EARLY DETECTION OF FATIGUE DAMAGE BY RESONANT ULTRASONIC SENSORS IN STEEL COMPONENTS OF BRIDGES*
George A. Alers, National Institute of Standards and Technology 325 Broadway, Boulder, Colorado 80303 (303) 497-7899
Detection of fatigue damage in less than half of the number of stress cycles needed to cause failure is a challenging problem to the nondestructive evaluation community. At the present time, a prediction of the remaining life in a steel undergoing cyclic loading can be made only after cracks are detected and their size can be measured. This cracked condition usually appears when 60 to 70 percent of the total number of stress cycles have been accumulated. Thus, the nondestructive evaluation challenge is to detect fatigue damage long before the first cracks can be detected.
Ultrasonic attenuation is known to be very sensitive to changes in dislocation structures and it has been monitored during fatigue tests by several researchers using high frequency pulseecho techniques. Their results, however, do not indicate any reproducible, systematic change in the interval between 10 and 70 percent of the life. By using a new resonant vibration technique on steels used in bridge construction (A36 type), it is possible to make measuements of the intrinsic damping of a steel at a sensitivity level much greater than is usually achieved in ultrasonic attenuation measurements by the pulse-echo technique. This new technique is described by Ward Johnson and G. A. Alers in Review of Scientific Instruments, Volume 68, pages 102-108, (January 1997). It involves excitation of resonant shear modes of a cylindrical test sample AND a means of trapping the vibrations in a short section at the center of the cylinder. Because of the trapping phenomenon, cyclic loads can be applied to the ends of the cylinder without affecting the damping capacity measurement. Thus, very small changes in damping can be monitored during the entire fatigue life of the sample. Preliminary results show that the intrinsic damping increases with the number of fatigue cycles mostly in the early stages and changes little during the middle stages in qualitative agreement with previous studies.
In addition to the damping measurements, special facilities at NIST are being used to make precision measurements of the elastic constants and to monitor changes in the nonlinear acoustic response as fatigue damage accumulates. At present, it is too early to report on the results of these studies.
* The results presented here were supported in part by the Federal Highway Administration.
This is a work of the federal government and is not subject to copyright.