INVESTIGATION ON LAMB WAVE CHARACTERISTICS IN ONE-DIMENSIONAL HEXAGONAL QUASI-CRYSTAL NANO PLATES
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摘要: 准晶纳米结构在工程中经常承受以弹性波为代表的动态工作载荷,为深入研究其动态失效机制,研究了一维六方准晶纳米板中Lamb波的波动特性。基于修正的偶应力理论,推导出Bak模型下Lamb波的波动控制方程,使用勒让德正交多项式方法(LOPM)求解该动力学方程,计算得到其频散曲线和位移分布。研究了声子场和相位子场尺寸效应、声-相耦合效应对波动特性的影响。结果表明:尺寸效应使声子模态和相位子模态相速度增大;声-相耦合效应显著地增大了声子模态中的相位子位移振幅和相位子模态中的声子位移振幅。研究结果为准晶纳米结构的设计和无损检测奠定了一定的理论基础。Abstract: Quasi-crystal nano structures are often subjected to dynamic loads represented by elastic waves in engineering. In order to study their dynamic failure mechanism, Lamb waves in one-dimensional hexagonal quasi-crystal nano plates are investigated. Based on the Bak model, dynamic equations of Lamb waves are derived in the context of the modified couple stress theory. The Legendre orthogonal polynomial method is employed to solve the dynamic equations, and dispersion curves and displacement distributions are calculated. The size effect in phonon and phason fields, and the phonon-phason coupling effect on the wave characteristics are studied. The results show that the size effect increases the phase velocities of phonon modes and phason modes; the phonon-phason coupling effect significantly increases the amplitudes of phason displacements in the phonon modes and the amplitudes of phonon displacements in the phason modes. The results lay a theoretical foundation for the design and nondestructive testing of quasi-crystal nano structures.
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Keywords:
- quasi-crystal /
- Lamb wave /
- size effect /
- phonon-phason coupling effect /
- displacement distribution
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SHECHTMAN等[1]在1984年发现了一类不同于晶体和非晶体的准周期长程平移对称性和非晶体旋转对称性[2]的新固体物质,即准晶(quasi-crystal,简称为QC),促进了物理学、晶体学的重大发展。由于其奇特的内部结构,准晶材料具有许多优异的性能,如低摩擦系数、低附着力、高耐磨性、高耐腐性和高硬度等[3-4],在表面涂层材料、薄膜、增强复合材料等领域具有广泛的应用前景[5]。
室温下块体准晶呈现高脆性,极易产生裂纹等缺陷[6],给大体积块状准晶的制备带来了很大的挑战,在一定程度上限制了准晶作为结构材料的工程应用。随着纳米材料制备技术的不断发展,国内外学者先后制备出准晶纳米颗粒增强材料和准晶层状纳米结构[7-8],弥补了准晶高脆性的缺点,并成功应用于发动机的隔热部件[9]。准晶纳米材料(quasi-crystal nano material)具有颗粒尺寸小、表面能高等一系列的特点,因而表现出特有的尺寸效应,并对其力学性能产生重要影响。实验表明:与传统的纳米晶体相比,准晶纳米结构具有更高的硬度、弹性模量、抗疲劳强度和抗拉强度[9-10]。因此,准晶纳米材料及其结构受到包括力学工作者在内的众多学者的关注。
准晶纳米结构的力学行为研究对其工程应用具有十分重要的意义。然而,当结构尺寸达到微米或纳米级时,尺寸效应将严重影响结构的静态和动态性能[11]。为了准确地分析其力学行为,需采用考虑尺寸效应的非经典连续介质力学理论,如修正的偶应力理论[12]、非局部理论[13]、非局部应变梯度理论[14]和表面效应理论[15]。基于修正的偶应力理论,LI和XIAO[16]研究了压电准晶纳米梁结构的自由振动。基于非局部理论,HUANG等[17]研究了多层准晶纳米圆板在初始应力和纳米尺度相互作用下的响应。基于非局部应变梯度理论,LI等[18]建立了多层二维准晶纳米板的响应模型,揭示了尺寸参数和尺寸效应的影响规律。基于表面效应理论,ZHAO和GUO[19]研究了层状压电准晶结构中增强纳米椭圆孔的反平面剪切问题。
目前,准晶纳米结构的研究主要集中在静力学方面,动力学方面的研究相对较少,尤其是波动问题。随着准晶纳米结构研究的不断深入,其动力学问题也引起越来越多的关注。ZHANG等[20]基于修正的偶应力理论,研究了简支多层二维十方准晶纳米板的三维弯曲变形和振动响应。基于非局部理论,WAKSMANSKI等[21]和LI等[22]分别研究了一维和二维准晶纳米板的自由振动。HUANG等[23]基于非局部理论和表面弹性理论,建立了准晶纳米圆盘致动器在机电载荷作用下的动力学模型。
准晶纳米结构在工程中经常承受以弹性波为代表的动态工作载荷,研究其波动特性,可以进一步了解其动态失效机制。基于此,基于修正的偶应力理论,推导出Bak模型下Lamb波的波动控制方程,使用勒让德正交多项式方法[24-25]研究了一维六方准晶纳米板中Lamb波的波动特性。揭示了尺寸效应、声-相耦合效应对准晶纳米板中Lamb波特性的影响规律。
1 问题描述及公式推导
考虑一个水平无限大的一维六方准晶纳米板,板厚为h,如图1所示。假设准周期方向沿板厚方向,导波沿x方向传播。假设其上下表面应力自由。
Bak模型下声子场和相位子场的控制方程[26]为:
\begin{split} & \frac{{\partial {{\boldsymbol{T}}_{xx}}}}{{\partial x}} + \frac{{\partial {{\boldsymbol{T}}_{{\textit{z}}x}}}}{{\partial {\textit{z}}}} = \rho \frac{{{\partial ^2}{{\boldsymbol{u}}_x}}}{{\partial {t^2}}} ,\\& \frac{{\partial {{\boldsymbol{T}}_{x{\textit{z}}}}}}{{\partial x}} + \frac{{\partial {{\boldsymbol{T}}_{{\textit{z}}{\textit{z}}}}}}{{\partial {\textit{z}}}} = \rho \frac{{{\partial ^2}{{\boldsymbol{u}}_{\textit{z}}}}}{{\partial {t^2}}} ,\\& \frac{{\partial {{\boldsymbol{H}}_{{\textit{z}}x}}}}{{\partial x}} + \frac{{\partial {{\boldsymbol{H}}_{{\textit{z}}{\textit{z}}}}}}{{\partial {\textit{z}}}} = \rho \frac{{{\partial ^2}{{\boldsymbol{w}}_{\textit{z}}}}}{{\partial {t^2}}} \end{split} (1) 式中:ui和wz分别为声子和相位子位移;Tij和Hij分别为声子和相位子应力;ρ为材料密度。
基于修正的偶应力理论和准晶的线弹性理论[12, 26],不考虑体力,一维六方准晶偶应力板的本构方程可以写为:
\begin{split} & {{\boldsymbol{T}}_{xx}} = \left({C_{11}}\frac{{\partial {{\boldsymbol{u}}_x}}}{{\partial x}} + {C_{13}}\frac{{\partial {{\boldsymbol{u}}_{\textit{z}}}}}{{\partial {\textit{z}}}} + {R_1}\frac{{\partial {{\boldsymbol{w}}_{\textit{z}}}}}{{\partial {\textit{z}}}}\right){\textit{π}} \left({\textit{z}}\right), \\[-2pt]& {{\boldsymbol{T}}_{{\textit{z}}x}} = \left({C_{55}}\left(\frac{{\partial {{\boldsymbol{u}}_x}}}{{\partial {\textit{z}}}} + \frac{{\partial {{\boldsymbol{u}}_{\textit{z}}}}}{{\partial x}}\right) + {R_3}\frac{{\partial {{\boldsymbol{w}}_{\textit{z}}}}}{{\partial x}}\right){\textit{π}} \left({\textit{z}}\right) - \frac{1}{2}\left(\frac{{\partial {{\boldsymbol{M}}_{xy}}}}{{\partial x}} + \frac{{\partial {{\boldsymbol{M}}_{{\textit{z}}y}}}}{{\partial {\textit{z}}}}\right) ,\\[-2pt]& {{\boldsymbol{T}}_{x{\textit{z}}}} = \left({C_{55}}\left(\frac{{\partial {{\boldsymbol{u}}_x}}}{{\partial {\textit{z}}}} + \frac{{\partial {{\boldsymbol{u}}_{\textit{z}}}}}{{\partial x}}\right) + {R_3}\frac{{\partial {{\boldsymbol{w}}_{\textit{z}}}}}{{\partial x}}\right){\textit{π}} \left({\textit{z}}\right) + \frac{1}{2}\left(\frac{{\partial {{\boldsymbol{M}}_{xy}}}}{{\partial x}} + \frac{{\partial {{\boldsymbol{M}}_{{\textit{z}}y}}}}{{\partial {\textit{z}}}}\right) ,\\[-2pt]& {{\boldsymbol{T}}_{{\textit{z}}{\textit{z}}}} = \left({C_{13}}\frac{{\partial {{\boldsymbol{u}}_x}}}{{\partial x}} + {C_{33}}\frac{{\partial {{\boldsymbol{u}}_{\textit{z}}}}}{{\partial {\textit{z}}}} + {R_2}\frac{{\partial {{\boldsymbol{w}}_{\textit{z}}}}}{{\partial {\textit{z}}}}\right){\textit{π}} \left({\textit{z}}\right), \\[-2pt]& {{\boldsymbol{H}}_{{\textit{z}}x}} = \left({R_3}\left(\frac{{\partial {{\boldsymbol{u}}_x}}}{{\partial {\textit{z}}}} + \frac{{\partial {{\boldsymbol{u}}_{\textit{z}}}}}{{\partial x}}\right) + {K_2}\frac{{\partial {{\boldsymbol{w}}_{\textit{z}}}}}{{\partial x}}\right){\textit{π}} \left({\textit{z}}\right) - \frac{1}{2}\left(\frac{{\partial {{\boldsymbol{N}}_{xy}}}}{{\partial x}} + \frac{{\partial {{\boldsymbol{N}}_{{\textit{z}}y}}}}{{\partial {\textit{z}}}}\right) ,\\[-2pt]& {{\boldsymbol{H}}_{{\textit{z}}{\textit{z}}}} = \left({R_1}\frac{{\partial {{\boldsymbol{u}}_x}}}{{\partial x}} + {R_2}\frac{{\partial {{\boldsymbol{u}}_{\textit{z}}}}}{{\partial {\textit{z}}}} + {K_1}\frac{{\partial {{\boldsymbol{w}}_{\textit{z}}}}}{{\partial {\textit{z}}}}\right){\textit{π}} \left({\textit{z}}\right) \end{split} (2) \begin{split} & {{\boldsymbol{M}}_{xy}} = G{l_1^2}\left(\frac{{\partial {{\boldsymbol{\theta}}_x}}}{{\partial y}} + \frac{{\partial {{\boldsymbol{\theta}}_y}}}{{\partial x}}\right){\textit{π}} \left({\textit{z}}\right),\;{{\boldsymbol{M}}_{{\textit{z}}y}} = G{l_1^2}\left(\frac{{\partial {{\boldsymbol{\theta}}_{\textit{z}}}}}{{\partial y}} + \frac{{\partial {{\boldsymbol{\theta}}_y}}}{{\partial {\textit{z}}}}\right){\textit{π}} \left({\textit{z}}\right) \\& {{\boldsymbol{N}}_{xy}} = K{l_2^2}\left(\frac{{\partial {{\bf\textit{φ}}_x}}}{{\partial y}} + \frac{{\partial {{\bf\textit{φ}}_y}}}{{\partial x}}\right){\textit{π}} \left({\textit{z}}\right),\;{{\boldsymbol{N}}_{{\textit{z}}y}} = K{l_2^2}\left(\frac{{\partial {{\bf\textit{φ}}_{\textit{z}}}}}{{\partial y}} + \frac{{\partial {{\bf\textit{φ}}_y}}}{{\partial {\textit{z}}}}\right){\textit{π}} \left({\textit{z}}\right), \end{split} (3) 式中:Mij和Nij分别为声子场和相位子场偶应力张量;Cij、Ri和Ki分别为声子场弹性常数、声-相耦合系数和相位子场弹性常数;偶应力理论引入了与微观结构相关的材料参数,即弯扭模量,弯扭模量与剪切模量之比的平方根l反映了偶应力固体中的尺寸效应,l被称为材料长度尺寸参数,l1和l2分别为声子场和相位子场材料长度尺寸参数;G为平均剪切弹性模量,G=(C44+C55+C66)/3;K=K2;θi和φi分别为声子场和相位子场旋转矢量,θi和φi分别取决于声子位移和相位子位移,即:
\begin{split} & {{\boldsymbol{\theta}}_x} = 0.5({{\boldsymbol{u}}_{{\textit{z}},y}} - {{\boldsymbol{u}}_{y,{\textit{z}}}}),\quad {{\bf\textit{φ}}_x} = 0.5({{\boldsymbol{w}}_{{\textit{z}},y}} - {{\boldsymbol{w}}_{y,{\textit{z}}}}), \\& {{\boldsymbol{\theta}}_y} = 0.5({{\boldsymbol{u}}_{x,{\textit{z}}}} - {{\boldsymbol{u}}_{{\textit{z}},x}}),\quad {{\bf\textit{φ}}_y} = 0.5({{\boldsymbol{w}}_{x,{\textit{z}}}} - {{\boldsymbol{w}}_{{\textit{z}},x}}) ,\\& {{\boldsymbol{\theta}}_{\textit{z}}} = 0.5({{\boldsymbol{u}}_{y,x}} - {{\boldsymbol{u}}_{x,y}}),\quad {{\bf\textit{φ}}_x} = 0.5({{\boldsymbol{w}}_{y,x}} - {{\boldsymbol{w}}_{x,y}}) \end{split} (4) 式中下标中的逗号表示偏导。
在式(2)、式(3)中引入由Heaviside函数组成的矩形窗口函数π(z)[27],即:
{\textit{π}} \left( {\textit{z}} \right) = H\left( {\textit{z}} \right) - H\left( 0 \right) = \left\{ \begin{aligned} & 1,\;\;{0 {\leqslant} {\textit{z}} {\leqslant} h} \\& 0,\;\;{{\text{其他}}} \end{aligned} \right. (5) 从而将上下表面应力自由的边界条件(在z=0和z=h,{{\boldsymbol{T}}_{{\textit{z}}x}} = {{\boldsymbol{T}}_{{\textit{z}}{\textit{z}}}} = {{\boldsymbol{H}}_{{\textit{z}}{\textit{z}}}} = {{\boldsymbol{M}}_{{\textit{z}}x}} = {{\boldsymbol{N}}_{{\textit{z}}y}} = 0)代入到本构方程。因此,在下面的计算中无需考虑边界条件。这种处理边界条件方法的正确性已经得到详细论证[27]。
对于沿x方向传播的Lamb波,声子和相位子位移分量的谐波解为:
\begin{split} & {{\boldsymbol{u}}_x} = U({\textit{z}}){{\rm{e}}^{{\rm{i}}kx - {\rm{i}}\omega t}},\;{{\boldsymbol{u}}_{\textit{z}}} = W({\textit{z}}){{\rm{e}}^{{\rm{i}}kx - {\rm{i}}\omega t}}, \\& {{\boldsymbol{w}}_{\textit{z}}} = \gamma ({\textit{z}}){{\rm{e}}^{{\rm{i}}kx - {\rm{i}}\omega t}},\;{{\boldsymbol{u}}_y} = {{\boldsymbol{w}}_x} = {{\boldsymbol{w}}_y} = 0 \end{split} (6) 式中:U(z)、W(z)和 \gamma ({\textit{z}}) 分别为x、z方向的声子位移振幅和z方向的相位子位移振幅;k为波数;ω为角频率。
将式(2)~式(4)和式(6)代入式(1),可得如下的波动微分控制方程:
\begin{split} & \{ {\textit{π}} ({\textit{z}})[ - {k^2}{C_{11}}U({\textit{z}}) + ({C_{55}} + 0.25{k^2}l_1^2G)U''({\textit{z}}) - 0.25 \cdot \\& l_1^2GU''''({\textit{z}}) + {\rm{i}}k({C_{13}} + {C_{55}} - 0.25l_1^2G{k^2})W'({\textit{z}}) + 0.25 \cdot \\& {\rm{i}}kl_1^2GW'''({\textit{z}}) + {\rm{i}}k({R_3} + {R_1})\gamma '({\textit{z}})] + {\textit{π}} '({\textit{z}})[({C_{55}} + 0.25 \cdot \\& {k^2}l_1^2G)U'({\textit{z}}) - 0.5l_1^2GU'''({\textit{z}}) + {\rm{i}}k(({C_{55}} - 0.25{k^2}l_1^2G) \cdot \\& W({\textit{z}}) + 0.5l_1^2GW''({\textit{z}}) + {R_3}\gamma ({\textit{z}}))] + {\textit{π}} ''({\textit{z}})( - 0.25l_1^2G \cdot \\& U''({\textit{z}}) + 0.25{\rm{i}}kl_1^2GW'({\textit{z}}))\} = - \rho {\omega ^2}U({\textit{z}}) \end{split} (7) \begin{split} & \{ {\textit{π}} ({\textit{z}})[{\rm{i}}k({C_{13}} + {C_{55}} - 0.25{k^2}l_1^2G)U'({\textit{z}}) + 0.25{\rm{i}}kl_1^2\cdot \\& GU'''({\textit{z}}) - {k^2}({C_{55}} + 0.25{k^2}l_1^2G)W({\textit{z}}) + ({C_{33}} + \\& 0.25{k^2}l_1^2G)W''({\textit{z}}) - {k^2}{R_3}\gamma ({\textit{z}}) + {R_2}\gamma ''({\textit{z}})] + {\textit{π}} '({\textit{z}}) \cdot\\& [{\rm{i}}k({C_{13}}U({\textit{z}}) + 0.25l_1^2GU''({\textit{z}})) + ({C_{33}} + 0.25{k^2}l_1^2 \cdot\\& G)W'({\textit{z}}) + {R_2}\gamma '({\textit{z}})]\} = - \rho {\omega ^2}W({\textit{z}}) \end{split} (8) \begin{split} & \{ {\textit{π}} ({\textit{z}})[{\rm{i}}k({R_1} + {R_3})U'({\textit{z}}) - {k^2}{R_3}W({\textit{z}}) + {R_2}W''({\textit{z}}) -\\& {k^2}({K_2} - 0.25{k^2}l_2^2K)\gamma ({\textit{z}}) + {K_1}\gamma ''({\textit{z}}) - 0.25 \cdot\\& {k^2}l_2^2K\gamma ''({\textit{z}})] + {\textit{π}} '({\textit{z}})[{\rm{i}}k{R_1}U({\textit{z}}) + {R_2}W'({\textit{z}}) + \\& {K_1}\gamma '({\textit{z}}) - 0.25{k^2}l_2^2K\gamma '({\textit{z}})]\} = - \rho {\omega ^2}\gamma ({\textit{z}}) \end{split} (9) 式中,上角标(' )为对z的求导。
将U(z)、W(z)和γ(z)展开成勒让德多项式级数的形式:
\begin{split} & U\left( {\textit{z}} \right) = \sum\limits_{m = 0}^\infty {p_m^1} {Q_m}\left( {\textit{z}} \right),W\left( {\textit{z}} \right) = \sum\limits_{m = 0}^\infty {p_m^2} {Q_m}\left( {\textit{z}} \right), \\& \gamma \left( {\textit{z}} \right) = \sum\limits_{m = 0}^\infty {r_m^{}} {Q_m}\left( {\textit{z}} \right) \end{split} (10) 式中: p_m^i\left( {i = 1,2} \right) 和 r_m^{} 为待定系数;{Q_m}\left( {\textit{z}} \right) = \sqrt {\dfrac{{2m + 1}}{h}}\cdot {P_m}\left( {\dfrac{{2{\textit{z}} - h}}{h}} \right)中 {P_m} 为第m阶勒让德多项式。理论上,m取值为0~∞;但实际上,当m取到有限值M时,多项式已经收敛。
式(7)~式(9)左右两边同乘 {Q_j}({\textit{z}}) ,j取从0~M。然后,利用勒让德多项式的正交性对z从0~h进行积分,得到:
A_{11}^{j,m}p_m^1 + A_{12}^{j,m}p_m^2 + A_{13}^{j,m}{r_m} = - {\omega ^2}M_m^jp_m^1 (11) A_{21}^{j,m}p_m^1 + A_{22}^{j,m}p_m^2 + A_{23}^{j,m}{r_m} = - {\omega ^2}M_m^jp_m^2 (12) A_{31}^{j,m}p_m^1 + A_{32}^{j,m}p_m^2 + A_{33}^{j,m}{r_m} = - {\omega ^2}M_m^jr_m^{} (13) 将式(11)~式(13)写成矩阵形式,可得:
\begin{split} & \left[ {\begin{matrix} {A_{11}^{j,m}}&{A_{12}^{j,m}}&{A_{13}^{j,m}} \\ {A_{21}^{j,m}}&{A_{22}^{j,m}}&{A_{23}^{j,m}} \\ {A_{31}^{j,m}}&{A_{32}^{j,m}}&{A_{33}^{j,m}} \end{matrix}} \right]\left[ {\begin{matrix} {p_m^1} \\ {p_m^2} \\ {{r_m}} \end{matrix}} \right] = {\omega ^2}\left[ {\begin{matrix} {M_m^j}&0&0 \\ 0&{M_m^j}&0 \\ 0&0&{M_m^j} \end{matrix}} \right]\left[ {\begin{matrix} {p_m^1} \\ {p_m^2} \\ {{r_m}} \end{matrix}} \right] \end{split} (14) 式中, A_{\alpha \beta }^{j,m} (α, β=1, 2, 3)和 M_m^j 为非对称矩阵中的元素,可根据式(7)~式(9)得到,详见附录。
式(14)转化为待求解的矩阵特征值问题,特征值为ω2。声子和相位子位移分量可由特征向量计算得到。由Vph=ω/k可得到其相速度。
2 数值结果及讨论
根据第1节理论推导,使用Mathemathica软件编写相应的计算机程序,计算得到了一维六方准晶纳米板中Lamb波的频散曲线和位移分布。一维六方准晶纳米板的材料参数[28]如表1所示。微观结构尺寸参数的测量还未建立统一的标准规范,确定材料长度尺度参数的相关实验数据很少。因此,为了研究偶应力效应的影响,采用尺寸参数l1、l2与物体尺寸(如板厚h)的比值来反映尺寸效应[16, 29-30],这个比值通常分布在0和1之间。因此,定义反映声子场和相位子场尺寸效应的无量纲参数L1和L2,即L1=l1/h,L2=l2/h。除特殊说明,下面的计算中L1和L2同时取0.1。
表 1 一维六方准晶纳米板的材料参数[28]Table 1. Material parameters of one-dimensional hexagonal quasi-crystal nano plate[28]材料参数 数值 材料参数 数值 声子场弹性常数C11/GPa 234.33 声子场弹性常数C13/GPa 66.63 声子场弹性常数C33/GPa 232.22 声子场弹性常数C44/GPa 70.19 声子场弹性常数C55/GPa 70.19 声子场弹性常数C66/GPa 88.46 相位子场弹性常数K1/GPa 122 相位子场弹性常数K2/GPa 24 声-相耦合系数R1/GPa 0.8846 声-相耦合系数R2/GPa 0.8846 声-相耦合系数R3/GPa 0.8846 材料密度ρ(kg·m−3) 4186 2.1 方法验证
目前,鲜见关于一维六方准晶偶应力板中波动特性研究的文献。为了验证计算方法的正确性,假设L1=L2=0.2,Ri=Ki=0,将准晶偶应力板简化为晶体偶应力板,并与文献[29]中氮化铝偶应力板结果进行比较。其中,氮化铝材料的横波波速CT和纵波波速CL分别为6220 m/s和11225 m/s。相应的相速度频散曲线如图2所示。从图2可以看到,本文的结果和文献[29]的结果一致。
2.2 收敛性分析
众所周知,勒让德正交多项式方法的收敛性取决于其阶数M。为此,计算了不同M时前2阶声子模态和前2阶相位子模态的相速度Vph,结果如表2所示。如果Vph随M的增加不再变化,可以认为结果是收敛的。从表2可以看出,当M取12时所有的Vph已经收敛。此外,高频的收敛速度比低频慢。从表2可以得到,至少前M/3阶已经收敛。为了保证下面的算例结果都是收敛的,本文M=20。
2.3 Lamb波频散曲线
首先,研究了声-相耦合效应对Lamb波波动特性的影响,图3给出了准晶偶应力板和相应晶体偶应力板中相速度频散曲线(Ri=Ki=0,其他参数不变)。可以看到,晶体板中点线和准晶板中一些曲线几乎重合。主要由于准晶中的声子场与晶体中的弹性场在动态变形中扮演着相似的角色。与晶体结构中弹性模态波动特性类似的模态被称为声子模态,定义为A0,S0···。与晶体偶应力板不同的是,由于相位子场的存在,准晶偶应力板中出现了另一类模态,这类模态称为相位子模态,定义为P0,P1···。
表 2 前2阶声子模态和前2阶相位子模态的相速度VphTable 2. Phase velocity Vph of the first two phonon modes and the first two phason modes/(km·s−1) 模态 无量纲波数kh M=9 M=10 M=11 M=12 P0 1 2.397 11 2.397 11 2.397 11 2.397 11 5 2.467 36 2.467 36 2.467 36 2.467 36 10 2.676 49 2.676 49 2.676 49 2.676 49 P1 1 17.132 30 17.132 30 17.132 30 17.132 30 5 4.211 32 4.211 32 4.211 32 4.211 32 10 3.190 84 3.190 84 3.190 84 3.190 84 A0 1 1.815 79 1.815 78 1.815 78 1.815 78 5 3.785 42 3.785 31 3.785 30 3.785 30 10 4.498 43 4.498 42 4.498 40 4.498 40 S0 1 7.144 18 7.144 18 7.144 18 7.144 18 5 5.181 13 5.1810 5.181 00 5.181 00 10 4.851 47 4.851 18 4.851 10 4.851 10 从图3中可以看到,点线和声子模态几乎完全重合,主要是由于表1中声-相耦合常数Ri远远小于声子场和相位子场的弹性常数。然后,研究了声-相耦合效应对准晶偶应力板中相速度的影响。当Ri增大时,图4给出了kh=3时前3阶模态相速度变化曲线。其他模态曲线类似,这里不再给出。可以看到,随着声-相耦合效应的增大,声子模态相速度值增大,而相位子模态相速度值减小。
接下来,研究了尺寸效应对频散曲线的影响。为了研究声子场尺寸效应对频散曲线的影响,当L2=0.1,L1变化时,图5给出了准晶偶应力板中频散曲线。可以看到,声子场尺寸效应使声子和相位子模态的相速度增大。这是因为修正的偶应力理论中引入了一个旋转梯度张量[30],使得准晶偶应力板的刚度增大。此外,声子场尺寸效应对声子模态的影响远远大于相位子模态。
然后,研究了相位子场尺寸效应对频散曲线的影响,当L1=0.1,L2变化时,图6给出了相位子场尺寸效应对频散曲线的影响。与图5一样,相位子场尺寸效应也增大了声子和相位子模态的相速度。此外,与图5相反的是,相位子场尺寸效应对声子模态的影响远远小于相位子模态。我们推测这种现象是由弱声-相耦合效应引起的,即声-相耦合系数Ri远小于声子场弹性常数Cij和相位子场弹性常数Ki。
为了验证上述推测,将Ri增大到10 Ri,图7(a)给出了L1变化时P0模态的频散曲线,图7(b)给出了L2变化时A0模态的频散曲线。对比图5(b)和图6(b),可以看到,声子场尺寸效应对P0模态的影响和相位子场尺寸效应对A0模态的影响都得到增强。
2.4 位移分布
本节研究了声-相耦合效应对位移分布的影响。当kh=2时,图8和图9分别给出了声-相耦合系数Ri变化时P0和A0模态中声子和相位子位移分量的分布曲线。其他模态也有类似的变化,这里不再给出。可以看到,对于P0模态,随着声-相耦合系数的增大,声子位移振幅U和W增大,相位子位移振幅 \gamma 减小。而对于A0模态,声-相耦合效应的影响正好相反。因为随着声-相耦合系数增大,声子场和相位子场的耦合效应增强,声子模态中相位子位移振幅和相位子模态中声子位移振幅增大。根据能量守恒定律,声子模态中声子位移振幅和相位子模态中相位子位移振幅减小。此外,声-相耦合效应对P0模态中声子位移振幅的影响远大于相位子位移振幅,对A0模态中相位子位移振幅的影响远大于声子位移振幅。
2.5 应力分布
最后,为了验证矩形窗口函数π(z)处理边界条件的正确性,图10给出了当kh=2时A0模态中声子和相位子应力分布。可以看到,当z=0和z=h时,{{\boldsymbol{T}}_{{\textit{z}}x}} = {{\boldsymbol{T}}_{{\textit{z}}{\textit{z}}}} = {{\boldsymbol{H}}_{{\textit{z}}{\textit{z}}}} = 0,满足上下表面应力自由的边界条件。
3 结论
基于修正的偶应力理论和准晶的线弹性理论,研究了一维六方准晶纳米板中Lamb波的波动特性。与文献[29]结果对比,验证了计算结果的正确性。研究了声子场和相位子场尺寸效应、声-相耦合效应对波动特性的影响。根据数值算例,可以得到以下结论:
(1)声子场尺寸效应和相位子场尺寸效应都使声子和相位子模态相速度增大。但弱声-相耦合效应导致声子场尺寸效应对相位子模态的影响和相位子场尺寸效应对声子模态的影响十分微弱。
(2)声子模态的相速度随着声-相耦合效应增大而增大,相位子模态的相速度随着声-相耦合效应增大而减小。
(3)随着声-相耦合效应增大,相位子模态中声子位移振幅显著增大,相位子位移振幅略微减小。而对于声子模态,声-相耦合效应对声子位移和相位子位移振幅的影响正好相反。
附录:
\small \begin{split} & A_{11}^{m,j} = - {k^2} \times {C_{11}} \times u[m,0,0,j] + {C_{55}} \times (K[m,0,1,j] + u[m,0,2,j]) + 0.25 \times {L_1^2} \times G \times ({k^2} \times u[m,0,2,j] - u[m,0,4,j] - \\&\qquad 2 \times K[m,0,3,j] - Y[m,0,2,j] + {k^2} \times K[m,0,1,j]),\\& A_{12}^{m,j} = i \times k \times ({C_{55}} \times (K[m,0,0,j] + u[m,0,1,j]) + {C_{13}} \times u[m,0,1,j] + 0.25 \times {L_1^2} \times G \times (u[m,0,3,j] - {k^2} \times\\&\qquad u[m,0,1,j] + 2 \times K[m,0,2,j] + Y[m,0,1,j] - {k^2} \times K[m,0,0,j]) ,\\& A_{13}^{m,j} = i \times k \times ({R_1} \times u[m,0,1,j] + {R_3} \times (u[m,0,1,j] + K[m,0,0,j])) ,\\& A_{21}^{m,j} = i \times k \times ({C_{13}} \times (u[m,0,1,j] + K[m,0,0,j]) + {C_{55}} \times u[m,0,1,j] - 0.25 \times {L_1}^2 \times G \times ({k^2} \times u[m,0,1,j] - u[m,0,3,j] - K[m,0,2,j])), \\& A_{22}^{m,j} = - {k^2} \times {C_{55}} \times u[m,0,0,j] + {C_{33}} \times (K[m,0,1,j] + u[m,0,2,j]) - 0.25 \times {k^2} \times {L_1}^2 \times G \times ({k^2} \times u[m,0,0,j] - u[m,0,2,j] - K[m,0,1,j]) ,\\ & A_{23}^{m,j} = - {k^2} \times {R_3} \times u[m,0,0,j] + {R_2} \times (u[m,0,2,j] + K[m,0,1,j]), \\& A_{31}^{m,j} = i \times k \times ({R_3} \times u[m,0,1,j] + {R_1} \times (u[m,0,1,j] + K[m,0,0,j])), \\& A_{32}^{m,j} = - {k^2} \times {R_3} \times u[m,0,0,j] + {R_2} \times (u[m,0,2,j] + K[m,0,1,j]) ,\\& A_{33}^{m,j} = - {k^2} \times {K_2} \times u[m,0,0,j] + {K_1} \times (u[m,0,2,j] + K[m,0,1,j]) - 0.25 \times {k^2} \times {L_2}^2 \times K \times ({k^2} \times u[m,0,0,j] - u[m,0,2,j] - K[m,0,1,j]) ,\\& {M_{m,j}} = - \rho \times u[m,0,0,j]。 \end{split} 其中:
\small \begin{split} & u[m,i,l,j] = \int_a^b {{\textit{π}} ({\textit{z}}){{\textit{z}}^i}{Q_m}({\textit{z}})\frac{{{\partial ^l}{Q_j}({\textit{z}})}}{{\partial {{\textit{z}}^l}}}{\rm{d}}{\textit{z}}} ,\\& K[m,i,l,j] = \int_a^b {{{\textit{z}}^i}{Q_m}({\textit{z}})\frac{{\partial {\textit{π}} ({\textit{z}})}}{{\partial {\textit{z}}}}\frac{{{\partial ^l}{Q_j}({\textit{z}})}}{{\partial {{\textit{z}}^l}}}{\rm{d}}{\textit{z}}} ,\\& Y[m,i,l,j] = \int_a^b {{{\textit{z}}^i}{Q_m}({\textit{z}})\frac{{{\partial ^2}{\textit{π}} ({\textit{z}})}}{{\partial {{\textit{z}}^2}}}\frac{{{\partial ^l}{Q_j}({\textit{z}})}}{{\partial {{\textit{z}}^l}}}{\rm{d}}{\textit{z}}} 。 \end{split} -
表 1 一维六方准晶纳米板的材料参数[28]
Table 1 Material parameters of one-dimensional hexagonal quasi-crystal nano plate[28]
材料参数 数值 材料参数 数值 声子场弹性常数C11/GPa 234.33 声子场弹性常数C13/GPa 66.63 声子场弹性常数C33/GPa 232.22 声子场弹性常数C44/GPa 70.19 声子场弹性常数C55/GPa 70.19 声子场弹性常数C66/GPa 88.46 相位子场弹性常数K1/GPa 122 相位子场弹性常数K2/GPa 24 声-相耦合系数R1/GPa 0.8846 声-相耦合系数R2/GPa 0.8846 声-相耦合系数R3/GPa 0.8846 材料密度ρ(kg·m−3) 4186 表 2 前2阶声子模态和前2阶相位子模态的相速度Vph
Table 2 Phase velocity Vph of the first two phonon modes and the first two phason modes
/(km·s−1) 模态 无量纲波数kh M=9 M=10 M=11 M=12 P0 1 2.397 11 2.397 11 2.397 11 2.397 11 5 2.467 36 2.467 36 2.467 36 2.467 36 10 2.676 49 2.676 49 2.676 49 2.676 49 P1 1 17.132 30 17.132 30 17.132 30 17.132 30 5 4.211 32 4.211 32 4.211 32 4.211 32 10 3.190 84 3.190 84 3.190 84 3.190 84 A0 1 1.815 79 1.815 78 1.815 78 1.815 78 5 3.785 42 3.785 31 3.785 30 3.785 30 10 4.498 43 4.498 42 4.498 40 4.498 40 S0 1 7.144 18 7.144 18 7.144 18 7.144 18 5 5.181 13 5.1810 5.181 00 5.181 00 10 4.851 47 4.851 18 4.851 10 4.851 10 -
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