Composites Science and Technology 65 (2005) 1791–1799
`
`COMPOSITES
`SCIENCE AND
`TECHNOLOGY
`
`www.elsevier.com/locate/compscitech
`
`A hollow fibre reinforced polymer composite encompassing
`self-healing and enhanced damage visibility
`
`Jody W.C. Pang, Ian P. Bond *
`
`University of Bristol, Department of Aerospace Engineering, QueenÕs Building, University Walk, Bristol BS8 1TR, UK
`
`Received 15 July 2004; received in revised form 8 March 2005; accepted 11 March 2005
`Available online 19 April 2005
`
`Abstract
`
`The aim of this study was to develop a novel fibre reinforced plastic which employed a biomimetic approach to undertake self-
`repair and visual enhancement of impact damage by a bleeding action from filled hollow fibres. The results of flexural testing have
`shown that for the lay-up investigated, a significant fraction of flexural strength lost after impact damage can be restored by the self-
`repairing effect of a healing resin stored within hollow fibres.
`The release and infiltration of an UV fluorescent dye from fractured hollow fibres into damage sites within the internal structure
`of the composite has been successfully demonstrated. It has been correlated with respect to the ultrasonic C-scan NDT/NDE tech-
`nique and shown to be an effective method of quickly and easily highlighting damage at the surface that requires further investiga-
`tion. This could be of particular benefit where rapid visual inspection of large surface areas (e.g., wing skins) is required.
`Ó 2005 Elsevier Ltd. All rights reserved.
`
`Keywords: A. Polymer–matrix composites; A. Smart materials; A. Fibres; C. Damage tolerance; Self-repair
`
`1. Introduction
`
`The field of fibre reinforced composite materials has
`grown rapidly since their introduction such that over
`20 million tons are now produced every year for a vari-
`ety of aerospace and other applications. However, con-
`cerns remain about the structural integrity of composite
`materials following impact loading, as such materials
`are susceptible to cracks or delaminations that form
`deep within the structure. These cracks are extremely
`difficult to detect and repair by conventional methods
`is often impossible. In addition to compromising the
`materialÕs structural properties, these cracks also pro-
`vide sites for activities such as moisture swelling which
`further degrade material performance [1]. Low velocity
`
`* Corresponding author. Tel.: +44 117 928 8662; fax: +44 117 927
`2771.
`E-mail address: I.P.Bond@bristol.ac.uk (I.P. Bond).
`
`0266-3538/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
`doi:10.1016/j.compscitech.2005.03.008
`
`impact damage can cause a substantial reduction in
`the undamaged structural strength of polymer–matrix
`composites. Such damage may be caused by dropped
`tools, ground handling equipment and hailstones. If this
`damage occurs on a macroscopic level it may be easily
`detected and repaired, but microscopic damage such as
`matrix micro-cracking, fibre–matrix debonding and
`delamination is more insidious and may go unnoticed
`and unrepaired [2] giving rise to barely visible impact
`damage (BVID). One of the key factors limiting current
`design allowables is the strain at which there will be no
`growth of BVID. Self-repairing composites offer the po-
`tential for a substantial improvement in resistance to
`delamination propagation, allowing the outstanding
`properties of fibre reinforced plastics to be more fully
`exploited.
`The concept of self-repair is that a damaged structure
`is repaired by materials already contained within it,
`analogous to the biological healing process in living
`organisms. The key is that no external action is required,
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`unlike conventional repair. The technology must sense
`and respond to damage, restoring the materialÕs perfor-
`mance without affecting the overall properties of the sys-
`tem. This would make the material safer, more reliable,
`longer lasting, and require less maintenance and thus re-
`duce costs.
`The use of functional components stored inside com-
`posite materials to restore physical properties after dam-
`age has been advocated by several workers. Dry [3–6]
`adapted the concept of a biological self-healing ap-
`proach, i.e., bleeding, for use in a concrete. This idea in-
`volved the storage of repair components inside vessels
`distributed within a concrete specimen which after sus-
`taining damage will release a repair medium. Methyl
`methacrylate liquid was used inside hollow porous poly-
`propylene fibres within the concrete, and released from
`the fibres to reduce concrete permeability. A further
`investigation was undertaken into the release of crack-
`adhering adhesive from hollow glass pipettes into the
`concrete after flexural testing. The adhesive loaded sam-
`ple demonstrated an ability to carry 20% more load
`under a subsequent flexural test. Li et al. [7] developed
`the self-repairing concept and applied it once again to
`cementitious composites. ÔSuperglueÕ (ethyl cyanoacry-
`late) was used as the healing agent within 500 lm diam-
`eter hollow glass tubes. The Ôcapillary effectÕ was first
`introduced as a method of filling a hollow glass fibre
`with healing agent.
`The application of a self-repairing concept to fibre
`reinforced polymer composite materials has been dis-
`cussed and demonstrated as feasible by several workers.
`Healing agent storage methods have been developed
`based on the use of hollow tubes and fibres, particles
`and microcapsules, all of which can provide an integral
`healing agent storage capacity. Dry et al. [8–10] has
`investigated damage-associated matrix microcracking.
`A single repair fibre was embedded in a polymer–matrix
`and tests performed to visually verify the release of re-
`pair agent. Motuku et al. [2] developed the concept by
`considering different critical parameters, such as method
`of storage (glass, copper and aluminium tubing), and
`healing agents (vinyl ester 411-C50 and EPON-862
`epoxy). The suitability of glass tubing in allowing the re-
`lease of healing agent into the matrix after fibre break-
`age was proven. In the work by Dry and McMillan
`[10] and Motuku et al. [2], dye release accompanied
`the healing agent, however, this combination resulted
`in an inability to cure and thus no improvement in
`mechanical properties was reported.
`Bleay et al. [11] conducted studies with a composite
`material self-repairing system. Hollow glass fibre com-
`posites were filled with an X-ray opaque dye penetrant
`and one and two-part curing resin systems. These were
`then assessed for an ability to perform self-repair and
`enhance damage detection. A vacuum assisted capillary
`action filling technique was developed and used to suc-
`
`cessfully fill the hollow fibres. A variety of treatments
`were used to draw the resin out of the fibres after im-
`pact. The most effective was shown to be the simulta-
`neous application of heat and vacuum. The damage
`detection method was improved by using a X-ray opa-
`que dye. It not only showed the damaged area, but also
`the ingress of dye penetrant into the damage zone after
`impact testing. The post-repair compression strength
`after impact
`testing showed about a 10% strength
`improvement.
`introduced a method of
`Zako and Takano [12]
`impregnating small particles (50 lm) of thermoplastic
`adhesive in a glass/epoxy composite laminate. The cure
`the epoxy matrix was 110 °C. The
`temperature of
`embedded thermoplastic particles melted when damaged
`composites were subsequently heated to 120 °C for
`10 min on a hot plate. In subsequent three point bend
`testing, the load–displacement curve indicated that stiff-
`ness was recovered in the repaired specimen.
`White et al. [13], Kessler et al. [14–16], Brown et al.
`[17–19] have all taken a different approach by embed-
`ding microcapsules of monomer healing agent through-
`out a polymer–matrix. These microcapsules fracture and
`release healing agent upon damage. The healing agent
`(DCPD – dicyclopentadiene monomer) moves through
`the matrix and contacts an embedded particulate cata-
`lyst (GrubbsÕ catalyst), initiating ÔRing Opening Metath-
`esis PolymerisationÕ [20] and healing the damage. The
`unique feature of this healing concept is that it uses a
`live catalyst, thus enabling multiple healing events. Early
`efforts [13–15,17] of injecting catalysed monomer manu-
`ally into damaged plain weave DCB specimens saw heal-
`ing efficiencies of up to 67% relative to the virgin
`fracture toughness. This was reduced to 19% when the
`particulate catalyst was directly embedded into the ma-
`trix. More recently, Kessler et al. [16] and Brown et al.
`[17–19] have also carried out an investigation into the ef-
`fect of size and concentration of the catalyst and micro-
`capsules on fracture toughness and also optimised the
`microcapsule surface morphology, rupture and healing
`agent release behaviour. Results found that the DCPD
`healing agent worked very well at a temperature of
`80 °C [16] giving a maximum healing efficiency of
`>70% [19].
`However, all approaches pose some problems for
`developing a self-healing composite. Kessler and White
`[14] report that the virgin toughness of a specimen de-
`creases slightly when an unintended catalyst cluster is
`found in the matrix. These clusters also contribute to
`unstable crack propagation. Also, in Zako et al. [12] re-
`search showed that the voids left after embedded ther-
`moplastic particles were melted and filled a damaged
`area had an erratic effect on the integrity of the material.
`Hollow glass fibres were seen to provide a good combi-
`nation of storage function and mechanical reinforce-
`ment by Bleay et al. [11]. They avoided some of the
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`detrimental effects on mechanical performance of the
`host composite associated with particles or microcap-
`sules and acted as both a reinforcement and healing
`agent reservoir. However, their self-healing effectiveness
`was shown to be limited by the amount of healing resin
`that could be stored internally.
`The approach taken in this study, here, requires the
`deployment of specially developed hollow fibre rein-
`forcement with large internal volume to maximise the
`storage capacity [21–24]. Hollow glass fibre is an ideal
`medium for storing healing components as it can simul-
`taneously act as structural reinforcement and potentially
`offers many other benefits to composite materials [25–
`27].
`During a damage event some of these hollow fibres
`will fracture, thus, initiating two processes. Firstly, the
`enhanced visualisation of the damage site by seepage
`of a highly conspicuous medium (e.g., UV fluorescent
`dye), thus, aiding the practical
`inspection for BVID
`and identifying areas for permanent repair. Secondly,
`the recovery of properties by ÔhealingÕ whereby a repair
`agent passes from within any broken hollow fibres to
`infiltrate the damage zone and acts to ameliorate its ef-
`fect on mechanical properties. This repair process will
`act to reduce the critical effects of matrix cracking and
`delamination between plies and, most importantly, pre-
`vent further damage propagation.
`It is worth noting that in conventional fibre rein-
`forced plastics, the role of a fibre is to add strength
`and stiffness to the polymer–matrix. The introduction
`of fibre multi-functionality to provide addition roles is
`an attractive but currently unavailable option [28] and
`heralds the move towards ÔsmarterÕ materials.
`
`2. Experimental approach
`
`The use of hollow fibres to contain a repair medium
`has proved difficult to implement to date, largely due
`to the unavailability of high quality structural hollow fi-
`bres [11]. The in-house manufacture and application of
`such fibres, eliminates the need to incorporate supple-
`mentary vessels which compromise composite structural
`performance, disrupt fibre regularity, act as discontinu-
`ities and reduce useful fibre volume fraction. Various
`methodologies for imparting self-repair are possible,
`including one-part resins, two part resins in alternating
`plies, and resin in hollow fibres with the associated hard-
`ener in microcapsules dispersed within the matrix.
`The aim of this study was to employ a biomimetic ap-
`proach and fabricate a composite with a ÔbleedingÕ abil-
`ity. The material used in this
`study comprised
`unidirectional hollow glass fibres (60 lm external diam-
`eter, 50% hollow fraction) in an epoxy matrix in combi-
`nation with conventional E-glass/epoxy. A 0°/90° lay-up
`ensured that uncured resin or hardener (mixed with UV
`
`fluorescent dye) could be infiltrated into the fibre lumens
`without combination. Uncured epoxy resin resided
`within the 0° layers and hardener within the 90° layers.
`A series of test specimens were produced both with and
`without resin/dye infiltrated hollow fibre plies. A repre-
`sentative impact damage site was formed in the centre of
`each specimen and healing was allowed to take place un-
`der various ÔhealingÕ regimes to determine the efficacy of
`repair prior to four point bend testing.
`
`2.1. Specimen preparation
`
`Borosilicate glass tubing [Schott DURANÒ] is drawn
`down into 60 lm external diameter, 50% hollow fraction
`fibre using the bespoke fibre making facility at Bristol
`University. This was created through previous collabo-
`rative work with DERA Farnborough (now QinetiQ)
`and BAE Systems. It has the capability to draw preci-
`sion solid, hollow and novel shaped glass fibres down
`to 10 lm diameter with >50% hollowness, and process-
`ing glasses up to 1500 °C. With careful choice and con-
`trol of preform dimensions, preform feed rate, fibre
`draw rate and furnace temperature, a highly consistent
`and concentric hollow fibre is produced with ±1 lm
`accuracy [21–24], Fig. 1. Hollow fibres of smaller diam-
`eter (down to 30 lm) can be drawn but were found to be
`inconsistent, in terms of their retained hollowness, i.e.,
`<25% hollow, and impractical to manufacture in the
`quantities needed.
`A resin film infusion process is used to produce hol-
`low glass fibre/epoxy preimpregnated tape (prepreg).
`Hexcel 913 epoxy resin is used as the matrix material.
`The prepreg contained a nominal gross fibre volume
`fraction (Vf) of 61.5%. Six laminates of 18 plies (nom-
`inal thickness 2 mm) were manufactured using a hand
`lay-up process and cured according to manufacturers
`recommendations. The lay-up chosen was {[90°/0°](solid),
`[90°/0°/90°/0°](hollow), [90°/0°/90°](solid)}S to position the
`hollow plies in the sub-surface of the laminate and pro-
`vide a fully symmetric arrangement and avoid any detri-
`mental residual stresses. A [90°/0°] lay up for the hollow
`glass plies ensures that uncured epoxy resin (plus fluo-
`rescent dye) and hardener can be infiltrated into the 0°
`and 90° plies, respectively. The solid plies were commer-
`cially supplied E-glass/913 epoxy resin. These E-glass fi-
`bres are typically 12 lm in diameter and possess a
`similar stiffness to the hollow fibres.
`The six panels were cut into 80 mm (length) · 25 mm
`(width) · 2 mm (depth) specimens (Fig. 2) using a dia-
`mond saw, then a water filled ultrasonic bath was used
`to remove any cutting debris from inside the hollow fi-
`bre lumens. Care was taken to fully dry specimens after
`this cleaning process. Four groups of specimens (B, C,
`D, E) had the 0° hollow plies filled with dilute epoxy re-
`sin repair agent (MY750 Ciba-Geigy + 30%/vol ace-
`tone) and the 90° plies filled with corresponding
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`Fig. 1. Optical micrographs of fibres and composites manufactured at Bristol (a) hollow glass fibres of 60 lm external diameter with a hollowness of
`50% and (b) the same fibres within a Hexcel 913 epoxy matrix.
`
`80mm
`
`32mm
`
`64mm
`
`25mm
`
`2mm
`
`Hollow glass fibre plies
`
`Fig. 2. Lay-up configuration and dimensions for 4-point bend flexural testing.
`
`hardener. The resin and hardener weight gain per spec-
`imen was recorded and used later to normalise the flex-
`ural strength test data to an equivalence of 1%/weight
`for all specimens.
`Fibres were filled using a vacuum assisted liquid infil-
`tration technique. After thorough cleaning, the speci-
`mens were oriented such that one end of either the 0°
`or 90° exposed fibre ends were immersed, to a depth
`
`of a few millimetres, in resin or hardener, respectively.
`A vacuum was then applied to the opposite end of the
`specimen. The combination of capillary action and vac-
`uum saw liquid drawn into the fibres within a short per-
`iod of time. The fibres are then sealed using a rapid
`room temperature cure epoxy putty (ITW DevconÒ Ma-
`gic BondTM) which is manually inserted a few millime-
`tres into the fibre lumens.
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`
`2.2. Mechanical testing
`
`Six specimen groups (A–F) were prepared in order to
`establish the mechanical behaviour before and after
`pseudo-impact damage on a self-healing hybrid solid/
`hollow glass fibre reinforced composite. An objective
`of the study was to establish the efficiency of repair after
`a period of time had elapsed. Thus, a series of tests were
`undertaken at prescribed time intervals. Four-point
`bend flexural testing, according to ASTM 790M-93,
`was chosen for simplicity.
`Four specimen groups (B, C, D, E) were filled with
`repairing agent and two groups (A&F) were not. The
`latter represented undamaged and damaged states,
`respectively. The five specimen groups (B, C, D, E, F)
`were subjected to impact damage by a process of inden-
`tation using a hardened steel hemi-spherical end of
`4.63 mm diameter with the specimen back face sup-
`ported by a steel ring, as shown in Fig. 3. An Instron
`1341 servohydraulic machine was used for both indenta-
`tion and 4-point bend testing. A PC based data acquisi-
`tion system was used for all mechanical
`testing.
`Indentation was performed under load control at a
`crosshead displacement rate of 3 mm/min to a maxi-
`mum load of 1200 N. This corresponds to an impact en-
`ergy of 0.6 J if the area under the load–displacement
`curve is integrated.
`In order to ascertain the effect of time on repair effi-
`ciency, specimen groups B, C, D and E were stored in a
`desiccator for periods of 0, 3, 6 and 9 weeks before being
`subject to damage (via indentation) and flexural testing.
`Immediately after indentation, these specimen groups
`were allowed to undergo a process of self-healing for
`24 h at ambient temperature as this had previously been
`established [29] as the most simple and effective healing
`regime. The four-point bend flexural testing was con-
`
`ducted to investigate the efficiency of a bleeding compos-
`ite to effect a self-repair. Fig. 2 gives a schematic of the
`test geometry. A displacement rate of 3.4 mm/min was
`used for the flexural testing. The impact damaged face
`of the specimens was oriented such that it was subject
`to compressive loading.
`
`2.3. Enhancing damage visibility
`
`To enhance the ÔbleedingÕ process, a conspicuous
`medium (e.g., UV fluorescent dye) can be added to the
`healing resin within the hollow fibres to aid inspection
`for BVID. In order to investigate, validate and calibrate
`this enhancement of damage visibility, ultrasonic scan-
`ning (C type) was employed to compare with the pro-
`posed
`ultra-violet mapping
`technique
`(UVMT).
`Ultrasonic C-scans are widely used as a reliable non-
`destructive method for composite materials inspection.
`Thus, it is an ideal method to assess the reliability and
`effectiveness of the UVMT technique.
`Twenty-five specimens were prepared according to
`the manufacturing process reported above, with the
`exception that an UV fluorescent dye penetrant (Ardrox
`985) was added to the fibres instead of repair resin or
`hardener. These specimens were then divided into five
`groups and subjected to indentation as described above.
`Five different indentation forces were applied to the
`specimens; 800, 1000, 1200, 1400 and 1600 N. The
`equivalent impact energies are shown in Table 1. Two
`damage sites were created on each specimen in order
`to provide an average result of ten damage sites for each
`impact energy. The damage created by the indentation
`process was then measured using the UVMT and then
`ultrasound C-scan. The former consists of recording
`magnified digital images of the damage site under ul-
`tra-violet
`illumination. Efforts were then made to
`
`Fig. 3. Optical micrograph of cross-section through impact damaged hybrid solid glass/hollow glass/epoxy laminate.
`
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`Table 1
`Correlation of indentation load and impact energy
`
`Indentation force (N)
`
`Energy absorbed (J)
`
`800
`1000
`1200
`1400
`1600
`
`0.25
`0.43
`0.62
`0.80
`1.13
`
`measure and correlate the resulting damage maps from
`each specimen using the two techniques and image anal-
`ysis (ImageProÒ) software.
`
`3. Results and discussion
`
`3.1. Indentation behaviour
`
`Fig. 3 shows a cross-section through an uninfiltrated
`specimen (group F) after indentation, illustrating inter-
`face delamination, matrix cracking and hollow fibre
`fracture. The majority of the impact induced damage
`is localised within or adjacent to the hollow fibre plies,
`thus, creating an ideal situation for self-repair by the un-
`cured resin within the hollow fibres. The fracture of hol-
`low fibres in the 0° and 90° plies and the mixing of resin
`and hardener allows initiation of the curing process
`whilst simultaneously promoting infiltration of the local
`matrix cracks and delamination by capillary action. A
`key aspect of this whole self-healing process is that the
`
`impact energy must be of a sufficient threshold value
`to fracture hollow fibre plies. This threshold value can
`be tailored for any application by the constituents, num-
`ber and positioning of the repair agent bearing layers
`within the laminate stack.
`
`3.2. Four-point bend flexural testing
`
`Fig. 4 and Table 2 show the results of four-point
`bend flexural testing for the six specimen groups. In or-
`der to provide a fairer comparison of the test data, flex-
`ural strengths for groups B–E have been normalised to
`a nominal 1%/weight repair resin content within the
`hollow fibres. This can be justified as the resin content
`directly affects the extent of damage repair after impact
`and thus the resulting flexural strength. All testing was
`undertaken with the damaged face of the specimen sub-
`ject to compressive loading. This was because resin re-
`pair would have negligible effect on the fibre dominated
`tensile face.
`It is clear that impact has a serious effect on flexural
`strength, as specimen group F (damaged, uninfiltrated)
`shows a 25% reduction compared to group A (undam-
`aged, uninfiltrated). If a process of self-repair is intro-
`duced (group B) immediately post-manufacture,
`it is
`clear that a significant proportion (93%) of flexural
`strength can be restored. This is probably attributable
`to an extensive penetration of damage crack paths (see
`Fig. 3) by the repair resin before the viscosity rise asso-
`ciated with cure progression precludes this process.
`This self-repairing mechanism is not proposed as a
`
`F dam.
`
`E 9wks
`
`D 6wks
`
`C 3wks
`
`B 0wks
`
`A undam.
`
`900
`
`800
`
`700
`
`600
`
`500
`
`400
`
`300
`
`200
`
`100
`
`0
`
`Flexural Strength (MPa)
`
`Fig. 4. Results of flexural testing for damaged, undamaged and self-repaired specimens after various storage periods (bars denote standard
`deviation).
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`Table 2
`Results of four-point bend flexural testing for all specimen groups
`
`Sample
`identity
`
`A
`B
`C
`D
`E
`F
`
`Sample condition prior to testing
`
`Number of
`samples
`
`Mean flexural
`strength (MPa)
`
`Standard deviation
`(MPa)
`
`Percentage undamaged
`state (%)
`
`Undamaged
`Stored 0 weeks, damaged & repaired
`Stored 3 weeks, damaged & repaired
`Stored 6 weeks, damaged & repaired
`Stored 9 weeks, damaged & repaired
`Damaged
`
`14
`8
`7
`8
`8
`8
`
`733
`682
`546
`574
`404
`547
`
`45
`125
`112
`114
`117
`57
`
`100
`93
`75
`78
`55
`75
`
`permanent measure to eradicate the effects of damage
`within a composite but to provide a means to inhibit fur-
`ther damage propagation.
`Specimens groups B–E were used to assess the rate of
`degradation of the repair resin effectiveness over time.
`Each group of specimens was stored for different periods
`(0, 3, 6 and 9 weeks) before being damaged, allowed to
`self-repair for 24 h under ambient conditions and then
`tested in flexure. The efficiency of repair is seen to dete-
`riorate markedly over a 9-week period (albeit with a sig-
`nificant degree of scatter). After a 9-week period had
`elapsed (group E) self-repair was no longer seen to oc-
`cur. Flexural strength is then shown to be equivalent
`to a damaged and unrepaired material (group F).
`This is probably attributable to the self-repairing
`agent failing to bleed out from the fractured fibres.
`Much of this behaviour can be caused by the use of an
`unoptimised repair resin which includes additional com-
`ponents (UV fluorescent dye and acetone). Although,
`mixing acetone and fluorescent dye with resin led to
`no observable physical change in the short term, other
`physical or chemical processes could still occur over an
`extended period of time. The modified resin is likely to
`experience discernible deterioration with time, or be
`unsuited to storage in an uncured state for long periods.
`Specimen groups C, D and E evidently failed to undergo
`self-repair, and this is probably attributable to several
`factors.
`In this research, a 30%/wt acetone dilutent was added
`to the MY750 epoxy healing resin to reduce the viscos-
`ity. This addition is likely to have altered the resin chem-
`istry,
`inhibiting
`the polymerisation process
`and
`shortening the molecular chains. The presence of ace-
`tone in epoxy is also likely to change the macromolecu-
`lar
`structure
`and/or
`the
`cross-link density. An
`investigation into the influence of solvent content in
`polymer reinforced matrix materials by Buehler and
`Seferis [30] demonstrated that acetone in resin precur-
`sors could lead to alteration of resulting physical prop-
`erties. A similar finding was obtained by Hong and
`Wu [31] who claimed that the presence of acetone can
`alter the reaction mechanism and cure speed of an epoxy
`system due to the temperature variation resulting from
`the heat absorbed by solvent evaporation. The conse-
`
`quences of such interactions, within this study, could
`have rendered the healing resin ineffective after a short
`period of only 9 weeks.
`The quantity of the repair agent stored inside the
`composite
`is
`critical
`to the
`self-healing process
`[11,13,15]. It was found that repair agent weight gain
`is highly variable between specimens, due to some of
`the hollow fibre cores being blocked during the specimen
`preparation process. Glass fragments in the fibre ends
`could have stopped repair resin infiltrating the fibre even
`after cleaning in an ultrasonic bath.
`The reason for the failure of self-healing occurring over
`the 9-week test period is unlikely to be attributable to any
`one factor. The mechanism of healing via a bleeding pro-
`cess from hollow fibre reservoirs requires several stages,
`the absence of any one can result in no repair. However,
`these results indicate the importance of choosing an
`appropriate repair resin which offers ease of infiltration
`into hollow fibres, the ability to infiltrate and repair a
`damage zone, simple and controllable cure characteristics
`and adequate mechanical properties once cured.
`
`3.3. Visual enhancement of damage
`
`An important aspect in the development of ÔbleedingÕ
`fibre composites is to provide visual enhancement of
`damage, in particular BVID. The bleeding action of a
`highly conspicuous dye into the numerous cracks and
`fissures created by a damage event serves to decorate
`these sites, increasing their ease of detection in NDT/
`NDE. This could be of particular benefit where rapid vi-
`sual inspection of large surface areas (e.g., wing skin
`panels) is required.
`Fig. 5 compares three typical views of a damaged
`(0.8 J) specimen identical to those described previously,
`but containing a UV fluorescent dye (Ardrox 985)
`within the hollow fibres. Figs. 5(a) and (b) show the
`respective front (side of impact) and back face views un-
`der UV illumination, while Fig. 5(c) shows an ultrasonic
`C-scan of the same damage site. It is clear that the use of
`a UV dye is very effective in highlighting a damage site.
`Also, it appears from Figs. 5(b) and (c) that the damage
`shown using UVMT correlates very well with that from
`C-scan. This is further verified by Fig. 6 which attempts
`
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`J.W.C. Pang, I.P. Bond / Composites Science and Technology 65 (2005) 1791–1799
`
`UVMT-front face
`
`UVMT-back face
`
`C-scan
`
`300
`
`250
`
`200
`
`150
`
`100
`
`50
`
`0
`
`Damage Area (mm2)
`
`0.2
`
`0.3
`
`0.4
`
`0.5
`
`0.8
`0.7
`0.6
`Impact Energy (J)
`
`0.9
`
`1
`
`1.1
`
`1.2
`
`Fig. 6. Correlation of damaged area measured by UVMT (from front
`and back faces) and ultrasonic C-scan.
`
`ing a rapid and easy technique for highlighting suspect
`areas for further NDT/NDE.
`
`4. Conclusions
`
`A biomimetic approach has been used to develop and
`demonstrate a self-repairing, enhanced damage visibil-
`ÔbleedingÕ composite which provides an effective
`ity,
`way to recover mechanical strength and highlight con-
`cealed damage after an impact damage event.
`The results of flexural testing have shown that for the
`lay-up investigated, a significant fraction of lost flexural
`strength can be restored by the self-repairing effect of a
`repair agent stored within hollow fibres. The Ôself-repairÕ
`is dependent upon uncured resin (in the 0° plies) com-
`bining with the hardener (in the 90° plies) as a result
`of fibre fracture in both these layers. This self-repairing
`mechanism is not proposed as a permanent measure to
`eradicate the effects of damage within a composite but
`to provide a means to inhibit further damage propaga-
`tion. The ability of self-repair has been shown to deteri-
`orate significantly over time as the repair resin degrades.
`Further work is needed to optimise the repair resin used
`within the fibres to provide increased environmental sta-
`bility and effective service life.
`The release and infiltration of an UV fluorescent dye
`from fractured hollow fibres into damage sites within
`the internal structure of the composite has been success-
`fully demonstrated. It has been correlated with respect
`to the ultrasonic C-scan NDT/NDE technique and
`shown to be an effective method of quickly and easily
`highlighting damage at the surface that requires further
`investigation. This could be of particular benefit where
`rapid visual inspection of large surface areas (e.g., wing
`skin panels) is required.
`Further work is currently ongoing to refine both the
`self-repairing and damage enhancement processes by
`the use of
`tailored resins and dyes which provide
`
`(a) Front face (impact face) view using UVMT.
`
`(b) Back face view using UVMT.
`
`(c) View using C-scan.
`
`Fig. 5. Comparison of Ultra-Violet Mapping Technique (UMVT)
`viewed from (a) front and (b) back faces of specimen and (c) ultrasonic
`C-scan after impact damage of 0.8 J (i.e., indentation@1400 N). Note:
`images not to scale.
`
`to quantify and compare the damage areas after various
`impact energies, measured using UVMT and C-scan.
`Measurement of damage on the back face using UVMT
`closely correlates to C-scan, with a reasonably uniform
`discrepancy of 25%, for the impact energies investi-
`gated. Measurement of damage area from the front face
`using UVMT is less distinctive. However, it is useful in
`finding and marking a damage site on the surface, offer-
`
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`1799
`
`improved repair properties, damage enhancement and
`environmental stability/longevity.
`
`References
`
`[1] White SR, Sottos NR, Geubelle PH, Moore JS, Kessler MR,
`Sriram SR, et al. Autonomic healing of polymer composites.
`Nature 2001;409(6822):794–7.
`[2] Motuku M, Vaidya UK, Janowski GM. Parametric studies on
`self-repairing approaches for resin infused composites subjected
`to low velocity impact. Smart Mater Struct 1999;8:623–38.
`[3] Dry C. Alteration of matrix permeability, pore and crack
`structure by the time release of internal chemicals. In: Proceed-
`ings: advance in cementitious materials. Gaithersbury, Mary-
`land: American Ceramic Society; 1990. p. 729–68.
`[4] Dry C. Matrix cracking repair and filling using act