Public Roads: A Journal of Highway Research and Development, Vol. 37. No. 8

Leone, Anthony; McGogney, Charles; Barton, J. Robert; Bean, Leonard; Solomon, David; Stockfisch, Charles R.; Taylor, James I.; Disque, Earl A.; Steere, L.B. · 1974 · ROSA P / United States. Government Printing Office

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Summary

This paper addresses the critical engineering challenge of nondestructively detecting and characterizing fatigue cracks in steel bridge structures. The research was motivated by the 1967 collapse of the Silver Bridge, which highlighted the danger of undetected fatigue cracks growing to critical sizes. Existing inspection methods were deemed inadequate for finding small, hidden cracks in older bridges subjected to modern traffic loads, particularly because they required extensive surface preparation, skilled operators, or could not detect flaws beneath fasteners and welds. To solve this, the Federal Highway Administration (FHWA) initiated a development program soliciting bids from 103 contractors. The study evaluated fifteen nondestructive inspection techniques against rigorous field-adaptability criteria, including portability, ruggedness, and minimal operator skill requirements. Five methods—Barkhausen noise analysis, eddy current, magnetoabsorption, magnetic field disturbance, and ultrasonics—were selected for experimental investigation using bridge specimens with known fatigue cracks. The experiments varied parameters such as frequency, probe size, and surface conditions (including painted surfaces) to assess detection sensitivity and reliability. The results indicated that Barkhausen noise and magnetoabsorption were difficult to interpret due to residual stresses in welds, while eddy current methods suffered from sensitivity to lift-off and welding stresses. Ultrasonics offered the best potential for rapid surveying, capable of detecting cracks through attachments like cover plates, though it required surface preparation and couplant. Magnetic field disturbance techniques proved effective for precisely locating crack length. Consequently, the authors developed a complementary two-instrument system: the Acoustic Crack Detector (ACD), an ultrasonic device for rapid surveying, and the Magnetic Crack Definer (MCD), a magnetic device for precise crack definition. The ACD uses automatic signal analysis and digital display, while the MCD uses a simple on-off light indicator. The significance of this work lies in the creation of a practical, lightweight, battery-powered inspection system that requires minimal operator training. Laboratory and field evaluations, including a trial in Idaho under adverse weather conditions, demonstrated that the ACD/MCD system could reliably detect cracks as small as 0.5 inches from distances of 3 to 10 feet. The system allows for rapid, economical inspections of fatigue-prone regions without extensive surface preparation, addressing the need for efficient maintenance of the nation’s aging steel bridge inventory. The FHWA contracted for additional prototypes for extensive field testing by multiple states to further validate the system’s utility.

Key finding

The complementary use of ultrasonic acoustic crack detection for surveying and magnetic field disturbance for precise crack definition provides the most effective nondestructive method for inspecting steel bridges for fatigue cracks.

Methodology

lab_experiment

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• roadway lighting effects
• rail grade crossings