TSA based evaluation of fatigue crack propagation in steel bridge members

Abstract

Fatigue crack propagation in aging steel bridges has become a serious problem. Nondestructive evaluation of fatigue damage propagation is necessary to ensure safety and to estimate the remaining life of the bridges. Conventionally employed nondestructive testing (NDT) techniques such as visual testing, magnetic particle testing and ultrasonic testing are time- and labor- consuming techniques, further these NDT techniques cannot be used to directly evaluate the remaining strength of the bridges. Thermoelastic stress analysis (TSA) using infrared thermography has been widely used as an effective full-field experimental stress measurement technique. In this study, TSA was applied for on-site measurement of stress distributions around fatigue cracks, and the future crack propagation behavior was estimated by the fracture mechanics approach. Experimental studies were conducted for laboratory specimens which modeled a part of welded structure in steel bridges. The stress intensity factors were calculated from stress distributions measured by TSA technique. Relationship between stress intensity factor ranges and crack propagation rates was obtained. It is found that the obtained relationship shows a good correspondence with the Paris law. Further TSA technique was applied to evaluate the effectiveness of repair or reinforcement for defective portions. Severity reduction in stress distribution around the fatigue crack after treatment was confirmed for actual steel bridge members by TSA. Crack propagation rate was estimated from the stress intensity factor calculated from on-site stress measurement data. As the result, 55% reduction in crack propagation rate was ascertained indicating the positive effect of the crack repair.