Researchers at the US Los Alamos National Laboratory have shown that enlarging the ends of short fibres used in composite materials simultaneously increases the overall toughness and strength of the material.
The finding impacts a problem material scientists have been trying to solve for decades: how to get effective load transfer between fibres and the surrounding matrix without making the composite more brittle, as happens when the fibres are tightly bonded to the matrix.
The special fibres, shaped like a dog bone, anchor into the matrix at each end but bond only weakly with the matrix along their length. This allows the fibres to help carry the load. The experimenters designed the shape and size of the enlarged fibre ends so they don’t experience the stresses that usually snap fibres and limit a short-fibre composite’s performance.
The bone-shaped fibres were developed from commercially available polyethylene stock and mixed in a polyester matrix. Another composite was made from the same materials, but without enlarging the ends of the fibres.
Standard, straight fibres can pull free of the matrix material if the fibres bond weakly with the surrounding matrix. If, on the other hand, the fibres bond strongly with the matrix, they can snap under the high stresses generated by a crack in the matrix. The bone-shaped fibres connect mechanically with the matrix predominantly at their ends. They have a weak interface, and so don’t experience extreme stress, but remain anchored at their ends and so still help carry the load felt by the composite.
Composites developed for the experiment were subjected to forces to the point of failure and examined microscopically.
The composite with the bone-shaped fibres significantly outperformed the straight-fibre composite for both toughness and strength (toughness measures the amount of energy required to damage the composite; strength measures the composite’s resistance to pressure).
The bone-shaped fibre composite was much more resistant to the propagation of cracks; the fibres would actually bridge the crack, refusing to let go. Inspection showed that even though a crack in the matrix had snaked through the sample, the sample remained intact overall.
Composite makers have successfully used long, continuous fibres to increase strength and toughness, but these materials require special, more expensive manufacturing techniques. Short-fibre composites have been long preferred because they are compatible with standard manufacturing processes.