Name: finding the plastic neutral axis in bending
Uploaded: 2024-05-23T12:59:43.000Z
Duration: 1 min 21 s
Channel: JoVE

Si consideri un'asta che subisce una deformazione plastica dovuta alla flessione e ha un piano di simmetria. Le sollecitazioni sono distribuite uniformemente sopra e sotto l'asse neutro, con una sollecitazione di -σy sopra l'asse neutro e +σy sotto l'asse neutro.
In questo caso, l'asse neutro dell'asta non coincide con il baricentro dell'asta.
Per localizzare l'asse neutro, considerare la risultante delle forze di compressione sopra l'asse neutro R1 e la risultante delle forze di trazione R2 sotto l'asse neutro.
Le forze risultanti di compressione e trazione formano una coppia, che è equivalente alla coppia applicata all'asta. Ciò mostra che l'asse neutro divide l'asta in sezioni trasversali di aree uguali.
La linea d'azione delle forze di compressione e trazione risultanti passa attraverso i baricentri delle due aree uguali. 
Se la distanza tra questi due baridi è definita come d, allora il momento plastico dell'asta è espresso come metà del prodotto dell'area della sezione trasversale, l'entità della sollecitazione e d.

deformazioni plastiche con un unico piano di simmetria

Quando un elemento strutturale subisce una deformazione plastica dovuta alla flessione, è fondamentale comprendere la posizione dell'asse neutro e la distribuzione delle sollecitazioni. Questo elemento, caratterizzato da un unico piano di simmetria, presenta una distribuzione uniforme delle sollecitazioni, con sollecitazioni negative sopra l'asse neutro e lo stress positivo sotto. In particolare, l'asse neutro non si allinea con il baricentro della sezione trasversale. Questo disallineamento è tipico nei casi in cui la sezione trasversale non è rettangolare o presenta gravi deformazioni.

Le forze risultanti generate dalla flessione dell'elemento vengono analizzate per individuare l'asse neutro. Le forze di compressione agenti al di sopra dell'asse neutro e le forze di trazione al di sotto di esso generano risultanti di uguale grandezza ma direzioni opposte, formando una coppia. Questa configurazione indica che l'asse neutro divide la sezione trasversale in due aree uguali, ciascuna delle quali contribuisce equamente all'equilibrio delle forze.

Il momento plastico dell'elemento M_p, un parametro strutturale critico, deriva dalla relazione tra l'area della sezione trasversale, il carico di snervamento del materiale e la distanza tra i baricentri delle aree d create dall'asse neutro.

<img alt="Equation 1" src="/files/ftp_upload/15634/15634_Equation_1.svg" style="height: 28px;">

Il calcolo del momento plastico è essenziale per prevedere il momento massimo che la sezione può sopportare prima che si verifichino deformazioni plastiche significative. Questa considerazione è particolarmente cruciale per le componenti strutturali in aree caricate dinamicamente, come le zone sismiche, sottolineando come le proprietà dei materiali e la geometria della sezione trasversale influenzano la capacità di una struttura di sopportare carichi di flessione.

Deformazioni plastiche con un unico piano di simmetria

Si consideri un'asta che subisce una deformazione plastica dovuta alla flessione e ha un piano di simmetria. Le sollecitazioni sono distribuite uniformemente sopra e sotto ...

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

Education

Didattica

JoVE Core

Mechanical Engineering

Ingegneria Meccanica

Unit 1

Unità 1

Bending

Flessione

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 rectangular or when the member has deformed strongly. To locate the neutral axis, the bending forces are replaced by resultants. The compressive force above the axis and the tensile force below it act with equal size and opposite direction.

These two resultants form a couple, which is a pair of equal and opposite forces that create bending. This balance shows that the plastic neutral axis divides the cross-section into two equal areas. Each half contributes equally to force equilibrium. The plastic moment, written as Mp, depends on the cross-sectional area, the material yield stress, and the distance d between the centroids of the two areas made by the neutral axis.

Finding the plastic moment is important because it shows the largest moment the section can carry before major plastic deformation begins. That makes it useful for predicting how structural members respond to bending loads. It is especially important for parts used in dynamic loading conditions, such as seismic zones, where material properties and cross-sectional shape affect bending capacity.

Finding the Plastic Neutral Axis in Bending

finding-the-plastic-neutral-axis-in-bending

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 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 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,...

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 the plastic neutral axis, stress distribution, and plastic moment define bending capacity in a member with one plane of symmetry.

Finding the Plastic Neutral Axis in Bending | 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

Finding the Plastic Neutral Axis in Bending

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