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The use of a material for a specific application is governed by
considerations on the expected conditions during its
lifetime. For aerospace applications for instance, lightness,
reliability and thermal stability ofthe material are of major
importance. No material will ever possess all the desired
properties at the same time. Therefore the engineer is left
with optimising the materials and minimise the impact ofthe
ever-present 'disadvantageous' properties. By introducing the
potential of 'Self Healing' in a material, the lifetime and
reliability could be significantly improved. Introduction of
safety factors in the design could be minimised, leading to
leaner structures. Making an aircraft from concrete may seem
far-fetched, but isn't that not what Aerospace Engineering is
all about? Jules Verne is now our reality.
by: Alexander Schmets, Delft Centre for Materials
R E P A I R T A K E S T I M E . . .
A l l existing materials accumulate damage during service i n an application. Sooner or later this w i l l lead to service failure, unless one repairs or replaces the damaged part. However, we know from biological materials that taking a period of rest (low load) can lead to the decrease of damage. Such rest periods can be rather long. As it is commonly known, soccer players w i t h torn tendons need many months of recovery before they can rejoin their team, although minimal load activities like walking can start w i t h i n a few weeks after the healing process has started. These recovery processes that we know from the biological world are also a very desirable property for man-made materials, although for many apphcations recovery times of months would be less attractive.
It was already explained i n the Leonardo Times of December 2004 that, quite recently, material researchers have succeeded i n man-ufacturing materials that possess this proper-ty of damage decrease over time. The para-mount example is the epoxy-system of Scott White's team, at the University of Illinois, who succeeded i n creating a material that shows decrease of damage over time by the embedding of spherical repair units i n the system. The term 'self healing material' was coined at that time. Since then many other systems have been identified to show self healing behaviour, and the work of White was rapidly followed up at other institutes. For instance at the department of Aerospace Engineering of the University of Bristol where Ian Bond's team extended the original idea of hollow spheres by developing a novel
[source: ESA]
fibre reinforced plastic. By storing a liquid resin i n the hollow reinforcement fibres a repair process is triggered after impact load-ing of the material (fig. i ) . The latter glass fibre system is now being further explored and optimised (repair times) i n a larger European Space Agency (ESA) program to study technology for self-healing spacecraft. At the same time a large program on Self Healing Materials, including all material classes, is starting i n the Netheriands. It is reasonable to speak ofthe birth of a new field of research and technology. That this is not yet another 'fad' i n the world of science and technology, is underiined by Dr. 'Les' Lee of the US Air Force Office of Scientific Research (AFOSR)"... Our dream of creating self-reliant aerospace structures may be roughly a decade away f r o m becoming operational i n the field. Still, the potential benefit to taxpay-ers, builders and customers is tremendous. Having a new breed of composite materials with self-healing, self-coohng and self-diag-nosis capabilities w i l l enable airplanes and space platforms to sur-vive longer, fail less often, be more reliable and cost effective ..."
. . . B U T T I M E C A N B E O N O U R S I D E A S W E L L
Whereas i n the aforementioned materials the property of self healing was an actively designed feature of the material, it is well known that certain man-made constructions such as Roman aqueducts or gothic churches survive for centuries without falling apart.
One of the main reasons for this longevity of these structures is the inherent self-healing capacity of the binders used for cementing the buildmg blocks together (fig. 2). This self healing i n cementitious structures is evidently a slow process, but fast enough to neutralise cracks that occur through nor-mal use of the object.
This intrinsic self healing effect is not pres-ent i n the concrete that is used nowadays. The approach to damage is to make the con-crete stronger and stiffer, and therefore the excess of calcium that is needed i n the con-crete for the intrinsic self healing effect is not available anymore. Since tenders for large infrastructural projects increasingly require both building and maintenance rather than only the delivery of a build construction, interest i n the intrinsic self-heahng property of concrete is gaining rapidly
It is clear that the pace at which self healing is needed depends on the specific application and its predicted load cycle, i.e. the projected forces that the structure wiU have to carry over its hfe cycle. In this sense self healing is not only a property that can be designed for a certain material: it is a different approach towards damage and the f u l l life cycle of a material i n a specific application. The design of self healing capacity into a material should include its foreseeable use and the influences i t w i l l undergo.
For instance the healing effect i n a plastic developed at the University of California by Fred W u d l only occurs when the material is heated after a crack has developed (fig. 3). This material should ideally be applied i n sit-uations where temperature changes are a natural consequence of the application (e.g. a rotating spaceship). The heat that is required doesn't necessarily originate form environ-mental effects; also the heat produced by the impact of a projectile can trigger the healing effect (fig. 4). A n illustrative example of this is the sawing of the plastic React-a-SeaF (fig. 5). The reader is invited to invent an applica-tion for materials with this surprising prop-erty.
S E L F H E A L I N G I N T E L E C O M M U N I C A T I O N S
Let's consider telecommunications to illus-trate the potential impact of self healing materials on society. Downstream one has the mobile telephone. Their display is of major importance for their practical use. A damaged or scratched display w i l l require for replacement of the device. So a scratch »
figure i: Self Healing Glass Fibre Reinforced Plastics (GFRP) for aircraft stinchiral applications. Hollow glass fibres of 60 micrometer outside diameter (upper image) are
embedded in an epoxp matiix (lower image). Afier (quasi static) impact damage this spstem recovers to 86% of the base material's flexural strength. This is achieved bi! filling part of the fibres with liquid 'repair resin' and others with 'hardener'. Impact damage is repaired bp breaking of the fibres, bleeding repair resin and hardener in the damaged
area. At room temperature recovery up to more than 80% ofthe original flexural strength takes more than 24h. [source: Dr. 1. Bond, University of Bristol]
figure 2: Inherent self healing explains tl\e solidi-ty over time of this i8th century bridge in Amsterdam (lower left): the porosity ofthe bricks leads to the precipitation of dissolved calcite in cracks (lower right). In this way the develop-ment of catastrophic damage is halted. The Mihara bridge in Hokkaido (Japan) is construct-ed from a concrete that keeps cracks small enough to allow for self healing. This leads to a weight reduction of 4.0% and a longer expected lifetime compared to the same bridge constructed from 'traditional concrete',
[source: Prof R. van Hees, TUDelft]
figure 3: A self-mending plastic: a crack recovers after applying heat to the specimen. The heat supply should ide-ally be a natural consequence ofthe specific application in which the plastic serves. If during service life the tempera-ture never gets to the self-mending temperatempera-ture, the plastic is not self healing in this application,
[source: ProfF. Wudl, Uitiversity of California]
HEAT
figure 4: Heat produced by an impacting bullet is used for self healing the 'impact wound', [source: A. Schmets]
figure j : Self-healing of sawed React-a-Seal". Images progress clockivise from the upper-left. Upon completion of theftdl cut, the two pieces are healed together and provide a bond between the two pieces, [source: S. Kalista, Virginia Polytechnic Institute]
healing display would be a real asset for mobile telephones.
More frustrating than scratches at the sur-face of the mobile's display, is an empty bat-tery. Any user of mobile telephones knows that the reload cycles of the batteries get ever shorter over time, leaving the user some-times powerless. This effect is inherent to the use of lithium-ion technology for mobile applications. The use of this technology is necessary because light weight is important to mobile telephone applications (to illus-trate this: for I ampere/hour 3.85 gram of lead is needed but only 0.26 grams of lithi-um). The working of such a lithium battery is shown i n figure 6. When the battery deliv-ers energy lithium moves from the anode, through the electrolyte, to the anode. When recharged the reverse process takes place. So lithium should be reversibly stored i n the cathode (fig. 7 a). However, by storing lithi-u m i n the cathode, the lattice expands. Repeated expansions lead to structural changes that at certain places i n the material wiU block the path along which hthium dif-fuses: the battery looses capacity and needs to be recharged more frequently (fig. 7b). From the point of self healing one should strive to overcome this problem, by lookmg for ways to restore the open diffusion path-ways while the battery is functioning. However, this is not the way that batteries
are nowadays developed: merely optimising for capacity rather than for recyclability. This is certainly a direction to pursue, not i n the least because better recharging character-istics of batteries would make them suitable for automotive applications. Actually, the same approach would help the automotive sector also i n another way: well known hydrogen storage materials (LaNis) embrit-tle for the same reason as for the batteries. Finally, to take a last example that impacts telecommunications, a telecommunications sateUite that is less susceptible to the impact of micrometeoroids or orbital debris would have a better operational reliability and increased mission life-time. So, just from these simple examples it is clear that the property of self healing would definitely affect the telecommunications sector and our society i n general.
S E L F H E A L I N G M A T E R I A L S A T T U D E L F T
At the TU Delft the possibilities for seh heal-ing materials have been recognised, and a joint research effort has recently started, approaching the challenge at different levels. At the faculty of Civil Engineering the possi-bilities for self heahng concrete are research-ed, while at the Reactor Institute the self healing effect i n a special aluminium alloy is being characterized.
The approach of White and Bond described above is followed at DelftChemTech, where work is done on microvascular networks for healing agents w i t h a fractal geometry (simi-lar to the artery system i n the human body; fig. 8). At the faculty of Aerospace Engineer-ing research is done on the possibilities for self healing intermetallic (i.e. metal and poly-mer) composites and self healing elastomer nanocomposites. As well as the necessary modeUing of self heahng mechanisms. Remarkably, ahhough dedicated research i n this new field is just taking off, the first com-mercial applications are already available at the market. I n December 2005 Nissan has introduced the first self healing car coating for its top segment. They claim that cars painted w i t h this coating w i l l have only one-fifth the abrasions caused by a car-washing machine compared w i t h a car covered w i t h conventional clear paint. Scratches from car-washing machines account for the majority of scratches to painted car surfaces (fig. 9). So returning to the question posed i n the title: W i l l aircraft ever be constructed using concrete? Concrete would obviously be the last option considered when choosing an appropriate material for building aircraft. However, as materials develop continuously, sometimes the impossible may even become true. Wasn't that the same for Jules Verne's 'ridiculous' ideas? 7<
figure 6: Rechargeable lithium-ion battery. When being charged, all amws reverse direc-tion, [source: A. Schmets]
figure ja: Crystal structure of a lithium-ion cathode material. Clearly visible are the diffiision pathways for the lithium ions (golden spheres) through an oxygen (lit-tle blue spheres) structure, [source: A. Schmets]
figure 7b: In cycling the battery lithium jumps in and out 'oxygen-cells'. Repeated loading/unloading leads to changes in the structure, blocking lithiimi's diffiision pathways and hence compromising on the fimction ofthe material as a cathode, [source: A. Schmets[
Cathode tuii/empty b»nery
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figure 8 (left): At DelftChemTech researchers are building afi-actal artery system for healing continuous healing agent delivery. This system closely resembles the hierarchy of for instance circulatory system within the human body, [source: A. Schmets]
figure 9 (right): Self healing car coating
by Nissan. "... with 'Scratch Guard Coat' a car's scratched surface will return to its original state anyivherefivm one day to a week, depending on temperature and the depth ofthe scratch." ]source: NissanJ
N e w s c r a t c h e s O n e w e e k later
