高压物理学报   2018, Vol. 32 Issue (5): 054102.  DOI: 10.11858/gywlxb.20180502.

研究论文

引用本文 [复制中英文]

周佳华, 杨强, 韩志军, 路国运. 轴向荷载下功能梯度材料圆柱壳的动力屈曲[J]. 高压物理学报, 2018, 32(5): 054102. DOI: 10.11858/gywlxb.20180502.
[复制中文]
ZHOU Jiahua, YANG Qiang, HAN Zhijun, LU Guoyun. Dynamic Buckling of Functionally Graded Cylindrical Shells under Axial Loading[J]. Chinese Journal of High Pressure Physics, 2018, 32(5): 054102. DOI: 10.11858/gywlxb.20180502.
[复制英文]

基金项目

国家自然科学基金(11372209)

作者简介

周佳华(1992—), 女, 硕士研究生, 主要从事非线性动力屈曲研究. E-mail: 18234133316@163.com

通信作者

杨强(1962—), 男, 博士, 副教授, 主要从事非线性动力屈曲研究. E-mail: yangqiang62@126.com

文章历史

收稿日期:2018-01-08
修回日期:2018-05-22
轴向荷载下功能梯度材料圆柱壳的动力屈曲
周佳华 1, 杨强 1, 韩志军 1, 路国运 2     
( 1. 太原理工大学力学学院, 山西 太原 030024
2. 太原理工大学建筑与土木工程学院, 山西 太原 030024 )
摘要:基于Donnell壳体理论和经典板壳理论,利用Hamilton变分原理得到轴向荷载作用下材料属性呈幂律分布的功能梯度材料圆柱壳的动力屈曲控制方程。根据圆柱壳周向连续性设出径向位移的函数表达,利用分离变量法求解得到功能梯度材料圆柱壳在轴向荷载作用下的动力屈曲临界荷载的解析表达式和屈曲解。利用MATLAB软件编程计算,讨论了径厚比、梯度指数、环向模态数、轴向模态数等对功能梯度材料圆柱壳动力屈曲临界荷载的影响。结果表明:圆柱壳的临界荷载随临界长度的增加而减小;冲击端为夹支的临界荷载比冲击端为简支的临界荷载大,说明约束条件对临界荷载有较大影响;圆柱壳的临界荷载随着模态数的增加而增大,说明临界荷载越大,高阶模态越易被激发;屈曲模态图随着模态数的增加而复杂化。
关键词功能梯度材料    屈曲临界荷载    圆柱壳    Hamilton原理    模态数    

功能梯度材料(Functionally Graded Materials, FGM)的概念是1984年在航空飞机计划中首次提出的[1],FGM的特性在于它的组成和结构随着体积的变化而变化,从而导致材料相应性质发生改变。因其材料特性呈幂律分布[2-3],FGM被广泛应用于工程领域,如航空航天、机械工程、生物医学等。圆柱壳在联合荷载作用下的屈曲分析备受学术界关注[4-5]。目前,对FGM板壳的研究较为深入[6-8]。Beni等[9]利用改进的偶应力理论,对FGM圆柱壳在不同边界条件下的动力屈曲进行了分析;Kargarnovin等[10]研究了轴向荷载作用下FGM圆柱壳的动力屈曲;Sofiyev等[11]研究了横向压力下功能梯度正交各向异性圆柱壳的动力屈曲,推导出基于一阶剪切变形理论的功能梯度正交各向异性圆柱壳的稳定性和相容性方程;Khazaeinejad等[12]研究了弹性模量在厚度方向上连续变化的FGM圆柱壳在复合外压和轴向压缩载荷作用下的动力屈曲;Khalili等[13]研究了横向冲击载荷作用下FGM圆柱壳的动力屈曲;Alashti等[14]对变厚度FGM圆柱壳外压和轴向压缩的动力屈曲问题进行了分析。

基于以上研究,本研究讨论了FGM圆柱壳在轴向荷载作用下的动力屈曲。根据Donnell壳体理论和经典板壳理论,利用Hamilton变分原理得到FGM圆柱壳的动力屈曲控制方程;采用分离变量法求得动力屈曲临界荷载表达式;通过MATLAB软件计算动力屈曲临界荷载,讨论由不同材料(陶瓷和钛、陶瓷和铁、陶瓷和铜)组成的FGM圆柱壳的径厚比(R/h)、梯度指数(k)、环向模态数(m)、轴向模态数(n)等对临界荷载的影响。

1 FGM圆柱壳的屈曲控制方程

图 1所示,FGM圆柱壳长度为l,半径为R,总厚度为h,选取柱坐标系(x, θ, z),其相应位移为(u, v, w)。FGM的材料属性(弹性模量E、密度ρ、泊松比μ等)呈幂律分布[2-3],表示为

$ P\left( z \right) = \left( {{P_1} - {P_2}} \right){\left( {\frac{{2z + h}}{{2h}}} \right)^k} + {P_2} $ (1)
图 1 圆柱壳坐标系统 Fig.1 Cylindrical shell coordinates

式中:P为物性参数,下标“1”和“2”分别代表组分1和组分2;k为梯度指数,k∈(0, ∞)。圆柱壳内任意点的物性参数为

$ \left\{ \begin{array}{l} E\left( z \right) = \left( {{E_1} - {E_2}} \right){\left( {\frac{{2z + h}}{{2h}}} \right)^k} + {E_2}\\ \rho \left( z \right) = \left( {{\rho _1} - {\rho _2}} \right){\left( {\frac{{2z + h}}{{2h}}} \right)^k} + {\rho _2}\\ \mu \left( z \right) = \left( {{\mu _1} - {\mu _2}} \right){\left( {\frac{{2z + h}}{{2h}}} \right)^k} + {\mu _2} \end{array} \right. $ (2)

根据Donnell壳体理论,圆柱壳的小挠度几何方程为

$ \begin{array}{*{20}{c}} {\left\{ \begin{array}{l} {\varepsilon _x} = \varepsilon _x^0 + z{K_x}\\ {\varepsilon _\theta } = \varepsilon _\theta ^0 + z{K_\theta }\\ {\gamma _{x\theta }} = \gamma _{x\theta }^0 + z{K_{x\theta }} \end{array} \right.,}&{\left\{ \begin{array}{l} u = {u_0} - z\frac{{\partial w}}{{\partial x}}\\ v = {v_0} - z\frac{{\partial w}}{{R\partial \theta }}\\ w = {w_0} \end{array} \right.,}&{\left\{ \begin{array}{l} \varepsilon _x^0 = \frac{{\partial {u_0}}}{{\partial x}}\\ \varepsilon _\theta ^0 = \frac{{\partial {v_0}}}{{R\partial \theta }} - \frac{{{w_0}}}{R}\\ \gamma _{x\theta }^0 = \frac{{\partial {u_0}}}{{R\partial \theta }} - \frac{{\partial {v_0}}}{{\partial x}} \end{array} \right.,}&{\left\{ \begin{array}{l} {K_x} = - \frac{{{\partial ^2}{w_0}}}{{\partial {x^2}}}\\ {K_\theta }{\rm{ = }} - \frac{{{\partial ^2}{w_0}}}{{{R^2}\partial {\theta ^2}}}\\ {K_{x\theta }}{\rm{ = }} - \frac{{2{\partial ^2}{w_0}}}{{R\partial x\partial \theta }} \end{array} \right.} \end{array} $ (3)

式中:ε为正应变,γ为切应变,上、下标“0”表示壳体中面,K为壳体曲率。

根据经典板壳理论,FGM圆柱壳的内力N与内力矩M可表示为

$ \left( {\begin{array}{*{20}{c}} {{N_x}}\\ {{N_\theta }}\\ {{N_{x\theta }}}\\ {{M_x}}\\ {{M_\theta }}\\ {{M_{x\theta }}} \end{array}} \right) = \left( {\begin{array}{*{20}{c}} {{A_{11}}}&{{A_{12}}}&{{A_{16}}}&{{B_{11}}}&{{B_{12}}}&{{B_{16}}}\\ {{A_{21}}}&{{A_{22}}}&{{A_{26}}}&{{B_{21}}}&{{B_{22}}}&{{B_{26}}}\\ {{A_{16}}}&{{A_{26}}}&{{A_{66}}}&{{B_{16}}}&{{B_{26}}}&{{B_{66}}}\\ {{B_{11}}}&{{B_{12}}}&{{B_{16}}}&{{D_{11}}}&{{D_{12}}}&{{D_{16}}}\\ {{B_{21}}}&{{B_{22}}}&{{B_{26}}}&{{D_{21}}}&{{D_{22}}}&{{D_{26}}}\\ {{B_{16}}}&{{B_{26}}}&{{B_{66}}}&{{D_{16}}}&{{D_{26}}}&{{D_{66}}} \end{array}} \right)\left( {\begin{array}{*{20}{c}} {\varepsilon _x^0}\\ {\varepsilon _\theta ^0}\\ {\gamma _{x\theta }^0}\\ {{K_x}}\\ {{K_\theta }}\\ {{K_{x\theta }}} \end{array}} \right) $ (4)

式中:AijBijDij(i, j=1, 2, 6)分别为FGM圆柱壳的拉伸刚度、耦合刚度和弯曲刚度系数矩阵分量。$ {A_{11}} = {A_{22}} = \int_{ - h/2}^{h/2} {\frac{{E\left( z \right)}}{{1 - {\mu ^2}\left( z \right)}}} {\rm{d}}z $, $ {A_{12}} = {A_{21}} = \int_{ - h/2}^{h/2} {\frac{{\mu \left( z \right)E\left( z \right)}}{{1 - {\mu ^2}\left( z \right)}}} {\rm{d}}z $, $ {A_{66}} = \int_{ - h/2}^{h/2} {\frac{{E\left( z \right)}}{{2\left[ {1 + \mu \left( z \right)} \right]}}} {\rm{d}}z $, $ {B_{11}} = {B_{22}} = \int_{ - h/2}^{h/2} {\frac{{E\left( z \right)}}{{1 - {\mu ^2}\left( z \right)}}z\;}{\rm{d}}z $, $ {B_{12}} = {B_{21}} = \int_{ - h/2}^{h/2} {\frac{{\mu \left( z \right)E\left( z \right)}}{{1 - {\mu ^2}\left( z \right)}}z\;} {\rm{d}}z $, $ {B_{66}} = \int_{ - h/2}^{h/2} {\frac{{E\left( z \right)}}{{2\left[ {1 + \mu \left( z \right)} \right]}}z\;} {\rm{d}}z $, $ {D_{11}} = {D_{22}} = \int_{ - h/2}^{h/2} {\frac{{E\left( z \right)}}{{1 - {\mu ^2}\left( z \right)}}{z^2}\;} {\rm{d}}z $, $ {D_{12}} = {D_{21}} = \int_{ - h/2}^{h/2} {\frac{{\mu \left( z \right)E\left( z \right)}}{{1 - {\mu ^2}\left( z \right)}}{z^2}\;} {\rm{d}}z $, $ {D_{66}} = \int_{ - h/2}^{h/2} {\frac{{E\left( z \right)}}{{2\left[ {1 + \mu \left( z \right)} \right]}}{z^2}\;} {\rm{d}}z $。FGM圆柱壳的力学性能为各向同性[15],那么:A16=A26=B16=B26=D16=D26=0。

对于圆柱壳,系统的应变能(不考虑剪力)为

$ U = \frac{1}{2}\int_0^{2{\rm{ \mathsf{ π} }}} {\int_0^l {\left( {{N_x}\varepsilon _x^0 + {N_\theta }\varepsilon _\theta ^0 + {N_{x\theta }}\gamma _{x\theta }^0 + {M_x}{K_x} + {M_\theta }{K_\theta } + {M_{x\theta }}{K_{x\theta }}} \right)} R{\rm{d}}x{\rm{d}}\theta } $ (5)

动能为

$ T = \frac{1}{2}\int_{ - h/2}^{h/2} {\int_0^{2{\rm{ \mathsf{ π} }}} {\int_0^l {\rho \left( z \right)\left[ {{{\left( {\frac{{\partial u}}{{\partial t}}} \right)}^2} + {{\left( {\frac{{\partial v}}{{\partial t}}} \right)}^2} + {{\left( {\frac{{\partial w}}{{\partial t}}} \right)}^2}} \right]R{\rm{d}}x{\rm{d}}\theta {\rm{d}}z} } } $ (6)

外力功为

$ W = \frac{1}{2}\int_0^{2{\rm{ \mathsf{ π} }}} {\int_0^l {N\left( t \right){{\left( {\frac{{\partial {w_0}}}{{\partial x}}} \right)}^2}} R{\rm{d}}x{\rm{d}}\theta } $ (7)

Hamilton变分原理为

$ {\rm{ \mathsf{ δ} }}\int_{{t_2}}^{{t_1}} {\left( {T - U + W} \right){\rm{d}}t} = 0 $ (8)

将(3)式~(7)式代入(8)式中,由Donnell壳体理论可知,圆柱壳内力沿环向均匀分布,忽略中面位移[16],由u0v0w0的变分系数为零,整理得到FGM圆柱壳的动力屈曲控制方程为

$ \begin{array}{l} 4{I_0}\frac{{{\partial ^2}{w_0}}}{{\partial t}} - {I_2}\left( {\frac{{{\partial ^4}{w_0}}}{{{R^2}\partial {\theta ^2}\partial {t^2}}} + \frac{{{\partial ^4}{w_0}}}{{\partial {x^2}\partial {t^2}}}} \right) = - 4{A_{22}}\frac{{{w_0}}}{{{R^2}}} - 4\frac{{{B_{12}}}}{{{R^2}}}\frac{{{\partial ^2}{w_0}}}{{\partial {x^2}}} - 4\frac{{{B_{22}}}}{R}\frac{{{\partial ^2}{w_0}}}{{{R^2}\partial {\theta ^2}}} - {D_{11}}\frac{{{\partial ^4}{w_0}}}{{\partial {x^4}}} - \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;2\left( {{D_{12}} + 2{D_{66}}} \right)\frac{{{\partial ^4}{w_0}}}{{{R^2}\partial {x^2}\partial {\theta ^2}}} - {D_{22}}\frac{{{\partial ^4}{w_0}}}{{{R^4}\partial {\theta ^4}}} + N\left( t \right)\frac{{{\partial ^2}{w_0}}}{{\partial {x^2}}} \end{array} $ (9)
2 控制方程的求解

设径向位移表示为[17]

$ w = Y\left( x \right)T\left( t \right){{\rm{e}}^{{\rm{i}}m\theta }} $ (10)

将(10)式代入(9)式中,分离变量得

$ \left\{ \begin{array}{l} {Y^{\left( 4 \right)}} = {\alpha ^2}Y'' + {\beta ^2}Y = 0\\ \ddot T - \lambda T = 0 \end{array} \right. $ (11)

其中

$ {\alpha ^2} = \frac{1}{{{D_{11}}}}\left[ {4\frac{{{B_{12}}}}{{{R^2}}} + \frac{2}{{{R^2}}}\left( {{D_{12}} + 2{D_{66}}} \right){m^2} - N\left( t \right) - {I_2}\lambda } \right] $ (12)
$ {\beta ^2} = \frac{1}{{{D_{11}}}}\left[ {4\frac{{{A_{22}}}}{{{R^2}}} + 4\frac{{{B_{22}}}}{{{R^3}}} + \frac{{{D_{22}}}}{{{R^4}}}{m^4} + 4{I_0}\lambda - \frac{{{I_2}}}{{{R^2}}}{m^2}\lambda } \right] $ (13)

α4>4β2>0且λ>0时,圆柱壳屈曲[18-20],其动力屈曲解为

$ Y\left( x \right) = {C_1}\sin \left( {{k_1}x} \right) + {C_2}\cos \left( {{k_1}x} \right) + {C_3}\sin \left( {{k_2}x} \right) + {C_4}\cos \left( {{k_2}x} \right) $ (14)

式中:C1~C4为系数,$ {k_1} = \sqrt {\frac{{{\alpha ^2} - \sqrt {{\alpha ^4} - 4{\beta ^2}} }}{2}} , {k_2} = \sqrt {\frac{{{\alpha ^2} + \sqrt {{\alpha ^4} - 4{\beta ^2}} }}{2}} $

(14)式满足下列两种边界条件:

(1) 对于一端夹支另一端固支的圆柱壳,其边界条件为

$ \left\{ \begin{array}{l} Y\left( 0 \right) = Y'\left( 0 \right) = 0\\ Y\left( l \right) = Y'\left( l \right) = 0 \end{array} \right. $ (15)

(2) 对于一端简支另一端固支的圆柱壳,其边界条件为

$ \left\{ \begin{array}{l} Y\left( 0 \right) = Y''\left( 0 \right) = 0\\ Y\left( l \right) = Y'\left( l \right) = 0 \end{array} \right. $ (16)

将(14)式代入(15)式中,整理得到如下齐次线性方程组

$ \left( {\begin{array}{*{20}{c}} 0&1&0&1\\ {{k_1}}&0&{{k_2}}&0\\ {\sin \left( {{k_1}l} \right)}&{\cos \left( {{k_1}l} \right)}&{\sin \left( {{k_2}l} \right)}&{\cos \left( {{k_2}l} \right)}\\ {{k_1}\cos \left( {{k_1}l} \right)}&{ - {k_1}\sin \left( {{k_1}l} \right)}&{{k_2}\cos \left( {{k_2}l} \right)}&{ - {k_2}\sin \left( {{k_2}l} \right)} \end{array}} \right)\left( {\begin{array}{*{20}{c}} {{C_1}}\\ {{C_2}}\\ {{C_3}}\\ {{C_4}} \end{array}} \right) = 0 $ (17)

若要(17)式有非平凡解,其系数行列式必为零,于是

$ 2{k_1}{k_2} - 2{k_1}{k_2}\cos \left( {{k_1}l} \right)\cos \left( {{k_2}l} \right) - \left( {k_1^2 + k_2^2} \right)\sin \left( {{k_1}l} \right)\sin \left( {{k_2}l} \right) = 0 $ (18)

k1k2可得

$ \left\{ \begin{array}{l} k_1^2 + k_2^2 = \left( {n_1^2 + n_2^2} \right){{\rm{ \mathsf{ π} }}^2}/{l^2} = {\alpha ^2}\\ k_1^2k_2^2 = n_1^2n_2^2{{\rm{ \mathsf{ π} }}^4}/{l^4} = {\beta ^2} \end{array} \right. $ (19)

将(19)式代入(12)式和(13)式中,得一端夹支另一端固支时FGM圆柱壳动力屈曲临界荷载Ncr,即

$ {N_{{\rm{cr}}}} = \frac{{{D_{11}}{{\rm{ \mathsf{ π} }}^2}\left( {n_1^2 + n_2^2} \right)}}{{{l^2}}} - \frac{{{B_{12}}}}{{{R^2}}} + \frac{{2\left( {{D_{12}} + 2{D_{66}}} \right){m^2}}}{{{R^2}}} + \frac{{{h^2}\left( {{D_{11}}n_1^2n_2^2{{\rm{ \mathsf{ π} }}^4}{R^4} - {A_{22}}{l^4}{R^2} + 4{B_{22}}{m^2}{R^2}{l^4} - {D_{22}}{l^4}{m^4}} \right)}}{{{l^4}{R^2}\left( {{h^2}{m^2} - 12{R^2}} \right)}} $ (20)

式中:n1=n=1, 2, 3, …;m=1, 2, 3, …;n2=n+2。

同理可得一端简支另一端固支时FGM圆柱壳动力屈曲临界荷载

$ {N_{{\rm{cr}}}} = \frac{{{D_{11}}{{\rm{ \mathsf{ π} }}^2}\left( {n_1^2 + n_2^2} \right)}}{{{l^2}}} - \frac{{{B_{12}}}}{{{R^2}}} + \frac{{2\left( {{D_{12}} + 2{D_{66}}} \right){m^2}}}{{{R^2}}} + \frac{{{h^2}\left( {{D_{11}}n_1^2n_2^2{{\rm{ \mathsf{ π} }}^4}{R^4} - {A_{22}}{l^4}{R^2} + 4{B_{22}}{m^2}{R^2}{l^4} - {D_{22}}{l^4}{m^4}} \right)}}{{{l^4}{R^2}\left( {{h^2}{m^2} - 12{R^2}} \right)}} $ (21)

此时,n1=n=1, 2, 3, …;m=1, 2, 3, …;n2=n+1。

将FGM退化成金属材料,得到金属材料圆柱壳动力屈曲临界荷载

$ {N_{{\rm{cr}}}} = \frac{{{D_{11}}{{\rm{ \mathsf{ π} }}^2}\left( {n_1^2 + n_2^2} \right)}}{{{l^2}}} + \frac{{2{D_{12}}{m^2}}}{{{R^2}}} + \frac{{{h^2}\left( {{D_{11}}n_1^2n_2^2{{\rm{ \mathsf{ π} }}^4}{R^4} - {A_{22}}{l^4}{R^2} - {D_{22}}{l^4}{m^4}} \right)}}{{{l^4}{R^2}\left( {{h^2}{m^2} - 12{R^2}} \right)}} $ (22)

(22)式与文献[16]中的表达式相同。

根据(10)式,取一端夹支另一端固支时圆柱壳动力屈曲解的表达式[16]

$ w = T\left( t \right)\left[ {\sin \left( {\frac{{{n_1}{\rm{ \mathsf{ π} }}x}}{l}} \right) - \frac{{{n_1}}}{{{n_2}}}\sin \left( {\frac{{{n_2}{\rm{ \mathsf{ π} }}x}}{l}} \right)} \right]\sin \left( {m\theta } \right) $ (23)

将(23)式代入控制方程(9)式中,计算并化简整理得到临界荷载表达式

$ {N_{{\rm{cr}}}} = \frac{{\left( {n_1^2 + n_2^2} \right){D_{11}}{{\rm{ \mathsf{ π} }}^2}}}{{{l^2}}} + \frac{{2{m^2}\left( {{D_{12}} + 2{D_{66}}} \right)}}{{{R^2}}} $ (24)

(24)式与不考虑转动惯量时用分离变量得到的结果相同,此时n1=n=1, 2, 3, …; m=1, 2, 3, …; n2=n+2。同理可得当边界条件为一端简支另一端固支时的临界荷载表达式,与(24)式相同,此时n1=n=1, 2, 3, …; m=1, 2, 3, …; n2=n+1。

3 算例分析

采用MATLAB软件编程,对FGM圆柱壳动力屈曲临界荷载进行计算。讨论由不同材料(陶瓷-钛、陶瓷-铁、陶瓷-铜)组成的FGM圆柱壳(见图 2)的径厚比(R/h)、梯度指数(k)、环向模态数(m)、轴向模态数(n)对临界荷载Ncr的影响。基本材料参数如表 1所示。

图 2 材料沿壁厚分布 Fig.2 Distribution of material along wall thickness
表 1 材料参数 Table 1 Material parameters

图 3表示n=1、m=2、k=1、R/h=20时,不同材料组成下Ncr与临界长度l(本研究中临界长度即为圆柱壳长度)的关系曲线。从图 3可以看出:Ncrl的增加而减小;当l<0.5 m时,Ncrl的增加而迅速减小;当l>0.5 m时,Ncrl的增大缓慢减小,且逐渐趋于常数;同一l下,陶瓷-铜的Ncr最大,陶瓷-铁次之,陶瓷-钛的Ncr最小。以下均以陶瓷-钛为例进行讨论。

图 3 不同材料组成下临界荷载与临界长度的关系 Fig.3 Critical load vs.critical length under different material composition conditions

图 4图 5分别表示冲击端为夹支和简支时n=1、m=2、k=1时不同R/hNcrl的关系曲线。可以看出:当l增加时,Ncr减小,且逐渐趋于常数;在同一l下圆柱壳的Ncr随着R/h的增大而减小;当R/hl一定时,冲击端为夹支时的Ncr明显比冲击端为简支时的Ncr大,说明约束条件对Ncr有较大影响。

图 4 冲击端为夹支时不同径厚比下临界荷载与临界长度的关系 Fig.4 Critical load vs.critical length under clamped edge and different diameter-thickness ratios conditions
图 5 冲击端为简支时不同径厚比下临界荷载与临界长度的关系 Fig.5 Critical load vs.critical length under simple support and different diameter-thickness ratios conditions

图 6图 7分别表示冲击端为夹支和简支条件下n=1、m=2、R/h=20时不同kNcrl的关系曲线。可见:FGM圆柱壳的Ncr随着l的增加而减小;在同一l下,FGM圆柱壳的Ncr随着k的增加而增加;当l<0.5 m时,Ncrl的增加迅速减小,当l>0.5 m时,Ncrl的增加缓慢减小并逐渐趋于常数;当k=1且l一定时,冲击端为夹支时的Ncr明显比冲击端为简支时的Ncr大,再次说明约束条件对Ncr的影响较大。

图 6 冲击端为夹支时不同梯度指数下临界荷载与临界长度的关系 Fig.6 Critical load vs.critical length under clamped edge and different gradient indexes conditions
图 7 冲击端为简支时不同梯度指数下临界荷载与临界长度的关系 Fig.7 Critical load vs.critical length under simple support and different gradient indexes conditions

图 8图 9分别表示冲击端为夹支和简支,n=1、k=1、R/h=20时不同mNcrl的关系曲线。可以看出:当l在一定范围内时,Ncrl的增加而迅速减小,超出这一范围后Ncrl的增加而缓慢减小且逐渐趋于常数;同一l下,随着m的增大,FGM圆柱壳的Ncr增大,表明Ncr越大,高阶模态越易被激发。当m=6且l一定时,冲击端为夹支时的Ncr明显比冲击端为简支时的Ncr大,表明约束条件对Ncr有较大影响。

图 8 冲击端为夹支时不同环向模态数下临界荷载与临界长度的关系 Fig.8 Critical load vs.critical length under clamped edge and different circumferential modal number conditions
图 9 冲击端为简支时不同环向模态数下临界荷载与临界长度的关系 Fig.9 Critical load vs.critical length under simple support and different circumferential modal number conditions

图 10图 11分别表示冲击端为夹支和简支,m=1、k=1、R/h=20时不同nNcrl的关系曲线。图 10图 11显示:Ncr随着l的增加而减小;不同n条件下,Ncrl的增加逐渐趋于同一值;当l<1 m时,在同一lNcrn的增加而增加,说明Ncr越大,高阶模态越容易被激发。

图 10 冲击端为夹支时不同轴向模态数下临界荷载与临界长度的关系 Fig.10 Critical load vs.critical length under clamped edge and different axial modal number conditions
图 11 冲击端为简支时不同轴向模态数下临界荷载与临界长度的关系 Fig.11 Critical load vs.critical length under simple support and different axial modal number conditions

图 12为不同环向模态数m下FGM圆柱壳的动力屈曲模态。可以看出:随着m的增大,圆柱壳的模态变得越来越复杂,俯视图由单一形变为多瓣形;当m=6时,俯视图为12瓣形。

图 12 不同环向屈曲模态图(n=1;m=1, 2, 3, 4, 5, 6) Fig.12 Different circumferential buckling modes (n=1;m=1, 2, 3, 4, 5, 6)

图 13为不同轴向模态数n下FGM圆柱壳的动力屈曲模态图。由图 13可知:随着n的增加,模态图变得越来越复杂。由FGM圆柱壳的俯视图可知,各阶模态数下动力屈曲模态图为轴对称。

图 13 不同轴向屈曲模态图(m=2;n=1, 2, 3, 4, 5, 6) Fig.13 Different axial buckling modes (m=2;n=1, 2, 3, 4, 5, 6)
4 结论

(1) 根据Donnell壳体理论和经典板壳理论,由Hamilton变分原理得到轴向荷载作用下FGM圆柱壳的动力屈曲控制方程。

(2) 由圆柱壳周向连续性设出径向位移的周向形式,并用分离变量法得到不同约束条件下FGM圆柱壳动力屈曲临界荷载的表达式和屈曲解式。

(3) 利用MATLAB对临界荷载进行计算,得到:在轴向模态数(n)、环向模态数(m)、梯度指数(k)、径厚比(R/h)一定的情况下,同种材料组成的圆柱壳的临界荷载随着临界长度的增加而减小;在nmkl一定的情况下,临界荷载随着径厚比的增大而减小;在nmR/hl一定的情况下,临界荷载随着梯度指数k的增加而增加;不同约束条件下,冲击端为夹支的临界荷载大于冲击端为简支的临界荷载,表明约束条件对临界荷载有较大影响;圆柱壳的临界荷载随模态数的增加而增大,表明临界荷载越大,越容易激发高阶模态;圆柱壳的动力屈曲模态随模态数的增加变得更为复杂。

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Dynamic Buckling of Functionally Graded Cylindrical Shells under Axial Loading
ZHOU Jiahua 1, YANG Qiang 1, HAN Zhijun 1, LU Guoyun 2     
( 1. College of Mechanics, Taiyuan University of Technology, Taiyuan 030024, China;
2. College of Architecture and Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China )
Abstract: Based on the Donnell shell theory and classical shell theory, we established the dynamic buckling governing equation of functionally graded cylindrical shell under axial load using Hamilton principle.According to the expression of the radial displacement based on the circumferential continuity of cylindrical shell, we also obtained the dynamic buckling critical load expression and the buckling solution of functionally graded cylindrical shell under axial loading using the separation variable method.Using MATLAB, we performed the numerical analysis of functionally graded cylindrical shells, and discussed the influence of the diameter-thickness ratio, the gradient index, the number of circumferential mode and axial mode on the critical load of dynamic buckling.The results show that the critical load of cylindrical shells decreases with the increase of the critical length.The constraint conditions have effects on the critical load, and the critical load of the clamped edges is higher than that of the simple support.Moreover, the critical load of cylindrical shells grows as the modal number increases, indicating that the higher the critical load is, the easier the high-stage mode excites.The dynamic buckling modal diagram becomes more complicated as the modal number increases.
Keywords: functionally graded material    buckling critical load    cylindrical shell    Hamilton principle    modal number