1-
Introduction :
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).
2–
Material choice :
Fiber and matrix have been chosen accordingly to previous studies performed for small plates :
- reinforcement with UltraHighModulus pitch-based carbon fabric.
- matrix composed of aluminum alloy with a low solidification temperature, so as to ensure a low time of contact between fibers and liquid aluminum during infiltration.
3-
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.
Preform manufacturing :
Rolling of fabrics on the mandrel
Preform in the mould before closing (With titane sheet)
Infiltration parameters :
Conclusion :
The two steps of manufacturing of carbon fiber reinforced aluminum alloys (wrapping and infiltration) are now well-mastered for single shape elements (tubes, plates,..) either thin or thick.
2.3-
CTE measurement
Conclusion :
CTE = 0.1 +- 0.24 μm/m/°C on [+20°C;+50°C] (3 cycles, 2 samples).
3–
Internal stress
3.1 –
Expansion curves
Experimental curves on as-received samples
3.2 –
Internal stress measurement
Results :
- Axial stress is always tensile, while radial and hoop stress are compressive
- Thermal cycling decreases internal stress (axial, radial and hoop)
- the lower is the cold temperature, the more internal stress is relaxed
3.3–
MicroYield Stress measurement
Sample in the Global Device
Coupling beetwen applied stress (alternatively tensile and compressive) and residual strain
Effect of 2 high level of repeated tensile/compressive stress
Conclusion :
- tensile and compressive MicroYield Strenght are low : the material is hysteretic
- tensile / compressive MicroYield Strenght is just sufficient for stable structure : 56 MPa
- no effect of strength hardening or strength softening
3.4
Conclusion
MicroYield Strenght should be increase by thermal cycling. Experience is planned in the beginning of 2001.
4-
Linkage studies
4.1–
Feasibility of a new stable linkage
tube-plate superposition principle
Feasibilty has been acquirred.
4.2–
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.
4.3–
Friction betwwen MMC and metal (or MMC)
Results have shown that friction coefficient is low (about 0.1) compared to classical metal / metal interface (about 0.2). This can be sensitively improved by an adequat surface treatment
