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In Applied mechanics, bending (also known as flexure) characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element.
The structural element is assumed to be such that at least one of its dimensions is a small fraction, typically 1/10 or less, of the other two.^{[1]} When the length is considerably longer than the width and the thickness, the element is called a beam. For example, a closet rod sagging under the weight of clothes on clothes hangers is an example of a beam experiencing bending. On the other hand, a shell is a structure of any geometric form where the length and the width are of the same order of magnitude but the thickness of the structure (known as the 'wall') is considerably smaller. A large diameter, but thinwalled, short tube supported at its ends and loaded laterally is an example of a shell experiencing bending.
In the absence of a qualifier, the term bending is ambiguous because bending can occur locally in all objects. Therefore, to make the usage of the term more precise, engineers refer to a specific object such as; the bending of rods,^{[2]} the bending of beams,^{[1]} the bending of plates,^{[3]} the bending of shells^{[2]} and so on.
A beam deforms and stresses develop inside it when a transverse load is applied on it. In the quasistatic case, the amount of bending deflection and the stresses that develop are assumed not to change over time. In a horizontal beam supported at the ends and loaded downwards in the middle, the material at the overside of the beam is compressed while the material at the underside is stretched. There are two forms of internal stresses caused by lateral loads:
These last two forces form a couple or moment as they are equal in magnitude and opposite in direction. This bending moment resists the sagging deformation characteristic of a beam experiencing bending. The stress distribution in a beam can be predicted quite accurately even when some simplifying assumptions are used.^{[1]}
In the EulerBernoulli theory of slender beams, a major assumption is that 'plane sections remain plane'. In other words, any deformation due to shear across the section is not accounted for (no shear deformation). Also, this linear distribution is only applicable if the maximum stress is less than the yield stress of the material. For stresses that exceed yield, refer to article plastic bending. At yield, the maximum stress experienced in the section (at the furthest points from the neutral axis of the beam) is defined as the flexural strength.
The EulerBernoulli equation for the quasistatic bending of slender, isotropic, homogeneous beams of constant crosssection under an applied transverse load is^{[1]}
where is the Young's modulus, is the area moment of inertia of the crosssection, and is the deflection of the neutral axis of the beam.
After a solution for the displacement of the beam has been obtained, the bending moment () and shear force () in the beam can be calculated using the relations
Simple beam bending is often analyzed with the EulerBernoulli beam equation. The conditions for using simple bending theory are:^{[4]}
Compressive and tensile forces develop in the direction of the beam axis under bending loads. These forces induce stresses on the beam. The maximum compressive stress is found at the uppermost edge of the beam while the maximum tensile stress is located at the lower edge of the beam. Since the stresses between these two opposing maxima vary linearly, there therefore exists a point on the linear path between them where there is no bending stress. The locus of these points is the neutral axis. Because of this area with no stress and the adjacent areas with low stress, using uniform cross section beams in bending is not a particularly efficient means of supporting a load as it does not use the full capacity of the beam until it is on the brink of collapse. Wideflange beams (Ibeams) and truss girders effectively address this inefficiency as they minimize the amount of material in this understressed region.
The classic formula for determining the bending stress in a beam under simple bending is:^{[5]}
where
The equation is valid only when the stress at the extreme fiber (i.e., the portion of the beam farthest from the neutral axis) is below the yield stress of the material from which it is constructed. At higher loadings the stress distribution becomes nonlinear, and ductile materials will eventually enter a plastic hinge state where the magnitude of the stress is equal to the yield stress everywhere in the beam, with a discontinuity at the neutral axis where the stress changes from tensile to compressive. This plastic hinge state is typically used as a limit state in the design of steel structures.
The equation above is only valid if the crosssection is symmetrical. For homogeneous beams with asymmetrical sections, the axial stress in the beam is given by
where are the coordinates of a point on the cross section at which the stress is to be determined as shown to the right, and are the bending moments about the y and z centroid axes, and are the second moments of area (distinct from moments of inertia) about the y and z axes, and is the product of moments of area. Using this equation it is possible to calculate the bending stress at any point on the beam cross section regardless of moment orientation or crosssectional shape. Note that do not change from one point to another on the cross section.
For large deformations of the body, the stress in the crosssection is calculated using an extended version of this formula. First the following assumptions must be made:
Large bending considerations should be implemented when the bending radius is smaller than ten section heights h:
With those assumptions the stress in large bending is calculated as:
where
When bending radius approaches infinity and , the original formula is back:
In 1921, Timoshenko improved upon the EulerBernoulli theory of beams by adding the effect of shear into the beam equation. The kinematic assumptions of the Timoshenko theory are:
However, normals to the axis are not required to remain perpendicular to the axis after deformation.
The equation for the quasistatic bending of a linear elastic, isotropic, homogeneous beam of constant crosssection beam under these assumptions is^{[7]}
where is the area moment of inertia of the crosssection, is the crosssectional area, is the shear modulus, and is a shear correction factor. For materials with Poisson's ratios () close to 0.3, the shear correction factor for a rectangular crosssection is approximately
The rotation () of the normal is described by the equation
The bending moment () and the shear force () are given by
The dynamic bending of beams,^{[8]} also known as flexural vibrations of beams, was first investigated by Daniel Bernoulli in the late 18th century. Bernoulli's equation of motion of a vibrating beam tended to overestimate the natural frequencies of beams and was improved marginally by Rayleigh in 1877 by the addition of a midplane rotation. In 1921 Stephen Timoshenko improved the theory further by incorporating the effect of shear on the dynamic response of bending beams. This allowed the theory to be used for problems involving high frequencies of vibration where the dynamic EulerBernoulli theory is inadequate. The EulerBernoulli and Timoshenko theories for the dynamic bending of beams continue to be used widely by engineers.
The EulerBernoulli equation for the dynamic bending of slender, isotropic, homogeneous beams of constant crosssection under an applied transverse load is^{[7]}
where is the Young's modulus, is the area moment of inertia of the crosssection, is the deflection of the neutral axis of the beam, and is mass per unit length of the beam.
For the situation where there is no transverse load on the beam, the bending equation takes the form
Free, harmonic vibrations of the beam can then be expressed as
and the bending equation can be written as
The general solution of the above equation is
where are constants and
The mode shapes of a cantilevered Ibeam  

In 1877, Rayleigh proposed an improvement to the dynamic EulerBernoulli beam theory by including the effect of rotational inertia of the crosssection of the beam. Timoshenko improved upon that theory in 1922 by adding the effect of shear into the beam equation. Shear deformations of the normal to the midsurface of the beam are allowed in the TimoshenkoRayleigh theory.
The equation for the bending of a linear elastic, isotropic, homogeneous beam of constant crosssection beam under these assumptions is ^{[7]}^{[9]}
where is the polar moment of inertia of the crosssection, is the mass per unit length of the beam, is the density of the beam, is the crosssectional area, is the shear modulus, and is a shear correction factor. For materials with Poisson's ratios () close to 0.3, the shear correction factor are approximately
For free, harmonic vibrations the TimoshenkoRayleigh equations take the form
This equation can be solved by noting that all the derivatives of must have the same form to cancel out and hence as solution of the form may be expected. This observation leads to the characteristic equation
The solutions of this quartic equation are
where
The general solution of the TimoshenkoRayleigh beam equation for free vibrations can then be written as
The defining feature of beams is that one of the dimensions is much larger than the other two. A structure is called a plate when it is flat and one of its dimensions is much smaller than the other two. There are several theories that attempt to describe the deformation and stress in a plate under applied loads two of which have been used widely. These are
The assumptions of KirchhoffLove theory are
These assumptions imply that
where is the displacement of a point in the plate and is the displacement of the midsurface.
The straindisplacement relations are
The equilibrium equations are
where is an applied load normal to the surface of the plate.
In terms of displacements, the equilibrium equations for an isotropic, linear elastic plate in the absence of external load can be written as
In direct tensor notation,
The special assumption of this theory is that normals to the midsurface remain straight and inextensible but not necessarily normal to the midsurface after deformation. The displacements of the plate are given by
where are the rotations of the normal.
The straindisplacement relations that result from these assumptions are
where is a shear correction factor.
The equilibrium equations are
where
The dynamic theory of plates determines the propagation of waves in the plates, and the study of standing waves and vibration modes. The equations that govern the dynamic bending of Kirchhoff plates are
where, for a plate with density ,
and
The figures below show some vibrational modes of a circular plate.
