Centrifugal model tests and numerical simulations for barrier dam break due to overtopping

The present study focuses on the breaching process and failure of barrier dams due to overtopping. In this work, a series of centrifugal model tests is presented to examine the failure mechanisms of landslide dams. Based on the experimental results, failure process and mechanism of barrier dam due to overtopping are analyzed and further verified by simulating the experimental overtopping failure process. The results indicate that the barrier dam will develop during the entire process of overtopping in the width direction, whereas the breach will cease to develop at an early stage in the depth direction because of the large particles that accumulate on the downstream slope. Moreover, headcut erosion can be clearly observed in the first two stages of overtopping, and coarsening on the downstream slope occurs in the last stage of overtopping. Thus, the bottom part of the barrier dam can survive after dam breaching and full dam failure becomes relatively rare for a barrier dam. Furthermore, the remaining breach would be smaller than that of a homogeneous cohesive dam under the same conditions.

Effects of seepage pressure on the mechanical behaviors and microstructure of sandstone

Surrounding rocks of underground engineering are subjected to long-term seepage pressure,which can deteriorate the mechanical properties and cause serious disasters.In order to understand the impact of seepage pressure on the mechanical property of sandstone,uniaxial compression tests,P-wave velocity measurements,and nuclear magnetic resonance(NMR)tests were conducted on saturated sandstone samples with varied seepage pressures(i.e.0 MPa,3 MPa,4 MPa,5 MPa,6 MPa,7 MPa).The results demonstrate that the mechanical parameters(uniaxial compressive strength,peak strain,elastic modulus,and brittleness index),total energy,elastic strain energy,as well as elastic strain energy ratio,decrease with increasing seepage pressure,while the dissipation energy and dissipation energy ratio increase.Moreover,as seepage pressure increases,the micro-pores gradually transform into meso-pores and macro-pores.This increases the cumulative porosity of sandstone and decreases P-wave velocity.The numerical results indicate that as seepage pressure rises,the number of tensile cracks increases progressively,the angle range of microcracks is basically from 50-120to 80-100,and as a result,the failure mode transforms to the tensile-shear mixed failure mode.Finally,the effects of seepage pressure on mechanical properties were discussed.The results show that decrease in the effective stress and cohesion under the action of seepage pressure could lead to deterioration of strength behaviors of sandstone.

Formation of Tianwen-1 landing crater and mechanical properties of Martian soil near the landing site

After landing in the Utopia Planitia,Tianwen-1 formed the deepest landing crater on Mars,approximately 40 cm deep,exposing precious information about the mechanical properties of Martian soil.We established numerical models for the plume-surface interaction(PSI)and the crater formation based on Computational Fluid Dynamics(CFD)methods and the erosion model modified from Roberts’Theory.Comparative studies of cases were conducted with different nozzle heights and soil mechanical properties.The increase in cohesion and internal friction angle leads to a decrease in erosion rate and maximum crater depth,with the cohesion having a greater impact.The influence of the nozzle height is not clear,as it interacts with the position of the Shock Diamond to jointly control the erosion process.Furthermore,we categorized the evolution of landing craters into the dispersive and the concentrated erosion modes based on the morphological characteristics.Finally,we estimated the upper limits of the Martian soil’s mechanical properties near Tianwen-1 landing site,with the cohesion ranging from 2612 to 2042 Pa and internal friction angle from 25°to 41°.

Trigger mechanism of loess-mudstone landslides inferred from ring shear tests and numerical simulation

Whereas loess-mudstone landslides are widely distributed and frequently occurred at the loess Plateau,this type of landslides is hard to detect due to its particularity,and easily generates serious losses.To clarify the shear characteristics and formation mechanism of loess-mudstone landslides,field investigations,ring shear tests and numerical simulation analyses were performed on the loess specimens collected from the Dingjiagou landslide in Yan’an city,China.The test results showed that both the peak strength and residual strength of slip zone soils have a decreasing tendency with moisture content,while the increasing of normal stress caused an increase in the shear strength.These phenomena indicate that the rise in the moisture content induced by precipitation or the decreasing of normal stress due to excavation activities would result in the weakening of slip zone soils.Numerical simulations of the evolution process of slope failure using the finite element method were conducted based on the Mohr–Coulomb criterion.It was found that the heavy precipitation played a more important role in the slope instability compared with the excavation.In addition,the field investigation showed that loess soils with well-developed cracks and underlying mudstone soils provide material base for the formation of loess-mudstone landslides.Finally,the formation mechanism of this type of landslides was divided into three stages,namely,the local deformation stage,the penetration stage,the creeping-sliding stage.This study may provide a basis for understanding the sliding process of loess-mudstone landslides,as well as guidelines for the prevention and mitigation of loess-mudstone landslides.

COMPUTER NUMERICAL SIMULATION OF MECHANICAL PROPERTIES OF TUNGSTEN HEAVY ALLOYS

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Experimental and numerical study of buckling-restrained braces configured with out-of-plane eccentricity under cyclic loading

This study focuses on variations in the hysteretic behavior of buckling-restrained braces(BRBs)configured with or without out-of-plane eccentricity under cyclic loading.Quasi-static experiments and numerical simulations were carried out on concentrically and eccentrically loaded BRB specimens to investigate the mechanical properties,energy dissipation performance,stress distribution,and high-order deformation pattern.The experimental and numerical results showed that compared to the concentrically loaded BRBs,the stiffness,yield force,cumulated plastic ductility(CPD)coefficient,equivalent viscous damping coefficient and energy dissipation decreased,and the yield displacement and compression strength adjustment factor increased for the eccentrically loaded BRBs.With the existence of the out-of-plane eccentricity,the initial yield position changes from the yield segment to the junction between the yield segment and transition segment under a tensile load,while the initial high-order buckling pattern changes from a first-order C-shape to a secondorder S-shape under a compressive load.

Numerical simulation of thermo-mechanical fatigue properties for particulate reinforced composites

In this paper, a two dimensional Voronoi cell element, formulated with creep, thermal and plastic strain, is applied for the numerical simulation of thermo-mechanical fatigue behavior for particulate reinforced composites. The relation between mechanical fatigue phases and thermal fatigue phases influences the thermo-mechanical fatigue behavior and cyclic creep damage. The topological features of micro-structure in particulate reinforced composites, such as the orientation, depth-width ratio, distribution and volume fraction of inclusions, have a great influence on thermo-mechanical behavior. Some related conclusions are obtained by examples of numerical simulation.

Mechanical Properties of Nano-CaCO_(3) and Basalt Fiber Reinforced Concrete:Experiments and Numerical Simulations

In this study,the compressive,split tensile,and flexural strengths of concrete with nano-CaCO_(3) only were compared with those of concrete with nano-CaCO_(3) and basalt fibers through field experiments,and the underlying mechanisms were analyzed by the Scanning Electron Microscope (SEM) techniques.On the mesoscale,a damage model of concrete was established based on the continuum progressive damage theory,which was used to investigate the influence of different lengths and contents of fibers on the mechanical properties of concrete.Then,the experimental and numerical simulation results were compared and analyzed to verify the feasibility of model.The results show that nano-CaCO_(3) can enhance the compressive strength of the concrete,with an optimal content of 2.0%.Adding basalt fibers into the nano-CaCO_(3) reinforced concrete may further enhance the compressive,split tensile,and flexural strengths of the concrete;however,the higher content of basalt fiber can not lead to higher performance of concrete.The optimal length and content of fiber are 6 mm and 0.20%,respectively.The SEM result shows that the aggregation of basalt fibers is detrimental to the mechanical properties of concrete.The numerical simulation results are in good agreement with the experimental results.

Grain refinement of 5A06 aluminum alloy welds caused by tungsten inert gas welding arc characteristics under Ar-He alternating gas supply conditions:a coupled method of water-cooled copper plate test/numerical simulation

The welding arc,as a carrier for the conversion of electrical energy to thermal energy,has a direct impact on the quality of welding by its properties and states.In the tungsten inert gas(TIG)welding process under the condition of Ar-He alternating gas supply,the arc is alternately converted between Ar arc and He arc with an alternating gas supply cycle,which has obvious arc change characteristics.The FLUENT software was used to numerically simulate the characteristics of the TIG arc under the condition of alternating gas supply,and the arc temperature field,arc pressure,electric potential and current density distribution under the condition of alternating gas supply were obtained.Combined with the real-time data of arc pressure measured by the water-cooled copper plate with holes,it is proved that the TIG arc has obvious dynamic characteristics under the condition of Ar-He alternating gas supply.This unique dynamic TIG arc acts on the 5A06 aluminum alloy weld,causing the molten pool to stir,resulting in uniform microstructure and grain refinement at the weld,and thereby improving the mechanical properties of the welded joint.

Determination of elastic and plastic mechanical properties of dentin based on experimental and numerical studies

The aim of this study is to investigate the change of mechanical properties of human dentin due to aging and spatial variation. Sections of coronal dentin are made from human molars in three groups： young, mid-aged, and old patients. A nanoindentation test is conducted from regions near the pulp to the dentin-enamel junction （DE J） to evaluate the load-depth indentation response and determine Young＇s modulus and hardness. Based on the loading and unloading load-displacement curves in nanoindentation, a numerical model of plastic damage is used to study the plastic and the damage behaviors and the contribution to the degradation in the unloading stiffness. The experimental results show that Young＇s modulus of the inner dentin is significantly lower than that of outer dentin in each age group. Compared with the young dentin, the old dentin has greater hardness and Young＇s modulus with similar spatial variations. The magnitudes of the yield strength and the damage variable are also affected by aging and vary with spatial locations. In the same age group, the yield strength in inner dentin is lower than those in middle and outer dentin, more damage occurs with similar spatial variations, and the yield strength of young dentin is generally lower and causes more damage compared with those in both the mid-aged and old groups.

Generation of 3D random meso-structure of soil-rock mixture and its meso-structural mechanics based on numerical tests

The mesoscopic failure mechanism and the macro-mechanical characteristics of soil-rock mixture(S-RM) under external load are largely controlled by S-RM's meso-structural features. The objective of this work is to improve the three-dimensional technology for the generation of the random meso-structural models of S-RM, for randomly generating irregular rock blocks in S-RM with different shapes, sizes, and distributions according to the characteristics of the rock blocks' size distribution. Based on the new improved technology, a software system named as R-SRM3 D for generation and visualization of S-RM is developed. Using R-SRM3 D, a three-dimensional meso-structural model of S-RM is generated and used to study the meso-mechanical behavior through a series of true-triaxial numerical tests. From the numerical tests, the following conclusions are obtained. The meso-stress field of S-RM is influenced by the distribution of the internal rock blocks, and the macro-mechanical characteristics of S-RM are anisotropic in 3D; the intermediate principal stress and the soil-rock interface properties have significant influence on the macro strength of S-RM.

Numerical simulations on cutting of frozen soil using HJC Model

Numerical simulation is known as an effective method for mechanical properties during frozen soil excavation.In order to reveal the development of cutting force,effective stress and cutting fragments in frozen silt during the cutting process,we introduce an explicit finite element program LS-DYNA to establish a two-dimensional numerical model of the frozen soil cut.We also use the Holmquist-Johnson-Cook(HJC)damage constitutive model for simulating the variation of soil mechanical properties according to the strong dependence between the cutting tool and frozen silt during the process with different cutting depths,angles and velocities.Meanwhile,a series of experimental results are acquired of frozen silt cutting to prove the application of the HJC model during simulation of cutting force variations.The result shows that the cutting force and fragment size are strongly influenced by cutting depths and cutting velocities increased,and the maximum effective stress at points where the tool contacts frozen soil during the cutting process.In addition,when the cutting angle is 52°,the cutting force is the smallest,and the cutting angle is optimum.Thus,the prediction of frozen soil mechanical properties on the cutting process by this model is conducive to selecting machinery equipment in the field.

Approach for Obtaining Material Mechanical Properties in Local Region of Structure Based on Accurate Analysis of Micro-indentation Test

The hot or cold processing would induce the change and the inhomogeneous of the material mechanical properties in the local processing region of the structure,and it is difficult to obtain the specific mechanical properties in these regions by using the traditional material tensile test.To accurately get actual material mechanical properties in the local region of structure,a micro-indentation test system incorporated by an electronic universal material test device has been established.An indenter displacement sensor and a group of special micro-indenter assemblies are estab-lished.A numerical indentation inversion analysis method by using ABAQUS software is also proposed in this study.Based on the above test system and analysis platform,an approach to obtaining material mechanical properties in the local region of structures is proposed and established.The ball indentation test is performed and combined with the energy method by using various changed mechanical properties of 316L austenitic stainless steel under differ-ent elongations.The investigated results indicate that the material mechanical properties and the micro-indentation morphological changes have evidently relevance.Compared with the tensile test results,the deviations of material mechanical parameters,such as hardness H,the hardening exponent n,the yield strength σy and others are within 5%obtained through the indentation test and the finite element analysis.It provides an effective and convenient method for obtaining the actual material mechanical properties in the local processing region of the structure.

Active-passive radial-additive friction stir repairing of mechanical hole out of dimension tolerance of AZ31 magnesium alloy

In order to avoid the depth increasing of repaired hole and eliminate the super-fine grain band in stir zone by radial-additive friction stir repairing(R-AFSR), a solid-state repairing technique of active-passive radial-additive friction stir repairing(AP-RAFSR) assisted by the truncated cone-shaped filling material was proposed in this study. The mechanical hole out of dimension tolerance of AZ31 magnesium alloy was chosen as the repaired object. The results indicated that the AP-RAFSR process rather than the R-AFSR process avoided the kissing bond in the bottom of the repairing interface under the condition of the tool pin length equal to the height of the standard mechanical hole.The continuously-distributed and large-length super-fine grain bands were eliminated in the stir zone by AP-RAFSR. The maximum tensile and compressive-shear strengths of repaired hole by AP-RAFSR reached 190.6 MPa and 138.9 MPa at 1200 rpm respectively, which were equivalent to 97.7% and 89.6% of those of the standard mechanical hole. This AP-RAFSR process assisted by the truncated cone-shaped filling material provides a new technique to obtain a no-depth-increasing, defect-free and high-strength repaired mechanical hole.

TC4钛合金表面固体粒子冲蚀损伤行为及机理研究

目的探究TC4钛合金在固体粒子冲蚀下的损伤行为,揭示钛合金表面的冲蚀机理。方法采用自主搭建的冲蚀试验装置,以TC4钛合金为研究对象,综合运用正交试验法和控制变量法开展常温干砂粒冲蚀试验。通过扫描电镜综合分析冲蚀区域表面和截面的微观形貌,采用能谱仪分析冲蚀区域元素的组成,利用电子天平测量冲蚀磨损质量;讨论工艺参数对钛合金损伤行为及冲蚀机理的影响。结果相较于冲砂量和冲蚀角度,冲蚀距离对冲蚀量的影响更大;冲蚀量随着冲蚀角度的增加呈先增大后减小的趋势,在40°附近达到峰值。结合数值模拟和试验研究结果发现,钛合金的损伤形式和冲蚀机理与冲蚀角度密切相关,在低攻角时形成了较狭长的犁沟和挤压唇,发生了明显的塑性变形,容易产生二次撞击,表现为微切削机制;在中攻角时,微切削与锤击效应共存,损伤最为严重;在冲蚀角度为90°时,形成了较多的撞击坑、挤压唇及少量的疲劳剥层,主要为锤击效应引起的疲劳破坏。在低攻角时,磨料动能损失较小,随着冲蚀角度的增加,磨料动能损失增大,且在不同攻角下均有破碎磨料嵌入基体。结论冲蚀距离和冲砂量对钛合金冲蚀损伤的影响较为显著,冲蚀角度会影响钛合金的冲蚀机理,此研究结论可为钛合金结构件抗冲蚀设计提供理论依据。

土动力学与岩土地震工程

社会发展需求的增长、工程建造能力的提升以及气候变化等环境因素的改变带来的新震害现象、技术手段和工程需求,为土动力学及岩土地震工程研究提出了新挑战并创造了新机遇。文章重点针对近年来土动力学与岩土地震工程领域的一些新现象、新手段和新挑战进行综述,聚焦于土的动力特性、砂土地震液化、土的动力测试和岩土地震工程试验、岩土地震工程数值模拟以及岩土工程抗震等方面,分析当前理论研究和工程实践所面临的新问题和新挑战,梳理近年来所取得的一些最新进展,并对今后的研究方向进行探讨。

基于数值模拟和机器学习的岩石力学性能预测

针对传统方法在岩石力学性能研究中的不足,提出一种兼具准确、高效的有限元数值模拟和多目标机器学习的岩石力学性能预测方法。采用连续损伤力学方法对岩石力学试验过程细观有限元模拟,得到机器学习的样本数据;以单轴抗拉/抗压强度、弹性模量、泊松比和围压为特征变量,以应力-应变矩阵和损伤变量矩阵为目标变量,采用粒子群-随机森林算法建立岩石三轴力学性能预测模型;在此基础上,开发岩石三轴力学性能预测系统,实现应力-应变曲线和破坏形态的直观显示。该方法在岩石力学实验教学中取得了良好的效果,也为工程设计和计算提供了有益的参考。

带定位榫盾构隧道管片接头剪切力学行为研究

外套于盾构隧道管片螺栓的空心圆锥筒定位抗剪构件定位榫能有效控制盾构管片错台量和错台值,与管片、螺栓共同形成“管片-定位榫-弯螺栓”接头.为探明盾构隧道管片受压工况、“管片-定位榫-弯螺栓”接头在剪切荷载作用下的结构力学特征和破坏机理,以内径7.5 m、外径8.3 m、弯螺栓连接的盾构隧道衬砌环的纵向接头为例,首先,建立三维有限元模型,对轴力分别为196、392、588和784 kN4种工况的接头剪切破坏全过程进行数值模拟;然后,对接头剪切破坏过程、定位榫和螺栓力学及变形特征、管片接缝面变形破坏特征进行分析;最后,采用原型剪切破坏试验对数值模拟结果进行印证和补充研究.研究结果表明:盾构隧道“管片-定位榫-弯螺栓”接头剪切破坏全过程分为接触面静摩擦、定位榫弹性变形、定位榫塑性错动和螺栓剪切破坏4个阶段;定位榫弹性变形阶段,定位榫表现为“压扁”形态,定位榫塑性错动阶段,定位榫拱顶外侧首先出现塑性屈服区,随着剪切荷载的增加,塑性区将扩展到定位榫端部和横断面内大部分区域,塑性破损过程中定位榫将超过其承载能力的外部剪切荷载转移给螺栓;螺栓杆体受力从定位榫塑性错动阶段开始,最终加载侧杆体将发生“拉直”破坏、固定侧杆体将发生弯剪破坏;管片接缝面混凝土从定位榫塑性错动阶段后期出现塑性区,初始位于榫槽内水平两侧,随着剪力增加,塑性区将向周边扩展,最终与管片内弧面手孔塑性区连通,形成“手孔-螺栓孔-手孔”塑性破坏域.

铸造镁合金凝固过程数值模拟研究进展

镁合金以其轻量化、高比强度和良好的阻尼性能在汽车、航空航天等领域应用广泛。数值模拟通过再现铸造成形过程中的各类宏观和微观物理过程,可以调控组织、减少缺陷、提高力学性能和优化铸造工艺参数。本文综述了铸造镁合金在枝晶和共晶凝固组织模拟,偏析、气孔和热裂等缺陷模拟,以及力学性能预测等方面的研究现状,简要介绍了近两年在铸造镁合金成形工艺模拟方面的研究进展。最后,指出了当前铸造镁合金数值模拟研究存在的问题及发展方向。

不同温度条件下相变石蜡力学性能分析

对3种不同相变温度的石蜡材料(PCM-28、PCM-18和PCM-10)在不同温度下的基本力学性能参数进行了系统的压缩实验,计算出对应的弹性模量和泊松比。结果表明:PCM-18和PCM-10的抗压强度随温度的降低而有所提高,但PCM-28的抗压强度随温度的降低反而呈现出下降趋势。在单轴压缩条件下PCM-28和PCM-18的应力-应变曲线主要经历了压实阶段、弹性变形阶段、裂纹扩展阶段和裂纹后破坏阶段4个明显阶段,最终表现为脆性破坏。PCM-10在经历压实阶段和弹性变形阶段后进入塑性阶段,曲线发展近似水平,且整个过程无明显裂纹出现。