Self-healing composites: A state-of-the-art review

Composites Part A 121 (2019) 474–486

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Composites Part A journal homepage: www.elsevier.com/locate/compositesa

Review

Self-healing composites: A state-of-the-art review a,⁎

b

b

Nand Jee Kanu , Eva Gupta , Umesh Kumar Vates , Gyanendra Kumar Singh a b c

T c

S.V. National Institute of Technology, Surat, India Amity University, Uttar Pradesh, India Technical and Vocational Education and Training Institute, Ethiopia

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-healing Carbon nanotube sheets (CNS) Graphene Microcapsules

Failure happens in composites after prolonged degradation process due to micro cracks. Thereafter repairing is not possible at remote locations in the way to enhance reliability and endurance of composites. Self-healing composites are fabricated to heal cracks and damages if there will be any to restrict failure and enhance the longevity of structures. Therefore, maintenance task is quite simplified. This review aims to provide summarized information on the recent developments in the field of self-healing composites. Initially, fabrication and characterization techniques have been reviewed as much as possible for self-healing carbon fiber laminates, microcapsules containing rejuvenator along with graphene/hexamethoxymethylmelamine (HMMM) hybrid shells and supramolecular elastomer. This paper also outlines numerical methods in its middle section to explore functionality recoveries in self-healing composites and to study improvement in mechanical properties of these smart composites. Thereafter, applications of shape memory alloy and shape memory polymer in advanced CNTs reinforced self-healing composites are also discussed. Composite material with carbon nanotube sheets (CNS) is discussed as sustainable self-healing material as it can maintain its temperature-similar to living species. Future application will be based on these smart self-healing composites and thus it becomes important to us to compile this review article.

1. Introduction Self-healing composites can restore their structural integrity during failure e.g. self-healing abilities in living species. In composites, the long-time degradation process results into micro cracks which in turn causes a failure [1]. Thereafter, repairing is indeed needed to enhance reliability and endurance of composites [2–8]. However, incorporation of self-healing properties in composites may not accomplish the selfhealing task unless not triggered externally. Based on that, self-healing is classified in two groups: (a) autonomic (without intervention); and (b) non autonomic (with human intervention). After embodiment of encapsulated healing agents into polymer matrix, self-healing capabilities can be improved thereby (as demonstrated in Fig. 1). While designing self-healing composites, discharge of healing agents and other concerned factors should be controlled properly. Microcapsule embedment or microencapsulation is related to enclosing micron-sized particles, which in turn isolates and assures these from the external environment [9–12]. In literature [13,14] it is found that the microencapsulated healing agents are used for polyester matrix to accomplish self-healing task. It is in 2001, when Prof. Scot White has practically demonstrated about self-healing composites [15]. Morphology of

⁎

encapsulated dicyclopentadiene (DCPD) and Grubb’s enzymes have been demonstrated in literature [16–19]. When dicyclopentadiene (DCPD) is made association with the Grubbs’ enzymes which is diffused in the epoxy resin, a ring like opening metathesis polymerization (ROMP) [20,21] is initiated and a eminently cross-linked tough polycyclopendiene is thereafter forming which actually heals the damage. In this way significant improvement in terms of fracture can be achieved as compared to the original specimen [18]. Further, authors have involved encapsulated catalyst as reported [22]. Keller et al. [23] have induced polydimethylsiloxane (PDMS)-based self-healing elastomers incorporating two kinds of microcapsules such as resin pellet and an initiator pellet. Impact of size of microcapsules on the self-healing ability is further studied by White et al. [24]. In literature, White et al. have demonstrated about fabrication of self-healing polymer laminates instead of using catalysts [25]. After this, many researchers around the globe have reported in this field [26–31]. Yin et al. have synthesized two-component healing system which consists of urea–formaldehyde microcapsules containing epoxy (30–70 μm in diameter) and CuBr2 (2MeIm)4 (imidazole metal salt complex) latent hardner to provide to provide self-healing ability in epoxy-based composites. Here, latent hardner is quite soluble in epoxy and the cracked sites are repaired

Corresponding author. E-mail address: [email protected] (N.J. Kanu).

https://doi.org/10.1016/j.compositesa.2019.04.012 Received 22 November 2018; Received in revised form 4 February 2019; Accepted 9 April 2019 Available online 09 April 2019 1359-835X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Self-healing formula using encapsulated microcapsules [1]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

self-healing concept, alternative approach is explored by White et al. [44,45] which rely on a centralized grid (microvascular grid) for distribution of healing agents into polymeric systems in an uninterrupted pathway. However, the manufacturing process is quite tedious and in that approach it is problematic to get artificial composites with networks for many applications. Polymeric systems with microvascular grids have been fabricated by incorporating chemical catalysts. Upon healing the polymer and removing the scaffold, the healing agent will be wicked into the microvascular routes [46–49]. After initiation of ring opening metathesis polymerization (ROMP); eminently cross-linked tough polycyclopendiene (as demonstrated in Fig. 3) is conceived which actually heals the damage [20,21].

through curing of released epoxy [32]. n case of microcapsule-based self-healing approach, uncertainty in accomplishing partial healing is of much concern as it has limited amount of healing agent. It is also not known when healing agent will be finished completely. To accomplish this task, another type of liquid healing agent is developed as hollow fiber embedment by Dry and coworkers [13,33,34]. Large diameter capillaries are embedded into resins by Motuku et al., but these iterations are eventually failed [35]. Belay et al. however, have utilized hollow glass fibers so called Hollex fibers filled with resin [36]. On the other hand Bond and coworkers have managed to revise the manufacturing of hollow glass fibers [37] and utilized fibers as the capsules for liquid healing agents [38–43]. The diameter of borosilicate glass fibers are ranging from 30 to 100 μm and reported to have space around 55%. These hollow fiber-based selfhealing composites (as shown in Fig. 2) containing hollow fibers, can restore healing agents which in turn could heal around 97% of its basic flexural strength. Advantages of such self-healing material fabrication include availability of greater volume of healing agent to heal damage and also possible usages of various activation types of resin as well as feasibility of visual detection of the damaged site, etc. However limitations of this approach will be based on the fact that fibers have to be broken to discharge the healing agent and low-viscosity resin is utilized to facilitate fiber infiltration as well as further application of hollow glass fibers in CFRP laminates will head to coefficient of thermal expansion (CTE) discrepancy which in turn is an issue. To avoid the short supply of a healing agent in microcapsules-based

2. Fabrication and characterization of self-healing composites Cross-linking of polymeric composites is accomplished to get improved mechanical attributes such as high stiffness, solvent resistance, thereby improved fracture toughness which in turn can resentfully disturb the healing efficiency of polymers. These composites are brittle indeed and have the tendency to get crack [50–52]. Ghezzo et al., have fabricated carbon fiber composites using thermally reversible eminently cross-linked polymeric matrices through a custom resin transfer molding (RTM) method. Here, the laminate resin is bis-maleimide tetrafuran (2MEP4F) which is incorporated after combining monomers such as furan (4F) and maleimide (2MEP) and that too at high temperatures. Further, resulting material quality and 475

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Fig. 2. Self-healing approach using hollow fibers [1]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

which contain rejuvenator along with graphene/hexamethoxymethylmelamine (HMMM) hybrid shells, using two-stride self-assembly procedure as demonstrated in Fig. 6. As depicted, the entire action is split within four strides. At first, a copolymer of styrene maleic anhydride (SMA) powder is mixed with water at 50 °C and pH value is adjusted to 10 using NaOH solution. Further the rejuvenator is emulsified for 10 min as demonstrated in Fig. 6 (a, b). In second stride, HMMM is combined drop wise into the mixture with a 400 r.min−1 stir as demonstrated in Fig. 6 (c, d) to initiate polymerization and further forming into a shell in third stride. HMMM prepolymer as well as graphene mixture is combined with a 300 r.min−1 stir and then temperature is elevated to 80 °C. They have waited till 2 h while reducing temperature to 20 °C as demonstrated in Fig. 6 (e, f). At the end in fourth stride, they have washed microcapsules with pure water after filtering it out of the mixture as demonstrated in Fig. 6 (g) [54]. Further, composites in emulsion are characterized using biological microscope while SEM is utilized to characterize the outer of dried microcapsules. Shell thickness is measured using ultramicrotomy and Fourier transform infrared spectra are utilized to characterize the chemical structure of microcapsule specimen. Also, Walther et al. have synthesized self-healing supramolecular elastomers after mixing supramolecular pseudo-copolymer and graphene as demonstrated in Fig. 7 [55]. Here, graphene derivatives are mixed with polymers by ultrasonic dispersion. Graphene and monomer are mixed and polymerized finally by addition of initiators under in situ polymerization approach [56–68]. It is also reported about synthesis of intrinsically healable, reduced graphene oxide (RGO)-reinforced polymer film using layer-by-layer (LBL) self assembly method. The LBL process is adaptable approach to synthesize nanocomposites [69,70]. The branched poly(ethyleneimine) is grafted with ferrocene groups (bPEI-Fc) and reduced graphene oxide is modified with β-cyclodextrin (β-CD) (represented as RGO-CD) for complexes as demonstrated in Fig. 8 (b, c).

Fig. 3. ROMP of DCPD [20]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

healing efficiency have been characterized using noninvasive X-ray phase contrast micro-tomography and in turn they have been quite successful in partial healing of extended cracks which has occurred in specimen that are kept at 90 °C for 2 h as demonstrated in Fig. 4 [53]. Through their research the authors have clearly demonstrated that the material will display a drop of stiffness when the temperature would way the glass transition temperature of the matrix polymer. It shows that an ideal healing cycle should be carried at temperatures that must not affect the architectural integrity of the composites as demonstrated in Fig. 5 where material storage modulus, E′, normalized with its value at room temperature, E′(RT), and Tan Delta (loss modulus/storage modulus) have been plotted as functions of temperature [53]. Xinyu Wang et al. have demonstrated synthesis of microcapsules 476

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Fig. 4. RTM synthesis of self-healing CFRP composites [53]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Self-consistent field/density functional theory model is preferred while utilizing nanoparticles to model self-healing panels [6,82]. Freeenergy of system can be demonstrated using Eq. (2)

Ffree = Fdiblock + Fparticle entropic + Fenthalpic

(2)

where free energy of diblock entropic is demonstrated using Eq. (3)

Fdiblock = (1 − ϕP ) ln[V (1 − ϕP ) Qd, partition] − (1 V )

∫ dc [wA (c)field φA (r ) + wB (c)field φB (c)]

(3)

where Qd, partition is partition function subject towA (c ) and wB (c ) fields Particle entropic contributions to free energy is demonstrated using Eq. (4) Fig. 5. Dynamic mechanical analysis of [0/90/0] 2MEP4F laminated composite at 1 Hz and temperature rate of 3 C/minute [53]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fparticle entropic =

− (1 V )

(4)

where Qp is partition function subject to ρp (c ) ; ρp (c ) is particle centre circulation; ψFMT is setric energy contribution; α is diblock-to-particle volume ratio Enthalpic reactions are demonstrated using Eq. (5)

Healing efficiency of any self-healing composites [71], can be written using Eq. (1)

fhealed − fdamaged fvirgin − fdamaged

∫ dc [wp (c) ρp (c)particle centre

− ρp (c )particle centre ψFMT (c )]

3. Functionality recoveries in self-healing composites

η=

ϕP ⎛ Qpartition α ⎞ ln ⎜ ⎟ α ⎝ VϕP ⎠

(1)

Fenthalpic = (1 V )

∫ dc [χAB NφA (c) φB (c) + χAP NφA (c) φP (c)

+ χBP NφB (c ) φP (c )]

where f is any property of interest such as fracture toughness, peak fracture load, strain energy, etc. It has been found that cracks are healed using borosilicate glass and composite manufactured so far, is found to have improved resistance to oxidation. Dispersed thermoplastic particles have been used to heal wide-open cracks [72]. This self-healing process is known as close-thenheal because wide-open cracks are closed after utilizing the shape memory effect of the shape memory polyurethane (SMPU) fibres prior to healing. Composites used in aerospace applications are critically assessed for their self-healing abilities using Eq. (1). Further, the equation holds good for fracture toughness improvement too. Investigations are also done for impact and fatigue improvement and 75% with respect to damage load of a microcapsules (which consist of epoxy resin) is recovered [73,74]. Fracture toughness improvement is referred more than peak damage load improvement to analyze healing capability. The microcapsules-based healing approach for low-velocity impacts is quite popular and analyzed in many research papers so far [75–80]. Peak load (53%) and energy to peak load (86%) as improvement after a further impact, are observed. Further, investigations have been carried out on high-velocity impacts to find whether a self-healing ionomeric polymer would be an alternative to aluminium plies (for protecting space structures from space debris) [81]. Researchers have been successful in their attempts as damages are healed significantly.

− (1 V )

∫ dr [HA (c) NφA (c) + HB (c) NφB (c)

+ HP (c ) NφP (c ) φP (c )]

(5)

where χi, j is limit between i, and j ; HA (c ), HB (c ), and HP (c ) represent A, B and particle exteriors synergy one at a time. Stiffness, E and Poisson’s ration, v as per lattice spring model is demonstrated using Eq. (6)

Estiffness =

[5k (2k + 3c ] −(k − c ) ; v= (4k + c ) (c + 4k )

(6)

where k is central force constant and c is noncentral force constant that define elastic properties of model Crack damage is healed time and again [83–85] after supply of regular stock of healing agents in self-healing composites. Fracture toughness analysis is done using four point bending equation for first crack as demonstrated in Eq. (7)

Kk =

EC3 t ⎛ ⎝

P (a − b) y ⎞2 lim Y (D) 4I ⎠ D→0

(7)

where E is stiffness of layer/membrane; t is thickness of layer; P is applied force when damage is happened; a, and b are spacing between 477

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Fig. 6. Self-healing microcapsules fabricated using two-stride self-assembly method [54]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Illustration of mixing of diaminotriazine (DAT) functionalized polyglycidols (PG) and cyanuric acid (CA) functionalized PG. Thermally reduced graphene oxide (TRGO), is combined to heated supramolecular pseudo-copolymer, which enhances photothermal effect [55]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 478

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Fig. 8. (a) Synthesis of self-healing antideterioration coating [69], (b) composites of bPEI-Fc, PAA and (c) RGO-CD, and synthesis and healing of PAA/ bPEI-Fc and RGO-CD films [70]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4. Applications

exterior and interior supports; I is moment of inertia of beam of composite. Healing efficiency of crack in this case is demonstrated using Eq. (8)

KIcHealed KIcVirgin

η=

Self-healing polymer composites are used as an alternative to metal alloys for protecting space structures from space debris [81]. It has been found that with the addition of shape memory alloy (SMA) in selfhealing carbon composites, damage tolerance under repeated impacts is reduced. Therefore, use of SMA wires is an encouraging approach to render better impact resistance to aerospace GFRP laminates, but it is not necessarily for CFRP composites. It is evident that self-healing coatings are promising as these would automatically recover any damage. Self-healing coatings are nowadays having applications in protecting aircraft panels from corrosion and minor impacts [87]. After combining a shape memory polymer with carbon nanotubes (CNTs) and a cross ply (0/90/90/0) carbon fabric, self-healing CFRP laminates are fabricated using a high-pressure molding mechanism. It is reported to have improved fatigue resistance and therefore, it is used as an aerospace material nowadays [88]. Graphene is a two-dimensional sp2 c-atom and has improved electrical as well as thermal conductivity. It is quite suitable for application in surface coating, electrical devices and biological as well as pharmaceutical area [89–103]. Under distinct DC voltage circumstances such as 40–70 V range, electrically triggered shape improvement ratio

(8)

Further, equation is simplified and demonstrated using Eq. (9)

η=

P Healed PVirgin

(9)

Ratio of interfacial shear strength to brick fracture strength [86] is demonstrated using Eq. (10)

w (σ − σO ) ⎛ ⎞ = B τO ⎝ h ⎠opt

(10)

where σB, σO, and τO are tensile fracture strength, peak tensile and shear cohesive forces of the cement The tensile fracture strength is given using Eq. (11)

σc =

1 (σB + σO ) 2

(11) 479

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(a) Poly(vinyl alcohol)/graphene (wt%) represented as

(b) Poly(vinyl alcohol)/graphene (wt%) represented as

[PVAc/Gr (3%)]

[PVAc/Gr (4.5%)]

Fig. 9. Faster improvement response with increase in graphene content [104]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. (a) Demonstration of laminate E, (b) Illustration of laminate C, (c) CNT porous ply, (d) random-discontinuous cotton fibres and CNT ply, and (e) woven carbon fibres and CNT ply [105]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

[105]. Vaporization of sacrificial components (VaSC) and resin infusion have been utilized to fabricate fibre-reinforced composites (FRCs) incorporating hollow vessels and heating components. Manually embedment of Poly (lactic acid) PLA conciliatory fibres such as 300 μm of VascTech fibres, has been incorporated in eight plies of woven glass fibres having area density of 290 g m−2 for respective ply in a squarewave-like pattern. Layer by layer deposition of the reinforced fibres and remaining untreated reinforced fibres, and conductive plies in the particular sequence (as demonstrated in Fig. 10), has been done [105]. In Fig. 10, two kinds of laminates such as random-discontinuous cotton-FRCs (type E with sub groups E1 and E2) having four sheets of cotton breather tissue, and woven carbon-FRCs (type C with sub groups C1 and C2) having four sheets of woven carbon fibres; have been tested out. Where E1 is random-discontinuous cotton string laminates and E2 is random-discontinuous cotton string laminates embedded with porous CNT layer, however C1 is woven carbon fiber laminates and C2 is woven carbon fibre laminates embedded with porous CNT ply [105].

is plotted against recovery time as demonstrated in Fig. 9 (a, b) [104]. It is found that increase in graphene content initiates recovery response. 5. Sustainable self-healing at ultra-low temperatures in structural composites In self-healing composites impressive healing efficiencies can be achieved when circumstances are favorable. Under adverse conditions, healing might not be possible as observed in case of drastic low ambient temperature. In the report authors have demonstrated about structural composite which is quite capable to maintain its temperature to bring sustainable self-healing efficiency as seen in case of some animals who maintain a constant temperature to allow enzymes to remain active. Certain laminate has been embedded with three-dimensional hollow vessels with the ambition of transporting and discharging healing agents, and further a porous conductive material to provide heat internally to defrost and promote healing reactions. The authors have claimed to achieve a healing efficiency over 100% at about −60 °C 480

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Fig. 11. (a) Temperature, sample in relation to time and voltage, (b) thermal distribution in the laminate baked by CNS in an ultra-low temperature environment, and (c) de-icing laminate with CNS [105]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 12. (a) Healing efficiency of laminate having CNS and CFS, and (b) displacement-load relation for CNS specimen [105]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

electricity to increase the interior temperature. That too is difficult as the copper foam has less resistivity. More electrical current of 55 A (in this case) is supposed to be required to generate the heat essential to keep the healing agent active in the way. After 24 h of healing at around −60 °C, the recovered mechanical properties of the samples are studied (as demonstrated in Fig. 12 (a, b)). At the end authors have concluded that for laminate with CNS, an average healing efficiency of 107.7% in terms of fracture energy and 96.22% in terms of peak load could be achieved. The maximum healing efficiency for fracture energy is found to be 141% (Fig. 12). In this way these results indicate that the laminates with CFS/CNS are having capabilities to self-heal at ultra-low temperatures [105]. Few self-healing circumstances have been discussed in case of

The steady-state temperatures of the samples have been maintained in the range 20–85 °C as demonstrated in Fig. 11 (a) after keeping voltage in the range 10–16 V. It is sufficiently high to enable healing in 24 h as demonstrated in Fig. 11 (b). As a result of the utilization of a low current such as 200 mA and its nice thermal conductivity, there is no severe heat concentration. Fig. 11 (c) shows that the laminate is able to be completely de-iced in 90 s and thereafter it is demonstrating the efficiency of carbon nanotube sheet (CNS) [105]. It is found that the laminate with copper foam sheet (CFS) is capable to maintain the temperature in the range 5–20 °C. It is also reported about design using CFS as 24 h is not long enough for it to heal completely, in spite of the healing agent is effective in the temperature range. One possible solution to this problem is raising the power of

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oven for 2 h [106]. Also, researchers have fabricated ultrafast infrared (IR) laser-triggered self-healing laminate that can heal itself on increasing temperature from 30 °C to 150 °C in few seconds [107] as demonstrated in Fig. 13. As functionalized graphene nanosheets (FGNS) have IR absorbing capacity. Using microwaves, self-healing can be accomplished as reported in case of covalently cross-linked reduced functionalized GO/PU laminates [108]. Self-healing is also achieved using other wavelengths of radiation as found in case of gold nanoparticles reinforced poly(ε-caprolactone) which have been coated using RGO and silver nanowires [109]. Here, 91% improvement is achieved in terms of exterior conductivity and tensile strength. Even self-healing can be accomplished at room temperature as reported in case of a mussel-inspired electroactive chitosan/GO laminate hydrogel and in this way it becomes more comprehensive [110–114]. A thermo-reversible elastomer (HBN-GO) has been synthesized and is reported to have efficient self-healing efficiency at room temperature without any enzymes [114]. Further, stress-strain curves have been plotted and tensile strength is examined for different specimens as demonstrated in Fig. 14. Here, 60% improvement is achieved in terms of tensile strength which is indeed remarkable improvement. Hydrogel based on β-cyclodextrin (β-CD) and N,N-dimethylacrylamide could heal itself at 37 °C and it has application as anticancer drug carrier [63]. Because camptothecin (CPT) contents (as loaded and cumulated) when discharged in β-CD, are found to be better than pristine

Fig. 13. FGNS nanocomposite is self-healed to 96% in terms of stiffness [107]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

graphene/polymer laminates to restore initial properties. Silicone rubber (SR)-graphene nanoplatelets (GNPs) laminate has been manufactured which can heal itself by thermal annealing, up to 250 °C in any

Fig. 14. Stress-strain curves of self-healing laminates (a) HBN-1% GO, (b) HBN-2% GO, (c) HBN-4% GO at room temperature consequent to distinct healing time; (d) stress-strain curves of HBN-2% GO consequent to 10 min healing time [114]. 482

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Fig. 15. (a) Polyborosiloxane (PBS) having discharged cumulate CPT [63], (b) dorsal muscle having hydrogel electrodes implanted into it and electrodes are connected with sensors which can detect signal, (c) hydrogel, (d) illustration of recording of signal using hydrogel electrodes from rabbit’s muscle when troubled [115]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Repairing of such structural materials at remote locations is tedious task. However, self-healing composites can be used in such conditions to ensure longevity of structures. Self-healing materials have specific potential to initiate running maintenance of micro cracks. Self-healing capabilities are improved by microencapsulation of healing agents into polymer matrix. Damage is healed through eminently cross-linked tough polycyclopendiene which is formed after initiation of the ring like opening metathesis polymerization (ROMP). Self-healing ability is also achieved in epoxy-based composites after the implementation of two-component healing system which consists of urea-formaldehyde microcapsules and CuBr2 latent hardner. To overcome the limitations of microcapsule-based self-healing approach (as discussed in Section 1), hollow glass fibers are utilized as the capsules for liquid healing agents. In this way, basic flexural strength is restored up to 97% after successful healing. Further, a centralized microvascular grid has been developed for distribution of healing agents into polymeric systems in a continuous pathway to avoid the limitations of short supply of a healing agent in microcapsules-based self-healing concept. Fabrication of carbon fiber laminates using thermally reversible highly cross-linked polymeric matrices through a custom resin transfer molding (RTM) method has been done successfully to partially heal the extended cracks. Further, X-ray phase contrast micro-tomography has been used in this case (as discussed in Section 2), to characterize material quality and healing efficiency. Synthesis of microcapsules which contain rejuvenator along with graphene/hexamethoxymethylmelamine (HMMM) hybrid shells using two-stride self-assembly procedure has been done. Further, characterizations have been done using biological microscope, SEM for the outer of dried microcapsules, ultramicrotomy for measurement of shell thickness and Fourier transform infrared spectra for the chemical structure of microcapsule specimen in this case (as discussed in Section 2). Synthesis of self-healing supramolecular elastomers has been done after mixing supramolecular pseudo-copolymer and graphene by

graphene hydrogel as demonstrated in Fig. 15 (a). Self-healing hydrogels are quite effective while healing injury spontaneously [115] as illustrated in Fig. 15 (b, c, and d). 6. Future scopes Indeed it is fact that self-healing composites have bright future in the field of innovative product research. Many researchers are putting their efforts to recover functional properties in materials after healing the damages through these smart composites (as discussed in Table 1). Still, the field of self-healing composites has few limitations in understanding healing mechanism and thereby stability of healing functionality. Identification of damages and further healing are main challenges for the self-healing composites. After survey, it is impossible to ignore graphene/polymer laminates which are having self-healing capabilities in spite of many challenges towards their use in practical applications. Since, improvement in selfhealing capabilities and mechanical properties, both are two different conditions. Therefore, maintaining balance between these in a graphene/polymer laminate is still a challenging task. Also, it is equally essential to improve compatibility of polymer and graphene after modification of graphene, while retaining natural properties of graphene as much as possible. This review also encourages researchers to investigate improvement in impact resistance in aerospace composites after addition of shape memory alloy (SMA). Carbon fiber based self-healing composites are required to be used in space applications to overcome fatigue, micro cracks and impact resistance related issues. 7. Conclusions It is evident that structural failures happen due to micro cracking and hidden damages in materials. Further, high maintenance charges restrict the acceptance of various composites in different applications. 483

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[107] [108] [110]

[106]

ultrasonic dispersion. While under in situ polymerization mode, Graphene and monomer are mixed and polymerized afterwards by addition of initiators. On the other hand, synthesis of intrinsically healable, reduced graphene oxide (RGO)-reinforced polymer film is achieved using layer-by-layer (LBL) self assembly method (as discussed in Section 2). Numerical methods have also been discussed briefly to explore functionality recoveries in self-healing composites. Initially, Self-consistent field/density functional theory model is preferred while utilizing nanoparticles to model self-healing panels as shown in Eq. (2). Elastic properties such as stiffness and poison’s ratio can be calculated after substituting suitable values of central and noncentral force constants as shown in Eq. (6). Fracture toughness can be analyzed using four point bending equation as shown in Eq. (7). Further, tensile fracture strength is studied using Eq. (11). Above all, these equations are very helpful while investigating improvement in mechanical properties of selfhealing composites (as discussed in Section 3). Self-healing composites are reported to have improved fatigue resistance after addition of certain new materials such as shape memory polymer with carbon nanotubes (CNTs) and cross-ply carbon fabric. Damage tolerance under repeated impacts is reduced after addition of shape memory alloy (SMA) in self-healing carbon composites. Nowadays, self-healing coatings are used to protect aircraft panels from corrosion and minor impacts (as discussed in Section 4). The review is compiled to have much enlightenment to the research in the field of self-healing composites. For this, research progress of selfhealing composites have been reviewed to motivate researchers towards sustainable self-healing at ultra-low temperatures in structural composites, light triggered self-healing action in functionalized graphene nanosheets (FGNS), study of rapid and efficient self-healing thermo-reversible elastomer (HBN-GO) through various stress-strain curves, and self-healing hydrogels applications as biomaterials to heal injury spontaneously. Self-healing ability has been considerably achieved at ultra-low temperatures after addition of vessels and a porous conductive layer into composite materials. The composite materials with CNS can self-heal cracks more effectively. Wang Y et al. have achieved sustainable self-healing efficiency over 100% in fracture energy and 96.22% in peak load in FRCs at temperature about −60 °C (as discussed in Section 5).

Tensile strength Stress and Young’s modulus Compressive stress IR Microwaves Contact (room temperature)

96% of tensile strength 93% (mechanical properties) –

87% of tensile strength

8. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding

Diels-Alder chemistry Diels-Alder chemistry π-π stacking, hydrogen bonds

Nand Jee Kanu, Research Scholar, S. V. National Institute of Technology, Surat, India; Eva Gupta, Research Scholar, Amity University, Uttar Pradesh, India; Umesh Kumar Vates, Associate Professor, Amity University, Uttar Pradesh, India; and Gyanendra Kumar Singh, Faculty, Technical and Vocational Education and Training Institute, Ethiopia; have not been funded in any way to carry out the review activities. Authors contribution statements Nand Jee Kanu, Research Scholar, S. V. National Institute of Technology, Surat, India, is pursuing PhD in Mechanical Engineering and has reviewed and compiled the work. Eva Gupta, Research Scholar, Amity University, Uttar Pradesh, India, is pursuing PhD in Electrical Engineering and has reviewed papers on above topic Umesh Kumar Vates, Associate Professor, Amity University, Uttar Pradesh, India has completed his PhD in Mechanical engineering from IIT Dhanbad (An Institute of National Importance). His role is as expert

Graphene/PU RFGO/PU composites Chitosan/GO Hydrogel

SR/GNP composite

Reversible bonds

Tensile strength

Fluid mechanics and machine tool control system Flexible electronics Strain sensors Electroactive tissue engineering applications

[57] Robotics Tensile strength PAA-GO-Fe3+ Hydrogel

Hydrogen bonds, electrostatic interaction P(AM-co-DAC)/GO Hydrogels

Ionic binding

Contact and immersed in FeCl3/ HCl Thermal annealing

[56] –

> 92% of tensile strength, > 99% of tensile strain, and > 93% of toughness Almost 100% Tensile strength, tensile strain, and toughness Drop water

Reference Applications Healing efficiency (%) Property of interest Self-healing condition Mechanism of self-healing Material

Table 1 Recovery of functional properties after using self-healing composites in various applications.

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