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

Si consideri un'asta soggetta a forze eccentriche uguali e opposte. Queste forze vengono applicate a una distanza orizzontale di a e a una distanza verticale di b dall'asse baricentrico principale dell'asta.
Ogni forza eccentrica è uguale alla forza centrica e due momenti di coppia con i bracci dei momenti che sono distanti a e b dall'asse baricentrico principale dell'asta.
Utilizzando il principio di Saint-Venant, i carichi equivalenti possono essere utilizzati per determinare la distribuzione della sollecitazione nella sezione dell'asta. L'analisi è valida quando la sezione non è vicina a nessuna delle estremità dell'asta.
Il principio di sovrapposizione determina le sollecitazioni dovute alle forze centriche e alle coppie di flessione e mostra che la sollecitazione varia linearmente lungo la sezione. 
A seconda della geometria dell'asta e della linea d'azione dei carichi eccentrici, la sollecitazione totale può avere lo stesso segno o il segno opposto in tutta la sezione. La linea lungo la sezione per la quale l'entità della sollecitazione è zero è nota come asse neutro della sezione.

caso generale di carico assiale eccentrico

General Case of Eccentric Axial Loading

La flessione asimmetrica si verifica quando il momento flettente applicato a un elemento strutturale non è allineato con il suo asse principale. Questo disallineamento porta a complesse distribuzioni delle sollecitazioni e modelli di deflessione che differiscono dalla flessione simmetrica, essenziali per progettare strutture in grado di resistere a diverse condizioni di carico.

Consideriamo un'asta soggetta a forze uguali e opposte applicate lungo una linea che non coincide con l'asse neutro dell'asta. Nella flessione asimmetrica, come qui descritta, l'asse neutro— dove lo stress è pari a zero—non si allinea necessariamente con gli assi geometrici della sezione trasversale. Ciò si traduce in momenti attorno a più assi, che contrastano l'eccentricità delle forze. La distribuzione delle tensioni dipende dalla relazione tra il carico applicato e le proprietà geometriche della sezione. La posizione dell'asse neutro è determinata assicurando che la somma dello stress normale attraverso la sezione sia uguale a zero.

Il momento di coppia nella flessione asimmetrica si riferisce ai momenti causati dalle forze che non passano attraverso il baricentro della sezione trasversale. Questi momenti provocano la flessione attorno a più assi e sono fondamentali nel determinare la distribuzione dello stress attraverso l'elemento.

Il limite proporzionale è il livello di stress oltre il quale il materiale si deforma in modo non lineare, segnando la fine del comportamento elastico. Il prodotto dell'inerzia misura la covarianza delle coordinate degli elementi dell'area della sezione trasversale rispetto agli assi. Se gli assi si allineano con gli assi baricentrici della sezione, semplificando il calcolo dello stress, l'asse neutro coinciderà con tali assi, rendendoli assi principali di flessione.

Si consideri un'asta soggetta a forze eccentriche uguali e opposte. Queste forze vengono applicate a una distanza orizzontale di a e a una distanza verticale di b ...

Education

Didattica

JoVE Core

Mechanical Engineering

Ingegneria Meccanica

Unit 1

Unità 1

Bending

Flessione

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

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mechanical engineering

Unsymmetrical Bending and Neutral Axis

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