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Effect of temperature
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Creep: Plastic deformation at high
temperature
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Effect of temperature on fatigue
In high temperature fatigue, there is a transition from fatigue failure to creep
failure as the temperature increases (creep dominates at high
temperatures).
Coarse grained metal has higher fatigue strength where creep dominates.
Fine grained metal has higher fatigue strength at low temperatures.
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Thermal fatigue
Thermal fatigue occurs when metal is subjected to high and low temperature,producing fluctuating cyclic thermal stress.
Normally occurs in high temperature equipment.
Low thermal conductivity and high thermal expansion properties are critical.
The thermal stress developed by a temperature change T is
=ET
Where is linear thermal coefficient of expansion
E is elastic modulus
T is change in temperature
If failure occurs by one application of thermal stress, the condition is called
thermal shock
Low thermal expansion - for many FRPs, leading to good dimensional stability
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Thermal fatigue
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Effect of temperature & Stress on
Creep curves
The higher the temperature, the greater the creep rate.
The higher the stress, the greater the creep rate.
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High temperature Mechanical tests
Different tests to evaluate high temperature properties based on the time
scale of the service requirements,
i. High temperature tensile test:- Completed in few minutes to produce
stress versus strain curves at specific temperatures. Useful for short term
applications such as Rocket parts.
ii. Creep test:- Measures dimensional changes accurately at constant high
temperature and constant load or stress. Useful for long term
applications which are strain limited, such as Turbine blades.
iii. Stress rupture test:- Measures time to failure at specified stress and
temperature. Useful for applications where some strain can be toleratedbut failure must be avoided, such as large furnace housings.
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High temperature Mechanical tests
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Thermal Expansion The energy addition to a material in the form of heat increases the thermal
vibration of the atoms in their lattice sites.
The thermal expansion is a direct result of a greater separation distancebetween the centers of adjacent atoms as the thermal vibration of individualatoms increases with increasing temperature.
for ceramics and glasses < for metals due to the shape of the energy well.
The ceramics and glasses generally have deeper wells (higher bonding
energies) due to their ionic and covalent bond natures; therefore less atomicseparation with the increasing temperature.
The elastic modulus is directly related to the derivative of the bonding energycurve near the bottom of the well. The deeper the well, the larger derivative
and greater the elastic modulus.
expansiothermaloftcoefficienlinear ]./[ CmmmmLdT
dL o
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Thermal Expansion
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Thermal conductivity
Electrons can be viewed as a particle as well as a wave. For a wave, any
structural disorder can behave as an obstacle for the movement of thewaveform. The increasing vibration of the crystal lattice at increasing
temperature results in a decrease in thermal conductivity.
Similarly, the structural disorder due to chemical impurities results in a
decrease in thermal and electrical conductivities. As result metal alloys tend
to have lower thermal conductivities than pure metals.
For ceramics and polymers, lattice vibrations, also wave-like in nature, are
similarly impeded by structural disorder. Thermal conductivities for
amorphous polymers are lower than crystalline polymers
The thermal conductivities of ceramics and polymer are further reduced due
to the presence of porosities.
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Thermograph
Infrared thermography with pulse heating mode
Higher surface temperature just over the defect
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Thermal shock: Fracture of materials due to a temperature change, usually suddencooling.
Thermal shock
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With the inevitable presence of flaws at
the surface, the created tensile stress at
the surface will cause a brittle fracture
of the material
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Moisture effect
1) The amount of absorbed moisture is dependent upon the matrix material and
the relative humidity.
2) Elevated temperature speeds the rate of moisture absorption. Absorbed
moisture reduces the matrix-dominated mechanical properties.
3) Absorbed moisture also causes the matrix to swell. This swelling relieves
locked-in thermal strains from elevated-temperature curing.4) These strains can be large, and large panels fixed at their edges can buckle
due to the swelling strains.
5) During freeze-thaw cycles, the absorbed moisture expands during freezing
and can crack the matrix.
6) During thermal spikes, absorbed moisture can turn to steam. When theinternal steam pressure exceeds the flat wise tensile strength of the
composite, the laminate delaminates.
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Moisture effect
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Absorption of Moisture for Polymer
Composites
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Effects of Temperature and Moisture on
Strength of Carbon~Epoxy
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Effects of Temperature and Moisture on
Strength of Carbon~Epoxy
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Eff f T d M i
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Epoxy matrices and adhesives, in fact almost all thermoset resins, absorb moisturefrom the atmosphere, which degrades their elevated temperature matrix-
dependent properties
Effects of Temperature and Moisture on
polymers
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