Bending - Wikipedia

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 thin-walled, 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,[tiga] the bending of shells[2] and so on.Quasi-static bending of beams[edit]

A beam deforms and stresses develop inside it when a transverse load is applied on it. In the quasi-static 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 over-side of the beam is compressed while the material at the underside is stretched. There are two forms of internal stresses caused by lateral loads:Shear stress parallel to the lateral loading plus complementary shear stress on planes perpendicular to the load direction;Direct compressive stress in the upper region of the beam, and direct tensile stress in the lower region of the beam.

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 when some simplifying assumptions are used.[1]Euler–Bernoulli bending theory[edit]

Element of a bent beam: the fibers form concentric arcs, the top fibers are compressed and bottom fibers stretched.

In the Euler–Bernoulli 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 tertekan experienced in the section (at the furthest points from the neutral axis of the beam) is defined as the flexural strength.

Consider beams where the following are true:The beam is originally straight and slender, and any taper is slightThe material is isotropic (or orthotropic), linear elastic, and homogeneous across any cross section (but not necessarily along its length)Only small deflections are considered

In this case, the equation describing beam deflection () can be approximated as:

where the second derivative of its deflected shape with respect to is interpreted as its curvature, is the Young's modulus, is the area moment of inertia of the cross-section, and is the internal bending moment in the beam.

If, in addition, the beam is homogeneous along its length as well, and not tapered (i.e. constant cross section), and deflects under an applied transverse load , it can be shown that:[1]

This is the Euler–Bernoulli equation for beam bending.

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 Euler–Bernoulli beam equation. The conditions for using simple bending theory are:[4]The beam is subject to pure bending. This means that the shear force is zero, and that no torsional or axial loads are present.The material is isotropic (or orthotropic) and homogeneous.The material obeys Hooke's law (it is linearly elastic and will not deform plastically).The beam is initially straight with a cross section that is constant throughout the beam length.The beam has an axis of symmetry in the plane of bending.The proportions of the beam are such that it would fail by bending rather than by crushing, wrinkling or sideways buckling.Cross-sections of the beam remain plane during bending.

Deflection of a beam deflected symmetrically and principle of superposition

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 tertekan 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 tertekan and the adjacent areas with low tertekan, 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. Wide-flange beams (I-beams) and truss girders effectively address this inefficiency as they minimize the amount of material in this under-stressed region.

The classic formula for determining the bending stress in a beam under simple bending is:[lima]

whereExtensions of Euler-Bernoulli beam bending theory[edit]Plastic bending[edit]

The equation is valid only when the tertekan at the extreme fiber (i.e., the portion of the beam farthest from the neutral axis) is below the yield tertekan of the material from which it is constructed. At higher loadings the stress distribution becomes non-linear, 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.Complex or asymmetrical bending[edit]

The equation above is only valid if the cross-section is symmetrical. For homogeneous beams with asymmetrical sections, the maximum bending stress in the beam is given by[6]

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 tertekan at any point on the beam cross section regardless of moment orientation or cross-sectional shape. Note that do not change from one point to another on the cross section.Large bending deformation[edit]

For large deformations of the body, the stress in the cross-section is calculated using an extended version of this formula. First the following assumptions must be made:Assumption of flat sections – before and after deformation the considered section of body remains flat (i.e., is not swirled).Shear and normal stresses in this section that are perpendicular to the normal vector of cross section have no influence on normal stresses that are parallel to this section.

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 is the normal force is the section area is the bending moment is the local bending radius (the radius of bending at the current section) is the area moment of inertia along the x-axis, at the place (see Steiner's theorem) is the position along y-axis on the section area in which the stress is calculated.

When bending radius approaches infinity and , the original formula is back:.Timoshenko bending theory[edit]

Deformation of a Timoshenko beam. The normal rotates by an amount which is not equal to .