**Abstract**

Self-healing materials are the next-generation materials for high-performance structures. To reduce the fatigue and subsequent probability of failure along with extended service life of polymer and polymer composites, the self-healing concept has great potential. Today, polymeric composites are structural matrix and prone to failure against cyclic mechanical and thermal loading. Significant degradation of polymeric structures at surficial sites can be measured by barely visible impact damage (BVID), but internal micro-cracks are not easily detectable. Various damage modes make major damage sites in composites and further lead to catastrophic failure of the structure. On-site repairing of microscopic or macroscopic damages in polymer composites is a value-added function that is offered by self-healing techniques. Different extrinsic methods including encapsulation, hollow fiber embedment, and vascular methods are preferred, and some intrinsic, dynamic bonding is created by reversible covalent networks and supramolecular interaction based on H-bonding, metal-ligand, and ionomers. This chapter is preferred on the new trends and challenges regarding the structural health monitoring of polymeric composites against external mechanical and environmental impacts and extended service life and performance by utilizing self-healing strategies.

**Keywords:** intrinsic and extrinsic self-healing, covalent reversible network, supramolecular interactions

## **1. Introduction**

Polymers and fiber-reinforced polymers (FRPs) are common as structural materials due to lightweight, easy processability, and constancy against adverse environmental impacts. However, mechanical properties are associated with many variables including service time, operating temperature and pressure, molecular weight, and constitution of matrix. The long-term durability, high performance and reliability are major challenges for polymeric architecture. Limiting factors of polymer composite is relatively poor performance under impact loading due to lack of plastic deformation, which is a most prominent aspect of any vehicle component design. Low-velocity and high-velocity impacts are the critical issues for FRPs. These impacts influence on mechanical strength and stiffness along with dimensional stability. In metals, the impact energy is dissipated through elastic and plastic deformations, so structural integrity is retained intact. However, in FRPs the impact energy is dissipated in the form of damages in matrix. The impact damages in FRPs affect mainly the internal integrity as compared to superficial visible zone. On cyclic mechanical and thermal loadings, stress is applied on a matrix which released in the

form of micro-cracking. Micro-cracks degrade polymeric properties inevitably and result into the failure. Single or multiple damage modes result into damage sites and are required to repair the components to continue to the service. If microcracks are untreated, they lead to larger macroscopic cracks, and finally catastrophic loss of the structure is an adverse outcome. To resist these failures, the new-generation materials having autonomic healing capability to damage repair are needed to develop. Conventionally, different lay-up repair techniques are adopted in thermoset.

Similarly, in thermoplastics which are known for their mechanical performances, service temperature, and solvent resistance, fusion bonding techniques are used to recover mechanical integrity.

However, these repair strategies are very costly, time-consuming, followed by complicated procedure, and assisted only by expert technicians. So, we required such unique functionality which intimates the damages at nascent stage and sometimes repaired the damages. Self-healing of cracks is an eminent and efficient possible solution of these issues. In healing systems, a damage incident triggers the internal processes that generate the healing response which cured the damages. The bio-systems have damage detection and subsequent prevention techniques; those are source of innovation to design such functionality by introducing the self-healing functionality into artificial materials. These techniques are demanding for manned materials and structures and assisted by the biomimetic approaches. The major concern is focused on recovery of mechanical properties of polymer composites against quasi-static fracture. Initially, Mercier developed a self-healing rubber that can reseal on puncture damage [1]. The concept of healing is progressed with developing efficient vehicles and other systems such as space suits to protect from micrometeoroid impacts [2]. Currently, damage healing of polymer structure is being demonstrated via different approaches, which include extrinsic and intrinsic methodologies. Extrinsic methodology is being performed via encapsulation [3–8], hollow fibers [9–11], microvascular systems [12–16] and intrinsic damage healing is offered by reversible covalent bonding [17–21] and supramolecular interactions [22–30]. In extrinsic methodology, healing is restricted only once, and the delivery of healing precursor's amount is limited. To overcome the above concern, different intrinsic healing systems are developed that offer multiple healing at macroscopic damages. Multiple healing of the same crack is achieved by microvascular 3D system, thermoreversible networks, and supramolecular interaction. These strategies help to design various self-healing systems. In literature, various high-performance systems such as self-healing coating, self-healing ceramics, and self-healing metals are reported. Self-healing nanocomposites are also reported. Self-healing technology provides public safety and reduced maintenance cost of the structure. Healing approaches offer longer lasting with faulttolerant components across various fields including coatings, electronics, robotics, transportations, energy, etc. In the following section, the author is mainly focused to elaborate current trends and the leading research field of remendable polymers for structural composites.
