Self Healing Systems in Aircraft

7/26/2019 Self Healing Systems in Aircraft

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SELF HEALING SYSTEMS IN AIRCRAFT

Contents:

1. Abstract

2. Introduction. Causes o! aircra!t !ai"ure

#. se"!$%ea"in& 'ateria"s

(. t)*es o! %ea"in& *o")'ers

+. se"!$%ea"in& conce*ts

,. Ad-anta&es

. /isad-anta&es

0. Conc"usion

1.Future sco*e

11.Re!erences

Abstract:

A new technique that mimics healing processes found in nature could enable

damaged aircraft to mend themselves automatically, even during a flight. Self-

healing materials are a class of materials that have the structurally incorporated

ability to repair damage caused by mechanical usage over time. The inspiration

comes from biological systems, which have the ability to heal after being

wounded. Initiation of cracks and other types of damage on an aircraft has beenshown to change thermal, electrical, and acoustical properties, and eventually lead

to whole scale failure of the aircraft sometimes. or a material to be defined as

self-healing, it is necessary that the healing process occurs without human

intervention.

Self-healing, how it works! If a tiny hole"crack appears in the aircraft #e.g. due to

wear and tear, fatigue, a stone striking the plane etc.$, epo%y resin would &bleed&

from embedded vessels near the hole"crack and quickly seal it up, restoring

structural integrity. 'y mi%ing dye into the resin, any &self-mends& could be made toshow as colored patches that could easily be pinpointed during subsequent ground

inspections, and a full repair carried out if necessary. In tests, the self-healed

composite material regained as much as () percent of its original strength.


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As well as the obvious safety benefits, this breakthrough could make it possible to

design lighter airplanes in the future. This would lead to fuel savings, cutting costs

for airlines and passengers and reducing carbon emissions too.

Introduction:

Self-healing can be de*ned as the ability of a material to heal #recover"repair$

damages automatically and autonomously, that is, without any e%ternal

intervention. +any common terms such as self-repairing, autonomic-healing, and

autonomic-repairing are used to de*ne such a property in materials. Incorporation

of self-healing properties in manmade materials very often cannot perform the self-

healing action without an e%ternal trigger. Thus, self-healing can be of the

following two types $ autonomic #without any intervention$,2)nonautonomic

#needs human intervention"e%ternal triggering$.

Causes o! aircra!t !ai"ure:

ommonly, failures are associated with stress concentrations, which can occur for

several reasons including

. /esign errors, #e.g. the presence of holes, notches, and tight fillet radii$.

0. The microstructure of the material may contain voids, inclusions etc.

1. orrosive attack of the material, #e.g. pitting, can also generate a local stressconcentration$.

ommon failure modes of aircraft

atigue.

/uctile or overload.

orrosion.

Stress corrosion.

2ydrogen embrittlement.

3%cessive yielding.

4verheating"fire.

There are types of failures in aircraft also namely


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$normal failure.

0$medium failure,

1$catastrophic failure.

A normal failure is a small failure which occurs generally in every 561 hours ofan aircraft and the aircraft can complete its mission if it is a normal failure.

A medium failure is a some more damaged than the normal and immediate

assistance is required and the aircraft should be landed.

atastrophic failure is the failure in which the entire aircraft goes down any

assistance is also waste and the aircraft should be removed.

Se"!$%ea"in& 'ateria"s:

Self-healing materials are a class of smart materials that have the structurally

incorporated ability to repair damage caused by mechanical usage over time.

Self-healing polymers and fiber-reinforced polymer composites possess the

ability to heal in response to damage wherever and whenever it occurs in the

material.

All classes of polymers, from thermosets to thermoplastics to elastomers,

have potential for self-healing.

igure shows the stretchable self-healing polymer and the figure 0 showsthe micro capsule releasing a healing agent.


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igure stretchable self-healing polymer

Fi&ure 2:+icro capsules releasing a healing agent

7ormally what happens is if any modes of failure occur structurally then the

gas or liquid comes from the tube and structural damaged gets sealed.

The inspiration comes from biological systems, which have the ability to

heal after being wounded. Initiation of cracks and other types of damage on

a microscopic level has been shown to change thermal, electrical, and

acoustical properties, and eventually lead to whole scale failure of the

material. 8sually, cracks are mended by hand, which is unsatisfactory

because cracks are often hard to detect. A material that can intrinsically

correct damage caused by normal usage could lower costs of a number of

different industrial processes through longer part lifetime, reduction of

inefficiency over time caused by degradation, as well as prevent costs


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incurred by material failure. or a material to be strictly defined as self-

healing, it is necessary that the healing process occurs without human

intervention. Some e%amples shown below, however, include healing

polymers that require intervention to initiate the healing process.

T)*es o! %ea"in& *o")'ers:

o")'er brea3do4n:rom a molecular perspective, traditional polymers

yield to mechanical stress through cleavage of sigma bonds. 9hile newer

polymers can yield in other ways, traditional polymers typically yield

through homolytic or heterolytic bond cleavage. The factors that determine

how a polymer will yield include type of stress, chemical properties

inherent to the polymer, level and type of solvation, and temperature. rom a

macromolecular perspective, stress induced damage at the molecular level

leads to larger scale damage called microcracks. A microcrack is formedwhere neighboring polymer chains have been damaged in close pro%imity,

ultimately leading to the weakening of the fiber as a whole.

Ho'o")tic bond c"ea-a&e:

:olymers have been observed to undergo homolytic bond cleavage through

the use of radical reporters such as /::2 #0,0-diphenyl--picrylhydra;yl$

and :+7' #pentamethylnitrosoben;ene.$ 9hen a bond is cleaved

homolytically, two radical species are formed which can recombine to repair

damage or can initiate other homolytic cleavages which can in turn lead tomore damage. igure shows the 2omolytic bond cleavage of polymer

#methyl methacrylate$.

igure homolytic bond cleavage of polymer #methyl methacrylate$

Hetero")tic bond c"ea-a&e:


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:olymers have also been observed to undergo heterolytic bond cleavage

through isotope labeling e%periments. 9hen a bond is cleaved

heterolytically, cationic and anionic species are formed which can in turn

recombine to repair damage, can be quenched by solvent, or can react

destructively with nearby polymers, figure 0 shows the 2eterolytic bondcleavage of polyethylene glycol.

Fi&ure 2:2eterolytic bong cleavage of polyethylene glycol

Re-ersib"e bond c"ea-a&e:

ertain polymers yield to mechanical stress in an atypical, reversible

manner. /iels-Alder-based polymers undergo a reversible cycloaddition,

where mechanical stress cleaves two sigma bonds in a retro /iels-Alder

reaction. This stress results in additional pi-bonded electrons as opposed to

radical or charged moieties.

Su*ra'o"ecu"ar brea3do4n:

Supramolecular polymers are composed of monomers that interact non-

covalently. ommon interactions include hydrogen bonds, metal

coordination, and van der 9aals forces. +echanical stress in supramolecular

polymers causes the disruption of these specific non-covalent interactions,

leading to monomer separation and polymer breakdown.

Re-ersib"e %ea"in& *o")'ers:


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e%ternal stimulus for it to occur. or a reversible healing polymer, if the

material is damaged by means such as heating and reverted to its

constituents, it can be repaired or =healed= to its polymer form by applying

the original condition used to polymeri;e it.

Co-a"ent") bonded s)ste':

/iels-Alder and


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There are different methods to effect healing that are applicable for each

individual mode of damage as well as each unique damaged material. Self-

healing has been demonstrated by some of these conceptual approaches

apsule-based healing systems.

ascular healing systems.

Intrinsic healing polymers.

arbon 7ano tubes #7Ts$.

+icroencapsulated 2ealing systems.

2ollow tube approach

SBI:S

/irect Ink 9riting

/iscrete channels

Ca*su"e$ based %ea"in& s)ste':

apsules containing the healing agents and other chemicals are distributed

throughout the material. If a breakage occurs, the capsules release their contents

causing a chemical reaction to =heal= the breakage. If the amount of damage is


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microscopic, capsule-based is the best option

Fi&ure 1:Sequester healing agents in capsule-based system.

5ascu"ar %ea"in& s)ste's:

ascular systems use networks of refillable channels #like capillaries, veins and

arteries$ within the material to deliver the healing agent to the site of damage and

polymeri;es to the breakage point. ascular networks offer e%ceptional healing

efficiency and vast possibilities.


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Fi&ure 2:


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one maDor factor to take into account is that the closer the tubes are together, the

lower the strength will be, but the more efficient the recovery will be. A sandwich

structure is a type of discrete channels that consists of tubes in the center of the

material, and heals outwards from the middle. The stiffness of sandwich structures

is high, making it an attractive option for pressuri;ed chambers. or the most partin sandwich structures, the strength of the material is maintained as compared to

vascular networks. Also, material shows almost full recovery from damage.

/irect In3 6ritin&:

The /irect Ink 9riting #/I9$ technique is a controlled e%trusion of viscoelastic

inks to create three-dimensional interconnected networks. It works by first setting

organic ink in a defined pattern. Then the structure is infiltrated with a material like

an epo%y. This epo%y is then solidified, and the ink can be sucked out with amodest vacuum, creating the hollow tubes.

Microca*su"e %ea"in&:

This method is similar in design to the hollow tube approach. +onomer is

encapsulated and embedded within the thermosetting polymer. 9hen the crack

reaches the microcapsule, the capsule breaks and the monomer bleeds into the

crack, where it can polymeri;e and mend the crack.

igure 1 /epiction of crack propagation through microcapsule-imbedded material.

+onomer microcapsules are represented by pink circles and catalyst is shown by

purple dots.

A good way to enable multiple healing events is to use living #or unterminated

chain-ends$ polymeri;ation catalysts. If the walls of the capsule are created too

thick, they may not fracture when the crack approaches, but if they are too thin,

they may rupture prematurely. In order for this process to happen at roomtemperature, and for the reactants to remain in a monomeric state within the

capsule, a catalyst is also imbedded into the thermoset. The catalyst lowers the

energy barrier of the reaction and allows the monomer to polymeri;e without the

addition of heat. The capsules #often made of wa%$ around the monomer and the

catalyst are important maintain separation until the crack facilitates the reaction.


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There are many challenges in designing this type of material. irst, the reactivity of

the catalyst must be maintained even after it is enclosed in wa%. Additionally, the

monomer must flow at a sufficient rate #have low enough viscosity$ to cover the

entire crack before it is polymeri;ed, or full healing capacity will not be reached.

inally, the catalyst must quickly dissolve into monomer in order to reactefficiently and prevent the crack from spreading further.

This process has been demonstrated with dicyclopentadiene #/:/$ and Erubbs&

catalyst #ben;ylidene-bis#tricyclohe%ylphosphine$dichlororuthenium$. 'oth /:/

and Erubbs& catalyst are imbedded in an epo%y resin. The monomer on its own is

relatively unreactive and polymeri;ation does not take place. 9hen a microcrack

reaches both the capsule containing /:/ and the catalyst, the monomer is

released from the coreFshell microcapsule and comes in contact with e%posed

catalyst, upon which the monomer undergoes ring opening metathesispolymeri;ation #


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Carbon Nano tubes 7CNTs8:

'y infusing a polymer with electrically conductive carbon 7ano tubes once a crack

is located, an electrical charge is sent to the area in order to heat up the carbon7ano tubes and in turn melt an embedded healing agent that will flow into and seal

the crack with a (5 percent recovery in strength.

Fi&ure #:arbon 7ano tubes healing system

Ad-anta&es:

3ffectively reducing the weight of planes and thus both their fuel needs andcarbon emissions.

2igher volume of healing agent is available to repair damage.

/ifferent activation methods"types of resin can be used.

isual inspection of the damaged site is feasible.

2ollow fibers can easily be mi%ed and tailored with the conventional

reinforcing fibers.

/isad-anta&es:

ibers must be broken to release the healing agent

Bow-viscosity resin must be used to facilitate fiber infiltration.


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+ultistep fabrication is required.

Conc"usion:

Self-healing approaches applied in composite materials to-date have

primarily been bio inspired.

A more recent advance is the detailed study of natural healing to allow true

biometric self-healing.

Tailored placement of healing components and the adoption of biometric

vascular networks for self-healing are very active research topics at the

cutting edge of self-healing.

Future sco*e:

A self-healing aircraft could be available in the near future, an epo%y

resin that bleedsC from embedded vessels near the holes or cracks and

quickly seals them up, restoring structural integrity.

As well as the obvious safety benefits, this breakthrough could make it

possible to design lighter aero planes in the future.

This would lead to fuel savings, cutting costs for airlines and passengers and

reducing carbon emissions too.

Re!erences:

1. S4a*an 9u'ar G%os% Se"!$%ea"in& Materia"s: Funda'enta"s; /esi&n

Strate&ies; and A**"icationsac; 9at%erine First Se"!$Hea"in& Coatin&s 7%tt*:? ? 444.

tec%no"o&) re-ie4. co'? business? 2112?@ a!8. tec%no"o&)re-ie4.co'.

/ece'ber 12; 2.

. Se"!$%ea"in& 'ateria" Source: %tt*:??en.4i3i*edia.or&?4?indeB.*%*@

o"did+220((21.

#. 6einer; S. and 6a&ner; H./. 71008 Annua" Re-ie4 o! Materia"s

Science; 2.
http://en.wikipedia.org/w/index.php?oldid=622955021http://en.wikipedia.org/w/index.php?oldid=622955021http://en.wikipedia.org/w/index.php?oldid=622955021http://en.wikipedia.org/w/index.php?oldid=622955021


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Self Healing Systems in Aircraft

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