Bending of plates, or plate bending, refers to the
deflection
Deflection or deflexion may refer to:
Board games
* Deflection (chess), a tactic that forces an opposing chess piece to leave a square
* Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers
Mechanics
* Deflection ...
of a plate perpendicular to the plane of the plate under the action of external
force
In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
s 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
Definitions
For a thin rectangular plate of thickness ,
Young's modulus
Young's modulus (or the Young modulus) is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Youn ...
, and
Poisson's ratio
In materials science and solid mechanics, Poisson's ratio (symbol: ( nu)) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value ...
, we can define parameters in terms of the plate deflection, .
The
flexural rigidity
Flexural rigidity is defined as the force couple required to bend a fixed non- rigid structure by one unit of curvature, or as the resistance offered by a structure while undergoing bending.
Flexural rigidity of a beam
Although the moment M(x) ...
is given by
:
Moments
The
bending moment
In solid mechanics, a bending moment is the Reaction (physics), reaction induced in a structural element when an external force or Moment of force, moment is applied to the element, causing the element to bending, bend. The most common or simplest ...
s per unit length are given by
:
:
The twisting moment per unit length is given by
:
Forces
The
shear force
In solid mechanics, shearing forces are unaligned forces acting on one part of a Rigid body, body in a specific direction, and another part of the body in the opposite direction. When the forces are Collinearity, collinear (aligned with each ot ...
shear stress
Shear stress (often denoted by , Greek alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...
is given by
:
Strains
The bending strains for small-deflection theory are given by
:
:
The
shear strain
In mechanics, strain is defined as relative deformation, compared to a position configuration. Different equivalent choices may be made for the expression of a strain field depending on whether it is defined with respect to the initial or the ...
for small-deflection theory is given by
:
For large-deflection plate theory, we consider the inclusion of membrane strains
:
:
:
Kirchhoff–Love plate theory
The Kirchhoff–Love theory of plates is a two-dimensional mathematical model that is used to determine the stresses and deformations in thin plates subjected to forces and moments. This theory is an extension of Euler-Bernoulli beam theory ...
for plates the governing equations areReddy, J. N., 2007, Theory and analysis of elastic plates and shells, CRC Press, Taylor and Francis.
:
and
:
In expanded form,
:
and
:
where is an applied transverse load per unit area, the thickness of the plate is , the stresses are , and
:
The quantity has units of
force
In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
per unit length. The quantity has units of moment per unit length.
For
isotropic
In physics and geometry, isotropy () is uniformity in all orientations. Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence '' anisotropy''. ''Anisotropy'' is also ...
,
homogeneous
Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...
, plates with
Young's modulus
Young's modulus (or the Young modulus) is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Youn ...
and
Poisson's ratio
In materials science and solid mechanics, Poisson's ratio (symbol: ( nu)) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value ...
these equations reduce toTimoshenko, S. and Woinowsky-Krieger, S., (1959), Theory of plates and shells, McGraw-Hill New York.
:
where is the deflection of the mid-surface of the plate.
Small deflection of thin rectangular plates
This is governed by the
Germain
Germain may refer to:
*Germain (name), including a list of people with the name
*Germain Arena, the former name of an arena in Estero, Florida
*Germain Racing, a NASCAR racing team
*Germain Amphitheater, a concert venue in Columbus, Ohio
*Paris Sa ...
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. Here is the distance of a point from the midplane of the plate.
The governing equation in coordinate-free form is
:
In cylindrical coordinates ,
:
For symmetrically loaded circular plates, , and we have
:
Therefore, the governing equation is
:
If and are constant, direct integration of the governing equation gives us
:
where are constants. The slope of the deflection surface is
:
For a circular plate, the requirement that the deflection and the slope of the deflection are finite
at implies that . However, need not equal 0, as the limit
of exists as you approach from the right.
Clamped edges
For a circular plate with clamped edges, we have and at the edge of
the plate (radius ). Using these boundary conditions we get
:
The in-plane displacements in the plate are
:
The in-plane strains in the plate are
:
The in-plane stresses in the plate are
:
For a plate of thickness , the bending stiffness is