Effect of refilling time on microstructure and mechanical properties of friction spot welded LY12 aluminum alloy

Friction spot welding （FSpW） was successfully used to produce joints of LY12 aluminum alloy. The effects of refilling time on microstructure and mechanical properties of FSpW joints were systematically studied. Results show that the cross-section of FSpW joint presents a basin-like morphology. A white bonding ligament exists in the center of the joint. The stir zone can be clarified into sleeve affected zone and pin affected zone based on different grain sizes. With increasing the refilling time from 2. 0 s to 3.5 s, grains in the stir zone become coarser, microhardness of the joint decreases and tensile shear failure load of the joint firstly increases and then decreases. The maximum tensile shear failure load of 8 130 N is attained when the refilling time is 3.0 s. Shear-plug fracture mode and shear fracture mode can be observed in the tensile shear tests. The maximum hardness of 169. 7 HV is attained in the joint center when the refilling time is 2. 0 s.

Effect of the time-weakening friction law during the nucleation process

Based on dynamic rupture simulations on a planar fault in a homogeneous half-space, we investigated the nucleation processes using the time-weakening friction law. Both the characteristic time and the rupture speed in the nucleation asperity play an important role in determining rupture behaviors on a fault plane following the time-weakening friction law, with which rupture starts from a single point in the nucleation asperity and propagates at a given speed toward the boundary of the nucleation area. Rupture with a small characteristic time or a large rupture speed in the nucleation asperity propagates earlier from the hypocenter. Rupture following the slipweakening friction law requires a smaller radius of nucleation patch to have similar rupture front contours of the time-weakening friction law. Even if the rupture velocity in the nucleation patch of the time-weakening friction law increases to infinity, the peak slip rate in the nucleation asperity is smaller than that of the slip-weakening law. The peak ground velocity distributions of ruptures following the two friction laws are also compared.

DYNAMIC INTERACTION OF PLANE WAVES WITH A UNILATERALLY FRICTIONALLY CONSTRAINED INCLUSION-TIME DOMAIN BOUNDARY ELEMENT ANALYSIS

A 2D time domain boundary element method(BEM)is developed to solve the transient scattering of plane waves by a unilaterally frictionally constrained inclusion.Coulomb friction is assumed along the contact interface.The incident wave is assumed strong enough so that localized slip and separation take place along the interface.The present problem is in effect a nonlinear boundary value problem since the mixed boundary conditions involve unknown intervals (slip,separation and stick regions).In order to determine the unknown intervals,an iterative technique is developed.As an example,we consider the scattering of a circular cylinder embedded in an infinite solid.

Grey Relation between Nonlinear Characteristic and Dynamic Uncertainty of Rolling Bearing Friction Torque

The rolling bearing friction torque which is characterized by its uncertainty and nonlinearity affects heavily the dynamic performance of a system such as missiles, spacecrafts and radars, etc. It is difficult to use the classical statistical theory to evaluate the dynamic evaluation of the rolling bearing friction torque for the lack of prior information about both probability distribution and trends. For this reason, based on the information poor system theory and combined with the correlation dimension in chaos theory, the concepts about the mean of the dynamic fluctuant range （MDFR） and the grey relation are proposed to resolve the problem about evaluating the nonlinear characteristic and the dynamic uncertainty of the rolling bearing friction torque. Friction torque experiments are done for three types of the rolling bearings marked with HKTA, HKTB and HKTC separately; meantime, the correlation dimension and MDFR are calculated to describe the nonlinear characteristic and the dynamic uncertainty of the friction torque, respectively. And the experiments reveal that there is a certain grey relation between the nonlinear characteristic and the dynamic uncertainty of the rolling bearing friction torque, viz. MDFR will become the nonlinear increasing trend with the correlation dimension increasing. Under the condition of fewer characteristic data and the lack of prior information about both probability distribution and trends, the unitive evaluation for the nonlinear characteristic and the dynamic uncertainty of the rolling bearing friction torque is realized with the grey confidence level of 87.7%-96.3%.

An Accurate and Computationally Efficient Explicit Friction Factor Model

The implicit Colebrook equation has been the standard for estimating pipe friction factor in a fully developed turbulent regime. Several alternative explicit models to the Colebrook equation have been proposed. To date, most of the accurate explicit models have been those with three logarithmic functions, but they require more computational time than the Colebrook equation. In this study, a new explicit non-linear regression model which has only two logarithmic functions is developed. The new model, when compared with the existing extremely accurate models, gives rise to the least average and maximum relative errors of 0.0025% and 0.0664%, respectively. Moreover, it requires far less computational time than the Colebrook equation. It is therefore concluded that the new explicit model provides a good trade-off between accuracy and relative computational efficiency for pipe friction factor estimation in the fully developed turbulent flow regime.

On the Arrow of Time

The arrow of time is one of the most difficult questions in modern physics. In this paper, a damping vibration equation is discussed, which shows that this equation reflects the arrow of time. We have further discussed that the concept of “the arrow of time” in this equation is quite different from the thermodynamic arrow.

赛车道软土路基PHC桩负摩阻力中性点的时间效应

武汉赛车场跑道下伏的深厚软土层给桩基的设计与施工带来不小阻碍,桩基的负摩阻力及其中性点的确定对工程的稳定性具有非常重要的影响。为检验武汉赛车场跑道软土路基的预应力高强度混凝土管桩(PHC)设计的合理性以及为后续桩基改进方案的提供技术支持,选择典型地层开展了堆载作用下桩与桩间土的沉降监测,根据桩与土的相对沉降变化规律研究负摩阻力中性点的时间效应。结果表明,该场地的PHC桩的桩周土沉降约是桩沉降的6倍,具有明显的负摩阻力特性,桩周软弱土的下限深度为13.0 m;随着时间增加,中性点向上移动;中性点深度随时间变化具有三阶段特征,采用指数函数的中性点时间效应预测模型能较好反映中性点初始值、中性点变化规律与速率以及最终稳定值,与规范法比较,预测模型计算的中性点深度具有更高的准确性,由此得到了两个试验场地的中性点稳定值分别为4.43 m与5.72 m。研究还表明三向土工格栅能有效降低桩间土的沉降,也能降低桩-土的差异沉降,使中性点上移,减小负摩阻力,提高桩的承载力。本研究对了解负摩阻力对桩基性能发挥有一定的参考价值。

Control of friction oscillator by Lyapunov redesign based on delayed state feedback

The stability and boundedness of mechanical system have been one of important research topics. In this paper ultimate boundedness of a dry friction oscillator, belonging to nonsmooth mechanical system, is investigated by proposing a controller design method. Firstly a sufficient condition of the stability for the nominal system with delayed state feedback is derived by constructing a Lyapunov-Krasovskii function. The delayed feedback gain matrix is calculated by applying linear matrix inequality method. Secondly on the basis of the delayed state feedback, a continuous function is designed by Lyapunov redesign to ensure that the solutions of the friction oscillator system are ultimately bounded under the overall control. Moreover, the ultimate bound can be adjusted in practice by choosing appropriate parameter. Accordingly friction-induced vibration or instability can be controlled effectively. Numerical results show that the pro- posed method is valid.

摩擦阻尼器组合支座的隔震优化对比

为解决一种摩擦阻尼器组合隔震支座的有效性及计算方法的问题,研究了用于结构设计有限元建模的组合元件计算模型,包括水平向、竖向刚度和滞回模型。使用有限元软件SAP2000,对比研究了布置有该组合支座的优化隔震方案和传统橡胶支座的隔震优化方案在高宽比不同的高层框架剪力墙筒体结构的抗震性能。结果表明:与传统隔震支座方案相比,组合隔震支座隔震方案可适当延长结构自振周期,减小隔震层侧移,有效降低支座拉应力,提高支座的抗倾覆能力和支座耗能,还可有效降低上部结构的层间剪力及侧移等地震响应,证明组合隔震支座方案较传统隔震支座方案综合性收效明显。

机体振动信号时频特性与燃烧压力相关性分析

为了探明机体振动信号与缸内燃烧状态间的相关性,本文通过改变转速、转矩、润滑油温度及配缸间隙形成不同的燃烧状态,利用连续小波变换算法得到了振动信号时频域特性,并与燃烧压力峰值进行对比分析。结果表明:随转速增加,燃烧压力峰值出现波动,8 kHz以上成分的能量总体呈增加的趋势;随转矩增加,燃烧压力峰值及振动信号的能量均增加;随润滑油温度升高,20 kHz以上主要频率成分的能量增加;随配缸间隙增加,10kHz以上频率成分的能量总体呈增加的趋势。

后处理工艺对抗静电滤料性能的影响

选择基布中嵌织导电纱线的基布导电型和面层中混入导电纤维的面层导电型2种典型抗静电滤料作为基材,进行聚四氟乙烯(PTFE)乳液浸渍处理和PTFE微孔膜覆膜处理,得到面密度和厚度均匀的复合滤料。再基于电阻率、静电压衰减时间及摩擦带电电压3项指标,从静电荷的消散及积聚2个角度,探究了后处理工艺对抗静电滤料性能的影响。试验结果表明:浸渍处理后,滤料抗静电性能提升,2种滤料的迎尘面表面电阻率和体积电阻率均降低且降幅小于1个数量级,基布导电型滤料和面层导电型滤料的迎尘面静电压半衰期分别由0.40 s、0.23 s缩短至0.38 s、0.20 s,摩擦时间为60 s时摩擦带电电压分别由1043 V、596 V减小至668 V、67 V。覆膜处理后,滤料抗静电性能下降,基布导电型滤料迎尘面表面电阻率升高且升幅为1个数量级、体积电阻率升高但升幅小于1个数量级,面层导电型滤料迎尘面表面电阻率和体积电阻率均升高且升幅为4个数量级,基布导电型滤料和面层导电型滤料迎尘面静电压半衰期分别由0.40 s、0.23 s延长至0.53 s、0.77 s,摩擦时间为60 s时摩擦带电电压分别由1043 V、596 V增大至1952 V、920 V。

An extended time-dependent numerical model of the mild-slope equation with weakly nonlinear amplitude dispersion

In the present paper, by introducing the effective wave elevation, we transform the extended elliptic mild-slope equation with bottom friction, wave breaking and steep or rapidly varying bottom topography to the simplest time-dependent hyperbolic equation. Based on this equation and the empirical nonlinear amplitude dispersion relation proposed by Li et al. （2003）, the numerical scheme is established. Error analysis by Taylor expansion method shows that the numerical stability of the present model succeeds the merits in Song et al. （2007）＇s model because of the introduced dissipation terms. For the purpose of verifying its performance on wave nonlinearity, rapidly varying topography and wave breaking, the present model is applied to study： （1） wave refraction and diffraction over a submerged elliptic shoal on a slope （Berkhoff et al., 1982）; （2） Bragg reflection of monochromatic waves from the sinusoidal ripples （Davies and Heathershaw, 1985）; （3） wave transformation near a shore attached breakwater （Watanabe and Maruyama, 1986）. Comparisons of the numerical solutions with the experimental or theoretical ones or with those of other models （REF/DIF model and FUNWAVE model） show good results, which indicate that the present model is capable of giving favorably predictions of wave refraction, diffraction, reflection, shoaling, bottom friction, breaking energy dissipation and weak nonlinearity in the near shore zone.

A linear complementarity model for multibody systems with frictional unilateral and bilateral constraints

The Lagrange-I equations and measure differential equations for multibody systems with unilateral and bilateral constraints are constructed. For bilateral constraints, frictional forces and their impulses contain the products of the filled-in relay function induced by Coulomb friction and the absolute values of normal constraint reactions. With the time-stepping impulse-velocity scheme, the measure differential equations are discretized. The equations of horizontal linear complementarity problems （HLCPs）, which are used to compute the impulses, are constructed by decomposing the absolute function and the filled-in relay function. These HLCP equations degenerate into equations of LCPs for frictional unilateral constraints, or HLCPs for frictional bilateral constraints. Finally, a numerical simulation for multibody systems with both unilateral and bilateral constraints is presented.

地震下三塔悬索桥主缆-鞍座摩擦滑移行为及其影响

三塔悬索桥中塔两侧主缆并未直接与锚锭连接,中塔处主缆的纵向约束较弱,地震下主缆-鞍座易发生显著的摩擦滑移。为了揭示三塔悬索桥中主缆-鞍座摩擦滑移及其对结构地震响应的影响规律,本文依托浔江大桥实际工程,建立了三塔悬索桥的非线性有限元模型,利用摩擦单元重现了主缆-鞍座界面的摩擦滑移行为,揭示了地震下主缆在边塔和中塔处的不同滑移变位,分析了摩擦系数对于界面滑移、主塔内力等响应量的影响规律。分析结果表明:摩擦系数大于0.1,在设计地震下主缆-索鞍界面无摩擦滑移,而在罕遇地震和极罕遇地震下,有一定的摩擦滑移,最大摩擦滑移量随着摩擦系数增大而减小;主缆-索鞍界面在地震作用下会发生明显滞回现象,最大可提供约2.0%的等效阻尼比;边塔顶剪力对塔底弯矩的贡献率达到了50%~70%,而中塔由于设置阻尼器,剪力贡献率仅为20%~40%;适当减小摩擦系数有利于降低塔的响应,但摩擦系数降低至0.001时,主缆将失去对塔变形的约束能力,将显著改变塔的变形模式与动力效应。

浸泡条件下页岩力学及摩擦特征实验

针对页岩水力压裂过程中的断层滑移问题,通过室内实验,分析了浸泡条件下页岩的力学及摩擦特征,研究了页岩摩擦因数演化规律。研究结果表明:1)页岩抗压强度、抗拉强度、弹性模量和黏聚力随浸泡时间的增加而降低;页岩泊松比随浸泡时间的增加而增加,内摩擦角变化规律不明显。2)页岩摩擦因数随浸泡时间、法向应力和取心夹角的增加而降低,浸泡6 h内页岩摩擦因数降幅较大,后趋于稳定。3)界面压力大于5 MPa后,页岩的摩擦因数不随界面压力的增加而变化,仅随浸泡时长的增加而降低。4)水力压裂结束后,摩擦因数持续降低,断层仍存在滑移风险,缩短压裂作业时长可避免摩擦系数持续降低对断层滑移的影响。研究结果可为页岩断层滑移预测提供科学依据。

水润滑轴承超长时间磨损后特性变化研究

对水润滑轴承超长时间磨损后的特性变化进行研究,分析转速、载荷、运行时间对高分子复合材料水润滑轴承摩擦性能的影响规律。通过跑合、低速启停、低速连续运行、高速启停、高速连续运行等项目,累计运行时间达到600 h,分析超长运行时间对高分子复合材料水润滑轴承摩擦因数和磨损量的影响。研究发现:高分子复合材料水润滑轴承的摩擦因数随着运行时间的推移逐渐减小并趋于稳定;在初始试验阶段,水润滑轴承的磨损量逐渐增大,随着试验时长的延长,水润滑轴承的摩擦副得以充分磨合,磨损速率逐渐趋于稳定;超长磨损时间会引起高分子复合材料水润滑轴承摩擦因数的波动,并且会导致轴承磨损量逐渐增加,但在稳定磨损阶段,摩擦因数和磨损量趋于稳定。

四主缆大跨悬索桥抗震性能分析及减震措施优化

以燕矶长江大桥为工程背景,首先,采用动力非线性时程法分析了燕矶长江大桥抗震性能,并研究了设置电涡流-摩擦组合型阻尼器和黏滞阻尼器对大跨悬索桥抗震性能的影响;然后,对附加电涡流-摩擦组合型阻尼器摩擦力、阻尼系数和阻尼指数开展了参数敏感性分析;最后,从耗能角度分析地震作用下电涡流-摩擦组合型阻尼器减震特点.结果表明:在E2地震作用下,桥塔关键截面抗弯能力均大于弯矩需求;在“纵向+竖向”地震作用下梁端纵向位移较大.设置塔梁处纵桥向阻尼器后,可有效降低主桥梁端纵向位移;增大摩擦力、阻尼系数与降低阻尼指数均可提升梁端纵向地震响应的控制效果,但参数变化对桥塔控制截面的地震响应影响较小.阻尼系数较大时,电涡流阻尼主导了电涡流-摩擦组合型阻尼器耗能,相较于黏滞阻尼器,电涡流-摩擦组合型阻尼器对梁端纵向位移控制效果更好.

考虑齿面摩擦的斜齿轮副时变啮合刚度计算方法

斜齿轮由于具有传动平稳等众多优点,在高速重载场合广泛应用。斜齿轮副在啮合时,不可避免地受到摩擦力影响。因此在计算斜齿轮副的时变啮合刚度时,有必要将齿面摩擦计入其中。先通过切片法将斜齿轮切成直齿轮微元薄片,再根据能量法计算微元薄片的啮合刚度,最后沿齿宽方向进行积分累加,获取斜齿轮副精准时变啮合刚度。研究斜齿轮副时变啮合刚度在零摩擦、定摩擦和时变摩擦作用下的变化规律,揭示时变摩擦作用下齿面粗糙度、转矩、转速、齿宽等参数对斜齿轮副时变啮合刚度的作用机理。研究结果表明:在摩擦力的作用下,斜齿轮副时变啮合刚度会发生变化;定摩擦作用下,斜齿轮副的时变啮合刚度会减小,且定摩擦因数越大,时变啮合刚度越小;时变摩擦作用下,斜齿轮副的时变啮合刚度会随齿面粗糙度增大而减小,分别随输入转矩、转速和齿宽的增大而增大。

机体表面振动信号影响因素关联性分析

为了研究机体表面振动信号各影响因素的影响规律,在不同转速、转矩、润滑油温度及配缸间隙下,对比分析振动信号的峰值、方差、标准差及均方根等4个时域参数以及小波包分解得到的各频段能量的变化趋势。结果表明:随着转速的升高,燃烧压力峰值出现波动,在活塞惯性力的主导作用下,振动信号时域特征参数及频段5以上频率成分的能量呈增加的趋势;燃烧压力峰值随着转矩的增加而增大,振动信号时域特征参数及各频段能量均呈增加的趋势;随着润滑油温度的升高,在润滑油阻尼及燃烧压力的双重作用下,振动信号时域特征参数幅值整体呈降低的趋势,频段12以上频率成分的能量呈增加的趋势;随着配缸间隙的增加,密封性降低导致燃烧压力峰值减小,但由于活塞二次运动加剧,导致振动信号时域特征参数及频段5以上频率成分的能量呈现增加的趋势。

基于摩擦摆支座的核安全壳结构隔震效果研究

核安全壳是防止核裂变产物泄漏的最后一道屏障,属于I类抗震物项,但近些年以日本福岛地震为代表性的超设计基准地震频发,故安全壳的抗震性能值得关注。基底隔震可以有效降低结构的地震响应,摩擦摆隔震支座因其优良的隔震性能已被广泛应用于桥梁、房屋建筑中,但目前尚未有核安全壳的应用实例。为此本文基于ANSYS有限元软件建立摩擦摆支座隔震和非隔震安全壳结构整体模型,通过输入相同地震动对比分析核安全壳结构的频率、位移、加速度以及剪力等指标,研究摩擦摆支座在核安全壳结构隔震中的应用可行性。分析结果表明:隔震结构基本周期相比不隔震结构显著提高,且隔震结构的层间相对位移、加速度和剪力值等指标均小于非隔震结构,进一步说明隔震支座有利于提高核安全壳的抗震性能。