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Self healing fiber reinforced epoxy composite: solvent epoxy resin filled hollow glass fiber

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https://www.eduzhai.net International Journal of Composite M aterials 2013, 3(6): 145-155 DOI: 10.5923/j.cmaterials.20130306.02 Self-healing Fiber-reinforced Epoxy Composites: Solvent-epoxy Filled Hollow Glass Fibers Narinder K. Mehta Department of Chemical Engineering, University of Puerto Rico, M ayagüez, PR 00681 USA Abstract Functional repair co mponents encased inside hollow glass fibers (HGFs) are being considered as self-healing systems for advanced composite structures. This study describes the resulting compressive residual strength of impacted composite laminates embedded with solvent-epoxy resin-filled HGFs, wh ich provided a self-healing functionality. The healing efficiency was found to be 73-88% after 10 days self-recovery time; it was highest for the embedded HGFs containing ethyl cyanoacrylate as the healing agent. This work has provided evidence that for the HGFs lay-up investigated, a significant fraction of the lost comp ression strength can be successfully restored by the self-repairing effect of the healing chemicals, suggesting that self-repair is possible for advanced composite structures. Keywords Self-healing, A. Poly meric-matrix Co mposites, A. Smart Materials, B. Damage To lerance 1. Introduction Advancements in materials research have resulted in remarkable g rowth in the field of s mart, inventive materials[1-6] capable of mechanical/damage healing of engineering structures in response to an applied stimu lus. New developments in the state-of-the art research have come out in recent years on self-healing fiber-reinforced structural polymer co mposite embedded a variety of novel solvents and chemical filled glass fibers and microcapsules among others[7-12]. Recent work on autonomic healing[11] at roo m temperature y ielded as much as 45% recovery of virg in interlaminar fracture toughness, while healing at 80 ℃ increases the recovery to over 80%. A co mprehensive review[1] in the field of stimuli-responsive healable materials suggests the examination o f the healing of polymeric materials, briefly discussing the conventional techniques for repair and maintenance of composite mater ia ls . A p ers p ect iv e o n t h e u s e o f mu lt id i men s io n al microvascular network for healing applications for future self-healing approaches using a biomimetric technique is discussed[10]. Again, hollow fiber and microencapsulation approaches are new types of self-healing technology and have been emerg ing at an increase rate over the last decade. The int ention is to stimulate debate and rein force the importance of a mult idisciplinary approach in the exciting * Corresponding author: narinderk.mehta@upr.edu (Narinder K. Mehta) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved field. Shape memory poly mer fibers[12] have also been used in a b io mimetic t wo-step self-healing system for healing microscopic cracks. Also a new healing mechanism is proposed[7] for co mposite networks, based on the dissipation of constraints that hinder fluctuation of network ju n ctio n s . The focus in the development of these materials is to address their susceptibility to impact damage. Damage to the polymer matrices at the macroscopic level leads to matrix micro -cracking and delamination within the bulk of the composite and thus alters its mechanical properties. Even thermal, electrical and acoustic properties change as matrix cracks are init iated. Matrix micro-cracking, fiber-matrix debonding and delamination are dangerous because a micro -crack may act as a nucleation site; it may propagate, resulting in pre mature ma jor structural fa ilure if the da mage is not detected and repaired. Hence, it becomes a necessity to autonomically repair cracks within a co mposite to retain the structural integrity and extend the lifet ime of the materia l. Fiber-reinforced epo xy-based polymer co mposites are leading the way to improve the efficiency and sustainability of structures. Highly cross-linked poly meric materials are required to achieve the mechanical strength desired for these structures; however, these types of materials tend to be britt le and are prone to the development of cracks through normal usage, which may result in catastrophic failure. Low energy impact can result in a drastic reduction in the composite strength, elastic moduli, structural durability and damage tolerance characteristics. However, these self-healing polymeric materials offer the ability to extend the functional lifetime of these structures. It is crucial to be concerned about the comp lexity of impact damage initiat ion and propagation in composite 146 Narinder K. M ehta: Self-healing Fiber-reinforced Epoxy Composites: Solvent-epoxy Filled Hollow Glass Fibers matrices because the repair strategy will have to cope with several mechanisms and factors that can affect the nature and magnitude of the impact damage. Structural failu res have pushed the scientific co mmunity to exp lore mechanisms to retard and repair microscopic cracks before irreversib le damage is done to the material. Smart materials are able to achieve self-healing[10, 13-15] either by reversib ly bonding the broken structure or via an embedded healing agent in the composite structure. It is ideal to utilize a methodology that can be executed quickly and effectively on site. Matrix cracking can be repaired by sealing the crack with an embedded resin or appropriate solvents. Bleeding of embedded functional chemicals doped with a UV dye may serve as a biomimet ic (12) simu lation for synthetic self-repair systems. Our aim is to ascertain the efficiency of the healing reagents with the dye infiltrating the damaged site and bridging the fractured surfaces. This feasibility study is focused on experimental wo rk on the self-healing of impacted epoxy co mposites embedded with HGFs containing healing agents spiked with a fluorescent dye. The role of the fiber in reinfo rced plastics is to add strength and stiffness to the polymer matrix, but healing agents[5] stored in HGFs and placed inside the composite matrix provide the ability to repair a damaged matrix independently of any external stimulus. The performance of the self-healing method described above is evaluated using compression mechanical testing of the impacted laminate specimens. The results regarding the extent of the healing efficiency and the recovery of the original mechanical integrity under impact stress (comp ression) are presented. 2. Methods and Materials 2.1. Self-healing Concept The concept of self-repair in co mposite structures is similar to the biological healing process in living organis ms; the key is that no external action is required. The technology must sense and respond to damage to restore the system’s original propert ies. Self-healing is a novel alternative to damage tolerant design and removes the need to perform temporary repairs to damaged structures; at the same time , it provides an opportunity to improve designs, allowing for fiber-reinforced co mposites. Self-healing laminate, in this work, is defined as a standard composite la minate with self-healing materials (filled HGFs) placed in layers at predetermined spaces and introduced at critica l interfaces[6]. Early damage detection is important, as is the ability to restore structural strength and stiffness to a composite structure after damage has occurred. The novel concept is that of “self-repair”, where a damaged structure is repaired by materials already contained within the structure. This repair involves sealing the impact cracks, followed by solvent-epoxy-amine curing of the cracks, leading to a recovery of mechanical p roperties similar to that of the un-cracked co mposite. One approach to mechanically simu lated self-healing systems utilizes epo xy resin (a composite matrix material) as the healing agent because it is probable that there is enough residual amine (hardener) available in the co mposite matrix to help the polarization process to occur, thus filling the crack and restoring the strength of the material. The healing mechanism involves wetting[15, 16] o f the impacted poly mer surface using solvents. The embedded self-healing chemicals in HGFs are released in the crack plane upon impact damage and help restore the initial fracture properties. When HGFs are broken by impact, the released solvent swells the nearby matrix and thus allows residual amines to further cross link with residual epo xy functionalities. However, addit ional epo xy resin within the broken HGFs (if embedded) aug ments the chance for additional cross-linking and may help bond new thermoset to the original matrix interface. The important point to consider is the ability of the repair agent to adequately reach the damaged site and commence healing. 2.2. Reagent Grade Org anic Sol vents Solvents assist in the healing behavior mainly by wetting the polymer surface and swelling the bulk poly mer. Self-healing chemicals can repair damage in thermosets through chemical reactions at room temperature, or in an autonomic way using filled HGFs or microcapsules with appropriate solvent-epoxy mixtures or stand-alone chemicals. It is important that the chemicals do not react with the epoxy and have a low viscosity in liquid form for easy filling of the hollow fiber tubes. Not all solvents are suitable as healing agents. Solvents with dielectric constants (DCs) ranging fro m 5-50 are ideal. Encouraging self-repair recovery results with the use of aprotic polar solvent-filled microcapsules embedded in thermoset composites have been reported in the literature[16, 17]. This suggested a more detailed exp loration of the feasibility of using some aprotic polar solvents in combination with the epoxy resin in HGFs to serve as an autonomic self-healing system. The hydrogen bond accepting ability of the aprotic polar solvents[16] is the probable exp lanation for the self-healing recovery. The reacting epoxy resin contains a large number of free hydroxy l groups that serve as hydrogen bond donors. The dipole-dipole interaction between hydrogen bond accepting solvents and the free available amines in the composite also influences the epoxy-amine curing reaction. The s elf-healing system requires that the solvent be available in the epoxy matrix because the solvent plasticizes the epoxy resin to allow mo lecular mobility, promot ing further curing reactions and cross-lin king. 2.2.1. Ethyl Phenyl Acetate (EPA) and Nit robenzene Polar aprotic solvents such as EPA, chlorobenzene and nitrobenzene are good candidates compared to other solvents such as non-polar hexane, which does not provide any indication of recovery of mechanica l integrity[16]. Due to its International Journal of Composite M aterials 2013, 3(6): 145-155 147 toxicity, ch lorobenzene was not considered in this work. EPA and nitrobenzene have DCs in the range of ~5-32 and a similar polarity to that of chlorobenzene (DC ~6), wh ich has been previously shown to provide self-healing to composites after impact damage. Approximately 19-80 wt% of these solvents mixed with epo xy resin were used to fill the HGFs. 2.2.2. Methacrylic Ac id High-impact toughness is an important physical property of a class of copolymers known as ionomers, where the bulk properties are governed by ionic interactions within the polymer. It has been suggested that acid groups in the mo lecular backbone of this material provide the synergy of the thermo-mechanical properties necessary for the healing behavior[1, 18]. The acid may allow adequate heat generation during impact, providing elastic retardation (viscoelastic response) and sealing, which may yield impact reversal in poly mer co mposites. 20 wt% methacrylic acid-epo xy resin was used to fill the HGFs for evaluation following an impact event on co mposite laminate specimens. 2.2.3. Ethyl Cyanoacrylate (Superglue) This commercial thermoplastic co mpound was selected to serve as a sealing/healing agent for filling the HGFs. This is a cement-type material and has natural britt leness. It was envisioned that the tensile cracking o f the epo xy matrix and the breaking of the glass fibers may stimu late the curing mechanis m in this stand-alone healing system. Th is product (as supplied) had a low viscosity (<30 cP) with high air curing properties. 2.3. Epoxy/ Hardener System Epo xy is widely used as a polymer in co mposites for diverse industry/engineering applications. Most fiber-reinforced epoxy thermosets use two-part components that create a glassy polymer network with a high material strength and stiffness when cured. These composites are brittle and prone to matrix micro-cracking under service load conditions. The function of the matrix is to secure the reinforcement and to facilitate load-transfer to the fibers. The epoxy matrix used was SC 15 Part A (a mixture of diglycidyl ether of bisphenol A, aliphatic dig lycidyl ether: 10-20% and epo xy toughener: 10-20%), and the hardener was SC 15 Part B (a mixture of cycloaliphatic amine: 65-90% and proprietary amine: 10-35%). They were mixed in the stoichio metric ratio of 100 parts epoxy resin to 30 parts amine hardener. These products are specialized chemicals manufactured for high impact ballistic studies. 2.4. Capsule-embedded Speci mens These specimens are the result of ongoing work at the Army ’s CERL-ERDC co mposite laboratory to evaluate the self-healing recovery performance of impacted specimens embedded with microcapsules. The panels were made with a urea-formaldehyde capsule shell filled with SC-15 Part A epoxy resin. The capsules were sprinkled on each of the E-glass fiber sheets (10 and 15 wt %) before fabricating the composite laminates. The compressive residual strength testing was performed similar to the HGF-embedded s p ecimen s . 2.5. Hollow Glass Fi bers (HGFs ) Impact-induced damage can often be d ifficu lt to detect and frequently gives rise to internal damage in the fo rm of delamination and matrix cracking. However, s mart revealing systems have been developed to detect damage within the composite. A liquid repair system can be embedded within brittle-walled containers (like HGFs) that are fractured during impact, allowing the liquid healing agent to flow into the areas of damage where it subsequently cures. HGF-reinforced g lass plies provide some degree of shielding under impact, absorbing energy and protecting the underlying material; they are also ideal to store self-healing functional materials. If a fluorescent dye[19] is incorporated within the co mpounds in HGFs, it p rovides enhanced visualizat ion of the damaged site by the bleed ing action[20] of a highly crack-penetrating mediu m to the micro-crack site. In some cases, acetone can be used to reduce the resin viscosity to facilitate the capillary action during the infiltrat ion process and the bleeding effect during the s elf-rep air. Hollow fibers are an ideal mediu m to provide a good combination of the storage and mechanical reinforcement functions of healing agents. A healing agent passes from within any broken HGF to infiltrate the impact-damaged zone and acts to improve the mechanical p roperties. This repair process will help reduce the critical effects of matrix cracking and delamination between plies, thus preventing further damage propagation. HGFs are the preferred choice for self-repair co mpared to embedded capsules due to their ability to store functional agents that impart a self-healing functionality to the laminate. Poly meric materials with fillers may perfo rm poorly under impact loading. HGFs filled with a healing agent would not only add the desired strength (no reinforcing function) to the system but would permit self-healing of the damage, thanks to the materials embedded within the resin matrix in the composite structure. Following a mechanical stimu lus (fracture), the healing agent would bleed into the damaged site to initiate repair. 2.5.1. Filling HGFs with Healing Agents HGFs have been shown to augment the structural performance of co mposite structures without creating sites of weakness within the composite. To imp lement the repair mechanis m of HGFs, a practical technique must be developed for filling them with the repair chemicals. The HGF diameter, wall thickness and fiber hollowness, as well as the viscosity of the healing materials and the healing kinetics, p lay an important role in the healing process. Kimb le Chase borosilicate glass capillary tubes of ~ 08-1.1 mm diameter x 10 cm length were used for this study. 148 Narinder K. M ehta: Self-healing Fiber-reinforced Epoxy Composites: Solvent-epoxy Filled Hollow Glass Fibers To fill an HGF tube with lo w viscosity fluids, the glass capillary was held almost horizontally with one of the open ends placed into the sealing agent, which flo ws into the glass by capillary action. A hand-held pump was used, which resulted in a d ifficu lt and messy scenario for filling the hollow fibers. Ethyl cyanoacrylate[14] was used as one of the healing agents to fill HGFs, supplied by the vendor in a small plastic bottle. The bottle served as an exceptional mechanis m to fill the hollo w fiber tubes with different mixtu res of solvent-epoxy healing agents. After filling, both ends of the tube were sealed with epo xy putty cured rapid ly at room temperature (RT), inserted manually a few millimeters into the fiber lu mens to prevent leaking of the healing mixture and react ion with air. EPA, nitrobenzene and methacrylic acid in comb ination with epo xy-resin were used to fill the HGFs, whereas ethyl cyanoacrylate was used as a stand-alone compound for filling the fiber tubes before adhering them to the fiber surface using epoxy glue. 2.6. Speci men Fabricati on A standard resin film infusion process was used to fabricate the HGF-embedded epoxy co mposite laminates, which consisted of eight unidirectional plies to obtain a RT-cu red thickness of ~0.5 mm with a stacking[21,22] sequence of[45°/ 0°/-45°/90°]symmetric. The designations represent a single layer (~ 4.57 x 4.57 cm of E-g lass woven fabric with the warp and weft fibers oriented at the specified angles. Four precut sheets were placed in a pre-fabricated mo ld as per the above fiber orientation pattern. A precut carton panel with ~ 39.1 x 1.52 cm sections was placed on top of the four E-glass stacked sheets. Eleven hollo w fiber tubes filled with solvent-epoxy mixtures were placed vertically (linear) into each of the 15 sections in a setting pattern of five tubes in the middle and three tubes on both sides, separated by ~ 1.27 cm of space, as shown in the figure 1 below. The lay-up was chosen to position the HGFs in the sub-surface of the E-glass fiber plies to provide a fully symmetric arrangement to avoid any detrimental residual stresses in the composite matrix. After p lacing the tubes on the fiber surface, they were adhered using epoxy glue; the glued HGFs were then allowed to dry overnight. The remain ing four E-glass laminate sheets were placed over the set-up before starting the infusion process using a stoichiometric mixture of epo xy resin and hardener. The panel was left undisturbed for a curing of 2-3 days at the laboratory temperature. The whole fabricated panel (Figure 2) was precision cut to have fifteen ~ 10.1x15.2 cm specimens for co mpression testing after impact. 2.7. Impact Damage (Indentation) of the S pecimens To establish the efficiency of the self-healing after impact damage, the specimens were prepared to establish the compressive residual strength behavior before and after the prescribed time intervals. The specimens were subjected to drop weight impact testing according to ASTM D7136D 7136M prior to the application of the compressive force. The specimens back side was subjected to impact damage by a process of indentation using a hardened steel tube with a hemi-spherical end ~ 4.63 mm in diameter falling fro m a height of ~ 122 cm with the specimen held tightly on a metal surface. A rep resentative impact damage site was formed in the center of each specimen, and healing was allowed to take place at ambient laboratory temperature fo r a sufficient self-healing time before the co mpression testing to determine the effectiveness of the self-repair of the impacted site. Figure 1. Lay-up of the embedded HGFs in the composite laminate International Journal of Composite M aterials 2013, 3(6): 145-155 149 Figure 2. Solvent-epoxy filled (with a fluorescent dye) HGFs glued on E-glass fiber sheets before infusion of the resin and hardener mixture 3. Results 3.1. Indentati on Behavior The majority of the impact-induced damage will be localized with in or adjacent to the hollow fiber plies, thus creating an ideal scenario for self-repair agents (containing uncured resin) leaking out of the broken hollow fibers. The initiat ion of curing simu ltaneously promotes the infiltration of the healing agents in the local crack matrix by capillary action. The presence of solvents may alter the reaction mechanis m and the curing speed due to temperature variations resulting fro m the heat absorbed by solvent evaporation. This may result in rendering the healing resin ineffective after a short period of t ime. 3.2. Eval uation of Damage Initi ation As the back side of the specimens was indented, visual inspection revealed that the damage was widespread, with each ply interface having some level of damage throughout the thickness of the laminate near the area of the embedded HGFs. The damage probably will be interconnected; hence the self-healing materials will be able to penetrate into the damage plane. A key aspect of the whole self-healing process is that the impact energy must be of a threshold value to fracture the HGFs. The bleeding of the dye fro m within the fractured HGFs into the damaged areas is visib le to some extent (Illustration 3) under a UV light source, thus demonstrating the effectiveness of the dye as a method for damage visualizat ion. However, a mo re sophisticated instrumental setup, such as C-UV scan time controlled photography, is needed to comment on the dispersion-bleeding of the dye into the crack plane. 3.3. Damage Anal ysis Overview: The impact will probably create a localized crushing zone followed by shear cracks and delaminations of increasing length through the thickness of the laminate, culminating in the largest delamination at the back face between the final plies of the stack. The HGF clusters may cause the delamination to deviate fro m its path due to the fiber clusters and the resulting resin-rich regions causing a weakness in the laminate. Instead of passing directly through an HGF/ matrix interface, the propagating crack may deviate around it, causing fiber rupture and the release of healing agents. HGFs are normally ruptured as a delamination propagates along a ply interface, suggesting that their susceptibility to fracture is similar to that of the matrix. Intra-ply shear cracks normally lin k mu ltip le delaminations. The presence of these cracks is essential for self-repair, as they provide connectivity between different damage sites in the laminate and facilitate healing at mu ltip le interfaces. Additionally, shear cracks would be expected to generate a large capillary action to transport healing components, as the capillary force generated is inversely proportional to the radius of the opening between the two interfaces. 3.4. Compressive Resi dual Strength after Impact To establish the baseline mechanical performance and damage levels anticipated in the HGF-embedded composite panels, compression testing after the impact tests was carried out on precut impacted and non-impacted test specimens. The purpose of this compression force test was to evaluate the ability of the healing solvent-epoxy resin mixture to repair matrix cracking and delamination damage. Because the back face is the primary location of the impact damage, it was loaded in compression mode for every impacted sample. The ASTM D6264/D 6264M test method was used to test the residual compression properties of the multid irect ional E-glass polymer matrix co mposite laminate specimens. The method utilizes a flat, rectangular ~10.2 x 15.2 cm co mposite laminate plate, p reviously subjected to a damaging event. Applied force data were recorded while loading. Susceptibility to damage fro m concentrated out-of-plane forces is one of the major design concerns of structures made of advanced composite laminates. Co mpressive residual strength data obtained by this method may reflect the damage tolerance characteristics of an un-stiffened monolithic skin that resists out-of-plane deformat ion. A PC-based data acquisition system was used for all compression testing. Ult imate co mpressive residual strength, psi = Pmax / A Pmax = maximu m fo rce prior to failu re, N[lb·30.5 cm] A = cross-sectional area = h (2.54 c m) x w (2.54 c m) x T (2.54 cm2) 3.4.1. Co mpressive Residual Strength Data The ASTM residual co mp ression methodology approach to assess the effect of self-healing was used to directly compare the effect of the HGF layers on the baseline laminate and assess the healing capability. Inclusion of the HGFs will increase the thickness of the laminate around the areas where they are embedded. It may reduce the fiber fracture due to localized crushing of the HGFs under the 150 Narinder K. M ehta: Self-healing Fiber-reinforced Epoxy Composites: Solvent-epoxy Filled Hollow Glass Fibers impact site; this will appear as a darker central reg ion of the type seen in Figure 3 A-D. All testing was undertaken with the damaged face of the specimen subjected to co mpressive loading. This was because the resin repair would have a negligible effect on the fiber-do minated tensile face. Virgin specimen (3-A) Impacted and compressed specimen (3-B) Un-impacted specimen (3-C) Figure 3A-D. specimens Impacted specimen (3-D) Virgin, compressed, un-impacted and impacted test The residual compressive strength data for the triplicate test specimens, which were randomly selected from a pool of 15 specimens (mother panel) so that none of the specimens was a neighbor of any other specimen, are given in Table 1 and shown in Figures 4-5. As shown in Figure 4, the average virgin co mpressive strength of all the test specimens of various panels was approximate ly 20x103 psi and decreased to the level of approximately 13x103 -17x103 psi after immed iate impact damage. In general, the standard deviation of the test data (triplicates) was less than 5%, which is attributed to the architectural pattern of the embedded hollow fibers, which may have scattered and deviated the mu lti-dimensional co mpressive force around the HGFs-matrix edges due to a non-uniform d istribution and thickness of the resin. Comparatively, the strength for the virgin HGF-embedded specimens was considerably lower, which was mainly attributed to the presence of HGFs in the thermoset matrix. Identi ficati on of composite s peci mens in Table 1 Figure 4 demonstrates that the immediate impacted panel T6 specimens (80 wt% EPA-epo xy) demonstrated impact healing recovery after five days of healing time, whereas panel T10 specimens (80 wt% EPA-epo xy HGFs and individual hardener HGFs) demonstrated compressive strength recovery after 10 days of healing time. This diffe rence may be attributed to the geometry of the setting of the HGFs in the panels because panel T10 had alternate HGF filled with hardener in the pattern of the HGFs embedded in the laminate. The results suggest[16] that the amount of residual amines present inside the matrix are not sufficient to provide self-healing after impact and that a slight excess of the amine (stoichiometric ratio) is needed to provide the extra curing of the crack plane after a fracture. To p rove this hypothesis[16], HGFs containing hardener in contact with solvent-epoxy HGFs were embedded together as per the geometry in this work to fabricate more laminates with other polar solvents. A low five-day strength recovery was obtained for the specimens embedded with HGFs containing 33.4 wt% nitrobenzene-epo xy (panel T9), 80 wt% EPAepoxy (panel T10) and 50 wt% methacrylic acid-epo xy (panel T11) in co mbination with indiv idually filled hardener HGFs. However, the impacted specimens of panels T9, T10 and T11 left to heal fo r ten days demonstrated a drastic increase in strength recovery, which strongly suggested that the presence of an extra amount of hardener instead of a stoichiometric amount is needed for improving the impact-damage healing as a function of healing time. Hence, it is appropriate to suggest the necessity and importance of inserting indiv idual HGFs filled with hardener in the lay-up pattern in combination with solvent-epoxy filled HGFs.] International Journal of Composite M aterials 2013, 3(6): 145-155 151 Table 1. Compressive Residual Strength and Self-Healing Efficiency T5, Baseline T3 T4 T6 T10 T7 T11 T8 T9 Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Average St an dard Dev iat ion Ultimate Residual Compressive Strength (psi) Self-Healing Virgin Immediate 5 days 10 days 20343.25 720.20 18070.50 1552.05 19512.49 809.01 20666.61 835.29 18323.93 865.18 21533.53 526.88 21478.98 416.03 20438.86 846.47 19451.96 1386.10 13890.61 1172.74 13891.13 288.01 17780.68 776.25 14995.44 721.16 13921.44 1005.49 16383.30 892.90 15066.85 527.52 16401.74 112.68 14980.77 511.37 - - - - 15029.99 13728.62 1728.41 190.78 16151.47 15767.08 1128.93 1105.52 15361.91 15252.73 1840.29 1399.91 12564.90 13515.07 517.51 1767.97 15943.03 16725.66 468.19 734.05 14262.04 14675.08 380.80 434.18 17777.07 18056.06 749.07 560.47 14990.01 15333.66 912.37 43.13 Average % Residual St ren gth Lost Compared to Virgin Immediat e Impact Specimen % Lost (-) -31.72% 5.76% -23.13% 1.59% -8.88% 3.98% -27.44% 3.49% -24.03% 5.49% -23.92% 4.15% -29.85% 2.46% -19.75% 0.55% -22.99% 2.63% Average % Residual Strength Lost Compared to Impacted 5 day 10 day healing healing time time % Gain (+) / Loss (-) % Gain (+) / Loss (-) - - - - 8.20% -1.17% 12.44% 1.27% -9.16% -11.32% 6.35% 6.84% 2.44% 1.72% 12.27% 9.11% -9.74% -2.92% 3.72% 14.07% -2.69% 2.09% 2.86% 4.60% -5.34% -2.60% 2.53% 3.04% 8.39% 10.09% 4.57% 3.15% 0.06% 2.36% 6.09% 0.29% % Self-Healing Efficiency Compared to Virgin Immediate 5 day 10 day Impact heal heal time time 68.28% 5.76% 76.87% 1.59% 91.12% 3.98% 72.56% 3.49% 75.97% 5.49% 76.08% 4.15% 70.15% 2.46% 80.25% 0.55% 77.01% 2.63% - - - - 83.17% 75.97% 9.56% 1.06% 82.78% 80.81% 5.79% 5.67% 74.33% 73.80% 8.90% 6.77% 68.57% 73.76% 2.82% 9.65% 74.04% 77.67% 2.17% 3.41% 66.40% 68.32% 1.77% 2.02% 86.98% 88.34% 3.66% 2.74% 77.06% 78.83% 4.69% 0.22% Average 26203.81 20088.71 18559.87 - P97 St an dard Dev iat ion 924.77 609.57 117.42 - Average 25802.85 18416.43 19076.37 - P98A St an dard Dev iat ion 572.39 1123.48 1057.07 - -23.34% -7.61% - 2.33% 0.58% - -28.63% 3.58% - 4.35% 5.74% - 76.66% 70.83% - 2.33% 0.45% - 71.37% 73.93% - 4.35% 4.10% - 152 Narinder K. M ehta: Self-healing Fiber-reinforced Epoxy Composites: Solvent-epoxy Filled Hollow Glass Fibers Figure 4. Ultimate Compressive Residual Strength after Impact As shown in Figure 4, the capsule-embedded specimens provide higher average compressive residual strength compared to the embedded HGFs filled with healing agents. It is instructive to compare the effect of the impact damage on the residual strength. Capsule-embedded panel 98A specimens and ethyl cyanoacrylate filled HGF-embedded panel T8 specimens statistically demonstrate similar compressive residual strength recovery performance after 10 days of healing t ime; however, the self-recovery healing efficiency as a function of healing time was far superior for ethyl cyanoacrylate (Figure 6). Figure 5 demonstrates the percent loss in the immed iate compressive residual strength of the specimens after impact; it was found to be 22-30% for the majority of the HGF-embedded test specimens, whereas it was 23-28% for the capsule-embedded panel 97 and 98A specimens. Panel T4 specimens de monstrated unusual data, which may be the result of improper filling of the HGFs in the in itial stages of the fabrication of the first panel for this work. Table 1 demonstrates that for certain specimens, a significant proportion of residual strength was restored (gained) after impact and five or 10 days of healing compared to the immediate impacted specimens. Ethyl cyanoacrylate HGF-embedded panel T8 specimens showed the maximu m gain in the co mpressive residual strength; 8.4 and 10.1% after five and ten days of healing time, respectively. For panel specimens embedded with HGFs containing methacrylic acid-epo xy (T7) and nitrobenzene epoxy (T9), the average residual strength gain was 2.1 and 2.4%, respectively, after ten days of recovery time, whereas the gain after eight days was 3.6% for 15 wt% loading of the epoxy-filled capsules embedded in the P 98A laminate. It should be stated that if one is looking at the materials on the basis of the recovery performance (co mpressive residual strength) immediately after impact, the ideal scenario is to embed capsules instead of HGFs in the composite laminates; however, the cost of capsules is enormous co mpared to HGFs. The reality of self-healing efficiency after impact as a function of recovery time to regain the virgin strength, as demonstrated by the data, is in favor of HGFs embedded in the laminates. The recovery in the compressive residual strength is attributable to the bleeding of healing agents into the damaged crack paths before the viscosity rise associated with cure progression hinders the healing process. 3.4.2. Self-healing Efficiency Data The self-healing efficiency is defined as the fracture compressive load ratio[23] of the healed and the virgin fracture toughness. Figure 6 demonstrates an average self healing or crack efficiency of 88% for ethyl cyanoacrylate International Journal of Composite M aterials 2013, 3(6): 145-155 153 (panel T8) specimens, 79% for nitrobenzene-epo xy (panel T9) specimens and 76% for methacrylic acid-epo xy (panel T7) specimens after 10 days of recovery time, whereas 15 wt% capsules provided a 74% self-healing efficiency on average after eight days of recovery time. More data are needed to evaluate the performance of the chemicalcontaining capsules as potential candidates for self-healing of thermoset composites after impact damage. On the basis of the test results, it is realistic to suggest that for impact damage recovery applications, solvent-epoxy filled HGFs embedded in epoxy composite laminates is extremely cost-effective and should be a preferred choice, as HGFs provide an enhanced, quick and autonomic self healing of the cracked and damaged thermoset matrix. Figure 5. % Loss Residual Strength after Impact (Immediate) Figure 6. Self-Healing of Impacted Composite Specimens (HealedComp.Strength / VirginComp.Strength) 154 Narinder K. M ehta: Self-healing Fiber-reinforced Epoxy Composites: Solvent-epoxy Filled Hollow Glass Fibers 4. Conclusions This work has shown that a hollow g lass fiber self-healing approach can be used to repair advanced composite structures. It has shown that HGFs containing solvent-epoxy mixtu res as self-repair agents can be embedded into composite laminates. A series of mechanical tests were undertaken to evaluate the influence of the incorporated HGFs on the compressive strength. The results have shown that for the lay-up of HGFs investigated, a significant fraction of lost compressive strength can be restored by the self-repairing effect of the solvent-epoxy/chemical agents stored within the embedded HGFs. The self-repair mechanis m is not a permanent solution, but provides a means to inhibit further damage propagation. The results have indicated that the inclusion of HGFs gives an initial comp ressive strength reduction of 19-30%. It was found that the baseline laminate and the laminates containing HGFs had comparab le low energy impact damage tolerance both in terms of damage size and immediate residual compressive failure strength (typically 70-80%). After self-healing of the damaged impact site fo r five and 10 days, it was found that the self-healing efficiency of the healed specimens with different co mbinations of solvent-epoxy mixtures inside the HGFs ranged fro m 73 to 88% co mpared to undamaged virgin laminate specimens. The results suggest that self-healing is possible for advanced composite structures using aprotic polar solvents and chemicals. The passive system demonstrated heals matrix micro -cracks and may prevent crack reopening and further crack gro wth. The self-repair ability may deteriorate over time as the repair chemicals such as resin degrade. Further work is needed to optimize the selection of healing agents, composite and resins, including ionomers within HGFs, to provide increased environmental stability and effective service life. Efforts are also needed to refine both the self-repairing and damage enhancement processes (equipment) by using tailored resins and dyes which may provide improved repair properties. One must recognize that curing by ethyl cyanoacrylate on contact within the mouth of the capillary may prevent the use of this type of material as a repair resin in s mall diameter HGF systems. However, careful manipulat ion of the materials may provide excellent results, as demonstrated in this work. We used a compound with a reduced viscosity of ~5 cP, and it prov ided encouraging data to further explo re the use of this compound as a stand-alone self-repair agent. The research has indicated the importance of choosing an appropriate combination of repairing resin and solvents to offer ease of infiltrat ion into the damaged site, excellent curing characteristics and adequate mechanical properties once cured. Ku mar and Dr. Larry D. Stephenson of the U. S. Army’s ERDC-CERL, Champaign, Illinois for the summer funding support and to William Lewis for h is help in carry ing out the immense work-load involved in this short summer pro ject. The research was supported in part by an appointment to the 2010 Facu lty Research Participation Program at the U. S. Army Engineer Research and Development Center, Corps of Engineers Construction Engineering Research Laboratory (ERDC-CERL), ad ministered by the Oak Ridge Institute of Science and Education (ORAU/ORISE) through an interagency agreement between the U. S. Depart ment of Energy (DOE) and ERDC-CERL. REFERENCES [1] M urphy EB, Wudl F, The world of smart healable materials. Progress in Polymer Science 2010;35:223-51. [2] Wu DY, M eure S, Solomon D, Self-healing polymeric materials: A review of recent developments. Prog. Polym. Sci. 2008;33:479-522. [3] Hayes SA, Jones FR, M arshiya K, Zang W, A self-healing thermosetting composite material. Composite: Part A 2007;38:1116-20. [4] Zako M , Takano N, Intelligent materials system using epoxy particles to repair microcracks and delamination damage in GFRP: Journal of Intelligent M aterials Systems and Structures. 1999;10:83641. [5] Tan PS, Zhang M Q, Bhattacharyya D, Processing and performance of self-healing materials. M aterials Science and Engineering 2004;4:012017. doi:10.1088/1757-899X/4/1/01 2017. [6] Jody M , Pang WC, Bond IP, A hollow fiber reinforced polymer composite encompassing self-healing and enhanced damage visibility. Compos. Sci. Technol. 2006;65:1791-9. [7] Ciferri Alberto, Healing and self-healing polymers: composite network visited. Polym. Chem., 2013. [8] Sonada K, Self-healing of interfacial de-bonding in fibre reinforced polymers, Polymer Science and Technology 2013:T 40@0, pp. T45-T50. [9] Huang L., Yi N., Zang Wu, Zang Q., Huang y., M a Y., Chen Y., Advanced M aterials 2013:25 (15) pp. 2224-2228. [10] Aissa B., Therriault D., Haddad E., Jamroz W., Self-healing M aterials Systems: Overview of M ajor Approaches and Recent Developed technologies, Advances in M aterials Science and Engineering 2012: doi:101155/2012/854203. [11] M ajchrzak M ., Hine P., Khosravi E., An autonomous self healing system based on ROM P of nonboronene dicarboximi de monomers. 2009: http://dx. doi.org/10.1016/j.progpolymx ci.2009.10.006. ACKNOWLEDGMENTS The author (retired professor) is grateful to Dr. Ashok [12] Guogiang Li., Oludayo A., Harper M ., Effect of strain hardening of shape memory polymer fibers on healing efficiency of thermosetting polymer composites. 2012: http://dx.doi. Org/10.1016/j.polymer.2012.12.046.

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