Effect of impulse and bedding on impact toughness of coal

In order to understand the properties of impact toughness of coal at different impact speeds,and the change of impact toughness of lump coal to joint directivity of lump coal,a series of impact tests were conducted on Beijing Da'anshan Lump Coal at different impact speeds and in different impact direction.Through analyzing the test result,it is shown that the change of testing samples is similar when impact is exerted on the vertical bedding and the parallel bedding when the impulse is less than 20 N.s,and the difference increases with the impulse increasing when the impulse is more than 20 N·s.At the same time,the expanding energy of fracture in samples increases with its expanding speed,and the expanding energy of fracture has close relation with the impact direction of the tested samples.And the difference of impact toughness of lump coal produced by different im- pact direction increase with the impact speed.The fracture surface of lump coal when im- pact is exerted on the vertical bedding is smooth and the broken block number is fewer; but the fracture surface of lump coal when impact is exerted on the parallel bedding isn't smooth and the broken block number is more,which inflects impact toughness of coal is sensitive to some deficiency.

Investigation of Impact Toughness of Electromagnetic Centrifugal Cast High Speed Steel

The effect of external magnetic field on the solidification structure and impact toughness of high-speed steel(HSS)produced by electromagnetic centrifugal casting(EMCC)have been investigated.It was found that the solidification structure could be improved effectively by electromagnetic stirring,which was characterized by elimination of the inclined columnar dendrites and refinement of the grain size.The results may be related to the changes of the fluid layers of liquid-solid phase interface.Moreover,the impact toughness of the test steel increased with the exciting current increasing from 12 to 18 A,but decreased when the current was above 18 A.SEM analysis showed that impact fracture surface was dimple fracture due to effect of the refined equiaxed grains when the current was 18 A,which was considered an important reason for the improvement of impact toughness.

真三轴应力条件下冻结钙质黏土能量特征及影响因素分析

为深入探讨在真三轴应力路径及多因素交互作用条件下冻结钙质黏土的能量特征及其影响程度,本研究借助ZSZ-2000型真三轴冻土试验平台,采用正交试验的设计方法,开展冻结钙质黏土真三轴压缩试验研究。研究结果揭示:在初始压密阶段及线弹性阶段,冻结钙质黏土的输入能相对较低,增长速率缓慢;随着加载过程的深入,输入能逐步增加,其增长速率亦随之提高;至破坏阶段,输入能-等效应力曲线呈现出明显的跃升特征,此时输入能的急剧上升,说明输入能曲线斜率的变化是冻结钙质黏土内部裂纹非稳定发展的微观表现。同时,输入能曲线的突然急升亦指示了失稳破坏的发生。进一步通过极差分析及AHP层次分析可得一致的输入能影响因素优劣次序:含水率w、中主应力系数b、温度T、含盐量φ、围压σ3。含水率对输入能的负向影响显著,温度和中主应力系数b对输入能有着显著的正向影响关系,随着中主应力系数b值的增加,其对冻结试样输入能的影响程度呈减弱的趋势且存在一个最优中主应力系数b=0.33使试样输入能增幅达到最大。

Influence of Rare Earth Elements on Microstructure and Mechanical Properties of Cast High-Speed Steel Rolls

The influence of rare earth （RE） elements on the solidification process and eutectic transformation and mechanical properties of the high-V type cast, high-speed steel roll was studied. Test materials with different RE additions were prepared on a horizontal centrifugal casting machine. The solidification process, eutectic structure transformation, carbide morphology, and the elements present, were all investigated by means of differential scanning calorimetry （DSC） and scanning electron microscopy energy dispersive spectrometry （SEM-EDS）. The energy produced by crack initiation and crack extension was analyzed using a digital impact test machine. It was found that rare earth elements increased the tensile strength of the steel by inducing crystallization of earlier eutectic γ-Fe during the solidification process, which in turn increased the solidification temperature and thinned the dendritic grains. Rare earth elements with large atomic radius changed the lattice parameters of the MC carbide by forming rare earth carbides. This had the effect of dispersing longpole M C carbides to provide carbide grains, thereby, reducing the formation of the gross carbide and making more V available, to increase the secondary hardening process and improve the hardness level. The presence of rare earth elements in the steel raised the impact toughness by changing the mechanism of MC carbide formation, thereby increasing the crack initiation energy.

Q355/SKH9高速钢激光重频焊接接头冲击韧性分析

文章采用激光焊接技术制造Q355/SKH9高速钢焊接接头,并开展V型缺口冲击韧性实验。研究结果表明:当试验温度下降后,接头受到了更小冲击吸收能量作用,焊接接头形成了明显梯度冲击韧性;在BM中存在大量回火板条型马氏体组织,HAZ热处理时内部形成了不同焊接热循环过程;在BM和HAZ断口部位形成了许多撕裂棱,表现为准解理断裂;断HAZ具备优异韧性,达到了比BM更优的韧性;经过回火处理后,板条马氏体与粒状贝氏体获得了更强冲击韧性;BM保持相对稳定的显微硬度与剪切强度,HAZ形成了大梯度变化的显微硬度与剪切强度;经过焊接热循环处理与之后回火过程,HAZ基体与晶界部位形成了更多碳化物,增大HAZ强度与硬度。

Dynamic constitutive modeling of frozen soil under impact loading

In this study, an isotropic hardening constitutive model is presented to study the behavior of frozen soil under impact loading. In plasticity, a modified Drucker–Prager yield function is adopted. Based on the experimental investigations at different strain rates and different temperatures by means of split Hopkinson pressure bar, the Drucker–Prager criterion has been modified with consideration in the effect of strain rate, and the model parameters have been determined. Compared the constitutive model with the experimental results, the predicted tendencies of the model corresponded well to the test curves.

高速铁路装配式桥墩抗震性能研究

为研究装配式桥墩在高速铁路桥梁中的适应性,以京雄高铁固霸大桥装配式桥墩为原型,在足尺拟静力试验基础上,采用ANSYS软件进行有限元数值模拟,并与试验数据进行比较;采用验证后的有限元模型对实际装配式成桥墩的抗震性能,以及一座采用装配式桥墩的两跨连续梁桥在地震波作用下的时程响应进行分析。研究表明:试验装配墩与实际成桥装配墩的滞回曲线、骨架曲线、刚度退化特征、耗能能力及残余变形均基本一致,且均与现浇墩差别不大,说明套筒连接未对桥墩的滞回性能和恢复力特征值产生明显影响;考虑桩-土效应后,装配墩的滞回环面积减小,初期刚度和滞回耗能分别下降36%和23.5%;采用装配墩的连续梁桥相同模态下的自振频率比采用现浇墩时略小,但最多只相差2.7%,而桩-土效应则使自振频率下降多达19%;在实际地震波作用下,装配墩连续梁桥与采用现浇墩时的墩顶最大位移、顺桥向墩底弯矩和剪力均接近,但桩-土效应使顺桥向墩顶最大位移增加9.4%,顺桥向墩底弯矩和剪力分别减小41%和40.9%。研究结果表明,采用装配式桥墩的高速铁路桥梁与现浇墩桥梁的抗震性能相比未明显下降。

压-剪耦合冲击下冻土力学行为与能量演化研究

在寒区工程施工中,冻土不可避免地会承受压-剪耦合荷载的作用。为了研究冻土在压-剪耦合冲击加载下的力学性能及其能量演化规律,设计了4种不同倾角(0°、3°、5°和7°)的冻土试样,并利用分离式霍普金森压杆在3组不同冻结温度(-23、-15、-7℃)下对其进行冲击实验。结果表明:压-剪耦合冲击加载下冻土的破坏过程可以分为弹性阶段、塑性阶段和破坏阶段;随着温度由-23℃升至-7℃,0°倾角下冻土的抗压强度下降了41.4%,表现出明显的温度敏感性,而当倾角由0°增加至7°时,-23℃下冻土的抗压强度降低了21.0%,表现出一定的加载路径依赖性;冻土的破坏面随着温度降低不断向外扩展;冻土的破坏过程伴随着能量积累、耗散和释放,且能量耗散密度随着温度的升高和倾角的增加而降低。

The Effect of Loading Rates on Crack Dynamic Behavior Under Medium-Low Speed Impacts

Rock structures are often subjected to dynamic loads,such as blasts,impacts and earthquakes,and their loading rates differ largely.To investigate the effect of loading rates on the dynamic behavior of crack propagation,impact tests were conducted on large single-cleavage semicircle compression(LSCSC)specimens using a drop weight impact test system.Five types of rock materials were selected to prepare the LSCSC specimens,and crack propagation gauges were mounted along the crack propagation paths to measure crack initiation time and propagation speeds.Finite element models were established by using ABAQUS code,and the dynamic stress intensity factors(SIFs)were calculated.The curves of dynamic SIFs versus time were obtained,and the initiation toughness was determined by using these curves and the initiation time measured in the impact tests.The results show that loading rate has a significant effect on crack propagation behavior,and both the crack propagation speed and initiation toughness increase with the loading rate,whereas the delayed fracture time decreases with the increase in loading rate.

侵彻体撞击冻土摩擦升温计算方法研究

针对水冰探测过程中需要改变其物态以便于质谱分析的问题,提出了侵彻体撞击目标接触头处摩擦升温的计算方法。将侵彻体撞击冻土划分为“冻土”和“融土”阶段,高速移动热源摩擦升温法和侵彻体撞击冻土阻力法结合,提出了侵彻体撞击“冻土”阶段弹头处摩擦升温计算方法。利用仿真软件,得到了侵彻体以低于400 m/s的速度撞击时头部的升温曲线,与理论计算曲线基本一致,验证了摩擦升温函数的正确性。根据传热学理论,得到了系列速度下侵彻体撞击“冻土”阶段物态体积的改变量。

1990~2022年气候变化对玉树地区季节性冻土的影响

利用1990~2022年玉树地区5个观测站的冻土深度、地温、气温、气温日较差、积雪日数、降水和日照等观测资料,采用现代气候诊断分析方法,对冻土特征及其变化进行了分析。结果表明,近33a来:(1)各站年平均气温呈明显的上升趋势;(2)冻土开始结冻日期杂多最早,依次为杂多、治多、曲麻莱、玉树、囊谦;完全消融日期囊谦最早,依次为囊谦、玉树、杂多、治多、曲麻莱;年冻土深度从深到浅依次为曲麻莱、治多、杂多、玉树、囊谦,与年平均气温从低到高的次序一致;(3)冻土呈退化趋势;(4)影响冻土的主要气象因子为气温、气温日较差、地温、积雪日数;(5)冻土不同时期降水量、日照与地温相关性比较复杂,对冻土的冻结与消融的影响,存在时间与地域的差异。

土壤冻融交替生态效应研究进展

土壤冻融作用是高纬度和高海拔地带性土壤热量动态的一种表现形式。国际上关于冻融的研究多集中在北方高纬度地带,特别是苔原、泰加林和北极生态系统,越来越集中在全球变化对冻融生态系统土壤过程的效应方面。已有研究表明冻融作用会引起土壤团聚结构破坏并导致冻融侵蚀、土壤溶液中养分浓度升高而导致土壤养分流失、土壤解冻后还可能导致土壤呼吸和N素矿化以及一些痕量气体短时间释放增加等,这些研究表明了冻融过程对土壤物理、化学、生物等各方面的效应。我国有大面积北方季节性地带冻土和青藏高原高海拔冻土,而在土壤冻融作用及生态效应方面的工作较少,值得关注和深入研究。

少量铈对高硼高速钢微观组织与力学性能的影响

借助光学显微镜、扫描电镜和X射线衍射仪研究少量铈对高硼高速钢微观组织和力学性能的影响。结果表明:少量铈的添加能显著提高高硼高速钢的冲击韧性和磨损性能,其冲击韧性值(ak)由6.7 J/cm2增加到14.3 J/cm2,少量铈的存在使高硼高速钢中鱼骨状硼碳化物基本消失,大部分羽毛状硼碳化物熔断,粒状硼碳化物增多,硼碳化物分布更加均匀,初生奥氏体晶粒明显细化。

冲击速度对冻土冲击韧度的影响

为研究不同冲击速度条件下冻土冲击韧度的变化特性，以及冻土在高应变速率下的强度特性，采用传统冲击试验机对冻结兰州黄土、兰州中砂和纯冰进行了不同冲击速度、不同温度条件下有缺口冲击试验研究。试验结果发现裂纹在试样中扩展所需能量随速度的增加而增加，并与试验样品种类密切相关。随着温度降低和锤速的增加，试样冲击韧度的敏感性也随之加大。

冻土冲击韧度与凿碎比能关系的试验研究

冻土的冲击韧度与试块破坏断口的平整度反映了冻土的冲击特性。冻土凿碎比能是表示凿碎单位体积冻土所消耗的功,它反映了冲击破碎冻土的能力。以冻结粘土为试样,利用摆锤式冲击试验仪进行不同温度、不同冲击能量的带缺口的冲击实验,利用凿测器进行了冻土凿碎比能试验。实验结果表明,冻结粘土的冲击韧度随温度降低而增大,随冲击能量的增加而增大。随温度的降低,试样的破坏断口的凹凸不平度增大,其范围在±3mm之间。随着温度的降低,冲击凿入深度越来越浅,冻土的硬度越来越大,钻进程度越来越难。通过回归分析,凿碎比能随着冲击韧度的增加而增大,且在冲击韧度一定的情况下,随着凿入次数的增加,凿碎比能增加的值越来越小,凿碎比能与冲击韧度的相关性较好,表明用冻土的冲击韧度来反映凿碎比能是可行的。

残余奥氏体量对高钒高速钢性能的影响

利用改变高钒高速钢淬火加热温度和回火温度获得不同残余奥氏体量 ,研究了残余奥氏体量对硬度、冲击韧性和磨粒磨损性能的影响。结果表明 :淬火加热温度升高 ,残余奥氏体量增加 ;回火温度升高 ,残余奥氏体量减少。随残余奥氏体量增加 ,硬度呈二次曲线状先升高后降低 ,冲击韧性近似直线升高 ,磨损量呈二次曲线状先下降后升高。残余奥氏体量为 2 0 %～ 4 0 % (vol% )时 ,高钒高速钢的硬度高 ,冲击韧性适中 ,磨粒磨损耐磨性最好。

硬度及冲击韧性对高钒高速钢磨损稳定性的影响

通过改变热处理工艺获得不同硬度及冲击韧性、含钒10%的高钒高速钢试样,提出了磨损稳定性的概念及采用磨损稳定性因子(K)量化磨损稳定性的方法,研究了硬度和冲击韧性对高钒高速钢的磨损稳定性影响.结果表明:随着硬度增加,磨损稳定性及其耐磨性提高;随着冲击韧性提高,虽然耐磨性降低,但磨损稳定性有所提高.在本文试验条件下,随着残余奥氏体变化量(ΔA)增加,高钒高速钢的磨损稳定性下降.

稀土对高碳高速钢组织和性能的影响

在成分为Fe-5%V-5%W-5%Mo-5%Cr-3%Nb-2%Co-2%C的高碳高速钢中添加稀土,研究了稀土对高碳高速钢铸态组织、热处理组织和力学性能的影响。结果显示:稀土处理使高碳高速钢的奥氏体晶粒和共晶组织明显细化,共晶组织中片层状碳化物变短、变细。热处理后,共晶碳化物大部分变成团球状且分布均匀。稀土处理高碳高速钢的硬度和红硬性略有增加,冲击韧性提高37.81%,达到10.17 J/cm2,分析了稀土在高碳高速钢中的作用机理及改善合金性能的机制。

基于Johnson-Cook模型的冻土动态本构关系

采用分离式霍普金森压杆(SHPB),对于-17℃冻土进行了应变率约350、600、800、1 000和1 200 s-1的单轴冲击实验。获得了其相应应变率下的应力应变关系。发现其没有明显的屈服现象,具有显著的应变率效应,其峰值应力与最终应变均随加载应变率增大而增大,并且具有一定的线性关系。引入含损伤的Johnson-Cook模型,描述-17℃冻土的应力应变关系,发现在600～1 200 s-1的加载应变率范围内,该模型具有较好的适用性。

冲量及层理对煤块冲击韧度的影响

为了研究不同冲击速度条件下块煤冲击韧度的变化特征,以及块煤节理方向性对块煤冲击韧度的变化特性。采用传统的冲击试验机对北京大安山煤矿的块煤进行了不同冲击速度、不同的冲击方向条件下的有缺口冲击试验。试验结果表明,当冲量小于25N.s时,沿着垂直层理方向和平行层理方向施加冲量后,试样的变化形态十分接近,而在此之后两者的差异随冲量的增加而增大。同时发现裂纹在试样中扩展所需能量随速度的增加而增加,裂纹扩展速度的所需能量与冲击方向密切相关,并且这种由不同结构产生的冲击韧度的差异随冲击速度的增加而加大。对于沿着垂直节理方向施加冲量的块煤产生的断裂面平整,破碎的块数较少;而对于沿着平行节理方向施加冲量的块煤,断裂面也就不平整,破碎的程度严重,碎块数较多。这反映了冲击韧度对煤块的某些缺陷很敏感。