The main issue with a parts’ life cycle is failure by fatigue which ultimately comes through crack propagation. Step forward fracture mechanics which is a relatively new science that dives into the mechanism of crack formation and spread. Thankfully there are now different scenarios to inspect and treat cracks in parts made from classic alloys but not many of them can be used as a default guideline for other non-alloy materials. When it comes to composites in general, and woven or laminated ones in particular, most methods devised to deal with cracks in alloys can’t be applied without greater damage being brought to the component. Fortunately, the research and engineering community is investigating innovative ways to deal with this problem and one of the promising techniques is the autonomic healing concept using self healing composites.
Autonomic healing and self healing composites
Failure by debonding and delamination are the most likely scenarios for laminated and woven composites. To repair such damage, bolting or patching are common practices, but they don’t restore the full properties of the composites and create their own array of problems. Besides, they are costly in respect of the time and expertise needed to set them up.
Autonomic healing, or self-healing, is a way to make use of the procedure of matrix formation on a microlocal scale, to mend cracks on their plane of propagation. The technique is rather simple: introducing microcapsules within the original plies or weaves that will contain either pristine resin or catalysts. Once the crack starts spreading, it will hit the capsules unleashing the resin and the polymerizing agent. Upon mixing, the polymer is formed and mends the fracture, thus clogging the crack, stopping its propagation and preventing failure. Hey presto, job done!
Further optimisation
The concept can be further optimised by locating the microcapsules in the critical regions of wear, which happen to be easily identified in the case of structural composites, thanks to the neat stacking or arrangement and the understanding of acting loads. Research conducted so far has shown a recovery ranging between 20% and almost 50% of the crack impact when the polymerization occurs at room temperature. If the heat is increased, the healing can be up to 80%.
While parts can’t be expected to constantly bathe in a 80C-100C environment, there are two factors that can actually provide a localised amount of heat and help in the microlocal regions of cracking. One of the characteristics of polymers is self-heating due to fatigue, and cracks formation and propagation is an exothermic operation that results in local heat at the crack’s tip and surfaces.
There is also another side to this evolutionary technique: the microcapsules and their positions throughout the composite which will inevitably alter its pristine properties. Therefore the material will not deliver the same toughness than the one predicted under its unfortified state. In addition to that, dispersion of the resin and the chemical agent along the dimensions within the composite can be tricky to manage and require an analysis on its own to determine optimised spread and dimensions. Such analysis often yields changes in the composite itself, such as modification of the plies’ thickness or weave configuration.
Compromise required with self healing composites
Self-healing requires many compromises when it comes to large scale cracks. Currently, the main path in research is the application of the technique on a microscopic level. Treating microcracks eliminates cracks at the source although it doesn’t require a high concentration of microcapsules nor much spread. Besides, the self-healing composites’ properties will be much closer to the pristine ones and won’t require any configuration change.
Microcracks do require a micromechanical approach though, and micromechanics is even newer as a discipline than fracture mechanics. Researchers are still developing the technique to study heterogeneous structures and describe their properties accurately. To improve self-healing composite and make up for their shortcomings, the study of microcracks, the optimization of microcapsules in dimensions, concentration or spread are the main challenges ahead. All of this ultimately relies on the understanding of the micromechanics behind the structures, another field where we have yet to expand our knowledge. All in all self healing composites are extremely useful but further research is required to get the best out of this area of mechanical engineering.
