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NIST Puts New Testing Device to Work on Recovered WTC Steel Shows How Steel Responds to High Stress, High Temperature

For Immediate Release: June 25, 2003

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Two researchers testing WTC steel.A new instrument at the National Institute of Standards and Technology that operates like an air-powered battering ram has been pressed into service to study steel salvaged from the World Trade Center (WTC), a key element in the agency's two-year building and fire safety investigation of the Sept. 11 disaster.

Initially planned for a project to determine the properties of materials to improve computer programs that predict how well machine tools will perform particular metal-cutting jobs, the one-of-a-kind instrument was described in a presentation June 23, during the Fifteenth Symposium on Thermophysical Properties in Boulder, Colo.

Sometimes called a Kolsky bar or a split-Hopkinson pressure bar, the peculiar-looking instrument can supply data key to understanding how steel and other materials respond to high-stress, high-temperature conditions. "With the NIST Kolsky bar apparatus, we will measure how each of the various types of steel used in the WTC buildings' structural components deform under high-impact conditions, akin to those caused by the aircraft that struck the towers," explains NIST metallurgist Richard Fields. “That data will help us model the response and behavior of the steel in the towers during the two impacts.”

The instrument is a 21st century incarnation of a 1913 innovation devised to measure how materials respond to pressure under dynamic conditions.

Starting with a customary Kolsky bar, a diverse team of NIST researchers and technicians conceived several high-tech enhancements that make the resulting instrument unique.

Key among them is a controlled means to rapidly heat sample materials, at rates of up to 50,000 degrees Celsius (90,000 degrees Fahrenheit) per second.

Another extra, a high-resolution heat-sensing microscope designed by NIST physicist Howard Yoon, measures the temperature of a 1-millimeter diameter region of the sample about every millionth of a second. And every thousandth of a second, a thermal-imaging camera records temperatures about every 5 micrometers (millionths of meter) across the surface of the sample. About 20 readings are taken over an area equivalent to the diameter of a human hair.

During a high-impact event, much of the mechanical energy is converted into heat, which can affect how materials respond to stress. For example, when a manufactured part is milled or otherwise cut from a metal bar, the temperature in some sections of the workpiece "can increase from room temperature to 1,000 degrees Celsius (1,800 degrees Fahrenheit) in a fraction of a second," explains NIST manufacturing engineer Richard Rhorer, the project leader.

"How materials will behave under extreme conditions may not be evident from properties data gathered under normal conditions,” he adds.

The business end of this high-tech array is the Kolsky bar itself. It consists of two painstakingly machined, mounted and aligned steel bars, each measuring 1.5 meters (about 5 feet) long and 15 millimeters (0.6 inch) in diameter. Made of hardened specialty steel, the bars are arranged end to end. Disks of sample materials are sandwiched in the split between the bars.

An air gun situated just beyond the far end of the first bar propels a striker rod, which reaches speeds up to 40 meters (130 feet) per second. When the striker hits it, the bar does not move immediately. Rather, says Rhorer, the collision creates a fast-traveling disturbance called a strain wave that races down the steel bar at 5,000 meters (16,400 feet) per second—equivalent to over 11,000 miles per hour. Upon impact, this strain wave rapidly compresses the sandwiched specimen.

The sample reflects back some of the wave's energy, and some of the wave is transmitted through to the other bar. Strain gages mounted at the center of both bars capture information needed to determine how stress, strain and temperature affect the sample material’s behavior.

Such information can be crucial to efforts to model and simulate materials performance during high-impact events and, ultimately, to improve the design and performance of systems that must endure such extremes. Examples range from enhancing the crashworthiness of automotive materials to increasing the protective capacity of armor and from reducing tool wear during machining to withstanding impact loading during earthquakes.