Advances in materials technologies have largely been responsible for major performance improvements in many engineering structures and continue to be the key in determining the reliability, performance and cost effectiveness of such systems. Lightweight, high strength and high stiffness fibre-reinforced polymer (FRP) composite materials are leading contenders to improve the efficiency and sustainability of many forms of transport. In addition, they offer immense scope for incorporating multifunctionality due to their hierarchical internal architecture. One limiting factor in their wider exploitation is relatively a poor performance under impact loading, a crucial aspect of any safety critical design, leading to a significant reduction in strength, stiffness and stability.
For example, carbon fibre-reinforced plastic (CFRP) used in aerospace applications is typically assigned an allowable strain level of less than 0.4% (Richardson & Wisheart 1996
; Zhou 1998
) whereas commercially available carbon fibres typically have a strain to failure of approximately 1.5%. This results in conservative design and higher weight structures. The inability to plastically deform results in energy absorption via the creation of matrix cracks and delaminations, which can be difficult to detect visually. Self-healing has the potential to mitigate the damage resulting from an impact event, thereby providing an opportunity to improve the design allowables for FRPs or offer other benefits such as reduced maintenance and inspection schedules.
Conceptual inspiration from nature is not new, and many engineering approaches can be considered to have been inspired by observing natural systems. The healing potential and repair strategies of living organisms is increasingly of interest to designers seeking lower mass structures with increased service life who wish to progress from a conventional damage tolerance philosophy. Naturally occurring ‘materials’ have evolved into highly sophisticated, integrated, hierarchical structures that commonly exhibit multifunctional behaviour (Curtis 1996
). Inspiration and mimicry of these microstructures and micromechanisms offer considerable potential in the design and improvement of material performance (Kassner 2005
), but many of the biological processes involved are extremely complex. Bioinspired self-healing using hollow fibres embedded within a structure has been investigated at different length-scales in several materials by various authors, e.g. bulk concrete (Li et al. 1989
; Dry 1994
; Dry & McMillan 1996
), bulk polymers (Chen et al. 2002
; Kassner 2005
) and polymer composites (Dry 1996
; Motoku et al. 1999
; Zako & Takano 1999
; Bleay et al. 2001
). The latter has seen exciting developments in recent years, e.g. (Kessler & White 2001
; White et al. 2001
; Kessler et al. 2003
; Brown et al. 2004
; Pang & Bond 2005a
; Trask & Bond 2006
), using the inspiration of biological self-healing applied with broadly traditional engineering approaches.
Hollow glass fibres (HGFs; Hucker et al. 1999
) are used in preference to embedded microcapsules (Brown et al. 2003
; Rule et al. 2005
) because they offer the advantage of being able to store functional agents for self-repair as well as integrating easily with and acting as a reinforcement. A typical hollow fibre self-healing approach used within composite laminates could take the form of fibres containing a one-part resin system, a two-part resin and hardener system, or a resin system with a catalyst or hardener contained within the matrix material (Bleay et al. 2001
). A schematic of these approaches is shown in .
A bespoke HGF making facility (Hucker et al. 1999
) has been used to produce HGF between 30 and 100
μm diameter with a hollowness of approximately 50%, . These are then embedded within either glass fibre-reinforced plastic (GFRP) or CFRP and infused with uncured resin to impart a self-healing functionality to the laminate. During a damage event, some of these hollow fibres will fracture, thus initiating the recovery of properties by ‘healing’ whereby a repair agent passes from within any broken hollow fibres to infiltrate the damage zone, and acts to ameliorate the critical effects of matrix cracking and delamination between plies and, most importantly, prevent further damage propagation. This release of repair agent mimics the bleeding mechanism in biological organisms (e.g. human thrombosis).
Figure 2 Typical hollow glass fibre (35μm external diameter with 55% hollowness fraction).
The exact nature of the self-healing method will depend upon (i) the nature and location of the damage, (ii) the choice of repair resin, and (iii) the influence of the operational environment. The self-healing fibres can be introduced within a laminate as additional plies at each interface, at damage critical interfaces or as individual filaments spaced at predetermined distances within each ply. In order to more fully understand and optimize the healing process, two parallel studies were undertaken in glass- and carbon-reinforced epoxy systems, respectively. A translucent glass/epoxy laminate provides good visualization of damage occurrence and the healing process when viewed with transmission microscopy, however, a carbon/epoxy laminate is opaque and therefore, to enhance visualization, a UV fluorescent dye (Ardrox 985) was added to the healing resin in both studies.