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Abstract
Aiming at the self-diagnosis and self-healing problems of composite materials, inspired by the self-healing mechanism of organisms, this paper proposes a damage self-healing method for composite structures based on bionic optical fiber. The paper introduces the designed structure and material composition of the bionic optical fiber, and the self-healing principle of the bionic optical fiber-composite structure. Finally, the three-point bending method is used to conduct damage self-healing experimental research on the composite material, and the influence of the embedding of the bionic optical fiber on the structural properties of the composite material is also analyzed. The repaired images of the composite structure obtained by the microscope can be seen that the physical structure repair is well. Moreover, the damage self-healing experimental results show that the self-repair efficiency of the flexural properties of the composite structure is 65.05%. The proposal of bionic optical fiber has important significance and practical application value for the research on the damage self-healing of composite materials.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Composite material is composed of two or more than two kinds of heterogeneous or special-shaped material. It has the advantages of high specific strength, high modulus, impact resistance and fracture resistance [1]. In November 1998, the “21st Century Materials Research, Technology and Education Symposium in the Service of Society in the United States and the Asia-Pacific Region” was held in Hawaii, and a branch of biological materials was proposed. Intelligent composite structures refer to materials and processes inspired by biological principles. The research fields include biosensors, biochips, and structures. The aim is to predict and repair the damage. It is possible to autonomously monitor damage and repair it online, thereby extending the service life of the material [2]. Different from the traditional repair method, the damage self-healing of intelligent composite structure is like the self-repair of biological tissue. Taking the self-healing function of biological bones as an example, when a biological bone is damaged, the blood vessels at the damaged site are ruptured, and blood flows from the crack to form a blood clot. The blood clot connects the crack to avoid further expansion of the wound. Immediately, new bone tissue will be produced in and around the crack, and the new bone tissue will fuse with the surrounding native bone tissue, and gradually grow into normal bone under the action of the biological matrix, completing the damage self-healing of the bone. Inspired by such a self-healing mechanism of organisms, scientists from various countries have introduced this mechanism into the composite structure, providing a novel and effective way and new research ideas for the damage self-healing of the composite structure. How to imitate the self-healing mechanism of organisms, realize the self-healing of composite intelligent structure damage is the key and difficult problem that needs to be solved urgently in the research on bionic self-healing of composite intelligent structure.
In general, according to whether the repair agent is embedded or not, composite bionic self-repair methods can be divided into two categories: intrinsic and extrinsic composite self-repair. The intrinsic self-healing method refers to a system in which the composite material can self-repair under external action without adding any additional media into the composite material. The non-intrinsic composite material self-repairing is a self-repairing method by embedding repair media (including microcapsules, hollow fibers, etc.). For example, when a crack occurred in the microcapsule or hollow fiber, the repairing agent is released from the microcapsule or hollow fiber, and the repairing agent reacts with the crack surface to bond together and self-healing is achieved [3–5]. At present, the researched extrinsic self-repair methods mainly include thermoplastic materials, microcapsules, hollow fibers, vascular-like, nanofibers, and carbon nanotube self-repair.
Since the 21st century, research on the use of microcapsules for damage self-repair of composite materials was very deep, represented by White SR, Brown EN and Kessler MR [6–8]. They dispersed and buried the microcapsules and Grubbs catalyst into the composite structure. The results of the study showed that the repair agent flowed out near the broken microcapsules and polymerized with the similar catalyst to repair the cracks. The repair efficiency was 38% to 90%. In the past ten years, Chinese scholars, such as Yong Sun, Zhuo Ni, Liying Wan et.al [9–11] have also begun to conduct research on microcapsule bionic self-repair. They also achieved great results. In 1992, Dry C [12] proposed a hollow fiber self-healing method to embed a 10 cm long hollow fiber filled with adhesive and 100 µL solvent in an epoxy resin-based composite material. Under external load, the fiber ruptures, and the adhesive gathers. Bonding occurs at the cracks to prevent further expansion of damage cracks. In 2014, Zhang H [13] et al. etched micron holes on insulating glass and used these holes as containers for epoxy resin and amine solution (curing agent) to realize the self-healing of epoxy-based composites. Through experiments, the repair rate of this self-repair system is as high as 93%. In 2016, Lee MW [14] et al. embedded nanofibers containing epoxy resin and curing agent into a polymer matrix. Experimental results showed that the fibers can be broken by stretching to release epoxy resin and curing agent. The curing reaction occurs to repair cracks, the rigidity of this material can be restored or even improved after repair, and it has good self-repair performance. In 2016, Quigley E [15] proposed a self-healing carbon nanotube/epoxy nanocomposite material on the nanometer scale, in which the healing agent dicyclopentadiene was injected into the carbon nanotubes. The results showed that when the self-healing nanotube/epoxy nanocomposite is damaged and cracks occur, the crack will be prevented from further expansion. In 2017, P. Li and Y. Liu [16] studied the damage self-healing of resin matrix composites based on microtubular network carrier. In 2019, Hart et al. [17] studied on the healing of impact damage of hollow fiber reinforced composite beams by bending test reinforced composite beams. They used a new air-assisted drug delivery scheme, and applied two portions of epoxy resin and amine healing agent to the impact damage beam specimens. The results show that the strength of the specimen after damage healing is restored by 47%, and the modulus is restored by 83%. In 2020, R. Shen et al. [18] studied a multi-diameter repair agent delivery system in order to achieve multi-point self-healing of scratches on composite surface coatings. S. Wei [19] successfully prepared DCPD / PA coaxial fiber by electrospinning, and added it into CF / EP composite as a nanofiber membrane interlayer to prepare a high strength and toughness self-healing composite DCPD/PA-CF/EP. The presence of nanofibers increased the flexural strength, fracture strain and fracture work of the composites by 10.59%, 11.26% and 53.69%, respectively. In 2021, some experts proposed that the 3D microvascular network generated by the 3D printing method is located inside the composite specimen [20]. The healing efficiency of the composites was investigated by mechanical tensile and creep tests. The experimental results show that the maximum healing rate of the tensile strength of the specimen can reach 89%. C. Kunakorn [21] et al. prepared self-healing composites using bromobutyl rubber (BIIR) and natural rubber (NR) blends as raw materials, filled with carbon nanotubes (CNT) and carbon black (CB). BIIR was modified by ionic liquid (IL) and butimidazole (IM). BIIR and NR were mixed in the ratio of 70:30 and 80:20 to achieve the best self-healing effect. In 2022, Chen Liang et. al [22] prepared an epoxy resin composite insulating medium containing one-component UV-sensitive microcapsules. When cracks occurred on the surface of epoxy resin, the mechanical stress concentrated on the capsule shell and ruptured it. The core photosensitive repair agent flowed out to fill the damage channel, and the curing self-healing of the damage channel was realized under external UV excitation.
Through inquiring and reading many literatures, in the field of biomimetic self-repair of composite material structure damage, the most researched is microcapsule and hollow fiber implantation method. Microcapsule and hollow fiber can solve the self-repair problem very well, and the repair efficiency is also very high. But the repair cannot be supplemented in time. That is said there is no continuity, and it has no perception of the external environment. In the actual application process, it is necessary to take the initiative to discover existing problems and threats from the external environment. In 1998, the research group of Academician Tao Baoqi [23,24] from Nanjing University of Aeronautics and Astronautics firstly introduced hollow fiber applied to damage repair of composite material. The hollow optical fiber has a better ability to perceive external effects, and it can carry a repair agent to repair the damage. But in this repair system, two-component epoxy resin glue is used as the self-healing material, and a repair fluid injection device is designed to provide enough repair fluid. The design of the entire structural device is more complicated. In addition, the hollow optical fiber they use is larger in diameter. It also has a greater impact on the structural properties of composite materials, which is difficult to achieve in practical engineering applications. In recent years, Zhimin Zhao et al. [25–27] proposed a self-healing method of composite structure damage based on liquid core fiber. Light curing agent is used as repair material. Liquid core fiber not only can realize the self-sensing of composite structure, but also realize the damage self-healing of composites.
Based on the above literature review results and inspiration of biological self-healing mechanism, we proposed the bionic optical fiber, which used photocuring materials instead of the glass core or the hollow core. And it is applied to the damage self-healing of composite structures. It is a new method for the damage self-healing of composite structure. In this paper, the designed structure of bionic optical fiber and the damage self-healing principle of composite structure are introduced in detail. The damage self-healing experiments are carried out and the repair efficiency is given.
2. Bionic optical fiber
The structure of bionic optical fiber is mainly composed of coating, cladding, light windows, and fiber core. The cladding is quartz glass. The coating material is carbon black. Plastic fiber is used as light window. Photocuring material is the core of bionic fiber. The function of the coating is to prevent the core from curing prematurely. The designed structure of bionic optical fiber is shown in Fig. 1.
The bionic fiber structure designed in this paper is composed of liquid core fiber and plastic fiber. Liquid core fiber is composed of quartz glass tube and liquid core, with an outer diameter of 0.3 mm and an inner diameter of 0.2 mm. Plastic fiber is used as cylindrical window and closes the fiber port. The core refractive index of plastic fiber is 1.45. Its diameter is 0.15–0.2 mm. It can be coupled to the quartz glass tube. The solidified adhesive was applied to the joint of plastic optical fiber and liquid core fiber. The plastic fibers not only serve as light windows, but also couple light into bionic fibers. The fiber coating includes two types of material: the light screening agent and adhesives. Carbon is selected as light screening agent adding about 1%. The adhesive consists of epoxy resin and curing agent. In order to improve the bonding strength of adhesive, coupling agent was added into adhesive. The self-made bionic optical fiber is shown in Fig. 2.
For bionic optical fiber, fiber-core is the most important component. The light-curable materials are used as the fiber-core. The photocuring system is generally composed of photo initiator, oligomer, and epoxy active diluent, which can absorb ultraviolet light and generate free radicals or cations, thus triggering polymerization and cross-linking reaction between oligomer and active diluent to form reticular structure film. In this paper, bisphenol-A epoxy acrylates and polyester acrylates were selected as oligomers, IBOA and 1, 6 hexanediol diacrylates (HDDA) as active diluent, 1173, 184 and TPO as composite photo initiators. In the preparation of photocurable materials, the properties of fiber core materials such as viscosity, complete curing time (curing rate), curing hardness and adhesion are considered comprehensively. Finally, the photocuring agent composed of oligomer, active diluent, and photo-initiator with a ratio of 5:3:0.4 is used as fiber-core. The refractive index of the prepared optical curing agent was 1.525 and the wavelength of curing light source (Ultraviolet curing lamp) is 365–410 nm.
3. Self-healing principle
The bionic optical fiber with light-curable core will be applied in damage self-healing of the composite structures. This idea comes from the self-perception, auto response and self-healing properties of organisms. We are wondering whether composite materials can have self-sensing, self-reactive and self-healing properties like organisms. Bionic optical fiber makes it be possible. The sensing characteristics of optical fiber are the basis for the self-diagnosis of composite structures, and the light-curable core is the structural basis for the damage self-healing of composite materials. This paper focuses on the application of bionic fiber in self-healing of composite structures.
In this paper, the damage self-healing of composite structures is researched by taking the epoxy resin matrix composite plate as the experimental object. The damage self-healing process are shown in Fig. 3. The bionic fiber is embedded in the surface layer of the composite structure to form the composite structure based on the bionic fiber.
When the composite material is damaged and cracks occur at the same time as the optical fiber, the light-curing agent will flow out and infiltrate into the crack. The preparation can be cured under ultraviolet light at the time of the injury, then the crack is repaired.
4. Experimental method and preparation
4.1 Experimental method
The experiment is to make the specimen damaged or achieve the predetermined deflection value with a constant loading rate through the three-point bending when the specimen is unconstrained supported [28]. In the whole process, the loads applied on the composite laminates were measured to determine the bending strength. The bending strength of the composite plate is the bending stress of the failure load or maximum load under the bending failure, the schematic diagram of the three-point bending test device [29] is shown in Fig. 4, the sectional view of the composite plate is shown in Fig. 5.
Fig. 4. The schematic diagram of the three-point bending test device test bearing 2. pressure head 3. composite plate 4. liquid core optical fiber h thickness of composite plate F load pressure l span length L length of composite plate P load)
The calculation of bending strength is as follows:
Suppose M is the concentrated load bending moment of composite plate, W is the section modulus, σ is bending strength. They are given as Eq. (1): [30]
So, the bending strength is given as Eq. (2):
4.2 Experimental preparation
The composite materials plate prepared in the experiment is shown in Fig. 6. Bionic fiber is embedded in the surface layer of composite plate. The length of composite laminates is 10 cm, the thickness h is 2.45 mm, the width b is 15.24 mm, the length of embedded bionic fiber is 10 cm, and the buried depth is 0.35 mm.
In order to study on the influence of the embedded fiber on the structural performance of composite materials, composite structural plates were prepared without embedded biomimetic fibers. The final experimental samples were prepared as shown in Table 1. Sample 1 is a composite plate without embedded any fiber. Sample 2 is a composite plate with an embedded bionic fiber, and the diameter of the embedded optical fiber is 0.36 mm, embedded in the surface layer of composite plate. Three groups are prepared for each sample.
Table 1. Experimental Samples
The experimental device is shown in Fig. 7. In Fig. 7, one end of the fiber in the composite plate is connected to a communication light source and the other end is connected to an optical power meter. The wavelength of communication light source is 632.8 nm. The bending and damage experiments of composite plates were carried out by three-point bending method. When the structure is bent until the crack occurs, the repair light source will be turned on. The bending strength of composites could be calculated and recorded.
5. Experimental results analysis and discussion
5.1 Influence of an embedded fiber on structural performance of composites
In this paper, the influence of the embedded bionic optical fiber on the structural performances of the composite material is analyzed through the bending strength of the composite material plate. Three groups of composite material plates were taken for each group of samples for experiment, and the average value of the bending strength of the three groups of composite material plates is taken as the experimental result. Bending test results of composite plates are shown in Table 2. The results in Table 2 show that the flexural strength of the composite board without embedded bionic optical fiber is 139.3 MPa, while after embedding the bionic fiber, the flexural strength of the composite board is reduced to 138.5 MPa. It reduced 0.5%. The reason is that the volume fraction of embedded fibers is small. The volume of one fiber is only 0.35% of the sample 1. Therefore, the embedding of a single bionic fiber has little influence on the bending properties of composite structures.
Table 2. Bending test results of composite laminates
5.2 Damage self-healing of composite structures
Evaluation of composite damage self-healing results should include two aspects. One is the physical structural damage self-healing of the composite materials, and the other is the structural performance repairing of the composite materials. Only the flexural properties of composite structures are discussed in this paper.
In the experiment, in order to evaluate the damage and self-healing of the physical structure of the composite material, we used a microscope to collect the damage and repair result images of the composite material structure. The magnification of the microscope is 20. The images collected by the microscopic optical system are shown in Fig. 8.
Figure 8(a) is the crack damage structure image of sample 2. It shows that when a crack occurs, the liquid core flows out of the fiber and fills the crack. Figure 8(b) is an image of the composite structure damage repaired by the repair light source. It can be seen from Fig. 8(b) that the outflowing liquid core is solidified and the structural damage of the composite is repaired. But it also can be seen that the crack is not filled. The reason is the cure shrinkage effect.
Images of experimental results show that when the composite is damaged, the liquid core flows out and into the crack. With light curing, the liquid core can repair cracks in real time. However, due to the curing shrinkage effect of photocurable materials, the composite structure cannot be completely repaired.
In addition, the flexural strength of the composite structure is used as the performance index, and the repair efficiency of the composite structure is used to evaluate the repair performance of the composite structure. Here, the repair efficiency of the flexural strength of the sample is expressed by Eq. (3):
In Eq. (3), ${\sigma _{healed}}$ represents the bending strength value of the sample after repair, ${\sigma _{virgin}}$ the bending strength of the original sample. The ratio is the repair efficiency of the sample. Three-point bending tests were performed on the composite panel sample 2 before and after the damage self-healing. The self-healing efficiency of the composite structure sample was obtained, and the results are shown in Table 3.
As can be seen from Table 3, the repair efficiency of composite structure is 65.05%. It shows that the flexural properties of composite plate structure can be restored to 65.05% of the original under the radiation of repairing light source. The self-healing efficiency of bionic fibers for composite structures is not very high. The reasons for this result are analyzed and discussed from the following aspects.
- (1) Size of damage crack. The damage degree of the composite structure is higher and the crack is larger, and more repair agent is needed. However, the size bionic fiber is small. The limited repair agent contained in the small size bionic fiber cannot completely repair the large damage crack. It will reduce the repair efficiency.
- (2) Shrinkage characteristics of the repair agent material. It can also be seen from the collected images that the cracks are not completely repaired due to the gravity factor and the shrinkage characteristics of the repair agent. It also affected the repair efficiency of the structural properties of the composite.
- (3) The adhesion of the repair agent. The viscosity of the repair agent and the adhesion of the composite material are contradictory. When preparing the light curing agent, the light curing agent is required to have good fluidity. That is, the viscosity is low. Therefore, it is also an important factor affecting the repair efficiency of composite materials.
Table 3. The repair efficiency results
6. Conclusions
A bionic optical fiber with a special structural design and surface layer damage self-healing method for composite structures were introduced in this paper. The repair effects are evaluated from two aspects: the physical repair of the composite structure and the repair efficiency of the bending performance of the composite structure. It could be seen from the images of the repaired structure that the material was repaired after the repair agent is cured. However, the crack is not repaired completed because of the limited repair agent and its curing shrinkage characteristics. Three-point bending tests were performed on the composite materials plates before and after the damage self-healing. The experimental results showed that the flexural properties of composite plate structure can be restored to 65.05% of the original. In order to improve the repair efficiency of the bionic optical fiber to the composite structure, the solution is to improve the performance of the curing agent. The damage self-repair of composite structures based on the bionic fiber is a new experimental exploration. Of course, more in-depth research is needed to improve the repair efficiency of composite materials. We will continue the following research in the future.
- (1) Continue to research on performance of the curing agent to improve repair efficiency.
- (2) Evaluate the repair efficiency of composite materials based on bionic optical fiber through other performance parameters, such as ratio of strength to density, specific modulus, fatigue resistance, not just bending strength.
- (3) Improve the structure design of bionic optical fiber to make it more suitable for damage self-repair of composite structures.
- (4) Study on the sensing performance of bionic fiber to realize the damage self-diagnosis of composite structures.
Funding
2021 "Science and Education Integration" project of Jinling Institute of Technology (No. 2021KJRH21); 2021 Innovation and Entrepreneurship Training program for university students of Jinling Institute of Technology (No. 202113573011Z); School-level research fund incubation project of Jinling Institute of Technology (No. jit-fhxm-2002); Ph.D. Project supported by the Jinling Institute of Technology (NO. jit-b-201814, No. jit-b-202012); Natural Science Foundation of Jiangsu Province (No. BK20190112).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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