Name: fatigue failure in repeated loading
Uploaded: 2024-03-07T13:22:37.000Z
Duration: 1 min 21 s
Channel: JoVE

La fatigue se produit lorsque les matériaux, soumis à des charges répétées pendant des milliers ou des millions de cycles, se rompent à un niveau de contrainte bien inférieur à la résistance à la rupture du matériau.
La défaillance par fatigue présente une fragilité, même dans des matériaux normalement ductiles.
La fatigue peut se produire en raison de charges fluctuantes, telles que la flexion continue de tiges ou de fils d’acier minces au même endroit ou les vibrations produites par des roues de pompe déséquilibrées.
Le diagramme des contraintes en fonction du nombre de cycles de charge pour l’acier indique que les applications à fortes contraintes nécessitent moins de cycles pour provoquer une rupture. Cependant, à mesure que la contrainte maximale diminue, le nombre de cycles nécessaires à la rupture augmente.
Le niveau de contrainte diminue jusqu’à la limite d’endurance à laquelle la défaillance ne se produit pas, même pour un nombre indéfiniment élevé de cycles de chargement.
Pour les métaux non ferreux, comme l’aluminium, les contraintes à la rupture diminuent avec l’augmentation des cycles de chargement.
La défaillance par fatigue commence souvent par des imperfections microscopiques, augmentant jusqu’à ce que le matériau cède sous charge. Les conditions de surface ont un impact significatif sur l’endurance, les éprouvettes usinées s’avérant plus durables.

fatigue

La fatigue se produit lorsque les matériaux se rompent sous des charges répétées ou fluctuantes, même à des niveaux de contrainte bien inférieurs à leur résistance à la rupture statique. Cela entraîne généralement une rupture fragile, même pour les matériaux ductiles. Il s'agit d'une considération essentielle dans la conception de machines et de composants structurels soumis à des charges répétitives ou variables. La nature de ces charges peut aller de charges fluctuantes comme des roues de pompe déséquilibrées provoquant des vibrations répétées d'une fine tige d'acier d'avant en arrière au même endroit, ce qui peut conduire à une rupture par fatigue.

Le nombre de cycles de chargement nécessaires pour provoquer la rupture d'un échantillon peut être déterminé expérimentalement pour n'importe quel niveau de contrainte maximum. Cela conduit à une courbe de contrainte en fonction du nombre de cycles de chargement, fournissant des informations significatives sur les propriétés de fatigue du matériau. Par exemple, la courbe contrainte typique en fonction du nombre de cycles de chargement pour l'acier indique que les applications à contraintes élevées nécessitent moins de cycles pour provoquer une rupture. Cependant, à mesure que la contrainte maximale diminue, le nombre de cycles nécessaires à la rupture augmente jusqu'à atteindre la limite d'endurance. Il est crucial de noter que la rupture par fatigue commence souvent par une fissure ou une imperfection microscopique, se propageant jusqu'à ce que le matériau ne puisse plus supporter la charge maximale. Les éprouvettes usinées et polies ont tendance à avoir une limite d'endurance plus élevée que les composants laminés, forgés ou corrodés.

Fatigue

La fatigue se produit lorsque les matériaux, soumis à des charges répétées pendant des milliers ou des millions de cycles, se rompent à un niveau de contrainte bien inférieur à celui du matériau ...

Education

Enseignement

JoVE Core

Mechanical Engineering

Génie mécanique

Unit 1

Unité 1

Stress and Strain - Axial Loading

Contraintes et déformations - Chargement axial

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

fatigue failure in repeated loading

Fatigue failure happens when materials rupture after repeated or fluctuating loads. The stress can be far below the material’s static breaking strength. Even ductile materials can fail in a brittle way under fatigue.

This behavior matters in machines and structural parts that face repetitive or changing loads. One example is an unbalanced pump impeller that creates vibrations. Another is a thin steel rod or wire bent back and forth in the same spot until it breaks.

The number of loading cycles needed for failure can be measured for a chosen maximum stress level. These results are shown on a stress versus number of loading cycles curve. For steel, higher stress usually means fewer cycles to rupture. As the maximum stress drops, the number of cycles needed for rupture rises until the endurance limit is reached.

Fatigue failure often starts at a tiny crack or imperfection. The crack grows until the material can no longer carry the maximum load. Machined and polished specimens usually have a higher endurance limit than rolled, forged, or corroded components.

Fatigue Failure in Repeated Loading

normal strain under axial loading

Axial Stress and Strain in a Rod

Axial stress and strain in a rod describe how a material changes length when a force is applied along its axis. The force may stretch the rod or compress it. In mechanics of materials, this deformation is called normal strain. Normal strain is found by dividing the change in length by the original length. This gives a unitless ratio that shows how much the material deforms under load. A load-deformation diagram can show elongation, but it cannot predict how another rod will deform. When a...

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stress-strain diagram - brittle materials

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Fatigue failure happens when materials rupture after repeated or fluctuating loads. The stress can be far below the material’s static breaking strength. Even ductile materials can fail in a brittle way under fatigue. This behavior matters in machines and structural parts that face repetitive or changing loads. One example is an unbalanced pump impeller that creates vibrations. Another is a thin steel rod or wire bent back and forth in the same spot until it breaks. The number of loading...

deformation of member under multiple loadings

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poisson's ratio

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saint-venant's principle

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key terms and concepts

Learn how repeated or fluctuating loads cause fatigue failure, why cracks grow over time, and how stress cycles and endurance limit affect materials.

Fatigue Failure in Repeated Loading | 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

Fatigue Failure in Repeated Loading

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