CC BY
0Accepted Manuscript
Optimisation of Epoxy Blends for use in Extrinsic Self-Healing Fibre-Reinforced Composites
D.T. Everitt, R. Luterbacher, Dr. T.S. Coope, Prof. I.P. Bond, Dr. R.S. Trask, Prof. D.F. Wass
PII: S0032-3861(15)00216-5
DOI: 10.1016/j.polymer.2015.02.047
Reference: JPOL 17660
To appear in: Polymer
Received Date: 18 December 2014
Revised Date: 25 February 2015
Accepted Date: 26 February 2015
Please cite this article as: Everitt DT, Luterbacher R, Coope TS, Bond IP, Trask RS, Wass DF, Optimisation of Epoxy Blends for use in Extrinsic Self-Healing Fibre-Reinforced Composites, Polymer (2015), doi: 10.1016/j.polymer.2015.02.047.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Optimisation of Epoxy Blends for use in Extrinsic Self-
2 Healing Fibre-Reinforced Composites
4 Daniel T. Everitt, Rafael Luterbacher, Tim S. Coope, Richard S. Trask, Duncan F. Wass, Ian
5 P. Bond
7 D. T. Everitt, R. Luterbacher, Dr .T. S. Coope, Prof. I. P. Bond, Dr. R. S. Trask
8 Advanced Composites Centre for Innovation and Science, Department of Aerospace
9 Engineering, University of Bristol, Queen's Building, BS8 1TR, United Kingdom
10 E-mail: daniel.everitt@bristol.ac.uk
12 Prof. D. F. Wass
13 School of Chemistry, University of Bristol, BS8 1TH, United Kingdom
16 Keywords: self-healing materials, toughened epoxies, composite materials
18 Abstract. A range of epoxy blends were investigated to determine their mechanical properties
19 and suitability for use as healing agents for the repair of fibre-reinforced polymer (FRP)
20 composites. Key requirements for an effective healing agent are low viscosity, and good
21 mechanical performance. A base epoxy resin was selected and blended with a variety of
22 diluents and a toughening agent, and the physical and mechanical properties of the resulting
23 polymers were investigated. Single lap shear strengths of up to 139% of the base epoxy
24 values were demonstrated, while double cantilever beam testing showed specimens healed
25 with optimised epoxy blends can provide recoveries in fracture toughness of up to 269%,
26 compared to 56% in specimens healed with the base epoxy resin. Cross-ply FRP laminate
27 tensile specimens were used to highlight the potential to recover stiffness decay caused by
intraply cracking. Following infusion of the damage via embedded vascules, the toughened epoxies were capable of providing complete recovery of stiffness.
1. Introduction
Self-healing materials have attracted a considerable amount of research interest in recent years, with significant advances in both intrinsic and extrinsic healing methods [1,2]. Healing mechanisms have been developed for a range of host materials, ranging from polymers [3-5] to cementitious materials [6]. Another area of particular interest has been fibre-reinforced polymer (FRP) composite materials, which are used widely in the aerospace and renewable energy industries. Efforts to construct civil aircraft from these lightweight materials are expanding in order to reduce fuel consumption and CO2 emissions. Due to the complex damage and failure mechanisms seen in FRPs [7], composite aircraft are currently built with a high level of redundancy, reducing potential structural mass savings. In the long-term, a shift from the current philosophy of 'no damage growth' to a more advanced damage-tolerant design approach is envisioned. The realisation of smarter materials and structures will further this aim, with self-sensing and self-repairing technologies being key areas of research. Within the research developments of self-healing FRPs, the vascular network [8-14] approach has largely dominated in recent years. The laminar nature of composites makes the manufacture of 1- and 2-dimensional networks relatively simple [12,15]; 3-D networks have also been demonstrated [11,16]. Healing has been achieved by both the injection of premixed healing agents [17] and post-damage mixing of a 2-part system [18,19]. Despite a body of research into the nature of vascules themselves, their arrangement within FRPs and demonstration of their healing performance, relatively little focus has been given to the nature of the healing agent that is deployed via the network. Dicyclopentadiene (DCPD), in combination with first generation Grubbs' catalyst, was the first extrinsic healing agent
1 used in FRPs [20] and has been successfully proven in a variety of configurations [21-23].
2 However, the high price of the Ruthenium catalyst and its sensitivity to air and moisture
3 prohibits use in larger scale applications. Cyanoacrylates [24] and polyesters [25] have
4 recently been considered as healing agents in FRPs, however the majority of healing agents
5 in use up to date have been epoxy based systems [9,15,26-32]. Low viscosity is a key
6 requirement for the healing agents in order to achieve effective infusion of damage. A variety
7 of non-reactive diluents such as ethyl phenylacetate (EPA) [26] and acetone [9], as well as
8 reactive diluents such as alkyl glycidyl ether [27] have been used. In some cases
9 commercially available low-viscosity epoxies such as those used for resin infusion [15,28] or
10 potting compounds [29,30] have also been employed. However, these epoxies are often
11 unsuitable as structural materials or possess other characteristics e.g. prolonged cure cycle,
12 which make them sub-optimal for use as healing agents in high performance composite
13 structures.
14 Herein we aim to demonstrate an epoxy resin optimised for use as a healing agent for
15 infusion of damage within FRPs via a vascular network. The key requirements were
16 identified as: (1) low viscosity, to enable infusion into low-volume damage; (2) good
17 compatibility with the host matrix, to ensure effective re-bonding of fracture surfaces; and (3)
18 high toughness, to prevent further damage in the healed region. Typical commercial low-
19 viscosity, and often low molecular weight, epoxies lack toughness in particular, which is
20 addressed here through the precipitation of toughening particles into the resin and the use of
21 reactive diluents. This research, therefore, builds upon the existing body of research into
22 toughened epoxies by considering a DGEBA based resin system with a carboxyl-terminated
23 butadiene acrylonitrile (CTBN) adduct [33-35].
24 A baseline diglycidyl ether bisphenol A (DGEBA) based resin system, Epon 828
25 (Polysciences, Inc.), was investigated in combination with a range of reactive diluents and a
1 toughening agent, HyPox RA840 (Emerald). The studied reactive diluents were D.E.R.736
2 from Dow Chemicals (a mixture of epichlorohydrin and propylene glycol, (further referred to
3 as A)) and poly(propylene glycol)diglycidyl ether (further referred to as B). A summary of
4 resin constituents is provided in Figure 1. Epon 828 was selected as a baseline resin due to
5 the absence of additives or diluents of unknown concentration, as found in many commercial
6 resin systems, along with its fundamental similarity to a number of commercial prepreg resin
7 systems [36]. HyPox RA840 (further referred to as RA840) is a DGEBA based resin system
8 with a carboxyl-terminated butadiene acrylonitrile (CTBN) adduct on 19% of epoxy
9 monomers. Upon reaction with the hardener (DETA), CTBN is displaced from the epoxy
10 monomer, which precipitates particles into the resulting polymer and contributes to
11 increasing the toughness of the cured self-healing polymer.
12 Figure 1. Chemical structures of (i) diglycidyl ether bisphenol A (DGEBA), (ii)
13 carboxyl-terminated butadiene acrylonitrile (CTBN), (iii) epichlorohydrin, propylene glycol
14 (D.E.R 736) (A), (iv) poly(propylene glycol) diglycidyl ether (B), (v) diethylenetriamine
15 (DETA)
1 Cured polymer blends were assessed for their degree of cure, glass transition temperature (Tg)
2 and lap shear strength, as well as their capacity to recover strength and Mode I fracture
3 toughness in FRP double cantilever beam (DCB) specimens.
4 In order to demonstrate the healing capabilities that are possible through optimisation of the
5 healing agent, mitigation of transverse damage was carried out with some resin blends within
6 cross-ply FRP laminate tensile specimens. FRPs can fail due to a variety of causes, and
7 certain failure modes can be distinctive such as fibre failure, interlaminar failure
8 (delaminations) or intralaminar failure (fibre debonding or matrix cracking) [37]. Self-healing
9 within FRPs is currently limited to repairing the polymeric matrix material and the main
10 focus has been on the interlaminar failure due to the ease by which this larger scale damage
11 can be infused [26]. However, matrix cracking is both the initiating point for delaminations
12 and also the mechanism by which delaminations migrate into adjacent ply interfaces.
13 In order to determine the effectiveness of the new healing resins developed herein, FRP
14 specimens were designed to encourage the controlled formation of damage which can then be
15 repaired by infusion of the resin blends. The first damage mode for cross-ply laminates [38]
16 (e.g. a laminate with a lay-up of [0°n/90°m]s) is matrix cracking. With increasing load the
17 density of matrix cracking in the 90° layer increases, until a load is reached that results in
18 failure of the 0° fibres. Damage to the transverse lamina results in a global stiffness
19 reduction, which can be mitigated by infusion and curing of a healing agent. While manual
20 injection of the healing resin is carried out here, the vasculature could be employed as part of
21 an autonomous healing network. The development of such a system was outside the scope of
22 this work, however a system similar to the one described by Norris et al. [39] or Minakuchi et
23 al. [40] could be easily implemented. In addition, future work is planned to use the developed
24 healing agent in conjunction with catalysts such as scandium triflate [4]. As the healing
1 agents are DGEBA based, a similar encapsulation strategy as described by [5] could be
2 followed, enabling the use as an autonomous self-healing solution.
3 2. Experimental
4 2.1 Materials
5 Poly(propylene glycol) diglycidyl ether, D.E.R 736 (Dow Chemicals) and diethylenetriamine
6 (DETA) were purchased from Sigma-Aldrich UK. Epon 828 was purchased from
7 Polysciences, Inc. Europe. Hypox RA840 (Emerald) was provided by Hubron Speciality Ltd.
8 IM7/8552 carbon/epoxy and 913 E-glass/epoxy pre-impregnated tape were purchased from
9 Hexcel.
10 2.2 Differential Scanning Calorimetry
11 Resin blends were thoroughly mixed and approximately 10 mg was quickly transferred to
12 Tzero aluminium hermetic pans. A TA Q200 DSC was used to study cure behaviour via
13 modulated dynamic scans. A heating rate of 10 0C min-1 and a temperature range of
14 0 0C - 250 0C was used. Nitrogen purge gas flow rate = 50 ml min-1. Tg values were obtained
15 over a temperature range of -30 0C - 200 0C with a heating rate of 3 oc min-1.
16 2.3 Viscometry
17 All viscosity readings were taken on a Brookfield DV-E viscometer at 24 ± 10C. Blends were
18 mixed thoroughly and readings were taken before addition of the hardener until stable, with
19 final reported readings taken immediately after adding the hardener component.
1 2.4 Tensile Specimen Manufacture & Testing
2 Type IV tensile specimens were prepared according to ASTM D 638 [41]. Additive layer
3 manufacturing was used to create a positive mould from which silicone moulds were cast.
4 Resin blends were mixed, degassed and poured into the moulds before an aluminium top
5 plate was applied to ensure a flat surface. Specimens were cured for 1 hour at 45 0C and left
6 for a minimum of 5 days at ambient temperature before testing.
7 Testing was conducted on a Shimadzu AGS-X universal test machine fitted with a calibrated
8 10 kN load cell at a rate of 20 mm min-1.
9 2.5 Single Lap Shear Specimen Manufacture & Testing
10 Carbon fibre / epoxy unidirectional (UD) prepreg (IM7 8552, Hexcel, UK) panels were
11 manufacture by hand layup. A cure cycle of 60 minutes at 1100C followed by 120 minutes at
12 180°C was carried out, both under 700 kPa pressure, as per the manufacturer's specifications.
13 Substrate panels were cut to size (75 mm x 135 mm x 2.8 mm, 20 plies) on a diamond coated
14 saw. End tabs of the same material were bonded using Elantech epoxy adhesive.
15 Bonding surfaces of each substrate were grit blasted and cleaned prior to addition of the
16 investigated healing agent. Two substrate panels were then clamped and cured before cutting
17 to final size (75 mm x 25 mm x 2.8 mm). Specimens were found by microscopy to have a
18 bond thickness of approximately 50 |im. A schematic of the specimen geometry can be seen
19 in Figure 2.
Elantech Adhesive
Epoxy Resin
Substrate
ca. 50 jLim
-03 -rvi -
End Tab
Figure 2. Schematic of the SLS specimens. Dimensions are given in mm.
Testing was carried out with reference to ASTM D 5868 [42] and AITM 1-0019 [43] on a Schenck Hydropuls PSA universal test machine fitted with a calibrated 75 kN load cell at a rate of 2 mm min-1. A lower test rate as indicated by the ASTM standard was selected in order to compare to former tests. The initial grip to grip separation during testing was 75mm. Lap shear strength (MPa) was calculated by dividing the failure load by the bonded area, nominally 25 mm x 25 mm.
aSLS —
10 In which Pmax, w and l refer respectively to the failure load (N), width (mm) and length (mm)
11 of the bond area.
12 2.6 Mode I Specimen Manufacture, Healing & Testing
13 Carbon fibre / epoxy unidirectional (UD) prepreg (IM7/8552, Hexcel, UK) panels (600 mm x
14 180 mm x 3.4 mm, 24 plies) featuring a 15 |im release film insert were manufactured by hand
15 lay-up so as to result in an initial delamination length (a0) of 50 mm. A cure cycle of 60
16 minutes at 110°C followed by 120 minutes at 180°C, both under 700 kPa pressure in an
17 autoclave, was carried out, as per the manufacturer's specifications. Specimens were cut to
size (160 mm x 20 mm x 3.4 mm) on a diamond coated saw and piano hinges were bonded onto grit blasted surfaces using an Elantech epoxy cured at ambient temperature overnight as preparation for testing. A schematic is provided in Figure 3.
Figure 3. Schematic depicting the hinged DCB specimens. The PTFE insert creates a precrack length (a0) of 50 mm from the point of loading. Dimensions are given in mm.
Testing was carried out according to ASTM D 5528 [44] on a Shimadzu AGS-X universal test machine fitted with a calibrated 1 kN load cell at a rate of 5 mm min-1. Cracks were propagated for approximately 50 mm. A Labview™ script was used to acquire the delamination length via high resolution images (to an accuracy greater than ±0.5 mm), displacement and load data.
Specimens were healed after the initial fracture via injection of the healing resin onto the fracture surface. Specimens were then taped closed and cured for 1 hour at 450C and left for a minimum of 5 days at ambient temperature prior to repeat testing.
Load P (in N), displacement 8 (in mm), crack length a (in mm), specimen width b (in mm) and specimen thickness t (in mm) were used to determine Mode I strain energy release rate GIC (in J m- ) via the modified beam theory (MBT) method [44]. A compliance calibration factor (A) is used to correct for any rotation at the crack front, which causes effective elongation of the delamination (a + |A|). A is determined from the gradient (m) and intercept
(c) of a plot of the cube root of compliance (C ) against crack length:
CTack Ropagation Region
3 PS 2 b{a + \A\)
2 Healing efficiencies (Q are given as percentages in terms of fracture toughness (Gic) and
3 peak load (P).
JICHea1ed
'^Pristine
^Healed ^Pristine
5 2.7 Cross-Ply Manufacture, Healing & Testing
6 For ease of damage identification via back light illumination, glass fibre-reinforced polymer
7 composites were selected. Panels (220 mm x 150 mm) of unidirectional 913/E-Glass (Hexcel,
8 UK) were manufactured by hand lay-up. The manufacturer's recommended cure cycle of
9 125°C for 1 hour and 700 kPa was followed. Self-healing specimens had a PTFE coated NiCr
10 wire (The Scientific Wire Company) incorporated in a cut out of the 9O2/O2 plies (refer to
11 Figure 4) in order to manufacture the vascular network (Procedure B as described by Norris
12 et al. [10]). This interface was selected in order to assure that the transverse damage
13 occurring in the 90° plies will intersect with perpendicular with the vascule. After cure, the
14 wire was removed and end tabs were bonded using Elantech epoxy adhesive. Specimens were
1 then cut to size on a diamond saw (200 mm x 20 mm x 2.2 mm) (see
Vascule Location 00.5 /
/ o fNI
- (Sli-
3 Figure 5).
90 90 90 90 90. 90 _9Q
_HL 0 0 0 0
5 Figure 4. Layup of cross-ply laminates. A 0.5 mm diameter vascule is placed such that it will
6 provide connectivity with any transverse damage formed in the 900 plies.
Vascule Location 00.5 /
/ o IN
9 10 11 12
Figure 5. Cross-ply laminate geometry.
Cross-ply laminates were tested with reference to ASTM D3039 [45] at a rate of 2 mm min-1 on a Schenck Hydropuls PSA universal testing machine equipped with a calibrated 75 kN load cell. Specimens for healing were loaded to 15 kN (ca. 70% of the failure load) and unloaded. Strain was measured on the central 50 mm of the specimen using a video
extensometer (Imetrum). The modulus was determined on the loading (Epnstine) and unloading (Edamaged) parts, between 20 MPa and 100 MPa.
Prior to infusion of the healing agent, the edges of the specimens were sealed. Healing agent was delivered to the vascular network with a syringe pump (Nexus) for 10 minutes at a flow rate of 0.2 ml min"1, and the specimens were cured and retested. The modulus was once again determined upon loading (Eheaied).
The healing performance was determined, as follows, in accordance with Blaiszik etal. [1]:
^healed Edamaged Epristine ~ Edamaged
3. Results & Discussion 3.1 Polymer Blends
Diethylenetriamine (DETA) was used to cross-link all blends discussed herein. Hardener concentration was determined for each blend based on epoxide-equivalent weight (EEW) calculations [46]. Concentrations of different epoxies and hardener used in each blend are shown in Table 1. The effect of non-stoichiometric mixing on resin properties will be investigated in future work.
Amine H eq. wt. :
Mw of amine # active hydrogens
pph of amine
Amine H eq. wt. x 100 Blend EEW
EEWs of resin blends were calculated as so:
Blend EEW
Total wt.
wt.a ËËWi
wt.b EEWh
1 Where a range of EEWs was provided for a given healing agent, the mean value was used.
2 Given that low viscosity was identified as a key characteristic of any target healing resin, and
3 the high viscosity of RA840 (440 000 cP), a maximum of 20 wt% RA840 was used.
4 Tripathi et ah [47] found that 20 wt% of CTBN within a DGEBA-based resin was optimum
5 for increasing toughness; thus, a similar value was targeted for the blends presented herein.
6 Rubber particles precipitated upon curing of the Epon 828 + RA840 blend by DETA can be
7 seen in Figure 6. Typical particles are approximately 50 |im in diameter.
9 Figure 6. Particles of CTBN precipitated by 20 wt% HyPox RA840 within Epon 828 epoxy
10 resin hardened with DETA.
11 3.2 Thermal Analysis
12 Epon 828 has a recommended cure cycle of 7 days at ambient temperature. Here, a cycle of 1
13 hour at 45°C followed by a minimum of 5 days at ambient temperature was used for all
14 blends. Optimisation of the cure cycle was outside the scope of this work, however,
15 preliminary analyses indicated that significantly faster curing is possible at higher
16 temperatures. It is thought that higher cure temperature will also result in higher Tg values.
1 Further investigation of alternative cure cycles will be carried out in the future. Modulated
2 DSC analysis was used to confirm complete curing of all blends; plots are shown in Figure 7.
10 11 12
25 20 15 10 5 0 -5
351 Jg"1
• Initial
----Post 1 Hour 45C
Post 1 Hour 45C + 5 Days RT
—i— 50
100 150 Temperature [°C]
Figure 7. Overlaid MDSC traces showing completion of resin cure after 1 hour at 450C and 5 days at room temperature for Epon 828/RA840 50B. Equivalent plots were obtained for all
blends.
The influence of diluent concentration on glass transition temperature (Tg) was also investigated by DSC. Tg data are summarised in Table 1. Tg drops rapidly as diluent concentration increases. This observation is consistent with mechanical testing data showing increasing ductility of the resulting polymers. In the case of A the increased ductility is attributed to the reduction in chain length caused by the mono-functional epoxy, epichlorohydrin, and presence of the non-reactive diluent, propylene glycol [48,49]. Diluent B, poly(propylene glycol) diglycidyl ether, is a bifunctional epoxy monomer which, due to its length and lack of bulky groups, increases polymer chain flexibility
1 3.3 Mechanical Testing
10 11 12
3.3.1 Lap Shear
Lap shear testing was carried out with reference to ASTM 5868 [42] and AITM 1-0019 [43]. Lap shear testing was selected as an efficient method to screen a high number of candidate polymer blends for lap shear strength and to investigate a mixed mode loading condition on the adhesive layer [50]. This loading condition, therefore, corresponds to a more realistic damage propagation scenario in FRP structures.
Variation in single lap shear (SLS) strength as a function of diluent concentration can be seen in Figure 8 and Table 1. Introducing 20 wt% of the toughening agent RA840 showed an increase in SLS strength to 148% of baseline Epon 828 values. SLS strength peaks at 29 MPa with 40 wt% of diluent A and at 28 MPa with 30 wt % of diluent B, within Epon 828 / RA840 (20 wt%). SLS decreases rapidly after these peaks.
10 20 30 40
Diluent Concentration [%]
Figure 8. Influence of varying concentration of reactive diluents on single lap shear strength of specimens bonded with RA840 toughened Epon 828. Baseline Epon 828 performance is
included for comparison.
ACCEPTED MANUSCRIPT
1 Table 1. Summary of properties of each epoxy blend. *All viscosity readings were carried out immediately after addition of the hardener at 24 ±
2 10C. |ASTM D 638, minimum of 4 specimens. *ASTM D 5868, minimum of 3 specimens.
Blend ID Epon 828 (wt %) HyPox RA840 (wt %) Diluent (wt %) A B Hardener Concentration (pph) Tg (0C) Viscosity (cP)* Tensile Strength / MPa SLS* / MPa
Epon 100 0 0 0 12 115 1900 73 ± 11 12 ± 0.4
EponRA840 80 20 0 0 10.8 105 2210 52 ± 4 21 ± 4
20A 60 20 20 0 10.6 82 1203 52 ± 8 18 ± 3
30A 50 20 30 0 10.5 48 680 62 ± 3 25 ± 1
40A 40 20 40 0 10.4 45 568 39 ± 6 29 ± 1
50A 30 20 50 0 10.2 9 357 14 ± 1 18 ± 2
20B 60 20 0 20 9.5 79 1416 32 ± 2 24 ± 28
30B 50 20 0 30 8.8 47 750 46 ± 8 28 ± 1
40B 40 20 0 40 8.1 42 702 48 ± 1 20 ± 1
50B 30 20 0 50 7.4 34 358 4 ± 0.3 8 ± 1
1 3.3.2 Mode I Fracture Mechanics Evaluation
2 Determination of self-healing performance of epoxy blends was carried out via double
3 cantilever beam (DCB) testing according to ASTM Standard D5528 [44]. Specimens were
4 fractured, unloaded, healed and retested to failure.
5 Table 2 provides a summary of pristine and healed DCB performance, along with healing
6 efficiency values, which are also summarised in Figure 9. Specimens healed with the baseline
7 Epon 828 achieved a mean peak load healing efficiency (ZP) of 84%, however, the fracture
8 toughness of the healed specimens was poor, just 56% of the baseline material. Upon addition
9 of 20 wt% of RA840 (Epon828+RA840) these values improved to Zp = 169%, Zgic = 87%
10 respectively, demonstrating the effectiveness of the additive at increasing failure load.
11 However, only a relatively small improvement was seen with fracture toughness; from 56%
12 recovery with Epon 828 to 87% with RA840 toughened Epon 828.
ro a) x
OB ■ Epon 828
0 10 20 30 40
Diluent Concentration [%]
o it LU
OB ■ Epon 828
10 20 30 40 50 Diluent Concentration [%]
13 Figure 9. Influence of concentration of diluents A and B in Epon+RA840 on healing
14 efficiency in terms of i) initial fracture toughness (ZGIC) and ii) peak load ( ZP). Baseline Epon
15 data are also included.
1 Upon addition of reactive diluents A or B to the Epon828+RA840 blend, mixed viscosity
2 reduced significantly, as shown in Table 1. This effect was seen to improve lap shear strength
3 up to a critical point (28 MPa) after which resin performance fell rapidly, as shown in Figure
4 8. DCB testing allows investigation of the impact of diluents on resin fracture toughness and
5 healing efficiency. Diluent loadings below 30 wt% were not investigated as their viscosities
6 were considered too high (> 1200 cP) for successful damage infusion within more realistic
7 composite specimens. The addition of 30 wt% of diluent resulted in a sharp increase in Zgic.
8 Further increasing diluent concentration resulted in a reduction to ZGIC relative to the peak
9 obtained at 30 wt%. In terms of DCB failure load recovery, however, the impact of diluent
10 concentration was shown to be much smaller than in the case of fracture toughness (Figure
11 9). Increasing diluent concentration from 0% to 30% does not cause Zp to vary significantly
12 from values seen in specimens healed with Epon+RA840. Therefore, it is apparent that the
13 presence of RA840 can significantly increase failure load recovery (from 87% with plain
14 Epon, to 169% when 20 wt% of RA840 is also present), whereas the presence of diluents can
15 significantly increase fracture toughness recovery from below 100% to over 250%.
16 Representative load-displacement behaviour for specimens healed with Epon 828, 30B and
17 50B (Table 1) compared with a baseline specimen can be seen in Figure 10. Here, the brittle
18 nature of specimen failure when healed with the baseline resin (Epon 828) is apparent. In
19 comparison, the addition of 30 wt% of either diluent A (30A) or B (30B) is seen to
20 significantly increase ductility and lead to stable crack propagation (Figure 11), with high
21 fracture toughness recoveries of around 250%. At 50 wt% of diluent this ductility remains,
22 but lower healing efficiencies of 200% fracture toughness recovery are obtained (Figure 12).
—Pristine —Healed: Epon —Healed: 30B
Healed: 50B
8 [mm]
Figure 10. Representative load displacement plots of (a) pristine IM7 8552 DCBs and specimens healed with (b) Epon 828, (c) RA840 toughened Epon 828 with 30 wt% of reactive diluent B (30B) and (d) RA840 toughened Epon 828 with 50 wt% of reactive diluent
B (50B).
--Healed: ЗОВ
Pristine
1000 S00 i 600 400 200 0
-Healed: ЗОВ
■Pristine
70 SO a [mm]
Figure 11. i) Representative load-displacement plots for a DCB specimen before and after healing with 30B. A peak load recovery (Zp) of 153% is observed. ii) Fracture toughness vs. crack length for the same specimen before and after healing with 30B. A fracture toughness
recovery (ZGIC) of 250% is observed.
--Healed: 50B -Pristine
ST 600
(3 400
i - Healed: 50B
■ Pristine
-♦-♦-♦
70 80 a [mm]
Figure 12. i) Representative load-displacement plots for a DCB specimen before and after healing with 50B. A peak load recovery (ZP) of 130% is observed. ii) Fracture toughness vs. crack length for the same specimen before and after healing with 50B. A fracture toughness
recovery (ZGC) of 203% is observed.
ACCEPTED MANUSCRIPT
2 Table 2. Summary of peak load and fracture toughness values obtained from pristine and healed DCB specimens. A minimum of 3 tests were
3 carried out per data point.
Diluent (wt%)
Pristine
Healed
Blend ID 828 (wt%) RA840 (wt%) A B Peak Load (P) / N Initial Fracture Toughness (Gic) / Jm-2 Peak Load (P) / N Mean Efficiency (Çp) Initial Fracture Toughness (GIC) / Jm-2 Mean Efficiency (Zgic)
Epon 100 0 0 0 42 ± 4 279 ± 21 37 ± 2 88% 158 ± 77 56%
EponRA840 80 20 0 0 44 ± 6 271 ± 35 76 ± 10 169% 235 ±100 87%
30A 50 20 30 0 43 ± 1 294 ± 37 68 ± 2 159% 764 ± 24 270%
40A 40 20 40 0 43 ± 2 255 ± 4 60 ± 5 141% 680 ± 123 266%
50A 30 20 50 0 38 ± 3 285 ± 23 68 ± 2 180% 701 ± 75 246%
30B 50 20 0 30 40 ± 6 280 ± 63 71 ± 12 177% 762 ± 148 272%
40B 40 20 0 40 40 ± 3 299 ± 42 61 ± 6 152% 592 ± 176 198%
50B 30 20 0 50 43 ± 3 272 ± 5 57 ± 6 131% 552 ± 112 203%
1 4. Repair of Transverse Damage in Cross-Ply Laminates
2 Cross-ply laminates vulnerable to the development of transverse matrix cracking were used to
3 further demonstrate the capacity for using the developed epoxy blends for repair of FRPs.
4 Manual infusion of the damage via a vascule was carried out in this case. While outside the
5 scope of this work, integration into an autonomous healing system is feasible. Either using
6 the vascule as a pressure sensor [39] or an additional integrated strain sensor system [40]
7 could trigger the mixing and release of the healing agent into the vascule. Then a heating
8 solution such as an incorporated resistive heating mat could provide a localised healing
9 solution. The infusion of a low viscosity red dye penetrant (Ardrox 996PA) clearly visualised
10 the connectivity between the vascule and the transverse damage (Figure 13 i).
11 Table 3 summarises the healing performance for the cross-ply FRP laminate infusion tests.
12 The resins 40A, 30B and 50B were used to heal damage cross-ply specimens. Full recovery
13 of stiffness was achieved with all resin systems.
Figure 13.i) Here the injection of dye penetrant (Ardrox 996PA) through the longitudinal vascule results in 'bleeding' from the side of the specimen (via damage in the 90° plies), demonstrating connectivity between the centrally located vascule and internal damage. ii) Transverse matrix damage spanning the test specimen width in a cross-ply laminate, resulting from tensile loading. iii) Infusion of healing resin into a damaged cross-ply specimen.
Section 2.7 provides details of specimen and test configuration. The development of matrix cracking plays a crucial role in both delamination initiation and migration in FRPs, and is therefore a key target damage state for self-healing solutions. In the case of DCB specimens [10] the vascule is located in the damage plane, thereby ensuring easy wet-out of the fracture surface. However, in the case of the cross ply laminates, the damage intersects the vascule perpendicularly. This intraply damage is much smaller in volume than in the case of delaminations in DCB specimens, therefore, the resin flow into these damage locations is
perturbed in comparison to that in DCBs. This limited connectivity between vasculature and damage, combined with trapped air in some locations, gives rise to incomplete infusion during healing, and is responsible for the high standard deviations observed for the achieved healing efficiencies.
Table 3. Summary of the healing performance. A minimum of 5 tests were carried out per
data point
Diluent
Epon Hypox (wt%) Healing
Blend ID 828 RA840 Efficiency (Z)
(wt%) (wt%) A B
40A 40 20 40 0 106% ± 31%
30B 50 20 0 30 114% ± 38%
50B 30 20 0 50 119% ± 30%
The investigated healing agent blends were observed to fully recover stiffness in all cases. Thus, healing agent formulation and selection will be driven more by requirements for failure strength and fracture toughness performance alongside physical and processing characteristics e.g. viscosity, cure kinetics, Tg.
5. Conclusions
It was found that by addition of 20 wt% of HyPox RA840 to Epon 828 epoxy, the precipitation of 50 ^m diameter toughening particles significantly improved the tensile strength in single lap shear, and fracture toughness in Mode I double cantilever beam tests. From a constant concentration of 20 wt% HyPox RA840, increasing the concentration of a reactive diluent (Dow Chemicals' D.E.R 736 (A) or poly(propylene glycol) diglycidyl ether (B)) while reducing the concentration of Epon 828 showed to increase polymer ductility,
1 resulting in an increasingly ductile failure in Mode I (DCB) testing. DCB testing also
2 revealed that the addition of these reactive diluents significantly increased fracture toughness
3 (G[C). It has been demonstrated that the investigated epoxies are highly effective at repairing
4 DCB specimens. Furthermore, the potential for full recovery of stiffness in specimens
5 affected by transverse matrix damage was demonstrated by the infusion of cross-ply FRP
6 laminates with the developed healing agent blends.
7 Overall, blend 30B (20wt% HyPox RA840, 50wt% Epon 828, 30wt% diluent B) was found
8 to offer the best range of physical and mechanical properties; fulfilling the requirement for
9 low viscosity while offering high strength and toughness. Therefore, the investigation and
10 optimisation of this polymer blend has identified the true potential to realising high
11 performance vascularised self-healing FRP composites, via an extrinsic method [39,40].
12 Acknowledgements
13 The authors would like to thank the EPSRC (EP/G036772/1) and Fundacio Obra Social La
14 Caixa for funding. We would also like to thank Dr Julie Etches for her assistance with DCB
15 data collection, Ian Chorley and Glenn Wallington for their assistance in the laboratory and
16 Martin F. Cicognani from Hubron Speciality for his support and for providing the HyPox
17 RA840.
1 References
2 [1] Blaiszik BJ, Kramer SLB, Olugebefola SC, Moore JS, Sottos NR, White SR. Self-Healing
3 Polymers and Composites. Annual Review of Materials Research 2010;40:179-211.
4 doi:10.1146/annurev-matsci-070909-104532.
5 [2] Yuan, Y. C., Yin, T., Rong, M. Z., Zhang MQ. Self healing in polymers and polymer
6 composites. Concepts, realization and outlook: A review. eXPRESS Polymer Letters
7 2008;2:238-50. doi: 10.3144/expresspolymlett.2008.29.
8 [3] Wu DY, Meure S, Solomon D. Self-healing polymeric materials: A review of recent
9 developments. Progress in Polymer Science 2008;33:479-522.
10 doi :10.1016/j .progpolymsci.2008.02.001.
11 [4] Coope TS, Mayer UFJ, Wass DF, Trask RS, Bond IP. Self-Healing of an Epoxy Resin Using
12 Scandium(III) Triflate as a Catalytic Curing Agent. Advanced Functional Materials
13 2011;21:4624-31. doi:10.1002/adfm.201101660.
14 [5] White SR, Sottos NR, Geubelle PH, Moore JS, Kessler MR, Sriram SR, et al. Autonomic
15 healing of polymer composites. Nature 2001;409:794-7. doi:10.1038/35057232.
16 [6] Wu M, Johannesson B, Geiker M. A review: Self-healing in cementitious materials and
17 engineered cementitious composite as a self-healing material. Construction and Building
18 Materials 2012;28:571-83. doi:10.1016/j.conbuildmat.2011.08.086.
19 [7] Greenhalgh ES. Failure Analysis and Fractography of Polymer Composites. Woodhead
20 Publishing; 2009.
21 [8] Dry C. Procedures developed for self-repair of polymer matrix composite materials.
22 Composite Structures 1996;35:263-9. doi:10.1016/0263-8223(96)00033-5.
23 [9] Pang JWC, Bond IP. "Bleeding composites"—damage detection and self-repair using a
24 biomimetic approach. Composites Part A: Applied Science and Manufacturing 2005;36:183-8.
25 doi:10.1016/j.compositesa.2004.06.016.
26 [10] Norris CJ, Bond IP, Trask RS. Interactions between propagating cracks and bioinspired self-
27 healing vascules embedded in glass fibre reinforced composites. Composites Science and
28 Technology 2011;71:847-53. doi:10.1016/j .compscitech.2011.01.027.
29 [11] Toohey KS, Sottos NR, Lewis J a, Moore JS, White SR. Self-healing materials with
30 microvascular networks. Nature Materials 2007;6:581-5. doi:10.1038/nmat1934.
31 [12] Trask RS, Bond IP. Biomimetic self-healing of advanced composite structures using hollow
32 glass fibres. Smart Materials and Structures 2006;15:704-10. doi:10.1088/0964-
33 1726/15/3/005.
34 [13] Williams HR, Trask RS, Weaver PM, Bond IP. Minimum mass vascular networks in
35 multifunctional materials. Journal of the Royal Society, Interface / the Royal Society
36 2008;5:55-65. doi:10.1098/rsif.2007.1022.
Bleay SM, Loader CB, Hawyes VJ, Humberstone L, Curtis PT. A smart repair system for polymer matrix composites. Composites Part A: Applied Science and Manufacturing 2001;32:1767-76. doi:10.1016/S1359-835X(01)00020-3.
Williams G, Trask RS, Bond IP. A self-healing carbon fibre reinforced polymer for aerospace applications. Composites Part A: Applied Science and Manufacturing 2007;38:1525-32. doi:10.1016/j.compositesa.2007.01.013.
Hansen CJ, Wu W, Toohey KS, Sottos NR, White SR, Lewis JA. Self-Healing Materials with Interpenetrating Microvascular Networks. Advanced Materials 2009;21:4143-7. doi:10.1002/adma.200900588.
Coope TS, Wass DF, Trask RS, Bond IP. Metal Triflates as Catalytic Curing Agents in Self-Healing Fibre Reinforced Polymer Composite Materials. Macromolecular Materials and Engineering 2013. doi:10.1002/mame.201300026.
Williams HR, Trask RS, Bond IP. Self-healing sandwich panels: Restoration of compressive strength after impact. Composites Science and Technology 2008;68:3171-7. doi:10.1016/j.compscitech.2008.07.016.
Hamilton AR, Sottos NR, White SR. Self-healing of internal damage in synthetic vascular materials. Advanced Materials (Deerfield Beach, Fla) 2010;22:5159-63. doi:10.1002/adma.201002561.
Kessler M., White S. Self-activated healing of delamination damage in woven composites. Composites Part A: Applied Science and Manufacturing 2001;32:683-99. doi:10.1016/S1359-835X(00)00149-4.
Sanada K, Yasuda I, Shindo Y. Transverse tensile strength of unidirectional fibre-reinforced polymers and self-healing of interfacial debonding. Plastics, Rubber and Composites 2006:6772.
Kessler M., Sottos N., White S. Self-healing structural composite materials. Composites Part A: Applied Science and Manufacturing 2003;34:743-53. doi:10.1016/S1359-835X(03)00138-6.
Patel AJ, Sottos NR, Wetzel ED, White SR. Autonomic healing of low-velocity impact damage in fiber-reinforced composites. Composites Part A: Applied Science and Manufacturing 2010;41:360-8. doi:10.1016/j.compositesa.2009.11.002.
Fifo O, Ryan K, Basu B. Glass fibre polyester composite with in vivo vascular channel for use in self-healing. Smart Materials and Structures 2014;23:095017. doi:10.1088/0964-1726/23/9/095017.
Zainuddin S, Arefin T, Fahim a., Hosur MV, Tyson JD, Kumar A, et al. Recovery and improvement in low-velocity impact properties of e-glass/epoxy composites through novel self-healing technique. Composite Structures 2014;108:277-86. doi:10.1016/j.compstruct.2013.09.023.
Coope TS, Wass DF, Trask RS, Bond IP. Repeated self-healing of microvascular carbon fibre reinforced polymer composites. Smart Materials and Structures 2014;23:115002. doi:10.1088/0964-1726/23/11/115002.
1 [27] Patrick JF, Hart KR, Krull BP, Diesendruck CE, Moore JS, White SR, et al. Continuous self-
2 healing life cycle in vascularized structural composites. Advanced Materials (Deerfield Beach,
3 Fla) 2014;26:4302-8. doi:10.1002/adma.201400248.
4 [28] Williams HR, Trask RS, Bond IP. Self-healing composite sandwich structures. Smart
5 Materials and Structures 2007;16:1198-207. doi:10.1088/0964-1726/16/4/031.
6 [29] Trask RS, Norris CJ, Bond IP. Stimuli-triggered self-healing functionality in advanced fibre-
7 reinforced composites. Journal of Intelligent Material Systems and Structures 2013;25:87-97.
8 doi:10.1177/1045389X13505006.
9 [30] Norris CJ, Bond IP, Trask RS. Healing of low-velocity impact damage in vascularised
10 composites. Composites Part A: Applied Science and Manufacturing 2013;44:78-85.
11 doi:10.1016/j.compositesa.2012.08.022.
12 [31] Yin T, Zhou L, Rong MZ, Zhang MQ. Self-healing woven glass fabric/epoxy composites with
13 the healant consisting of micro-encapsulated epoxy and latent curing agent. Smart Materials
14 and Structures 2008;17:015019. doi:10.1088/0964-1726/17/01/015019.
15 [32] Yin T, Rong MZ, Zhang MQ, Zhao JQ. Durability of self-healing woven glass fabric/epoxy
16 composites. Smart Materials and Structures 2009;18:074001. doi:10.1088/0964-
17 1726/18/7/074001.
18 [33] Tripathi G, Srivastava D. Effect of carboxyl-terminated poly(butadiene-co-acrylonitrile)
19 (CTBN) concentration on thermal and mechanical properties of binary blends of diglycidyl
20 ether of bisphenol-A (DGEBA) epoxy resin. Materials Science and Engineering: A
21 2007;443:262-9. doi:10.1016/j.msea.2006.09.031.
22 [34] Thomas R, Yumei D, Yuelong H, Le Y, Moldenaers P, Weimin Y, et al. Miscibility,
23 morphology, thermal, and mechanical properties of a DGEBA based epoxy resin toughened
24 with a liquid rubber. Polymer 2008;49:278-94. doi:10.1016/j.polymer.2007.11.030.
25 [35] Bagheri R, Marouf BT, Pearson R a. Rubber-Toughened Epoxies: A Critical Review. Polymer
26 Reviews 2009;49:201-25. doi:10.1080/15583720903048227.
27 [36] Wang Q, Storm BK, Houmoller LP, Vej NB. Study of the Isothermal Curing of an Epoxy
28 Prepreg by Near-Infrared Spectroscopy. Journal of Applied Polymer Science 2003;87:2295-
29 305.
30 [37] Cantwell WJ, Morton J. The significance of damage and defects and their detection in
31 composite materials: A review. The Journal of Strain Analysis for Engineering Design
32 1992;27:29-42. doi:10.1243/03093247V271029.
33 [38] Lundmark P, Varna J. Damage evolution and characterisation of crack types in CF/EP
34 laminates loaded at low temperatures. Engineering Fracture Mechanics 2008;75:2631-41.
35 doi:10.1016/j.engfracmech.2007.03.004.
36 [39] Norris CJ, White JAP, McCombe G, Chatterjee P, Bond IP, Trask RS. Autonomous stimulus
37 triggered self-healing in smart structural composites. Smart Materials and Structures
38 2012;21:094027. doi:10.1088/0964-1726/21/9/094027.
1 [40] Minakuchi S, Sun D, Takeda N. Hierarchical system for autonomous sensing-healing of
2 delamination in large-scale composite structures. Smart Materials and Structures
3 2014;23:115014. doi: 10.1088/0964-1726/23/11/115014.
4 [41] ASTM International. ASTM Standard D 638, 2003, "Tensile Properties of Plastics," ASTM
5 International, West Conshohocken, PA, 2003, DOI: 10.1520/D0638-10, www.astm.org. n.d.
6 [42] ASTM International. ASTM Standard D 5868, 2001, "Lap Shear Adhesion for Fiber
7 Reinforced Plastic (FRP)," ASTM International, West Conshohocken, PA, 2003, DOI:
8 10.1520/D5868, www.astm.org. n.d.
9 [43] Airbus. AITM 1-0019 Determination of lap shear tensile strength of composite joints. In-plane
10 shear by Lap-Shear. Airbus Internal Test Method n.d.
11 [44] ASTM International. ASTM Standard D 5528, 2007, "Mode I Interlaminar Fracture
12 Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites," ASTM
13 International, West Conshohocken, PA, 2003, DOI: 10.1520/D5528, www.astm.org. vol. 01.
14 n.d.
15 [45] ASTM International. ASTM Standard D 3039-14, 2014, "Standard Test Method for Tensile
16 Properties of Polymer Matrix Composite Materials," ASTM International, Conshohocken, PA,
17 2003, DOI: 10.1520/D3039, www.astm.org. n.d.
18 [46] Dow Plastics. Liquid Epoxy Resins. 1999.
19 [47] Tripathi G, Srivastava D. Effect of carboxyl-terminated poly(butadiene-co-acrylonitrile)
20 (CTBN) concentration on thermal and mechanical properties of binary blends of diglycidyl
21 ether of bisphenol-A (DGEBA) epoxy resin. Materials Science and Engineering: A
22 2007;443:262-9. doi:10.1016/j.msea.2006.09.031.
23 [48] Petrie EM. Epoxy Adhesive Formulations. 1st ed. McGraw-Hill Professional; 2005.
24 [49] Pritchard G (Polymer S and T. Plastics Additives: An A-Z Reference. 1st ed. Chapman & Hall;
25 1998.
26 [50] Tong L. Strength of adhesively bonded single-lap and lap -shear joints. Int J Solids Structures
27 1998;35:2601-16.
