LECTURE 15. Thermal properties of materialsaml883.wdfiles.com/local--files/notes/Lecture15.pdf ·...
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AML 883 Properties and selection of engineering materials
LECTURE 15. Thermal properties of materials
M P GururajanEmail: [email protected]
Room No. MS 207/A3 Phone: 1340
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Thermal stress
● Thermal distortion is a common phenomenon: doors jam; bearings seize; overhead tranmission lines sag; railway tracks bend and buckle in exceptionally hot weather.
● Equipment design for making highperformance computer chips, for example: loss of accuracy due to changes in temperature makes it very difficult task
● Why thermal distortion?
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Differential thermal expansion● Mismatch between the thermal expansion of
two materials – if there be constraints, results in stresses
● Suppose, we give a thin layer coating to a component (to impart wear resistance, or resistance to corrosion, or oxidation)
● Deposition – typically at high T● On cooling, the substrate and film shrink by
different amounts – leading to stress● How to calculate this residual stress?
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Thermal stresses in thin filmsx
L0
T0
Massive body substrate
Thin film
T0
Separate
0L
1T
Cool to T1
Stretch film RebondL (1 T)
L (1 T)0
0
1
2
Strain ( – ) T
1 2
T1 1
T
Stress E ( – ) T/(1+)1 21 1
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Stress cracking of thin films
Optical micrograph of cracked silica film on silicon substrate – U Mich homepage
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Stress cracking of thin films
● Stress can be large enough to crack the surface film
● Car windows on steel frame, say: mismatch could be a factor of four or five
● Put a compliant material in between – rubber, for isntance
● Not always possible – put a layer or layers so that the expansion coefficients change gradually – case study of thermal barrier coatings
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Thermal sensing and actuation
● Thermal expansion can be used to sense (measure temperature) and to actuate )open or close valves or electric circuits)
● Bimaterial strips – strips of two materials with different expansion coefficients in specific geometry to accentuate the small thermal displacement so that temperatures can be accurately measured
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Bimetallic sensor and actuator
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Thermal gradients
T1 T1
T0
Distance from surface
Centre line
T
T1
0
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Shock resistance
● Temperature gradient (even in a material with a single expansion coefficient) can lead to sresses – Glass toughening, for example
● Stresses could be as high to cause fracture in brittle materials
● Thermal shock resistance (in K or degree C) is the maximum sudden change of temperature to which such a material can be subjected without damage
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● J D Eshelby● Stresses due to
transformational strains, dislocations, fracture, ...
● Well versed in Sanskrit, lover of old books
● Selftaught – no formal training
● A story about why he did not do experiments: Alex King – MRS Bulletin Image courtesy: Biographical memoirs of FRS
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Distortions due to thermal gradients
● While heat front diffuses in, temperature gradient gives rise to thermal stresses
● Material remains elastic after uniform heating has been achieved, the stresses fade away
● If the material partially yields, stresses never go away – surface yielded and centre hasn't
● Residual stress – a major problem● Time – important variable – how long to cool?
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Thermal diffusivity
● Time is determined by thermal diffusivity● Bigger the conductivity, faster the diffusivity● However, larger the heat capacity per unit
voume, more heat has to diffuse in out out of unit volume to change the temperature
a=/Cp
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Thermal diffusivity
● The characteristic distance “x” metres that heat diffuses in time “t” seconds is related by an (approximate) formula:
● Choosing materials: low expansion and high conductivity
x2≃2at
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Conduction with strength● Heat exchanger – typically, one of the fluids is
under pressure● Heat conduction – efficient if the walls are
thinner● Strength of wall – better if it is thicker● What is the material index? ● Derivation is left as an excercise● Answer:● Metals and some ceramics: metals because
can be shaped into thin walls easily
y
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Insulation
● Central heating and airconditioning costs energy
● Good insulation can save you lots of money!● For a given wall thickness, power consumption
is minimized by choosing materials with the lowest possible conductivity
● Foams!● Maximum service temperature: metals foams● Still higher temperature: ceramic foams
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Storage heaters● Demand of electricity: high during the day than
night● Not economic for electricity companies to
reduce output● Charge less for offpeak electricity● Storage heater – to expolit this – Use cheap
eelctricity, heat a large block of material, and, later extract heat from it by blowing air over it!
● More technical roles also – simulation of reentry vehicles and supersonic aircraft
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Storage heaters
● What is the material index?● Material (of lowest cost) with the largest heat
capacity that can withstand the operating temperature!
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Change of phase as thermal buffer
● Keep something at fixed temperature without external power
● Use the fact that the latent heat of fusion or melting either absorbs or gives heat at constant temperatureImage courtesy: Splung.com
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Shape memory!
● Superelastic materials● Shapememory materials – Distort the material;
it changes phase; below critical temperature, retains the shape with enormous ditortion; above critical temperature, structure reverts back to original structure and hence original shape!
● What if room temperature is above the critical temperature?
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Superelasticity
● Say, you sit on an eyeglass frame, by mistake● As soon as you get up, it goes back to original
shape!● Superelastic – since strained to 100% or more,
springs back to original shape!
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Thermal properties
● Themal expansion● Thermal conductivity● Thermal diffusivity● Specific heat capacity● Along with density, yield strength, ... can be
made use of in materials property charts to choose materials for applications – many of which, we spoke about
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Whither?
● What are the origins of thermal properties?● How do we manipulate them?