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(Strain ,ε)
Figure 5.12: Coefficient of Thermal Expansion (CTE) of composite at different
points
0.0003
0.00025
0.0002
0.00015
0.0001
0.00005
Gradient C3 = 23.94 µε / °C
0
20 25 30 35 40 45
-0.00005
(Temperature °C)
The relative thermal expansion ΔL/L of a glass fibre composite as a function of
temperature T is shown in Figure 5.12. These five curves start at room temperature
(24°C) and stop at 40°C only after taking into consideration the safety of the maximum
resistance of the strain gauge adhesives to heating. Next they were corrected by being
subtracted from the thermal output of composite obtained in the earlier section.
Theoretically, the thermal expansion of any engineering material should be
linear and the slope of the curve or CTE (
Gradient C4 = 22.10 µε / °C
Gradient C1 = 10.03 µε / °C
Gradient C2 = 3.88 µε / °C
181
should be constant if using
the same material. However, from Figure 5.12, all the curves do not show perfect linear
expansions. This was due to some anticipated errors which occurred during the
measurements. Since the test was carried out in an open space, the temperature was not
fully controllable and subject to temperature fluctuations.
Corrected C3 (C3-TO)
Corrected C4 (C4-TO)
Corrected C1 (C1-TO)
Corrected C2 (C2-TO)
Composite Thermal
Output (TO)
There are several reasons why gradients C1 and C2 are lower than C3 and C4;
First, the thermal strain was measured at a different strain rate. Due to the limitations of
the surface temperature detector, the actual temperature inside the composite layer was