轴对称平面应力状态屈服轨迹上的应变变化规律

本文将诸薄筒件成形工序,按照主应力的大小顺序、找出其在轴对称平面应力状态屈服轨迹上所对应的区域,进而给出无摩擦条件ε_θ、ε_ρ和ε_τ随角φ=tg~(-1)σ_ρ/σ_θ的变化规律。文章还给出了锥面成形工序在有摩擦条件下,ε_θ、ε_ρ和ε_τ沿φ角的分布曲线。     

摩擦系数对齿轮接触应力分布的影响

从J—C公式(两平行圆柱体同时受到法向和切向赫芝压强时的应力分布)出发,在计算机上用优化方法计算了不同摩擦系数下,最大切应力τ_(max),等效应力σ_e和正交切应力τ_(XZ)的极大值位置。结果表明,当摩擦系数分别为0.18和0.24时,τ_(max)和σ_e的最大值位置到达表面。当z>b时,摩擦系数的变化对τ_(max)和σ_e的分布没有显著影响。当摩擦系数增大时,动态的正交切应力τ_(XZ)由对称循环状态向脉动循环状态过渡,而τ_(XZ)的极大值位置的x坐标及应力变程没有显著变化。还在计算结果的基础上对接触疲劳破坏的力学条件进行了讨论。     

在较弱的矩条件下最近邻预测问题中条件风险估计量的收敛速度

设(X,θ)是随机向量,X∈R~d、θ∈R~1;(X_1,θ_i)是(X,θ)的i·i·d·随机样本,i=1,…,n;L_n是平方损失下最近邻(NN)预测的条件风险。设是L_n的估计量,其中θ_(nj)是按训练样本(X_1,θ_1),…,(X_(j-1),θ_(1-1)),(X_(1+1),θ_(1+1))…(X_n,θ_n)与观察到的X_j对θ_j所作的NN预测。周知,, a·s·其中R~*是Bayes风险。本文在较弱的矩条件Eθ~2(log~+|θ|)~δ<∞(δ>0)下,得到了收敛到2R~*的速度。     

平方损失下最近邻预测问题中条件风险估计量的强收敛性

设(X,θ)与(X_1,θ_1),…,(X_n,θ_n)是独立同分布的随机向量,它们的取值空间为R~d×R~1,其中d是一个正整数。这里,(X_1,θ_1),…,(X_n,θ-n)是训练样本。又,已经观察到X的值,现在要依据它们来预测θ的值。     

磁悬浮高速转子真空计原理的研究

本文介绍磁悬浮高速转子真空计(简称磁悬规),详细叙述了该计的工作原理。磁悬规是通过测量气体分子对高速转子的阻尼造成的转速衰减来测量气体压强。测出两个相邻的时间间隔τ_n,τ_(n+1)(在每个时间间隔上转子转动次数相同),利用P=c_o/σ(τ_(n+1)-τ_n)/τ_(n+1)·τ_n公式,计算出气体压强,其中c_o、σ为仅与转子和气体种类有关的常数。该计具有良好的测量重复性,在1Pa～10~(-4)Pa的压强范围中,三年内测量重复性零散小于1％。测量时不改变被测气体的成份与压强。它是一种有发展前途的真空计。     

短纤维增强树脂基复合材料强度和模量的各向异性

研究了短纤维增强聚合物复合材料(SFRP)的强度和模量的各向异性。纤维的平均长度l_(mean)对纤维增强有效因子有很大的影响。在Φ=0°,纤维增强有效因子随着l_(mean)的增加而增加;当Φ=90°,纤维有效因子在Θ不太大时随着l_(mean)增加而增加,当Θ很大时,纤维有效因子值变成零。θ_(mean)对纤维有效因子也有很重要的影响。在Φ=0°,Θ不太大(<40°)时,纤维有效因子随着θ_(mean)增加而减小;而当Θ很大(>50°)时,则随着θ_(mean)的增加而增大。在Φ=90°,Θ不太大时,纤维有效因子随着θ_(mean)增加而减小;而当Θ很大时,纤维有效因子值为零。模式纤维长度对复合材料的强度无明显影响。大l_(mean)对应高的复合材料模量。当Θ接近60°时,平均纤维长度对模量的影响不大。模式纤维长度对模量的影响相对比较小,而对于一个很小的l_(mean),影响则比较明显。当Θ很小时,θ_(mean)值越小,模量值越高;相反,Θ很大时,θ_(mean)值越小,模量值越低。     

中孔炭对蛋白质的吸附

对人体血清蛋白质(HSA)的吸附进行了热效应和动力学的研究,按照其在准均匀介质中的扩散过程,描述了HSA吸附动力学,研究表明该过程服从方程a=kL1/2,在研究合成活性炭(SCN)对HSA的吸附时(在磷酸盐缓冲液中),发现该过程伴随有放热行为(每吸附一克蛋白质放热约为50焦耳)。由放热速率和蛋白质吸附速率之比即((?)_q/(?)_τ)/((?)_a/(?)_τ)来表达热效应测量结果。研究了温度、离子强度、起始蛋白质浓度对((?)_q/(?)_a)_τ时间变量的影响,研究表明在被吸附的HSA分子中没有明显的结构形态变化的部分随着起始溶质浓度和离子强度的增大而增加。     

相依样本下带有讨厌参数的经验似然比置信区域

本文将条件θ_0=EH(X,μ)推广到更一般的估计方程EH(X,θ_0,μ)=0,并且在样本为强平稳φ混合序列情形下讨论带有讨厌参数的经验似然比置信区域。     

参量相控发射阵的研制

本文介绍参量阵在信息传送、水下电话、海底剖面测量等应用的近年发展,以及参量相控发射阵在提高参量声呐性能方面的潜力:阐述矩形参量相控发射阵的设计和性能计算,给出测试结果。所研制阵的两个原频声波频率分别为f_L=41～45kHz,f_H=46～49kHz。差频声波频率f_N=3～8kHz。波束宽度θ_(-3dB)=6.7°,随f_s变化很小。在f_s=7kHz时,差频声源级SL_s超过186dB,参量增益G约为-41dB。在相控信号激励下,差频波束与原频波束同步扫描,在相控角θ_0不大时(θ_0≤12°),θ_(-3dB)几乎不随θ_0变化,当θ_0=12°时,SL_s平均下降2.2dB,可满足多波束发射的需要。     

Dependence of mixed-mode crack propagation on the variation of σr and τrθ

The variation of the direction θ_p of crack propagation in mixed mode with the angle β of the inclined crack is shown to fit very well to the variation of θ_r, of the bisector of the angle of directions θ_r1 and θ_r2 of max σ_r and sufficiently well to that of θ_rθ, bisector of the angle of directions θ_rθ1 and θ_rθ2 of min τ_rθ < 0 and max τ_rθ > 0. The variation of σ^cr/σ^cr_I, causing initiation of propagation, with β as predicted by the max σ_θ criterion, fits very well to that of a critical loading evaluated by considering the combined “opening” effect of σ and τ_rθ along the circumference of a small circle centered at the crack tip. The above results may be used as criteria of mixed-mode fracture based on σ_r and τ_rθ.     

Transient heat transfer to liquid helium from bare copper surfaces in a vertical orientation — I: Film boiling regime

An experiment on transient heat transfer is described in which partial quench and recovery in a composite superconductor is simulated. Results show that transient heat transfer in the film boiling regime can be expressed as the sum of the steady state term, q_s(τ), and a term that characterizes heating or cooling associated with a thin layer of helium vapour created at the test surface. Specifically the transient heat transfer rate is given as a function of τ by:

$\text{q}{}_{\mathrm{t}}\text{(θ) = q}{}_{\mathrm{s}}\text{(θ) + a(θ)}\text{d}\text{(θ)}\text{d}\text{t}$

.This is in a form directly usable for the computation of partial quench and recovery processes in a composite superconductor.The proportionality function a(τ) represents an effective heat capacity of the vapour layer. Like q_s(τ) it is strongly dependent on surface conditions. Because of this term, transient heat transfer in the film boiling regime can be much higher in the heating cycle and much lower in the cooling cycle than steady state heat transfer: it is hysteretic with temperature cycling.The function a(τ) also quantitatively determines the vapour layer thickness as a function of temperature. The effect of channel gap on transient heat transfer is qualitatively interpreted in terms of the layer thickness relative to the channel size. A criterion is given for a proper choice of channel sizes in superconducting magnet windings.     

Apparent Active Adsorption Force at a Solid-Liquid Interface with Active Adsorption

The paper presents a new relationship betweenthe three surface tensions on the solid-liquid-vaporinterface, γ_(sl)-γ_(sv)+γ_(lv)cosθ=βin order to understandthe wetting on the liquid-solid interface in the caseof active adsorption.The authors suggest a newforce“apparent active adsorption force”βto takepart in the balance at the three interface lines ofcontact in the solid-liquid-vapor phases,its dimen-sion isβ=Σα_iRT(Γ_i~(sl)-Γ_i~(sv)+Γ_i~(lv)cosθ),and its directionis dependent on the sign of β,whenβis a positive,the direction is agree with surface tension of the sol-id-vapor interface γ and vice versa.     

Lubich Second-Order Methods for Distributed-Order Time-Fractional Differential Equations with Smooth Solutions

This article is devoted to the study of some high-order difference schemes for the distributed-order time-fractional equations in both one and two space dimensions. Based on the composite Simpson formula and Lubich second-order operator, a difference scheme is constructed with $\mathscr{O}(τ^2+h^4+σ^4)$ convergence in the $L_1$($L_∞$)-norm for the one-dimensional case, where $τ$, $h$ and $σ$ are the respective step sizes in time, space and distributed-order. Unconditional stability and convergence are proven. An ADI difference scheme is also derived for the two-dimensional case, and proven to be unconditionally stable and $\mathscr{O}(τ^2|lnτ|+h^4_1+h^4_2+σ^4)$ convergent in the $L_1$($L_∞$)-norm, where $h_1$ and $h_2$ are the spatial step sizes. Some numerical examples are also given to demonstrate our theoretical results.     

Notch effects on photoelastic determination of mixed-mode stress intensity factors

An investigation into the notch effect on the photoelastic determination of the mixed mode stress intensity factors is presented. This accomplished by comparing the isochromatic loops generated for a crack and an ellipse in an infinite plate. The generated mathematical loops are based on the exact solutions. A method of analysis is proposed and used to correct the distorted maximum angle $\text{θ}{}_{\mathrm{m}}$ and maximum shear stress $\text{τ}{}_{\max }$ due to the notch effect. The results obtained from the photoelastic measurements based on the proposed method compare favourably with those by an earlier investigation.     

Finite element analysis of postbuckling and delamination of composite laminates using virtual crack closure technique

The two-dimensional and three-dimensional parametric finite element analysis (FEA) of composite flat laminates with two through-the-width delamination types: 0₄/(±θ)₆//0₄ and 0₄//(±θ)₆//0₄ (θ = 0°, 45°, and “//” denotes the delaminated interface) under compressive load are performed to explore the effects of multiple delaminations on the postbuckling properties. The virtual crack closure technique which is employed to calculate the energy release rate (ERR) for crack propagation is used to deal with the delamination growth. Three typical failure criteria: B-K law, Reeder law and Power law are comparatively studied for predicting the crack propagation. Effects of different mesh sizes and pre-existing crack length on the delamination growth and postbuckling properties of composite laminates are discussed. Interaction between the delamination growth mechanisms for multiple cracks for 0₄//(±θ)₆//0₄ composite laminates is also investigated. Numerical results using FEA are also compared with those by existing models and experiments.     

曲线裂纹裂尖SIFs等效分析的广义参数Williams单元确定方法

基于Williams级数建立的含直线裂纹平面弹性体裂尖应力强度因子（Stress Intensity Factors，简记SIFs）分析的广义参数Williams单元（简记W单元），因裂纹面必须满足σ_θ=0、τ_ρθ=0（θ=±π）的边界条件，而曲线裂纹面无法满足该边界条件使得W单元不能直接适用。该文针对此问题，将含曲线裂纹平面弹性体裂尖局部区域进行等效处理，以裂尖为圆心，选取合适的半径建立等效区，即截取斜率呈单调变化的曲线微段，在微段两端引切线交叉成折线段，以该折线段近似代替曲线微段。算例分析表明:当等效区的尺寸取建议值时，该文方法的计算结果精度较高，证明了该文改进方法在分析曲线裂纹尖端SIFs的合理性，同时给出了等效区尺寸的建议值，即ψ_min=5π/6，且等效区截取曲线长度宜大于原曲线长度的1/5。     

2(1/4)Cr-1 Mo钢的短裂纹疲劳断裂特征

本文用弹塑性断裂力学分别研究了在空气和氢中 ,压力容器器壁的承载材料 2 ( 1 / 4 )Cr— 1 Mo钢的短裂纹低频疲劳特性 ,结果表明 :都可用 da/ d N =C(ΔJ) n 的关系式计算上述两环境中的短裂纹扩展速率 .裂纹呈穿晶扩展 .氢明显增加了 2 ( 1 / 4 ) Cr— 1 Mo钢的断裂敏感性     

The cracking of composites consisting of discontinuous ductile fibres in a brittle matrix — effect of fibre orientation

The effect of the orientation of metal wires on the opening of a crack in a brittle-matrix composite has been studied. The force arising from the plastic bending of a wire which is weakly bonded to the matrix and which crosses the matrix crack at an angleθ to the crack face normal has been measured in model resin-wire composites and good agreement is found with a simple theory based on the calculation of the force needed to produce a plastic hinge in a cantilever beam. The force passes through a maximum at a small crack opening, of the order of one wire diameter, and decreases with further crack opening. The The largest force is obtained for a value ofθ of approximately 45°. For wires whose length approaches the critical length, the force and the total work of fracture arising from the bending of the wire are small compared to the values arising from the interfacial shear stress resisting pull-out; the contributions due to bending and interfacial shear stress are of comparable magnitudes for wires which are approximately one-fifth of the critical length.