thermal and moisture effect

7/28/2019 Thermal and Moisture Effect

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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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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


polymers

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thermal and moisture effect

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