A high degree of dimensional stability is often required for aircraft and spacecraft components. In order to evaluate polymeric composite materials for aerospace applications the coefficients of thermal expansion (CTE) and moisture expansion (CME) have to be determined.
Coefficient of Moisture Expansion (CME) is defined as the ratio of change in strain and mass variation due to the moisture desorption or absorption. For the determination of the CME the change of length and the mass loss has to be measured with high accuracy.
The swelling/shrinkage of CFRPs due to moisture uptake/release is very small – CME values of CFRPs are in the range of <5E-5/wt% H2O in the fibre direction and about 1E-3/wt% H2O normal to the fibres.- And therefore length variation data can be obtained with high resolution /high accuracy measurement systems only.
Beside the need for high accuracy, the process of mass loss and shrinkage due to humidity loss is very slow. Depending on different material parameters the process from fully saturated to fully dry can take weeks and months. Measuring the full range of humidity content is extremely time consuming and therefore cost intensive and decreases measurement accuracy due to time drift of the measurement systems.
AAC determines CME by measuring length change and mass loss of samples in situ in vacuum. The length variation is determined by two laser interferometers, one focused to the front and one to the rear end of the sample, which is thermally stabilized with a thermostat. Nevertheless, the temperature distribution within the vacuum chamber is measured by pt100 thermoresistors in order to estimate the effective measurement errors due to CTE. The mass loss is determined by a high resolution balance operated in a vacuum chamber and recorded online. The ratio of length to mass variation is the CME in m/m/wt% H2O.
The main advantage of AAC CME test method is a highly accurate measurement within some days of a process, which takes typically weeks or months. AAC measures the mass loss in vacuum with an accuracy of 1 µg and the length change with an accuracy of 10nm. Both measurements are done in vacuum and are therefore close to real space conditions.
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