Scientists from the University of Nebraska-Lincoln (UNL) have proposed a new technique for combating the failure of high-performance fibers found in body armor and aerospace engineering.
A high-performance fiber usually consists of three hierarchies: nanoscopic tendrils that are thousands of times thinner than human hair; microscopic, tightly packed bundles of those tendrils; and the macroscopic fiber that those bundles make up.
Though there has been much research on how fibers respond on the nano- and macroscale, none had figured out how to measure the interactions among the microscopic bundles.
Now UNL doctoral graduate Taylor Stockdale – under the supervision of professor Yuris Dzenis – and colleagues at the U.S. Army Research Laboratory (ARL) have devised a new technique to etch miniscule T-shaped notches into the top of the fiber and peel back its surface while it was being stretched.
They have done this while avoiding the perturbations that invalidate measurements captured by other techniques – the nanoscopic equivalent of walking a tightrope without disturbing it, the scientists opine.
With the fiber’s guts revealed, the team was then able to employ more familiar methods, using a nano-indenting instrument to measure the forces separating adjacent bundles and a sophisticated microscope to image those bundles tearing apart.
Having done that, the team set out to compare the behavior of two common high-performance fibers: a Kevlar fiber consisting of rigid polymer chains and another, more flexible polyethylene fiber.
Dzenis and his colleagues were especially interested in analyzing fiber fibrillation, the tendency of bundles to tear not at the same point but at different points along the length of a fiber, leading to bundle pullout and fiber failure.
Because no team had ever managed to quantify the separation among bundles, that process, much like the bundles themselves, had remained hidden beneath the surface.
The team’s experiments revealed that substantially less energy was needed to separate bundles in the flexible-chain polyethylene fiber than in the stiffer Kevlar fiber, helping clarify why fibrillations propagated much farther along the length of the former fibers than the latter.
The resulting data, and the technique that yielded it, could help inform future computational models and eventually help optimize manufacturing processes that lead to more-resistant, longer-lasting fibers.
Image and content: University of Nebraska-Lincoln