William
04-22-2006, 08:43 AM
Self-healing CF (http://www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=60200&strSite=MDSite&Screen=CURRENTISSUE&CatID=3)
So how long do you think it will take for this to make it into the cycling industry? Frames and esp forks could benefit. It would certainly make me feel better about trying out CF. ;)
William
******************************
Aerospace engineers are on the cusp of developing composites that can repair themselves.
Jean M. Hoffman
Senior Editor
Engineers are continuing to use more fiber-reinforced composites, especially for spacecraft structures. This includes solar arrays, optical benches, and antenna systems that must withstand the rigors of interstellar travel.
SPACE COMPOSITES
Composites used for spacecraft typically have a polymer matrix made from thermosetting (epoxies, phenolics, and polyimides) resins reinforced with aramid, carbon, or glass fibers. But there is a common weakness with most composite structures: They don't tolerate impacts, i.e., they have low fracture toughness and don't resist crack propagation.
Unlike metals that can absorb a significant amount of impact, thermoset composites absorb energy by elastically deforming or fracturing. Impact loads typically generate microcracks in the "through thickness" direction where damage tolerance in composites is at its lowest, explains Hugo Williams and Ian Bond, researchers at University of Bristol's Department of Aerospace Engineering, in the U.K.
For space composites, temperature extremes, as well as micrometeroid strikes — dust grains traveling at several kilometers/second — start small cracks. These cracks often develop on the opposite side of the surface that gets struck or deep within the material at the fiber/matrix interface, says the Bristol team. This so-called barely visible impact damage, or BVID, makes detection difficult or impossible. And once cracks form, the composite will likely lose strength, stiffness, and stability. BVID also leaves the structure prone to further damage. Over time, microcracks get larger, weakening the spacecraft until a catastrophic failure is inevitable.
To minimize BVID failures, aerospace engineers build in strain safety factors of 5% or more. But adding safety factors comes with a price: Thicker composite walls, for example, add weight and can degrade performance.
Researchers may be on the brink of developing composite laminates that will let spacecraft components automatically mend nicks, scrapes, and cracks. The trick, says the Bristol team, is to devise composites with mechanical microvascular systems. These engineered veins "bleed" adhesivelike polymers that cure to a hard structure inside microcracks before the cracks have a chance to grow into a more serious breach.
The Bristol team is working with laminate composites with micro/nanoencapsulated healing agents throughout the matrix. As a microcrack propagates through the matrix, it eventually encounters and breaks a microcapsule filled with polymer resin. Capillary forces push the released polymer along the open crack front. As the front moves through the structure, it encounters and breaks another microcapsule containing a curing agent or catalyst.
As the polymer and catalyst mix, the self-healing reaction or polymerization solidifies the mixture inside the crack, stopping its progression. Polymerizing the resin forms a structure that, according to the Bristol team, nearly restores the composite to its original strength. And as long as there are still unbroken microcapsules of both the polymer and its catalyst, the composite will still be able to repair future damage.
To build these biomimetic structures, the researchers took a somewhat counterintuitive approach. They construct laminates with hundreds of fragile, hollow glass-fiber (filament) reinforcements. The hollow filaments crack easily when the composite is damaged, releasing either polymer or catalyst.
The filaments were made at Bristol University. They sport a 60-m m OD and a 30-m m ID. This gives the filament a 50% hollow fraction. This so-called 50/60-filament configuration stems from previous work and makes it easy to fill filaments with repair agent. Plies made from hollow filaments used in proof-of-concept trials have estimated gross-volume fractions on par with plies used in conventional spacecraft composites.
Plies with the hollow filaments are placed between conventional solid-glass-fiber-reinforced plies. Researchers fill half of all the hollow filaments with polymer resin and the other half with its catalyst. For future work, the Bristol team would like to tailor filament diameters and hollow fractions to match specific threats the composite will likely see. For example, larger-diameter filaments may be best suited to repair impact damage, while smaller diameter filaments repair internal cracking.
There are two key benefits to self-healing superstructures on spacecraft. They could double spacecraft life, cutting mission costs in half. And doubling spacecraft life may let mission planners aim for destinations farther out in the Solar System.
The key to finding the right self-repair systems for space composites, the Bristol team contends, is to first define the environmental threats the spacecraft will likely encounter.
To characterize environmental threats, the team conducted lab and space tests. These included low-Earth orbit (LEO), circular low-Earth orbit (CLEO), and highly elliptical-orbit (HEO) experiments. The goal was to identify the synergistic effects that high vacuum, atomic oxygen, UV radiation, impacts with cosmic debris, and thermal cycling have on the material.
BIOMIMETIC COMPOSITES
Because environmental threats from space cause internal as well as external damage, the Bristol researchers investigated placing self-healing, hollow filament plies at different depths within the composite. Three locations were considered:
Surface mounted. This approach is applicable to repairing surface damage due to atomic oxygen and UV exposure as well as, micrometeoroid and orbital debris (MOD) impacts.
Intermediate. This position is one or two plies from the outer surface. Putting filaments with self-healing agents here may help repair surfaces as well as internal damage.
Midline. Filaments placed here repair internal damage due to thermal cycling and MOD impacts.
Cont...
So how long do you think it will take for this to make it into the cycling industry? Frames and esp forks could benefit. It would certainly make me feel better about trying out CF. ;)
William
******************************
Aerospace engineers are on the cusp of developing composites that can repair themselves.
Jean M. Hoffman
Senior Editor
Engineers are continuing to use more fiber-reinforced composites, especially for spacecraft structures. This includes solar arrays, optical benches, and antenna systems that must withstand the rigors of interstellar travel.
SPACE COMPOSITES
Composites used for spacecraft typically have a polymer matrix made from thermosetting (epoxies, phenolics, and polyimides) resins reinforced with aramid, carbon, or glass fibers. But there is a common weakness with most composite structures: They don't tolerate impacts, i.e., they have low fracture toughness and don't resist crack propagation.
Unlike metals that can absorb a significant amount of impact, thermoset composites absorb energy by elastically deforming or fracturing. Impact loads typically generate microcracks in the "through thickness" direction where damage tolerance in composites is at its lowest, explains Hugo Williams and Ian Bond, researchers at University of Bristol's Department of Aerospace Engineering, in the U.K.
For space composites, temperature extremes, as well as micrometeroid strikes — dust grains traveling at several kilometers/second — start small cracks. These cracks often develop on the opposite side of the surface that gets struck or deep within the material at the fiber/matrix interface, says the Bristol team. This so-called barely visible impact damage, or BVID, makes detection difficult or impossible. And once cracks form, the composite will likely lose strength, stiffness, and stability. BVID also leaves the structure prone to further damage. Over time, microcracks get larger, weakening the spacecraft until a catastrophic failure is inevitable.
To minimize BVID failures, aerospace engineers build in strain safety factors of 5% or more. But adding safety factors comes with a price: Thicker composite walls, for example, add weight and can degrade performance.
Researchers may be on the brink of developing composite laminates that will let spacecraft components automatically mend nicks, scrapes, and cracks. The trick, says the Bristol team, is to devise composites with mechanical microvascular systems. These engineered veins "bleed" adhesivelike polymers that cure to a hard structure inside microcracks before the cracks have a chance to grow into a more serious breach.
The Bristol team is working with laminate composites with micro/nanoencapsulated healing agents throughout the matrix. As a microcrack propagates through the matrix, it eventually encounters and breaks a microcapsule filled with polymer resin. Capillary forces push the released polymer along the open crack front. As the front moves through the structure, it encounters and breaks another microcapsule containing a curing agent or catalyst.
As the polymer and catalyst mix, the self-healing reaction or polymerization solidifies the mixture inside the crack, stopping its progression. Polymerizing the resin forms a structure that, according to the Bristol team, nearly restores the composite to its original strength. And as long as there are still unbroken microcapsules of both the polymer and its catalyst, the composite will still be able to repair future damage.
To build these biomimetic structures, the researchers took a somewhat counterintuitive approach. They construct laminates with hundreds of fragile, hollow glass-fiber (filament) reinforcements. The hollow filaments crack easily when the composite is damaged, releasing either polymer or catalyst.
The filaments were made at Bristol University. They sport a 60-m m OD and a 30-m m ID. This gives the filament a 50% hollow fraction. This so-called 50/60-filament configuration stems from previous work and makes it easy to fill filaments with repair agent. Plies made from hollow filaments used in proof-of-concept trials have estimated gross-volume fractions on par with plies used in conventional spacecraft composites.
Plies with the hollow filaments are placed between conventional solid-glass-fiber-reinforced plies. Researchers fill half of all the hollow filaments with polymer resin and the other half with its catalyst. For future work, the Bristol team would like to tailor filament diameters and hollow fractions to match specific threats the composite will likely see. For example, larger-diameter filaments may be best suited to repair impact damage, while smaller diameter filaments repair internal cracking.
There are two key benefits to self-healing superstructures on spacecraft. They could double spacecraft life, cutting mission costs in half. And doubling spacecraft life may let mission planners aim for destinations farther out in the Solar System.
The key to finding the right self-repair systems for space composites, the Bristol team contends, is to first define the environmental threats the spacecraft will likely encounter.
To characterize environmental threats, the team conducted lab and space tests. These included low-Earth orbit (LEO), circular low-Earth orbit (CLEO), and highly elliptical-orbit (HEO) experiments. The goal was to identify the synergistic effects that high vacuum, atomic oxygen, UV radiation, impacts with cosmic debris, and thermal cycling have on the material.
BIOMIMETIC COMPOSITES
Because environmental threats from space cause internal as well as external damage, the Bristol researchers investigated placing self-healing, hollow filament plies at different depths within the composite. Three locations were considered:
Surface mounted. This approach is applicable to repairing surface damage due to atomic oxygen and UV exposure as well as, micrometeoroid and orbital debris (MOD) impacts.
Intermediate. This position is one or two plies from the outer surface. Putting filaments with self-healing agents here may help repair surfaces as well as internal damage.
Midline. Filaments placed here repair internal damage due to thermal cycling and MOD impacts.
Cont...