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John Binner - Week 4 - Coggle Diagram
John Binner - Week 4
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Thermal expansion
The general term used to describe the change in dimensions that occurs with most materials as the temperature changes.
coefficient of thermal expansion, a, = (delta L/ original L)/ delta T
The basis of thermal expansion is that with increasing thermal energy, the atoms will oscillate further from their equilibrium lattice positions
As we know, many applications expose materials to a range of temperatures.
A large temperature gradient or a mismatch in behaviour between two adjacent materials (e.g. ceramic and metal can result in high stresses that distort the metal if the ceramic is strong or fracture the ceramic if it is weak.
Some materials can be specifically designed to have specific thermal expansion coefficients to form hermetically sealed devices.
Thermal shock
Thermal shock refers to the thermal stresses that occur in a component as a result of exposure to a temperature difference between different regions, typically the surface and the interior. The peak stress usually occurs at the surface during cooling according to the equation
thermal stress = elastic modulus (E) thermal coefficient of expansion (a) temperature difference / 1 - Poisson's ratio (v)
The thermal stress increases as the elastic modulus and the thermal coefficient of expansion increase as the imposed temperature difference increases
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Temperature difference can also be decreased by also changing the system design and the heat flow conditions
In general, a small amount of cracks and porosity can increase the thermal shock resistance significantly, as it significantly increases the difficulty of crack propagation.
Creep
Usually refers to deformation under constant stress as a function of temperature and time. A typical creep curve has 4 distinct regions - secondary (linear, steady state) creep is the most easily used to predict the life of components
Creep mechanisms (especially at elevated temperatures) can include dislocation movement and grain boundary sliding. Grain boundary separation can also occur.
As both temperature and stress increase, creep rate increases, and the time spent in steady state creep decreases (not good - makes it much more difficult to predict behaviour at higher temperatures).
Microstructure effects - whilst grain boundary sliding is often an undesirable factor behind creep in polycrystalline materials, creep rate is significantly faster in ceramics with a glassy grain boundary phase, especially at elevated temperatures, where the phase softens. The presence of porosity can also increase creep rate.
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