys

The carbon and nitrogen free new alloys which were composed of supersaturated martensitic microstructure with high dislocation density before the creep test have been investigated systematically. These alloys were produced from the new approach which raised creep strength by the utilization of the reverse transformed austenite phase as a matrix and intermetallic compounds such as Laves phase and mu-phase as precipitates during heating before the creep test. It is important that these alloys are independent of any carbides and nitrides as strengthening factors. The high temperature creep test over 700 °C exceeds 50,000 h, and the test is continuous. Creep behavior of the alloys is found to be different from that of the conventional high-Cr ferritic steels. The addition of boron to the alloy pulled the recrystallization temperature up in the high temperature, and it became a creep test in the un-recrystallization condition, and the creep property of high temperature over 700 °C was drastically improved. The minimum creep rates of Fe–Ni alloys at 700 °C are found to be much lower than those of the conventional high Cr ferritic heat resistant steels, which is due to fine dispersion strengthening useful even at 700 °C in these alloys. As a result it became clear that the value for 100,000 h was exceeded at 700 °C and 100 MPa calculated from the Larson-Miller parameter at C = 20.