Joseph Caspermeyer, joseph.caspermeyer@asu.edu

(480) 965-8382

Research points way to better composite materials

Stress. Strain. Fatigue. Cracking. The forces that often bring our blood pressures to boil can also be the bane of existence for scientists developing new materials for structural applications.

With the increasing demands for stronger, lighter and more reliable materials, composite materials are emerging as the materials of choice. These materials are a “composite” of two or more materials, and the reinforcement, typically in fibrous form, is very strong and stiff. The matrix is the continuous phase that holds the fibers together. It is typically softer and more ductile.

Now, Nik Chawla, an Ira A. Fulton School of Engineering chemical and materials professor, and doctoral student Matt Kerr have developed, for the first time, a more refined test to measure the fatigue of high-performance ceramic fibers. These ceramic fibers are being deployed in several applications in the form of fabrics, ropes or reinforcing composite materials used to make lightweight and high performance transmission cables for high-voltage power lines. The results were published in January’s Journal of the American Ceramic Society.

Chawla examined the fatigue behavior of two high-performance, commercially available ceramic fibers made by 3M: one made of aluminum oxide and the other an aluminum-silicon-boron-oxide composite. Adding oxygen to the different components tends to make the material stiffer.

"If I want to make a composite material, I need to understand how the fiber fatigues before I can do that,” Chawla says. "The main thing we’ve shown is that they do fatigue. We are very excited about that finding because I think it’s going to change the way that people design and look at the behavior of composites.”

To make a composite high voltage cable, the fibers are first wrapped in a matrix, the glue that holds the fibers together.

"There’s been a lot of work done on fatigue of composites, and people always attribute damage to the matrix,” Chawla says. “Nobody’s looked at the fatigue of the fibers themselves.”

It is one thing to measure fatigue in an iron rod or steel girder. What made Chawla’s accomplishment all the more impressive is that each ceramic fiber is just 10 microns in diameter – as thin as the width of a human hair. These single filaments possess very high strength and stiffness when they are woven to reinforce materials much like making a shirt or fabric.

Chawla was interested in studying the effects that normal wear and tear would have on the fibers with a method called a “cyclic loading test,” which repeatedly produces a small force on the fiber.

"One of the nice things about the facilities we have at ASU is a microforce testing system, where we can for the first time test these fibers for cyclic loading,” Chawla says. “And we can take a single filament, the diameter of your hair, and grip it – not just pull it to failure – but actually apply a controlled cyclic load and look at the fatigue behavior.”

In the next phase of his research, Chawla wants to look at the high-temperature behavior of the fibers and correlate the fatigue behavior of the fiber with that of the matrix to try to come up with a global fatigue behavior of the composite material.

"We should have all of the parts now,” Chawla says. “The one that was missing was the fiber fatigue. So, if we integrate all of the parts, we should be able to come up with a constitutive, composite behavior.”

The more long-term impact of the work will affect the design of lightweight, high-strength composites, which will allow for designers to better predict the failures of given materials.

Caspermeyer, with the Fulton School of Engineering, can be reached at (480) 965-8382 or (joseph.caspermeyer@asu.edu).