板壳结构中的非线性兰姆波

鉴于常规超声检测技术对分布式材料细微损伤和接触类结构损伤的检测效果不佳,近年来非线性超声技术逐渐引起广泛关注.超声波在板壳结构中通常以兰姆波的形式进行传播,然而由于兰姆波的频散及多模特性,使得非线性兰姆波的理论和实验研究进展缓慢.本文从经典非线性理论出发,总结了源于材料固有非线性诱发的非线性兰姆波的理论和实验两个方面的研究进展,井综述了兰姆波的二次谐波发生效应在材料损伤评价方面的若干应用;从接触声非线性理论出发,讨论了目前由于接触类结构损伤诱发的非线性兰姆波的研究现状.最后展望了非线性兰姆波的未来研究重点及发展趋势.     

相关性和显著性结合的制导工具误差系数分离方法

针对惯性制导工具误差分离准确度较低的问题,提出了一种相关性和显著性结合的惯性制导工具误差系数分离方法。首先,完整、准确的惯性测量系统误差模型是实现弹道导弹制导工具误差准确分离的基础,依次建立了陀螺仪和加速度计的高阶误差模型,可分离的误差系数达到63项,以此模型为基础,建立遥外测速度误差矩阵方程。其次,对于环境函数矩阵的复共线问题,给出了结构矩阵相关时的遥外测速度误差重构模型和系数计算方法,在经过相关性检验后的模型基础上,结合显著性检验方法,突出因子和输出之间的显著性关系,简化了模型,分离出了更精确、显著的制导工具误差系数。最后,将使用相关性和显著性结合得到的误差系数代入遥外测速度误差模型中进行补偿,得到补偿后的速度残差基本在±0.2 m/s之内,补偿效果较好。     

Microcrack localization using a collinear Lamb wave frequency-mixing technique in a thin plate

A novel Lamb wave frequency-mixing technique is proposed for locating microcracks in a thin plate,which does not require the resonance condition of Lamb wave mixing and can accurately locate the microcracks through only one-time sensing.Based on the bilinear stress-strain constitutive model,a two-dimensional finite element(FE)model is built to investigate the frequency-mixing response induced by the interaction between two primary Lamb waves and a microcrack.When two primary Lamb waves of A0 and S0 modes with different frequencies excited on the same side of the plate simultaneously impinge on the examined microcrack,under the modulation of the contact acoustic nonlinearity,the microcrack itself can be deemed as the secondary sound source and it will radiate the Lamb waves of new combined frequencies.Based on the time of flight of the generated A0 mode at difference frequency,an indicator named normalized amplitude index(NAI)is defined to directly locate the multi-microcracks in the given plate.It is found that the number and location of the microcracks can be intuitively visualized by using the NAI based frequency-mixing technique.It is also demonstrated that the proposed frequency mixing technique is a promising approach for the microcrack localization.