Name: stress tensor and principal stresses
Uploaded: 2024-05-23T12:59:54.000Z
Duration: 1 min 21 s
Channel: JoVE

L’état général de stress fait référence aux diverses forces et pressions qu’un objet subit.
Considérons un tétraèdre dont une face est perpendiculaire à une droite OA et les autres faces parallèles aux plans de coordonnées.
Les aires des faces parallèles sont déterminées en multipliant l’aire de la face XYZ par les cosinus directeurs de la droite OA.
L’état de contrainte au point O, défini par diverses composantes de contrainte, influence les forces exercées sur chaque face.
Les forces exercées sur la face XYZ comprennent à la fois une force normale et une force de cisaillement. La somme de toutes les forces le long de OA est nulle, ce qui conduit à l’équation de contrainte normale.
L’équation de contrainte normale est sous une forme quadratique avec les cosinus de direction, ce qui permet de sélectionner des axes de coordonnées qui simplifient l’équation. Les axes de coordonnées, appelés axes principaux de contrainte, dépendent de l’état de contrainte au point O.
En conséquence, les plans de coordonnées sont connus comme les principaux plans de contrainte, et les contraintes normales sont les principales contraintes au point O.

l’état de contrainte générale

General State of Stress

L'état général de contrainte au sein d'un matériau peut être représenté avec précision à l'aide d'un tenseur de contrainte. Ce tenseur encapsule les forces internes répartis au sein d'un matériau soumis à des forces ou déformations externes.

Plus précisément, considérons un élément tétraédrique où une face, étiquetée XYZ, est perpendiculaire à la ligne OA et les faces restantes s'alignent sur les axes de coordonnées avec le point O comme origine. À tout moment, comme le point O, le tenseur des contraintes peut être utilisé pour déterminer les composantes des contraintes sur n'importe quel plan passant par O. Ce tenseur est important pour comprendre comment les matériaux réagissent dans diverses conditions de chargement en fractionnant les forces en composantes normales et de cisaillement sur les faces du tétraèdre.

Les aires des faces alignées par coordonnées du tétraèdre sont calculées en multipliant l'aire de la face XYZ par les cosinus directeurs λ_x, λ_y, et λ_z de la droite OA. Ces cosinus relient l'orientation de la face aux axes de coordonnées, facilitant ainsi la résolution des forces, essentielle à la conception des matériaux et de la structure. La condition d'équilibre, selon laquelle la somme de toutes les forces le long de OA est égale à zéro, conduit à l'équation de contrainte normale exprimée sous une forme quadratique avec des cosinus directeurs.

<img alt="Equation 1" src="/files/ftp_upload/15656/15656_Equation_1.svg" style="text-align: center; height: 30px;">

Cette forme identifie les axes principaux de contrainte. Si un nouveau système de coordonnées est défini sur la base des cosinus directionnels, les termes de contrainte de cisaillement disparaissent, simplifiant ainsi le tenseur des contraintes. Ces axes définissent les plans principaux où les contraintes de cisaillement disparaissent et les contraintes normales, appelées contraintes principales, sont maximisées. Comprendre ces composantes de contraintes est essentiel pour prédire les modes de défaillance des matériaux et améliorer la conception structurelle.

<img alt="Equation 2" src="/files/ftp_upload/15656/15656_Equation_2.svg" style="text-align: center; height: 19px;">

L’état de contrainte générale

L’état général de stress fait référence aux diverses forces et pressions qu’un objet subit.
Considérons un tétraèdre dont une face est perpendiculaire à une droite...

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Mechanical Engineering

Génie mécanique

Unit 1

Unité 1

Transformations of Stress and Strain

Transformations des contraintes et des déformations

KEY TERMS AND CONCEPTS

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stress tensor and principal stresses

The stress tensor describes the internal forces inside a material. It is used when a material is under external force or deformation. At a point such as O, the tensor can show the stress components on any plane through that point.

A tetrahedral element helps show how these forces act. One face, XYZ, is perpendicular to the line OA. The other faces line up with the coordinate axes, and point O is the origin. The stress on the tetrahedron is split into normal and shear components on each face.

The areas of the coordinate-aligned faces come from the area of face XYZ and the direction cosines of line OA. These direction cosines are λx, λy, and λz. They link the orientation of the face to the coordinate axes and help resolve the forces.

Force balance along OA gives the equilibrium condition, where the sum of all forces in that direction is zero. This leads to the normal stress equation in quadratic form with direction cosines. That form identifies the principal axes of stress.

If a new coordinate system is chosen using those direction cosines, the shear stress terms disappear. The result is a simpler stress tensor. The new axes are the principal planes, where shear stress is zero and the normal stresses are the principal stresses. These ideas help predict failure modes and support structural design.

Stress Tensor and Principal Stresses

transformation of plane stress

Plane Stress Changes with Rotation

Plane stress changes with rotation in a material element. The analysis looks at a prismatic element inside a cube under plane stress. As the element turns, the normal stress and shearing stress on its faces change in both size and direction. These changes are described with trigonometric functions of the rotation angle. The forces on the rotated faces are compared with the forces on the original faces that are perpendicular to each principal axis. The equilibrium equations use only the forces...

principal stresses

Stress Circle for Principal and Shear Stress

The stress circle shows how normal stress and shearing stress change with angle on a transformed plane. Its center, C, lies on the vertical axis and represents the average normal stress. The radius shows the amount of stress variation as the plane rotates. Points A and B, where the circle crosses the horizontal axis, mark the maximum and minimum normal stresses. At these points, shearing stress is zero. These points define the principal stress planes, where only normal stress, called principal...

principal stresses: problem solving

Finding Principal Planes and Stress Limits

Principal planes and principal stresses are found by analyzing two planes that meet at right angles under shearing, tensile, and compressive stresses. The goal is to use the given stress values to describe how the material is loaded and to locate the planes where the stress state changes. To find the principal planes, use the formula that relates the plane angle to twice the shearing stress divided by the difference between tensile and compressive stresses. After substituting the known stress...

mohr&#39;s circle for plane stress

Reading Stress Components with Mohr's Circle

Mohr's circle is a graphical way to read the stress state at a point in a material under plane stress. It turns normal stress and shearing stress into a two-dimensional picture. This makes stress transformation easier to analyze. The method uses Cartesian coordinates. The horizontal axis shows normal stress, written as σ, and the vertical axis shows shearing stress, written as τ. Two points, A and B, are plotted from the stresses on the element. Point A is at (σx, -τxy), and point B is at (σx,...

general state of stress

The stress tensor describes the internal forces inside a material. It is used when a material is under external force or deformation. At a point such as O, the tensor can show the stress components on any plane through that point. A tetrahedral element helps show how these forces act. One face, XYZ, is perpendicular to the line OA. The other faces line up with the coordinate axes, and point O is the origin. The stress on the tetrahedron is split into normal and shear components on each face. <br...

yield criteria for ductile materials under plane stress

yield-criteria-for-ductile-materials-under-plane-stress

Tresca and Von Mises Yield Criteria

Ductile materials can yield when they are loaded beyond a safe stress level. Engineers use yield criteria to check whether structural elements and machine parts can handle the expected load without permanent deformation. The yield point is first found with a tensile test, which measures how the material responds to uniaxial stress. A tensile test is not enough when a part faces biaxial or plane stress conditions. In those cases, more advanced criteria are needed to predict failure. One common...

transformation of plane strain

Plane Strain Under Rotated Axes

Plane strain changes when the coordinate axes are rotated. This matters for long bars and similar elongated structures that are loaded uniformly on their sides. In these members, one dimension is much larger than the others, so deformation is mainly studied in a single plane. At a point such as O, the key strain components lie along the x and y axes. The strain along the z-axis is negligible in plane strain. Together, the x and y strain components describe the deformation state in the xy-plane.

mohr&#39;s circle for plane strain

Principal and Shear Strains on Mohr’s Circle

Mohr’s circle for plane strain shows how normal strain and shear strain change on a graph. The horizontal axis is normal strain, written as ε, and the vertical axis is shear strain, written as γ. It uses two plotted points, X and Y, to represent the strain state. Point X has coordinates (εx, -γXY), and point Y has coordinates (εY, γXY). The center of the circle, O, gives the average normal strain. The radius comes from the relationship between normal strain and shear strain, so the circle...

three-dimensional analysis of strain

Principal Strain Directions in 3D

Three-dimensional strain analysis shows how a material changes shape when it is loaded. It is especially useful for elastic, homogeneous materials. A small cubic element inside the material can expand or contract along principal stress axes. That change turns the cube into a rectangular parallelepiped, which makes the deformation easier to picture. The principal stress axes are orthogonal, or at right angles to each other. Along these directions, the stress does not create shear inside the...

measurements of strain

How Strain Gauges Measure Deformation

Strain describes how a material deforms when a force is applied. Normal strain compares the change in length with the original length. Engineers use this measure to track stretching or compression in materials. For higher accuracy, they often use electrical strain gauges. A strain gauge has a conductive wire on a paper backing that sticks to the surface of the material. It works through the piezoresistive effect, which means the wire’s electrical resistance changes when the material is bent,...

key terms and concepts

Learn how a stress tensor resolves normal and shear stress on a tetrahedral element, and how principal axes and principal stresses are found.

Stress Tensor and Principal Stresses | JoVE K12

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Stress Tensor and Principal Stresses

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