Researchers at the Department of Energy's Los Alamos National Laboratory have shown that enlarging the ends of short fibers used in composite materials simultaneously increases the overall toughness and strength of the material.
Composite materials are used widely in the automotive, aerospace, civil engineering and other industries requiring lightweight but structurally sturdy parts.
The Los Alamos finding impacts a problem material scientists have been trying to solve for decades: how to get effective load transfer between fibers and the surrounding matrix without making the composite more brittle, as happens when the fibers are tightly bonded to the matrix.
The special fibers, shaped like a cartoon dog bone, anchor into the matrix at each end because of their shape but bond only weakly with the matrix along their length. This allows the fibers to help carry the load. The experimenters designed the shape and size of the enlarged fiber ends so they don't experience the stresses that usually snap fibers and limit a short-fiber composite's performance.
"People have been trying to solve this problem for the last couple of decades," said Los Alamos' Yuntian Zhu, who leads the research effort. "We've shown that this fairly simple mechanical approach can provide a solution."
In a pair of scientific papers, Zhu and his colleagues in Los Alamos' Material Sciences and Technology Division described their experiment of developing bone-shaped fibers from commercially available polyethylene stock and mixing them in a polyester matrix. They made another composite from the same materials, but without enlarging the ends of the fibers.
Standard, straight fibers can pull free of the matrix material if the fibers bond weakly with the surrounding matrix. If, on the other hand, the fibers bond strongly with the matrix, they can snap under the high stresses generated by a crack in the matrix.
The bone-shaped fibers 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.
The composites developed for the experiment were subjected to forces to the point of failure and examined microscopically.
The composite with the bone-shaped fibers significantly outperformed the straight-fiber 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, or force spread over a given area).
The bone-shaped fiber composite was much more resistant to the propagation of cracks; the fibers 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.
The researchers are conducting additional experiments to adjust the shape of the fibers for optimal composite performance. One member of the team, Irene Beyerlein, is using computer modeling to better understand the experimental results and predict the outcome when the researchers use different materials or change the fiber design.
Composite makers have successfully used long, continuous fibers to increase strength and toughness, but these materials require special, more expensive manufacturing techniques. Short-fiber composites have been long preferred because they are compatible with standard manufacturing processes.
The Los Alamos team expects their bone-shaped fiber approach could also be used in reinforced concrete structures, such as roads, bridges and buildings.
One Los Alamos research paper on this topic is in press at Scripta Materialia; a second paper has been submitted to Acta Materialia.
Other Los Alamos staff members engaged in the research are James Valdez, Michael Stout, Shujia Zhou and Terry Lowe.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.
Materials provided by Los Alamos National Lab. Note: Content may be edited for style and length.
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