18.7
Espansione termica e stress termico: risolvere i problemi
Il Ponte Golden Gate di San Francisco è esposto a temperature che variano da -15 °C a 40 °C. Al suo punto più freddo, la campata principale del ponte…
Un filo metallico lungo 75 centimetri viene posto all'interno di una fornace a 450 gradi Celsius. Se la lunghezza del filo cambia a 75,77 centimetri e la temperatura ambiente è di 20 gradi Celsius, valutare il coefficiente di dilatazione lineare del materiale del filo.
Dopo il raffreddamento, entrambe le estremità del filo vengono fissate a un supporto e posizionate all'interno del forno a 200 gradi Celsius. Valutare lo stress termico generato se il modulo di Young è 7,0 x 10 Pascal e identificare il materiale del filo.
Il coefficiente di dilatazione lineare del filo dipende dalla lunghezza iniziale, dal gradiente di temperatura e dalla variazione di lunghezza ed è calcolato come 2,4 x10-5 per gradi Celsius.
Inoltre, lo stress termico generato può essere calcolato utilizzando il modulo di Young, il gradiente di temperatura e il coefficiente di dilatazione lineare del materiale del filo ed è calcolato come 3,02 x102 megapascal.
I valori del coefficiente di dilatazione e del modulo di Young suggeriscono che il filo è molto probabilmente fatto di alluminio.
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Q1: How do you calculate the coefficient of linear expansion from experimental data?
The coefficient of linear expansion is calculated using the change in length, initial length, and temperature gradient. The formula relates these three variables as a proportionality constant. For example, a 75-centimeter wire expanding to 75.77 centimeters over a 430-degree Celsius temperature change yields a coefficient of 2.4 × 10⁻⁵ per degree Celsius, indicating the material's thermal expansion rate.
Q2: What is thermal stress and how does it develop in constrained materials?
Thermal stress occurs when an object is prevented from expanding or contracting despite temperature changes. It develops in two steps: first, calculate the free thermal expansion, then determine the stress needed to compress or extend the object back to its original length. For a wire with both ends fixed at 200 degrees Celsius, thermal stress reaches 3.02 × 10² megapascals using Young's modulus and the linear expansion coefficient.
Q3: How does initial length affect the amount of thermal expansion in a material?
Thermal expansion is directly proportional to the initial length of a material. Longer objects expand more than shorter ones when exposed to the same temperature change. The Golden Gate Bridge's main span of 1275 meters expands by 0.84 meters between extreme temperatures, demonstrating how substantial length changes can be in large structures despite relatively small expansion coefficients.
Q4: Why do engineers use expansion joints in large structures like bridges?
Expansion joints accommodate the observable length changes caused by thermal expansion without creating excessive stress at any single point. By distributing thermal expansion across many small joints rather than allowing one large deformation, engineers prevent structural damage and maintain safety. This design approach is essential for structures like the Golden Gate Bridge that experience significant temperature variations.
Q5: How can you identify a material based on its thermal and elastic properties?
Material identification combines the coefficient of linear expansion with Young's modulus values. For instance, a wire with a coefficient of 2.4 × 10⁻⁵ per degree Celsius and Young's modulus of 7.0 × 10¹⁰ Pascals is most likely aluminum. These characteristic property values serve as fingerprints for distinguishing different metals and alloys.
Q6: What role does Young's modulus play in calculating thermal stress?
Young's modulus measures a material's resistance to elastic deformation and is essential for calculating thermal stress in constrained objects. It quantifies how much stress is required to produce a specific strain when the material cannot freely expand. Combined with the linear expansion coefficient and temperature change, Young's modulus determines the final thermal stress magnitude in fixed-end systems.
Q7: How does temperature gradient magnitude affect thermal expansion calculations?
The temperature gradient, or change in temperature, is directly proportional to thermal expansion. Larger temperature differences produce greater length changes. The Golden Gate Bridge experiences a 55-degree Celsius temperature range, producing a 0.84-meter length change, while the furnace wire problem involves a 430-degree Celsius gradient, resulting in much larger expansion relative to its initial length.