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Blast-resistant glass research places third

A man holds a piece of small square sample of glass in front of his face.

Hua Zhu holds a sample of the glass fiber reinforced composite interlayer that he developed with mechanical engineering professor Sanjeev Khanna. The research won third place at a National Science Foundation workshop for his work in producing a new, thinner variety of blast-resistant glass.

A University of Missouri College of Engineering student in mechanical and aerospace engineering won third place at a National Science Foundation (NSF) workshop for his work in producing a new, thinner variety of blast-resistant glass. The March workshop, “Durability of Polymers and Composites,” was part of a larger NIST conference on “Service Life Prediction of Polymeric Materials.”

Doctoral candidate Hua Zhu was one of 11 students invited from around the country. There he presented research on a new glass fiber reinforcement technique he developed under the direction of  Professor Sanjeev Khanna,

“Bomb attacks often occur in different parts of the world,” Zhu said. “One of the most significant effects is that the blast will break nearby windows of buildings, and those fragments can seriously hurt people.”

Of the 508 injuries sustained after the Oklahoma City bombing in 1995, Zhu notes that more than 200 were related to broken glass. Zhu’s research promises to make windows safer by resisting the shattering and scattering of glass fragments during an explosion.

“The final objective is to make a  new blast-resistant glass. It’s still under testing, but it has performed pretty well against a medium-intensity blast,” Zhu said.

A medium intensity blast is one with a peak overpressure of 5-10 pounds per square inch (psi). Explosions of this grade are more than capable of shattering windows, scattering debris and inducing significant trauma to bystanders. Conventional blast-resistant windows are typically made with a plastic interlayer called polyvinyl butyral (PVB) sandwiched between sheets of tempered glass. The final product is often more than an inch thick, and a cumbersome reinforcement to a building’s weakest link.

“They couldn’t solve the problem, which is how to make the glass fiber material transparent, so they just used PVB,” said Zhu. “For decades, nobody has made a new interlayer. We need a thinner transparent interlayer, such as a transparent glass fiber reinforced composite interlayer, and that’s what we’re trying to develop.”

Zhu and Khanna have developed a method that seems to solve the problem. Their “glass fiber reinforced composite interlayer” is only around 1/16-1/8 inch thick, and has been employed with new chemical additives that preserve the window’s transparency.

Before now, fiberglass, which is stronger than PVB, has not been viable as an interlayer for blast-resistant windows because of its opacity. Using new chemical additives, Khanna and Zhu have been able to match the refractive index of the polymer to the glass fibers, allowing light to pass unscattered through the fiberglass interlayer. By sandwiching this interlayer between glass sheets, the glass is reinforced, but still maintains as much light transmittance as a car window.

The  new blast-resistant glass with the transparent fiberglass interlayer is still under testing and has held up against small-scale blasts similar to bomb delivered by hand. The standard glass layers are tempered and do shatter, but the fragments stay bonded to the clear fiber interlayer which prevents the window from being punctured completely — a remarkable feat for a pane less than half an inch thick.

“We’re still testing, and we need to work on scaling up the size of our glass,” said Zhu. “Our method is cheaper than conventional blast-resistant glass. I think ultimately we would like to put this in the market.”