Future Work

For these modified compact tension specimens to become practical for further use, much analysis needs to be completed. The first step would be to do a comprehensive mechanical analysis of the behavior of these samples. Experiments like digital image correlation may be useful for conducting this analysis, but pairing with computational techniques is recommended. This analysis would be useful for beginning to understand exactly how different damages correspond to sample compliance. If compliance can be accurately predicted for any crack geometry (matrix crack length and debonding size), then the energy release correlating to different crack growths would be better understood.

For any further experimental work, a return to a glass fiber system as first proposed by Jones [31] would be required. Glass fibers have a modulus approximately 10x lower than that of carbon fibers. This difference in modulus makes it more difficult to fracture the fibers and allows

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for greater load drops due to debonding. Another advantage of glass fiber is that debonding damage is much easier to observe in situ on glass fibers than on carbon fibers. These factors together would make for easier experimentation that could allow for some of the important questions about the relationships between damage geometry, energy release, and compliance to be evaluated.

Another interesting question remaining after this study is the cause of the high healing performance in the model composite reference testing. Many potential reasons for this improvement in healing have been considered, but none have been experimentally or computationally verified. First, most healing testing that has been conducted is done in mode I. The presence of the tow in the model composite ensures that other loading modes are present in this test. Without a better understanding of how fracture mode impacts healing performance, it is impossible to confirm or refute the potential that changes in fracture mode could account for improved healing performance.

Healing of interfaces is another potential for the improvement in healing. It was found by Jones et. al. [31] that near perfect healing of fiber/matrix interfaces can be achieved. This is a significant improvement over most of the healing that has been demonstrated for glassy systems in literature. However, the healing observed in that interfacial study peaked at 100%, which means in cannot fully explain the >100% healing observed in the model composite reference test.

A final potential cause of the increase in healing performance is solvent swelling of the thermoplastic creating pressure by being constrained between the epoxy and carbon fibers. It is possible that swelling of the thermoplastic due to the presence of solvent could create pressure that serves to strengthen the interface. This pressure would have to be overcome before the re-initiation of damage. To test this hypothesis, a different healing system would have to be used that heals without expanding.

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Finally, any further experimentation with this sample type should be conducted using a different self-healing system. Foremost, the opaque samples make any sort of optical analysis of the damage and its repair nearly impossible. Additionally, while learning about if and how a protocol works, it would be logical to select a healing system that does not have as complicated of a preparation process or as long of a healing cycle.

46 5 References

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49 A APPENDIX A

DIGITAL IMAGE CORRELATION

Digital image correlation (DIC) is an experimental technique for finding the displacements on the surface of materials during mechanical testing. It is a promising experimental avenue for continuing development of the model composite test. Some preliminary DIC experimentation was conducted using both the thermoplastic toughened epoxy and 8605 test system.