广义波导不变量

海洋波导中声场在空间-频率域形成明暗相间的干涉条纹,Chuprov根据距离-频率(r-ω)域干涉条纹定义了波导不变量。一直以来,科研工作者探讨波导不变量定义式均未考虑声源-接收器方位角对波导不变量β值的影响,传统的声场干涉研究也忽略方位角变化对干涉条纹的影响。通过实验和仿真发现,对于我国近海大陆架常见的水平缓变楔形波导,方位角变化对干涉条纹的影响显著,因此将Chuprov给出的仅考虑传播距离变化对干涉条纹影响的波导不变量定义式推广为同时考虑方位角变化对干涉条纹影响的波导不变量定义式,并将之称为广义波导不变量。通过理论推导、数值仿真及海上实验数据分析发现,在水平缓变楔形波导环境下,传统波导不变量定义式无法解释干涉现象时,广义波导不变量可很好地解释数值仿真及海上实验声场干涉现象。     

运动目标低频矢量宽带谱干涉结构及其应用研究

随着安静型技术的飞速进步,舰船和潜艇的辐射噪声总源级越来越低,但其低频成分很难消除。推导了低频矢量声场双曲线方程形式的声场干涉条纹方程,建立了双曲线参数与目标运动参数的关系。分别利用射线理论和波动理论模拟了低频声场,计算了运动目标声压和水平、垂直振速的干涉谱图,采用短时傅里叶变换与霍夫变换结合的方法对干涉结构图进行处理,估计舰船目标的运动参数。仿真结果表明,利用运动目标的低频矢量宽带干涉谱可以有效估计出目标的运动参数,同时,垂直振速的干涉谱图参数估计结果较前两种谱图误差小。     

宽带噪声目标声场干涉结构图的时空变换

水听器接收的运动目标宽带噪声的频率-时间I(ω,t)声场干涉结构图随着目标的运动会发生相应的变化。这个变化将引起I(ω,t)中干涉条纹的弯曲,使得I(ω,t)和频率-距离干涉结构图不再有简单的线性映射关系。在深入分析宽带噪声运动目标频率-时间I(ω,t)和相应频率-距离I(ω,r)声场干涉结构间的非线性映射关系的基础上,提出了一种从I(ω,t)到线性I(ω,r)的时空变换方法(Time-Space Linear Transformation,TSLT)。数值仿真实验结果表明,TSLT可以有效地消除目标运动的影响,从I(ω,t)中恢复出能真实反映频率-距离声场干涉结构规律的I(ω,r)。最后给出利用运动目标宽带噪声声场干涉结构估计波导不变量的应用例。     

利用Hough变换提取波导不变量

针对浅海声场频率．距离干涉结构特性,利用Hough变换图像处理技术提取能反映干涉结构特性的波导不变量,该算法把频率．距离上的干涉条纹变换到参数空间的对应点,利用一段时间内的接收信号,在低信噪比下也能提取出干涉条纹的斜率。将实际海试数据分析结果与理论计算结果做比较,结果吻合。     

波导不变量和航向角联合估计方法研究

波导不变量是一个与环境特性和传播特性有关的物理量,对其准确估计有重要意义.结合LOFAR图和方位-时间历程图,提出了利用Hough变换进行波导不变量和目标航向角联合估计算法,该算法无需知道运动目标的距离信息,不要求目标有最近通过距离.提出了综合Hough变换参数估计算法,较提取单条纹参数估计算法有更好的稳健性,但增加了计算开销.仿真研究和海试数据分析均验证了算法的可行性,并表明该参数估计算法有较高的估计精度.     

基于波导不变量的双阵元被动探测方法研究

利用一种水声信道统计不变特征(波导不变量)实现被动探测。采用双阵元,分别对每一个阵元接收的宽带噪声信号进行LOFAR分析,对LOFAR图利用Hough变换等图像处理方法进行参数估计,得到波导不变量和曲线(文中讨论抛物线)参数估计值,进而运用几何关系估计出运动目标的航向角、速度和距离信息。利用Matlab仿真验证了该方法的正确性。     

希尔伯特变换实时全息干涉条纹相位提取

实时全息干涉法可以观察记录整个测试过程中条纹图的动态变化,传统相位提取算法只适合于静态干涉条纹图相位的提取.根据实时全息干涉条纹和希尔伯特变换的特点,提出了利用希尔伯特变换提取实时全息干涉条纹相位值的方法,采用了高通滤波的方法减少背景光强的影响,对铝片受力变形实验中实时全息干涉条纹的相位变化分布进行了提取.实验表明:希尔伯特变换法适合于动态条纹的相位提取,可以自动提取实时全息干涉测量过程中全场各点在任意两个时刻间的相位变化值,且测量结果与实时全息干涉条纹人工分析结果一致.     

一种基于阵列不变量的浅海声源深度分类方法

浅海负跃层环境下波导不变量可以用于声源深度分类,但是当声源相对于接收阵无径向距离变化时,波导不变量值难以估计。针对距离已知且相对于接收阵无径向距离变化的声源,提出一种基于阵列不变量的声源深度分类方法,可适用于浅海负跃层环境。所提方法利用阵列不变量估计波导不变量,进而实现声源深度分类。计算机仿真结果表明,当垂直阵位于负跃层以下且声源距离已知时,利用阵列不变量估计出的波导不变量随声源深度的变化明显,可用于目标深度分类。     

工业机器人视觉与平面运动参数的估计

本文提出了用二维Hotelling变换对平面运动参数进行估计的一种方法,并进行了模拟实验。首先提取出图象序列中各图象目标的轮廓特征,然后利用Hotelling变换使各轮廓的方位标准化,从而可求得目标的位置和旋转角等运动参数。实验得到了较为满意的结果。     

自由曲面上曲线物理空间和参数空间互相转换算法

根据曲面曲线上的点与曲面参数域之间的对应关系,提出了一种实用的物理空间与参数空间的转换算法.曲面和曲线离散后,用迭代法求得曲线离散点列对应的曲面参数值,以求得的参数值为型值点在曲面参数平面上构造插值曲线,然后根据插值曲线细分曲面曲线可得到曲面曲线在曲面参数平面上更加精确的投影曲线.计算实例表明投影曲线映射到三维空间与原曲面曲线高度一致.     

Initial sintering stage of thoria powder investigated by emanation thermal analysis

The initial stage of free sintering of thoria (ThO₂) has been investigated by means of emanation thermal analysis. Thoria powder (having surface area of 18 m² g^–1) was prepared from thorium oxalate (homogeneously labelled by²²⁸Th) by heating to 500 ° C. The release of²²⁰Rn was studied at various temperatures. The curves of radon diffusion release measured during isothermal heating of thoria powder at four temperatures (705, 735, 780 and 825 ° C) have been used for investigating the kinetics of the surface changes of thoria powder during the initial stage of sintering. The experimental data evaluated obey the kinetic law E _p S _eff = const ×t ⁿ , whereE _p is the emanating power due to the diffusion of radon in intergranular space and open pores,S _eff is the effective surface area,t is time andn equals 0.64. The effective surface areaS _eff has been proposed as a new parameter characterizing the active surface of powdered solid and its changes during sintering.     

Accuracy and precision in the estimation of peak areas: The effects of apodization

Weiss, G.H., Kiefer, J.E. and Ferretti, J.A., 1988. Accuracy and precision in the estimation of peak areas: the effects of apodization. Chemometrics and Intelligent Laboratory Systems, 4: 223-229.The problem of finding optimal apodization functions for the numerical estimate of peak areas is investigated. The formalism in such studies is presented and the effects are analyzed for a set of functions of the type exp[(-t/T_f)^α], where α = 1, 2 are the most frequently used values of this parameter. A measure of generalized error that combines bias and instrumental noise effects in peak area measurements is used to compare different apodization functions. It is found that increasing the value of α always decreases the generalized error but at the cost of requiring more precise knowledge of T₂*. Effects of varying other parameters available to the experimenter are also studied.     

Improved accuracy of frequency-domain photon migration close to light source using delta–P1 approximation of the adjustable parameter f

The δ–P1 approximation can improve the accuracy of the diffusion equation in the near field. Usually, the method to determine the fractional forward-scattering peak f in the δ–P1 approximation is to compare the first two moments of the Henyey–Greenstein phase function and those of the δ–Eddington phase function. The error relative to Monte Carlo estimation still exists within a mean free path using the value f above. To overcome this defect, we put forward a new method to determine the parameter f for improving the accuracy of frequency-domain photon migration in the δ–P1 approximation if the detector is close to a light source. Results demonstrate that the adjustable f δ–P1 approximation can accurately predict photon fluence rate at distances less than half the transport mean free paths from the source.     

Nonlinear dynamics of structures assembled by bolted joints

Summary The nonlinear transfer behaviour of an assembled structure such as a large lightweight space structure is caused by the nonlinear influence of structural connections. Bolted or riveted joints are the primary source of damping compared to material damping, if no special damping treatment is added to the structure. Simulation of this damping amount is very important in the design phase of a structure. Several well known lumped parameter joint models used in the past to describe the dynamic transfer behaviour of isolated joints by Coulomb friction elements are capable of describing global states of slip and stick only.The present paper investigates the influence of joints by a mixed experimental and numerical strategy. A detailed Finite Element model is established to provide understanding of different slip-stick mechanisms in the contact area. An advanced lumped parameter model is developed and identified by experimental investigations for an isolated bolted joint. This model is implemented in a Finite Element program for calculating the dynamic response of assembled structures incorporating the influence of micro- and macroslip of several bolted joints.List of symbols a acceleration - E ₀, E_t material moduli - F ₀ mass weighted excitation force - F _t tangential joint force - F generalized force - F _exc ^* excitation force - F _exc amplitude of excitation force - F _C0 spring element force - F _R0 friction element force - K _A, K_B normal stiffness - K _t tangential stiffness - L length of contact area - M _t transmitted joint torque - m _red reduced mass - p normal contact pressure - r effective radius - q generalized coordinate - z internal variable - x coordinate in the contact area - u relative displacement - u relative velocity - relative angle - friction coefficient - damping ratio - material parameter - ₀ equivalent slip limit - microslip parameter - excitation frequency Dedicated to Prof. Dr. Dr. h. c. Franz Ziegler on the occasion of this 60th birthday     

Blended infinite elements for paraboblic boundary value problems

A common method for numerically approximating two-point parabolic boundary value problems of the form u_t = L[u]+f(u) defined of the semi-infinite strip S = [0, 1]×[0, ∞] is to first discretize the spatial operator in the differential equation and then solve for the time evolution. Such an approach typically involves solving a system of algebriaic equations at a sequence of time steps. In this paper we take a different approach and subdivide S into a collection of semi-infinite substrips S_i = [x_i, x_i+1]×[0, ∞], and use blending function techniques to derive finite parameter functions e_i(x, t) defined on S_i. Spectral matching methods are used in deriving e_i to ensure that (u ? e_i) can be made small on S_i. Galerkin's method, with associated integration sover the entire space-time domain S, is then used to generate approximations to u(x, t) based upon the so defined infinite element (e_i, S_i). Approximations are hence found for all (x, t) in S by solving one well structed system of algebraic equations. We apply the method to several linear and non-linear problms.     

Measurement of frequency deviations in FM oscillations by means of a frequency meter

Summary It follows from the theoretical and experimental investigations that the error of measurement by the above method for a zero intermediate frequency and a sinusoidal modulation is equal to 100%, i.e., it drops with a rising modulation index. The deviation of the intermediate frequency from zero has a small effect on the measurement results [the additional error due to this factor is of the order of (f _if/f)²]. Since the error of the frequency counter can normally be neglected, the only source of error for a large modulation index, in fact, consists of the modulating function deviation from a sinusodial shape. For a small nonlinear distortion coefficient this error is of the same order as the coefficient.     

Deformation and failure of adhesive bonds under shear loading

Experiments have shown that certain mechanical properties can be greatly enhanced when a material is stressed while under tight spatial constraint. In this work, the post-yield behaviour of brittle and ductile epoxy resins used as thin adhesive bonds was determined using the napkin ring shear test. Real-time observations of the deformation in the bond as well as SEM post-failure analysis were employed to gain information on the failure process. The complete stress-strain histories of the adhesives were established for bond thicknesses ranging from the micrometre level up to values large enough to expose the bulk properties. The most dramatic variations occurred for the ultimate shear strain, _f; for the brittle adhesive, _f increased by over 30-fold relative to the bulk material when the bond thickness, t, was decreased to a few micrometres. Experimental evidence and analytical considerations suggest that the decline of _f with t was due to premature bond failure caused by tensile microcracks or voids that were formed in the interlayer during loading, with the specific _f versus t relationship being a mere reflection of the variations in the degree of stress concentration at the tip of the flaws. The astonishingly large value of _f (i.e. 2.8–3.4) found for the brittle epoxy in the micrometre thickness range, is believed to represent the intrinsic shear strain of this material.     

An investigation of the effect of high-temperature cyclic straining and creep loading on tensile mechanical properties of GH4145/SQ alloy

The effect of the reverse cyclic straining and the creep loading on the resultant tensile mechanical properties, such as the strength parameter (σ_0.2 and σ_b), the ductile parameter (δ and ϕ_f) and the composite parameter of the strength and ductility, the static toughness (U_t), for the precipitation-strengthened nickel-based superalloy GH4145/SQ used for high-temperature turbine and valve studs/bolts in power plant was investigated systematically at a temperature of 538°C. The experimental results show that in the case of the reverse cyclic straining both σ_0.2 and σ_b increases at early stages of cyclic straining and, after reaching their saturated values, σ_0.2 remains relatively constant until about 90% of fatigue life, while σ_b exhibits continuous reduction up to a level equal to the maximum applied stress amplitude. With the increasing number of straining cycles, both δ and ϕ_f as well as U_t decrease significantly until final fracture. In the case of creep loading the strength parameters (σ_0.2 and σ_b) tend to increase, as a whole, while the ductile parameters (δ and ϕ_f) and the static toughness (U_t) exhibit continuous decrease characterization as the amount of the creep deformation increases. The variation of the aforementioned various tensile mechanical properties during cyclic straining and creep loading of the alloy was further discussed by means of the observations of the deformation microstructures as well as the examinations of the fracture features of the specimens.     

The stress relaxation of the 1,2‐syndiotactic polybutadiene thermoplastic elastomer

Abstract

The stress relaxation of a low‐crystallinity type thermoplastic elastomer (TPR), 1,2‐syndiotactic polybutadiene, was studied. By using the continuous and intermittent stress relaxation method, the Co⁶⁰ r‐ray irradiated samples were tested from 30°C to 106°C under air, nitrogen or oxygen atmosphere. The relative stress relaxation curve of this low crystallinity kind of elastomer can be expressed by two Maxwellian decay terms.

f(t)/f(0)=ø ₁ exp (‐k ₁ t) + ø ₂ exp (‐k ₂ t)

Since there are only a few C=C double bonds in the polymer main chain, the oxidation of C=C in the side chain does not influence the strength of the stress relaxation. Thus the chemical relaxation can be ignored in this kind of thermoplastic elastomer. These two Maxwellian terms are both of physical stress relaxation, namely, flexible chain slippage and crystalline domain disintegration (breaking), respectively: