Self-healing materials are an emerging class of smart materials that are capable of repairing themselves from damage, either spontaneously or under a stimulus such as light, heat, or the application of a solvent. Intended for an audience of researchers in academia and industry, this book addresses a wide range of self-healing materials and processes, with emphasis on their performance in the space environment. This revised, expanded and updated second edition addresses the key concepts of selfhealing processes, from their occurrences in nature through to recent advances in academic and industrial research. It includes a detailed description and explanation of a wide range of materials and applications such as polymeric, anticorrosion, smart paints, and carbon nanotubes. Emphasis is given to performance in the space environment, addressing vacuum, thermal gradients, mechanical vibrations, and space radiation. Innovations in controlling self-healing materials for space debris mitigation are also covered. The book concludes with a comprehensive outlook into the future developments and applications of self-healing materials.
Inspec keywords: polymers; space debris; fibre optic sensors; composite materials; biomimetics; impact testing; materials preparation; aerospace materials; intelligent materials
Other keywords: space environment; fibre sensors; polymers; healing mechanisms; self-healing materials; impact tests; natural processes; natural systems; COPV; finite element analysis; orbital space debris simulation; advanced fabrication processes; composites
Subjects: Nondestructive materials testing methods; General topics in manufacturing and production engineering; Aerospace industry; Materials testing; Intelligent materials; Testing; Other topics in aerospace; Ballistics and mechanical impact (mechanical engineering); Intelligent materials (engineering materials science); Other methods of preparation of materials; Polymers and plastics (engineering materials science); Textbooks; Engineering mechanics; Composite materials (engineering materials science); General electrical engineering topics; Engineering materials
This book begins with a description of the methods for evaluating different self-healing approaches efficiencies. Next some examples of different approaches proposed to heal the thermoplastic systems are discussed and this is followed by covering the preparation and characterisation of self-healing of thermosetting systems. A particular emphasis is given to space applications. The book concludes by considering future research.
Natural systems, such as biological materials, have the ability to sense, react, regulate, grow, regenerate and heal themselves. Biological materials constitute most of the bodies of plants and animals around us. They allow cells to function, eyes to capture and interpret light, plants to respond to the light and animals to move or fly. This multitude of operations has always inspired mankind to make materials and devices, which simplify many of our day-to-day functions. One remarkable property of natural materials and structures is their ability of self-sealing and self-healing. Many animals and plants regenerate tissues or even whole organs after injury. However, biological repair processes are generally very complex and an adaption into a technical system is not easy. Recent advances in chemistry and microand nanoscale fabrication techniques have enabled biologically inspired technical systems that mimic many of these remarkable functions. For example, self-cleaning surfaces are based on the super-hydrophobic effect, which causes water droplets to roll off with ease, carrying away dirt and debris. Design of these surfaces is based on the hydrophobic microand nanostructures of a lotus leaf. In this chapter, the most successful strategies are examined, and future research directions, opportunities and outlooks are discussed. As example, the self-cleaning phenomenon and the self-healing of human wound are described in detail.
Modelling of the nature is developed at three levels. At the first level, a relatively simple approach is used to imitate a natural function such as healing human skin, which can then be used for healing a crack. At the second level, various models are created to produce a multifunctional component, for example, a biomimetic of shark skin, which can be adopted for a swimming suit that gives an increase in the swimming speed. At the same time the textile would be capable of repairing itself after a scratch or a puncture. At the third level, a model will be based on a more complex design. Most of the models developed try to predict and optimise self-healing behaviour of materials at the first level. Some of the most interesting work in that area is reviewed in Sections 3.1 and 3.2 and gives an example of such modelling with finite element analysis (FEA). No work has been reported on the second level models. Recently several approaches at third level modelling have been proposed and developed. A brief summary of these models is reviewed in Section 3.3.
Since the first report of the self-repairing composites systems in the literature [1], a conventional strategy was developed by embedding a microencapsulated liquid healing agent and solid catalytic chemical materials within a polymer matrix. Thus, when there is damage induced cracking in the matrix, the microcapsules release their encapsulated liquid healing agent into the crack planes. All the materials involved must be carefully engineered. For example, the encapsulation procedure must be chemically compatible with the reactive healing agent, and the liquid healing agent must not diffuse out of the capsule shell during its shelf-life. At the same time, the microcapsule walls must be resistant to the processing conditions of the host composite. At the same time excellent adhesion with the cured polymer matrix has to be maintained to ensure that the capsules rupture upon composite fracture.
Various methods are used for the evaluation of the healing efficiency. One of the three fracture modes (i.e., Mode I, II or III, Figure 5.1) is induced in two sets of devices. The first set of devices includes samples from the original host material; the second set includes samples containing the self-healing agent and the catalyst. After the healing process is completed, a standard test is performed to compare the two sets of the devices. A second test can be run in parallel, or separately, to validate the results from the first test. Some of the common tests used to measure self-healing efficiency are as follows: stretching the sample up to its rupture; three and four-point flexure bend tests; indentation tests; ballistic test with projectile; hypervelocity impact test; accelerated aging damage tests.
In this chapter, the main experimental results obtained to date on the selfhealing composite materials are reviewed. The review starts with the nanostructuration of the ruthenium Grubbs' catalyst (RGC) by means of the laser ablation process, followed by the encapsulation of the 5-ethylidene-2norbornene (ENB) liquid monomer into small capsules and the fabrication of three-dimensional (3D) microvascular nanocomposite beams by microfluidic infiltration. Special attention is given to the use of single-wall carbon nanotubes (SWCNT) material as reinforcement of the ENB healing agent from the perspective of obtaining a self-healing composite material with improved mechanical properties and, at the same time, having a fast ring-opening metathesis polymerisation (ROMP) reaction with high mechanical properties.
The space environment is quite hostile to structural materials. The lifetime of space-craft is determined by the environmentally induced degradation of the structural materials. The self-healing systems discussed in this book chapter are applied mainly to polymer matrix-based materials. They usually include a variety of epoxies, polyimides, polysulfones and phenolics. In particular, cyanate-ester resins have been considered due to their lower hygrostrain/outgassing compared to the first generation of epoxy matrices. Carbon, glass and aramid fibres have also been used as reinforcing filaments within composite space structures. Aramid fibres are often employed as a `buffer' within a shielding system against damage induced by micrometeoroid impacts.
The presence in space of micrometeoroids and orbital debris, particularly in the lower Earth orbit, presents a continuous hazard to orbiting satellites, spacecraft and the International Space Station. Space debris includes all nonfunctional man-made objects and fragments in Earth orbit. As the amount of debris continues to grow, the probability of collisions that could lead to potential damage will consequently increase. In this book chapter, the feasibility of self-healing of impacted composites in space is discussed.
In the book chapter, the authors review results regarding the mitigating of the impacts of space small debris on composite overwrapped pressure vessels (COPV) by using fibre sensors monitoring and self-repairing materials. The small debris impact dynamic was detected and monitored with fibre Bragg gratings (FBG) sensors at very fast acquisition frequencies, up to 500 MHz (2 ns), measuring the variation of the total reflected signal by the FBG. The acquisition system was based on commercially available products.
We have presented a series of recent results related to the various self-healing concepts and systems. Research of self-healing materials is an active and exciting field, with an increasing number of articles published every year. This research covers a wide spectrum of different materials and methods such as healing of concrete structures using embedded glass fibres and the more recent work on healing using shape memory alloy wires in a polymer composite, and/or the use of a multidimensional microvascular network for the healing applications.
