Merenje zaostalih napona bez razaranja u konstrukcionim delovima mostova

Abstract

Bridges are vital in our society for uninterrupted transportation of goods and people on roads and railways and timely maintenance and repair of bridges are of outmost importance. The residual stresses have a significant effect on the process of the initiation and propagation of the fatigue cracks in welded elements and are responsible for many bridge failures.Knowledge of residual stresses, their distribution and their nature is, therefore, of paramount importance in all stages of bridge's design, building and maintenance. Among nondestructive methods for residual stress measurements the use of ultrasonic waves is gaining popularity and acceptance. A portable instrument, UltraMARS that is capable of measuring residual stresses in materials either averaged through thickness or in surface and subsurface layers using ultrasonic waves of different frequencies and displaying the results in a form of a continuous curve on the screen of the instrument was developed and used successfully in many investigations [1, 2]. The main principles of operation and used methodology are briefly discussed, with actual measurement examples using the bulk, the surface and the subsurface presented. A new transducer for measurement of surface and subsurface stresses with a variable base between the ultrasonic wave sender and receiver was designed and manufactured recently. By changing the distance between the sender and receiver it is possible to obtain nondestructively information on residual stress distribution through a certain range of thicknesses of the interrogated materials and structures. Results of calibration of the new variable base ultrasonic transducer (VBUT) for a number of selected materials will be presented. The results of residual stresses measured in structural details of a bridge that was damaged as well as in a number of welded bridges before and after application of improvement treatment used to beneficially redistribute the residual stresses are also presented. The obtained data on residuals stress distribution had proven that the nondestructive ultrasonic method for measurement of residual stresses is a practical and useful tool in maintenance and repair of bridges.