Aerospace engineers are on the cusp of developing composites that can repair themselves.
Outgassing: Under high vacuum, carbon-fiberreinforcedepoxy composites lose moisture. This dimensionally destabilizes laminated composites. This loss of dimensional stability can be many times greater than that caused by large temperature fluctuations, it affects which self-healing resin is used. Outgassing must take precedence over the coefficient of thermal expansion mismatch between the repair agent and the composite's matrix, say researchers at University of Bristol's Department of Aerospace Engineering, in the U.K.
Atomic oxygen: In low-Earth orbit (LEO), the atmosphere consists of 80% atomic oxygen and 20% nitrogen. Atomic oxygen, at this altitude (186 miles or 300 km) has a density of up to 8 3 109 atoms/cm3. Spacecrafts in LEO travel at nearly 5 mile/sec (8 km/sec). As the spacecraft collides with atomic oxygen, impingement kinetic energy is reportedly about 5 eV. Collateral damage of the impacts, reports the Bristol team, erodes leading surfaces of spacecraft by as much as 30% in epoxybased composites. Depending on stacking sequences in the epoxy composite, six years of atomic oxygen erosion reduces average thickness from 0.00035 to 0.0052 in. (9 and 132 m m).
In an experiment a unidirectional, 16-layer composite had a 5 to 10% reduction in flexural strength after exposure to LEO. A thinner, four-layer laminate with alternating 0 and 45° plies suffered nearly a 50% drop in flexural strength, reports the Bristol team. Tests show that atomic oxygen also degrades tensile, compressive, and shear strengths as well as the optical, thermal, and electrical properties of polymer-based composites, regardless of ply orientation and laminate thickness.
Micrometeoroids and orbital debris (MOD): Spacecraft in LEO are vulnerable to micrometeoroids traveling 19 km/sec, as well as man-made debris. The impact velocities of man-made objects, including solid propellant ash and paint flecks, along with derelict upper stage and dormant satellites, ranges from zero, for objects in the same orbit, to 6.8 miles/sec (11 km/sec) for those in retrograde orbits.
When MOD hit optical materials on spacecraft, damage ranges from heat-induced cratering at point of impact of to 20 3 the diameter of the debris to fractures in surrounding material. MOD-induced cracks can grow to 100 3 the diameter.
In contrast, MOD damage to composites typically consists of penetrating holes and adjacent surface damage, or some internal ply delamination, says the Bristol team. Internal damage is often anisotropic, following the fibers' structure. Complete penetrations, like bullet wounds, do more damage to the opposite surface or exit site. This includes surface spallation where the exit hole is 5 3 larger than the entry hole.
Thermal cycling: A key advantage of fiber-reinforced composites is that laminates can be tailored to increase strength and dimensional stability (near-zero coefficients of thermal expansion or CTE) through material selection. However, laminate CTE can change during a composite's service life due to the effects of outer space. Changes in CTE can lead to microcracking.
A complicating feature, says the Bristol team, is the combination of electron radiation and thermal fatigue. Electron radiation, which increases with altitude, makes composites more brittle. In turn, more microcracks will generate due to thermal changes.
Bristol University, Dept. of Aerospace Engineering, www.aero.bris.ac.uk
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