Fatigue Behavior of High Speed Steel Roll Materials for Hot Rolling by Laser Impacting

The fatigue behavior of high speed steel （HSS） roll materials for hot rolling was researched under watercooling conditions by laser impacting. The microstructure of HSS sample and the morphologies of fatigue samples were observed by scanning electron microscope. The phase structure was detected by XRD. The morphology of situ oxide scale was observed by optical microscope, and the expansion coefficient was measured by TGA. The experiment results indicate that the cracks come into being at the carbide-matrix interface, but there are no cracks in the matrix after many times of laser impacting treatment, for the situ sample taken from the fractured roll surface, big carbides are more sensitive to the fatigue, and peel off prior to small ones. The relevant fatigue mechanisms are also discussed.

Assessing the Impacts of High Speed Rail Development in China’s Yangtze River Delta Megaregion

This paper assesses the impacts of high speed rail (HSR) development in the Yangtze River Delta (YRD) Megaregion, China. After giving an introduction and conducting a literature review, the paper proposes a pole-axis-network system (PANS) model guiding the entire study. On the one hand, the HSR projects in the YRD Megaregion are expected to generate significant efficiency-related transportation and non-transportation benefits. As a result, the spillover effects from Shanghai and other major cities (poles) will greatly promote the urban and regional developments along the major HSR corridors (axes), and the entire megaregion will become more integrated economically, socially, and culturally. But, on the other hand, the HSR projects will also create serious social and geographic inequity issues, which need to be addressed as soon as possible in a proper way. This empirical study confirms the PANS model proposed.

Fabrication of TiB_2 composite powders coated with BN by high speed airflow impact

TiB2 powders coated with BN were prepared by Hybridization System making use of dry impact blending method to achieve powder surface modification. Parameters of coating were analyzed and the most appropriate condition was summarized. Scan electron microscope of JSM-5610LV and transmission electron microscope of H-600STEM/EDS were used to observe the microstructure of coated powders. Results show that treatment time, rotation speed, granularity ratio of TiB2 to BN, pretreatment of materials etc influence the coating results evidently. Mixing raw materials and coating with BN under the appropriate condition can get round TiB2/BN composite powder with smooth surface and compact coating layer.

Visualization of High-Speed Impact of Projectile in Granular Sheet with Destructive Collision of Particles

The impact and penetration of a projectile in a particle-laden space, which are expected to have frequently occurred during the formation of the solar system and will occur in the case of an impact probe for future planetary exploration, were experimentally simulated by using the ballistic range. A two-dimensional sheet made from small glass beads or emery powder was formed by the free-falling device through a long slit in the test chamber evacuated down to about 35 Pa. A polycarbonate projectile of a hemi-sphere-cylinder or sphere shape with the mass and diameter about 4 g and 25 mm, respectively, was launched at the velocity up to 430 m/s, and the phenomena were observed by the high-speed camera at 20,000 fps. From a series of images, the bow-shock-wave-like laterally facing U-shaped pattern over the projectile and the absence of particles in the trail behind it were clearly seen. At the impact of the particles on the projectile surface, fine grains were formed due to the destructive collision and injected outward from the projectile. The images obtained by different lighting methods including the laser light sheet were compared. The effects of the particle diameter, its material and the impact velocity were also investigated.

Visualization of High-Speed Impact of Penetrator into Icy Target

For application to exploration under the surface of icy objects in the solar system, the penetration of an impact probe into an icy target was experimentally simulated by using the ballistic range. Slender projectiles with a cylindrical body and various nose shapes were tested at the impact velocity 130 - 420 m/s. The motion of the penetrator, fragmentation of ice and crater forming were observed by the high-speed camera. It revealed that the crown-shaped ejection was made for a short time after the impact and then the outward normal jet-like stream of ice pieces continued for much longer time. The concave shape of the crater was successfully visualized by pouring the plaster into it. The two-stage structure, the pit and the spall, was clearly confirmed. The rim was not formed around the crater. Observation of the crater surface and the ice around the trace of the penetrator shows that both crushing into smaller ice pieces and recompression into ice blocks are caused by the forward motion of the penetrator. In case of a body with a flow-through duct, ice pieces entering the inlet at the nose tip were ejected from the tail, resulting in relaxation of the impact force. The correlation of the penetration distance and the crater diameter with the impact velocity was investigated.

Computation of Impact Factor of High-Speed Railway Bridge by KTX Train Riding Test

The design live load of railway is divided into common railway and high-speed railway separately inKorea. Accordingly, the Korean design specification of railway specifies the impact factor for common railway and high-speed railway respectively. The impact factor for high-speed railway is based on Eurocode. Since the impact factor criteria inKoreawere established by adopting those of the Eurocode and without dedicated investigation relying on research results reflecting the domestic circumstances, thorough examination should be implemented on these criteria. Therefore the evaluation of impact factor based on field tests is required. Both dynamic and static vertical displacements are necessary to compute the impact factor. The dynamic response can be obtained from the measurement of deflection of the bridge slab crossed by the firstKoreahigh-speed train (KTX, Korea Train eXpress) running at high-speed. The main difficulties encountered are in obtaining static response because static response corresponds to the response of the bridge when the train remains immobile on the bridge or crosses the bridge at speed slower than5 km/hr. This study introduces the static response derived by applying the moving average method on the dynamic response signal. To that goal, field measurements was conducted under train speeds of5 km/hr and ranging from100 km/hr to300 km/hr on Yeonjae Bridge located in the trial section of the Gyeonbu High-Speed Railway Line before its opening. The validity of the application of the moving average method is verified from comparison of measured static response and derived static response by moving average method. Moreover, evaluation is conducted on the impact factor computed for a bridge crossed by the KTX train running at operational speed.

VALENCE ELECTRON STRUCTURE OF HIGH PROPERTY HIGH SPEED RESISTANCE STEEL AND ITS COMPOSITION DESIGN

Theempiricalelectrontheory of solidsand molecules( EET) and theimproved TFDtheory wereapplied tocalculatethe phasestructurefactorsand interfaceconjunction factorsofcom mon alloying elementsin high speed impact resistance ( HSIR) steel. According tothese va lenceelectron structure parametersitis deduced thatthe alloying elements of high propertyHSIRsteelshould be Mn, Cr, Ni and Mo. The microscopicstructure ofthesteel with this compositionisthe mixtureof martensite, bainiteand austeniteafter quenching andlow tem peraturetempering.Itspropertiesare:σb = 1750 MPa,σ0 2 = 1460 MPa,δ5 = 12 7 % , AKV(at 40 ℃, cross direction) = 21J, which exceed the requirement of design. For the thick plateof high property HSIRsteel,itissuggestedto add alittleamountof Cu on thebasisofabovecomposition .

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.

微型反应堆高速撞击下核临界安全的初步分析

微型反应堆可作为月球表面动力、航天器动力等用于太空探索,反应堆发射前需要对发射事故的影响进行安全分析。反应堆高空坠落并撞击到混凝土是主要的临界安全分析场景,高速撞击后反应堆可能重返临界。本文以两种简化的反应堆模型高速撞击场景为例,利用连续介质力学有限元程序ABAQUS与粒子输运蒙特卡罗程序耦合,完成了纯燃料堆芯垂直撞击地面和带径向反射层和屏蔽层圆柱堆30°倾角撞击地面的模拟,预测了两种场景反应堆高速撞击下keff随时间变化的物理特性。结果表明:纯燃料反应堆垂直撞击地面的keff增加最高可达1000×10^(-5),而具有反射层和屏蔽层反应堆30°倾角撞击地面的keff增加最高为200×10^(-5)。均匀密度变化条件下,采用表面非结构网格与内部非结构网格的蒙卡程序的计算结果符合较好,内部非结构网格能够更真实地捕捉材料的非均匀密度变化效应。本文的研究为微型反应堆高速撞击下的临界安全研究奠定了重要基础。

基于赫兹接触模型的发动机封严涂层碰磨力计算与优化

目的提高航空发动机的推进效率,在压气机机匣上喷涂可以减少叶尖径向间隙的封严涂层。叶尖与涂层之间的碰磨力会导致涂层脱落,且会击伤叶片,需要对碰磨力进行分析。方法采用大气等离子喷涂技术制备4种不同硬度的AlSi-PHB(聚苯酯)封严涂层,通过表面硬度测试、弹性模量测试和高速碰磨试验,分别评价封严涂层的硬度、弹性模量,以及在高速碰磨过程中不同工况下涂层受到的碰磨力;基于赫兹接触模型对叶尖与涂层之间的碰磨力进行计算,通过激光共聚焦显微镜和扫描电子显微镜对碰磨后的涂层和叶尖进行分析,同时根据接触面形态特征和温度特征对赫兹模型进行优化。结果碰磨力与涂层的硬度、叶尖转速、叶尖切入速率有关,复杂的接触表面形貌和摩擦升温会导致理论计算值与实验值之间出现偏差。结论通过优化叶尖和涂层的接触系数,同时考虑摩擦升温对涂层弹性模量的影响,可将不同工况下碰磨力计算值与测量值之间的偏差控制在1%~11%,这项研究对于指导航空发动机封严涂层的设计具有重要意义。

轴向串联式吸能管的缓冲吸能特性

针对工程技术领域的碰撞载荷削峰减载问题,采用数值模拟与试验相结合的方法研究了轴向串联式吸能管的吸能特性:首先基于材料高速拉伸试验,构建吸能管的材料Johnson-Cook动态本构参数,并对拟合参数有效性进行评估;随后通过数值模拟与高速冲击试验研究高速撞击过程中吸能管的缓冲吸能特性,评估仿真与试验的一致性;最终通过数值模拟对吸能管轴向串联构型与单管构型之间的吸能评价指标开展对比分析。分析研究表明:数值模拟与冲击试验的变形模式、载荷曲线、吸能评价指标均吻合较好,材料性能参数准确,仿真预示方法有效,高速冲击试验方案合理可信;与相同结构参数的串联构型吸能管相比,单管构型吸能管在压缩过程中会出现非轴对称、不稳定的扭曲变形,单管构型的有效压缩行程减小了13%,峰值载荷提高了33.4%,撞击瞬间载荷提高了15%,平均压缩力提高了13%,载荷峰均比提高了17.7%;吸能管的串联构型是更为理想的缓冲吸能结构。

射弹高速撞击下结构冲击响应建模方法

装备受到外界打击可能造成失效,以往的研究多集中在打击部位的损伤破坏,而对于打击引起的冲击波及其在装备内传播研究较少,而后者同样能够导致装备失效。本文针对结构在高速撞击下的冲击响应问题开展了建模方法研究,提出了结构在高速打击下的冲击源建模方法,并通过理论分析给出了建模方法的适用条件。利用传统的有限元技术,结合本文的冲击源建模方法,可以有效地对结构高速撞击下的冲击响应进行仿真,通过试验验证了方法的适用性。

高速跨介质入水多相流动与流固耦合特性研究综述

高速跨介质入水问题广泛存在于海洋工程、航空航天等领域,入水多相流动与流固耦合作用机理是该领域的研究热点,对于跨介质航行器的载荷预报、弹道稳定性评估、结构强度校核以及安全性设计等具有重要意义。本文围绕高速跨介质入水所涉及的基础关键力学问题,重点论述了高速跨介质多相流动与空泡演化、冲击载荷与降载方法、运动稳定性与流固耦合响应、流固耦合数值研究方法等方面的研究现状,并针对目前仍存在的问题提出了建议,旨在为高速跨介质入水的相关研究与设计等提供参考。

计及高风速和低风速影响的海上风电机组两阶段滚动优化维护策略

针对复杂天气变化影响下海上风电机组维护决策问题,提出了考虑高风速与低风速影响的海上风电机组两阶段滚动优化机会维护策略。首先,提出考虑高风速冲击影响的维护役龄回退模型,量化实际部件退化与及维护成本关系。其次,建立了考虑低风速停机事件维护机会影响的海上风电机组长短期两阶段预防性机会维护模型,滚动更新长期预防性机会维护阈值。最后仿真结果表明,本文所提模型能够有效降低海上风电机组停机成本以及故障损失,提高机组寿命周期内的维护效益。

预制孪晶对AZ31镁合金绝热剪切敏感性的影响

为了分析不同孪晶体积分数对材料绝热剪切敏感性的影响,预制具有不同孪晶体积分数的AZ31镁合金。采用分离式霍普金森压杆在200℃、944 s^(-1)条件下对AZ31镁合金帽状试样进行高速冲击试验,采用电子背散射衍射仪和光学显微镜对试样在高应变速率变形前后的微观组织进行观察,计算了试样发生绝热剪切时的吸收能量,比较了绝热剪切带内外的显微硬度。结果表明:不同孪晶体积分数试样内均产生了绝热剪切带,随着孪晶体积分数的增加,绝热剪切带宽度和吸收能量减小,而绝热剪切敏感性增大。

气体射流冲击噪声的工程计算与仿真

气体射流冲击广泛存在于工业领域之中,气体射流冲击噪声不仅污染环境,还对身心健康产生影响。针对气体射流冲击噪声缺乏工程计算依据的问题,文章对气体射流冲击进行声压能分析,结合气体射流冲击噪声产生机理,对气体射流冲击噪声进行公式化推导,形成工程计算公式。在典型气速下,通过有限元仿真验证计算公式的有效性,结果可为气体射流冲击噪声仿真、工程应用提供依据。

高铁开通对物流业发展影响的实证研究

促进物流业的高质量发展是建设现代化产业体系的一个重要方面,也是内循环发展格局的重要环节,高铁网建设的不断进展使铁路运输能力得到质的飞跃,探讨和分析高铁货运以及高铁开通对物流业的影响具有现实意义和必要性。依据运输业和物流业发展现状及规划发展的相关文件,论证了高铁开通对物流业的影响机制,建立物流业综合水平测度指标,通过PSM-DID模型实证分析高铁对物流发展的影响,进行异质性检验、机制检验等。研究表明:高铁开通对城市区域物流发展水平整体上具有促进作用;高铁对物流业的影响与该城市的规模、等级、地理位置以及运输业市场结构密切相关;高铁对物流业的影响主要通过区域经济增长、货运运输量提升、物流人力资本提升等途径实现。

WFeNiMo高熵合金弹丸高速撞击钢靶的数值模拟研究

随着现代武器装备防护能力的日益增强,提高穿甲弹侵彻能力的需求越发强烈,对穿甲弹的高效毁伤能力提出了更高的要求,常规的穿甲弹材料已不能满足现今侵彻毁伤目标的需求。由于高熵合金因其成分设计灵活和优异的力学性能,有望作为穿甲弹芯材料取代贫铀和钨合金等。利用二级轻气炮开展了WFeNiMo高熵合金弹丸高速撞击钢靶实验,利用弹丸撞击铜网产生的电压信号,同步触发高速摄像机。利用高速摄像机采集的弹丸高速碰撞过程,确定撞击速度分别为1072m/s和1346m/s。靶板厚度分别为4mm和10mm,WFeNiMo高熵合金弹丸对钢靶的毁伤模式分别为成坑和穿透。同时,利用物质点法(MPM)对WFeNiMo高熵合金弹丸撞击钢靶过程进行数值模拟。分别从扩孔直径、侵彻深度、剩余弹丸长度和翻唇高度等方面,对比WFeNiMo高熵合金弹丸高速撞击钢靶实验结果和MPM数值模拟结果。研究结果表明:MPM数值模拟结果与实验结果数值上的平均误差约为2%,且MPM弹靶毁伤形貌与实验毁伤形貌吻合MPM适用于高速冲击的数值模拟,能够较好地模拟弹丸侵彻靶板过程,且具有较高的仿真精度,计算所需时间少。

2D-SiC/SiC复合材料的弹丸冲击损伤及冲击后拉伸性能

连续纤维增韧陶瓷基复合材料以其良好的高温性能而被广泛应用在航空发动机等高温部件处,但服役过程中容易受到异物碎片冲击,这受到了广泛关注。研究采用轻气炮对化学气相渗透(CVI)技术制备的2D-SiC/SiC试样进行弹丸冲击,利用高速摄像机记录冲击过程,使用光学显微镜和CT观察异物损伤(FOD)的表面和内部结构,探讨了2D-SiC/SiC复合材料的高速冲击损伤特性。结果表明,2D-SiC/SiC复合材料的高速冲击损伤形式包括锥形裂纹、层间分层、纤维断裂以及基体压溃等。通过表征损伤发现,试件背部损伤和边缘处分层损伤由反射拉伸波造成,随着冲击速度提升,弹丸与拉伸波共同作用会造成试件穿透,试件边缘分层损伤减弱。对高速冲击后的试件进行准静态拉伸试验,明确了剩余力学性能与冲击速度、弹丸直径的关系,表明剩余拉伸强度是表征冲击损伤程度的有效参数。同时,采用数字图像相关(DIC)方法获得拉伸过程中的应变分布,并结合不同弹丸直径、冲击速度冲击后的剩余拉伸强度,进一步对比探究不同变量对冲击损伤的影响。研究结果表明弹丸直径是高速冲击损伤程度的主要影响因素。

钢筋混凝土的负泊松比设计与抗高速冲击性能

为增强钢筋混凝土的抗高速冲击性能,对钢筋混凝土的内部骨架进行负泊松比设计,得到两种内凹角数量不同的六肋及星型钢筋混凝土。利用霍普金森压杆试验对不同类型的钢筋混凝土进行抗高速冲击性能测试,并结合数字散斑相关方法对其场变化及微观泊松比进行分析,得到内凹角数量对钢筋混凝土破坏形态、承载力及耗能能力的影响规律和机理。结果表明:六肋和星型钢筋混凝土在抗高速冲击作用下出现了负泊松比,分别为-0.50和-1.00;从应变场来看,六肋和星型钢筋混凝土的最大应变均出现在内凹角附近,且以内凹角为中心向四周逐渐减小;随着钢筋结构内凹角数量的增加,钢筋混凝土的变形能力和能量耗散能力显著增强。单位体积内六肋和星型钢筋混凝土的耗能分别为1.76×10^(5)和1.86×10^(5) J/m^(3),是方形钢筋混凝土的1.17倍和1.23倍,表明负泊松比钢筋混凝土的耗能能力随钢筋内凹角数量的增多而增强。