For most common engineering materials , the Coefficient of Thermal Expansion (CTE) is available in published data books. But for new materials such as alloys and composites, the CTE may need to be determined experimentally. This is normally done on a test bar made of the material with un-known CTE , by measuring the change in length of the bar for a given range of temperature change, and calculating the CTE in parts per million per deg F or C.
This video illustrates the use of strain gage and a temperature sensor to determine CTE on any material.
Strain gages are made with self temperature compensation to match the CTE of different materials and will tend to cancel out the expansion due to temperature, so for this exercise a Micro -Measurements type CEA-00-250UW-120 strain gage, with zero compensation is used, along with a Micro-Measurements type ETG-50B bondable temperature sensor. The temperature sensor has a non-linear output . This is made linear by the LST matching network unit type LST-10C-350D, which converts the temperature sensor into an equivalent strain gage half - bridge, that can be connected to a System 8000 strain gage data logger, and the output will be seen directly in deg C.
A small heat lamp is used to apply heat to the test beam, which is kept upside down to prevent direct heating of the strain gage and the sensor.
The gage and sensor are connected to a Micro Measurements system 8000 data logger. The readings are balanced for no-load and room temperature conditions, then the lamp is switched on, and the readings are monitored continuously. After some time, the test beam and sensor will reach stable conditions. The strain reading divided by the change in temperature will give the CTE in parts per million per degree C (ppm/degC), since the readings are in microstrain, and one microstrain is one part per million.
For this video, the test was done on aluminium for which the CTE is known. The result of the test matches the actual CTE of Aluminium fairly well, indicating this method can be used with confidence to find the CTE of any material.
The above test is deliberately limited to a small temperature change, within which the strain gage will have a linear output with respect to temperature.
While the temperature sensor and the strain gage output are not absolutely precise, and the measurement is affected by many factors, the method shown, and the result obtained, will give a reasonable indication of which STC number to choose for the strain gage to be used on the material under test, to get stable and accurate readings at or around room temperature.
However it must be remembered that strain gages will give their own output when temperature is changed, called apparent thermal output. For better accuracy, the result must be compared with the output of a strain gage from the same batch, bonded on a titanium Silicate test bar, subjected to the same temperature changes. Since Titanium Silicate is known to have zero thermal expansion, the strain gage on it will not give any output due to expansion, but will give an output due to its own temperature sensitivity. This will be the same thermal output from the gage on the test bar of the un-known CTE. The strain value from the gage on Titanium Silicate can be used as correction for apparent strain to the reading from the gage on the test bar with un-known CTE. The two test bars can be placed in a single chamber to ensure that both see the same temperature. Also, the two gages can be connected in a half-bridge configuration to automatically cancel out the apparent thermal output. The result will be the true physical expansion of the test bar of un-known CTE.
For a complete understanding of the above procedure, please refer to :
http://www.vishaypg.com/docs/11063/tn...