无损评价固体板材疲劳损伤的非线性超声兰姆波方法

从理论上分析了采用非线性超声兰姆波方法准确评价固体板材疲劳损伤的可行性。对于未承受周期性载荷作用的航空铝板,采用已建立的超声兰姆波实验系统测量了在其中传播的超声兰姆波的基波和二次谐波的幅频曲线,并对所测得结果给予了合理的物理解释。对两块给定的航空铝板试件施加拉-拉疲劳载荷作用,在超声兰姆波具有强烈非线性效应的条件下,对于不同的载荷循环次数,分别对两试件中超声兰姆波的基波和二次谐波的幅频曲线进行了测量。结果表明,在板材疲劳损伤的初期,超声兰姆波基波的应力波因子随循环次数的改变并不明显,且与循环次数的对应关系也不确定;然而,在周期性载荷作用的最初阶段,超声兰姆波二次谐波的应力波因子随循环次数的变化非常显著,并表现出明显的单调对应关系,表明超声兰姆波的非线性效应及相应的超声兰姆波二次谐波的应力波因子,可望能为固体板材早期疲劳损伤的准确评价提供一条有效途径。     

金属板疲劳损伤非线性兰姆波混频检测

针对金属板结构安全运行需要,开展了金属板结构疲劳损伤非线性兰姆波混频检测方法研究。通过数值仿真,研究了两列A₀兰姆波与材料损伤间的非线性相互作用。结果表明,两列共线A₀兰姆波在结构材料损伤处产生单向传播的和频S₀波,且和频波幅值随传播距离具有积累增长效应。对不同疲劳程度金属板试件进行了共线混频兰姆波检测实验,结果表明,和频波幅值随试件疲劳周数的增加呈单调递增趋势,提出的兰姆波混频技术可用于金属板结构疲劳程度的表征。研究工作为金属板结构疲劳损伤检测提供了可行的技术方案。     

基于 LabVIEW 声速测量系统研究声速与温湿度的关系

利用USB独立声卡结合虚拟仪器软件LabVIEW开发了声速测量系统,研究了声速与空气的温度、湿度之间的关系,得到了温湿度修正的声速经验公式。     

一种评价固体板材表面性质的非线性超声兰姆波方法

在基频与二倍频兰姆波相速度匹配但群速度失配的条件下, 通过选择适当的兰姆波二次谐波时域信号的测量起止时间, 可完全扣除换能器对二次谐波积分振幅测量所带来的影响。本文提出采用兰姆波二次谐波的积分振幅作为评价参量, 以实现对板材表面性质的准确评价。当板材表面性质发生改变时, 原本在理想表面条件下成立的基频与二倍频兰姆波相速度匹配的条件不再严格满足, 这将显著地影响到兰姆波的二次谐波发生效率, 相应的二次谐波积分振幅随表面性质的改变也将发生非常敏感的单调变化。实验结果表明,利用扣除换能器影响之后所测得的兰姆波二次谐波的积分振幅,可对板材表面性质的变化情况进行准确评价。     

基于模式分离的兰姆波逆时偏移成像

应用频率域逆时偏移方法实现各向同性和各向异性板中缺陷的兰姆波成像.由于缺陷引起的多模态散射信号会在重建图像中形成伪像,根据基本导波模式振动对称性的差别进行了模式分离预处理.基于多元阵列超声技术,开展了铝板和复合板内缺陷频率域逆时偏移超声成像方法的数值仿真研究.首先,建立有限元模型,采用环形传感器数值采集由缺陷引起的兰姆波散射信号,然后,将采集到的多模式散射信号进行模式分离处理,再将模式分离后的兰姆波散射信号经时间反转后并在相应的接收器处重新激励,在频域中运用格林函数反向传播兰姆波散射信号,获取监测区域的声场信息,与正向传播声场进行互相关,重建缺陷图像.首先对铝板中单缺陷以及复合材料板中相邻的两个相同缺陷进行数值仿真,对比有无模式分离处理的缺陷逆时偏移成像效果,体现出模式分离的重要性.在此基础上,采用逆时偏移方法对复合板材内位置邻近、深度不同的双缺陷进行识别.数值结果表明,模式分离预处理后的缺陷重建图像能够有效去除多模式干扰产生的伪像.文中提出的成像方法对各向同性板和各向异性板内缺陷的检测和成像具有很好的发展潜力,可以准确地探测多个缺陷的形状、尺寸和深度.     

兰姆波的非线性研究

本文结出兰姆波非线性的一些理论结果,与非色散导波如表面波、界面波的非线性性质相比,具有更大的复杂性,表现为与Kd值、波的对称性及波模式的阶次均有关,且兰姆波的二次谐波只能为对称波。     

用于储罐底板缺陷检测的超声兰姆波模式研究

从超声兰姆波的声场方程出发,得到板材表面离面位移(法向位移)为零的条件,即当超声兰姆波的相速度等于板材介质的纵波声速时,在板材表面的离面位移为零。在给定适当频厚积的条件下,分别数值模拟了仅有切向位移而无离面位移的A1、S1、A2和S2兰姆波模式在有液体负载的单层钢板中的传播情形。结果表明:离面位移为零的S2模式频散较小且对板中缺陷更为敏感。     

一种定征复合板材粘接层性质的非线性超声兰姆波方法

借助于兰姆波频散曲线及导波激发的模式展开分析方法,对基频兰姆波时域信号及二次谐波时域信号的发生过程进行了直观的论述。结合Ritec-SNAP系统的测量功能,详细分析了二次谐波时域脉冲包络积分表达式的物理意义;该积分表达式可表征基频兰姆波时域脉冲传播过程中的二次谐波发生效率,以及基频与二倍频兰姆波模式之间的频散程度。在基频与二倍频兰姆波相速度相等(或近似相等)的频率附近,实验观察到显著的且无模式混叠的二次谐波信号,显示出在兰姆波的传播过程中的确可存在强烈的非线性效应。对于三种不同粘接情形的复合板材,实验结果表明,采用本文引入的非线性兰姆波应力波因子,结合二次谐波幅频曲线峰值所对应的频率值,可有效地对板材粘接层性质进行表征。     

简谈空气中的声速与温度关系

讨论了驻波法测空气中声速的实验,分别用相位比较法和李萨如图法来确定超声波波长.在不同温度条件下测量了声速并与理论值进行比较,实验结果显示测量温度接近室温时实验值与理论值基本一致,测量温度与室温相差较大时实验值与理论值偏差较大.     

海底沉积物声速与物理性质的理论关系

为建立海底沉积物声速与物理性质的关系,引入密度变化比和弹性模量变化比为参数描述海底沉积物声速。密度变化比是描述声速的一个复合参数,由孔隙率、固相密度和海水密度三个物理参数表示;弹性模量变化比是描述声速的另一个复合参数,由孔隙率、固相体积模量和海水体积模量三个物理参数表示;声速公式可展开为这两个复合参数的泰勒多项式。两个复合参数的变化构成声速曲面,声速曲面包含了声速特性与沉积物性质的完整信息;而单参数声速公式是声速曲面上的一条曲线。数值计算与测试结果对比表明:把孔隙率-声速经验公式变换为参考声速乘以一个调制函数的标准形式,各个海区的孔隙率-声速经验公式的调制函数是相当接近的。论文导出的声速公式与Wood声速公式的区别在于弹性模量的模型不同。     

金属板中Lamb波波速与应力关系的实验研究

本文给出了金属板中Lamb波波速对同向应力变化率测量的装置,探讨分析了测量原理及误差,并对Lamb波的波速与应力关系进行了实验研究。结果表明Lamb波的波速对应力有较好的线性关系,Lamb波相速度对应力的变化率的实验值与理论值符合很好。     

金属薄板中Lamb波波速与静应力关系的理论研究

本研究基于各向同性金属材料在静压力作用下产生各向异性的事实,从固体中超声波的非线性理论出发,得到了在一定应力下固体的等效的二阶弹性常数,再利用各向异性板中的Lamb波理论推得了在静应力作用下与应力平行和垂直方向的Lamb波的频散方程,讨论了金属薄板中Lamb波波速与静应力的关系。     

Research on relationships between Lamb wave velocity and static stress in metal plate

On the fact that an isotropic metal solid produces anisotropic property in the state of static stress, based on the theory of the nonlinear acoustoelasticity, the equivalent second order elastic constants are calculated for metal plate with static stress. For the case of thin metal plate with stress, the two kinds of dispersion equation for Lamb waves propagating parallel and vertical to the direction of static stress are derived. Using the equations, the relationships between Lamb wave velocity and static stress in a metal plate are discussed.     

The group velocity variation of Lamb wave in fiber reinforced composite plate

Experimentally measured Lamb wave group velocities in composite materials with anisotropic characteristics are not the same as the theoretical group velocities which is calculated with the Lamb wave dispersion equation. This discrepancy arises from the fact that the angle between the group velocity direction and the phase velocity direction in anisotropic materials exists. Wave propagation in a composite material with anisotropic characteristics should be considered with respect to magnitude correction in addition to direction correction. In this study, S0 mode phase velocity dispersion curves are depicted with the variation of degree with respect to the fiber direction using a Lamb wave dispersion relation in the unidirectional, bidirectional, and quasi-isotropic composite plates. Slowness surface is sketched by the reciprocal value of the phase velocity curves. The magnitude and direction of the group velocity could be calculated from the slowness surface. The recalculated group velocities with consideration of the magnitude and direction from the slowness surface are compared with experimentally measured group velocities. The proposed method shows good agreements with theoretical and experimental results.     

兰姆波在自由端面上的反射

对于板材的内部缺陷,常用兰姆波作超声无损检测。兰姆波在板材自由端面上的反射呈现复杂现象,即使在纯模入射条件下,通常也要产生无数个其它模式的反射兰姆渡。由于兰姆波本征模式正交关系同时涉及位移和应力,使其反射问题的分析较复杂,我们采用了一种新方法,应用最小二乘原理,将兰姆波本征模式作为试验函数,实现了一种较简单的分析方法。在一般情形下,利用8位微处理机就可取得满意的结果。应用该方法,给出了兰姆波在自由端面上反射时各模式的反射系数,阐明了纯模反射条件,并发现了关于模式转换的一些有趣的新现象。     

Lamb wave reflection at the free edge of a plate

This study concentrates on the reflection of Lamb waves at the free end of a plate. The conversion phenomena are examined in detail over a large frequency range and the energy conversion coefficients are obtained by three different ways: theoretically, numerically (finite element method) and experimentally. The experimental energy determination is obtained from the measurement of the plate normal displacements, by mean of a laser interferometer. All results are in relatively good agreement and the energy balance between incident and reflected waves is discussed in each case.     

Local vibration of an elastic plate and zero-group velocity Lamb modes

Elastic plates or cylinders can support guided modes with zero group velocity (ZGV) at a nonzero value of the wave number. Using laser-based ultrasonic techniques, we experimentally investigate some fascinating properties of these ZGV modes: resonance and ringing effects, backward wave propagation, interference between backward and forward modes. Then, the conditions required for the existence of ZGV Lamb modes in isotropic plates are discussed. It is shown that these modes appear in a range of Poisson's ratio about the value for which the cutoff frequency curves of modes belonging to the same family intercept, i.e., for a bulk wave velocity ratio equal to a rational number. An interpretation of this phenomenon in terms of a strong repulsion between a pair of modes having a different parity in the vicinity of the cutoff frequencies is given. Experiments performed with materials of various Poisson's ratio demonstrate that the resonance spectrum of an unloaded elastic plate, locally excited by a laser pulse, is dominated by the ZGV Lamb modes.     

Lamb wave radiation from plate termination in a liquid

The liquid wedge method is used to excite leaky Lamb waves on an aluminium plate while the plate is immersed in water. The excitation occurs near the end of the plate so that sonic energy of the excited mode radiates into the liquid. A Schlieren method is used to observe the radiation pattern and a correlation is made between the radiation lobe structure and the symmetric or antisymmetric nature of the excited mode. Theoretical calculations of the transfer of energy from the plate to the liquid are made and agreement between theory and experiment is obtained.     

Lamb modes with multiple zero-group velocity points in an orthotropic plate

In this paper the spectrum of Lamb modes in an orthotropic plate is studied. Infinitely many modes are found which undergo a change of sign in their slopes twice, thus leading to two zero-group velocity points. A few modes possessing three such points also exist.