基于锥形喷注器的凝胶推进剂流变特性数值研究

为探讨锥形喷注器内凝胶推进剂的流变特性,开发了基于同步重叠相加(SOLA)交错网格算法的非牛顿不可压流体求解器,粘性本构方程采用剪切稀化幂率模型,并采用Sicilian-Hirt部分网格法处理固壁边界,开展了凝胶推进剂流变特性历变过程的数值模拟。结果表明:构造的数值模型和处理方法是适用的,数值解与理论值有较高的吻合度,误差仅在10%以内;锥形喷注器可有效加剧凝胶推进剂的剪切"稀化"作用,平均表观粘性可降低近70%;细圆管段的存在会不利于"稀化";随着收敛角度的增大,喷注器出口截面的平均表观粘度会随之降低,50°时出口平均表观粘性仅为10°时的50%,但也会增加喷注器中的压力损失。     

Vibration coupling effects and machining behavior of ultrasonic vibration plate device for creep-feed grinding of Inconel 718 nickel-based superalloy

Ultrasonic vibration-assisted grinding(UVAG) is an effective and promising method for machining of hard-to-cut materials. This article proposed an ultrasonic vibration plate device enabling the longitudinal full-wave and transverse half-wave(L2T1) vibration mode for UVAG.The characteristics of two-dimensional coupled vibration in different directions were analyzed on the basis of apparent elastic method and finite element method. Furthermore, a correction factor was applied to correct the freque...     

球轴承内圈剥落缺陷双冲击特征动力学建模

基于Hertz接触理论,以内圈滚道表面存在局部剥落的球轴承为研究对象,对球轴承局部剥落故障所激起的非线性激励机理进行研究,分析球轴承双冲击现象激励机理。将滚动体与球轴承内外圈非线性接触特性纳入考虑,提出时变位移激励与时变接触力激励相耦合的球轴承局部剥落双冲击现象动力学模型。研究中采用Runge-Kutta数值积分法求解该二阶非线性动力学微分方程,并对故障球轴承仿真信号进行双冲击时间间隔特征验证。在不同转速和内圈剥落尺寸条件下开展仿真与实验研究,通过仿真和实测信号的双冲击时间间隔与理论双冲击时间间隔对比分析,仿真信号的双冲击时间间隔与理论值对比的误差结果皆小于2%,与实测信号的双冲击时间间隔值对比的误差结果皆小于12%;同时,仿真信号与实测信号具有很好的相似性,从而验证了该动力学模型的有效性。      

Fracture behavior and self-sharpening mechanisms of polycrystalline cubic boron nitride in grinding based on cohesive element method

Unlike monocrystalline cubic boron nitride (CBN), polycrystalline CBN (PCBN) shows not only higher fracture resistance induced by tool-workpiece interaction but also better self-sharpening capability; therefore, efforts have been devoted to the study of PCBN applications in manufacturing engineering. Most of the studies, however, remain qualitative due to difficulties in experimental observations and theoretical modeling and provide limited in-depth understanding of the self-sharpening behavior/mechanism. To fill this research gap, the present study investigates the self-sharpening process of PCBN abrasives in grinding and analyzes the macro-scale fracture behavior and highly localized micro-scale crack propagation in detail. The widely employed finite element (FE) method, together with the classic Voronoi diagram and cohesive element technique, is used considering the pronounced success of FE applications in polycrystalline material modeling. Grinding trials with careful observation of the PCBN abrasive morphologies are performed to validate the proposed method. The self-sharpening details, including fracture morphology, grinding force, strain energy, and damage dissipation energy, are studied. The effects of maximum grain cut depths (MGCDs) and grinding speeds on the PCBN fracture behavior are discussed, and their optimum ranges for preferable PCBN self-sharpening performance are suggested.     

Orientational versus esthetical urban street morphology parameterization in Space Syntax

Current topological and geometrical distances in Space Syntax are based on the premise that each change of direction along a path is a mental cost because one loses orientation. This paper proposes to extend the analysis to the case in which esthetic and variety, rather than orientation, are the key elements of street selection. It is widely recognized that most people are attracted to curvy paths rather than straight ones; therefore, when one is not worried about losing orientation in her walk, or when the preferred path is well recognizable even when it requires changes of direction, we should adopt another criterion to weigh distances: morphoahestetic and networkvariety criteria are shown as potential parameterization for Space Syntax distances.     

SCC evaluation of a 2297 Al-Li alloy rolled plate using the slow-strain rate technique

The stress corrosion cracking (SCC) susceptibility of 2297 Al-Li alloy in 1 M NaCl + 0.01 M H₂O₂ solution (CP solution) and 1 M NaCl + 0.01 M H₂O₂ + 0.6 M Na₂SO₄ solution (CPS solution) was investigated by slow-strain rate tests at various strain rates ranging from 10⁻⁵ s⁻¹ to 10⁻⁷ s⁻¹. The roles of H₂O₂ and SO₄²⁻ in the corrosion process were estimated by potentiodynamic polarization and electrochemical impedance spectroscopy. 2297 Al-Li alloy does not fracture ascribed to SCC in CP solution, while it undergoes SCC in CPS solution. In CPS solution, with a decreasing strain rate from 10⁻⁵ s⁻¹ to 10⁻⁷ s⁻¹, the SCC susceptibility firstly rises and then declines exhibiting a peak value at a strain rate of 10⁻⁶ s⁻¹. H₂O₂ promotes the active dissolution while SO₄²⁻ lowers the corrosion rate. The SCC fracture is associated with a decline in the dissolution rate of the crack tip by SO₄²⁻, which leads to stress concentration. In CPS solution, a reduction in the local dissolution rate of the crack tip leads to stress concentration, resulting in SCC fracture. As the preferred initiation site for a crack, pits also show a noteworthy effect on SCC of 2297 Al-Li alloy.     

TC21-0.28%H钛合金微观组织及其高温变形行为研究

利用光学金相及X射线衍射,研究了TC21-0.28%H(质量分数,下同)钛合金的组织结构,通过热模拟压缩实验,研究了TC21-0.28%H钛合金在800～920℃温度范围和0.01～1s-1应变速率范围的高温变形行为,建立了钛合金高温变形本构方程。结果显示,与TC21钛合金相比,TC21-0.28%H钛合金β相比例显著增加,并且有新相马氏体α″与氢化物δ生成,TC21-0.28%H钛合金在α+β相区与β相区的变形激活能分别为233kJ/mol与153kJ/mol,软化机制为动态回复,与TC21钛合金相比,TC21-0.28%H钛合金变形激活能降低,热加工性能得到改善。     

重力跳动对多相流体系统的影响

研究了重力跳动对液桥表面及内部流体流动的影响, 以期明确重力跳动对多相流体系统的作用效果. 理论研究表明, 当重力跳动作用于液桥时, 液桥反响频率的大小取决于液桥尺寸和物性参数; 实验研究显示重力跳动引入的液桥表面振动、液桥内部流动与温度振动之间存在着三角耦合关系; 数值模拟结果揭示了重力跳动作用于液桥时液桥内部流体的流动结构. 另外, 对理论、实验与数值模拟结果进行了对比验证, 得到了吻合一致的结果.      

2.5维自愈合C/SiC复合材料的拉伸应力-应变关系

根据2.5维自愈合C/SiC复合材料的细观结构特点,基于界面开裂和基体开裂的损伤机理,分别建立了该材料经向和纬向拉伸的细观力学模型,进而得到了经向和纬向拉伸的非线性应力-应变关系,应力-应变曲线的预测结果与试验值吻合较好.结果表明:纬向应力-应变曲线的最大预测误差为9%,经向应力-应变曲线的最大预测误差约为10%,经向和纬向的应力-应变关系不同.该模型采用切线弹性模量和平均裂纹间距的方法,省去了以往模型复杂的应变计算过程,使模型得到简化,便于应用.      

基于漏斗试样的C250钢高温扭转低周疲劳行为

C250钢的扭转疲劳破坏是其主要失效形式之一。为了获得航空材料C250钢的扭转疲劳特性,并消除等直圆棒试样在扭转试验中产生的弊端,依据漏斗试样完成了C250钢在3种高温(150、200、350℃)环境下的扭转低周疲劳试验,获得了在扭转低周疲劳下的扭矩-名义扭转角曲线。基于试验结果,利用FAT方法分析得到了C250钢在3种高温环境下的材料循环本构关系,利用该循环本构关系对漏斗试样进行三维扭转有限元分析,获得漏斗根部剪切应力与扭矩之间的转换公式及漏斗根部剪切应变与名义扭转角之间的转换公式。基于以上试验与分析方法,得到了材料的剪切应变幅-倍循环次数曲线、剪切应力幅-循环分数曲线以及剪切应力-剪切应变的稳定滞回线,发现材料呈现出循环软化特性,并基于Manson-Coffin模型对材料寿命进行了分析。