A high degree of dimensional stability is often required for aircraft and spacecraft components. In order to evaluate materials for aeronautic and aerospace applications the Coefficients of Thermal Expansion (CTE) and the Coefficient of Moisture Expansion (CME) have to be determined. Especially in case of fibre reinforced polymer composites, the common dilatometers suffer from limited resolution. Therefore, this special device was developed and is able to measure expansions with high accuracy combined with long term stability.
CME is defined by the ratio of the length variation to the mass variation [%] due to moisture evaporation or absorption:
with initial length l0, initial mass m0, time dependent length Δl and mass variation Δm.
CME values of carbon fibre reinforced polymeres are in the range of <5x10-5 /wt%H2O in the fibre direction and about 1x10-3 /wt% H2O normal to the fibres. The very low values in the 0° direction require a high resolution and accuracy measurement system
For determination of CME as function of mass loss, i.e. water content, two tests are necessary:
- Moisture Expansion Measurement: to determine the expansion as function of time
- Thermal vacuum test II : to determine the mass loss as function of time
The length variation is determined by two laser micro-interferometers, one focused to the front and one to the rear end of the sample. The maximum sample size is 300mm x 30mm. However, other than rectangular geometries (e.g. tubes) can be taken into consideration. In any case with the available space the expected CME values of advanced composites can be determined. The complete system is operated in vacuum (<10-3 Pa ) and is mounted on one support frame. This support is also part of a vibration isolation required for the interferometers.
The measurement of length is based on micro-interferences of the light of the lasers (yellow). A small mirror is fixed on the specimen (red, approximately 220 mm long). The specimen is located in thermal vacuum environment (see image below: stainless chamber). Micro-interferometers are thermally separated, but in the same vacuum chamber.
A closed copper cylinder contains the sample holder and can be operated in the range of RT up to 70°C. The heating/cooling system consists of a fluid pipe and a heating wire. Thermal expansion effects are additionally minimized due to the use of low CTE materials like glass ceramic and Invar.
The laser sensors are mounted in the same vacuum chamber but they are thermally shielded. Furthermore the lasers are temperature stabilized by a seperate cooling system which is operated by a fluid thermostat at about 22°C.
For calculation of the Coefficient of Moisture Expansion (CME) an Advanced Outgassing Test has to be added, using the identical thermal vacuum.
CME-Test Facility Characteristics
On-line measurement of
- Expansion as a function of time and temperature in two axes
- Mass-loss as a function time and temperature (Thermal Vacuum Test)
- Mass spectroscopy of outgassing particles (Sublimation)
|Temperature||RT <> +70°C|
|Vacuum||Maximum 10-3 Pa|
|Specimens||Length: 150mm <> 220mm , Width: 20mm <> 25 mm, Thickness maximum 5 mm|
|Shrinkage||Δ l/l0 minimum10-6|
|Mass loss||Δm/m0 minimum 0,01%|