混凝土浇筑层面的渗透特性与微裂纹结构特征

混凝土的宏观渗透性能与其内部微裂纹结构密切相关。本文开展了一系列室内试验研究混凝土浇筑层面的微裂纹结构及渗透特性。采用不同间隔时间浇筑形成有层面的混凝土方法制备了6组混凝土试件,分别进行宏观渗透性能试验和荧光显微结构观测。试验结果表明:随着形成层面的混凝土浇筑间隔时间延长,混凝土的渗透系数、电通量、初始电流及交流电导都逐渐增大,表明混凝土浇筑层面的抗渗性能逐渐降低;荧光显微观测结果表明微裂纹长度服从对数正态分布;表征裂纹方向的渗透变量λ曲线亦随之由圆形逐渐趋向于椭圆,表明裂纹网络由各向同性逐渐趋于各向异性,同时裂纹网络的优势方向逐渐趋于顺浇筑层面方向;渗透变量曲线极值λmax与渗透系数、初始电流及等效电导隙宽呈线性正相关,相关系数分别为0.85、0.80和0.95,而λmax与交流电阻呈线性负相关,相关程度为0.76。

基于Lyapunov指数的非线性Lamb波的微裂纹检测

为了提高在背景噪声干扰下非线性Lamb对于结构微裂纹的检测精度,提出了利用Duffing振子和Lya-punov指数对噪声干扰下的非线性Lamb波特征进行增强与量化分析的方法。首先,采用了庞加莱图确定Duffing系统外策动力参数;其次,将周期延拓滤波后的非线性Lamb波输入调整好的Duffing系统中,对系统输出时间序列进行相空间重构,计算出相应的最大Lyapunov指数。通过多个模型数据的仿真分析结果表明,即使在噪声干扰情况下,Lyapunov指数与裂纹大小也存在着良好的线性关系。该方法对噪声干扰下的微裂纹缺陷识别具有明显的优势,对提高非线性Lamb波的检测灵敏度具有重要意义。

不等幅值循环荷载下冻土残余应变研究及其CT分析

基于不等幅值循环荷载作用下低温三轴振动试验资料,研究了冻结粉质粘土在分级往返加荷情况下的残余应变产生条件。根据冻土的残余应变与其温度、振次、含水量等因素的关系曲线,得到了随着温度的降低残余应变值减小、随着振动次数的增加残余应变值增大;含水量的变化对其影响很小的结论。根据冻土试样震融沉前后CT测试数据及扫描图像,定量地分析了土样结构微裂纹发展、密度变化规律,并对冻土特有的震融沉形成机理给予解释。

Failure mechanism of as-cast Mg-6Zn-2Er alloy during tensile test at room temperature

Tensile test of the as-cast Mg-6Zn-2Er alloy was conducted at room temperature. The results indicate that the alloy is inclined to failure when the strain reaches 5.6%. The coarse secondary phases are responsible for the failure, especially for the Mg3Zn3Er2 phase （W-phase）. It is indicated that the existence of the W-phase activates the stress concentrations due to the incapacity of W-phase for the load transfer, which results in the void at the inner of the W-phase. In comparison, the interface between the matrix and the secondary phase is stable. In conclusion, the characters of the secondary phases with respect to size, distribution, morphology and type, play an important role in the plastic deformation behavior of the alloy.

Microstructure and room temperature tensile property of as-cast Ti44Al6Nb1.0Cr2.0V alloy

The nominal Ti44Al6Nb1.0Cr2.0V alloy was newly designed and prepared by vacuum consumable melting technique with the ingot sizes of d225 mm×320 mm. The results show that the average lamella colony size is 780-1830 μm. This as-cast alloy has a modified near lamellar(M-NL) structure that is composed of mainly larger(α2+γ) lamella colonies and smaller(B2+equiaxed γ) blocky morphology. It exhibits the moderate tensile properties at room temperature, in which the Region(5) yields the ultimate tensile strength(UTS) about 499 MPa and the elongation about 0.53%. The obvious brittle fracture characteristics and trans-granular interlamellar fracture are the predominant modes. After room temperature tensile testing, there are some <101] and a few 1/2<112] superdislocations in the γ phase. The as-cast microcrack is the main factor to deteriorate the tensile property, which results in the premature fracture, poor ductility and few dislocations. The addition of Nb, Cr and V can decrease stacking fault energy(SFE) obviously, which is helpful to enhancing the ductility of the alloy.

Effects of Mineral Composition and Microstructure on Crack Resistance of Sintered Ore

Vickers indentation test was used to study the effects of mineral composition and microstructure on crack resistance of sintered ore, and the initiation and propagation of cracks in different minerals contained in sintered ore were examined. The results indicate that the microstructure of calcium ferrites is a major factor influencing crack resistance of sintered ore. Finer grain size of calcium ferrite will lead to higher cracking threshold and better crack resistance of sintered ore. The formation of calcium ferrite with fine grain size during sintering process is favorable for crack resistance of sintered ore.

Microstructure-Fatigue Crack Propagation Kinetics Relationships of Rail Steels

Microstructural analysis and fatigue crack propagation behavior of three types of rail steels, was performed. These are premium pearlitic, austenitic manganese （AM） and bainitic rail steels. Rectangular un-notched and notched test specimens were machined from railheads of each material using electrical discharge machining （EDM） and used for the mechanical properties and fatigue evaluation respectively. Bainitic steel has the highest yield strength, ultimate strength, and strain to failure as compared to both pearlitic and austenitic manganese steels. Fatigue studies showed that the crack speed for the bainitic steel is lower than that for the pearlitie and the AM steels over the entire range of the energy release rate. The bainitic steel exhibits a higher rate of crack deceleration in the second stage, as indicated by the lower slope of the fatigue crack propagation kinetics curve in comparison with the pearlitic and manganese rail steels. This attests to the superior fatigue damage tolerance of the bainitic rail steel in comparison to pearlitic and austenitic manganese rail steels. Microstructural analysis of the three rail steels revealed that bainitic steel has a more intricate structure than AM and pearlitic steels. AM steel shows very few signs of being work hardened or toughened, which usually increases the mechanical properties of the material. As the number of alloying elements increase, the microstructure of the steel becomes more complex, resulting in the increase of mechanical properties and fatigue fracture resistance of bainitic rail steel.

Influence of Temperatures on Creep Behavior of Pt-Al Coated Third-Generation Low-Cost Single Crystal Superalloy

The influence of applied temperatures on the creep rupture life of the third-generation low-cost single crystal(SX)superalloy with Pt-Al coating was evaluated.The creep damage was observed under the conditions of 1100℃/137 MPa,1120℃/137 MPa,and 1140℃/137 MPa.Results show that the properties of bare superalloy outperform those of coated superalloy under all test conditions.The most significant reduction in creep life reaches 50%when the test condition is 1100℃/137 MPa.At higher temperatures(1120 and 1140℃),the crack propagation rate in Pt-Al coatings to SX superalloy substrate decreases,thereby reducing the degradation degree of mechanical properties.Instead of the penetration into SX substrate,tip oxidation and Al diffusion of the coating cracks cause the formation of oxides,therefore leading to the slow degradation in microstructures of the substrate beneath the coating.At 1100℃,however,the microstructure of coating/SX superalloy substrate degrades due to the Al internal diffusion.This diffusion mechanism promotes the formation of harmful topologically close packed phases around 1100℃.At 1120 and 1140℃,the dislocation of SX superalloy substrate beneath the coating is relatively unchanged,compared to that in the inner superalloy.In contrast,the dislocation network of the substrate beneath the coating becomes sparse,and the number of superdislocations cutting intoγ′phases increases at 1100℃.

Multiscale approach to micro/macro fatigue crack growth in 2024-T3 aluminum panel

When two contacting solid surfaces are tightly closed and invisible to the naked eye, the discontinuity is said to be microscopic regardless of whether its length is short or long. By this definition, it is not sufficient to distinguish the difference between a micro- and macro-crack by using the length parameter. Microcracks in high strength metal alloys have been known to be sev- eral centimeters or longer. Considered in this work is a dual scale fatigue crack growth model where the main crack can be mi- cro or macro but there prevails an inherent microscopic tip region that is damaged depending on the irregularities of the micro- structure. This region is referred to as the ＂micro-tip＂ and can be simulated by a sharp wedge with different angles in addition to mixed boundary conditions. The combination is sufficient to model microscopic entities in the form of voids, inclusions, precipitations, interfaces, in addition to subgrain imperfections, or cluster of dislocations. This is accomplished by using the method of ＂singularity representation＂ such that closed form asymptotic solutions can be obtained for the development of fa- tigue crack growth rate relations with three parameters. They include： （1） the crack surface tightness o-＊ represented by Cro/Cr~ = 0.3-0.5 for short cracks in region I, and 0.1-0.2 for long cracks in region II, （2） the micro/macro material properties reflected by the shear modulus ratio/1＂ （=,L/micro/]-/macro varying between 2 and 5） and （3） the most sensitive parameter d＊ being the micro-tip characteristic length d＊ （=d/do） whose magnitude decreases in the direction of region I ---~II. The existing fatigue crack growth data for 2024-T3 and 7075-T6 aluminum sheets are used to reinterpret the two-parameter da/dN=C（AK）n relation where AK has now been re-derived for a microcrack with surfaces tightly in contact. The contact force will depend on the mean stress ~m or mean stress ratio R as the primary parameter and on the stress amplitude era as the secondary parameter.

Promoting the mechanical properties of Ti42Al9V0.3Y alloy by hot extrusion in the α+β phase region

Hot extrusion was conducted in the α+β phase region for promoting mechanical properties of Ti42Al9V0.3Y. The microstructures and tensile properties before and after hot extrusion were studied. The results show that the microstructure of the as-cast alloy mainly consists of massive γ phase in β matrix and the as-extruded alloy mainly consists of lamellar α2/γ, lamellar β/γ, and strip γ propagating from elongated β phase. In the as-cast alloy, the predominantly observed fracture mode is transgranular cleavage failure at room temperature and intergranular fracture at 650-750 °C. After hot extrusion, it transforms into transgranular cleavage-like failure, including translamellar cleavage and delamination. The excellent tensile properties of the as-extruded material are attributed to the obvious refined microstructure with broken YAl2 particles and the micro-crack shielding action of the TiAl lamellasome.