Cracking behaviors and strength of rock-like samples with steps of different geometries under shear stress

Steps are distinctive features for estimating the movement of the upper and lower block of faults.However,studies about the influence of steps as a special type of discontinuity on cracking behaviors and strength of rock masses are limited.In this research,rock-like samples with steps and preexisting flaws were fabricated.Step height h and the inclination angle of gentle slope of the stepαwere set to different values.Direct shear tests were conducted on these samples under different normal stresses.The experimental results reveal that the inclination angle of the gentle slope of the stepα,step height h,and normal stress have an influence on the strength,crack initiation,and crack propagation of the samples.The experimental results show that crack behaviors and shear strength were affected by step inclination anglesαand step height h.As the normal stress increases,the improvement of the strength of samples with a large step height is larger than that of samples with a small step height,the improvement of the strength of samples withαof 10°is larger than that of samples withαof 0°and-10°.The discrete element method was used to simulate the shear test.Numerical results show five different types of displacement vectors,which can be used to determine whether the cracks are tensile cracks or shear cracks.The above conclusions can provide help for estimating mechanical properties and failure modes of rock masses with steps of different geometries.

Influence of fly ash on early-age cracking behavior of high-flowing concrete

The effects of quality and content of fly ash on the early-age cracking behavior of high-flowing concrete(HFC)were investigated.The early-age cracking behavior of the HFC was analyzed by combining the tests of evaporation capacity and electrical resistivity of the HFC.In these tests,a modified flat-type specimen was adopted.The results show that the HFC will have a lower evaporation capacity when it is mixed with fine fly ash,while it will have a higher evaporation capacity when grade III fly ash is used as mineral admixture.And the electrical resistivity rate of HFC reduces with the increase of the content of fly ash.A nonlinear relationship exists between the cracking time of HFC and the minimum electrical resistivity.The early-age cracking behavior of HFC with fly ash can be enhanced by appropriately increasing the fine particle content and MgO,K2O,and SO3 contents of fly ash.The optimal content of fly ash,which makes a satisfied early-age cracking behavior of HFC,is obtained.And when the content of fly ash exceeds a critical value,the early-age cracking behavior of HFC will rapidly decrease.

Crack growth behavior of SRR99 single crystal superalloy under thermal fatigue

The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100℃. The crack growth curves as a function of the number of cycles were plotted. With the increase of peak temperature, the crack initiation life was shortened dramatically. Through optical microscopy （OM） and scanning electron microscopy （SEM） observation, it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles. The primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.

On-line diagnosis method of crack behavior abnormality in concrete dams based on fluctuation of sequential parameter estimates

Research on on-line diagnosis method of crack behavior abnormality in concrete dams can provide support for timely grasping abnormality state of the crack itself and achieving real-time monitoring of the dam safety.Considering that samples of crack effects in concrete dams increase actually over monitoring time,a superiority criterion for the on-line diagnosis is determined so as to detect the abnormality moments timely and reliably.By integrating the safety monitoring statistical model of crack effect variable with change point theory,a fluctuation method of regression coefficients is established for the on-line diagnosis.In addition,each abnormality moment is detected by the cumulative sum of regression model residuals.Results indicate that abnormality of crack behavior in concrete dams can be characterized by structural instability of crack monitoring model.And causes of crack behavior abnormality can be analyzed by the established method,which will play an important role in dam safety monitoring.Further,taking the crack in a concrete gravity-arch dam as an example,the scientific rationality and validity of the established on-line diagnosis method are confirmed.

Improvement of Cracking-resistance and Flexural Behavior of Cement-based Materials by Addition of Rubber Particles

By ring test and bend test, the improvement of waste tire rubber particles on the crack- resistance and flexural behaviors of cement-based materials were investigated. Test results show that the cracking time of the ring specimens can be retarded by the incorporation of rubber particles in the cement paste and mortar. The improvement in the crack-resistance depended on the rubber fraction. When the rubber fraction was 20% in volume, the cracking time was retarded about 15 h for the paste and 24 d for the mortar respectively. Flexural properties were evaluated based on the bend test results for both mortar and concrete containing different amount of rubber particles. Test results show that rubberized mortar and concrete specimens exhibit ductile failure and significant deformation before fracture. The ultimate deformations of both mortar and concrete specimen increase more than 2-4 times than control specimens.

Metallurgical modeling of microcrack repairment during welding nonferrous materials: non-equilibrium solidification behavior of weld pool (Ⅰ)

Metallurgical modeling of synergistic microcrack self-repairmen during welding single crystal and polycrystalline superalloys of high-temperature aerospace materials has been properly established. The idea of improvement of nickel-based superalloys weldability through non-equilibrium solidification behavior of backfill to self-repair arterial crack network is usefully proposed. Crystallographic control strategy of crack self-repairmen of fusion zone interdendritic solidification cracking and heat-affected zone （HAZ） intergranular liquation cracking is technically achievable, indicating that optimal niobium alloying beneficially refines weld microstructure, stabilizes the primary solidification path, increases the solidification temperature and concomitantly decreases the weld pool geometry. High-carbon grain boundary is more thermal stable and less contributes to incipient intergranular liquid film than that of low-carbon grain boundary. The theoretical predictions of cracking susceptibility are indirectly verified in a rather satisfactory manner. Additionally, the metallurgical modeling enhances predicative capabilities and thereby is readily applicable for other alloy systems.

Surface Cracking Behaviors of Ultra-Fine Grained Tungsten Under Edge Plasma Loading in the HT-7 Tokamak

Tests of the candidate plasma facing materials（PFMs） used in experimental fusion devices are essential due to the direct influence of in-situ plasma loading.A type of ultrafine grained（UFG） tungsten sintered by resistance sintering under ultra-high pressure（RSUHP） method has been exposed in the edge plasma of the HT-7 tokamak to investigate its performance under plasma loading.Under cychc edge plasma loading,the UFG tungsten develops both macro and micro cracks.The macro cracks are attributed to the low temperature brittleness of the tungsten material itself,while the micro cracks are generated from local intense power flux deposition.

DYNAMIC BEHAVIOR OF A CRACKED FLEXIBLE ROTOR SUPPORTED ON JOURNAL BEARINGS

The dynamic behavior of a cracked flexible rotor supported on three kinds ofjournal bearings is presented. Numerical experiments show that nonsynchronous responseswill happen due to the rotor crack, and the amplitudes of the nonsynchronous componentsbecome larger with the increase of crack. On the other hand, the fluid forces of journalbearings can suppress the nonsynchronous response. The (1/2) × or (3/2) × harmoniccomponent rarely appears for small crack near the rotating speed ratio Ω = 2Ωc or Ω =(2/3)Ωc. In the case of supercritical rotating speed, the additional 0× harmonic component is increased as the crack increases. The bearing parameters affect greatly the occur-rence of the nonsynchronous responses by means of exerting innuence on the critical spedand the stabi1ity of the system.

Fatigue Crack Propagation Behavior of TiNi_(50.6)Shape Memory Alloy

The fatigue crack propagation behavior of TiNi50.6 shape memory alloy was studied. The ex- periment results showed that the crack propagation properties of this alloy display difference and similarity in comparison with common metallic materials. Because of the stress concentra- tion there was stress induced martensite transformation (SIMT) near the crack tip though the nominal stress was lower than the threshold stress of SIMT. The position and the amount of SIMT was in situ observed by a quester remote measurement system (QRMS). The observation results showed that the position of SIMT was beside the crack tip and was not in the plastic zone of common metallic materials (in front of the crack tip). The SIMT zone at an angle of about 45°to the direction of the crack propagation, like a butterfly,appeared in the loading process, disappeared in the unloading process and grew larger with the increase of K. The crack propagation rate(da/dN) followed the linear law in lg-lg plot. Observation of the crack surface showed fatigue striation clearly. The relationship between the site and the size of the plastic zone and the SIMT zone is discussed and a model is given to explain both the similarity and the difference of the crack propagation property of TiNi50.6 to common metal materials.

The Crack of Turbine Blade Effect on the Dynamic Behavior of Turbine

In this report, the finite element method is applied to study the crack effect on the dynamic behavior of the turbine blade. There are four subjects reported in this paper. The first subject describes the material properties, the visual observation of the blade crack, the geometric dimensional measurement of the turbine blade. The second part will present the finite element models included single blade and a group of 6 blades. The third section will build up the full model of 60 blades. The full model will have four different types: the first type that has no crack in the blade root named “No crack”, the second type has the 126 mm2 crack area in the blade root and it is named “1 Line crack”, the 3rd type has 364 mm2 crack area in the crack root (named 2 Lines Crack), and the fourth type has 598 mm2 crack area in the root (named 3 Lines Crack). The location of crack area is modeled by applying the spring elements. The vibration frequencies for single blade and a group of 6 blades are higher without the crack in the blade root. For full model of 60 blades, the vibration frequencies are lower with the crack area in the blade root. When the crack area becomes bigger, the lower frequencies of the full blade system are observed.

Prevalence and Incidence of HIV and Sexual Risk Behaviors in Crack Users in the San Salvador Metropolitan Area, El Salvador

Objective: It is in order to estimate the prevalence and incidence of HIV, the frequency of sexual risk behaviors, and perceptions of available resources to prevent and treat HIV among crack users in the San Salvador Metropolitan Area. Methods: We conducted a survey of 420 crack users by using respondent-driven sampling to measure demographic characteristics, the quantity and frequency of drug use, history of STIs, including HIV, and experiences with organizations which provide prevention and treatment of HIV. Each participant offered a free and voluntary HIV test and was asked permission to share the results of the test with the study. Bernoullian modeling was used to estimate the prevalence and incidence of HIV among heterosexual males in this population. Results: The estimated prevalence was 7% (95% CI: 2.3% -9.8%) among participants who agreed to take the test and share the results, and 4.9% (95% CI: 2.8% -7.8%) assuming that those who did not take the test or share results were seronegative. Participants reported a high frequency of sexual risk behaviors. In addition, participants were reported to have little knowledge of organizations to prevent or treat HIV/AIDS;58% had never taken an HIV test prior to survey administration. Conclusions: Crack users in San Salvador are at high risk for HIV acquisition. HIV prevention interventions are urgently needed, especially interventions increasing access to HIV testing and prevention.

基于综合性能试验的混凝土早期开裂行为研究

为提高混凝土的早期抗裂性能,突破单因素改善其早期稳定性,进行了多因素补偿混凝土各性能缺陷方面的研究.基于调控混凝土配合比开展了胶凝材料水化热、混凝土早期收缩试验和大板加速开裂试验.通过相关收缩机理分析了不同混凝土拌和物的开裂行为;运用双参数Weibull分布分析试验结果,得出了混凝土大板裂缝经时开裂规律.试验结果表明:采用高效稳定型聚羧酸减水剂可以增加浆体的和易性,结合66.7%矿物掺合料(粉煤灰和矿渣粉与水泥质量比)、较好级配碎石和0.29的水胶比有利于延长混凝土的初裂时间和控制裂缝宽度,即提高混凝土的早期抗裂性能.同时,该调控手段也可以降低浆体的水化热,提高混凝土的抗压强度.此外,混凝土裂缝宽度的经时变化规律很好地服从双参数Weibull分布.

Numerical Simulation of the Behavior of Cracked Reinforced Concrete Members

Refined non-linear static or dynamic analyses of reinforced concrete structures require the knowledge of the actual force-displacement or bending moment-rotation curves of each structural member, which depend on the crack widths and on the crack pattern, and after all on the slip between concrete and reinforcing steel. For this reason the definition of improved local models taking into account all these local aspects is a fundamental prerequisite for advanced assessment of r.c. structures. A numerical procedure which allows to predict the relative displacement between steel reinforcement and the surrounding concrete in a reinforced concrete element, once assigned the stress in the naked steel bar and the bond-slip law is discussed. The method provides as final outcomes the sequence of crack openings and the individual crack widths, regardless of the particular bond-slip correlation adopted. The proposed procedure is implemented referring to two relevant experimental case studies, demonstrating that it is able to predict satisfactorily actual strain fields and slips along the investigated reinforced concrete elements.

Very High Cycle Fatigue Behaviors and Surface Crack Growth Mechanism of Hydrogen-Embrittled AISI 304 Stainless Steels

The influence of hydrogen embrittlement on the fatigue behaviors of AISI 304 stainless steel is investigated. The fatigue endurance limits of the untreated and hydrogen-embrittled materials were almost the same at 400 MPa, and hydrogen embrittlement had little influence even though the sample contained about 8.1 times more hydrogen. Thus, the sensitivity of hydrogen gas in this material is very low. A surface crack initiation, growth, coalescence, and micro ridge model is proposed in this study. Slip line formation?⇒microcrack formation?⇒increases in the crack width, and blunting of the crack tip as it grows?⇒formation of many slip lines because of deformation in the shear direction?⇒growth of the crack in the shear direction, forming micro ridges, coalescence with adjacent cracks ⇒?continuous initiation, growth, coalescence, and ridge formation of surface cracks and specimen breakage.

孔洞对疲劳裂纹扩展路径的影响

为了研究孔洞位置和尺寸对疲劳裂纹扩展路径的影响,利用ABAQUS有限元仿真软件建立改进CT试样的裂纹扩展仿真模型,采用扩展有限元法研究了在低周疲劳加载条件下Q235钢圆孔与裂纹尖端的角度、距离以及圆孔半径对裂纹扩展路径的影响,并通过引入裂纹偏转系数来定量描述不同参数对裂纹扩展路径偏转程度的影响。结果表明:当圆孔与裂纹尖端的夹角为45°时,裂纹偏转系数取得最大值;裂纹偏转系数随着孔洞半径的增加逐渐增大,随着圆孔与裂纹尖端距离的增大逐渐减小。

端钩型钢纤维混凝土简支梁受弯承载力试验

为改善普通钢筋混凝土梁自重大、易开裂、承载力低等缺点,在混凝土中常加入钢纤维,端钩型钢纤维,作为常见的一种高性能钢纤维得到广泛关注,中外学者针对端钩型钢纤维混凝土的基本力学性能开展了较多研究,而对端钩型钢纤维混凝土受弯构件的正截面受力性能有待深入研究。为研究端钩型钢纤维混凝土简支梁受弯性能,制作4根端钩型钢纤维混凝土梁及1根普通混凝土梁,对其进行受弯性能试验,根据试验得到的破坏形态、承载力、荷载-挠度曲线、荷载-应变曲线,分析端钩型钢纤维体积掺量对试件受弯承载力及破坏形态的影响。结果表明:端钩型钢纤维混凝土简支梁与一般混凝土简支梁受弯过程类似,均经历了弹性、开裂、带裂缝工作、破坏4个阶段;端钩型钢纤维混凝土简支梁与一般混凝土简支梁受弯过程均基本符合“平截面假定”;端钩型钢纤维限制了裂缝的产生和发展,使得纤维混凝土梁变形能力增强;与一般混凝土梁相比,端钩型钢纤维混凝土梁抗裂性及极限承载力得到提高,且构件承载力与钢纤维体积掺量基本呈现正相关;基于试验数据,对现行规范中开裂弯矩及极限承载力计算公式进行优化,开裂弯矩方面考虑对截面抵抗矩塑性影响系数进行修正,极限承载力方面引入纤维混凝土正截面承载力影响系数ζ,经修正计算值可较好吻合试验值。

钢纤维细石混凝土-钢组合板抗弯性能试验研究

钢纤维细石混凝土是一种具有优异力学性能和良好工作性的高性能混凝土复合材料。本试验针对正交异性钢桥面铺装层裂缝病害问题,对4组采用不同钢筋配筋率和保护层厚度的横桥向钢纤维细石混凝土-钢组合桥面板进行了抗弯破坏试验,研究了组合板的荷载-挠度关系与裂缝开展特征;用等效截面法计算了开裂应力、裂缝宽度和钢筋应力,验证了钢纤维细石混凝土-钢组合板的抗裂性能。结果表明:60 mm厚钢纤维细石混凝土铺装层在小配筋率(2.6%)和低于规范要求的保护层厚度(15 mm)条件下呈现出多缝开裂的破坏形态,具备较高的延性和开裂应力,满足规范和工程应用的要求。

基于DIC技术的自密实混凝土界面裂缝扩展规律研究

采用自密实混凝土作为堆石混凝土坝的抗冲磨层并与坝体进行一体化浇筑成型是一种新构造,其中抗冲磨层与坝体的界面性能至关重要。通过制作600 mm×600 mm×600 mm自密实混凝土(SCC)试件(C40SCC、C25SCC)及C40、C25SCC分层浇注试件(C40-C25SCC),结合数字图像技术(DIC)开展劈裂抗拉试验,分析了自密实混凝土界面裂缝扩展规律。结果表明:SCC断面破坏形态表现为骨料劈裂和剥落2种破坏形式;C40SCC、C25SCC及C40-C25SCC劈裂抗拉强度分别为1.861、1.416和1.362 MPa;基于DIC技术获取了裂缝相关参数,其中C40-C25SCC裂缝最大开口宽度、扩展时间均为最大,分别为0.125 mm、10.667 ms,C25SCC裂缝最大扩展速度最快,为10.12 m/s。研究表明:骨料的存在及粒径大小改变了试件破坏形态,影响了裂缝开口宽度及扩展速度变化。

水岩作用下不同煤体厚度组合体力学特性及损伤规律研究

为研究水岩作用下煤岩组合体力学行为及其界面不均匀破坏特征,揭示井下隔水煤岩柱变形及富水环境下煤岩层变形规律.对不同高度比例的岩-煤-岩组合体开展干燥和饱水条件下单轴压缩试验,分析组合体应力-应变、强度及弹性模量劣化规律,探究水岩作用下煤-岩交接面处应力分布关系,明确组合体变形破坏特征及过程.研究结果表明:水岩作用下煤体高度对组合体力学特征具有较大影响,在干燥与饱水状态下,组合体单轴抗压强度与弹性模量均随煤体高度的增加而减小,且试件的破坏形态随着煤体高度的增加由拉伸破坏转变成斜面剪切破坏,最终表现为煤体挤裂破坏.但组合体强度损失劣化度随着岩体层间煤体高度增加,先增大后减小,当煤∶岩高度比为1∶2时,组合体强度对水的敏感度最大并出现极大值,这主要与煤岩交界面高度位置有关.弹性模量呈现降速缓释趋势,主要与组合体组成岩石性质有关.水岩作用下煤体部分为组合体整体失稳破坏的触发点位,延续影响煤体两端岩体,由于交界面产生侧向平面约束力,使煤体轴向抵抗变形能力得到强化,但随着煤体高度的增加,煤体中部所受约束影响迅速衰减.组合体变形破坏渐进性过程分为四个阶段:裂隙孔隙压密阶段、裂隙产生并稳定发育阶段、裂隙加速发育并贯通阶段和破坏后阶段.

非晶合金纤维和钢纤维单/混掺增强混凝土开裂及断裂性能研究

为探索纤维混凝土抗裂性能,本文研究了不同类型纤维(平直型钢纤维、平直型非晶合金纤维、钢纤维+非晶合金纤维)和不同纤维体积掺量(1.0%,1.5%和2.0%)混凝土的早期开裂性能和硬化后的断裂性能。首先通过平板法试验并借助数字图像对混凝土的早期裂缝特征进行量化分析,评估了纤维混凝土的早期开裂性能;再通过带切口棱柱体试件的三点弯曲试验,基于双K断裂参数,分析并评估了混凝土硬化后的断裂性能。结果表明,钢纤维单掺和混掺纤维混凝土的早期抗裂性能最好;纤维掺量由1.0%增加至2.0%时,钢纤维、合金纤维和混掺纤维混凝土失稳韧度分别增加了58.9%、44.3%和55.5%,且掺量为2.0%的混掺混凝土断裂能是普通混凝土的11.8倍。钢纤维和非晶合金纤维可在混凝土硬化过程的不同时期发挥协同作用,两种纤维混掺不仅能阻止混凝土早期的塑性开裂,还可有效控制硬化后受拉裂缝的形成和发展,达到分阶段抗裂的目的。综合考虑混凝土的早期开裂性能、硬化后受拉性能以及断裂性能,体积掺量相同情况下混掺纤维混凝土整体性能更优。