Quantitative safety analysis using fracture mechanics and ultrasonic stress measurements

摘要

Abstract: Fracture mechanics can be applied to assess the safety of cracked bridge members. If crack length and stresses are known, the crack driving force (stress intensity factor, K) can be calculated. K was calculated for hot-rolled beams, as a function of crack length. K eventually becomes negative, indicating not further crack propagation. However a cracked girder will become compliant and 'shed' load to uncracked neighboring members. Our calculations show that the changes in both compliance and load-carrying capacity of the cracked girder are small until the girder is deeply cracked. A finite-element analysis of a cracked girder showed that by determining the bending stresses at about one beam depth from the crack it is possible to determine K. Measurement of these stresses was simulated in a field test. The method used small changes in sound speed to determine stress. The ultrasonic transducers used required no couplants and no surface preparation. They were also used to measure stresses in an integral backwall bridge. !8