Name: unsymmetrical bending and neutral axis
Uploaded: 2024-05-23T12:59:44.000Z
Duration: 1 min 12 s
Channel: JoVE

Considérons un membre soumis à des forces excentriques égales et opposées. Ces forces sont appliquées à une distance horizontale de a et à une distance verticale de b par rapport à l’axe centroïdal principal de l’élément.
Chaque force excentrique est égale à la force centrale et à deux moments de couple, les moments les bras étant éloignés a et b de l’axe centroïdal principal de l’élément.
En utilisant le principe de Saint-Venant, les charges équivalentes peuvent être utilisées pour déterminer la répartition de la contrainte au niveau de la section de l’élément. L’analyse se vérifie lorsque la section n’est pas proche de l’une ou l’autre des extrémités du membre.
Le principe de superposition détermine les contraintes dues aux forces centrales et aux couples de flexion, et montre que les contraintes varient linéairement le long de la section. 
En fonction de la géométrie de l’élément et de la ligne d’action des charges excentriques, la contrainte totale peut avoir le même signe ou un signe opposé sur toute la section. La ligne le long de la section pour laquelle l’amplitude de la contrainte est nulle est appelée axe neutre de la section.

cas général de chargement axial excentrique

General Case of Eccentric Axial Loading

Une flexion asymétrique se produit lorsque le moment de flexion appliqué à un élément structurel ne s'aligne pas avec son axe principal. Ce désalignement conduit à des répartitions de contraintes et à des modèles de déflexion complexes qui diffèrent de la flexion symétrique, qui sont essentiels pour concevoir des structures capables de résister à différentes conditions de charge.

Considérons une barre soumise à des forces égales et opposées appliquées le long d’une ligne qui ne coïncide pas avec l’axe neutre de la barre. En flexion asymétrique, comme décrit ici, l'axe neutre—où la contrainte est nulle—ne s'aligne pas nécessairement avec les axes géométriques de la section-transversale. Il en résulte des moments autour de plusieurs axes, qui contrecarrent l'excentricité des forces. La répartition des contraintes dépend de la relation entre la charge appliquée et les propriétés géométriques de la section. La position de l'axe neutre est déterminée en s'assurant que la somme des contraintes normales à travers la section est égale à zéro.

Le moment de couple en flexion asymétrique fait référence aux moments provoqués par des forces qui ne passent pas par le centroïde de la section. Ces moments entraînent une flexion autour de plusieurs axes et sont essentiels pour déterminer la répartition des contraintes sur l'élément.

La limite proportionnelle est le niveau de contrainte au-delà duquel le matériau se déforme de manière non-linéaire, marquant la fin du comportement élastique. Le produit d'inertie mesure la covariance des coordonnées des éléments de surface de la section par rapport aux axes. Si les axes s'alignent avec les axes centroïdaux de la section, simplifiant les calculs de contraintes, l'axe neutre coïncidera avec ces axes, ce qui en fera des axes principaux de flexion.

Cas général de chargement axial excentrique

Considérons un membre soumis à des forces excentriques égales et opposées. Ces forces sont appliquées à une distance horizontale de a et à une distance verticale de b ...

Education

Enseignement

JoVE Core

Mechanical Engineering

Génie mécanique

Unit 1

Unité 1

Bending

Flexion

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

unsymmetrical bending and neutral axis

Unsymmetrical bending happens when the bending moment on a structural member does not line up with its principal axis. This creates stress and deflection patterns that differ from symmetrical bending. Understanding this case helps explain how structures respond to different loading conditions.

The member in this situation is acted on by equal and opposite forces. The forces are placed along a line that does not pass through the neutral axis. The neutral axis is the line where stress is zero, and in unsymmetrical bending it may not match the geometric axes of the cross-section.

Because the load is eccentric, moments develop around more than one axis. These moments help balance the offset of the forces. The stress pattern depends on the relation between the applied load and the shape of the section.

The position of the neutral axis is found by setting the sum of the normal stresses across the section equal to zero. The couple moment in unsymmetrical bending comes from forces that do not pass through the centroid of the cross-section. This is why bending occurs about multiple axes.

The proportional limit is the stress level where material behavior stops being linear and elastic behavior ends. The product of inertia describes how area elements are spread relative to a set of axes. When the axes match the centroidal axes of the section, stress calculations become simpler, and the neutral axis aligns with them. In that case, these axes are the principal axes for bending.

Unsymmetrical Bending and Neutral Axis

bending

Pure Bending in Beams and Structures

Pure bending describes how a beam bends under equal and opposite moments. It is a key idea in structural mechanics. Engineers use it to study how prismatic members, such as beams, deform under symmetrical loads without direct forces on the member. In pure bending, the beam experiences bending without axial force or shear force. This makes it possible to predict the stress pattern across the cross-section. The bending stress depends on the bending moment, the moment of inertia of the...

symmetric member in bending

Bending Stress in a Symmetric Member

Bending stress in a symmetric member comes from a prismatic member that is loaded by two couples acting in opposite directions. A couple is made of two equal and opposite forces. It creates a moment, but it does not create a resultant force. To keep the member in equilibrium, internal forces appear at any section cut. These internal forces must balance the external couples. They are resolved into normal stress and shear stress components. Normal stress acts perpendicular to the...

deformations in a symmetric member in bending

deformations-in-a-symmetric-member-in-bending

Neutral Axis and Strain in Bending

A symmetric prismatic member in bending shows how neutral surfaces, strain, and stress work together. When equal and opposite couples bend the member, the straight lines on its wider faces become circular arcs centered at Point C. That curved shape helps show how deformation changes across the section. If the member is divided into tiny cubic elements, the main stress inside it is normal stress. This creates uniaxial stress conditions at any point. The bending pattern also reveals a neutral...

flexural stress

Bending Stress and the Neutral Axis

Bending stress in a symmetric member depends on how the material responds to a bending moment. Within the elastic limit, Hooke's Law applies, so stress is proportional to strain. That relationship helps describe how the member stretches and compresses as it bends. In a bent member, strain depends on distance from the neutral axis. The neutral axis is the central layer that has no longitudinal strain. Strain is zero at that axis and increases linearly toward the outermost fibers. Because...

deformations in a transverse cross section

deformations-in-a-transverse-cross-section

Poisson Effects in Beam Bending

Poisson effects in beam bending describe how a material changes shape in directions perpendicular to the applied force. When a material is under uniaxial stress, it stretches or contracts along the load and also changes in the crosswise directions. Poisson's ratio, written as v, measures the ratio of transverse strain to axial strain. As the material stretches, it may expand or contract in directions orthogonal to the load. These changes are not the same in every part of the material. In the...

bending of material: problem solving

Pipe Wall Thickness and Bending Strength

Pipe wall thickness and bending strength are compared by finding the ratio of the maximum bending moments for two metal pipes. Both pipes can withstand a maximum stress of 100 MPa. Each pipe has the same outer radius of 1.8 cm, but Pipe A has an inner radius of 1.5 cm and Pipe B has an inner radius of 1 cm. The first step is to calculate the moment of inertia for each pipe. The moment of inertia depends on the pipe dimensions and is found using the given equation for a hollow circular section.

bending of members made of several materials

bending-of-members-made-of-several-materials

Composite Beam Stress and Neutral Axis

Composite beam stress depends on the elastic properties of each material in the member. When a structural member is made of two materials with the same cross-sectional area, each material responds differently under load because its modulus of elasticity is different. To analyze this behavior, engineers use the transformed section concept, which compares the two materials in a single equivalent section. Hooke’s Law links stress and strain in each material. Stress is proportional to strain, but...

stress concentrations

Stress at Holes and Notches in Bending

Stress at holes and notches in bending shows how a material can carry more load in one small area than in the rest of the cross-section. For a symmetric member under pure bending, the stress is usually treated as evenly spread across the cross-section. That simple assumption works only when the cross-section is uniform and has no irregularities. When the shape changes, the stress pattern changes too. Notches, holes, and other discontinuities can raise the local stress above the nominal stress...

plastic deformations

Plastic Bending and the Neutral Axis

Plastic bending changes the shape of a structural member after the bending stress goes beyond the material’s yield strength. Before yielding, the member behaves in a linear elastic way. After yielding, the deformation is permanent. The strain at any point in the member is described in terms of the maximum strain. During elastic bending, the neutral axis, where strain is zero, matches the centroid. Under plastic conditions, that axis shifts away from the centroid. Finding its new position uses...

members made of elastoplastic material

Bending Stages of Elastoplastic Members

Elastoplastic members under bending show a clear change in stress as the load increases. For a rectangular cross-section, the first response is elastic. The stress follows Hooke's Law and forms a linear pattern across the section. In this early stage, stress grows from the neutral axis to the outer fibers. The outer fibers reach the elastic limit first because they carry the highest stress. The inner part of the section stays elastic while the bending moment remains in this range. When the...

plastic deformations of members with a single plane of symmetry

plastic-deformations-of-members-with-a-single-plane-of-symmetry

Finding the Plastic Neutral Axis in Bending

Plastic bending in a member with one plane of symmetry depends on the plastic neutral axis and the stress pattern across the cross-section. In this state, the stress is uniform above and below the neutral axis. The region above the axis is in compression, so it carries negative stress. The region below the axis is in tension, so it carries positive stress. The plastic neutral axis does not usually pass through the centroid of the cross-section. This shift can happen when the section is not...

residual stresses in bending

Bending, Yielding, and Residual Stress

Bending in elastoplastic members shows how a material responds during loading and unloading. At first, the member behaves elastically, so stress is proportional to strain and follows Hooke's Law. This stage continues until the bending moment becomes large enough to pass the yield strength. Once yield strength is exceeded, plastic deformation begins. Plastic deformation means the material does not fully return to its original shape. It leaves permanent strain and deformation even after the load...

eccentric axial loading in a plane of symmetry

eccentric-axial-loading-in-a-plane-of-symmetry

Eccentric Load Stress in Structural Members

Eccentric axial loading happens when an axial load is applied away from the centroidal axis of a structural member. In engineering, this can happen when parts are not directly aligned because of design or function. The load then creates both a direct axial force and a bending effect. The offset load produces a moment, or turning effect, that acts like a couple and must be balanced to keep the member in equilibrium. The internal stress across the cross-section is therefore a mix of uniform...

unsymmetric bending

Bending About Multiple Axes

Unsymmetrical bending happens when a bending moment does not line up with a member’s principal axis. This creates a stress pattern and deflection shape that are more complex than in symmetrical bending. These effects matter when engineers design structures for different loading conditions. In unsymmetrical bending, the neutral axis is the line where stress is zero. It does not always match the geometric axes of the cross-section. Its orientation depends on the relationship between the applied...

unsymmetric bending - angle of neutral axis

unsymmetric-bending-angle-of-neutral-axis

Unsymmetric Bending and Neutral Axis

Unsymmetric bending happens when a beam or other structural member bends in a plane that does not match its principal axes. It often appears when a load is applied at a non-ideal angle, which makes stress analysis more complex. When a bending moment is applied at an angle θ to the vertical axis of a symmetrical member, it can be split into components along the member’s principal centroidal axes. The stress from each component is found separately. Then the results are combined using the...

general case of eccentric axial loading

Unsymmetrical bending happens when the bending moment on a structural member does not line up with its principal axis. This creates stress and deflection patterns that differ from symmetrical bending. Understanding this case helps explain how structures respond to different loading conditions. The member in this situation is acted on by equal and opposite forces. The forces are placed along a line that does not pass through the neutral axis. The neutral axis is the line where stress is zero,...

bending of curved members - strain analysis

bending-of-curved-members-strain-analysis

Curved Beam Bending and the Neutral Axis

Curved beams and arches bend in a way that changes both their shape and their strain pattern. When a curved member that is symmetric about the y-axis is loaded by equal and opposite forces, its circular shape deforms and the center of curvature moves from C to C'. The new curve is tighter than the original one. A key idea in this bending analysis is the neutral axis. The neutral axis is a hypothetical line inside the material whose length does not change during bending. It does not experience...

bending of curved members - neutral surface

bending-of-curved-members-neutral-surface

Curved Beam Stress and Neutral Axis

Curved beams have a non-uniform stress pattern across the cross-section because of their curvature. The stress is not spread evenly the way it is in a straight beam. The neutral axis, or the line where stress is zero, does not pass through the centroid of the section. In a curved member, strain changes with distance from the neutral axis. Hooke’s law still links stress and strain within the elastic limit of the material. Because the strain changes in a non-linear way, the stress also changes...

key terms and concepts

Learn how eccentric forces create unsymmetrical bending, shift the neutral axis, and produce stress around multiple axes in structural members.

Unsymmetrical Bending and Neutral Axis | JoVE K12

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

Unsymmetrical Bending and Neutral Axis

Description

Transcript

Tags