Size-dependent thermomechanical vibration characteristics of rotating pre-twisted functionally graded shear deformable microbeams

A three-dimensional(3D)thermomechanical vibration model is developed for rotating pre-twisted functionally graded(FG)microbeams according to the refined shear deformation theory(RSDT)and the modified couple stress theory(MCST).The material properties are assumed to follow a power-law distribution along the chordwise direction.The model introduces one axial stretching variable and four transverse deflection variables including two pure bending components and two pure shear ones.The complex modal analysis and assumed mode methods are used to solve the governing equations of motion under different boundary conditions(BCs).Several examples are presented to verify the effectiveness of the developed model.By coupling the slenderness ratio,gradient index,rotation speed,and size effect with the pre-twisted angle,the effects of these factors on the thermomechanical vibration of the microbeam with different BCs are investigated.It is found that with the increase in the pre-twisted angle,the critical slenderness ratio and gradient index corresponding to the thermal instability of the microbeam increase,while the critical material length scale parameter(MLSP)and rotation speed decrease.The sensitivity of the fundamental frequency to temperature increases with the increasing slenderness ratio and gradient index,and decreases with the other increasing parameters.Moreover,the size effect can suppress the dynamic stiffening effect and enhance the Coriolis effect.Finally,the mode transition is quantitatively demonstrated by a modal assurance criterion(MAC).

Dynamic strength characteristics and failure criteria of anisotropically consolidated silt under principal stress rotation

以便识别批评性质和失败标准在原处在车辆或波浪下面的淤泥在 undrained 下面的装载、各向异性地巩固的淤泥周期的主要压力旋转与空柱体被学习动态测试。结果证明为有比 1.5 没有大的巩固比率的稍微各向异性地巩固的样品，结构崩溃， deviator 紧张和毛孔压力严厉地增加在倒塌以后失败。为各向异性地高度与比 1.5 大的巩固比率巩固了样品，紧张稳定地增加到高价值，它显示出可锻的失败的特征。4% 被建议是 deviator 污点的阀值价值决定倒塌的出现。在毛孔压力和 deviator 紧张之间的规范的关系能被力量功能为所有各向异性地巩固的样品相关。基于它，为各向异性地高度巩固了样品，力量功能曲线上的变化点的外观被建议签署失败。它能在这个点通过凸的毛孔压力被预言，压力在其到最终的毛孔的比率附近与巩固比率尽管有线性动态砍压力水平。并且相应 deviator 紧张在 3% 和 6% 之间。紧张失败标准能也被采用，但是污点的有限价值应该根据设计实践是坚定的。至于稍微各向异性地巩固的样品，转弯的点出现在倒塌以后。那么，失败被建议与倒塌的出现被定义，倒塌毛孔压力能与最终的毛孔压力和巩固比率被预言。

Influence of principal stress rotation of unequal tensile and compressive stress amplitudes on characteristics of soft clay

Soil behavior can reflect the characteristics of principal stress rotation under dynamic wave and traffic loads. Unequal amplitudes of tensile and compressive stresses applied to soils have complex effects on foundation soils in comparison with the pure principal stress rotation path. A series of undrained cyclic hollow torsional shear tests were performed on typical remolded soft clay from the Hexi area of Nanjing, China. The main control parameters were the tensile and compressive stress amplitude ratio(α) and the cyclic dynamic stress ratio(η). It was found that the critical η tended to remain constant at 0.13, when the value of the compressive stress amplitude was higher than the tensile stress amplitude. However, the influence of the tensile stress was limited by the dynamic stress level when α= 1.For obvious structural change in the soil, the corresponding numbers of cyclic vibration cycles were found to be independent of α at low stress levels and were only related to η. Finally, a new method for evaluating the failure of remolded soft clay was presented. It considers the influence of the tensile and compressive stresses which caused by complex stress paths of the principal stress rotation. This criterion can distinguish stable, critical, and destructive states based on the pore-water-pressure-strain coupling curve while also providing a range of failure strain and vibration cycles. These results provide the theoretical support for systematic studies of principal stress rotation using constitutive models.

Research on dynamic stabilityrotational speed range of rolling projectiles with different characteristics

Traditional dynamic stability analyses of the rolling projectiles are mainly based on solving the systems＇ transfer functions or angular motion＇ s homogeneous equations to obtain their charac- teristic roots. The solving processes of these methods are complex and lacking further analysis of the results. To solve this problem, Routh stability criterion is introduced to determine the stability of rolling missiles based on the transfer function model, and an important advantage of this method is that it is unnecessary to solve the system＇ s characteristic equation. Rotational speed ranges satisfy- ing the dynamic stability of rolling projectiles with four different characteristics are acquired, and the correctness of analysis results is verified by computing the system＇ s root locus. The analysis results show that the relation between stability and rotational speed for static stable missiles is opposite to that for spin-stabilized projectiles, and the relative size of gyroscopic effect and Magnus effect has an extremely important influence on the trend of the stability of the system with increasing rotational speed.

Two-dimensional face stability analysis in rock masses governed by the Hoek-Brown strength criterion with a new multi-horn mechanism

The face stability problem is a major concern for tunnels excavated in rock masses governed by the Hoek-Brown strength criterion.To provide an accurate prediction for the theoretical solution of the critical face pressure,this study adopts the piecewise linear method(PLM)to account for the nonlinearity of the strength envelope and proposes a new multi-horn rotational mechanism based on the Hoek-Brown strength criterion and the associative flow rule.The analytical solution of critical support pressure is derived from the energy-work balance equation in the framework of the plastic limit theorem;it is formulated as a multivariable nonlinear optimization problem relying on 2m dependent variables(m is the number of segments).Meanwhile,two classic linearized measures,the generalized tangential technique(GTT)and equivalent Mohr-Coulomb parameters method(EMM),are incorporated into the analysis for comparison.Surprisingly,the parametric study indicates a significant improvement in support pressure by up to 13%compared with the GTT,and as expected,the stability of the tunnel face is greatly influenced by the rock strength parameters.The stress distribution on the rupture surface is calculated to gain an intuitive understanding of the failure at the limit state.Although the limit analysis is incapable of calculating the true stress distribution in rock masses,a rough approximation of the stress vector on the rupture surface is permitted.In the end,sets of normalized face pressure are provided in the form of charts for a quick assessment of face stability in rock masses.

Pseudo-static stability analysis of rock slopes reinforced by passive bolts using the generalized Hoek-Brown criterion

The stability analysis of passive bolt-reinforced rock slopes under seismic loads is investigated within the framework of the kinematic approach of limit analysis theory.A pseudo-static method is adopted to account for the inertial forces induced in the rock mass by seismic events.The strength properties of the rock material are described by a modified Hoek-Brown strength criterion,whereas the passive bolts are modeled as bar-like inclusions that exhibit only resistance to tensile-compressive forces.Taking advantage of the ability to compute closed-form expressions for the support functions associated with the modified Hoek-Brown strength criterion,a rotational failure mechanism is implemented to derive rigorous lower bound estimates for the amount of reinforcement strength to prevent slope failure.The approach is then applied to investigating the effects of relevant geometry,strength and loading parameters in light of a preliminary parametric study.The accuracy of the approach is assessed by comparison of the lower bound estimates with finite element limit analysis solutions,thus emphasizing the ability of the approach to properly predict the stability conditions and to capture the essential features of deformation localization pattern.Finally,the extension of the approach to account for slipping at the interface between reinforcements and surrounding rock mass is outlined.

Experimental Verification of Geometric Criterion of Damage and Fracture by Grating Methods

Micro cracking in solid material is a phenomenon of solid plane bifurcation with incompatible micro-rotation. Therefore it is important to analysis the micro-rotation field in a strained material in fractute mechanics. On the basis of strain-rotation (S-R) decomposition theorem, the geometric criterion of cracking has been established. In order to verify its validity, using grating experimental method the thin compact tension specimens were investigated. The analysis results show a good agreement between the experimental and geometric criterion.

MEDIMAX CRITERION FOR ROTATION IN FACTOR ANALYSIS AND ITS APPLICATION IN MEDICAL RESEARCH

In factor analysis, due to the indeterminacy of factor solutions, in order to obtain the factors which can satisfy the so-called 'simple structure', the factor matrix often undergoes various rotations according to different criterion. In the case of orthogonal factor solutions, 'simple structure' implies that an objective matrix is a simple structured sparse matrix. In this matrix, many of the elements would near 0 while others would near ±1. For this purpose, the Varimax and Quartimax criteria have been proposed. However, a more flexible and simple method is that the variables in

基于多元经验模态分解的旋转机械早期故障诊断方法

针对旋转机械早期微弱故障诊断问题,提出了基于多元经验模态分解的旋转机械早期故障诊断新方法。首先将多个加速度传感器合理布置在轴承座的关键位置,同步采集多通道振动信息;再利用多元经验模态分解同时对多通道振动信号进行自适应分解,得到一系列多元IMF分量;最后,依据峭度准则和相关系数从中选取包含故障主要信息的IMF分量进行信号重构,提取故障特征。多元经验模态分解方法克服了EMD等方法在进行多通道数据融合时缺乏理论依据的局限性。仿真信号和旋转机械故障信号的实验结果表明,该方法明显优于EEMD方法,对齿轮和滚动轴承故障的检测精度更高,可以在强背景噪声情况下更好地提取出故障冲击特征。

油膜稳定性三维判据研究

本文讨论了滑动轴承油膜存在三个自由度时的稳定性准则。在给出轴心涡动三维运动方程的基础上 ,分析了三维油膜的作功状况 ,并进而给出了判别油膜在三维情况下稳定性的两种方法。最后采用迭代法对径向 -推力联合动静压轴承的稳定性进行了计算机求解 ,得到了其相应的三维稳定性参数。

轴流式压气机旋转失速信号的早期检测

早期检测航空发动机压气机旋转失速信号对于设计一种高效的防喘、消喘控制系统具有重要意义。通过对压气机旋转失速形成、发展、退出过程中压力信号频谱演化特性的分析,提出了一种基于FFT的压气机旋转失速信号检测判据。对某轴流式压气机压力测量信号的分析结果表明,该判据是准确、可靠和快速的,可以用于旋转失速信号的早期检测。

Drucker-Prager准则在拉剪区的修正

针对在岩石塑性力学领域广泛应用的Drucker-Prager准则(简称D-P准则)是一个压剪屈服准则,在拉剪受力状态下不适用的情况,提出在拉剪区修正D-P准则的2种可行性方案。第一种方案是在压剪区,仍采用D-P准则的锥面式,而在拉剪区及其邻近,采用一个球形屈服面代替原来的锥顶附近的锥面。第二种方案是使用一个双曲旋转面近似地代替D-P圆锥面。将这2种方法定义的新的修正的D-P准则简称为D-P-Y准则,它是一个三参数准则,不仅在压剪区符合三轴压缩实验结果,而且在拉剪区也可拟合抗拉强度的资料。同时,它还是一个在全区域处处光滑的正则函数,可以使用经典的正交法则和一致性条件建立本构关系,而不必做任何奇异点的处理,这给应用和编程都带来方便,具有广泛的应用前景。特别地,对于双曲型的D-P-Y准则,当相关参数取0时,可分别退化为D-P准则和Mises准则,因此,在编程中可只编入D-P-Y准则。

超固结黏土单调和耦合循环的剪切特性研究

针对超固结黏土空心试样,利用土工静力-动力液压三轴-扭转多功能剪切仪,在均等固结条件下进行了单调扭剪和三轴-扭转耦合循环剪切试验。试验结果表明:不同超固结比黏土的单调扭剪强度也可由正常固结黏土的单调扭剪强度得到,得到了不同超固结比下饱和黏土的强度及模量的退化规律;随着超固结比的增大,相同破坏循环次数的动应力比和临界循环应力比均线性增大;超固结比对耦合循环剪切的孔隙水压力的发展模式影响显著。参考Yasuhara的测量方法,采用荷载停止后继续采集孔压的方法可以更好地反映黏土在耦合循环荷载下产生的真实孔压和孔压的增长情况。提出的综合应变式同时考虑了剪切变化和正向偏差变形的共同效应,适合作为主应力连续旋转的耦合循环剪切试验的破坏标准。

颗粒破碎的微观力学模拟

基于颗粒尺度建立了考虑颗粒破碎过程中能量耗散的微观力学模型。为了模拟颗粒体的破碎过程和侧向应力的影响,把胶结在一起的三个等粒径的颗粒体当作颗粒破碎体。基于颗粒破碎准则和热力学原理,考虑了颗粒破碎过程中颗粒接触点之间的塑性滑移和颗粒转动以及侧向应力作用,通过颗粒破碎体内部颗粒之间的接触破碎来模拟颗粒体的破碎。计算结果表明:本文所提出的微观力学模型可以模拟胶结在一起的大颗粒的破碎过程;颗粒之间的微观力学参数对颗粒破碎过程中的力、塑性滑移、颗粒的转动角有不同程度的影响;力-位移关系曲线在破碎前是线性的,而颗粒破碎时竖向力迅速降低到较小值,并伴有颗粒的转动。

振动环境下过盈装配圆柱结构预紧防转判据研究

在振动环境下过盈装配圆柱结构预紧防转判据研究中,以Mooney-Rivilin本构模型描述缓冲橡胶垫层,以库伦滑动假设描述界面接触滑动,在ABAQUS中计算获得了任选接触点处于压剪变形状态与摩擦滑动状态分界的位移边界,并由此定义了最小滑移偏心距作为预紧有效性的判据,即振动环境下相对运动幅值小于此偏心距,内外层之间将不会产生相对转动,预紧防转装配有效。数值仿真试验和基于单轴振动台的正交相差相对运动试验表明,只有当相对运动幅值超过一定量值时,才出现层间转动现象,验证了最小滑移偏心距判据的合理性。

室内火灾旋转火焰理论分析与实验研究

根据涡量传输和火灾动力学等理论,分析室内火灾旋转火焰的形成机理,导出室内火灾产生旋转火焰的控制因素表达式。旋转火焰的产生需要足够的密度梯度、压力梯度和斜压效应。利用小规模室内火灾实验,研究室内火灾旋转火焰的基本特征,比较旋转与非旋转火焰火灾的基本参数。与非旋转火焰的室内火灾比较,旋转火焰室内火灾热烟气层温度、升温速率、燃烧速率、地板所受辐射热通量等的均值与峰值均有显著提高,且峰值出现较早。结合理论分析与实验研究结果,得到了室内火灾发生旋转火焰的条件判据。

混流泵叶片安放角对内涡结构及叶轮-导叶适应性的影响

转叶式混流泵的叶片安放角可以调节,当工况发生改变、叶片安放角调节后,叶轮-导叶的适应性将随之变化,进而影响到混流泵的水力性能.以一叶片安放角可取-4°,0°和4°的转叶式混流泵为研究对象,从涡结构的角度出发,探究叶片安放角改变对叶轮-导叶适应性的影响规律.研究结果表明:随着叶片安放角的增大,叶轮内部涡结构的大小在逐渐增大,且环绕在轮缘处的涡带直径逐渐增大,而导叶内通道涡数量在逐渐减少,叶轮和导叶外缘后盖板壁上的涡结构大小和强度在逐渐增大;混流泵强度较高的涡主要集中在叶片表面以及壁面附近;通过对叶片安放角的调节,转叶式混流泵能在不同的流量区间内保持高效运行,但随着叶片安放角的增大,由于涡结构逐渐增多,转叶式混流泵效率逐渐降低;导叶内通道涡的数量随着叶片安放角的降低而增加,从而导致转叶式混流泵的扬程变小;转叶式混流泵叶轮-导叶适应性在叶片安放角调节后产生变化,叶片安放角为-4°时,叶轮与导叶匹配度最好,而越偏离-4°,从叶轮进入导叶的涡结构越多,叶轮-导叶适应性越差.

旋转盘腔流动与换热试验准则

在分析空气系统旋转盘腔作用,及对其开展流动与换热试验研究目的的基础上,利用相似理论,推导出试验研究时应遵循的进气雷诺数、旋转雷诺数、普朗特数、旋转格拉晓夫数等相似准则及准则方程,解决了试验参数选取依据及匹配等问题,并建立了初步试验方法及数据处理方法。研究成果对进行相关试验研究具有一定的参考意义,但仍需通过试验研究进行不断补充与完善。

发散光长程空气传输受激转动拉曼散射效应

研究强激光长程空气传输受激转动拉曼散射(SRRS)效应,寻求抑制措施是高功率固体激光驱动器建造中需要解决的关键问题之一。采用数值模拟的方法研究了发散光长程空气传输SRRS效应的演化规律,通过几何近似理论推导了该效应有效建立时的强度距离乘积阈值和抑制判据。结果表明:发散光长程空气传输SRRS效应的建立速度小于平行光,二者阈值有理论关系;能够有效抑制SRRS效应的发散角设计判据由光束口径、激光强度及安全传输距离决定。

基于Routh-Hurwitz法的离心调速器稳定性分析

对机械运动中多自由度线性定常系统的运动稳定性分析有多种方法,但是这些方法不是受到使用条件的限制就是数学分析繁杂。利用劳斯-赫尔维茨判据(Routh-HurwitzCriterion)对机械系统中离心转速调节器的运动稳定性进行了分析,这种分析方法的优点就在于:建立系统运动微分方程的特征方程后,不必求解特征方程的根,只需知道根的符号就可判断系统的零解稳定性。结果表明使用Routh-Hurwitz方法分析运动稳定性问题更为简捷、实用。