Ultrasonic instrumentation for measuring applied stress on bridges

Abstract

The measurement of applied stress on bridges can provide valuable information on the condition of the structure. The conventional technique for measuring applied stress is with a strain gage. However, strain gages can be time consuming to install because first the surface must usually be prepared. On a bridge, paint removal will most likely be necessary as part of this surface preparation. When dealing with lead-based paints, which are considered hazardous waste, many time consuming removal procedures are required. Because of these factors, a device that measures applied stress without requiring paint removal could be useful. While a "clamp-on" strain gage can also be used to measure applied stress without requiring paint removal, this type of strain gage can not be used on some bridge details, such as webs of 1-beams and tops of box girders. An ultrasonic technique using non-contact electromagnetic transducers provides a possible method for applied stress measurement which is not limited by the same factors as those with conventional strain gages. The transducers operate through nonconductive and conductive (lead-based) paint and work on rusted, pitted surfaces. Our previous research developed a technique for measuring applied stresses on bridges with EMATs and included many laboratory tests. This paper describes field applications of the technique on actual bridge structures, as well as additional system testing and instrument calibration in the laboratory.

Structural health monitoring of a railway truss bridge using vibration-based and ultrasonic methods

“…The first application of ultrasonic monitoring of dynamic, live stresses in a bridge structure was described in [12]. Readings were based on the TOF measurement of low-frequency pulses of a surface wave propagating along the stress direction.…”

Report no.32 :

“…The program then looks for the two data points above and the two points below signal zero. It fits a straight line through these four points and calculates the zero crossing (Figure 12); this is taken to be the arrival time [10]. The search in all subsequent signals in a given measurement set is in the same time vicinity (for both polarizations).…”

“…The program fits a straight line to the four points immediately above and below the zero crossing. The intercept with the signed zero is the arrival time [10].…”

Sets of Piezoelectric Probeheads for Stress Evaluation with Subsurface Waves

“…For example, stress changes in a girder of a steel bridge, performed with electromagnetic-acoustic (EMAT) transducers transmitting and receiving surface waves are described in [13]. Measurements of stress changes in a railroad bridge using both surface and longitudinal waves with piezoelectric transducers are presented in [14].…”