July 6, 2001

Contact: Guillermo "Willy" Ramirez, (785) 864-2948.

Engineers master 'hearing' tests

LAWRENCE -- Do you hear what I hear?

Civil and environmental engineering students at the University of Kansas will be honing their "hearing" skills in the days to come. They are being taught to use a number of nondestructive evaluation methods, many of which use sound waves, to indicate the fitness of a structure or material.

Guillermo Ramirez, assistant professor of civil and environmental engineering is among the champions of nondestructive evaluation at KU. Nondestructive evaluation isn't really new, he said, but it is relatively new to contemporary civil engineers who are exploring the many ways it can benefit society.

"In the past when people built boats, they would go in with a hammer and tick, tick, tick, tick, tick, and just listen," said Ramirez, who teaches a course in nondestructive evaluation. To the experienced worker, the tone produced with the hammer could indicate flaws in the structure.

Many of those principles are now being applied to structures and materials that are a part of people's daily lives. But quality control measures today require something more accurate than a strong hammer and a good ear. Engineers are developing and honing additional methods for use in their arena. The emphasis on nondestructive evaluation could save millions of dollars, as well as help determine the safety of important structures.

The current market trend has been toward retrofitting existing structures instead of constructing new ones, Ramirez said. Civil engineers will need to use nondestructive evaluation to determine the current condition of structures as well as the effectiveness of retrofits. There also is a growing interest in using nondestructive evaluation to test existing structures for longer service lives and higher load requirements, he said.

Well-prepared civil engineers will have to possess a background in commonly available nondestructive evaluation methods and their limitations. "It's been left at the technician level. Most of the engineers don't know how to read the results," Ramirez said. Exposing engineering students to a variety of nondestructive evaluation methods, which can include x-ray, infrared and magnetic technologies, will have a great impact on the economy as well as the students' careers, Ramirez said.

"It will definitely make the students more marketable. To have someone on your staff who can give you the answers you are looking for is quite sought after. At the graduate level, it makes them even more desirable for research."

On a recent day students worked in labs at KU testing different materials by a variety of means.

Three graduate students "listened" to the waves generated by acoustic emissions. The sound waves are not in the audible range for humans, but instead are charted on a computer screen. The method, which can be used for bridge inspections, looks at the natural sound waves generated inside materials. A computer digitizes the wave and extracts certain values. Since each material generates a unique acoustic property -- a signature -- the students will be able to tell whether the material is exhibiting the expected characteristics. Unexpected results could indicate a flaw. This type of test will allow engineers to monitor the fitness of a material over time or while bearing loads.

In another lab, assistant professor of aerospace engineering Rick Hale demonstrated how an ultrasonic scan system can detect flaws in a material. The ultrasonic scan system uses high frequency pulses or waves, also outside the range of human hearing, to penetrate a material to discover flaws. The computer screen glows with the brightly color-coded information being collected. One section of the screen shows the ultrasonic wave as it travels through the material being tested. The wave doubles back to the source, but along the way flaws register as peaks and troughs on the wave image. Because the wave doubles back, there is an art to reading this image. The computer assists in some ways by creating pictures -- cross sections and depth readings -- that represent the object being evaluated. Engineers can look at representational cross sections and pinpoint where a flaw or obstacle resides in a material.

In yet another lab, three graduate students learned the finer points of impact echo testing.

A small hand-held device connected to a portable computer sends a low-frequency signal through a concrete slab that has been marked with a grid. By knowing the number of times the signal rebounds in a second and the velocity of wave propagation in the material, scientists and engineers can calculate the thickness of the material. Where there is a flaw, the thickness has changed and the signal does not follow the expected pattern.

This particular method doesn't require as much precision as the other methods, but it can be just as useful.

Using ultrasonic means on a large structure such as a concrete slab on a bridge deck would be relatively ineffective. The slab's thickness would be problematic, as would its composition, in allowing the high frequency waves to penetrate the structure. The longer, lower frequency wavelengths used in impact echo testing give engineers an average look at the structure. Considering that concrete slabs are not expected to be uniform throughout (there are aggregates and steel bars for example) this is important. What engineers might be looking for is a place of separation, where air or some other unexpected substance is in a place where it shouldn't be. The computer screen gives the students an instantaneous reading of the impact echo, showing flaws as a spike on the chart.

On this day, the students were learning the intricacies of the method -- how to use the device to get a good reading and how to maintain the equipment for accurate results.

"They'll get a feel of how difficult it is and how reliable it is. Reading a book is not enough. You have to get on your knees and do it," Ramirez said. In the hands of a civil engineer, nondestructive evaluation is a hearing aid with financial and social implications. The long-term benefits are safer buildings and lower cost of long-term maintenance. Nondestructive evaluation will help new structures be built less expensively and will help older structures last longer, Ramirez said.