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Toughening effect of nanocomposite-wall microcapsules on the fracture behavior of epoxy
Polymer 168 (2019) 104–115
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Toughening effect of nanocomposite-wall microcapsules on the fracture behavior of epoxy
T
Minoo D. Shokriana, Karim Shelesh-Nezhada,∗, Reza Najjarb a b
Division of Plastics and Composites Engineering, Department of Mechanical Engineering, University of Tabriz, Tabriz, 5166616471, Iran Polymer Research Laboratory, Faculty of Chemistry, University of Tabriz, Tabriz, 5166616471, Iran
H I GH L IG H T S
-CNT nanocomposite-wall microcapsule was synthesized using a new approach. • NH for obtaining calibration parameters of TDCB specimen were conducted. • Analyses capsules enhanced the fracture toughness of epoxy composite. • Hollow properties of epoxy significantly improved by adding NH -CNT microcapsules. • Fracture • Capsules developed crack tailing, plastic deformation and toughened the epoxy. 2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite-wall microcapsule Hollow microcapsule Epoxy NH2-CNT TDCB Fracture toughness Fracture energy
In this research the reinforcing effect of nanocomposite-wall microcapsules on the fracture toughness and fracture energy of epoxy under mode I was studied. The fracture specimens with the geometry of localized tapered double cantilever beam (TDCB) were used to take the advantage of crack length independent-measurement in determining the fracture properties. The crack length investigations were performed experimentally and numerically to extract the calibration parameters and also evaluate the optimal crack length of TDCB specimens. Three types of microcapsules including hollow microcapsule (CH), ethyl phenylacetate core microcapsule (CEPA) and NH2-CNT reinforced-wall microcapsule (CCNT) were synthesized and used as reinforcements. The scanning electron microscopy study of synthesized microcapsules proved the successful presence of CNTs in the shell and on the wall of CCNT microcapsules. The hollow capsule-loaded epoxy composite exhibited enhanced fracture properties as compared to the neat epoxy. Moreover, the SEM study of fracture surface of CH reinforced specimen revealed a characteristic crack tail toughening mechanism. The CEPA microcapsule decreased fracture properties of neat epoxy. The maximum improvement of fracture properties belonged to the specimen containing the CCNT microcapsules. It was observed that the composite-wall microcapsule was able to increase the fracture toughness and fracture energy of neat epoxy by 59% and 165%, respectively, due to the developments of considerable out-of-plane fracture and large plastic deformation.
1. Introduction Compounding different materials to introduce a new material with improved properties is always one of the attracting subjects, especially in the field of polymeric composites. The integration of fillers into polymers is accomplished with different aims such as decreasing cost, improving physical and mechanical performances, achieving selfhealing ability and so on. Polymeric materials and composites often experience micro and macro cracks in their applications. Nowadays, the fracture behavior of polymeric composites is considered to be one of
∗
their important mechanical properties. The fracture toughness and fracture energy of a composite in mode I are usually examined using compact tension (CT) and double cantilever beam (DCB) fracture tests. Mostovoy et al., in 1967 [1] proposed a tapered shape fracture specimen which was resulted from the combination of CT and DCB geometry specimens known as tapered double cantilever beam (TDCB) geometry. In the TDCB geometry, the fracture toughness and energy are independent of crack length and are proportional to the critical load [2]. In the next researches, side grooves with various angles were incorporated into the TDCB geometry to force the crack to grow along the
Corresponding author. E-mail address: [email protected] (K. Shelesh-Nezhad).
https://doi.org/10.1016/j.polymer.2019.02.027 Received 1 November 2018; Received in revised form 28 January 2019; Accepted 12 February 2019 Available online 13 February 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Capsules preparation setups of (a) CH, (b) CCNT, (c) CEPA microcapsules.
capsule/epoxy composite for two different sizes of UF-wall microcapsules. They established that the integration of microcapsules increased the virgin fracture toughness of epoxy due to crack pining and crack deflection mechanisms as well as crack tail formation. This increase in fracture toughness was higher in the case of smaller microcapsules. Moreover, slight improvements in the virgin fracture toughness of capsule/epoxy composite were reported by Jin et al. [9] and Ahangaran et al. [10]. On the other hand, according to Bailey et al. [11] the presence of microcapsules in the epoxy for the coating application caused heterogeneities in the matrix and reduced the tensile modulus and failure stress. It was also demonstrated by Jin et al. [6] that the addition of 117 μm-sized capsules (polyurethane–UF double-walled microcapsules) into epoxy reduced the virgin fracture toughness, particularly at higher capsule concentrations. Thus, reinforcing the traditional microcapsule with nanoparticles has been researched with the aims of improving mechanical properties of capsules and composites as well as developing the interfacial adhesion of capsules to the surrounding matrix. Chuanjie and Xiaodong [12] prepared clay reinforced UF-wall microcapsules. They improved the barrier properties of microcapsules and decreased the leakage of core material from the microcapsules. They also observed that the surface roughness of composite-wall microcapsule enhanced, and inferred that it could lead to the improvement of mechanical adhesion of the microcapsules to the surrounding matrix. Fereidoon et al. [13,14] synthesized nanocompositewall microcapsule reinforced with either carbon nanotube (CNT) or nano-alumina to improve the mechanical properties of traditional UFshell microcapsules. The elastic modulus of composites-wall microcapsules, extracted using nano-indentation technique, increased as compared to unreinforced capsules. Li et al. [15] demonstrated that by assembling functionalized-CNT on the wall of capsules, more hydroxyl groups were introduced owing to modification by dopamine, and this promoted the formation of covalent bonds between microcapsules and epoxy/hardener. To accurately study the toughening effects of microcapsules inclusion in polymer matrix requires an in-depth investigation of both chemical and mechanical aspects. In the present work, the virgin mode Ifracture properties of epoxy reinforced with various microcapsules were studied using TDCB specimens. In order to do this, unreinforced TDCB specimens with different crack lengths were examined both experimentally and numerically to calibrate the TDCB specimen and also extract calibration parameters for the further calculation of fracture properties. The fracture properties of neat epoxy extracted by TDCB test were also compared with those obtained using standardized CT test. Three types of microcapsules including UF-shell hollow microcapsule (CH), ethyl phenyl acetate (EPA) filled UF-shell microcapsule (CEPA) and EPA filled NH2-CNT/UF-shell microcapsule (CCNT) were synthesized as
Fig. 2. Geometrical parameters and dimensions of localized TDCB specimen (top) and localized part (bottom).(dimensions in [mm]).
center-line of specimen [3]. Some researchers also inspected the effect of different preparing methods of TDCB geometry, including laser-cutting and casting, on the resulted fracture toughness [4]. The TDCB geometry because of its advantage over the CT geometry has been recently incorporated to assess the self-healing phenomena in polymeric systems. Self-healing materials are defined as materials that are able to autonomically repair damage whenever and wherever it takes place. Though, different self-healing systems have been introduced so far, the capsule based self-healing materials are the common types of mechanical stimuli due to their simplicity and ease of preparation and application [5]. In these systems, the healing agent can be encapsulated in capsules and dispersed in a polymer matrix. However, the presence of microcapsules in a polymeric matrix, prior to the initiation of healing process, certainty affects the virgin mechanical properties of the matrix. The capsule size, shape, surface roughness, distribution as well as the nature of capsule core material (e.g. viscosity, surface tension, miscibility with shell material, etc.) affect the performances of polymeric matrix [6]. Brown et al. [7] established that the integration of urea formaldehyde (UF)-wall self-healing microcapsules increased the virgin fracture toughness of epoxy matrix as well as provided an efficient mechanism for self-healing. The improvement of fracture toughness was related to a change in the fracture plane morphology from mirror-like to hackle markings and increased subsurface micro-cracking. Blaiszik et al. [8] studied the fracture toughness of 105
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Fig. 3. (a) The silicone rubber molds for fabricating localized TDCB specimen, (b) placing localized part in the main mold before the second step of molding, (c) setup for TDCB testing, (d) CCNT reinforced TDCB specimen after breakage and (e) setup for CT testing.
the reinforcements. For the filled microcapsules, EPA solvent was applied as core, since this solvent has been used as a healing material in self-healing purposes in the literatures [16–18]. Fourier-transform infrared spectroscopy (FT-IR) was employed to verify the chemical structure of materials constituting the microcapsules. Field emissionscanning electron microscopy (FE-SEM) was utilized to study the morphology of microcapsules and also the fracture surfaces.
Table 1 Elastic properties of matrix. Material
Modulus (GPa)
Tensile strength (MPa)
Poisson ratio
Epoxy828
2.7 ± 0.09
42.82 ± 0.49
0.3a
a
[19].
2. Fracture theory of TDCB specimen According to the linear elastic fracture mechanics theory of Irwin and Kies, the strain energy release rate (G) over the thickness of propagating crack (b) is proportional to the change in compliance (C) with respect to crack length (a) variation. Therefore, fracture energy can be determined directly using the Eq. (1) [19].
G=
P 2 dC 2b da
(1)
where P is the applied load. The compliance (C) of the substrate beam is given by the crack opening displacement divided by the load (δ/p). The critical stress intensity factor, KIC, is also calculated using Eq. (2) [20].
K 2 IC = GIC E
(2)
where GIC is the critical fracture energy which is achieved by substituting PCritical for P in Eq. (1). The value of dC/da (Eq. (1)) for TDCB geometry is constant within a range of crack lengths which it can be precisely measured experimentally [4,21]. For the case of TDCB specimen with side grooves, an effective thickness, ben, is substituted for b in Eq. (1). According to Mostovoy assumption, the effective thickness can be defined by Eq. (3) [21,22].
Fig. 4. Illustration of TDCB boundary condition and refined mesh around the crack tip.
ben =
bbn
where bn is the thickness of grooved region. 106
(3)
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Table 2 Description and size of prepared urea-formaldehyde microcapsules. Average diameter ± SD (μm) 2.021 ± 0.357 1.210 ± 0. 931 1.021 ± 0.374
a b
Size range (μm) 1.794–3.690 0.664–4.126 0.269–2.059 30-42b (large)
Description
Shell
Hollow microcapsule Microcapsule containing self-healing core Composite-wall microcapsule containing self-healing core
a
UF UF UF/NH2-MWCNT
Core
Capsule code
– EPA EPA
CH CEPA CCNT
Urea-formaldehyde thermoset. Few microcapsules in corresponding size range was observed.
Fig. 5. FE-SEM images of: (a) CH and (b) CEPA microcapsules.
3. Materials and methods
800 rpm. Then, 4 aliquots of 3 ml distilled water (at 30 °C) were added to the solution every 15 min. In order to increase air bubbles in the solution which acted as molds for the formation of hollow capsules, argon (Ar) gas was bubbled into the solution during the capsule preparation. After continuous agitation at 800 rpm and in about 34–36 °C, for 2 h, the formed hollow microcapsules were rinsed with water and air dried. The filled microcapsules with EPA as the core material (CEPA) were produced using an in-situ polymerization technique in an oil-in-water emulsion. For preparing first solution, 0.75 g SDS, 0.2 g NH4Cl (3.7 mmol), 0.2 g resorcinol, 4 g formalin (49.4 mmol formaldehyde) and 50 ml sulfuric acid (0.5 M) were placed in a 100 ml round-bottomed flask and agitated for 5 min by magnetic stirrer to form a stabilized emulsion. Then, 3 g of EPA was gently added to the mixture to form an oil in water (O/W) emulsion. Next, 2 g (34 mmol) of urea was dissolved in 5 ml sulfuric acid (0.5 M) as the second solution and it was added dropwise into the reaction mixture (first solution) while stirring. Finally, the mixture was stirred at 1200 rpm for 24 h. After completion of UF-resin curing the resulting dispersion was centrifuged to separate capsules from the solution as well as sediment of polymeric particles. The resultant microcapsules were rinsed with distilled water, vacuumfiltered and air dried in ambient condition for 2 days. Nanocomposite-wall microcapsules (CCNT) were synthesized using almost the same conditions as described for CEPA. The first solution was prepared by the same procedure as for CEPA, but the second solution was prepared by dissolving of 0.15 g CNT, 0.3 g SDS and 2 g urea in 15 ml of H2SO4 (0.5 mol) in the beaker. Then, the solution was sonicated by tip of a probe type ultrasonic homogenizer (model of HD 3200, Bandelin) for 6 min at 50% amplitude (horn specific maximum amplitude of 170 μm) to achieve proper dispersion of CNTs. Later on, this solution (as the second solution) was slowly added to the emulsion containing formalin (the first solution as described for CEPA), while agitation. Finally, the encapsulation was accomplished under
3.1. Materials KER 828 epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) (Kumho P&B Chemicals, South Korea) and triethyltetramine (TETA) hardener (Hexion Specialty Chemicals, Inc.) with a resin: hardener weight ratio of 100:12 were used as the matrix materials. Amino functionalized multi wall carbon nanotube (NH2-MWCNT) was purchased from US Research Nanomaterials Inc. The length of MWCNTs was about 55 μm and their outer diameters ranged from 20 to 40 nm. Ethyl phenylacetate (EPA), ammonium chloride, urea, formalin (37% formaldehyde), resorcinol, tween 80, sodium hydroxide and sulfuric acid were purchased from Merck. Sodium dodecyl sulfate (SDS) was obtained from Dr Mojallali Co. All of the aforementioned materials were of high purity grade and used as received without any further purification. 3.2. Microcapsule preparation and dispersion Hollow microcapsules (CH) were produced by a two-step approach, following the procedure described in Ref. [9] with a few modifications. First, 5.51 g of formalin (containing 68 mmol formaldehyde) was placed in a 100 ml beaker and its pH was adjusted to 8.0 by adding of few drops of dilute NaOH and followed by adding of 2.05 g (68 mmol) of urea. The solution was placed in the water bath on a temperaturecontrolled hotplate-magnetic stirrer and slowly heated to 68 °C. After 1 h of reaction, the pre-polymer mixture in the beaker was cooled to ambient temperature, and 4 ml of 3 wt% tween 80 as surfactant and 10 ml distilled water were added to the beaker. The solution was then heated to 30 °C and the pH was adjusted to 2 by adding some drops of dilute sulfuric acid. Afterwards, 5 ml of distilled water (ca. 30 °C) was added to the solution and it was heated to 35 °C and agitated at 107
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Fig. 6. FE-SEM images: (a) a micron-sized CCNT capsules (b) presence of CNT on the surface of capsule (c) a larger crashed CCNT capsule (d) presence of CNT on the surface of large capsule (e) the core-missed CCNT capsule and (f) cross section of a fractured CCNT capsule.
microcapsules were coated with gold or platinum and the morphologies and sizes were analyzed using FE-SEM (MIRA3 FEG- TESCAN).
continuous agitation in ambient temperature at 1000 rpm for 24 h, followed by 2 h agitation at 40–42 °C. The same washing method as CEPA microcapsule was used. The weight ratio of CNT to the shell material of CCNT microcapsule was 6.1% assuming a complete reaction between urea and formalin. In Fig. 1 the setups used for preparation of different microcapsules are presented. FT-IR spectra were obtained by using FT-IR spectrometer (model of Tensor 27, Brucker) to identify the chemical structure of microcapsules and also verify the encapsulation of EPA. The synthesized
3.3. Fracture specimen preparation and testing To study the fracture toughness of epoxy reinforced with various microcapsules (hollow capsule and capsules filled with self-healing material), TDCB specimens were employed. Moreover, to minimize the amount of microcapsules consumption per sample, the localized TDCB 108
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Fig. 7. FT-IR spectra of NH2-CNT, EPA, CH microcapsule, CEPA microcapsule and CCNT microcapsule.
Fig. 8. Variations of compliance versus crack length for reference specimens.
10 min. The mechanical stirring and degasing were repeated again and the mixture was filtered to get the uniform capsule distribution. Afterwards, a stoichiometric amount of the hardener was added to the mixture, degased for 15 min and poured into the mold of localized part (Fig. 3(a)). To complete curing, all specimens (TDCB and CT specimens) were post-cured in an oven at 45 °C for 8 h. After curing, samples were precracked at the end of the notch, with a fret saw and a razorblade in order to introduce a sharp crack. Then, samples were pin loaded in uniaxial tension at a displacement rate of 0.3 mm/min in a tensile testing load frame (ZWICK Z100 testing machine) to crack grow and completely separate the specimen under mode I. Three replications were performed per specimen and average value was reported for each composition. Fig. 3(c) and (d) show the setup used for TDCB fracture test and CCNT reinforced broken TDCB specimen, respectively. According to Fig. 3 (d), there is no indication of plastic deformation and hence the linear elastic fracture mechanism can be assumed for the fracture of employed localized TDCB specimens. Fig. 3(e) presents the setup used for CT fracture test.
specimens were fabricated. The geometrical parameters and dimensions of localized TDCB specimen are presented in Fig. 2. A side groove angle of 90° was chosen. According to Fig. 3(a), two different silicone rubber molds were utilized and the specimens were fabricated using a two-step casting. First the localized portion of TDCB specimen containing either mixture of epoxy and microcapsules or neat epoxy as reference was casted in a single-part silicone mold. After a day at room temperature, the localized part was pulled out of first mold and inserted inside the main 2-half silicon mold (Fig. 3(b)) and then the surrounding of inserted localized part was filled with neat epoxy. The fracture properties achieved by using TDCB specimen for neat epoxy were also compared versus those of standardized compact tension (CT) specimen (ASTM D5045). The CT specimens with width of W = 34.49 ± 0.32 mm, the thickness of b = 8.28 ± 0.10 mm and the crack length of 0.45 W ≤ a≤0.55 W were fabricated using a silicon mold. According to previous researches, a base value equal to 10 wt% for the microcapsules inclusion in epoxy matrix was chosen since it is within the suitable range for healing applications [9,16,23,24]. The dispersion of microcapsules in the epoxy matrix was accomplished by heating of the epoxy resin to 32 °C to decrease viscosity of resin, followed by gentle addition of the microcapsules under mechanical stirring for 5 min. The mixture was then sonicated in an ultrasonic water bath (70 W Parsonic 2600s) for 10 min, followed by degasing for
3.4. Numerical modeling of calibration parameters for TDCB specimen To verify the experimental calibration parameters of reference 109
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Fig. 9. Comparison of FEM with experimental (a) critical fracture toughness and (b) critical fracture energy (c) discrepancy between experimental and simulation for KIC and GIC of reference specimens.
TDCB test with different crack lengths and in order to estimate crack length region with constant-K (or G), three-dimensional (3D) linear elastic finite element analyses of TDCB specimens were performed using ANSYS commercial finite element method (FEM) software. Polymer material was assumed to be linear, elastic and isotropic. Tensile properties of epoxy were extracted from the tensile test according to ASTM D638 [25] with the displacement rate of 5 mm/min and gauge length of 50 mm. Three replications were performed. Table 1 presents the tensile properties of the epoxy matrix. Fig. 4, shows the FEM model of TDCB specimen. Tetrahedrons-path conforming mesh was used for meshing. In order to capture a high stress gradient, the finite element meshing was strongly refined near the crack tip. Mesh independent study was also carried out to yield the appropriate size of mesh. A pin loaded, force boundary condition was applied on the upper hole of specimen. The maximum load that can be applied to each specimen was extracted from the experimental fracture tests. FE-fracture energy and fracture toughness were calculated using J-integral [26] and SIF (stress intensity factor) methods [27].
Fig. 10. Comparison of fracture toughness and fracture energy for unreinforced TDCB and CT specimens.
4. Results and discussion 4.1. Microcapsule morphology By following the encapsulation method proposed in the current work (section 3.2), hollow and filled microcapsules were successfully synthesized. The diameter of microcapsules extracted from SEM images using ImageJ are given in Table 2. Capsule diameters were calculated from a minimum of 60 measurements of observed capsules in the SEM micrographs. The SEM micrographs of the synthesized CH and CEPA microcapsules are depicted in Fig. 5. As it is evident in Fig. 5(a), CH microcapsule has a spherical shape with relatively smooth surface. This implies that tween 80 surfactant was able enough to create UF hollow microcapsules. Moreover, the injection of Ar gas during the microencapsulation of CH, entrapped more air bubbles and promoted the formation of individual capsules. For CEPA microcapsule, shape irregularity and aggregation
Fig. 11. Median load-crack opening curves of unreinforced and capsule reinforced TDCB specimens.
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Fig. 12. (a) Critical fracture toughness and (b) critical fracture energy obtained by inclusions of various microcapsules.
The results of FT-IR support the successful encapsulation of EPA as core material of CEPA and CCNT microcapsules as well as the preparation of three different microcapsules.
could be observed according to Fig. 5(b). It seems that the amount of SDS surfactant was a little high to form suitable micelles. The increase in the amount of surfactant has increased the total count of the micelles in the mixture, and as a consequence, has decreased the size of capsules [28]. At the same time, nano-sized capsules and nano-UF particles has settled on the surface of micro-sized capsules and attached to the microcapsules as conglomerates. As a result, the final size of microcapsules is increased as compared to the initial size. Fig. 6 illustrates the SEM micrographs of nanocomposite-wall microcapsules. As it is obvious from Fig. 6 (a), (b) and (d), the aminofunctionalized CNTs were entirely covered with UF and attached to the wall material of microcapsules. The average diameter of CCNT microcapsules according to Table 2 was about a micrometer. However few larger microcapsules were also observed in the SEM images. Moreover, most of the larger microcapsules damaged, broken (Fig. 6 (c)) and shrank (Fig. 6 (e)), and these led to missed-core microcapsules. Fig. 6 (d) shows a UF-covered CNT lying on the surface of a large CCNT capsule. The presence of CNTs inside the shell of microcapsules and thus good compatibility between functionalized CNT and UF as shell can be supported using Fig. 6(f) (as marked by circles). Although, the larger microcapsules are able to store higher core material as compared to smaller microcapsules and thus this could promote their potential use in self-healing applications [7], they are more prone to be damaged during synthesizing, handling and dispersing in the matrix.
4.3. Fracture analysis 4.3.1. Fracture parameters calibration (experimental and numerical approaches) In order to extract the calibration value of dC/da, several reference specimens (unreinforced TDCB) with different crack lengths were tested. The associated error of applying calibration value of reference specimen for all specimens was assumed to be marginal. Fig. 8 presents the variation of compliance with crack length for the reference TDCB specimens. To determine the compliance of each TDCB specimen, the best straight line was drawn according to the ASTM D5040 [29] on the load-crack opening displacement curve and the inverse slope of drawn line was considered as C. According to Fig. 8, the change in the compliance with respect to the crack length remains constant for the crack length in the range of 20–47 mm. The rapid increase at larger crack lengths is attributed to the occurrence of end effect. This disrupts the K-dominate field, and hence the linear elastic fracture mechanics theory is no longer valid when crack length exceeds 47 mm [30]. By performing a linear least square curve fitting on the compliance versus crack length data of Fig. 8, for the crack lengths of 20–47 mm, a slope value equals to 4.22 × 10−4 (N−1) was achieved as calibration value of dC/da. For epoxy 828 cured with DETA hardener, the dC/da was reported to be 2.29 × 10−4 (N−1) by Brown et al. [20,31]. Thus, it is worth noting that the calibration value of dC/da is also dependent on the material grade (comprising resin and hardener) of TDCB specimen and it should be extracted experimentally. Fig. 9 (a) and (b) presents the critical fracture toughness and critical fracture energy achieved by using both FE analysis and experimental tests. The experiential fracture properties were accomplished using the Eqs. (1)–(3) and applying abovementioned calibration value of dC/da. It seems that KIC and GIC stay fairly constant for the crack length smaller than about 32 mm. The slight variation of the fracture properties against crack length (for the crack length smaller than 32 mm) is attributed to the fact that Mostovoy's theory assumes a non-linear taper for the TDCB geometry, but for simplified fabrication of the mold a linear shape profile was used in manufacturing the TDCB specimen [1]. The values of KIC and GIC for the crack lengths larger than 32 mm significantly decrease. For clarification, the deviations of fracture energy and fracture toughness of experiments from those of FEMs are illustrated in Fig. 9 (c). Average of deviation between experiments and FE simulations for the specimens with crack lengths in the range of 20–40 is equal to 5.6% (for both KIC and GIC). Moreover, the prediction errors
4.2. FTIR spectra of microcapsules The FT-IR spectra of NH2-CNT, EPA (core material) and also synthesized CH, CEPA and CCNT microcapsules are shown in Fig. 7. Peaks at about 1535 cm−1 and 1655 cm−1 on FT-IR spectra of NH2-CNT belong to the stretching vibration of C=C and bending vibration of N–H in CNT, respectively. Moreover, the peak appeared at 3848 cm−1 is related to the amine group of amino functionalized CNT. The peak observed at about 1742 cm−1 (on FT-IR spectra of EPA), belongs to the stretching vibration of the carbonyl group of the EPA core, which has induced shoulders on the FT-IR results of CEPA and CCNT microcapsules, yet it has disappeared for CH due to lack of EPA core. Additionally, the absorption peak at approximately 725 cm−1 belongs to OOP (out-of-plane) bending of the aromatic C–H bonds of EPA. The absorption peaks of aromatic and aliphatic C–H bonds in EPA have also appeared at about 2877 cm−1 to 3050 cm−1. The absorption peaks of the microcapsules occurred at approximately 1562 and 1643 cm−1 are related to C=O stretching peaks, owing to the shell material of microcapsules (i.e. UF). In addition, the absorption peaks at approximately 3350 cm−1 to 3393 cm−1, represent the stretching modes of –OH and –NH in the UF resin. 111
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Fig. 13. FE- SEM images of fracture surface for (a, b) unreinforced, (c, d) CH/epoxy, (e, f) CEPA/epoxy TDCB specimens (crack propagation from top to bottom).
that the fracture properties achieved by both fracture tests are in good agreement.
of KIC and GIC as compared to experimental results have rapidly increased for the crack lengths larger than 38 and 35 mm respectively. Thus both experimental and numerical results demonstrate the limitation of TDCB geometry in the prediction of fracture properties for crack length larger than 32 mm. To validate the fracture parameters (achieved using the calibration value of TDCB specimen), the KIC and GIC of neat epoxy extracted using TDCB test (a = 30 mm) were compared with those of obtained by employing CT test (Fig. 10). The deviation of KIC and GIC of TDCB test from those of CT test are 2.8% and 5.4% respectively. Thus, it can be noted
4.3.2. Fracture properties of microcapsule reinforced epoxy According to the results of previous section, the crack length of reinforced TDCB specimens was selected about 30 mm. Fig. 11 represents the load-crack opening displacement for various TDCB samples comprising unreinforced and reinforced with different microcapsules. A brittle fracture in macro-scale and unstable crack growth occurred for all specimens during fracture tests. It seems that the inverse slopes of 112
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Fig. 14. FE-SEM image of fracture surface for epoxy/CCNT TDCB specimens.
According to the Fig. 13 (c), the fracture surface of CH reinforced specimen was rougher than that of neat epoxy. Moreover, the crack tail formation and micro-crack pinning were detected as toughening mechanisms (Fig. 13 (d)) [7,32]. For this composite, the crack during propagation broke the hollow capsules and/or turned around the capsules or agglomerated capsules and UF sediments to form crack tails. The aforementioned mechanisms developed a relatively tough fracture (in micro-scale) and led to the improved fracture properties, as it is demonstrated in Fig. 12. The integration of CEPA microcapsule into the epoxy matrix seems that couldn't provide a significant amount of plastic deformation and or crack tail formation according to Fig. 13 (e) and (f). Also the large capsule aggregations can be observed on the fracture surface. In contrast to the CH reinforced specimen, the aggregated CEPA capsules were disable to effectively dissipate crack energy. Cleaved microcapsules and nano-sized bumps on the inner surface of broken microcapsules could also be observed from the SEM image (Fig. 13 (f)). Thus, the weak fracture properties of CEPA filled specimen as compared to the neat epoxy (Fig. 12) were related to the morphology of CEPA capsules (Fig. 5 b) and their dispersion (Fig. 13 (e) and (f)) which led to the occurrence of brittle fracture surface in micro-scale as illustrated in Fig. 13 (e) and (f). Fig. 14 represents the fracture surface of composite-wall microcapsule (CCNT) reinforced TDCB specimen near the crack tip. FE-SEM study of fracture plane revealed a completely different topography of
curves (i.e. compliances) for all specimens are fairly different. The compliance of CEPA filled specimen is slightly higher than the neat epoxy, yet the compliance of CH and CCNT reinforced specimens are lower than pure epoxy (Fig. 11). The critical fracture toughness and fracture energy of unreinforced and capsule reinforced TDCB specimens are compared in Fig. 12 (a) and (b). The incorporation of CH microcapsule increased KIC and GIC about 17% and 37%, respectively, as compared to unreinforced specimen. However, the application of CEPA microcapsule decreased the fracture properties. The KIC and GIC of specimens increased to about 1.8 MPa m0.5 and 1.2 kJ/m2, corresponding to the 59% and 165% improvements, by integrating CCNT microcapsules (Fig. 12). It seems that, the fracture properties of reinforced specimens directly depend on the morphologies of microcapsules, and fracture surface of TDCB specimens which are presented and discussed in following section. 4.4. Fracture surface Fig. 13, presents the FE-SEM micrographs of fracture surfaces near the crack tip for the unreinforced, CH and CEPA capsule-reinforced specimens (the direction of crack propagation is from top to bottom). According to Fig. 13 (a), for the neat TDCB specimen, crack growth created river-lines parallel with crack direction on the fracture surface. The thin horizontal lines on major vertical lines in Fig. 13 (b) are the indications of shearing band formation and micro-cracks branching. 113
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to the research, authorship, and/or publication of this article.
fracture surface (Fig. 14(a)) as compared to those of CH and CEPA reinforced specimens (Fig. 13(c) and (e)). A rough fracture surface with ruptured microcapsules and large plastic deformation of matrix could be observed according to the Fig. 14. The large plastic deformation (Fig. 14(a)) and out-of-plane fracture indicate the occurrence of crack branching, which this can waste a large portion of crack growth energy [6,33]. According to Fig. 14 (b), (c) and (d), using higher SEM magnifications, amino-functionalized CNTs were observed in the capsule wall as well as inside the matrix around and near the capsules. The presence of CNTs in the vicinity of the cleaved microcapsules indicates the activation of interlocking mechanism of CNTs which are attached on the capsule shell. This leads to the increase of interfacial adhesion of microcapsules to the surrounding matrix. The existence of CNTs in the shell of microcapsules additionally increased the strength of capsule shell (Fig. 14 (b)). Moreover, CCNT reinforced specimen showed fairly better microcapsule dispersion compared to other specimens. Consequently, higher fracture toughness and fracture energy as compared to unreinforced, CH and CEPA reinforced specimens were attained for CCNT reinforced TDCB specimen, according to the Fig. 12.
Funding The author(s) received no financial support for the research, authorship, and/or publication of this article. References [1] T.R. Brussat, S.T. Chin, S. Mostovoy, Fracture mechanics for structural adhesive bonds, AFML-TR-77-163, 1978 Wright-Patterson Air Force Base, Ohio. [2] R.M. Lopes, R.D.S.G. Campilho, F.J.G. da Silva, T.M.S. Faneco, Comparative evaluation of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for estimation of the tensile fracture toughness of adhesive joints, Int. J. Adhesion Adhes. 67 (2016) 103–111 https://doi.org/10.1016/j.ijadhadh.2015.12.032. [3] W. Beres, A. Koul, R. Thamburaj, A tapered double-cantilever-beam specimen designed for constant-K testing at elevated temperatures, J. Test. Eval. 25 (6) (1997) 536–542 https://doi.org/10.1520/JTE11493J. [4] D.H. Kafagy, S.S. Khajotia, M.W. Keller, Evaluation of hybrid laser-cut and cast miniature fracture specimens, Polym. 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Jahanshahi, Effect of nanoparticles on the morphology and thermal properties of self-healing poly(urea-formaldehyde) microcapsules, J. Polym. Res. 20 (6) (2013) 1–8 https://doi.org/10.1007/s10965013-0151-3. [14] A. Fereidoon, M. Jahanshahi, M.G. Ahangari, M. Hemmati, Effect of nanoparticles on the micromechanical and surface properties of poly(urea–formaldehyde) composite microcapsules, Composites, Part B 56 (2014) 450–455 https://doi.org/10. 1016/j.compositesb.2013.08.071. [15] H. Li, Y. Ma, Z. Li, Y. Cui, H. Wang, Synthesis of novel multilayer composite microcapsules and their application in self-lubricating polymer composites, Compos. Sci. Technol. 164 (2018) 120–128 https://doi.org/10.1016/j.compscitech.2018.05. 042. [16] M.M. Caruso, B.J. Blaiszik, S.R. White, N.R. Sottos, J.S. Moore, Full recovery of fracture toughness using a nontoxic solvent-based self-healing system, Adv. Funct. Mater. 18 (13) (2008) 1898–1904 https://doi.org/10.1002/adfm.200800300. [17] A.-D.N. Celestine, N.R. Sottos, S.R. White, Autonomic healing of PMMA via microencapsulated solvent, Polymer 69 (2015) 241–248 https://doi.org/10.1016/j. polymer.2015.03.072. [18] W. Li, C.C. Matthews, K. Yang, M.T. Odarczenko, S.R. White, N.R. Sottos, Autonomous indication of mechanical damage in polymeric coatings, Adv. Mater. 28 (11) (2016) 2189–2194 https://doi.org/10.1002/adma.201505214. [19] R.J. Lemmens, Q. Dai, D.D. Meng, Side-groove influenced parameters for determining fracture toughness of self-healing composites using a tapered double cantilever beam specimen, Theor. Appl. Fract. Mech. 74 (2014) 23–29 https://doi. org/10.1016/j.tafmec.2014.06.011. [20] E.N. Brown, Use of the tapered double-cantilever beam geometry for fracture toughness measurements and its application to the quantification of self-healing, J. Strain Anal. Eng. Des. 46 (2011) 167–186 https://doi.org/10.1177/ 0309324710396018. [21] D.G. Gómez, F.A. Gilabert, E. Tsangouri, D.V. Hemelrijck, X.K.D. Hillewaere, F.E.D. Prez, W.V. Paepegem, In-depth numerical analysis of the TDCB specimen for characterization of self-healing polymers, Int. J. Solids Struct. 64–65 (0) (2015) 145–154 https://doi.org/10.1016/j.ijsolstr.2015.03.020. [22] S. Mostovoy, P. Crosley, E.J. Ripling, Use of crack-line-loaded specimens for measuring plane-strain fracture toughness, J. Mater. 2 (1967) 661–681. [23] T. Yin, M.Z. Rong, M.Q. Zhang, G.C. Yang, Self-healing epoxy composites –
5. Conclusions In the current research, the fracture properties, comprising fracture toughness and fracture energy, of epoxy reinforced with various microcapsules were studied using localized-TDCB specimens under mode I. Three types of microcapsules with urea-formaldehyde resin as wall material, including hollow microcapsule (without filling), filled with EPA as potential self-healing agent, and also CNT nanocomposite-wall microcapsule filled with EPA were synthesized. The crack length investigations were performed experimentally and numerically to extract the calibration parameters of TDCB geometry. The average of deviation error between experiment and numerical simulation results of fracture properties for the TDCB specimens with crack length in the range of 20–40 mm was approximately 5.6%. The calibration parameter of dC/ da for epoxy matrix was determined to be 4.22 × 10−4 (N−1) and it was verified by comparing the fracture properties achieved using TDCB test with those of CT fracture test. The SEM images of synthesized microcapsules and fractured specimens proved the successful preparation of hollow microcapsules as well as the presence of CNTs in the shell and on the wall of CCNT microcapsules. The effectiveness of CCNT microcapsules incorporation in the toughening of specimens was confirmed via SEM images and fracture test results. The fracture tests revealed that hollow capsule-loaded epoxy exhibited the enhanced fracture properties as compared to the neat epoxy due to the formation of tail structures in the polymer matrix. The non-uniform dispersion of CEPA microcapsule in the epoxy matrix and capsule aggregations led to the reduction in the fracture properties of CEPA filled TDCB specimen as compared to the neat epoxy. The integration of composite-wall microcapsules into epoxy was able to increase the fracture toughness and fracture energy by 59% and 165%, respectively. This result was attributed to the occurrence of out-of-plane fracture, high plastic deformation of matrix and interlocking of CNT, attached on the capsules, with surrounding matrix. The results indicating superior fracture performance of nanocomposite-wall microcapsule reinforced-epoxy are particularly useful for those researchers who apply microcapsules for the self-healing of brittle polymers and those who seek for multi-functional materials. Determining the self-healing performance and electrical behavior of CNT reinforced-wall microcapsules filled polymers are suggested for the future research trends. The numerical simulation of fracture behavior for capsule reinforced composites is also a noteworthy subject for future researches. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect 114
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preparation and effect of the healant consisting of microencapsulated epoxy and latent curing agent, Compos. Sci. Technol. 67 (2) (2007) 201–212 https://doi.org/ https://doi.org/10.1016/j.compscitech.2006.07.028. S. Neuser, V. Michaud, Fatigue response of solvent-based self-healing, Smart Materials 54 (2) (2014) 293–304 https://doi.org/10.1007/s11340-013-9787-5. Standard Test Method forTensile Properties of Plastics ASTM D 638 - 02a, ASTM International, West Conshohocken, 2003. C.F. Shih, B. Moran, T. Nakamura, Energy release rate along a three-dimensional crack front in a thermally stressed body, Int. J. Fract. 30 (1986) 79–102 https://doi. org/10.1007/BF00034019. ANSYS-Inc, ANSYS Help System, (2013) Release 15.0. L. Yuan, A. Gu, G. Liang, Preparation and properties of poly(urea–formaldehyde) microcapsules filled with epoxy resins, Mater. Chem. Phys. 110 (2–3) (2008) 417–425 https://doi.org/10.1016/j.matchemphys.2008.02.035. Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy
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Release Rate of Plastic Materials, ASTM International, West Conshohocken, 1999 ASTM D 5045 – 99. A.S. Jones, H. Dutta, Fatigue life modeling of self-healing polymer systems, Mech. Mater. 42 (4) (2010) 481–490 https://doi.org/10.1016/j.mechmat.2010.02.002. E.N. Brown, N.R. Sottos, S.R. White, Fracture testing of a self-healing polymer composite, Exp. Mech. 42 (4) (2002) 372–379 https://doi.org/10.1007/ BF02412141. H. Jin, G.M. Miller, N.R. Sottos, S.R. White, Fracture and fatigue response of a selfhealing epoxy adhesive, Polymer (2011) 1–7 https://doi.org/10.1016/j.polymer. 2011.02.011. T.S. Coope, D.H. Turkenburg, H.R. Fischer, R. Luterbacher, H.v. Bracht, I.P. Bond, Novel Diels-Alder based self-healing epoxies for aerospace composites, Smart Mater. Struct. 25 (8) (2016) 084010 https://doi.org/10.1088/0964-1726/25/8/ 084010.
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Toughening effect of nanocomposite-wall microcapsules on the fracture behavior of epoxy
T
Minoo D. Shokriana, Karim Shelesh-Nezhada,∗, Reza Najjarb a b
Division of Plastics and Composites Engineering, Department of Mechanical Engineering, University of Tabriz, Tabriz, 5166616471, Iran Polymer Research Laboratory, Faculty of Chemistry, University of Tabriz, Tabriz, 5166616471, Iran
H I GH L IG H T S
-CNT nanocomposite-wall microcapsule was synthesized using a new approach. • NH for obtaining calibration parameters of TDCB specimen were conducted. • Analyses capsules enhanced the fracture toughness of epoxy composite. • Hollow properties of epoxy significantly improved by adding NH -CNT microcapsules. • Fracture • Capsules developed crack tailing, plastic deformation and toughened the epoxy. 2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite-wall microcapsule Hollow microcapsule Epoxy NH2-CNT TDCB Fracture toughness Fracture energy
In this research the reinforcing effect of nanocomposite-wall microcapsules on the fracture toughness and fracture energy of epoxy under mode I was studied. The fracture specimens with the geometry of localized tapered double cantilever beam (TDCB) were used to take the advantage of crack length independent-measurement in determining the fracture properties. The crack length investigations were performed experimentally and numerically to extract the calibration parameters and also evaluate the optimal crack length of TDCB specimens. Three types of microcapsules including hollow microcapsule (CH), ethyl phenylacetate core microcapsule (CEPA) and NH2-CNT reinforced-wall microcapsule (CCNT) were synthesized and used as reinforcements. The scanning electron microscopy study of synthesized microcapsules proved the successful presence of CNTs in the shell and on the wall of CCNT microcapsules. The hollow capsule-loaded epoxy composite exhibited enhanced fracture properties as compared to the neat epoxy. Moreover, the SEM study of fracture surface of CH reinforced specimen revealed a characteristic crack tail toughening mechanism. The CEPA microcapsule decreased fracture properties of neat epoxy. The maximum improvement of fracture properties belonged to the specimen containing the CCNT microcapsules. It was observed that the composite-wall microcapsule was able to increase the fracture toughness and fracture energy of neat epoxy by 59% and 165%, respectively, due to the developments of considerable out-of-plane fracture and large plastic deformation.
1. Introduction Compounding different materials to introduce a new material with improved properties is always one of the attracting subjects, especially in the field of polymeric composites. The integration of fillers into polymers is accomplished with different aims such as decreasing cost, improving physical and mechanical performances, achieving selfhealing ability and so on. Polymeric materials and composites often experience micro and macro cracks in their applications. Nowadays, the fracture behavior of polymeric composites is considered to be one of
∗
their important mechanical properties. The fracture toughness and fracture energy of a composite in mode I are usually examined using compact tension (CT) and double cantilever beam (DCB) fracture tests. Mostovoy et al., in 1967 [1] proposed a tapered shape fracture specimen which was resulted from the combination of CT and DCB geometry specimens known as tapered double cantilever beam (TDCB) geometry. In the TDCB geometry, the fracture toughness and energy are independent of crack length and are proportional to the critical load [2]. In the next researches, side grooves with various angles were incorporated into the TDCB geometry to force the crack to grow along the
Corresponding author. E-mail address: [email protected] (K. Shelesh-Nezhad).
https://doi.org/10.1016/j.polymer.2019.02.027 Received 1 November 2018; Received in revised form 28 January 2019; Accepted 12 February 2019 Available online 13 February 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Capsules preparation setups of (a) CH, (b) CCNT, (c) CEPA microcapsules.
capsule/epoxy composite for two different sizes of UF-wall microcapsules. They established that the integration of microcapsules increased the virgin fracture toughness of epoxy due to crack pining and crack deflection mechanisms as well as crack tail formation. This increase in fracture toughness was higher in the case of smaller microcapsules. Moreover, slight improvements in the virgin fracture toughness of capsule/epoxy composite were reported by Jin et al. [9] and Ahangaran et al. [10]. On the other hand, according to Bailey et al. [11] the presence of microcapsules in the epoxy for the coating application caused heterogeneities in the matrix and reduced the tensile modulus and failure stress. It was also demonstrated by Jin et al. [6] that the addition of 117 μm-sized capsules (polyurethane–UF double-walled microcapsules) into epoxy reduced the virgin fracture toughness, particularly at higher capsule concentrations. Thus, reinforcing the traditional microcapsule with nanoparticles has been researched with the aims of improving mechanical properties of capsules and composites as well as developing the interfacial adhesion of capsules to the surrounding matrix. Chuanjie and Xiaodong [12] prepared clay reinforced UF-wall microcapsules. They improved the barrier properties of microcapsules and decreased the leakage of core material from the microcapsules. They also observed that the surface roughness of composite-wall microcapsule enhanced, and inferred that it could lead to the improvement of mechanical adhesion of the microcapsules to the surrounding matrix. Fereidoon et al. [13,14] synthesized nanocompositewall microcapsule reinforced with either carbon nanotube (CNT) or nano-alumina to improve the mechanical properties of traditional UFshell microcapsules. The elastic modulus of composites-wall microcapsules, extracted using nano-indentation technique, increased as compared to unreinforced capsules. Li et al. [15] demonstrated that by assembling functionalized-CNT on the wall of capsules, more hydroxyl groups were introduced owing to modification by dopamine, and this promoted the formation of covalent bonds between microcapsules and epoxy/hardener. To accurately study the toughening effects of microcapsules inclusion in polymer matrix requires an in-depth investigation of both chemical and mechanical aspects. In the present work, the virgin mode Ifracture properties of epoxy reinforced with various microcapsules were studied using TDCB specimens. In order to do this, unreinforced TDCB specimens with different crack lengths were examined both experimentally and numerically to calibrate the TDCB specimen and also extract calibration parameters for the further calculation of fracture properties. The fracture properties of neat epoxy extracted by TDCB test were also compared with those obtained using standardized CT test. Three types of microcapsules including UF-shell hollow microcapsule (CH), ethyl phenyl acetate (EPA) filled UF-shell microcapsule (CEPA) and EPA filled NH2-CNT/UF-shell microcapsule (CCNT) were synthesized as
Fig. 2. Geometrical parameters and dimensions of localized TDCB specimen (top) and localized part (bottom).(dimensions in [mm]).
center-line of specimen [3]. Some researchers also inspected the effect of different preparing methods of TDCB geometry, including laser-cutting and casting, on the resulted fracture toughness [4]. The TDCB geometry because of its advantage over the CT geometry has been recently incorporated to assess the self-healing phenomena in polymeric systems. Self-healing materials are defined as materials that are able to autonomically repair damage whenever and wherever it takes place. Though, different self-healing systems have been introduced so far, the capsule based self-healing materials are the common types of mechanical stimuli due to their simplicity and ease of preparation and application [5]. In these systems, the healing agent can be encapsulated in capsules and dispersed in a polymer matrix. However, the presence of microcapsules in a polymeric matrix, prior to the initiation of healing process, certainty affects the virgin mechanical properties of the matrix. The capsule size, shape, surface roughness, distribution as well as the nature of capsule core material (e.g. viscosity, surface tension, miscibility with shell material, etc.) affect the performances of polymeric matrix [6]. Brown et al. [7] established that the integration of urea formaldehyde (UF)-wall self-healing microcapsules increased the virgin fracture toughness of epoxy matrix as well as provided an efficient mechanism for self-healing. The improvement of fracture toughness was related to a change in the fracture plane morphology from mirror-like to hackle markings and increased subsurface micro-cracking. Blaiszik et al. [8] studied the fracture toughness of 105
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Fig. 3. (a) The silicone rubber molds for fabricating localized TDCB specimen, (b) placing localized part in the main mold before the second step of molding, (c) setup for TDCB testing, (d) CCNT reinforced TDCB specimen after breakage and (e) setup for CT testing.
the reinforcements. For the filled microcapsules, EPA solvent was applied as core, since this solvent has been used as a healing material in self-healing purposes in the literatures [16–18]. Fourier-transform infrared spectroscopy (FT-IR) was employed to verify the chemical structure of materials constituting the microcapsules. Field emissionscanning electron microscopy (FE-SEM) was utilized to study the morphology of microcapsules and also the fracture surfaces.
Table 1 Elastic properties of matrix. Material
Modulus (GPa)
Tensile strength (MPa)
Poisson ratio
Epoxy828
2.7 ± 0.09
42.82 ± 0.49
0.3a
a
[19].
2. Fracture theory of TDCB specimen According to the linear elastic fracture mechanics theory of Irwin and Kies, the strain energy release rate (G) over the thickness of propagating crack (b) is proportional to the change in compliance (C) with respect to crack length (a) variation. Therefore, fracture energy can be determined directly using the Eq. (1) [19].
G=
P 2 dC 2b da
(1)
where P is the applied load. The compliance (C) of the substrate beam is given by the crack opening displacement divided by the load (δ/p). The critical stress intensity factor, KIC, is also calculated using Eq. (2) [20].
K 2 IC = GIC E
(2)
where GIC is the critical fracture energy which is achieved by substituting PCritical for P in Eq. (1). The value of dC/da (Eq. (1)) for TDCB geometry is constant within a range of crack lengths which it can be precisely measured experimentally [4,21]. For the case of TDCB specimen with side grooves, an effective thickness, ben, is substituted for b in Eq. (1). According to Mostovoy assumption, the effective thickness can be defined by Eq. (3) [21,22].
Fig. 4. Illustration of TDCB boundary condition and refined mesh around the crack tip.
ben =
bbn
where bn is the thickness of grooved region. 106
(3)
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Table 2 Description and size of prepared urea-formaldehyde microcapsules. Average diameter ± SD (μm) 2.021 ± 0.357 1.210 ± 0. 931 1.021 ± 0.374
a b
Size range (μm) 1.794–3.690 0.664–4.126 0.269–2.059 30-42b (large)
Description
Shell
Hollow microcapsule Microcapsule containing self-healing core Composite-wall microcapsule containing self-healing core
a
UF UF UF/NH2-MWCNT
Core
Capsule code
– EPA EPA
CH CEPA CCNT
Urea-formaldehyde thermoset. Few microcapsules in corresponding size range was observed.
Fig. 5. FE-SEM images of: (a) CH and (b) CEPA microcapsules.
3. Materials and methods
800 rpm. Then, 4 aliquots of 3 ml distilled water (at 30 °C) were added to the solution every 15 min. In order to increase air bubbles in the solution which acted as molds for the formation of hollow capsules, argon (Ar) gas was bubbled into the solution during the capsule preparation. After continuous agitation at 800 rpm and in about 34–36 °C, for 2 h, the formed hollow microcapsules were rinsed with water and air dried. The filled microcapsules with EPA as the core material (CEPA) were produced using an in-situ polymerization technique in an oil-in-water emulsion. For preparing first solution, 0.75 g SDS, 0.2 g NH4Cl (3.7 mmol), 0.2 g resorcinol, 4 g formalin (49.4 mmol formaldehyde) and 50 ml sulfuric acid (0.5 M) were placed in a 100 ml round-bottomed flask and agitated for 5 min by magnetic stirrer to form a stabilized emulsion. Then, 3 g of EPA was gently added to the mixture to form an oil in water (O/W) emulsion. Next, 2 g (34 mmol) of urea was dissolved in 5 ml sulfuric acid (0.5 M) as the second solution and it was added dropwise into the reaction mixture (first solution) while stirring. Finally, the mixture was stirred at 1200 rpm for 24 h. After completion of UF-resin curing the resulting dispersion was centrifuged to separate capsules from the solution as well as sediment of polymeric particles. The resultant microcapsules were rinsed with distilled water, vacuumfiltered and air dried in ambient condition for 2 days. Nanocomposite-wall microcapsules (CCNT) were synthesized using almost the same conditions as described for CEPA. The first solution was prepared by the same procedure as for CEPA, but the second solution was prepared by dissolving of 0.15 g CNT, 0.3 g SDS and 2 g urea in 15 ml of H2SO4 (0.5 mol) in the beaker. Then, the solution was sonicated by tip of a probe type ultrasonic homogenizer (model of HD 3200, Bandelin) for 6 min at 50% amplitude (horn specific maximum amplitude of 170 μm) to achieve proper dispersion of CNTs. Later on, this solution (as the second solution) was slowly added to the emulsion containing formalin (the first solution as described for CEPA), while agitation. Finally, the encapsulation was accomplished under
3.1. Materials KER 828 epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) (Kumho P&B Chemicals, South Korea) and triethyltetramine (TETA) hardener (Hexion Specialty Chemicals, Inc.) with a resin: hardener weight ratio of 100:12 were used as the matrix materials. Amino functionalized multi wall carbon nanotube (NH2-MWCNT) was purchased from US Research Nanomaterials Inc. The length of MWCNTs was about 55 μm and their outer diameters ranged from 20 to 40 nm. Ethyl phenylacetate (EPA), ammonium chloride, urea, formalin (37% formaldehyde), resorcinol, tween 80, sodium hydroxide and sulfuric acid were purchased from Merck. Sodium dodecyl sulfate (SDS) was obtained from Dr Mojallali Co. All of the aforementioned materials were of high purity grade and used as received without any further purification. 3.2. Microcapsule preparation and dispersion Hollow microcapsules (CH) were produced by a two-step approach, following the procedure described in Ref. [9] with a few modifications. First, 5.51 g of formalin (containing 68 mmol formaldehyde) was placed in a 100 ml beaker and its pH was adjusted to 8.0 by adding of few drops of dilute NaOH and followed by adding of 2.05 g (68 mmol) of urea. The solution was placed in the water bath on a temperaturecontrolled hotplate-magnetic stirrer and slowly heated to 68 °C. After 1 h of reaction, the pre-polymer mixture in the beaker was cooled to ambient temperature, and 4 ml of 3 wt% tween 80 as surfactant and 10 ml distilled water were added to the beaker. The solution was then heated to 30 °C and the pH was adjusted to 2 by adding some drops of dilute sulfuric acid. Afterwards, 5 ml of distilled water (ca. 30 °C) was added to the solution and it was heated to 35 °C and agitated at 107
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Fig. 6. FE-SEM images: (a) a micron-sized CCNT capsules (b) presence of CNT on the surface of capsule (c) a larger crashed CCNT capsule (d) presence of CNT on the surface of large capsule (e) the core-missed CCNT capsule and (f) cross section of a fractured CCNT capsule.
microcapsules were coated with gold or platinum and the morphologies and sizes were analyzed using FE-SEM (MIRA3 FEG- TESCAN).
continuous agitation in ambient temperature at 1000 rpm for 24 h, followed by 2 h agitation at 40–42 °C. The same washing method as CEPA microcapsule was used. The weight ratio of CNT to the shell material of CCNT microcapsule was 6.1% assuming a complete reaction between urea and formalin. In Fig. 1 the setups used for preparation of different microcapsules are presented. FT-IR spectra were obtained by using FT-IR spectrometer (model of Tensor 27, Brucker) to identify the chemical structure of microcapsules and also verify the encapsulation of EPA. The synthesized
3.3. Fracture specimen preparation and testing To study the fracture toughness of epoxy reinforced with various microcapsules (hollow capsule and capsules filled with self-healing material), TDCB specimens were employed. Moreover, to minimize the amount of microcapsules consumption per sample, the localized TDCB 108
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Fig. 7. FT-IR spectra of NH2-CNT, EPA, CH microcapsule, CEPA microcapsule and CCNT microcapsule.
Fig. 8. Variations of compliance versus crack length for reference specimens.
10 min. The mechanical stirring and degasing were repeated again and the mixture was filtered to get the uniform capsule distribution. Afterwards, a stoichiometric amount of the hardener was added to the mixture, degased for 15 min and poured into the mold of localized part (Fig. 3(a)). To complete curing, all specimens (TDCB and CT specimens) were post-cured in an oven at 45 °C for 8 h. After curing, samples were precracked at the end of the notch, with a fret saw and a razorblade in order to introduce a sharp crack. Then, samples were pin loaded in uniaxial tension at a displacement rate of 0.3 mm/min in a tensile testing load frame (ZWICK Z100 testing machine) to crack grow and completely separate the specimen under mode I. Three replications were performed per specimen and average value was reported for each composition. Fig. 3(c) and (d) show the setup used for TDCB fracture test and CCNT reinforced broken TDCB specimen, respectively. According to Fig. 3 (d), there is no indication of plastic deformation and hence the linear elastic fracture mechanism can be assumed for the fracture of employed localized TDCB specimens. Fig. 3(e) presents the setup used for CT fracture test.
specimens were fabricated. The geometrical parameters and dimensions of localized TDCB specimen are presented in Fig. 2. A side groove angle of 90° was chosen. According to Fig. 3(a), two different silicone rubber molds were utilized and the specimens were fabricated using a two-step casting. First the localized portion of TDCB specimen containing either mixture of epoxy and microcapsules or neat epoxy as reference was casted in a single-part silicone mold. After a day at room temperature, the localized part was pulled out of first mold and inserted inside the main 2-half silicon mold (Fig. 3(b)) and then the surrounding of inserted localized part was filled with neat epoxy. The fracture properties achieved by using TDCB specimen for neat epoxy were also compared versus those of standardized compact tension (CT) specimen (ASTM D5045). The CT specimens with width of W = 34.49 ± 0.32 mm, the thickness of b = 8.28 ± 0.10 mm and the crack length of 0.45 W ≤ a≤0.55 W were fabricated using a silicon mold. According to previous researches, a base value equal to 10 wt% for the microcapsules inclusion in epoxy matrix was chosen since it is within the suitable range for healing applications [9,16,23,24]. The dispersion of microcapsules in the epoxy matrix was accomplished by heating of the epoxy resin to 32 °C to decrease viscosity of resin, followed by gentle addition of the microcapsules under mechanical stirring for 5 min. The mixture was then sonicated in an ultrasonic water bath (70 W Parsonic 2600s) for 10 min, followed by degasing for
3.4. Numerical modeling of calibration parameters for TDCB specimen To verify the experimental calibration parameters of reference 109
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Fig. 9. Comparison of FEM with experimental (a) critical fracture toughness and (b) critical fracture energy (c) discrepancy between experimental and simulation for KIC and GIC of reference specimens.
TDCB test with different crack lengths and in order to estimate crack length region with constant-K (or G), three-dimensional (3D) linear elastic finite element analyses of TDCB specimens were performed using ANSYS commercial finite element method (FEM) software. Polymer material was assumed to be linear, elastic and isotropic. Tensile properties of epoxy were extracted from the tensile test according to ASTM D638 [25] with the displacement rate of 5 mm/min and gauge length of 50 mm. Three replications were performed. Table 1 presents the tensile properties of the epoxy matrix. Fig. 4, shows the FEM model of TDCB specimen. Tetrahedrons-path conforming mesh was used for meshing. In order to capture a high stress gradient, the finite element meshing was strongly refined near the crack tip. Mesh independent study was also carried out to yield the appropriate size of mesh. A pin loaded, force boundary condition was applied on the upper hole of specimen. The maximum load that can be applied to each specimen was extracted from the experimental fracture tests. FE-fracture energy and fracture toughness were calculated using J-integral [26] and SIF (stress intensity factor) methods [27].
Fig. 10. Comparison of fracture toughness and fracture energy for unreinforced TDCB and CT specimens.
4. Results and discussion 4.1. Microcapsule morphology By following the encapsulation method proposed in the current work (section 3.2), hollow and filled microcapsules were successfully synthesized. The diameter of microcapsules extracted from SEM images using ImageJ are given in Table 2. Capsule diameters were calculated from a minimum of 60 measurements of observed capsules in the SEM micrographs. The SEM micrographs of the synthesized CH and CEPA microcapsules are depicted in Fig. 5. As it is evident in Fig. 5(a), CH microcapsule has a spherical shape with relatively smooth surface. This implies that tween 80 surfactant was able enough to create UF hollow microcapsules. Moreover, the injection of Ar gas during the microencapsulation of CH, entrapped more air bubbles and promoted the formation of individual capsules. For CEPA microcapsule, shape irregularity and aggregation
Fig. 11. Median load-crack opening curves of unreinforced and capsule reinforced TDCB specimens.
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Fig. 12. (a) Critical fracture toughness and (b) critical fracture energy obtained by inclusions of various microcapsules.
The results of FT-IR support the successful encapsulation of EPA as core material of CEPA and CCNT microcapsules as well as the preparation of three different microcapsules.
could be observed according to Fig. 5(b). It seems that the amount of SDS surfactant was a little high to form suitable micelles. The increase in the amount of surfactant has increased the total count of the micelles in the mixture, and as a consequence, has decreased the size of capsules [28]. At the same time, nano-sized capsules and nano-UF particles has settled on the surface of micro-sized capsules and attached to the microcapsules as conglomerates. As a result, the final size of microcapsules is increased as compared to the initial size. Fig. 6 illustrates the SEM micrographs of nanocomposite-wall microcapsules. As it is obvious from Fig. 6 (a), (b) and (d), the aminofunctionalized CNTs were entirely covered with UF and attached to the wall material of microcapsules. The average diameter of CCNT microcapsules according to Table 2 was about a micrometer. However few larger microcapsules were also observed in the SEM images. Moreover, most of the larger microcapsules damaged, broken (Fig. 6 (c)) and shrank (Fig. 6 (e)), and these led to missed-core microcapsules. Fig. 6 (d) shows a UF-covered CNT lying on the surface of a large CCNT capsule. The presence of CNTs inside the shell of microcapsules and thus good compatibility between functionalized CNT and UF as shell can be supported using Fig. 6(f) (as marked by circles). Although, the larger microcapsules are able to store higher core material as compared to smaller microcapsules and thus this could promote their potential use in self-healing applications [7], they are more prone to be damaged during synthesizing, handling and dispersing in the matrix.
4.3. Fracture analysis 4.3.1. Fracture parameters calibration (experimental and numerical approaches) In order to extract the calibration value of dC/da, several reference specimens (unreinforced TDCB) with different crack lengths were tested. The associated error of applying calibration value of reference specimen for all specimens was assumed to be marginal. Fig. 8 presents the variation of compliance with crack length for the reference TDCB specimens. To determine the compliance of each TDCB specimen, the best straight line was drawn according to the ASTM D5040 [29] on the load-crack opening displacement curve and the inverse slope of drawn line was considered as C. According to Fig. 8, the change in the compliance with respect to the crack length remains constant for the crack length in the range of 20–47 mm. The rapid increase at larger crack lengths is attributed to the occurrence of end effect. This disrupts the K-dominate field, and hence the linear elastic fracture mechanics theory is no longer valid when crack length exceeds 47 mm [30]. By performing a linear least square curve fitting on the compliance versus crack length data of Fig. 8, for the crack lengths of 20–47 mm, a slope value equals to 4.22 × 10−4 (N−1) was achieved as calibration value of dC/da. For epoxy 828 cured with DETA hardener, the dC/da was reported to be 2.29 × 10−4 (N−1) by Brown et al. [20,31]. Thus, it is worth noting that the calibration value of dC/da is also dependent on the material grade (comprising resin and hardener) of TDCB specimen and it should be extracted experimentally. Fig. 9 (a) and (b) presents the critical fracture toughness and critical fracture energy achieved by using both FE analysis and experimental tests. The experiential fracture properties were accomplished using the Eqs. (1)–(3) and applying abovementioned calibration value of dC/da. It seems that KIC and GIC stay fairly constant for the crack length smaller than about 32 mm. The slight variation of the fracture properties against crack length (for the crack length smaller than 32 mm) is attributed to the fact that Mostovoy's theory assumes a non-linear taper for the TDCB geometry, but for simplified fabrication of the mold a linear shape profile was used in manufacturing the TDCB specimen [1]. The values of KIC and GIC for the crack lengths larger than 32 mm significantly decrease. For clarification, the deviations of fracture energy and fracture toughness of experiments from those of FEMs are illustrated in Fig. 9 (c). Average of deviation between experiments and FE simulations for the specimens with crack lengths in the range of 20–40 is equal to 5.6% (for both KIC and GIC). Moreover, the prediction errors
4.2. FTIR spectra of microcapsules The FT-IR spectra of NH2-CNT, EPA (core material) and also synthesized CH, CEPA and CCNT microcapsules are shown in Fig. 7. Peaks at about 1535 cm−1 and 1655 cm−1 on FT-IR spectra of NH2-CNT belong to the stretching vibration of C=C and bending vibration of N–H in CNT, respectively. Moreover, the peak appeared at 3848 cm−1 is related to the amine group of amino functionalized CNT. The peak observed at about 1742 cm−1 (on FT-IR spectra of EPA), belongs to the stretching vibration of the carbonyl group of the EPA core, which has induced shoulders on the FT-IR results of CEPA and CCNT microcapsules, yet it has disappeared for CH due to lack of EPA core. Additionally, the absorption peak at approximately 725 cm−1 belongs to OOP (out-of-plane) bending of the aromatic C–H bonds of EPA. The absorption peaks of aromatic and aliphatic C–H bonds in EPA have also appeared at about 2877 cm−1 to 3050 cm−1. The absorption peaks of the microcapsules occurred at approximately 1562 and 1643 cm−1 are related to C=O stretching peaks, owing to the shell material of microcapsules (i.e. UF). In addition, the absorption peaks at approximately 3350 cm−1 to 3393 cm−1, represent the stretching modes of –OH and –NH in the UF resin. 111
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Fig. 13. FE- SEM images of fracture surface for (a, b) unreinforced, (c, d) CH/epoxy, (e, f) CEPA/epoxy TDCB specimens (crack propagation from top to bottom).
that the fracture properties achieved by both fracture tests are in good agreement.
of KIC and GIC as compared to experimental results have rapidly increased for the crack lengths larger than 38 and 35 mm respectively. Thus both experimental and numerical results demonstrate the limitation of TDCB geometry in the prediction of fracture properties for crack length larger than 32 mm. To validate the fracture parameters (achieved using the calibration value of TDCB specimen), the KIC and GIC of neat epoxy extracted using TDCB test (a = 30 mm) were compared with those of obtained by employing CT test (Fig. 10). The deviation of KIC and GIC of TDCB test from those of CT test are 2.8% and 5.4% respectively. Thus, it can be noted
4.3.2. Fracture properties of microcapsule reinforced epoxy According to the results of previous section, the crack length of reinforced TDCB specimens was selected about 30 mm. Fig. 11 represents the load-crack opening displacement for various TDCB samples comprising unreinforced and reinforced with different microcapsules. A brittle fracture in macro-scale and unstable crack growth occurred for all specimens during fracture tests. It seems that the inverse slopes of 112
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Fig. 14. FE-SEM image of fracture surface for epoxy/CCNT TDCB specimens.
According to the Fig. 13 (c), the fracture surface of CH reinforced specimen was rougher than that of neat epoxy. Moreover, the crack tail formation and micro-crack pinning were detected as toughening mechanisms (Fig. 13 (d)) [7,32]. For this composite, the crack during propagation broke the hollow capsules and/or turned around the capsules or agglomerated capsules and UF sediments to form crack tails. The aforementioned mechanisms developed a relatively tough fracture (in micro-scale) and led to the improved fracture properties, as it is demonstrated in Fig. 12. The integration of CEPA microcapsule into the epoxy matrix seems that couldn't provide a significant amount of plastic deformation and or crack tail formation according to Fig. 13 (e) and (f). Also the large capsule aggregations can be observed on the fracture surface. In contrast to the CH reinforced specimen, the aggregated CEPA capsules were disable to effectively dissipate crack energy. Cleaved microcapsules and nano-sized bumps on the inner surface of broken microcapsules could also be observed from the SEM image (Fig. 13 (f)). Thus, the weak fracture properties of CEPA filled specimen as compared to the neat epoxy (Fig. 12) were related to the morphology of CEPA capsules (Fig. 5 b) and their dispersion (Fig. 13 (e) and (f)) which led to the occurrence of brittle fracture surface in micro-scale as illustrated in Fig. 13 (e) and (f). Fig. 14 represents the fracture surface of composite-wall microcapsule (CCNT) reinforced TDCB specimen near the crack tip. FE-SEM study of fracture plane revealed a completely different topography of
curves (i.e. compliances) for all specimens are fairly different. The compliance of CEPA filled specimen is slightly higher than the neat epoxy, yet the compliance of CH and CCNT reinforced specimens are lower than pure epoxy (Fig. 11). The critical fracture toughness and fracture energy of unreinforced and capsule reinforced TDCB specimens are compared in Fig. 12 (a) and (b). The incorporation of CH microcapsule increased KIC and GIC about 17% and 37%, respectively, as compared to unreinforced specimen. However, the application of CEPA microcapsule decreased the fracture properties. The KIC and GIC of specimens increased to about 1.8 MPa m0.5 and 1.2 kJ/m2, corresponding to the 59% and 165% improvements, by integrating CCNT microcapsules (Fig. 12). It seems that, the fracture properties of reinforced specimens directly depend on the morphologies of microcapsules, and fracture surface of TDCB specimens which are presented and discussed in following section. 4.4. Fracture surface Fig. 13, presents the FE-SEM micrographs of fracture surfaces near the crack tip for the unreinforced, CH and CEPA capsule-reinforced specimens (the direction of crack propagation is from top to bottom). According to Fig. 13 (a), for the neat TDCB specimen, crack growth created river-lines parallel with crack direction on the fracture surface. The thin horizontal lines on major vertical lines in Fig. 13 (b) are the indications of shearing band formation and micro-cracks branching. 113
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to the research, authorship, and/or publication of this article.
fracture surface (Fig. 14(a)) as compared to those of CH and CEPA reinforced specimens (Fig. 13(c) and (e)). A rough fracture surface with ruptured microcapsules and large plastic deformation of matrix could be observed according to the Fig. 14. The large plastic deformation (Fig. 14(a)) and out-of-plane fracture indicate the occurrence of crack branching, which this can waste a large portion of crack growth energy [6,33]. According to Fig. 14 (b), (c) and (d), using higher SEM magnifications, amino-functionalized CNTs were observed in the capsule wall as well as inside the matrix around and near the capsules. The presence of CNTs in the vicinity of the cleaved microcapsules indicates the activation of interlocking mechanism of CNTs which are attached on the capsule shell. This leads to the increase of interfacial adhesion of microcapsules to the surrounding matrix. The existence of CNTs in the shell of microcapsules additionally increased the strength of capsule shell (Fig. 14 (b)). Moreover, CCNT reinforced specimen showed fairly better microcapsule dispersion compared to other specimens. Consequently, higher fracture toughness and fracture energy as compared to unreinforced, CH and CEPA reinforced specimens were attained for CCNT reinforced TDCB specimen, according to the Fig. 12.
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5. Conclusions In the current research, the fracture properties, comprising fracture toughness and fracture energy, of epoxy reinforced with various microcapsules were studied using localized-TDCB specimens under mode I. Three types of microcapsules with urea-formaldehyde resin as wall material, including hollow microcapsule (without filling), filled with EPA as potential self-healing agent, and also CNT nanocomposite-wall microcapsule filled with EPA were synthesized. The crack length investigations were performed experimentally and numerically to extract the calibration parameters of TDCB geometry. The average of deviation error between experiment and numerical simulation results of fracture properties for the TDCB specimens with crack length in the range of 20–40 mm was approximately 5.6%. The calibration parameter of dC/ da for epoxy matrix was determined to be 4.22 × 10−4 (N−1) and it was verified by comparing the fracture properties achieved using TDCB test with those of CT fracture test. The SEM images of synthesized microcapsules and fractured specimens proved the successful preparation of hollow microcapsules as well as the presence of CNTs in the shell and on the wall of CCNT microcapsules. The effectiveness of CCNT microcapsules incorporation in the toughening of specimens was confirmed via SEM images and fracture test results. The fracture tests revealed that hollow capsule-loaded epoxy exhibited the enhanced fracture properties as compared to the neat epoxy due to the formation of tail structures in the polymer matrix. The non-uniform dispersion of CEPA microcapsule in the epoxy matrix and capsule aggregations led to the reduction in the fracture properties of CEPA filled TDCB specimen as compared to the neat epoxy. The integration of composite-wall microcapsules into epoxy was able to increase the fracture toughness and fracture energy by 59% and 165%, respectively. This result was attributed to the occurrence of out-of-plane fracture, high plastic deformation of matrix and interlocking of CNT, attached on the capsules, with surrounding matrix. The results indicating superior fracture performance of nanocomposite-wall microcapsule reinforced-epoxy are particularly useful for those researchers who apply microcapsules for the self-healing of brittle polymers and those who seek for multi-functional materials. Determining the self-healing performance and electrical behavior of CNT reinforced-wall microcapsules filled polymers are suggested for the future research trends. The numerical simulation of fracture behavior for capsule reinforced composites is also a noteworthy subject for future researches. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect 114
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