Reliability Assessment of High-Risk Steel Bridges by Nondestructive Test Methods

Fatigue-crack related failures of high-risk (fracture-critical) steel bridges may be prevented through application of nondestructive tests (NDT). Economical reliability assessment of those structures requires 1) application of a suitable NDT method, 2) testing of fatigue-susceptible members, and 3 ) periodic retests of those members to preclude problems with growing or previously undetected flaws. Six conventional NDT techniques were tested in the laboratory using buttweld configured specimens having intentionally embedded cracks. Those NDT methods were 1) visual inspection, 2) magnetic-particle testing, 3) dyepenetrant testing, 4) florescent-enhanced surface testing, 5 ) eddy-current testing, and 6) ultrasonic testing. The other NDT methods provided better crack-detection capabilities than visual inspection. Visual inspections by different personnel produced low detection rates. Tests using the florescent magnetic-particle method missed several indications because of marginal test conditions. The other NDT methods detected all cracks. Laboratory tests revealed that the visually enhanced NDT methods could play useful roles in bridge inspection. Magnetic-particle and dye-penetrant testing were slow, but they would be beneficial for spot bridge inspections. Eddy-current testing was found to be the fastest NDT method and would be best for large-scale bridge inspections. Ultrasonic testing was useful for flaw sizing. Modern digital ultrasonic flaw detectors offer advantages in calibration and data recording that make them more advantageous than conventional units for large-scale bridge tests. A stand-alone, microcomputer-based data-acquisition and data processing system was designed and assembled to assist highway personnel in planning and prioritizing bridge inspections. It may be used to acquire live-load strain-gage readings from selected bridge members and to store the data in digital form. The data may be processed by seven sequential computer programs to yield 1) digitized stresses, 2) stress range-cycle histograms, 3) Miner equivalent stress ranges, 4) root-mean-square equivalent stress ranges, 5) weighted equivalent stress ranges from several tests, and 6) predictions of fatigue lives of structural members based on the AASHTO fatigue design curve for specific structural members. The system has been tested and is capable of providing crack-initiation data useful for scheduling or prioritizing nondestructive inspections of bridges. Histogram stress data derived from the programs may be used with commercially available fracture-mechanics software to predict growth rates of fatigue cracks and critical crack sizes for failure. That information may be mused to select the appropriate NDT method and periodic reinspection interval for fatigue-prone bridge details