Self-Healing Materials Systems as a Way for Damage Mitigation in Composites Structures Caused by Orbital Space Debris

1. Introduction

Polymers and structural composites are used in a variety of applications. However, these materials are susceptible to damage induced by mechanical, chemical, thermal, UV radiation, or a combination of these factors (Bucknall, Drinkwater, & Smith, 2004). When polymer composites used as structural materials become damaged, there are only a few methods available to attempt to extend their functional lifetime. Ideal repair methods are ones that can be executed quickly and effectively directly on damaged site, eliminating thereby the need to remove a component for repair. However, the mode of damage must also be taken into consideration as repair strategies that work well for one mode might be completely useless for another. For example, matrix cracking can be repaired by sealing the crack with resin, where fibre breakage would require new fibres replacement or a fabric patch to achieve recovery of strength. One of the earlier healing methods for fractured surfaces was “hot plate” welding, where polymer pieces were brought into contact above the glass transition temperature of the material, and this contact was maintained long enough for interdiffusion across the crack face to occur and restore strength to the material. It has been shown, however, that the location of the weld remains the weakest point in the material and thus the favourable site for future damage to occur (Liu, Lee, & Lu, 1993). For laminate composites, resin injection is often employed to repair damage in the form of delamination. This can be problematic, however, if the crack is not easily accessible for such an injection. For fibre breakage in a laminate composite, a reinforcing patch is often used to restore some of the strength to the material. Often, a reinforcing patch is used in conjunction with resin injection to restore the greatest amount of strength possible (Osswald & Menges, 2003). None of these methods of repair is an ideal solution to damage in a structural composite material. These methods are temporary solutions to prolong the lifetime of the material, and each of these repair strategies requires monitoring of the damage and manual intervention to enact the repair. This greatly increases the cost of the material by requiring regular maintenance and service.

Alternative healing strategies are therefore of great interest. Moreover, with polymers and composites being increasingly used in structural applications space, automobile, defence, and construction industries, several techniques have been developed and adopted by industries for repairing visible or detectable damages on the polymeric structures.

However, these conventional repair methods are not effective, for example, for healing invisible microcracks within the structure during its service life. In response, the concept of “self-healing” polymeric materials was proposed in the 1980s (Jud, Kausch, & Williams, 1981) as a means of healing invisible microcracks for extending the working life and safety of the polymeric components. The publications in the topic by Dry and Sottos (Dry & Sottos, 1993) in 1993 and then White et al. (2001) further inspired world interests in these materials (Kringos et al., 2011). Examples of such interests were demonstrated through US Air force (Carlson & Goretta, 2006) and European Space Agency (Semprimosching, 2006) investments in self-healing polymers.

Conceptually, self-healing materials have the built-in capability to substantially recover their mechanical properties after damage. Such recovery can occur autonomously and/or be activated after an application of a specific stimulus (e.g., heat, radiation, pressure, etc.). As such, these materials are expected to contribute greatly to the safety and durability of polymeric components without the high costs of active monitoring or external repair. Throughout the development of this new range of smart materials, the mimicking of biological systems has been used as a source of inspiration (since most materials in nature are themselves self-healing composite materials) (Varghese, Lele, & Mashelkar, 2006).