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[[Image:BendingCircularPlate.png| thumb | 300px | Bending of an edge clamped circular plate under the action of a transverse pressure. The left half of the plate shows the deformed shape while the right half shows the undeformed shape. This calculation was performed using [[Ansys]].]]
'''Bending of plates''' or plate bending refers to the [[Deflection (engineering)|deflection]] of a [[plate]] perpendicular to the plane of the plate under the action of external [[force]]s and [[Moment (physics)|moments]]. The amount of deflection can be determined by solving the differential equations of an appropriate [[plate theory]]. The [[stress (physics)|stress]]es in the plate can be calculated from these deflections.  Once the stresses are known, [[material failure theory|failure theories]] can be used to determine whether a plate will fail under a given load.
 
== Bending of Kirchhoff-Love plates ==
[[Image:PlateForcesMoments upd.png|thumb | 350px | Forces and moments on a flat plate.]]
In the [[Kirchhoff–Love plate theory]] for plates the governing equations are<ref name=Reddy>Reddy, J. N., 2007, '''Theory and analysis of elastic plates and shells''', CRC Press, Taylor and Francis.</ref>
:<math>
    N_{\alpha\beta,\alpha} = 0
</math>
and
:<math>
    M_{\alpha\beta,\alpha\beta} - q = 0
</math>
In expanded form,
:<math>
      \cfrac{\partial N_{11}}{\partial x_1} + \cfrac{\partial N_{21}}{\partial x_2} = 0 ~;~~
      \cfrac{\partial N_{12}}{\partial x_1} + \cfrac{\partial N_{22}}{\partial x_2} = 0
</math>
and
:<math>
      \cfrac{\partial^2 M_{11}}{\partial x_1^2} + 2\cfrac{\partial^2 M_{12}}{\partial x_1 \partial x_2} +
      \cfrac{\partial^2 M_{22}}{\partial x_2^2} = q
</math>
where <math>q(x)</math> is an applied transverse [[load]] per unit area, the thickness of the plate is <math>H=2h</math>, the stresses are <math>\sigma_{ij}</math>, and
:<math>
  N_{\alpha\beta} := \int_{-h}^h \sigma_{\alpha\beta}~dx_3 ~;~~
  M_{\alpha\beta} := \int_{-h}^h x_3~\sigma_{\alpha\beta}~dx_3~.
</math>
The quantity <math>N</math> has units of [[force]] per unit length.  The quantity <math>M</math> has units of [[Moment (physics)|moment]] per unit length.
 
For [[isotropic]], [[homogeneous]], plates with [[Young's modulus]] <math>E</math> and [[Poisson's ratio]] <math>\nu</math> these equations reduce to<ref name=Timo>Timoshenko, S. and Woinowsky-Krieger, S., (1959), '''Theory of plates and shells''', McGraw-Hill New York.</ref>
:<math>
  \nabla^2\nabla^2 w = -\cfrac{q}{D} ~;~~ D := \cfrac{2h^3E}{3(1-\nu^2)} = \cfrac{H^3E}{12(1-\nu^2)}
</math>
where <math>w(x_1,x_2)</math> is the deflection of the mid-surface of the plate.
 
In rectangular Cartesian coordinates,  
:<math>
  \cfrac{\partial^4 w}{\partial x_1^4} + 2\cfrac{\partial^4 w}{\partial x_1^2 \partial x_2^2} +
      \cfrac{\partial^4 w}{\partial x_2^4} = -\cfrac{q}{D} \,.
</math>
 
==Circular Kirchhoff-Love plates==
The bending of circular plates can be examined by solving the governing equation with
appropriate boundary conditions. These solutions were first found by Poisson in 1829.
Cylindrical coordinates are convenient for such problems.
 
The governing equation in coordinate-free form is
:<math>
  \nabla^2 \nabla^2 w = -\frac{q}{D} \,.
</math>
In cylindrical coordinates <math>(r, \theta, z)</math>,
:<math>
  \nabla^2 w \equiv \frac{1}{r}\frac{\partial }{\partial r}\left(r \frac{\partial w}{\partial r}\right) +
      \frac{1}{r^2}\frac{\partial^2 w}{\partial \theta^2} + \frac{\partial^2 w}{\partial z^2} \,.
</math>
For symmetrically loaded circular plates, <math> w = w(r)</math>, and we have
:<math>
  \nabla^2 w \equiv \frac{1}{r}\cfrac{d }{d r}\left(r \cfrac{d w}{d r}\right) \,.
</math>
Therefore, the governing equation is
:<math>
  \frac{1}{r}\cfrac{d }{d r}\left[r \cfrac{d }{d r}\left\{\frac{1}{r}\cfrac{d }{d r}\left(r \cfrac{d w}{d r}\right)\right\}\right] = -\frac{q}{D}\,.
</math>
If <math>q</math> and <math>D</math> are constant, direct integration of the governing equation gives us
<blockquote style="border: 1px solid black; padding:10px; width:530px">
:<math>
  w(r) = -\frac{qr^4}{64 D} + C_1\ln r + \cfrac{C_2 r^2}{2} + \cfrac{C_3r^2}{4}(2\ln r - 1) + C_4
</math>
</blockquote>
where <math>C_i</math> are constants.  The slope of the deflection surface is
:<math>
  \phi(r) = \cfrac{d w}{d r} = -\frac{qr^3}{16D} + \frac{C_1}{r} + C_2 r + C_3 r \ln r \,.
</math>
For a circular plate, the requirement that the deflection and the slope of the deflection are finite
at <math>r = 0</math> implies that <math>C_1 = C_3 = 0</math>.
 
===Clamped edges===
For a circular plate with clamped edges, we have <math>w(a) = 0</math> and <math>\phi(a) = 0</math> at the edge of
the plate (radius <math>a</math>).  Using these boundary conditions we get
<blockquote style="border: 1px solid black; padding:10px; width:530px">
:<math>
  w(r) = -\frac{q}{64 D} (a^2 -r^2)^2 \quad \text{and} \quad
  \phi(r) = \frac{qr}{16 D}(a^2-r^2) \,.
</math>
</blockquote>
The in-plane displacements in the plate are
:<math>
  u_r(r) = -z\phi(r) \quad \text{and} \quad u_\theta(r) = 0 \,.
</math>
The in-plane strains in the plate are
:<math>
  \varepsilon_{rr} = \cfrac{d u_r}{d r} = -\frac{qz}{16D}(a^2-3r^2) ~,~~
  \varepsilon_{\theta\theta} = \frac{u_r}{r} = -\frac{qz}{16D}(a^2-r^2) ~,~~
  \varepsilon_{r\theta} = 0 \,.
</math>
The in-plane stresses in the plate are
:<math>
  \sigma_{rr} = \frac{E}{1-\nu^2}\left[\varepsilon_{rr} + \nu\varepsilon_{\theta\theta}\right] ~;~~
  \sigma_{\theta\theta} = \frac{E}{1-\nu^2}\left[\varepsilon_{\theta\theta} + \nu\varepsilon_{rr}\right] ~;~~
  \sigma_{r\theta} = 0 \,.
</math>
For a plate of thickness <math>2h</math>, the bending stiffness is <math>D = 2Eh^3/[3(1-\nu^2)]</math> and we
have
<blockquote style="border: 1px solid black; padding:10px; width:430px">
:<math>
  \begin{align}
  \sigma_{rr} &= -\frac{3qz}{32h^3}\left[(1+\nu)a^2-(3+\nu)r^2\right] \\
  \sigma_{\theta\theta} &= -\frac{3qz}{32h^3}\left[(1+\nu)a^2-(1+3\nu)r^2\right]\\
  \sigma_{r\theta} &= 0 \,.
  \end{align}
</math>
</blockquote>
The moment resultants (bending moments) are
:<math>
  M_{rr} = -\frac{q}{16}\left[(1+\nu)a^2-(3+\nu)r^2\right] ~;~~
  M_{\theta\theta} = -\frac{q}{16}\left[(1+\nu)a^2-(1+3\nu)r^2\right] ~;~~
  M_{r\theta} = 0 \,.
</math>
The maximum radial stress is at <math>z = h</math> and <math>r = a</math>:
:<math>
  \left.\sigma_{rr}\right|_{z=h,r=a} = \frac{3qa^2}{16h^2} = \frac{3qa^2}{4H^2}
</math>
where <math>H := 2h</math>.  The bending moments at the boundary and the center of the plate are
:<math>
  \left.M_{rr}\right|_{r=a} = \frac{qa^2}{8} ~,~~
  \left.M_{\theta\theta}\right|_{r=a} = \frac{\nu qa^2}{8} ~,~~
  \left.M_{rr}\right|_{r=0} = \left.M_{\theta\theta}\right|_{r=0} = -\frac{(1+\nu) qa^2}{16} \,.
</math>
 
==Rectangular Kirchhoff-Love plates==
[[Image:RectangularPlateBending.svg|thumb | 250px | Bending of a rectangular plate under the action of a distributed force <math>q</math> per unit area.]]
For rectangular plates, Navier in 1820 introduced a simple method for finding the displacement and stress when a plate is simply supported. The idea was to express the applied load in terms of Fourier components, find the solution for a sinusoidal load (a single Fourier component), and then superimpose the Fourier components to get the solution for an arbitrary load.
 
===Sinusoidal load===
Let us assume that the load is of the form
:<math>
  q(x,y) = q_0 \sin\frac{\pi x}{a}\sin\frac{\pi y}{b} \,.
</math>
Here <math>q_0</math> is the amplitude, <math>a</math> is the width of the plate in the <math>x</math>-direction, and
<math>b</math> is the width of the plate in the <math>y</math>-direction.
 
Since the plate is simply supported, the displacement <math>w(x,y)</math> along the edges of
the plate is zero, the bending moment <math>M_{xx}</math> is zero at <math>x=0</math> and <math>x=a</math>, and
<math>M_{yy}</math> is zero at <math>y=0</math> and <math>y=b</math>.
 
If we apply these boundary conditions and solve the plate equation, we get the
solution
:<math>
  w(x,y) = \frac{q_0}{\pi^4 D}\,\left(\frac{1}{a^2}+\frac{1}{b^2}\right)^{-2}\,\sin\frac{\pi x}{a}\sin\frac{\pi y}{b} \,.
</math>
We can calculate the stresses and strains in the plate once we know the displacement.
 
For a more general load of the form
:<math>
  q(x,y) = q_0 \sin\frac{m \pi x}{a}\sin\frac{n \pi y}{b}
</math>
where <math>m</math> and <math>n</math> are integers, we get the solution
<blockquote style="border: 1px solid black; padding:10px; width:530px">
:<math> \text{(1)} \qquad
  w(x,y) = \frac{q_0}{\pi^4 D}\,\left(\frac{m^2}{a^2}+\frac{n^2}{b^2}\right)^{-2}\,\sin\frac{m \pi x}{a}\sin\frac{n \pi y}{b} \,.
</math>
</blockquote>
 
===Navier solution===
Let us now consider a more general load <math>q(x,y)</math>.  We can break this load up into
a sum of Fourier components such that
:<math>
  q(x,y) = \sum_{m=1}^{\infty} \sum_{n=1}^\infty a_{mn}\sin\frac{m \pi x}{a}\sin\frac{n \pi y}{b}
</math>
where <math>a_{mn}</math> is an amplitude.  We can use the orthogonality of Fourier components,
:<math>
  \int_0^a \sin\frac{k\pi x}{a}\sin\frac{\ell \pi x}{a}\text{d}x =
    \begin{cases} 0 & k \ne \ell \\ a/2 & k = \ell \end{cases}
</math>
to find the amplitudes <math>a_{mn}</math>.  Thus we have, by integrating over <math>y</math>,
:<math>
  \int_0^b q(x,y)\sin\frac{\ell\pi y}{b}\,\text{d}y =
    \sum_{m=1}^{\infty} \sum_{n=1}^\infty a_{mn}\sin\frac{m \pi x}{a}
    \int_0^b \sin\frac{n \pi y}{b} \sin\frac{\ell\pi y}{b}\,\text{d}y =
    \frac{b}{2}\sum_{m=1}^{\infty} a_{m\ell}\sin\frac{m \pi x}{a} \,.
</math>
If we repeat the process by integrating over <math>x</math>, we have
:<math>
  \int_0^b \int_0^a q(x,y)\sin\frac{k\pi x}{a}\sin\frac{\ell\pi y}{b}\,\text{d}x\text{d}y =
    \frac{b}{2}\sum_{m=1}^{\infty} a_{m\ell}
    \int_0^a \sin\frac{m \pi x}{a} \sin\frac{k\pi x}{a}\,\text{d}x =
    \frac{ab}{4} a_{k\ell} \,.
</math>
Therefore,
:<math>
  a_{mn} = \frac{4}{ab}
  \int_0^b \int_0^a q(x,y)\sin\frac{m\pi x}{a}\sin\frac{n\pi y}{b}\,\text{d}x\text{d}y \,.
</math>
Now that we know <math>a_{mn}</math>, we can just superpose solutions of the form given in
equation (1) to get the displacement, i.e.,
<blockquote style="border: 1px solid black; padding:10px; width:630px">
:<math> \text{(2)} \qquad
  w(x,y) = \sum_{m=1}^\infty \sum_{n=1}^\infty \frac{a_{mn}}{\pi^4 D}\,\left(\frac{m^2}{a^2}+\frac{n^2}{b^2}\right)^{-2}\,\sin\frac{m \pi x}{a}\sin\frac{n \pi y}{b} \,.
</math>
</blockquote>
 
====Uniform load====
Consider the situation where a uniform load is applied on the plate, i.e.,
<math>q(x,y) = q_0</math>.  Then
:<math>
  a_{mn} = \frac{4q_0}{ab}
  \int_0^a \int_0^b \sin\frac{m\pi x}{a}\sin\frac{n\pi y}{b}\,\text{d}x\text{d}y \,.
</math>
Now
:<math>
  \int_0^a \sin\frac{m\pi x}{a}\,\text{d}x = \frac{a}{m\pi}(1 - \cos m\pi) \quad\text{and}\quad
  \int_0^b \sin\frac{n\pi y}{b}\,\text{d}y = \frac{b}{n\pi}(1 - \cos n\pi)\,.
</math>
We can use these relations to get a simpler expression for <math>a_{mn}</math>:
:<math>
  a_{mn} = \frac{4q_0}{mn\pi^2}(1 - \cos m\pi)(1 - \cos n\pi) \,.
</math>
Since <math>\cos m\pi = \cos n\pi = 1</math> [ so <math>(1 - \cos m\pi) = (1 - \cos n\pi) = 0</math> ] when <math>m</math> and <math>n</math> are even, we can get an even simpler expression for <math>a_{mn}</math> when both <math>m</math> and <math>n</math> are odd:
:<math>
  a_{mn} = \begin{cases}
            0 & m~\text{or}~n~\text{even}, \\
            \cfrac{16q_0}{mn\pi^2} & m~\text{and}~n~\text{odd}\,.
            \end{cases}
</math>
Plugging this expression into equation (2) and keeping in mind
that only odd terms contribute to the displacement, we have
<blockquote style="border: 1px solid black; padding:10px; width:630px">
:<math>
  \begin{align}
  w(x,y) & = \sum_{m=1}^\infty \sum_{n=1}^\infty \frac{16 q_0}{(2m-1)(2n-1)\pi^6 D}\,\left[\frac{(2m-1)^2}{a^2}+\frac{(2n-1)^2}{b^2}\right]^{-2} \,\times\\
    & \qquad \qquad \quad \sin\frac{(2m-1) \pi x}{a}\sin\frac{(2n-1) \pi y}{b} \,.
  \end{align}
</math>
</blockquote>
The corresponding moments are given by
:<math>
  \begin{align}
    M_{xx} & = -D\left(\frac{\partial^2 w}{\partial x^2} + \nu \frac{\partial^2 w}{\partial y^2}\right) \\
          & = \sum_{m=1}^\infty \sum_{n=1}^\infty\frac{16 q_0}{(2m-1)(2n-1)\pi^4}\,
              \left[\frac{(2m-1)^2}{a^2}+\nu\frac{(2n-1)^2}{b^2}\right] \,\times\\
          &    \qquad \qquad \left[\frac{(2m-1)^2}{a^2}+\frac{(2n-1)^2}{b^2}\right]^{-2}
              \sin\frac{(2m-1) \pi x}{a}\sin\frac{(2n-1) \pi y}{b} \\
    M_{yy} & = -D\left(\frac{\partial^2 w}{\partial y^2} + \nu \frac{\partial^2 w}{\partial x^2}\right) \\
          & = \sum_{m=1}^\infty \sum_{n=1}^\infty\frac{16 q_0}{(2m-1)(2n-1)\pi^4}\,
              \left[\frac{(2n-1)^2}{b^2}+\nu\frac{(2m-1)^2}{a^2}\right] \,\times\\
          &    \qquad \qquad \left[\frac{(2m-1)^2}{a^2}+\frac{(2n-1)^2}{b^2}\right]^{-2}
              \sin\frac{(2m-1) \pi x}{a}\sin\frac{(2n-1) \pi y}{b} \,.
  \end{align}
</math>
The stresses in the plate are
:<math>
  \sigma_{xx} = \frac{3z}{2h^3}\,M_{xx} = \frac{12 z}{H^3}\,M_{xx} \quad \text{and} \quad
  \sigma_{yy} = \frac{3z}{2h^3}\,M_{yy} = \frac{12 z}{H^3}\,M_{yy} \,.
</math>
:{{multiple image
  | width    = 400
  | footer    = Displacement and stresses along <math>x=a/2</math> for a rectangular plate with <math>a=20</math> mm, <math>b=40</math> mm, <math>H=2h=0.4</math> mm, <math>E=70</math> GPa, and <math>\nu=0.35</math> under a load <math>q_0 = -10</math> kPa.  The red line represents the bottom of the plate, the green line the middle, and the blue line the top of the plate.
  | image1    = wx rectangularPlate.svg
  | caption1  = Displacement (<math>w</math>)
  | image2    = sxx rectangularPlate.svg
  | caption2  = Stress (<math>\sigma_{xx}</math>)
  | image3    = syy rectangularPlate.svg
  | caption3  = Stress (<math>\sigma_{yy}</math>)
  }}
<!--
<gallery widths=270px heights=270px perrow=2 caption="Displacement and stresses along <math>x=a/2</math> for a rectangular plate with <math>a=20</math> mm, <math>b=40</math> mm, <math>H=2h=0.4</math> mm, <math>E=70</math> GPa, and <math>\nu=0.35</math> under a load <math>q_0 = -10</math> kPa.  The red line represents the bottom of the plate, the green line the middle, and the blue line the top of the plate.">
file:wx rectangularPlate.svg|Displacement (<math>w</math>)
file:sxx rectangularPlate.svg|Stress (<math>\sigma_{xx}</math>)
file:syy rectangularPlate.svg|Stress (<math>\sigma_{yy}</math>)
</gallery>
-->
 
===Levy solution===
Another approach was proposed by Levy in 1899.  In this case we start with an
assumed form of the displacement and try to fit the parameters so that the
governing equation and the boundary conditions are satisfied.
 
Let us assume that
:<math>
  w(x,y) = \sum_{m=1}^\infty Y_m(y) \sin \frac{m\pi x}{a} \,.
</math>
For a plate that is simply supported at <math>x=0</math> and <math>x=a</math>, the boundary conditions
are <math>w=0</math> and <math>M_{xx} = 0</math>.  The moment boundary condition is equivalent to
<math>\partial^2 w/\partial x^2 = 0</math> (verify).  The goal is to find <math>Y_m(y)</math> such that
it satisfies the boundary conditions at <math>y = 0</math> and <math>y = b</math> and, of course, the
governing equation <math>\nabla^2 \nabla^2 w = q/D</math>.
 
====Moments along edges====
Let us consider the case of pure moment loading.  In that case <math>q = 0</math> and
<math>w(x,y)</math> has to satisfy <math>\nabla^2 \nabla^2 w = 0</math>.  Since we are working in rectangular
Cartesian coordinates, the governing equation can be expanded as
:<math>
  \frac{\partial^4 w}{\partial x^4} + 2 \frac{\partial^4 w}{\partial x^2\partial y^2}
  + \frac{\partial^4 w}{\partial y^4}  = 0 \,.
</math>
Plugging the expression for <math>w(x,y)</math> in the governing equation gives us
:<math>
  \sum_{m=1}^\infty \left[\left(\frac{m\pi}{a}\right)^4 Y_m \sin\frac{m\pi x}{a}
  - 2\left(\frac{m\pi}{a}\right)^2 \cfrac{d^2 Y_m}{d y^2} \sin\frac{m\pi x}{a}
  + \frac{d^4Y_m}{dy^4} \sin\frac{m\pi x}{a}\right] = 0
</math>
or
:<math>
  \frac{d^4Y_m}{dy^4}  - 2 \frac{m^2\pi^2}{a^2} \cfrac{d^2Y_m}{dy^2} + \frac{m^4\pi^4}{a^4} Y_m = 0 \,.
</math>
This is an ordinary differential equation which has the general solution
:<math>
  Y_m = A_m \cosh\frac{m\pi y}{a} + B_m\frac{m\pi y}{a} \cosh\frac{m\pi y}{a} +
  C_m \sinh\frac{m\pi y}{a} + D_m\frac{m\pi y}{a} \sinh\frac{m\pi y}{a} 
</math>
where <math>A_m, B_m, C_m, D_m</math> are constants that can be determined from the boundary
conditions.  Therefore the displacement solution has the form
<blockquote style="border: 1px solid black; padding:1px; width:800px">
:<math>
  w(x,y) = \sum_{m=1}^\infty \left[ 
  \left(A_m  + B_m\frac{m\pi y}{a}\right) \cosh\frac{m\pi y}{a} +
  \left(C_m  + D_m\frac{m\pi y}{a}\right) \sinh\frac{m\pi y}{a} 
      \right] \sin \frac{m\pi x}{a} \,.
</math>
</blockquote>
Let us choose the coordinate system such that the boundaries of the plate are
at <math>x = 0</math> and <math>x = a</math> (same as before) and at <math>y = \pm b/2</math> (and not <math>y=0</math> and
<math>y=b</math>).  Then the moment boundary conditions at the <math>y = \pm b/2</math> boundaries are
:<math>
  w = 0 \,, -D\frac{\partial^2 w}{\partial y^2}\Bigr|_{y=b/2} = f_1(x) \,,
  -D\frac{\partial^2 w}{\partial y^2}\Bigr|_{y=-b/2} = f_2(x) 
</math>
where <math>f_1(x), f_2(x)</math> are known functions.  The solution can be found by
applying these boundary conditions.  We can show that for the ''symmetrical'' case
where
:<math>
  M_{yy}\Bigr|_{y=-b/2} = M_{yy}\Bigr|_{y=b/2}
</math>
and
:<math>
  f_1(x) = f_2(x) = \sum_{m=1}^\infty E_m\sin\frac{m\pi x}{a}
</math>
we have
<blockquote style="border: 1px solid black; padding:1px; width:800px">
:<math>
  w(x,y) = \frac{a^2}{2\pi^2 D}\sum_{m=1}^\infty \frac{E_m}{m^2\cosh\alpha_m}\,
    \sin\frac{m\pi x}{a}\, \left(\alpha_m \tanh\alpha_m \cosh\frac{m\pi y}{a}
    - \frac{m\pi y}{a}\sinh\frac{m\pi y}{a}\right)
</math>
</blockquote>
where
:<math>
  \alpha_m = \frac{m\pi b}{2a} \,.
</math>
Similarly, for the ''antisymmetrical'' case where
:<math>
  M_{yy}\Bigr|_{y=-b/2} = -M_{yy}\Bigr|_{y=b/2}
</math>
we have
<blockquote style="border: 1px solid black; padding:1px; width:800px">
:<math>
  w(x,y) = \frac{a^2}{2\pi^2 D}\sum_{m=1}^\infty \frac{E_m}{m^2\sinh\alpha_m}\,
    \sin\frac{m\pi x}{a}\, \left(\alpha_m \coth\alpha_m \sinh\frac{m\pi y}{a}
    - \frac{m\pi y}{a}\cosh\frac{m\pi y}{a}\right) \,.
</math>
</blockquote>
We can superpose the symmetric and antisymmetric solutions to get more general
solutions.
 
====  Uniform and symmetric moment load ====
For the special case where the loading is symmetric and the moment is uniform, we have at <math>y=\pm b/2</math>,
:<math>
  M_{yy} = f_1(x) = \frac{4M_0}{\pi}\sum_{m=1}^\infty \frac{1}{2m-1}\,\sin\frac{(2m-1)\pi x}{a} \,.
</math>
:{{multiple image
  | width    = 400
  | footer    = Displacement and stresses for a rectangular plate under uniform bending moment along the edges <math>y=-b/2</math> and <math>y=b/2</math>.  The bending stress <math>\sigma_{yy}</math> is along the bottom surface of the plate.  The transverse shear stress <math>\sigma_{yz}</math> is along the mid-surface of the plate.
  | image1    = surfRecBMIso_w.png
  | caption1  = Displacement (<math>w</math>)
  | image2    = surfRecBMIso_sy.png
  | caption2  = Bending stress (<math>\sigma_{yy}</math>)
  | image3    = surfRecBMIso_syz.png
  | caption3  = Transverse shear stress (<math>\sigma_{yz}</math>)
  }}
The resulting displacement is
<blockquote style="border: 1px solid black; padding:1px; width:800px">
:<math>
  \begin{align}
  w(x,y) & = \frac{2M_0 a^2}{\pi^3 D}\sum_{m=1}^\infty
    \frac{1}{(2m-1)^3\cosh\alpha_m}\sin\frac{(2m-1)\pi x}{a} \times\\
  & \qquad \left[
        \alpha_m\,\tanh\alpha_m\cosh\frac{(2m-1)\pi y}{a} -\frac{(2m-1)\pi y}{a}
    \sinh\frac{(2m-1)\pi y}{a}\right]
  \end{align}
</math>
</blockquote>
where
:<math>
  \alpha_m = \frac{\pi (2m-1)b}{2a} \,.
</math>
The bending moments and shear forces corresponding to the displacement <math>w</math> are
:<math>
  \begin{align}
    M_{xx} & = -D\left(\frac{\partial^2 w}{\partial x^2}+\nu\,\frac{\partial^2 w}{\partial y^2}\right) \\
            & = \frac{2M_0(1-\nu)}{\pi}\sum_{m=1}^\infty\frac{1}{(2m-1)\cosh\alpha_m}\,
                \sin\frac{(2m-1)\pi x}{a}
                \left[
                  -\frac{(2m-1)\pi y}{a}\sinh\frac{(2m-1)\pi y}{a} + \right. \\
            & \qquad \qquad \qquad \qquad
              \left. \left\{\frac{2\nu}{1-\nu} + \alpha_m\tanh\alpha_m\right\}\cosh\frac{(2m-1)\pi y}{a}
                \right] \\
    M_{xy} & = (1-\nu)D\frac{\partial^2 w}{\partial x \partial y} \\
            & = -\frac{2M_0(1-\nu)}{\pi}\sum_{m=1}^\infty\frac{1}{(2m-1)
                    \cosh\alpha_m}\,\cos\frac{(2m-1)\pi x}{a}
              \left[\frac{(2m-1)\pi y}{a}\cosh\frac{(2m-1)\pi y}{a} + \right. \\
            & \qquad \qquad \qquad \qquad
              \left. (1-\alpha_m\tanh\alpha_m)\sinh\frac{(2m-1)\pi y}{a}\right] \\
    Q_{zx} & = \frac{\partial M_{xx}}{\partial x}-\frac{\partial M_{xy}}{\partial y} \\
            & = \frac{4M_0}{a}\sum_{m=1}^\infty \frac{1}{\cosh\alpha_m}\,
                \cos\frac{(2m-1)\pi x}{a}\cosh\frac{(2m-1)\pi y}{a}\,.
  \end{align}
</math>
The stresses are
:<math>
  \sigma_{xx} = \frac{12z}{h^3}\,M_{xx} \quad \text{and} \quad
  \sigma_{zx} = \frac{1}{\kappa h}\,Q_{zx}\left(1 - \frac{4z^2}{h^2}\right)\,.
</math>
 
=== Cylindrical plate bending ===
Cylindrical bending occurs when a rectangular plate that has dimensions <math>a \times b \times h</math>, where <math>a \ll b</math> and the thickness <math>h</math> is small, is subjected to a uniform distributed load perpendicular to the plane of the plate.  Such a plate takes the shape of the surface of a cylinder.
 
==== Simply supported plate with axially fixed ends ====
For a simply supported plate under cylindrical bending with edges that are free to rotate but have a fixed <math>x_1</math>.  Cylindrical bending solutions can be found using the Navier and Levy techniques.
 
==Bending of thick Mindlin plates==
For thick plates, we have to consider the effect of through-the-thickness shears on
the orientation of the normal to the mid-surface after deformation.  Mindlin's theory
provides one approach for find the deformation and stresses in such plates.  Solutions
to Mindlin's theory can be derived from the equivalent Kirchhoff-Love solutions using
canonical relations.<ref name=lim03>Lim, G. T. and Reddy, J. N., 2003, ''On canonical bending
relationships for plates'', International Journal of Solids and Structures, vol. 40,
pp. 3039-3067.</ref>
 
===Governing equations===
The canonical governing equation for isotropic thick plates can be expressed as<ref name=lim03/>
:<math>
  \begin{align}
    & \nabla^2 \left(\mathcal{M} - \frac{\mathcal{B}}{1+\nu}\,q\right) = -q \\
    & \kappa G h\left(\nabla^2 w + \frac{\mathcal{M}}{D}\right) =
      -\left(1 - \cfrac{\mathcal{B} c^2}{1+\nu}\right)q \\
    & \nabla^2 \left(\frac{\partial \varphi_1}{\partial x_2} - \frac{\partial \varphi_2}{\partial x_1}\right)
      = c^2\left(\frac{\partial \varphi_1}{\partial x_2} - \frac{\partial \varphi_2}{\partial x_1}\right)
  \end{align}
</math>
where <math>q</math> is the applied transverse load, <math>G</math> is the shear modulus, <math>D = Eh^3/[12(1-\nu^2)]</math>
is the bending rigidity, <math>h</math> is the plate thickness, <math>c^2 = 2\kappa G h/[D(1-\nu)]</math>,
<math>\kappa</math> is the shear correction factor, <math>E</math> is the Young's modulus, <math>\nu</math> is the Poisson's
ratio, and
:<math>
    \mathcal{M}  = D\left[\mathcal{A}\left(\frac{\partial \varphi_1}{\partial x_1} + \frac{\partial \varphi_2}{\partial x_2}\right)
    - (1-\mathcal{A})\nabla^2 w\right] + \frac{2q}{1-\nu^2}\mathcal{B}  \,.
</math>
In Mindlin's theory, <math>w</math> is the transverse displacement of the mid-surface of the plate
and the quantities <math>\varphi_1</math> and <math>\varphi_2</math> are the rotations of the mid-surface normal
about the <math>x_2</math> and <math>x_1</math>-axes, respectively.  The canonical parameters for this theory
are <math>\mathcal{A} = 1</math> and <math>\mathcal{B} = 0</math>.  The shear correction factor <math>\kappa</math> usually has the
value <math>5/6</math>.
 
The solutions to the governing equations can be found if one knows the corresponding
Kirchhoff-Love solutions by using the relations
:<math>
  \begin{align}
    w & = w^K + \frac{\mathcal{M}^K}{\kappa G h}\left(1 - \frac{\mathcal{B} c^2}{2}\right)
        - \Phi + \Psi \\
    \varphi_1 & = - \frac{\partial w^K}{\partial x_1}
    - \frac{1}{\kappa G h}\left(1 - \frac{1}{\mathcal{A}} - \frac{\mathcal{B} c^2}{2}\right)Q_1^K
    + \frac{\partial }{\partial x_1}\left(\frac{D}{\kappa G h \mathcal{A}}\nabla^2 \Phi + \Phi - \Psi\right)
    + \frac{1}{c^2}\frac{\partial \Omega}{\partial x_2} \\
    \varphi_2 & = - \frac{\partial w^K}{\partial x_2}
    - \frac{1}{\kappa G h}\left(1 - \frac{1}{\mathcal{A}} - \frac{\mathcal{B} c^2}{2}\right)Q_2^K
    + \frac{\partial }{\partial x_2}\left(\frac{D}{\kappa G h \mathcal{A}}\nabla^2 \Phi + \Phi - \Psi\right)
    + \frac{1}{c^2}\frac{\partial \Omega}{\partial x_1}
  \end{align}
</math>
where <math>w^K</math> is the displacement predicted for a Kirchhoff-Love plate, <math>\Phi</math> is a
biharmonic function such that <math>\nabla^2 \nabla^2 \Phi = 0</math>, <math>\Psi</math> is a function that satisfies the
Laplace equation, <math>\nabla^2 \Psi = 0</math>, and
:<math>
  \begin{align}
    \mathcal{M} & = \mathcal{M}^K + \frac{\mathcal{B}}{1+\nu}\,q + D \nabla^2 \Phi ~;~~ \mathcal{M}^K := -D\nabla^2 w^K \\
    Q_1^K & = -D\frac{\partial }{\partial x_1}\left(\nabla^2 w^K\right) ~,~~
    Q_2^K = -D\frac{\partial }{\partial x_2}\left(\nabla^2 w^K\right) \\
    \Omega & = \frac{\partial \varphi_1}{\partial x_2} - \frac{\partial \varphi_2}{\partial x_1} ~,~~ \nabla^2 \Omega = c^2\Omega \,.
  \end{align}
</math>
 
===Simply supported rectangular plates===
For simply supported plates, the ''Marcus moment'' sum vanishes, i.e.,
:<math>
  \mathcal{M} = \frac{1}{1+\nu}(M_{11}+M_{22}) = D\left(\frac{\partial \varphi_1}{\partial x_2}+\frac{\partial \varphi_2}{\partial x_2}\right) = 0 \,.
</math>
In that case the functions <math>\Phi</math>, <math>\Psi</math>, <math>\Omega</math> vanish, and the Mindlin solution is
related to the corresponding Kirchhoff solution by
:<math>
  w = w^K + \frac{\mathcal{M}^K}{\kappa G h} \,.
</math>
 
== Bending of Reissner-Stein cantilever plates ==
Reissner-Stein theory for cantilever plates<ref name=Reissner51>E. Reissner and M. Stein. Torsion and transverse bending of cantilever plates. Technical Note 2369, National Advisory Committee for Aeronautics,Washington, 1951.</ref> leads to the following coupled ordinary differential equations for a cantilever plate with concentrated end load <math>q_x(y)</math> at <math>x=a</math>.
:<math>
  \begin{align}
    & bD \frac{\mathrm{d}^4w_x}{\mathrm{d}x^4}  = 0 \\
    & \frac{b^3D}{12}\,\frac{\mathrm{d}^4\theta_x}{\mathrm{d}x^4} - 2bD(1-\nu)\cfrac{d^2 \theta_x}{d x^2} = 0
  \end{align}
</math>
and the boundary conditions at <math>x=a</math> are
:<math>
  \begin{align}
  & bD\cfrac{d^3 w_x}{d x^3} + q_{x1} = 0 \quad,\quad
  \frac{b^3D}{12}\cfrac{d^3 \theta_x}{d x^3} -2bD(1-\nu)\cfrac{d \theta_x}{d x} + q_{x2} = 0 \\
  & bD\cfrac{d^2 w_x}{d x^2} = 0 \quad,\quad  \frac{b^3D}{12}\cfrac{d^2 \theta_x}{d x^2} = 0 \,.
  \end{align}
</math>
Solution of this system of two ODEs gives
:<math>
  \begin{align}
    w_x(x) & = \frac{q_{x1}}{6bD}\,(3ax^2 -x^3) \\
    \theta_x(x) & = \frac{q_{x2}}{2bD(1-\nu)}\left[x - \frac{1}{\nu_b}\,
      \left(\frac{\sinh(\nu_b a)}{\cosh[\nu_b (x-a)]} + \tanh[\nu_b(x-a)]\right)\right]
  \end{align}
</math>
where <math>\nu_b = \sqrt{24(1-\nu)}/b</math>.  The bending moments and shear forces corresponding to the displacement
<math>w = w_x + y\theta_x</math> are
:<math>
  \begin{align}
    M_{xx} & = -D\left(\frac{\partial^2 w}{\partial x^2}+\nu\,\frac{\partial^2 w}{\partial y^2}\right) \\
            & = q_{x1}\left(\frac{x-a}{b}\right) - \left[\frac{3yq_{x2}}{b^3\nu_b\cosh^3[\nu_b(x-a)]}\right]
                \times \\
            & \quad \left[6\sinh(\nu_b a) - \sinh[\nu_b(2x-a)] +
                  \sinh[\nu_b(2x-3a)] + 8\sinh[\nu_b(x-a)]\right] \\
    M_{xy} & = (1-\nu)D\frac{\partial^2 w}{\partial x \partial y} \\
            & = \frac{q_{x2}}{2b}\left[1 -
                \frac{2+\cosh[\nu_b(x-2a)] - \cosh[\nu_b x]}{2\cosh^2[\nu_b(x-a)]}\right] \\
    Q_{zx} & = \frac{\partial M_{xx}}{\partial x}-\frac{\partial M_{xy}}{\partial y} \\
            & = \frac{q_{x1}}{b} - \left(\frac{3yq_{x2}}{2b^3\cosh^4[\nu_b(x-a)]}\right)\times
                \left[32 + \cosh[\nu_b(3x-2a)] - \cosh[\nu_b(3x-4a)]\right. \\
            & \qquad \left. - 16\cosh[2\nu_b(x-a)] +
                23\cosh[\nu_b(x-2a)] - 23\cosh(\nu_b x)\right]\,.
  \end{align}
</math>
The stresses are
:<math>
  \sigma_{xx} = \frac{12z}{h^3}\,M_{xx} \quad \text{and} \quad
  \sigma_{zx} = \frac{1}{\kappa h}\,Q_{zx}\left(1 - \frac{4z^2}{h^2}\right)\,.
</math>
If the applied load at the edge is constant, we recover the solutions for a beam under a
concentrated end load.  If the applied load is a linear function of <math>y</math>, then
:<math>
  q_{x1} = \int_{-b/2}^{b/2}q_0\left(\frac{1}{2} - \frac{y}{b}\right)\,\text{d}y = \frac{bq_0}{2} ~;~~
  q_{x2} = \int_{-b/2}^{b/2}yq_0\left(\frac{1}{2} - \frac{y}{b}\right)\,\text{d}y = -\frac{b^2q_0}{12} \,.
</math>
 
== See also ==
*[[Bending]]
*[[Infinitesimal strain theory]]
*[[Kirchhoff–Love plate theory]]
*[[Linear elasticity]]
*[[Mindlin–Reissner plate theory]]
*[[Plate theory]]
*[[Stress (mechanics)]]
*[[Stress resultants]]
*[[Structural acoustics]]
*[[Vibration of plates]]
 
== References ==
{{reflist}}
 
{{DEFAULTSORT:Bending Of Plates}}
[[Category:Continuum mechanics]]

Revision as of 02:39, 1 February 2014

Bending of an edge clamped circular plate under the action of a transverse pressure. The left half of the plate shows the deformed shape while the right half shows the undeformed shape. This calculation was performed using Ansys.

Bending of plates or plate bending refers to the deflection of a plate perpendicular to the plane of the plate under the action of external forces and moments. The amount of deflection can be determined by solving the differential equations of an appropriate plate theory. The stresses in the plate can be calculated from these deflections. Once the stresses are known, failure theories can be used to determine whether a plate will fail under a given load.

Bending of Kirchhoff-Love plates

Forces and moments on a flat plate.

In the Kirchhoff–Love plate theory for plates the governing equations are[1]

Nαβ,α=0

and

Mαβ,αβ−q=0

In expanded form,

∂N11∂x1+∂N21∂x2=0;∂N12∂x1+∂N22∂x2=0

and

∂2M11∂x12+2∂2M12∂x1∂x2+∂2M22∂x22=q

where q(x) is an applied transverse load per unit area, the thickness of the plate is H=2h, the stresses are σij, and

Nαβ:=∫−hhσαβdx3;Mαβ:=∫−hhx3σαβdx3.

The quantity N has units of force per unit length. The quantity M has units of moment per unit length.

For isotropic, homogeneous, plates with Young's modulus E and Poisson's ratio ν these equations reduce to[2]

∇2∇2w=−qD;D:=2h3E3(1−ν2)=H3E12(1−ν2)

where w(x1,x2) is the deflection of the mid-surface of the plate.

In rectangular Cartesian coordinates,

∂4w∂x14+2∂4w∂x12∂x22+∂4w∂x24=−qD.

Circular Kirchhoff-Love plates

The bending of circular plates can be examined by solving the governing equation with appropriate boundary conditions. These solutions were first found by Poisson in 1829. Cylindrical coordinates are convenient for such problems.

The governing equation in coordinate-free form is

∇2∇2w=−qD.

In cylindrical coordinates (r,θ,z),

∇2w≡1r∂∂r(r∂w∂r)+1r2∂2w∂θ2+∂2w∂z2.

For symmetrically loaded circular plates, w=w(r), and we have

∇2w≡1rddr(rdwdr).

Therefore, the governing equation is

1rddr[rddr{1rddr(rdwdr)}]=−qD.

If q and D are constant, direct integration of the governing equation gives us

w(r)=−qr464D+C1ln⁡r+C2r22+C3r24(2ln⁡r−1)+C4

where Ci are constants. The slope of the deflection surface is

ϕ(r)=dwdr=−qr316D+C1r+C2r+C3rln⁡r.

For a circular plate, the requirement that the deflection and the slope of the deflection are finite at r=0 implies that C1=C3=0.

Clamped edges

For a circular plate with clamped edges, we have w(a)=0 and ϕ(a)=0 at the edge of the plate (radius a). Using these boundary conditions we get

w(r)=−q64D(a2−r2)2andϕ(r)=qr16D(a2−r2).

The in-plane displacements in the plate are

ur(r)=−zϕ(r)anduθ(r)=0.

The in-plane strains in the plate are

εrr=durdr=−qz16D(a2−3r2),εθθ=urr=−qz16D(a2−r2),εrθ=0.

The in-plane stresses in the plate are

σrr=E1−ν2[εrr+νεθθ];σθθ=E1−ν2[εθθ+νεrr];σrθ=0.

For a plate of thickness 2h, the bending stiffness is D=2Eh3/[3(1−ν2)] and we have

σrr=−3qz32h3[(1+ν)a2−(3+ν)r2]σθθ=−3qz32h3[(1+ν)a2−(1+3ν)r2]σrθ=0.

The moment resultants (bending moments) are

Mrr=−q16[(1+ν)a2−(3+ν)r2];Mθθ=−q16[(1+ν)a2−(1+3ν)r2];Mrθ=0.

The maximum radial stress is at z=h and r=a:

σrr|z=h,r=a=3qa216h2=3qa24H2

where H:=2h. The bending moments at the boundary and the center of the plate are

Mrr|r=a=qa28,Mθθ|r=a=νqa28,Mrr|r=0=Mθθ|r=0=−(1+ν)qa216.

Rectangular Kirchhoff-Love plates

Bending of a rectangular plate under the action of a distributed force q per unit area.

For rectangular plates, Navier in 1820 introduced a simple method for finding the displacement and stress when a plate is simply supported. The idea was to express the applied load in terms of Fourier components, find the solution for a sinusoidal load (a single Fourier component), and then superimpose the Fourier components to get the solution for an arbitrary load.

Sinusoidal load

Let us assume that the load is of the form

q(x,y)=q0sin⁡πxasin⁡πyb.

Here q0 is the amplitude, a is the width of the plate in the x-direction, and b is the width of the plate in the y-direction.

Since the plate is simply supported, the displacement w(x,y) along the edges of the plate is zero, the bending moment Mxx is zero at x=0 and x=a, and Myy is zero at y=0 and y=b.

If we apply these boundary conditions and solve the plate equation, we get the solution

w(x,y)=q0π4D(1a2+1b2)−2sin⁡πxasin⁡πyb.

We can calculate the stresses and strains in the plate once we know the displacement.

For a more general load of the form

q(x,y)=q0sin⁡mπxasin⁡nπyb

where m and n are integers, we get the solution

(1)w(x,y)=q0π4D(m2a2+n2b2)−2sin⁡mπxasin⁡nπyb.

Let us now consider a more general load q(x,y). We can break this load up into a sum of Fourier components such that

q(x,y)=∑m=1∞∑n=1∞amnsin⁡mπxasin⁡nπyb

where amn is an amplitude. We can use the orthogonality of Fourier components,

∫0asin⁡kπxasin⁡ℓπxadx={0k≠ℓa/2k=ℓ

to find the amplitudes amn. Thus we have, by integrating over y,

∫0bq(x,y)sin⁡ℓπybdy=∑m=1∞∑n=1∞amnsin⁡mπxa∫0bsin⁡nπybsin⁡ℓπybdy=b2∑m=1∞amℓsin⁡mπxa.

If we repeat the process by integrating over x, we have

∫0b∫0aq(x,y)sin⁡kπxasin⁡ℓπybdxdy=b2∑m=1∞amℓ∫0asin⁡mπxasin⁡kπxadx=ab4akℓ.

Therefore,

amn=4ab∫0b∫0aq(x,y)sin⁡mπxasin⁡nπybdxdy.

Now that we know amn, we can just superpose solutions of the form given in equation (1) to get the displacement, i.e.,

(2)w(x,y)=∑m=1∞∑n=1∞amnπ4D(m2a2+n2b2)−2sin⁡mπxasin⁡nπyb.

Uniform load

Consider the situation where a uniform load is applied on the plate, i.e., q(x,y)=q0. Then

amn=4q0ab∫0a∫0bsin⁡mπxasin⁡nπybdxdy.

Now

∫0asin⁡mπxadx=amπ(1−cos⁡mπ)and∫0bsin⁡nπybdy=bnπ(1−cos⁡nπ).

We can use these relations to get a simpler expression for amn:

amn=4q0mnπ2(1−cos⁡mπ)(1−cos⁡nπ).

Since cos⁡mπ=cos⁡nπ=1 [ so (1−cos⁡mπ)=(1−cos⁡nπ)=0 ] when m and n are even, we can get an even simpler expression for amn when both m and n are odd:

amn={0morneven,16q0mnπ2mandnodd.

Plugging this expression into equation (2) and keeping in mind that only odd terms contribute to the displacement, we have

w(x,y)=∑m=1∞∑n=1∞16q0(2m−1)(2n−1)π6D[(2m−1)2a2+(2n−1)2b2]−2×sin⁡(2m−1)πxasin⁡(2n−1)πyb.

The corresponding moments are given by

Mxx=−D(∂2w∂x2+ν∂2w∂y2)=∑m=1∞∑n=1∞16q0(2m−1)(2n−1)π4[(2m−1)2a2+ν(2n−1)2b2]×[(2m−1)2a2+(2n−1)2b2]−2sin⁡(2m−1)πxasin⁡(2n−1)πybMyy=−D(∂2w∂y2+ν∂2w∂x2)=∑m=1∞∑n=1∞16q0(2m−1)(2n−1)π4[(2n−1)2b2+ν(2m−1)2a2]×[(2m−1)2a2+(2n−1)2b2]−2sin⁡(2m−1)πxasin⁡(2n−1)πyb.

The stresses in the plate are

σxx=3z2h3Mxx=12zH3Mxxandσyy=3z2h3Myy=12zH3Myy.
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Levy solution

Another approach was proposed by Levy in 1899. In this case we start with an assumed form of the displacement and try to fit the parameters so that the governing equation and the boundary conditions are satisfied.

Let us assume that

w(x,y)=∑m=1∞Ym(y)sin⁡mπxa.

For a plate that is simply supported at x=0 and x=a, the boundary conditions are w=0 and Mxx=0. The moment boundary condition is equivalent to ∂2w/∂x2=0 (verify). The goal is to find Ym(y) such that it satisfies the boundary conditions at y=0 and y=b and, of course, the governing equation ∇2∇2w=q/D.

Moments along edges

Let us consider the case of pure moment loading. In that case q=0 and w(x,y) has to satisfy ∇2∇2w=0. Since we are working in rectangular Cartesian coordinates, the governing equation can be expanded as

∂4w∂x4+2∂4w∂x2∂y2+∂4w∂y4=0.

Plugging the expression for w(x,y) in the governing equation gives us

∑m=1∞[(mπa)4Ymsin⁡mπxa−2(mπa)2d2Ymdy2sin⁡mπxa+d4Ymdy4sin⁡mπxa]=0

or

d4Ymdy4−2m2π2a2d2Ymdy2+m4π4a4Ym=0.

This is an ordinary differential equation which has the general solution

Ym=Amcosh⁡mπya+Bmmπyacosh⁡mπya+Cmsinh⁡mπya+Dmmπyasinh⁡mπya

where Am,Bm,Cm,Dm are constants that can be determined from the boundary conditions. Therefore the displacement solution has the form

w(x,y)=∑m=1∞[(Am+Bmmπya)cosh⁡mπya+(Cm+Dmmπya)sinh⁡mπya]sin⁡mπxa.

Let us choose the coordinate system such that the boundaries of the plate are at x=0 and x=a (same as before) and at y=±b/2 (and not y=0 and y=b). Then the moment boundary conditions at the y=±b/2 boundaries are

w=0,−D∂2w∂y2|y=b/2=f1(x),−D∂2w∂y2|y=−b/2=f2(x)

where f1(x),f2(x) are known functions. The solution can be found by applying these boundary conditions. We can show that for the symmetrical case where

Myy|y=−b/2=Myy|y=b/2

and

f1(x)=f2(x)=∑m=1∞Emsin⁡mπxa

we have

w(x,y)=a22π2D∑m=1∞Emm2cosh⁡αmsin⁡mπxa(αmtanh⁡αmcosh⁡mπya−mπyasinh⁡mπya)

where

αm=mπb2a.

Similarly, for the antisymmetrical case where

Myy|y=−b/2=−Myy|y=b/2

we have

w(x,y)=a22π2D∑m=1∞Emm2sinh⁡αmsin⁡mπxa(αmcoth⁡αmsinh⁡mπya−mπyacosh⁡mπya).

We can superpose the symmetric and antisymmetric solutions to get more general solutions.

Uniform and symmetric moment load

For the special case where the loading is symmetric and the moment is uniform, we have at y=±b/2,

Myy=f1(x)=4M0π∑m=1∞12m−1sin⁡(2m−1)πxa.
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After obtaining a Sale License (topic to authorities circumstances meant to guard individuals shopping for property in Singapore), he might www.rmmonline.com proceed to sell units in his growth. Funding in the property market of Singapore is likely one of the few investment choices the place utilizing the bank's cash could not be any easier. The power of expats, to make a down payment, leverage the capital and consequently improve total return on investment, is excessive in Singapore. PRs who own a HDB flat must sell their flat inside six months of buying a non-public residential property in Singapore. EVERLASTING residents (PRs) now face unprecedented limits on their skill to purchase property in Singapore. New Condo in 2013 March Foreigners are eligible for Singapore greenback mortgage loan. housing grant

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The resulting displacement is

w(x,y)=2M0a2π3D∑m=1∞1(2m−1)3cosh⁡αmsin⁡(2m−1)πxa×[αmtanh⁡αmcosh⁡(2m−1)πya−(2m−1)πyasinh⁡(2m−1)πya]

where

αm=π(2m−1)b2a.

The bending moments and shear forces corresponding to the displacement w are

Mxx=−D(∂2w∂x2+ν∂2w∂y2)=2M0(1−ν)π∑m=1∞1(2m−1)cosh⁡αmsin⁡(2m−1)πxa[−(2m−1)πyasinh⁡(2m−1)πya+{2ν1−ν+αmtanh⁡αm}cosh⁡(2m−1)πya]Mxy=(1−ν)D∂2w∂x∂y=−2M0(1−ν)π∑m=1∞1(2m−1)cosh⁡αmcos⁡(2m−1)πxa[(2m−1)πyacosh⁡(2m−1)πya+(1−αmtanh⁡αm)sinh⁡(2m−1)πya]Qzx=∂Mxx∂x−∂Mxy∂y=4M0a∑m=1∞1cosh⁡αmcos⁡(2m−1)πxacosh⁡(2m−1)πya.

The stresses are

σxx=12zh3Mxxandσzx=1κhQzx(1−4z2h2).

Cylindrical plate bending

Cylindrical bending occurs when a rectangular plate that has dimensions a×b×h, where a≪b and the thickness h is small, is subjected to a uniform distributed load perpendicular to the plane of the plate. Such a plate takes the shape of the surface of a cylinder.

Simply supported plate with axially fixed ends

For a simply supported plate under cylindrical bending with edges that are free to rotate but have a fixed x1. Cylindrical bending solutions can be found using the Navier and Levy techniques.

Bending of thick Mindlin plates

For thick plates, we have to consider the effect of through-the-thickness shears on the orientation of the normal to the mid-surface after deformation. Mindlin's theory provides one approach for find the deformation and stresses in such plates. Solutions to Mindlin's theory can be derived from the equivalent Kirchhoff-Love solutions using canonical relations.[3]

Governing equations

The canonical governing equation for isotropic thick plates can be expressed as[3]

∇2(ℳ−ℬ1+νq)=−qκGh(∇2w+ℳD)=−(1−ℬc21+ν)q∇2(∂φ1∂x2−∂φ2∂x1)=c2(∂φ1∂x2−∂φ2∂x1)

where q is the applied transverse load, G is the shear modulus, D=Eh3/[12(1−ν2)] is the bending rigidity, h is the plate thickness, c2=2κGh/[D(1−ν)], κ is the shear correction factor, E is the Young's modulus, ν is the Poisson's ratio, and

ℳ=D[𝒜(∂φ1∂x1+∂φ2∂x2)−(1−𝒜)∇2w]+2q1−ν2ℬ.

In Mindlin's theory, w is the transverse displacement of the mid-surface of the plate and the quantities φ1 and φ2 are the rotations of the mid-surface normal about the x2 and x1-axes, respectively. The canonical parameters for this theory are 𝒜=1 and ℬ=0. The shear correction factor κ usually has the value 5/6.

The solutions to the governing equations can be found if one knows the corresponding Kirchhoff-Love solutions by using the relations

w=wK+ℳKκGh(1−ℬc22)−Φ+Ψφ1=−∂wK∂x1−1κGh(1−1𝒜−ℬc22)Q1K+∂∂x1(DκGh𝒜∇2Φ+Φ−Ψ)+1c2∂Ω∂x2φ2=−∂wK∂x2−1κGh(1−1𝒜−ℬc22)Q2K+∂∂x2(DκGh𝒜∇2Φ+Φ−Ψ)+1c2∂Ω∂x1

where wK is the displacement predicted for a Kirchhoff-Love plate, Φ is a biharmonic function such that ∇2∇2Φ=0, Ψ is a function that satisfies the Laplace equation, ∇2Ψ=0, and

ℳ=ℳK+ℬ1+νq+D∇2Φ;ℳK:=−D∇2wKQ1K=−D∂∂x1(∇2wK),Q2K=−D∂∂x2(∇2wK)Ω=∂φ1∂x2−∂φ2∂x1,∇2Ω=c2Ω.

Simply supported rectangular plates

For simply supported plates, the Marcus moment sum vanishes, i.e.,

ℳ=11+ν(M11+M22)=D(∂φ1∂x2+∂φ2∂x2)=0.

In that case the functions Φ, Ψ, Ω vanish, and the Mindlin solution is related to the corresponding Kirchhoff solution by

w=wK+ℳKκGh.

Bending of Reissner-Stein cantilever plates

Reissner-Stein theory for cantilever plates[4] leads to the following coupled ordinary differential equations for a cantilever plate with concentrated end load qx(y) at x=a.

bDd4wxdx4=0b3D12d4θxdx4−2bD(1−ν)d2θxdx2=0

and the boundary conditions at x=a are

bDd3wxdx3+qx1=0,b3D12d3θxdx3−2bD(1−ν)dθxdx+qx2=0bDd2wxdx2=0,b3D12d2θxdx2=0.

Solution of this system of two ODEs gives

wx(x)=qx16bD(3ax2−x3)θx(x)=qx22bD(1−ν)[x−1νb(sinh⁡(νba)cosh⁡[νb(x−a)]+tanh⁡[νb(x−a)])]

where νb=24(1−ν)/b. The bending moments and shear forces corresponding to the displacement w=wx+yθx are

Mxx=−D(∂2w∂x2+ν∂2w∂y2)=qx1(x−ab)−[3yqx2b3νbcosh3[νb(x−a)]]×[6sinh⁡(νba)−sinh⁡[νb(2x−a)]+sinh⁡[νb(2x−3a)]+8sinh⁡[νb(x−a)]]Mxy=(1−ν)D∂2w∂x∂y=qx22b[1−2+cosh⁡[νb(x−2a)]−cosh⁡[νbx]2cosh2[νb(x−a)]]Qzx=∂Mxx∂x−∂Mxy∂y=qx1b−(3yqx22b3cosh4[νb(x−a)])×[32+cosh⁡[νb(3x−2a)]−cosh⁡[νb(3x−4a)]−16cosh⁡[2νb(x−a)]+23cosh⁡[νb(x−2a)]−23cosh⁡(νbx)].

The stresses are

σxx=12zh3Mxxandσzx=1κhQzx(1−4z2h2).

If the applied load at the edge is constant, we recover the solutions for a beam under a concentrated end load. If the applied load is a linear function of y, then

qx1=∫−b/2b/2q0(12−yb)dy=bq02;qx2=∫−b/2b/2yq0(12−yb)dy=−b2q012.

See also

References

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  1. ↑ Reddy, J. N., 2007, Theory and analysis of elastic plates and shells, CRC Press, Taylor and Francis.
  2. ↑ Timoshenko, S. and Woinowsky-Krieger, S., (1959), Theory of plates and shells, McGraw-Hill New York.
  3. ↑ 3.0 3.1 Lim, G. T. and Reddy, J. N., 2003, On canonical bending relationships for plates, International Journal of Solids and Structures, vol. 40, pp. 3039-3067.
  4. ↑ E. Reissner and M. Stein. Torsion and transverse bending of cantilever plates. Technical Note 2369, National Advisory Committee for Aeronautics,Washington, 1951.