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Basics of Self-Healing - State of the Art

CHAPTER MENU

• 1.1 Background
• 1.1.1 Adhesive Bonding for Healing Thermosetting Materials
• 1.1.2 Fusion Bonding for Healing Thermoplastic Materials
• 1.1.3 Bioinspired Self-Healing
• 1.2 Intrinsic Self-Healing
• 1.2.1 Self-Healing Based on Reversible Covalent Chemistry
• 1.2.1.1 Healing Based on General Reversible Covalent Reactions
• 1.2.1.2 Healing Based on Dynamic Reversible Covalent Reactions
• 1.2.2 Self-Healing Based on Supramolecular Interactions
• 1.2.2.1 Coordination Bonds
• 1.2.2.2 Ionic Associations
• 1.2.2.3 Hydrogen Bonds
• 1.2.2.4 Other Intermolecular Forces
• 1.2.2.5 Host-Guest Inclusion
• 1.3 Extrinsic Self-Healing
• 1.3.1 Self-Healing in Terms of Healant Loaded Pipelines
• 1.3.1.1 Hollow Tubes and Fibers
• 1.3.1.2 Three-Dimensional Microvascular Networks
• 1.3.2 Self-Healing in Terms of Healant Loaded Microcapsules
• 1.3.2.1 Methods of Microencapsulation
• 1.3.2.2 Healing Chemistries
• 1.4 Insights for Future Work
• References

1.1 Background

Polymers and polymer composites have been widely used in important engineering fields including aerospace, marine, automotive, surface transport, and sports equipment because of their advantages including light weight, good processibility, and chemical stability in any atmospheric conditions. However, long-term durability and reliability of polymeric materials are still problematic when they are used for structural applications [1, 2]. This is particularly true when impact resistance is concerned, which is a critical aspect of vehicle design. Their inability to undergo plastic deformation results in energy adsorption via the creation of defects and damage. Besides, exposure to a harsh environment would easily lead to degradation of polymeric components. Comparatively, microcracking or hidden damage is one of the fatal deteriorations generated either during manufacturing or in service as a result of mechanical stress or cyclic thermal fatigue (Figure 1.1). Its propagation and coalescence would bring about catastrophic failure of the materials and hence significantly shorten the lifetime of the structures.

Figure 1.1 Cohesive failures beside plated through holes on copper clad laminates, which used to be produced by thermal stress.

1.1.1 Adhesive Bonding for Healing Thermosetting Materials

Damaged composites or composite structures should be repaired when significant structural degradation is detected [3]. The routine repair procedures for thermosetting composites are shown in Scheme 1.1. Some damage to composites is obvious and easily assessed but in some cases the damage may first appear quite small, although the real damage is much greater. Impact damage to a fiber can appear as a small dent on the reinforced composite surface but the underlying damage can be much more extensive. The decision to repair or scrap is determined by considering the extent of repair needed to replace the original structural performance of the composite [4]. Other considerations are the repair costs, the position and accessibility of the damage, and the availability of suitable repair materials. Easy repairs are usually small or do not affect the structural integrity of the component. Complex repairs are needed when the damage is extensive and the original structural performance of the component needs to be replaced. The best choice of materials would be to use the original fibers, fabrics, and matrix resin. Any alternative would need careful consideration of the service environment of the repaired composite, i.e. hot, wet, and mechanical performance. The proposed repair scheme should meet all the original design requirements for the structure. Some repairs need specialist equipment of a workshop and some form of improvised repair is needed to return the component to a suitable repair workshop. A temporary repair, usually in the form of a patch, can be fixed to the component. Usually a "belt and braces" approach is taken to ensure safety until the component can be repaired at a later date.

Scheme 1.1 Flow chart of the key stages for thermosetting composite repair.

Most damage to fiber reinforced polymer composites is a result of low velocity (and sometimes high velocity) impact [5]. In metals the energy is dissipated through elastic and plastic deformations, and a good deal of structural integrity is retained. In polymer composites the damage is usually more extensive than that seen on the surface. Typical damage is summarized in the following. (i) Delamination following impact on a monolithic laminate. (ii) Laminate splitting, which does not extend through the full length of the part. Its influence on the mechanical performance depends on the length of split relative to the component thickness. (iii) Heat damage, a local fracture with separation of surface plies. Its effect on the mechanical performance depends on the thickness of the part. (iv) Dents in a sandwich structure. (v) Puncture damage in a sandwich structure. (vi) Bolt hole damage, which could be elongation of the hole causing laminate splitting, or damage to the upper plies.

Patch repair, a main technique based on adhesive bonding, involves covering or replacing of the damaged portions with a new material [5-7]. It restores the load path weakened or removed by damage or cracking, ideally without significantly changing the original load distribution. Reinforcements or doublers are used to replace lost strength or stiffness, correct design errors, or to improve performance [8]. Since the main purpose of composite repair is to fully support applied loads and to transmit applied stresses across the repaired area, the patch repair materials must overlap, and be adequately bonded to the plies of the original laminate. In this case, the thickness of the original laminate is made up of filler plies and the repair materials are bonded to the surface of the laminate. The advantages of this approach include the fact that it is quick and simple to do, and there is minimum preparation, while the repaired laminate is thicker and heavier than the original and very careful surface preparation is needed for good adhesion. The degree of property recovery is a function of bonding between the patch and the original material, the presence/orientation of reinforcing fibers and patch thickness [6,9-12].

In addition to patch repair, there are two similar techniques: (i) taper sanded or scarf repair; and (ii) step sanded repair. For the first one, an area around the hole is sanded to expose a section of each ply in the laminate. Sometimes one filler ply is added to produce a flatter surface. Taper is usually in the region of 30-60:1. Comparatively, the repaired version is only marginally thicker than the original. Each repair ply overlaps the ply that it is repairing, so a straighter and stronger load path is obtained. The freshly exposed surfaces help to achieve tight bonds at the interface. With respect to the second technique, the laminate is sanded down so that a flat band of each layer is exposed, producing a stepped finish. Typical steps are 25-50 mm per layer. Nevertheless, it is worth noting that the method needs high skill and is difficult to do.

To conduct bonded external patch repair for structural components, equipment and ancillaries must be employed (Figure 1.2). The vacuum bag is suited to components with thin sections and large sandwich structures. It involves the placing and sealing of a flexible bag over a composite lay-up and evacuating all the air under the bag. The removal of air forces the bag down onto the lay-up with consolidation pressure of 1?atm. The completed assembly, with vacuum still applied, is placed inside an oven with good air circulation, and the composite is produced after a relatively short cycle cure.

Figure 1.2 Typical repair processes of thermosetting composites represented by carbon fiber reinforcement polymer (CFRP).

Source: Reprinted from Kim et al. [4]. Copyright 2019, with permission from Elsevier.

1.1.2 Fusion Bonding for Healing Thermoplastic Materials

In general, the aforementioned thermosetting adhesive bonding is not directly transferable to thermoplastic polymer-based composite. Fusion bonding, or welding, a long-established technology in the thermoplastic industry, offers an effective way for rejoining fractured surfaces with thermal flowability [13]. Although welding may induce residual stresses if performed without adequate control, it eliminates the stress concentrations created by holes required for mechanical fasteners and so does thermosetting adhesive bonding. In addition, welding reduces processing times and surface preparation requirements [14]. However, the high content of carbon fiber reinforcement in the composites, resulting in high thermal and electrical conductivity, imposes difficulties such as uneven heating, delamination and distortion of the laminates. These problems become more difficult when bonding large components...