18.6
Contrainte thermique
Si la température d'un objet change tout en étant empêché de se dilater ou de se contracter, l'objet est soumis à une contrainte. La contrainte est co…
Un corps subit un stress lorsqu’il subit des variations de température, ce stress est connu sous le nom de stress thermique, qui peut même déformer durablement le corps.
Par exemple, un espace est intentionnellement maintenu entre deux joints d’une tige sur la voie ferrée pour éviter la déformation ou la flexion des tiges.
Pour calculer la contrainte thermique dans un objet, déterminez d’abord la dilatation ou la contraction thermique, puis calculez la contrainte.
Considérons une tige de longueur L0.. Lorsque de la chaleur est appliquée, la tige augmentera en longueur de delta 'L' et la variation fractionnaire de la longueur de la tige peut être évaluée.
Maintenant, la même tige est fixée aux deux extrémités. Lorsque la chaleur est appliquée, la tige subit une tension car elle ne peut pas se dilater. Pour restaurer la longueur d’origine de la tige, la contrainte peut être calculée.
Si l'on se souvient de la définition du module de Young, il s'agit du rapport entre la contrainte et la déformation.
Par conséquent, la contrainte thermique peut être déterminée en combinant les équations et en réarrangeant les termes.
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Q1: What is thermal stress and how does it develop in materials?
Thermal stress occurs when an object experiences temperature changes while prevented from expanding or contracting freely. The stress is compressive if the object would expand without constraint, or tensile if it would contract. This stress can be quite large and cause permanent deformation or damage to materials and structures.
Q2: How do engineers prevent thermal stress in infrastructure like railroad tracks?
Engineers design components to expand and contract freely by leaving deliberate gaps or expansion joints. For example, railroad tracks have intentional spaces between joints to prevent rod deformation. When gaps cannot be left, engineers must select compatible materials, such as using steel reinforcing rods in concrete because steel's coefficient of linear expansion nearly equals concrete's.
Q3: How is thermal stress calculated in a constrained object?
To calculate thermal stress, first determine the thermal expansion or contraction the object would undergo. Then calculate the resulting strain. Using Young's modulus, which is the ratio of stress to strain, you can determine thermal stress by combining and rearranging the thermal expansion and strain equations.
Q4: What real-world phenomena result from thermal stress?
Thermal stress explains many observable effects: rock and pavement weathering from ice expansion during freezing, railroad track and roadway buckling on hot days without expansion joints, power line sagging in summer and snapping in cold weather, cracks opening and closing in plaster walls, and glass cookware cracking from rapid or uneven cooling.
Q5: Why does a fixed rod experience strain when heated?
When a rod is fixed at both ends and heated, it cannot expand freely. The thermal expansion that would normally occur is prevented by the fixed constraints, creating internal strain within the material. This constraint-induced strain develops stress that can be calculated using Young's modulus and the material's thermal expansion coefficient.
Q6: What is the difference between compressive and tensile thermal stress?
Thermal stress is compressive when an object is prevented from expanding during heating, creating inward pressure. Thermal stress is tensile when an object is prevented from contracting during cooling, creating outward pulling forces. Both types can cause significant damage if the stress exceeds the material's strength limits.
Q7: Why is material compatibility important in composite structures?
When different materials are bonded together, they must have similar coefficients of linear expansion. If coefficients differ significantly, temperature changes create differential thermal stress at the interface, causing cracking or separation. Steel reinforcing rods in concrete work well because their expansion rates match, preventing stress-induced failure.