Getting tough on materials

Researchers have found they can toughen flexible composite materials, which can be used for cables and tethers, by creating multiple locations at which energy can be dissipated.

Georgia Institute of Technology aerospace engineers Drs. D. Stefan Dancila and Erian Armanios developed this concept of ‘redundant load paths.’ And now, a patent on the tailoring process for redundant load paths is in the final stage of being issued to both the researchers and the Georgia Tech Research Corporation.

While experiments continue to verify the concept with a variety of flexible composite materials, the researchers are investigating several applications. One is inflatable space structures, which are being evaluated for use with the International Space Station. Researchers could reinforce these structures with flexible composite webbing containing redundant load paths to help them better withstand an accidental internal pressure pulse.

Armanios and Dancila also see potential applications in the reinforcement of tethers, mountain climbing ropes and possibly crashworthy helicopter seat restraints.

‘We plan to evaluate several materials with appropriate combinations of properties,’ Dancila explained. ‘Then we will develop manufacturing technologies appropriate for each material system and create tailored structures aimed at various applications, such as fall-tolerant climbing ropes and reinforcement straps for inflatable structures.’

The researchers conceived the redundant load path concept from observing the tough tearing response of a plastic mesh bag, the kind that is often used to package fruit. When subject to load, it tears at one location and then another as applied force is increased. Finally, it gives in and fails completely, Dancila explained.

‘We combined this observation with our understanding of typical failure mechanisms in one-directional, fibre-reinforced composite materials,’ Dancila said. ‘We already knew that the adjacent failure of a small number of fibres can lead to the formation of a matrix crack that concentrates load on neighbouring fibres. These cracks precipitate the overall failure process.’

Composite materials made from these high-performance fibres can and do crack and then break. But researchers know that the energy it takes to break the material increases as the surface area of the fracture increases. In other words, lots of little cracks are better than one big one.

‘Based upon what we already knew, we set out to devise a tailoring concept that would induce a load redistribution mechanism forcing repeated fracture of fibres,’ Dancila explained. ‘This mechanism — redundant load paths — increases the amount of fracture surface generated, increases the energy dissipated and results in a ductile, yield-type response.’

To test the concept initially, researchers used glass-fibre-reinforced packaging tape to create a strap-like structure tailored with redundant load paths. They applied varying amounts of load to it and observed (with a slow motion camera) a succession of partial failures along the redundant load paths before the strap-like structure failed completely. The redundant load paths kept the load almost constant and required more energy to break the entire structure compared to an equivalent, untailored strap.

In later experiments, the researchers found that tests using the material system made from packaging tape verified their response model predictions for both quasi-static and dynamic loading. In quasi-static loading tests, researchers applied force using a servo-hydraulic testing machine. In dynamic loading, researchers used a custom-designed drop test stand.

While they continue to test the redundant load path concept with other material systems, Dancila and Armanios have discussed its application in the TransHab inflatable space structure with scientists at NASA’s Johnson Space Center. The researchers’ preliminary findings, which they presented earlier this year at a meeting of the American Institute of Aeronautics and Astronautics, indicates the tailoring concept is both applicable and beneficial for reinforcement of inflatable space structures with flexible composite webbing containing redundant load paths.

On the web at