In order to fit the thermo-mechanical stability domain of tubes on specifications, 3 thermal cycles have been tested, regarding to relaxation of internal stress and changes in macroscopic thermo-mechanical properties. Experimental expansion curves and microstructure observation show that thermal treatments permitts such a good fitting. For a better understanding of physical internal phenomena, internal stress has been measured by neutron diffraction on tube samples after each thermal treatment. Results show a significant decrease of stress due to cycling in cold temperature.
In order to decrease the absolute value of CTE of assemblies, a new concept of thermo-mechanical stable linkage has been developped, which consists in a common alumlinum infiltration of superposed carbon preforms. Structural bonding, which usually affects stability properties and impose surface treatments and polymerization, can so be avoided.
The study has been achieved through CNES (Centre National d’Etudes Spatiales) and French Ministry of Defense (DGA) supports.
Carbon fiber reinforced aluminum alloy is the Metal Matrix Composite (MMC) which properties suit best for thermally stable satellite structures : no moisture instability (compared to Carbon/Resin), high stiffness (compared to Carbon/Carbon), optimized coefficient of thermal expansion (CTE) to near-zero value (compared to SiC).
Material choice :
Fiber and matrix have been chosen accordingly to previous studies performed for small plates :
Feasibility of near-zero CTE MMC thin-walled tubes :
The lay-up has been defined with the two following criteria :
In order to reduce overall dimensions of small truss structures, thermally stable thin-walled (1.6mm) tubes with low diameter (32mm) have been manufactured by pressure infiltration casting.
Concerning the feasibility of thin-walled tubes, critical points are identified by comparison with already made thick-walled (5mm) tubes with large diameter (60mm) :
- Preform manufacturing : the thinner is the tube, the more difficult is the compaction of dry fabric, necessary to obtain the 60% fiber content.
- Infiltration parameters : the ratio length/diameter has been decreased, so that thermal behaviour should differ during infiltration.
- Material health : swelling of fabric in the mould could cause waving of fibers, which could increases standard deviation of CTE values. Moreover, fiber content is measured in different points to control homogeneity along the tube.
Infiltration parameters :
Internal stress measurement
MicroYield Stress measurement
MicroYield Strenght should be increase by thermal cycling. Experience is planned in the beginning of 2001.
Gluing MMC / metal
Results have shown delamination of MMC in each case. It has been shown that shear stress concentration in the glue due to the different stiffness of MMC and metal can cause delamination. The further is the ratio of stiffnesses from 100%, the earlier happens delamination of MMC.