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  Working Science

  People

  About

  Subscribe Free





























This issue...

  Brieflies

  View from the Inside

  Shocking Rocks

  PubSCIENCE

  Working Science

  People

  About

  Subscribe Free

Shocking Rocks

A New Method of Testing and a New Classification of Materials

by James Weber

During an earthquake shock, a concrete piling holding up a freeway overpass may be strained and then return to its original position. To the naked eye, it looks exactly as it did before but it may in fact be severely damaged. After the quake, we might depend upon the elasticity of the concrete piling; that is, its tendency to return to its original state before the stress. But if it doesn't and we are unaware of the damage, it could lead to disastrous consequences.

Cypress viaduct (Interstate 880
Aerial view of the Cypress Viaduct (Interstate 880) after the Loma Prieta earthquake in 1989. Concrete and metal pilings gave way and the upper highway collapsed onto the lower highway. (Source: National Information Service for Earthquake Engineering, University of California, Berkeley)

An acoustic method of testing such materials is proving to be very sensitive in detecting internal damage to building materials. This new method has led two researchers—Paul Johnson of Los Alamos National Laboratory and Robert Guyer of the University of Massachusetts—to propose a class of materials called "nonlinear mesoscopic elastic" (NME) materials, which differ from other materials in their reactions to acoustic vibrations.

Microscopic
The microscopic structure of Berea sandstone, showing a multitude of cracks and grains characteristic of NME materials. (Source: Paul Johnson, Los Alamos Seismic Research Laboratory; reprinted from the cover of Physics Today, April 1999)

Guyer and Johnson have been applying sound-wave shocks to various materials and analyzing the results spectroscopically. The results show that NME materials, such as sandstone or internally damaged concrete, often exhibit surprising effects from sound shocks. To use an analogy, if you strike a bell with a mallet, first gently and then harder, you would expect the bell to ring both times with a constant tone, just that the first time will be softer and the second will be louder. However, strike a bell with a crack in it, like the Liberty Bell, and you will find that the frequency actually goes down when you strike harder. This is an example of "nonlinearity" in the wave frequencies produced inside the bell when you strike it. NME materials show the same sort of nonlinear responses when exposed to acoustic waves, apparently absorbing and dispersing the stresses.

In an article about NME materials in the April 1999 issue of Physics Today, Guyer and Johnson describe a new model of the nonlinear responses of NME under acoustic stresses that was developed at the Los Alamos Nonlinear Elasticity Group. The mesoscopic experiments observe rocks such as sandstone that are composed of grains 20 microns (0.00079 inch) across. Besides sandstone, igneous and metamorphic rocks also exhibit nonlinear elasticity, primarily because of the many cracks inside the rocks. NME properties also extend to manmade materials that are internally damaged by stresses.

The nonlinear wave response of a material, used by Johnson and Guyer in studying NME materials, is the most sensitive manner now available to detect the onset and progression of damage to materials. A wide range of needed nondestructive testing can benefit from the nonlinear wave spectroscopy techniques and our resulting increased knowledge about material fatigue and damage studies after earthquakes, bridge-pillar fatigue, and for material fatigue at nuclear power plants. Application of the nonlinear wave detection method to assembly line processes could help eliminate damaged parts.

Future study of the concrete-curing process could also help produce damage-resistant concrete (journal article, pdf 291K). Of course, the techniques also promise a continued use in exploring the properties of metals, ceramics, and composites.

This work is supported by the Office of Basic Energy Science, Engineering and Geosciences. Contact: Paul Johnson, Los Alamos Seismic Research Center, (505) 667-8936, paj@lanl.gov; or Robert Guyer, University of Massachusetts, (505) 667-6488.

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