U.S. patent application number 12/594624 was filed with the patent office on 2010-05-13 for composite self-healing system.
Invention is credited to Benjamin Allen Dietsch, David Ernest Havens, Christopher Douglas Hemmelgarn, Anthony Louderbaugh, Thomas Wood Margraf, John Lewis Reed, JR., Logan Wayne Snyder.
Application Number | 20100119704 12/594624 |
Document ID | / |
Family ID | 40186228 |
Filed Date | 2010-05-13 |
United States Patent
Application |
20100119704 |
Kind Code |
A1 |
Hemmelgarn; Christopher Douglas ;
et al. |
May 13, 2010 |
COMPOSITE SELF-HEALING SYSTEM
Abstract
An advanced reflexive structure system is disclosed. The
reflexive system mimics the pain withdrawal reflex on which the
human body relies. The reflexive system incorporates a continuous
health and performance monitoring system via an embedded dielectric
film, an adaptive composite structure based on shape memory
composite material, and an intelligence system which will be
interfaced with both the health/performance sensors and the
adaptive structure. When activated shape memory polymer will
recover its structural integrity via shape recovery and a reptation
healing process. These features enable the use of SMP as an
adaptive structure in the proposed reflexive system. The
development of a reflexive system for structures will enable
increased safety and security and demonstrate a better
understanding of integrated performance systems. This reflexive
technology could find immediate implementation on all current and
future systems and future implementation on platforms such as the
International Space Station, Lunar, and Martian habitats.
Inventors: |
Hemmelgarn; Christopher
Douglas; (Miamisburg, OH) ; Margraf; Thomas Wood;
(Centerville, OH) ; Havens; David Ernest;
(Bellbrook, OH) ; Reed, JR.; John Lewis; (West
Salem, OH) ; Snyder; Logan Wayne; (Fairborn, OH)
; Louderbaugh; Anthony; (Cincinnati, OH) ;
Dietsch; Benjamin Allen; (Dayton, OH) |
Correspondence
Address: |
CORNERSTONE RESEARCH GROUP, INC.
2750 INDIAN RIPPLE ROAD
DAYTON
OH
45440
US
|
Family ID: |
40186228 |
Appl. No.: |
12/594624 |
Filed: |
April 11, 2008 |
PCT Filed: |
April 11, 2008 |
PCT NO: |
PCT/US08/60055 |
371 Date: |
October 5, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60911682 |
Apr 13, 2007 |
|
|
|
60911673 |
Apr 13, 2007 |
|
|
|
60911665 |
Apr 13, 2007 |
|
|
|
Current U.S.
Class: |
427/140 |
Current CPC
Class: |
B29C 35/0272 20130101;
B32B 43/00 20130101; B29C 73/18 20130101; B29C 73/22 20130101 |
Class at
Publication: |
427/140 |
International
Class: |
B05D 7/00 20060101
B05D007/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with U.S. Government support under
Contract No. NNL05AA97C awarded by National Aeronautics and Space
Administration and Contract No. NNL06AA07C awarded by the National
Aeronautics and Space Administration to Cornerstone Research Group
Inc. The U.S. Government has certain rights in the invention.
Claims
1. A method of healing a material comprising: a sensing means for
detecting damage at a damaged point in a material; a repair means
for repairing said material at said damaged point; and a control
means for selectively activating the repair means at the damaged
point.
2. The method of healing a material of claim 1 wherein said means
for detecting damage is a continuous health and performance
monitoring system.
3. The method of healing a material of claim 2 wherein said
continuous health and performance monitoring system is an embedded
piezoelectric sensor system.
4. The method of healing a material of claim 1 wherein said
material is an adaptive composite structure.
5. The method of healing a material of claim 4 wherein said
adaptive composite structure is a fibrous material in a dynamic
elastic modulus resin matrix.
6. The method of healing a material of claim 5 wherein said dynamic
elastic modulus resin matrix is a shape memory polymer matrix.
7. The method of healing a material of claim 1 wherein said
material is an adaptive resin structure.
8. The method of healing a material of claim 7 wherein said
adaptive resin structure is a dynamic elastic modulus resin
matrix.
9. The method of healing a material of claim 8 wherein said dynamic
elastic modulus resin matrix is a shape memory polymer matrix.
10. The method of healing a material of claim 1 wherein said repair
means activates and deactivates a self-healing mechanism in said
material.
11. The method of healing a material of claim 10 wherein said
self-healing mechanism is activated and deactivated with thermal
energy.
12. The method of healing a material of claim 10 wherein said
self-healing mechanism is activated and deactivated with
electromagnetic waves.
13. The method of healing a material of claim 12 wherein said
self-healing mechanism is activated and deactivated with visible
light.
14. The method of healing a composite material of claim 10 wherein
said self-healing mechanism is a combination of a first
self-healing mechanism and a second self-healing mechanism.
15. The method of healing a material of claim 14 wherein said first
self-healing mechanism is a shape memory effect.
16. The method of healing a material of claim 14 wherein said
second self-healing mechanism is reptation of said material across
said damaged point.
17. The method of healing a material of claim 1 wherein said
control means for selectively activating the repair means at the
damaged point is a computer control system interfaced with said
sensing mean, said repair means and said material.
18. The method of healing a material of claim 17 wherein said
computer control system: a. compares a baseline data set against a
first new data set; b. determines a healing process based on the
comparison between the baseline data set and first new data set; c.
creates a set of tasks capable of being carried out in the proper
sequence by a machine to execute said healing process; and d.
compares said baseline data set against a second new data set to
determine when said healing process is complete.
19. The method of healing a material of claim 1 wherein said means
for selectively activating the repair means at the damaged point is
by manual input from a user.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application Ser. No. 60/911,673 filed Apr. 13, 2007, U.S.
Provisional Application Ser. No. 60/911,682 filed Apr. 13, 2007,
and U.S. Provisional Application Ser. No. 60/911,665 filed Apr. 13,
2007.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This application relates to the self-repair of polymer and
polymer composite structures. The disclosed method of achieving
this is to design and incorporate smart, intelligent, and adaptive
systems into the structures composed of adaptive materials such as
dynamic modulus resins and composites. Such systems will allow for
continuous health and performance monitoring, fast and decisive
information processing to ensure that the system is "highly aware"
of its current health status and unparalleled in its adaptability
to damage. Currently, most state-of-the-art health monitoring
technologies can only deliver the indication of damage to the human
operator via visual display. An integrated system consisting of
smart, adaptive, and intelligent components will enable an advanced
system to sense and immediately recover from physical damage while
informing the operator of the situation but not requiring a
response.
[0005] 2. Description of Related Art
[0006] Dynamic Elastic Modulus Resins (DMR) are resins whose
elastic modulus changes with a change in temperature of the resin.
One such DMR is shape memory polymer (SMP). Shape memory materials
were first developed about twenty-five (25) years ago and have been
the subject of commercial development in the last fifteen (15)
years. Shape memory materials derive their name from their inherent
ability to return to their original "memorized" shape after
undergoing a shape deformation. There are principally two types of
shape memory materials, shape memory alloys (SMAs) and shape memory
polymers (SMPs).
[0007] SMAs and SMPs that have been pre-formed can be more easily
deformed to a desired shape above their glass transition
temperature (Tg). The SMA and SMP must remain below, or be quenched
to below, the Tg while maintained in the desired shape to "lock" in
the deformation. Once the deformation is locked in, the SMA,
because of its crystalline network, and the SMP, because of its
polymer network, cannot return to a relaxed state due to thermal
barriers. The SMA and SMP will hold its deformed shape indefinitely
until it is heated above its Tg, whereupon the SMA and SMP stored
mechanical strain is released and the SMA and SMP returns to its
pre-formed, or memory, state.
[0008] There are principally two types of plastics, thermoset
resins and thermoplastic resins, each with its own set of unique
characteristics. Thermoset resins, for example polyesters, are
liquids that react with a catalyst to form a solid, and cannot be
returned to their liquid state, and therefore, cannot be reshaped
without destroying the polymer networks. Thermoplastics resins, for
example PVC, are also liquids that become solids. But unlike
thermoset resins, thermoplastics are softened by application of
heat or other catalysts. Thermoplastics can be heated, reshaped,
heated, and reshaped over and over.
[0009] SMPs used in the presently disclosed method and devices are
unique thermosetting polymers that, unlike traditional
thermosetting polymers, can be reshaped and formed to a great
extent because of their shape memory nature and will not return to
a liquid upon application of heat. Thus by creating a shape memory
polymer that is also a thermosetting polymer, designers can utilize
the beneficial properties of both thermosetting and thermoplastic
resins while eliminating or reducing the unwanted properties. Such
polymers are described in U.S. Pat. No. 6,759,481 issued to Tong,
on Jul. 6, 2004 which is incorporated herein by reference. Other
thermoset resins are seen in PCT Application No. PCT/US2006/062179,
filed by Tong, et al on Dec. 15, 2006; and PCT Application No.
PCT/US2005/015685 filed by Tong et al, on May 5, 2005 of which both
applications are incorporated herein by reference.
[0010] Additionally DMRs and SMPs can self-heal through combination
of shape memory effects and a reptation process. Reptation theory
describes the snake-like large-scale motion of long-chain entangled
polymers across an interface, involving interfacial bonding across
a boundary. The diffusive motion of polymer segments across a
boundary is increased at high temperatures in which long-chain
polymers are embedded. Essentially this motion allows two polymer
surfaces to bond together along their interface when placed in
intimate contact above the Tg of the polymer. This phenomenon
commonly referred to as "healing" is essentially the interfacial
welding of two polymer surfaces through the inter-diffusion of the
polymer by motions across the interface via chain reptation-type
motions. This basic picture of reptation is by now experimentally
verified and well-established in many contexts.
[0011] This form of healing is commonly studied when polymers are
placed in contact above Tg. As samples are heated and expand, the
crack surfaces come into intimate contact and healing progresses
following the diffusion process described above, mending the crack.
The Tong patents mentioned above can be formulated by those of
skill in the art to include this self-healing feature.
[0012] There are three types of SMP's: 1) A partially cured resin,
2) thermoplastics, and 3) fully cured thermoset systems. There are
limitations and drawbacks to the first two types of SMP. Partially
cured resins continue to cure during operation and change
properties with every cycle. Thermoplastic SMP "creeps," which mean
it gradually "forgets" its memory shape over time. A thorough
understanding of the chemical mechanisms involved will allow those
of skill in the art to tailor the formulations of SMP to meet
specific needs, although generally fully cured thermoset resin
systems are preferred in manufacturing.
[0013] While SMA and SMP appear to operate similarly on the macro
scale, at the molecular scale it is apparent that the method of
operation of each is very different. The difference between SMA and
SMP at the molecular level is in the linkages between molecules.
SMA essentially has fixed length linkages that exist at alternating
angles establishing in a zigzag patterned molecular structure.
Reshaping is achieved by straightening the angled connections from
alternating angles to straight forming a cubic like structure. This
method of reshaping SMA material enables bending while limiting any
local strains within the SMA materials to less than eight percent
(8%) strain, as the maximum shape memory strain for SMA is eight
percent (8%). This eight percent (8%) strain allows for the
expansion or contraction of the SMA by only 8%, a strain that is
not useful for most industrial applications. Recovery to memory
shape is achieved by heating the material above a certain
temperature at which point the molecules return to their original
zigzag molecular configuration with significant force thereby
reestablishing the memory shape. The molecular change in SMA is
considered a metallic phase change from Austensite to Martensite
which is defined by the two different molecular structures.
[0014] SMP has connections between molecules with some slack. When
heated these links between connections are easily contorted,
stretched and reoriented due to their elastic nature as the SMP
behaves like an elastic material when heated, when cooled, the
shape is fixed to how it was being held. In the cooled state the
material behaves as a typical rigid polymer that was manufactured
in that shape. Once heated the material again returns to the
elastic state and can be reformed or return to the memory shape
with very low force. Unlike SMA which possesses two different
molecular structures, SMP is either a soft elastomer when heated or
a rigid polymer when cool. Both SMA and SMP can be formulated to
adjust the activation temperature for various applications.
Critical to the success of the currently claimed device is
thermoset SMP which provides an order of magnitude higher stiffness
than previous state-of-the-art thermoplastic SMPs. This added
stiffness coupled with high strain capability enables the
development and use of a highly useful composite tooling
technology.
[0015] Unlike SMAs, SMPs exhibit a radical change from a normal
rigid polymer to a flexible elastic and back on command. SMA would
be more difficult to use for most applications because SMAs do not
have the ease in changing the activation temperature as do SMP's.
SMAs would also have issues with galvanic reactions with other
metals which would lead to long term instability. The current
supply chain for SMAs is currently not consistent as well. SMP
materials offer the stability and availability of a plastic and are
more inert than SMAs. Additionally, when made into a composite SMPs
offer similar if not identical mechanical properties to that of
traditional metals and SMAs in particular. Throughout this
disclosure SMP and SMP composites are used interchangeably as each
can be replaced by the other depending on the specific design
requirements to be met.
[0016] The term "composite" is commonly used in industry to
identify components produced by impregnating a fibrous material
with a thermoplastic or thermosetting resin to form laminates or
layers. Generally, polymers and polymer composites have the
advantages of weight saving, high specific mechanical properties,
and good corrosion resistance, which make them indispensable
materials in all areas of manufacturing. Nevertheless,
manufacturing costs are sometimes detrimental, since they can
represent a considerable part of the total costs and are made even
more costly by the inability to quickly and easily repair these
materials without requiring a complete, and expensive, total
replacement. Because SMPs are resins, they can be used to make
composites, which are referred to in this application as SMP
composites.
[0017] Advanced composites, containing continuous fibers dispersed
in a resin matrix material, are widely used in aerospace, sports
equipment, infrastructure, automotive, and other industries both as
primary and secondary load-bearing structures. These composite
materials derive their excellent mechanical strength, stiffness,
and other properties from a combination of the resin and
reinforcement fibers used. The addition of reinforcements such as
continuous fiber, fiber mats, chopped fibers, fiberglass,
nanoparticles and other similar material is known. Even with
nanoparticles like carbon nanotubes and carbon nano-fillers a small
amount of these nano-fillers could dramatically alter the
properties of a matrix resin.
[0018] A recurring issue in product applications using materials
such as polymeric materials is that they tend to fail or degrade
due to mechanical fatigue, mechanical impact, oxidation due to
radiation or impurities, thermal fatigue, chemical degradation, or
a combination of these processes. The degradation can lead to
embrittlement of the polymer along with other adverse effects. The
embrittlement and associated cracking can advance to a point that
it causes product failure and associated replacement costs.
Thermoplastic and thermoset polymer systems used in products can be
particularly susceptible to these failures.
[0019] This problem is of great concern because of the widespread
and intensive use in modern society of polymers and polymer
composites in product components. Traditional approaches to
increasing the reliability of polymeric based components and
products have included a focus on suitable design enhancements and
the use of incrementally improved plastics.
[0020] One recently developed process to impart self-healing
capability to a polymer involves the incorporation of microcapsules
containing a healing agent in a polymer matrix. When a fracture
occurs in the polymer matrix in close proximity to the
microcapsules the associated stresses caused by the fracture
ruptures the microcapsules. As a consequence the healing agent is
released from the ruptured microcapsules and contacts the fracture
surfaces. At the same time the healing agent comes into contact
with a polymerization agent dispersed in the polymer matrix. The
polymerization agent is functionally active in the presence of
various chemicals including moisture in the air. When the
polymerization agent contacts the self-healing agent and promotes
polymerization of the healing agent resulting in filling the crack
planes of the fracture.
[0021] U.S. Pat. No. 7,285,306 issued on Oct. 23, 2007 to Parrish
discloses a self-healing system for an insulation material wherein
the self repair process is initiated by rupturing a plurality of
microcapsules disposed on the insulation material. When a plurality
of microcapsules is ruptured, reactants within the plurality of
microcapsules react to form a replacement polymer in a break of the
insulation material.
[0022] U.S. Pat. No. 7,108,914 issued on Sep. 19, 2006 to Skipor et
al. also discloses a self-healing polymer composition containing a
polymer media and a plurality of microcapsules of flowable
polymerizable material dispersed in the polymer media, where the
microcapsules of flowable polymerizable material containing a
flowable polymerizable material and have an outer surface upon
which at least one polymerization agent is chemically attached. The
microcapsules are effective for rupturing with a failure of the
polymeric media and the flowable polymerizable material reacts with
the polymerization agent when the polymerizable material makes
contact with the polymerization agent upon rupture of the
microcapsules.
[0023] The principal drawback of Parrish and Skipor is that once
the microcapsules have ruptured and repaired the insulation a
second break or damage point at or near the first break or damage
point cannot be as easily repaired because the replacement polymers
in the microcapsules will have been used in the first repair.
[0024] U.S. Pat. Nos. 6,261,360; 5,989,334; 5,660,624; 5,575,841;
and 5,5611,73 issued to Dry describe a cured composite matrix
having a plurality of hollow release vessels usually fibers
dispersed therein with the hollow fibers having a selectively
releasable modifying agent contained within them a means for
maintaining and modifying agent within the fibers until selectively
released and a means for permitting selective release of the
modifying agent from the hollow fibers into the matrix material in
response to at least one predetermined external stimulus. The cured
matrix materials have within them fibers capable of delivering
repair agents into the matrix wherever and whenever they are
needed.
[0025] While this engineered healing composite represents a very
exciting advance in the self-repair of materials, it is limited to
crack-type damage and would not be expected to heal the large sized
projectile damage (several mm or more in diameter) or repair damage
at the same point multiple times. The biggest difference between
these patent and the presently disclosed system is the fact that
the presently disclosed system is known to heal via a
thermo-mechanical response rather than by chemical reaction.
[0026] International Application No. PCT/US2005/0198 filed Jun. 6,
2005 describes a manual process to repair damage in a material
thought the application of a SMP or SMP composite patch. The
pressure sensitive adhesive placed on one side of the patch bonds
the patch to the damaged area, covering the damage. The SMP in the
patch allows a human operator to mold the patch to accurately fit
the product being repaired. This method is most useful for
aesthetic repairs to a product, not for structural repairs because
the damage area will remain and could propagate beyond the
boundaries of the patch at a future point. Additionally this device
and method of repair requires a separate piece of SMP or SMP
composite and a human operator to effect repairs.
DISCLOSURE OF THE INVENTION
[0027] An advanced sense and respond technology comprised of
multiple technologies to enhance survivability of future systems
constructed of lightweight resins or composites is disclosed. The
system is designed to react to detected damage with health
monitoring by locally activating shape recovery and healing
mechanisms of the adaptive polymer matrix or adaptive polymer
matrix composite structure through the use of proven health
monitoring technology to sense the location and significance of the
damage and new healable dynamic elastic polymers and dynamic
elastic polymers composites, an intelligent control system that
integrates the system of technologies seamlessly and characterize
the overall effectiveness.
[0028] Reflexive response to the structural damage is introduced
through the design of an electronic structural control system
intended to mimic the reflex action of the human body. Through this
system, a response to sensed structural damage will occur when a
specific damage threshold is reached. Data interpretation and
response will continue throughout the monitoring process by the
structural control system, but only when the sensed damage reaches
a limiting threshold will the reflexive repair system be
activated.
[0029] A method for designing a product which can detect damage to
the product, determine a course of action to heal the damage,
selectively activating the components needed to heal the damage,
allowing the material to heal, and detecting when healing is
complete is disclosed.
[0030] Replacing traditional metallic structures with composites
offers the end user increased functionality including higher
specific mechanical properties, customizable ply schedules for
tailored properties, and low coefficient of thermal expansion.
Introduction of composite materials into structural applications
does however result in specific design considerations including
failure mechanisms. The failure mechanism for composites, as
compared to traditional structural materials including concrete and
steel, are more difficult to predict and monitor for. This change
in failure mechanism results in the necessity for expensive, and
time consuming, non destructive evaluation (NDE) to monitor
structural health. This requirement for NDE drives up the cost of
composite integration therefore increasing the barrier to
entry.
[0031] If a damage or fatigue failure is located in the structure
the cost of repairing and or replacing the component can be
extremely high as a result of composites strength being derived
from continuous fiber paths. The breaking of these fiber paths due
to a repair can result in a weakening of the structure leading to
reduced service life. Potential problems also arise with composite
repairs made in place. If the structure needs to remain in place
during the repair, the potential exists to require in place heating
and pressure to cure the repair to the composite. Also, during the
repair of the composite time and money are lost as a result of the
structure not being in service.
[0032] One method of achieving this is to design and incorporate
smart, intelligent, and adaptive systems. Such systems will allow
for continuous health and performance monitoring, fast and decisive
information processing to ensure that the system is "highly aware"
of its current health status and unparalleled in its adaptability
to detect and repair damage. Currently, most state-of-the-art
health monitoring technologies can only deliver the indication of
damage to the human operator via visual display. An integrated
system consisting of smart, adaptive, and intelligent components
will enable an advanced mission system to sense and immediately
recover from physical damage on-the-fly while informing the
operator of the situation but not requiring a response. Currently
there is no method of recovering from physical damage due to an
unforeseen event.
[0033] Conventional solutions to the problem of composite failure
mechanisms and health monitoring have focused on integrating
sensors to monitor the health of the composite structure. Examples
of sensors that can be incorporated into a composite for monitoring
purposes include; strain gauges, piezoelectric fibers, and fiber
optics. These techniques provide an adequate level of structural
understanding, however still require a human operator to interpret
the results and make the appropriate decision about structural
capability. This process requires dedicated personnel and results
in time lags between when a structure is found to be sub-par and
when the structure can be repaired.
[0034] Upon determination from a human operator that a composite
structure is no longer operating at 100% capacity it is required
that either a new structure be fabricated to replace the failed
structure or a in place repair is required. Repairs that are made
while the structure is in place requires that the structure be
taken out of use while a composite engineer can determine the
appropriate fix for the structure and during the implementation of
that repair. Currently, composite repairs can be difficult as a
result of elevated temperatures required to cure composites as well
as the necessity for a force to be applied during the composite
cure for adequate fiber compaction. As a result of these
requirements, in place composite repair can be highly costly and
time consuming.
[0035] A better approach to composite repair is modular structural
design that if a failure occurs in the structure, a modular section
can be removed and replaced with a new section that can be
fabricated at a separate location. This approach eliminates the
necessity for costly in place curing equipment as well as the
potential elimination of having an engineer evaluate the structure
since it will be replaced with the exact material that was damaged.
This process still does require the structure to be monitored by a
human operator to determine if a failure has occurred as well as
taking the necessary actions required to complete the structural
repair. This approach also requires additional design time to
ensure that each aspect of the design is modular and has the
ability to be replaced section by section if required.
[0036] The reflexive system disclosed enables real time health
monitoring of in use composite structures as well as the ability to
repair damage thus restoring cold mechanical properties to the
structure. The integration of this system eliminates the need for a
dedicated user, or team of users, to monitor the results of the
integrated health monitoring system as well as offers an in place
repair capability. Having the repair mechanism in place greatly
reduces the amount of time for repair through the elimination of
needing to schedule a repair crew. Time is also saved through the
way that a repair is conducted. Rather than having to either cure a
composite in place or remove a section and mechanically fasten a
new section into place, repairs can be completed simply through the
application of heat to the structure. It is for these reasons that
both time and money are saved through this technology as well as a
minimization of structural down time.
[0037] The idea for self repairing composites utilizing
encapsulated fibers or beads containing polymers and polymerization
agent's are now widespread. However these systems are only as
reliable and useful as long as there is polymer remaining
encapsulated within the composite or polymer matrix. Additionally
once an area has repaired the damage with current systems there is
little if any polymer remaining to repair a second crack or failure
at the same point. Therefore there is a need for a method and
system for repairing a polymer or polymer composite repeatably
without the use of micro-encapsulated resins and polymers, without
the application of additional resin or composite, and without the
interaction of a human operator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 shows the preferred heater panel design.
[0039] FIG. 2 shows the first layer of the preferred heater panel
design.
[0040] FIG. 3 shows the second layer of the preferred heater panel
design
[0041] FIG. 4 shows the third layer of the preferred heater panel
design
[0042] FIG. 5 shows the fourth layer of the preferred heater panel
design
[0043] FIG. 6 shows an electrical schematic of a three heater by
three heater section of the preferred heater panel design.
[0044] FIG. 7 is a perspective drawing showing damage to an
aircraft wing.
[0045] FIG. 8 is perspective drawing showing the sensing system
used to detect damage in an aircraft wing.
[0046] FIG. 9 is a perspective drawing showing the heating system
selectively activating heaters to heal the damage in the aircraft
wing.
[0047] FIG. 10 is a perspective drawing showing the sensing system
detecting that the damage has been healed.
[0048] FIG. 11 is a perspective drawing showing that the damage to
the aircraft wing has been healed.
MODES FOR CARRYING OUT THE INVENTION
[0049] The presently disclosed method and devices utilizing the
method are an advanced reflexive structure technology system which
increase the survivability of systems constructed of lightweight
polymer and polymer composite material. Applications of this device
include a broad selection of high performance systems ranging from
aircraft and spacecraft to habitats for space stations and
interplanetary exploration in addition to commercial applications
such as motor vehicles and building components. The control system
for the reflexive structures mimics the pain withdrawal-reflex on
which the human body relies. This is important because quick
reaction is critical to survivability. This system combines a
damage sensing technology, a control system, a dynamic elastic
modulus resin (DMR) matrix or a composite in a dynamic elastic
modulus resin matrix, and a means for activating the dynamic
elastic modulus resin matrix.
[0050] Dynamic Elastic Modulus Resins and Composites
[0051] One element of the claimed system is an adaptive resin or
composite system. The term "composite" is commonly used in industry
to identify components produced by impregnating a fibrous material
with a thermoplastic or thermosetting resin to form laminates or
layers. Generally, polymers and polymer composites have the
advantages of weight saving, high specific mechanical properties,
and good corrosion resistance which make them indispensable
materials in all areas of manufacturing. The use of other fabrics
such as carbon nano-fibers, spandex, chopped fiber, random fiber
mat, fabric of any material, continuous fiber, fiberglass, or other
type of textile fabric can be used to replace carbon fiber in any
of the cited examples.
[0052] The preferred DMR to use in forming the resin system or
composites used in the presently claimed device is shape memory
polymer (SMP). SMP materials "heal" via two mechanisms. First, SMPs
have memorized shapes, allowing them to return to form upon thermal
activation above T.sub.g. Secondly, SMPs perform a reptation
process, where long polymer chains within a thermoset matrix will
move freely and entangle at temperatures above the T.sub.g of the
thermoset and thermoplastic. When the Tg of the SMPs are above the
Tg not only does the elastic modulus of the SMP decrease
dramatically, but additionally, the increase in temperature
increases the mobility of the long chain polymers within the SMP
across a boundary.
[0053] Because of the properties inherent in shape memory polymers,
composites utilizing shape memory polymer as the resin matrix can
be temporarily softened, reshaped, and rapidly hardened in
real-time to function in a variety of structural configurations.
SMPs can be fabricated with nearly any type of fabric, and creative
reinforcements can result in dramatic shape changes in functional
structures and SMPs have the additional benefit of being highly
machinable.
[0054] SMAs and SMPs that have been pre-formed, with a memory
shape, can be more easily deformed to a desired shape above their
glass transition temperature (Tg). The SMA and SMP must remain
below, or be quenched to below, the Tg while maintained in the
desired shape to "lock" in the deformation. Once the deformation is
locked in, the SMA, because of its crystalline network, and the
SMP, because of its polymer network, cannot return to its
pre-formed, or memory, shape due to thermal barriers. The SMA and
SMP will hold its deformed shape indefinitely until it is heated
above its Tg, whereupon the SMA and SMP stored mechanical strain is
released and the SMA and SMP returns to its pre-formed, or memory,
state.
[0055] Many sources claim that there is an interchangeable nature
between shape memory alloys (SMA) and shape memory polymers,
however, for many applications this is not so. The technical
difference between SMA and SMP at the molecular level is in the
linkages between molecules. SMA essentially has fixed length
linkages that exist at alternating angles establishing in a zigzag
patterned molecular structure. Reshaping is achieved by
straightening the angled connections from alternating angles to
straight forming a cubic like structure. Recovery to memory shape
is achieved by heating the material above a certain temperature at
which point the molecules return to their original zigzag molecular
configuration with significant force thereby reestablishing the
memory shape. Locally, no more that eight percent (8%) strain is
recoverable with the memory effect. The molecular change in SMA is
considered a metallic phase change from Austensite to Martensite
which is defined by the two different molecular structures.
[0056] SMP has connections between molecules with some slack. When
heated these links between connections are easily contorted,
stretched and reoriented due to their elastic nature as the SMP
behaves like an elastic material when heated, when cooled, the
shape is fixed to how it was being held. In the cooled state the
material behaves as a typical rigid polymer that was manufactured
in that shape. Once heated the material again returns to the
elastic state and can be reformed or return to the memory shape
with very low force. Unlike SMA which possesses two different
molecular structures, SMP is either a soft elastomer when heated or
a rigid polymer when cool. Both SMA and SMP can be formulated to
adjust the activation temperature for various applications.
[0057] Typical SMP materials can switch in elastic modulus by at
least three orders of magnitude becoming at least three orders of
magnitude softer than SMA materials. Although SMA materials can be
switched from a slightly softer state to a stiff state, they change
less than an order of magnitude remaining more stiff than SMP in
the stiff state. For example, this is like the difference between
inflating an aluminum soda can versus a common balloon where the
balloon will expand and conform dramatically at low pressure.
Conversely, an aluminum can would bulge and subsequently burst
after minor shape change and higher pressure. Another example to
consider would be the difference between shaping a soft sheet of
rubber versus a typical sheet of metal. SMP materials have been
shown to replicate complex shapes of large and small size.
[0058] SMP materials have been shown to replicate surfaces with
less than 10 nanometer root mean squared surface roughness. This
favorable performance characteristic enables class A finished
surfaces to be achieved without subsequent surface preparation and
painting typically necessary to achieve final surface qualities for
composites. Not only can SMA not approach the level of surface
quality performance of SMP, typical surface qualities necessary for
the composite industry are not achievable with corrugated SMA.
[0059] The cost per pound differences are more than an order of
magnitude greater for metals versus polymers and the difference in
density adds an additional factor of six times the cost, bringing
the cost advantage at the most general comparison level to 10 to 60
times in favor of SMP. This general comparison would be the best
approach in considering each material in the lowest commodity level
at extreme production volumes. At low volumes the cost advantage of
SMP over SMA becomes more significant. Lastly considering lifecycle
of the two materials used to make the same device, SMA devices may
offer more cycles than an SMP device in certain cases however this
advantage would be limited to no more than 2 times greater at best
and only in rare situations.
[0060] If SMA was used instead of SMP for the presently claimed
device the cost of the materials and production would be
significantly higher. Additionally it would require at least
100,000 to 1,000,000 psi of air pressure to assist the SMA to
return quickly to is memory shape and the crack would have to be
repaired by a human operator on the ground. Such sources of such
extremely high air pressure are not easily available and not
feasible in most commercial uses such as cars, aircraft, and homes.
Additionally, if the damage to an SMA results in over eight (8)
percent strain, it will be impossible for the SMA to fully recover
its original, memory, shape.
[0061] However, SMP in the soft state will require little, if any,
air pressure to return the SMP or SMP composite to its memory
shape. This amount of air pressure is readily available in most
manufacturing facilities and can easily be incorporated into a
commercial product. Finally, any attempt to replace SMP with thin
sheets of SMA would yield a non functional product as the stiffness
of the thin sheets of SMA would not provide adequate structural and
mechanical properties for most applications.
[0062] Integrating thermoplastics into existing SMP resins by
either dissolving commercially available thermoplastics in the
resin or by in-situ polymerizing thermoplastic monomers in the
resin during cure is difficult. Thermoplastics do not easily
dissolve in the resin; high temperatures and extended time periods
were required to dissolve relatively low amounts of thermoplastic
(two to five percent (2-5 (1/4) wt). The in-situ polymerization
process prevents time-consuming dissolving step; however, many new
variables arise with this process, including thermoplastic monomer
selection, molar ratio of monomers, total weight loading of
monomers, use and loading of initiator, and cure cycle selection.
Although thermoplastic networks were successfully established in
the SMP matrix, the extent of reaction and repeatability of the
polymerization posed issues requiring substantially more
developmental time than currently described polymers.
[0063] While the styrene based SMPs described in Tong '481 are
preferred, the most preferred resin system is an epoxide monomer
that resembles a long thermoplastic chain but with epoxide
functional groups. All monomers are low viscosity, easily dissolve
in the epoxy resin, and qualitatively enhances the "stickiness" and
"healing" ability of the material. The method of incorporating
mono-, di-, or multi-functional long chain epoxides in the epoxy
SMP matrix qualitatively gives the best healing properties. Many
such epoxides are available commercially to enhance further the
healing properties without sacrificing the mechanical properties of
the material. The Epoxy SMP resins used are disclosed in PCT
application number PCT/US2006/062179 and are commercially available
from CRG Industries in Beavercreek, Ohio, U.S.A. at
http://www.crg-industries.com.
[0064] Those of skill in the art should be able to select the
proper material for the application based on cost, mechanical
properties desired, and type of resin desired. The material
selection for the disclosed device involves the selection of a
resin system, the optimization of its mechanical properties, and
the incorporation of self-healing mechanisms into the resin. Based
on the application requirements and the wide range of resin systems
available, the resin system can be designed to exhibit a T.sub.g
between 0.degree. C. and 280.degree. C. The mechanical properties
will meet or exceed those of conventional resins of the same
chemistry. The elongation and recovery of the material above
T.sub.g can be designed to be between 0% and 100%, allowing most
minor and major deformations to return to original form.
[0065] The healing time is important to fully understand and
optimize the healing cycle. In optimizing the healing times the
system will only heat the damaged sections for the minimum required
time. In removing excess heating time the system will require less
power, therefore reducing the power draw. In addition to limiting
the power draw on the system this approach also minimizes the
structural change. Current reptation models show the time to heal
at a boundary to be five hundredths of a second. The full healing
time, which includes the time for the SMP or SMP composite to
return to its memory shape, is on average 3 minutes to heal fully.
The time difference is believed to stem from the time needed to
heat the composite, and to cheat low heat transfer coefficients of
composites.
[0066] Those of skill in the art should be able to find the time to
heat and repair a given system. First the thermal conductivity,
specific heat capacity and mass densities must be found in order to
correctly model the time to heat the system. An infrared camera can
be used to directly measure the composite. This method will utilize
the Kapton.RTM. heaters to heat the bottom of the composite system
while recording temperatures on the top side with the IR camera; in
addition to finding the time to repair the system the IR camera
will also check how evenly the composite is heated.
[0067] The currently claimed device can use either a thermoplastic
or thermoset DMR, DMR composite, SMP or SMP composite in the
process. The most preferred type of resin is a thermoset SMP which
provides an order of magnitude higher stiffness than previous
state-of-the-art thermoplastic SMPs. This added stiffness coupled
with high strain capability enables the development and use of a
highly useful material for use in the claimed process. Such
polymers are described in U.S. Pat. No. 6,759,481 issued to Tong,
on Jul. 6, 2004 which is incorporated herein by reference. Other
thermoset resins are seen in PCT Application No. PCT/US2006/062179,
filed by Tong, et al on Dec. 15, 2006; and PCT Application No.
PCT/US2005/015685 filed by Tong et al, on May 5, 2005 of which both
applications are incorporated herein by reference. All of these
polymers are commercially available from CRG Industries, Inc. in
Beavercreek, Ohio, U.S.A. at http://www.crg-industries.com.
[0068] Damage Detecting Sensors
[0069] Another element of the claimed system is the sensing system.
Any method or means of sensing damage that can be incorporated into
a product's structure will work. Most commercially available damage
sensing systems use embedded piezoelectric sensors and is the
preferred embodiment. One such system is made by Acellent
Technology, Inc. based in Sunnyvale, Calif. Acellent Technology's
sensor are the most preferred sensors to integrate into a DMR or
DMR composite that can correctly identify damage and alert a
controlling system of the location, amount, and type of damage
present. Those of skill in the art will be able to customize a
sensor system for a desired system. While placement of the damage
detecting sensors can either be internal or external to the DMR or
DMR composite, the preferred location is internal as discussed
below.
[0070] Activation Elements
[0071] Another element needed is the means to activate the DMR. As
used throughout this application the term "activate" means to
enable the DMR to switch from a high elastic modulus to a low
elastic modulus. As used throughout this application the term
"deactivate" means to enable the DMR to switch from a low elastic
modulus to a high elastic modulus.
[0072] The means for activating and deactivating the dynamic
elastic modulus resin (DMR) can be thermal, light, water,
electromagnetic radiation, and other means which will induce the
dynamic elastic modulus resin matrix to change its elastic modulus
from a hard state to a soft state and reverse that state upon
application of the opposite stimulus. For thermally activated DMRs
the stimulus can be the application and removal of heat. For
electromagnetic radiation activated DMRs the stimulus can be
application of one wavelength and energy of light and then the
application of a second wavelength and energy of light. The Tong
patents previously noted can create DMRs that can heal in a few
minutes or less.
[0073] There are multiple means of activating and deactivating a
DMR including, but not limited to, thermal energy, light, other
electromagnetic wave types, magnetism, water, exposure to certain
chemicals and substances, and other means which are known in the
art. The most preferred method of activating and deactivating a DMR
is thermal energy through the application and removal of a heat
source, most preferably through resistive heating elements embedded
in the product's structure.
[0074] Composite panels which integrate the activation means and
the DMR or DMR composite composing of discrete heating elements
through the incorporation of a foil etched Kapton.RTM. encapsulated
resistive heating element is the most preferred method of applying
and removing heat. The discrete heater should be integrated at the
top ply of the DMR or DMR composite and processed during the DMR or
DMR composite fabrication. To protect the thermoplastic coated
diodes and solder pads, a layer of Kapton.RTM. film is added over
the top of the heating element prior to processing. Encompassing
the diodes into the package allows the discrete heater to be placed
mid-ply in the composite and minimize heat energy lost to the
ambient, a concern that will only amplify when dealing with forced
convection during flight or other motion.
[0075] The added layer of Kapton.RTM. film is sealed to the
resistive heater using tape around the perimeter. The excess tape
is trimmed off leaving the bottom surface unaltered. The enclosed
heating element is then placed atop the composite ply schedule of
pre-preg carbon fiber and healable DMR matrix, and more preferred
SMP matrix, is pressed down using a squeegee. The composite is then
vacuum bagged and cured. Once the composite is cured the vacuum bag
is removed and the tape can be removed from the localized heater
exposing the solder points and diodes. As a result of the heater
and composite being co-cured there is good adhesion between the
Kapton.RTM. film and the composite panel.
[0076] The design of the preferred heater pad, 4, is shown in FIG.
1. The heater sections, 2, are evenly spread throughout the pad
with the electrical bus bars, 6 and 8, providing the necessary
paths for the flow of electricity to the individual heater
sections, 2.
[0077] The layers of the heater sections are shown in FIGS. 2-5.
The first layer, shown in FIG. 2, is a Kapton.RTM. layer, 10, with
holes, 14 and 12, in it for electrical connections to the other
layers. The leads of a diode, to prevent the two-way flow of
electricity through the heaters, is passed through the holes, 14
and 12, on the first layer, 10, and holes, 30 and 38, of the third
layer, 32, as shown in FIG. 4, and finally connected to the leads
of the heater, 18 and 16, on the fourth layer, 22, shown in FIG. 5.
FIG. 3 shows the bus bars, 38, used to allow current flow. These
bus bars are sandwiched between the first and third layers so that
the bus bar, 38, in FIG. 3 aligns with the hole, 26, in the third
layer, 32, and the connector, 20, in the fourth layer, 22. FIG. 4
is an electrically insulating Kapton.RTM. layer, 32, to prevent the
bus bars, 38, in FIG. 2, from contacting other bus bars, in FIG. 5.
The holes, 30 and 38, are lined up with holes, 14 and 12, on the
first layer so as to allow electrical connections between the
Kapton.RTM. layers.
[0078] The preferred design of the heating panel is an array of 64,
1 inch square resistive heating elements spaced at approximately
1.5 inch on center. The package is a 5 layer polyimide package to
insulate the heating elements from the composite, insulate the bus
bars from each other, and to protect the top surface from
environmental degradation. Each resistive heating element is
designed for 8 ohms resistance, which will provide a power density
of 2 watts/in 2 when supplied with 0.5 A at 4 volts. Using the
above power, localized heating of at least 115.degree. C. can be
achieved, while maintaining the temperature of surrounding elements
at approximately the current environmental temperature.
[0079] The preferred design uses bus bars capable of higher current
draw without heating, resulting in thinner traces. This allows a
user to place the heaters closer together, allowing a more finely
tuned activation area when repairing damage. With the integration
of the wires into the Kapton.RTM. package, the wires are run to a
central location similar to the sensing layer. The Kapton.RTM.
package will have receptors for solder points to attach surface
mounted molex connectors that will interface directly with the
intelligent control system.
[0080] Localized heating is achieved by selectively activating the
heating elements. Temperatures ranging between 115.degree. C. and
130.degree. C. can easily be achieved and higher temperatures are
possible with proper design. By increasing the power to the
resistive heating element, temperatures above 130.degree. C. can be
achieved; additionally the use of variously heating elements and
resin can also affect the temperatures. Those of skill in the art
will be able to determine what temperatures must be obtained by the
heating element to heat the entire composite panel to activation
temperature. Additionally those of skill in the art will be able to
determine the optimum placement in the composite for both uniform
thermal distribution and composite mechanical properties.
[0081] Those of skill in the art can also design a system that
reduces the parasitic weight of the discrete activation layer in
the DMR or DMR composite system. The referred way to reduce this
parasitic weight is to design heating elements with leads
integrated into a Kapton package. These leads were designed as
copper traces that were chemically etched away on a Kapton
substrate. Those of skill in the art will be able to design a
heating system which satisfies the needs of the system without
causing damage through improper heater spacing and thermal
gradients.
[0082] Incorporating the current design of the discrete heater into
the mid-ply of the composite would result in shorting of the
circuit due to the conductivity of the carbon fiber and the diodes
being surface mounted on exposed solder pads. Also, due to the
thickness of the diodes, reduced mechanical performance is also a
possibility as a result of continuous fibers deflecting
out-of-plane. While thin film transistor (TFT) networks to
distribute electric current to discrete points to resistively heat
the composite could be used, the technology is still new and bulky
that lower profile diodes and diode alternatives are preferred.
[0083] Integration of Activation Element, Sensing Element, and DMR
Element
[0084] The preferred panels of the reflexive structure were
fabricated using 3 k plain weave carbon fiber and styrene based
healable SMP resin system as described in U.S. Pat. No. 6,759,481
issued to Tong by integrating Acellent's SMART Layer.RTM. sensing
system at the mid-ply and the Kapton.RTM. discrete heating element
at the bottom or back ply of the panels, leaving the side of the
panel which will be exposed to the surrounding environment to be
the resin or composite. Incorporating the non-porous resistive
heating element into the composite ply schedule required the
development of a process to pre-preg styrene based SMP matrix
composites. The developed process used a vacuum assisted resin
transfer molding (VARTM) process to infuse carbon fiber fabric with
styrene SMP in a closed mold. The parts were cured for 1 hour and
45 minutes in an oven at 75.degree. C. to gel cure the resin. The
gel cured parts were then cut to shape and used in the composite
ply schedule as standard pre-preg material. The parts were then
vacuum bagged and fully cured in a closed mold. Precautions were
taken to protect exposed electrical connections in both the heater
and sensor layer by coating exposed connections with chemically
resistant tape.
[0085] Once the parts were fully cured they were demolded from the
vacuum bag and glass and the tape was removed exposing the
electrical leads. 22-guage wire leads were soldered to the discrete
heater on all 32 solder pads. Flat ribbon cable was soldered to the
Acellent SMART Layer.RTM. with a male connector at one end
interfacing with the Acellent hardware.
[0086] The ability of the sensing layer and Acellent health
monitoring system to identify location and magnitude of structural
change requires a determination of the optimal wave form to
identify structural damage. The system offers a variety of
potential wave forms for characterization including waves with 3,
5, and 10 peaks as well as bursts and chirps. The system also
offers the capability for user defined wave forms. Resolution
analysis on repeatable structural changes should be run to
determine which combination of wave forms has the highest
resolution of structural change.
[0087] Active scanning systems from Acellent can be used to drive a
3.times.3 sensor grid array that is integrated into the reflexive
system panels. This Acellent active imaging system collects data
from the sensor grid, and produces a 1/4 inch spatial resolution
data file that represents the mechanical fatigue of the respective
11-inch square inspection area.
[0088] The sensing layer is integrated at the mid-ply of the
composite ply schedule to optimize sensor resolution. The sensor is
integrated into the composite during the fabrication process to
minimize the potential for void introduction and optimize bonding.
The integration process consists of assembling the bottom plies of
the composite using the carbon/Stryene SMP pre-preg then placing
the sensor at the mid ply and assembling the remaining layers of
the composite ply schedule on top of the sensor. Pressure is then
applied between each layer with a hand squeegee to ensure good
adhesion between plies. The composite panel was then vacuum bagged
and cured completely.
[0089] The appropriate set of signal parameters allowing for the
generation of accurate data will only be accurate for the preferred
embodiment of a specific composite ply schedule of the composite
panels fabricated with the sensing layer at the mid-ply; however, a
process capable of reducing the amount of time needed to identify
optimal parameters is disclosed. By creating a baseline data set
and comparing that baseline to a future data set, a control system
can automatically determine where the damage has occurred, how big
the damage is, and the proper sequence to repair the damage.
[0090] This process requires an initial effort to run scans at a
variety of frequencies, gains, and signal types at each of the
respective angles the signal could travel. In the current sensor
layer, the piezoelectric sensors are equally spaced in the X and Y
directions limiting the angles the waves have to travel to
.+-.ninety degrees)(.+-.90.degree. and .+-.forty-five
degrees)(.+-.90.degree.. The scans are run on un-damaged panels to
allow for the user to analyze the data and identify trends as to
which set of parameters yields the highest sensitivity. The
sensitivity of the system can be identified through an analysis of
the sensed signal strength in an undamaged panel. Sets of
parameters yielding higher sensitivity will have higher received
signal strength in undamaged panels, while sets of parameters with
lower sensitivity will have lower received signal strength in
undamaged panels. The selection of sensitivity is directly related
to the type of and extent of damage being identified by the
system.
[0091] The preferred panels are capable of identifying and healing
damage of a least ninety-five percent (95%) of original form and
can be designed to heal one-hundred percent (100%) of original
form. The preferred panels fabricated are 12''.times.12'' squares
with integrated piezoelectric sensing layers at the mid-ply and
Kapton.RTM. encapsulated foil etched heating elements capable of
discrete heating co-cured at the top ply. The preferred DMR is a
styrene healable SMP resin system due to the maturity of the system
as well as its ability to consistently heal over ninety percent
(90%) of damage.
[0092] Using this process those of skill in the art will be able to
generate the proper set of scanning parameters for subsequent
panels quickly. This is important for the commercial fabrication of
the panels. The obvious design goal of this process is to develop a
set of panels with optimized performance of heater design and
minimize the parasitic weight of the panels through the reduction
of weight associated with non-structural components in other
healing systems.
[0093] The preferred design has minimized the thermal gradient and
thermal uniformity through the reduction of bus bar width. The bus
bars are 1/8 inch copper bus bars connected to the 1 inch center
heaters. The most preferred design incorporates foil etched leads
that homerun to a central location and are connected with surface
mount connecters. This modification reduces the weight penalty of
system integration through the elimination of 22-guage wire
conductors. To further add robustness to the design of the
integrated heater a conformal surface coating is applied to the
diodes to both protect the diode as well as provide a layer of
electrical insulation.
[0094] Based on the current reflexive system design the heater is
integrated at the top ply of the structure for multiple reasons
including ease of diode repair as well as a result of the lack of
electrical insulation in carbon fiber. One large trade-off of
integrating the heater at the top ply is the heat losses to the
surrounding ambient environment associated with the placement. A
much more efficient design is to integrate the heater at the
mid-ply of the composite to maximize the generated heat into the
composite structure.
[0095] To integrate the heater to the control system, Molex pin
connecter and 22 gage wire bundles are used. Following the damage
prioritization and selected healing cycle the control system
applies a voltage drop across the selected leads to generate heat.
This approach is the most streamlined approach, however due to the
design of the control system this approach only allows for the
application of 5V to each heater. To increase the range of
performance of the system, an interface running between the control
system and the discrete heater can be fabricated. The interface is
comprised of mechanical relays and a DC power supply that will
allow both a higher and lower voltage drop to introduce more or
less heat based on both the resin formulation and ambient
conditions. This interface will also allow a control system or
human operator to determine what power densities are required to
heal composites of different thicknesses and ply schedules.
[0096] An electrical schematic of the selective nature of the
heaters is shown in FIG. 6. The array of horizontal lines, D, E,
and F and vertical lines, A, B, and C in FIG. 6 represent bus bars
with the ability to supply power to locally heat the small
resistive heating elements once the bus bars are connected to a
power source. The bus bars can be connected to a power source
manually or through switches that turn the power on or off. The
heating elements are represented by the commonly accepted resister
symbol. Additionally, diodes are placed in-line with the resistive
heating elements and only allow the current to flow in one
direction through the circuit. The vertical and horizontal bus bars
are electrically insulated from each other where they meet. This
electrical isolation between the horizontal and vertical bus bars
allows current to only flow one way through the system of parallel
circuits resulting in localized activation without bleed-off to
surrounding heaters. For example, as shown in FIG. 6, if power is
connected to Bus Bars D and B, then current can only flow through
one heating element, 40, because of the in-line diode, 42.
Additionally, it will be apparent to those of skill in the art that
the amount of heat generated is controlled by the amount of power
applied to the elements.
[0097] The preferred embodiment consists of multiple components,
including an integrated piezoelectric sensor, a computer and
algorithm capable of analyzing the change in wave propagation to
identify damage, a polymer matrix composite with modified SMP resin
capable of healing with the application of heat, and a DC power
supply interface and mechanical relay instrument that allows to
vary the voltage drop across the resistive heating elements. This
system has demonstrated the ability to successfully identify
damage, heal the damage and verify a restoration of mechanical
properties.
[0098] The delivered control system has the ability to import data
generated from the Acellent structural health monitoring (SHM)
system, analyze and prioritize the damage based on user defined
criteria, determine the appropriate healing cycle, and apply a
voltage drop across the appropriate heaters all while keeping the
user informed of its status on a LCD monitor. The control system is
comprised of a custom printed circuit board, DC power supply and
associated capacitors, programmable microprocessor, custom
enclosure, and cooling fan.
[0099] As part of the functionality of the Acellent (SHM) system, a
variety of tailorable functions exist that dictate how well the
system monitors the health of the structure. These functionalities
are built into the system to allow for its application to multiple
structures comprised of varying shapes, sizes, and materials. The
functionality of the system allows the user to define such criteria
as wave form, frequency, gain, sampling points, and other
functionalities.
[0100] The first step in integration of the sensor system with the
composite structure is to tune in the SHM functionality to identify
varying types of damage. If the system is not tuned into the
structure properly, the SHM system will not identify the damage
with the proper magnitude and location. Due to the unlimited number
of potential combinations of variables, a matrix design approach
varying only one criterion at a time while keeping the others
constant is preferred. After each run it should be noted how well
each combination of variables identifies the damage location and
what the magnitude of damage was. The results of this effort will
yield a more complete understanding of the SHM system and how each
variable affects the accuracy of the scans. One critical piece of
information that is gained during this effort is the recognition
that when dealing with integrated SHM of an an-isotropic composite
structure the wave forms propagate differently when going in the
direction of the fibers, such as at zero (0) or ninety (90)
degrees, or when going at an angle to the fibers of forty-five
degrees) (.+-.45.degree.. The completion of this investigation
results in the identification of a combination of variables that
allow the SHM to use the same parameters between reflexive
composite structures fabricated using the same ply schedule and
processing methods. Thus for every type of ply schedule and
processing method, these tests should only be needed to be run
once.
[0101] A critical component to the successful integration of each
subcomponent into a functional reflexive system is the ability for
the control system to read, interpret and interoperate the output
data from the SHM system. To accomplish this task, an added
functionality of the SHM system to output an ASCII file of the raw
data comprised of location and magnitude information was devised by
Acellent Technologies. The ASCII files are named by date and a
post-script number that identifies which is the most recent data
file when more than one data file is created per day. The
intelligent structural control system then imports the raw data and
looks at the data based on user defined criteria that includes
damage magnitude, damage location, physical size of damage, and
other parameters that were deemed relevant. Based on these
criteria, the control system assigns a damage value to each damage
location for prioritization. The location with the highest damage
prioritization is healed first.
[0102] Control and Operation
[0103] In order to optimize the control system, it is necessary to
determine an appropriate algorithm to prioritize the damage
locations, if multiple impacts were to occur, as well as develop a
method to address damaged areas in the event that the damaged area
required more power to heal than is provided by the system. By
evaluating what would be most important to increasing the survival
of a product and then using a priority ranking system to weigh each
aspect an algorithm and system can be easily developed by those of
skill in the art. Aspects to consider in creating the algorithm and
system include, size, magnitude, location, and magnitude of
surrounding rankings. Using the algorithm, each damage area is
assigned a numerical value for direct comparison with other damage
sites. The control system then finds the highest number and begins
by healing that area first. Those of skill in the art should be
able to design an algorithm to prioritize the damage locations, if
multiple impacts were to occur, as well as develop a method to
address damaged areas in the event that the damaged area required
more power to heal than is provided by the system
[0104] The process is shown in FIGS. 7-11. In FIG. 7, a damaged
area, 50, is shown. The causes of the damage, or the products on
which the damage occur, are irrelevant, however, for the purposes
of this description the damage has occurred on the wing of an
aircraft. In FIG. 8, the sensors, 54, of the sensing system, have
detected damage in an area, 52, of the wing. The system will next
determine if the damage can be healed in one step or requires
multiple, sweep, healing. Upon identification of the damage
location with the highest prioritization, the control system will
then look at how many heaters are required to heal the damage at
that location. This step is completed by examining the physical
size of the damage and the magnitude of the surrounding areas of
the damage. To mimic an aircraft environment and to minimize the
stiffness change in the composite structure, limited the number of
heaters that can be activated at any given time to six (6), which
corresponds to a six (6) inch square physical area. If a damage
area requires more than six (6) heaters to be activated to heal the
damage the system beings a sweep healing process that activates six
(6) heaters for healing then activates six (6) adjacent heaters and
so forth until the entire damage area is healed. The system also
has a top end limit of heaters that the system will activate at any
given time before it recommends a system abort. If the damage
magnitude is too large or extreme for the reflexive system to
repair, the system will recommend an ejection or replacement of the
part.
[0105] After the scan of the damage area is complete, a file is
generated, downloaded and processed in order to determine the
location(s) requiring healing. The algorithm used for this
processing includes two modes. Mode one operation includes default
values for all parameters and thresholds. Mode two operation
prompts the user to select the parameters used for processing. The
results of this algorithm are passed to the "healing algorithm"
which selectively activates a 10 heater.times.10 heater power grid
of 12''.times.12'' in physical size in order to heat and thus heal
target areas as shown in FIG. 9. In. FIG. 9 the heaters, 56, are
selectively activated by the control system so that the damaged
area, 58, is raised above its Tg and healed.
[0106] While automatic control systems to determine the amount of
damage and the proper sequence of healing and activation are
preferred, the system can be designed so that a human user can
determine and activate the proper sequence. Alternatively the
system can be designed for control by an automatic system or user
input depending on the damage and available time to repair the
product.
[0107] Once the "Active Healing" cycle is complete, the user is
prompted to initiate another inspection of the product using the
Acellent system as shown in FIG. 10 after the temperature of the
panel has been lowered below the Tg. In FIG. 10, the sensors, 60,
determine that the area, 62, has been healed. Then the above
procedure is repeated until the healing algorithm determines that
no additional healing is needed as seen in FIG. 11. During this
entire process, the user is constantly updated with the status of
the healing via a graphical LCD display panel. A portable yet
powerful Rabbit Semiconductor microcontroller is the preferred
heart of the control system, and it has been used to demonstrate
the capability of monitoring the files and downloading the files as
needed for processing. The isolated power drive circuits have been
designed and tested as well. These include GMR (giant
magnet-resistive) sensors to serve as a low cost, and miniature
means for current measurement of all power lines feeding the
healing grid. The traditional means for current measurement include
bulky and expensive coil packages. This GMR current information
will be useful in identifying open power lines due to severe impact
damage. In the event of an open power line, power will be rerouted
to the surrounding closed lines for healing. This feature would be
used in the event of puncture damage or severe impact damage.
INDUSTRIAL APPLICABILITY
[0108] The claimed method and devices can be used to create a
self-healing system for aircraft, automobiles, buildings, other
structures, and any product that uses resins or composites.
* * * * *
References