Polymer embedded damage detection

Micro capsules in polymer show cracks and damage Micro capsules in polymer break open and dye the cracks to show damage in the polymer
Detecting micro-cracks or damage to structures has always been the bane of engineers. Here, Engineering Materials looks at research that could make hidden damage visible, as it happens.

Damage developing in a material can be difficult to see until something breaks or fails. However, researchers are developing polymers with damage indication systems, which automatically highlight areas that are cracked, scratched or stressed, allowing engineers to address problem areas before they turn in to more serious issues.

The early warning system would be particularly useful in applications like petroleum pipelines, air and space transport, and anywhere composite materials are used due to the difficulty in spotting internal damage in the fibres.

“Polymers are susceptible to damage in the form of small cracks that are often difficult to detect,” says researcher, Professor Nancy Sottos from the University of Illinois in the US. “Even at small scales, crack damage can significantly compromise the integrity and functionality of polymer materials. We’ve developed a very simple but elegant material to autonomously indicate mechanical damage.”

The researchers embedded tiny microcapsules of a pH-sensitive dye in an epoxy resin, so if the polymer forms cracks or suffers a scratch, stress or fracture, the capsules break open and the dye reacts with the epoxy. This causes a dramatic colour change from light yellow to a bright red, with no additional chemicals or activatorsrequired.

To help assess the extent of the damage, the deeper the scratch or crack, the more microcapsules are broken, and the more intense the colour. Even so, tiny microscopic cracks of only 10µm are enough to cause a colour change, letting the user know that the material has suffered some damage and could have lost some of its structural integrity.

“Detecting damage before significant corrosion or other problems can occur provides increased safety and reliability for coated structures and composites,” says co-researcher, Wenle Li.

The researchers demonstrated that the damage indication system worked for a variety of polymer materials, as well as applying it as a surface coating to different substrates including metals, composites and glasses. They also found that the system has long-term stability – with no microcapsule leaking to produce false positives, and no colour fading.

In addition to averting unforeseen and costly failure, another economic advantage of the microcapsule system is the low cost.

“A polymer needs only to be 5% microcapsules to exhibit excellent damage indication ability,” says Professor Sottos.

The researchers are exploring further applications for the indicator system with fibre-reinforced composites, as well as integrating it with the group’s previous work in self-healing systems.

“We envision this self-reporting ability can be seamlessly combined with other functions such as self-healing and corrosion protection to both report and repair damage,” she says. “Work is in progress to combine the ability to detect new damage with self-healing functionality and a secondary indication that reveals that crack healing has occurred.”

Author
Justin Cunningham

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