**2. Self-healing materials**

Self-healing is the capability of a material to recover from any kind of damage automatically without any external intervention as obtainable in biological systems or with external stimulation such as heat, light, electrical stimulus and solvent. The materials that exhibit self-healing without any external intervention or stimuli are said to be autonomic while self-healing that involves human or external influence to induce healing is said to be nonautonomic in nature [5, 6]. One of the major problems encountered in the use of materials in diverse fields is how to ensure their durability and minimize structural failures [7]. A self-healing material is therefore an artificial material designed with built-in ability to detect failure and respond automatically to restore partial or full properties or function of the structure after encountering in-service damage [3, 7].

This in-service damage, which is usually in form of micro-scratches, surface and internal cracks, voids or other defects [8, 9], is majorly responsible for failure in materials systems. Over time, these micro-cracks accumulate and grow until catastrophic failure of the entire product or system occurs. Since this source of failure normally initiates at the nanoscale level and progresses subsequently to the micro- and macroscale levels until failure occurs, an ideal self-healing material would without any external influence prevent initiation of failure at these small length scales or repair already nucleated damage, thereby restoring the original material properties in a shortest time [3, 7]. Since the greater of the in-service damage encountered in material systems is usually in form of micro-cracks, voids or other defects, the objective of designing self-healing materials is to impart them with the capabilities to prevent the initiation of micro-cracks and voids or fill and seal them automatically.

For so many years now, the strategies of fabricating synthetic materials with the capability to self-heal like a biological system or as envisaged above have been exploited greatly. This huge interest is anchored on benefits of self-healing in materials. These benefits include enhancing materials' service lifetime, reduction in replacement costs and improvement in product safety [7]. Great advances have been witnessed in creation of self-healing materials since the birth of the concept. The concept has been exploited in almost all materials classes including polymers, metals, ceramics, cements, coatings and composites [3]. The design strategies and processing routes involved in creating self-healing capabilities in these material classes are different, just like their self-healing reactions to damage encountered during their lifetimes. The next subsection takes a look at the creation of selfhealing abilities in these systems, the prevalent mode of failures and mechanisms of self-repairing.

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systems.

**3.2 Self-healing metals**

temperatures [23, 25].

*Exploits, Advances and Challenges in Characterizing Self-Healing Materials*

**3. Preparatory routes and mechanisms of self-healing**

**3.1 Self-healing polymers and polymer matrix composites**

defense, biomedicine and construction industries [7, 12, 13].

agents into polymer networks via microvascular channels [5, 16].

by favorable reaction kinetics and post-polymerization as in (b).

The self-healing concept has been most successful in the development of self-healing polymer-based systems [10, 11]. This emanates from the fast diffusion rate and high plasticity due to open-molecular structures in polymers, which facilitate diffusion of healing agents to fill and seal voids or micro-cracks [10] even at room temperature. Unlike metallic and ceramic systems, polymeric systems are light weight, chemically stable and can be easily processed [4]. These properties are exploited in developing efficient self-repairing polymers and polymer-based fiber-reinforced composites, which have applications in transportation, electronics,

Based on the strategies exploited to achieve self-repair, polymers are generally grouped into extrinsic and intrinsic self-healing systems [14]. Intrinsic self-repair is achieved by synthesis of smart polymers containing functional groups with the inherent ability to reversibly polymerize or cross-link their bonds in the presence of a stimulus like light or heat [15] and by so doing act as healing agents. The processes for obtaining extrinsic self-healing include (a) embedding microcapsules containing curable healing agents into polymer networks; and (b) incorporation of healing

The microcapsule in (a) could be in form of capsule containing healing agent

The mode of damage often encountered in polymers and structural composites

When compared to other material systems, it is much difficult to achieve selfhealing in metals [22–24]. This is as a result of their high melting temperatures and strong atomic bonds, which limit diffusion of healing agents/solute atoms to sites of damage at low temperatures [22, 23]. There is also further restriction due to the relative small size and volume of the solute atoms. As a result, rate of mass transport to fill damage sites is intrinsically low at the usual low operating

The major factor limiting the useful life of metals is the occurrence of internal damage such as voids and cracks during processing or service. These defects usually initiate as nano- or micro-cracks in the bulk or on the surface, grow and propagate and eventually lead to failure. The self-healing process in metals in response to crack initiation follows the sequence of diffusion or release or transport of healing agents or atoms into the void or crack to fill and seal it, thereby restoring partially or fully the mechanical properties such as fatigue strength, stiffness and fracture toughness.

is in form of matrix micro-cracking, fiber breakage or delamination and fibermatrix debonding [7, 20, 21]. Self-healing mechanism or recovery or recuperation takes place when a damage/crack is encountered and is healed by intrinsic polymerization or polymerization of healing agent as crack ruptures the capsules as in (a) or

and catalyst or twin microcapsules each containing a monomer/resin and its hardener [5, 16, 17] while that in (b) can be in form of fibrous composite architecture impregnated with a microvessel filled with reactive healants [18]. Unlike extrinsic routes where healing agents are consumed during the curing process and are not replenished, intrinsic approaches have the advantage of multiple healing of damage in the same area owing to reversible polymerization [19]. Self-healing has been exploited and accomplished in thermoplastic, thermosetting and elastomeric

*DOI: http://dx.doi.org/10.5772/intechopen.93031*
