Recovery Of Fracture Toughness Research Articles

Repair of acrylic/glass composites by liquid resin injection and press moulding

This paper presents repair methods for in-situ polymerised acrylic (Elium®)/glass fibre (GF) composites focusing on mode-I fracture toughness recovery. GF/acrylic composites were first subjected to double cantilever beam (DCB) tests to measure their Mode-I fracture toughness. The delaminated samples after DCB tests were repaired and rejoined. Two repair methods were performed: liquid resin injection and press moulding at two different temperatures (130 °C and 160 °C). The repaired samples were subjected to a second set of DCB tests. The fracture behaviours of the four specimen groups (virgin, resin-injected, pressed at 130 °C, and pressed at 160 °C) were evaluated in terms of strain energy release rates (GIC) during crack initiation and propagation. The results showed that specimens repaired by resin injection exhibited the highest GIC values, about 30 % higher than the virgin state, due to the formation of a semi-interpenetrating polymer network (semi-IPN) at the joining interface. Scanning electron microscopy images provided insight into distinctive fracture behaviours for each test group.

Poly(methyl methacrylate) as Healing Agent for Carbon Fibre Reinforced Epoxy Composites.

Self-healing materials offer a potential solution to the problem of damage to fibre-reinforced plastics (FRPs) by allowing for the in-service repair of composite materials at a lower cost, in less time, and with improved mechanical properties compared to traditional repair methods. This study investigates for the first time the use of poly(methyl methacrylate) (PMMA) as a self-healing agent in FRPs and evaluates its effectiveness both when blended with the matrix and when applied as a coating to carbon fibres. The self-healing properties of the material are evaluated using double cantilever beam (DCB) tests for up to three healing cycles. The blending strategy does not impart a healing capacity to the FRP due to its discrete and confined morphology; meanwhile, coating the fibres with the PMMA results in healing efficiencies of up to 53% in terms of fracture toughness recovery. This efficiency remains constant, with a slight decrease over three subsequent healing cycles. It has been demonstrated that spray coating is a simple and scalable method of incorporating a thermoplastic agent into an FRP. This study also compares the healing efficiency of specimens with and without a transesterification catalyst and finds that the catalyst does not increase the healing efficiency, but it does improve the interlaminar properties of the material.

Stimulated autogenous-healing capacity of fiber-reinforced mortar incorporating healing agents for recovery against fracture and mechanical properties

Abstract This study aimed to investigate the effect of autogenous healing capacity with the addition of expansive and crystalline agents on mechanical and fracture behaviors of fiber-reinforced (FR) mortar specimens with crack mouth opening displacement (CMOD) controlled test set-up. The experimental results of a self-healing approach of FR mortar were analyzed in terms of first cracking peak load (FCPL) increase, index of fracture toughness recovery (IFTR), and index of fracture energy recovery (IFER). Initially, the specimens were pre-cracked at different crack widths ranging from 30 μm to 200 μm after 28-days of curing. After pre-cracking, the specimens were kept in water for 56- and 120-day healing. A controlled three-point bending test (PBT) was applied on prism specimen having a central notch of 40 mm depth for pre-cracking as well as the post-conditioning stage for determining the FCPL. However, the crack surfaces were monitored by a high-range digital microscope and scanning electron microscope (SEM) to examine the nature of healed products near the damaged area. Test results revealed that a significant recovery of small cracks (≤50 μm) could be achieved for self-healing specimens by using healing agents (HA), while for large cracks (≥100 μm) partial recovery could be achieved after the 120-day healing period.

Development of self-contained microcapsules for optimised catalyst position in self-healing materials

Self-contained microcapsules for use in self-healing epoxy resin are successfully synthesized by suspension polymerization process. The microencapsulation of an epoxy resin using Polymethylmethacrylate (PMMA) as a shell material and the location of scandium triflate (Sc(OTf)3) as the catalyst into microcapsules shell during the microencapsulation processes is presented (PMMA/Sc(OTf)3-walled microcapsules). Spherical microcapsules of 80 μm in diameter with a liquid core content of 30 wt% (determined by HPLC) are produced. Catalyst location on microcapsules are assessed qualitatively by SEM-EADS and quantitatively by TGA showing high yields (⁓70 wt%). The evaluation of the healing efficiency was assessed in terms of fracture toughness recovery. PMMA/Sc(OTf)3-walled microcapsules showed an increased healing efficiency than that of conventional PMMA-walled capsule. The healing efficiency of the PMMA-walled capsules was 46.7 and 55.1% when the system healed at 80 and 120 °C, respectively. However, in the case of PMMA/Sc(OTf)3-walled microcapsules healing efficiency increased to 57.5 and 79.1% for the same healing temperatures.

Self-constructed nanodomain structure in thermosetting blend based on the dynamic reactions of cyanate ester and epoxy resins and its related property

A new strategy to obtain nanodomain structure in thermoset/thermoset blends was developed via different dynamic reactions of cyanate ester (CE) and epoxy (EP) resins in the presence of zinc (II) acetylacetonate and glutaric anhydride. The resulting polymer systems show high mechanical property mainly attributed to EP nanophase and the excellent interfacial interaction between EP nanophase and CE matrix. Owing to the broad glass transition, the polymer systems display thermally activated triple shape memory effects in the temperature ranges of 150–225 °C. Moreover, the systems exhibit thermal self-healing behavior resulting from the transesterification and possible reactions between the residual cyanate ester and epoxy groups, and a 78–83% recovery of fracture toughness for the resulting polymers can be achieved after the first healing schedule of 200 °C/2 h.

Self-healing isocyanate microcapsules for efficient restoration of fracture damage of polyurethane and epoxy resins

Isophorone diisocyanate-loaded microcapsules were synthesized with controllable size for self-healing of structural polymer materials. The obtained microcapsules showed good compatibility with polymer resins, e.g., one-component epoxy and multi-component polyurethane/water glass structural materials. During the processing of composite self-healing materials, the highly cross-linked composite capsule wall ensured the integrity of microcapsules and prevented the leakage and degradation of reactive liquid isocyanates effectively. A relatively larger amount of self-healing agent could be released to the local cracks with the increasing size and content of microcapsules in polymer matrix which improved the self-healing efficiency of the composites. Based on the microcapsules with a diameter of ~96 μm, the healing efficiency of epoxy composites containing 15 wt% of capsules and polyurethane/water glass composites containing 20 wt% of capsules could reach to 105 and 101% recovery of fracture toughness, respectively. The crack-healing effect was further verified by scanning electron microscopy. Meanwhile, a possible self-healing mechanism of microcapsules to the cracks of composite polymer materials was proposed.

Enhanced Mixing of Microvascular Self-Healing Reagents Using Segmented Gas-Liquid Flow.

Microvascular self-healing systems have previously been demonstrated to restore large-scale damage and achieve repeated healing of multiple damage events in polymers. However, the healing performance of these systems is often limited because thelaminar nature of flow in microchannels results in poor mixing of two-part self-healing reagents. In this paper, we introduce segmented gas-liquid flow (SGLF) to enhance the mixing of reagents in microvascular self-healing systems. In SGLF, discrete liquid slugs containing self-healing reagents are separated by gas bubbles while flowing through a single microchannel. Recirculating streamlines within the liquid slugs can enhance the mixing of miscible liquids such as healing reagents. We investigate the effect of SGLF on mixing and healing for a two-stage chemistry used to restore large-scale damage in thermoset polymers. Additionally, we employ SGLF to deliver an epoxy-thiol chemistry, enabling the repeated recovery of fracture toughness in glass fiber-reinforced composites. In both systems, the mixing of healing agents delivered by SGLF is enhanced compared to alternative microvascular delivery strategies. For the two-stage chemistry, SGLF increases the maximum damage size that can be healed by 25% compared to laminar single-phase flow. Furthermore, there are concomitant increases in the extent of polymerization and the mechanical properties of the restored material, including a fivefold increase in the peak load sustained during a push-out test. For the epoxy-thiol chemistry, SGLF enables multiple healing cycles with healing efficiency above 100%. On the basis of these results, we envision that SGLF could improve performance for a variety of microvascular self-healing systems with different host materials, damage modes, and healing chemistries.

A reconfiguring and self-healing thermoset epoxy/chain-extended bismaleimide resin system with thermally dynamic covalent bonds

Muti-functional shape memory materials based on epoxy resins (EP) with different concentration of chain-extended bismaleimide resins (CBMI) were fabricated. The resulting EP/CBMI polymers have high glass transition temperature (Tg: 93–125 °C), excellent thermal stability and good mechanical property. Unlike traditional shape memory polymers (SMPs), EP/CBMI SMPs are capable of shape reconfiguration and self-healing abilities. EP/CBMI polymers have high cycle-life shape memory property at high temperature, and their original shapes can be quickly recovered within 21 s by thermal activation method. Because of dynamic transesterification, EP/CBMI polymers can exhibit plasticity and reconfigure their shapes when heated to the temperature of dynamic transesterification (200 °C). For the same reason, EP/CBMI polymers also exhibit good healing behavior, and a 77.5–86.5% recovery of fracture toughness can be obtained after the fractured EP/CBMI polymers are healed at 200 °C/2 h.

Clinically Applicable Self-Healing Dental Resin Composites

ABSTRACTSelf-healing is one of the most desired material properties. Herein, we present the design and development of a new self-healing dental composite (SHDC) that can heal micro-cracks autonomously. The SHDC has two functional components in addition to contemporary dental composites: healing powder (HP) and healing liquid (HL) encapsulated in silica microcapsules. The autonomous healing is triggered by micro-cracks which fracture microcapsules in their propagation path and release the HL. As a consequence, the released HL dissolves and reacts with the HP, and then fill the micro-cracks with a cement-like new material. This 3-step crack-release-heal process prevents micro-cracks from causing restoration failure, thus improving the service life of dental restorative material. The mechanical performance of the SHDC prepared were evaluated in terms of elastic modulus and fracture toughness, which were in the upper level compared to commercial dental restorative materials, and the self-healing capability was confirmed through fracture toughness recovery test. In addition, the SHDCs were made with clinically-tested, biocompatible materials, which makes them readily applicable as medical devices.

Interlaminar fracture of CF/EP composite containing a dual-component microencapsulated self-healant

We present an experimental study of the self-healing ability of carbon fibre/epoxy (CF/EP) composite laminates with microencapsulated epoxy and its hardener (mercaptan) as a healing agent. Epoxy- and hardener-loaded microcapsules (average size large: 123μm; small: 65μm) were prepared by in situ polymerisation in an oil-in-water emulsion and were dry-dispersed at the ratio 1:1 on the surface of unidirectional carbon fabric layer. The CF/EP laminates were fabricated using a vacuum-assisted resin infusion (VARI) process. Width-tapered double cantilever beam (WTDCB) specimens were used to measure mode-I interlaminar fracture toughness of the CF/EP composites with a pre-crack in the centre plane where the microcapsules were placed. Incorporation of the dual-component healant stored in the fragile microcapsules provided the laminates with healing capability on delamination damage by recovering as much as 80% of its fracture toughness. It was also observed that the recovery of fracture toughness was directly correlated with the amount of healant covering the fracture plane, with the highest healing efficiency obtained for the laminate with large capsules.

Repeatable self-healing of an epoxy matrix using imidazole initiated polymerization

For the first time, repeatable self-healing was achieved in a cross-linked epoxy polymer by incorporating 2-ethyl-4-methylimidazole (24-EMI) into the matrix as a latent polymerization initiator. Upon material damage and infiltration of liquid EPON 8132 epoxy monomer healing agent into the crack plane, polymerization occurs in the damaged region with a moderate application of heat in the presence of the latent imidazole initiator. Using tapered double cantilever beam (TDCB) fracture testing, greater than 90% recovery of fracture toughness was observed over multiple healing cycles in samples containing 10 wt% 24-EMI, with up to 11 repeat healing cycles possible. The effect of incorporating the imidazole on the host epoxy fracture toughness, complex moduli and glass transition temperature was also investigated. As imidazole concentration increases, a reduction in glass transition temperature and an increase in fracture toughness of the host epoxy is observed.

Autonomic healing of PMMA via microencapsulated solvent

Fully autonomous, room temperature self-healing in PMMA is achieved for the first time through the use of microcapsules containing a solvent. Linear PMMA is embedded with microcapsules (ca. 300 μm) containing a liquid anisole solvent core and a small amount of linear PMMA polymer for healing of crack damage. Specimens containing a range of concentrations of microcapsules were fracture tested and then allowed to heal for 1–7 days at ambient conditions. The healing efficiency of the material is evaluated based on the recovery of fracture toughness and is shown to be dependent on both healing time and microcapsule concentration. A maximum healing efficiency of 89% is obtained for specimens containing 5 wt% solvent microcapsules after 3 days of healing.

Development of Composites with a Self-Healing Function

This research aimed to realize experimentally the facilevascular self-healing system in epoxy glass fibre reinforced composite. Using flexiblepolytetrafluoroethylene tubes as removable preforms, the channels were embeddedinto both neat epoxy resin and unidirectional glass-fibre reinforced epoxy laminate.Room temperature curable epoxy resin with a surfactant and an amine-basedhardener were the components of the binary healing agent. The specimens oftapered double cantilever beam geometry were subjected to Mode I fracture tests.Fracture of specimens released the healing agent from channels and triggeredself-healing process of the crack. Tested neat epoxy resin specimensdemonstrated recovery of fracture toughness ca. 70 % after 24 h of self-healingat 50°C. Unidirectional laminate specimens (250×23×1.2 mm) were made by vacuuminfusion method from two layers of glass yarns with 5 embedded channels aligningto reinforcing fibers. The channels were alternately filled with components of thehealing agent and then sealed. It was revealed that the embedded vascularchannels in specimens had very little effect on their elastic modulus. Theexperimental program included multiple three-point bending tests of specimensfor their initial damage and self-healing of specimens during their heat treatmentand following exposure at room temperature. Static and dynamic flexural moduli ofelasticity were determined by three-point bending and cantilever beam vibrationat all stages of the test program. The healing efficiency was evaluated as a relativechange of elastic modulus. The efficiency ca. 30 % was reached during 24 h at50°C and additionally increased up ca. 40 % after more than 3 weeks of roomtemperature exposure. The sealed healing agent was capable of maintaining thecapacity for self-healing for at least six months. The research resultsdemonstrated capacity of the macro-channel approach for self-healing realizationin multifunctional polymer composite materials. DOI: http://dx.doi.org/10.5755/j01.ms.21.1.5354

Design and development of novel reactive amine nanocontainers for a self-healing epoxy adhesive: self-repairing investigation using the lap shear test

Recovery of fracture toughness of a self-healing epoxy adhesive is achieved by using a novel amine nanocontainer.

Fracture Toughness Healing in Epoxy Containing Both Epoxy and Amine Loaded Capsules

The feasibility of self‐healing epoxy matrices using the same chemistry as the matrix material, namely a standard epoxy‐amine system, is demonstrated. The two‐component self‐healing system is encapsulated in core‐shell structures using standard in situ curing in a stirred emulsion and a novel microfluidic encapsulation technique for the epoxy monomer and the amine hardener, respectively. The recovery of fracture toughness after room temperature healing is above 50%, and increases to 100% when shape memory alloy wires are combined with a low temperature cure of the matrix to enhance crack closure. This healing ability is also preserved for higher curing temperatures, demonstrating the potential of this novel approach.

Thermally Stable Autonomic Healing in Epoxy using a Dual‐Microcapsule System

Self-healing is achieved with a dual-microcapsule system utilizing epoxy-amine chemistry in a high temperature cured thermosetting epoxy polymer. One capsule contains a modified aliphatic polyamine prepared by vacuum infiltration of polyoxypropylenetriamine into hollow polymeric microcapsules. The second capsule contains a difunctional epoxide and reactive diluent. Healing efficiency is accessed through recovery of fracture toughness and excellent long-term stability at ambient conditions is demonstrated.

Thermal annealing recovery of fracture toughness in HT9 steel after irradiation to high doses

The HT9 ferritic/martensitic steel with a nominal chemistry of Fe(bal.)12%Cr1%MoVW has been used as a primary core material for fast fission reactors such as FFTF because of its high resistance to radiation-induced swelling and embrittlement. Both static and dynamic fracture test results have shown that the HT9 steel can become brittle when it is exposed to high dose irradiation at a relatively low temperature (<430°C). This article aims at a comprehensive discussion on the thermal annealing recovery of fracture toughness in the HT9 steel after irradiation up to 3148dpa at 378504°C. A specimen reuse technique has been established and applied to this study: the fracture specimens were tested Charpy specimens or broken halves of Charpy bars (13×3×4mm). The post-anneal fracture test results indicated that much of the radiation-induced damage can be recovered by a simple thermal annealing schedule: the fracture toughness was incompletely recovered by 550°C annealing, while nearly complete or complete recovery occurred after 650°C annealing. This indicates that thermal annealing is a feasible damage mitigation technique for the reactor components made of HT9 steel. The partial recovery is probably due to the non-removable microstructural damages such as void or gas bubble formation, elemental segregation and precipitation.

Fracture behavior of a self-healing, toughened epoxy adhesive

A self-healing, toughened epoxy adhesive is demonstrated based on a commercial structural adhesive film. Self-healing is achieved via embedded microcapsules containing dicyclopentadiene monomer and solid particles of bis(tricyclohexylphosphine)-benzylidine ruthenium (IV) dichloride (Grubbs') catalyst. Recovery of fracture toughness is assessed through fracture testing of width tapered double cantilever beam (WTDCB) specimens. Healing efficiencies as high as 58% were achieved for 6.6wt% DCPD microcapsules and 10mg Grubbs’ catalyst. However, virgin fracture toughness is reduced with the addition of ca. 117μm diameter microcapsules as a result of suppression of the damage zone as revealed by transmission optical micrographs. The uniform dispersal of microcapsules throughout a rubber toughened epoxy adhesive formulated using EPON 828, piperidine and CTBN alleviated the suppression effect and demonstrated retention of virgin fracture toughness of adhesives.

Multiple crack healing of a Ti2AlC ceramic

A highly attractive self-healing material would be one which combines excellent mechanical properties with a multiple healing capability. Selfhealing ceramics have been studied for over 40 years to obtain some performance recovery and to prevent material failure during service, but so far only materials with the capability of single healing event per damage site have been realized. Here we report on a self-healing Ti2AlC ceramic capable of repeatedly repairing damage events. The Ti2AlC ceramic achieves at least seven healing cycles after repeated cracking at a given location. The main healing mechanism at high temperature is the filling of the cracks by the formation well adhering -Al2O3 and the presence of some rutile TiO2. For healed samples, the flexural strength returned or even slightly exceeded the virginal strength. The fracture toughness recovery has been quantified for multiple healing cycles. © 2012 Elsevier Ltd. All rights reserved.

Pressurized vascular systems for self-healing materials

An emerging strategy for creating self-healing materials relies on embedded vascular networks of microchannels to transport reactive fluids to regions of damage. Here we investigate the use of active pumping for the pressurized delivery of a two-part healing system, allowing a small vascular system to deliver large volumes of healing agent. Different pumping strategies are explored to improve the mixing and subsequent polymerization of healing agents in the damage zone. Significant improvements in the number of healing cycles and in the overall healing efficiency are achieved compared with prior passive schemes that use only capillary forces for the delivery of healing agents. At the same time, the volume of the vascular system required to achieve this superior healing performance is significantly reduced. In the best case, nearly full recovery of fracture toughness is attained throughout 15 cycles of damage and healing, with a vascular network constituting just 0.1 vol% of the specimen.