干涉雷达永久散射体识别方法的对比分析

基于永久散射体的干涉测量技术可以克服卫星合成孔径雷达应用于区域地表形变探测所面临的时空失相关和大气影响.永久散射体的有效识别是干涉系统中的一个关键环节.本文对永久散射体的两种识别方法, 即相关系数阈值法和振幅离差指数阈值法进行了理论上的分析和实验上的对比.以上海市局部地区作为研究对象, 采用26幅欧洲空间局的ERS-1/2卫星C波段SAR影像为源数据, 使用两种识别方法分别进行了永久散射体的提取, 结果表明, 振幅离差指数方法具有更为稳健的抗噪能力.     

Non‐linear finite element modal approach for the large amplitude free vibration of symmetric and unsymmetric composite plates

A theoretical analysis is presented for the large amplitude vibration of symmetric and unsymmetric composite plates using the non‐linear finite element modal reduction method. The problem is first reduced to a set of Duffing‐type modal equations using the finite element modal reduction method. The main advantage of the proposed approach is that no updating of the non‐linear stiffness matrices is needed. Without loss of generality, accurate frequency ratios for the fundamental mode and the higher modes of a composite plate at various values of maximum deflection are then determined by using the Runge–Kutta numerical integration scheme. The procedure for obtaining proper initial conditions for the periodic plate motions is very time consuming. Thus, an alternative scheme (the harmonic balance method) is adopted and assessed, as it was employed to formulate the large amplitude free vibration of beams in a previous study, and the results agreed well with the elliptic solution. The numerical results that are obtained with the harmonic balance method agree reasonably well with those obtained with the Runge–Kutta method. The contribution of each linear mode to the maximum deflection of a plate can also be obtained. The frequency ratios for isotropic and composite plates at various maximum deflections are presented, and convergence of frequencies with the number of finite elements, number of linear modes, and number of harmonic terms is also studied. Copyright © 2005 John Wiley & Sons, Ltd.     

地质B超中聚束波的振幅与频率特征研究

地质B超是基于医学上的B超思想,采用相控超声激发方式,使用多个波源,通过相控,使波在地下探测点聚焦,以增强能量,并采用连续扫描的观测方式成像.本文根据这种思想,分析了聚焦波的振幅特性和频率特性,由理论分析可知,聚束波相当于一个线性系统,具有低通滤波效应,当单波相位差较小时,聚束波能量显著增强,高频成分损失不大.     

热声发动机压力特性研究

热声发动机压力特性主要体现在压力波形、相位、压比、压力振幅以及振荡频率等方面,是衡量热声发动机性能的重要依据之一.实验研究了加热温度、充气压力、谐振管长度、工质种类、负载等对压比、压力振幅、频率等压力特性的影响,为更好地进行热声发动机的设计和应用奠定了基础.     

移动荷载作用下的桥梁振动及其TMD控制

为了全面地了解移动荷载作用下桥梁的振动机理及其调质阻尼器(TMD)控制,将列车简化成移动简谐力模型,对列车过桥时桥梁的振动形态幅频特性作了详细的探讨。然后论述了移动荷载作用下的TMD控制。给出了桥梁在不同速度下的幅频特性曲线以及TMD控制的质量比影响曲线,揭示了移动荷载作用下的桥梁振动及其控制的特点,同时为进一步的桥梁振动控制提供详尽的参考数据。     

具有随机系数和带间隙的自激振动问题的人工神经网络解法

利用人工神经网络技术(BP网络)研究具有随机系数和带有间隙的初轧机自激振动问题,提出了一种Runge-Kutta法和人工神经网络相结合的求解方法。即利用数值计算和BP网络建立随机系数和稳态振幅之间的关系,从而直接计算出稳态振幅的统计特性。计算结果表明:采用所提出的方法求解,通用性好且可提高计算精度,并得到了间隙与稳态振幅的均值有关,与标准离差无关的有用结论。     

120 mm×120 mm结晶器振动参数的优化

分析了120 mm×120 mm断面结晶器溅钢的主要原因,通过优化结晶器的振动参数,消除了结晶器溅钢,漏钢现象,脱模情况有所改观,取得了良好的效果.     

Plastic strain-controlled short crack growth and fatigue life

Constant plastic strain-controlled and constant stress-controlled tests were performed on smooth and lightly notched specimens machined from a massive forging of 42CrMo4 steel. Comparison of the fatigue life curves plotted as function of the plastic strain amplitude and stress amplitude shows a decisive role of plastic strain amplitude. Crack initiation and the kinetics of short crack growth were studied in constant plastic strain amplitude loading and the relation between the crack growth coefficient and plastic strain amplitude was established. This is equivalent to the Coffin–Manson law and shows that the Coffin–Manson law can be interpreted in terms of short crack growth.     

Time derivative equations for mode I fatigue crack growth in metals

Predicting fatigue crack growth in metals remains a difficult task since the available models based on the Paris law are cycle-derivative equations (da/dN), while service loads are often far from being cyclic. This imposes a cycle-reconstruction of the load sequence, which significantly modifies the load history in the signal. The main objective of this paper is therefore to propose a set of time-derivative equations for fatigue crack growth in order to avoid any cycle reconstruction. The model is based on the thermodynamics of dissipative processes. Its main originality lies in the introduction of a supplementary state variable for the crack, which allows describing continuously the state of the crack throughout any complex load sequence. The state of the crack is considered to be fully characterized at the global scale by its length a, its plastic blunting ρ, and its elastic opening. In the equations, special attention is paid to the elastic energy stored inside the crack tip plastic zone, since, in practice, residual stresses at the crack tip are known to considerably influence fatigue crack growth. The model consists finally in two laws: a crack propagation law, which is a relationship between dρ/dt and da/dt and which observes the inequality stemming from the inequality of Clausius Duhem, and an elastic–plastic constitutive behaviour for the cracked structure, which provides dρ/dt versus load and which stems from the energy balance equation. The model was implemented and tested. It successfully reproduces the main features of fatigue crack growth as reported in the literature, such as the Paris law, the stress ratio effect, and the overload retardation effect.     

高速调制光光折变自适应光外差探测系统实验研究

利用新型光折变自适应光外差探测系统，对相位畸变的高速振幅调制信号进行了光外差探测实验研究。获得了稳定的光外差输出．实验研究表明，提高本振光强度可提高光外差输出信号强度，所以该系统适用于对弱信号的探测；同时新型系统对高速振幅调制信号光有更高的探测灵敏度．因此，这种新型光折变自适应光外差探测系统可应用于空间相位畸变的时域高速调制弱信号光的外差探测．