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This is the final highway accident report (NTSB-HAR-71-1) from the National Transportation Safety Board, adopted in December 1970. It covers the collapse of the U.S. 35 bridge between Point Pleasant, West Virginia, and Kanauga, Ohio, on December 15, 1967, which killed 46 people. The Board found that the cause was a cleavage fracture in eyebar 330 at joint C13N. That fracture grew from a flaw produced over 40 years by stress corrosion and corrosion fatigue, in a spot that could not be seen or found by inspection.
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d. Three different possible mechanisms by which the critical flaw in eyebar No. 230 might have grown to critical size were explored; namely, fatigue cracking, hydrogen-stress-cracking, and stress-corrosion. The fatigue crack propagation rates were measured in dry laboratory air, at room temperature, in dry air at 32 degrees Fahrenheit, and in air moistened by a water spray. Results are presented in Figures 20a, and 20b, which show the rate of change of crack depth per cycle versus the stress intensity factor range for the imposed nominal stress range. It can be seen that this measure of crack propagation was virtually independent of the temperature and environmental factors tested. The nominal live load stress range in eyebar No. 330 was 12,500 psi from dead load to full live load. However, the maximum live load stress could be produced only by loading both lanes of the bridge from end to end with the full design loading, a situation of rare occurrence. A more typical maximum stress would probably be that associated with the loading at the time of collapse, which produced a live load unit stress in the shank of the eyebar of 4900 psi. The range of stress at the edge of the hole in the eyebar would be approximately three times as much, or 15,000 psi. Computations based on the fatigue crack propagation data indicates that to grow a defect from a small pit assumed approximately 0.020 inches deep to a crack 0.100 inches deep corresponding to the critical crack in eyebar No. 330 would require about 541,000 cycles of such loading. This is equivalent to 37 cycles of such loading per day for the forty years of life of the structure. The Battelle report concludes that while fatigue might have been a significant contributing factor, it was probably not in itself the primary method of crack propagation. Experiments were conducted to determine the hydrogen-stress-cracking susceptibility of the eyebar material. This mechanism of crack growth may exist in steels processed to a tensile strength level of the order of 110,000 psi which are subjected to continuous loading above some minimum level, and which are subjected to an environment which contains hydrogen which is free to diffuse through the lattice structure of the steel. It was therefore necessary to determine whether the steel could absorb enough hydrogen from the service environment and whether or not the stress level was above the critical level. Surface flawed specimens made of the material from the outer layers of eyebar No. 330, which because of its hardness would be most susceptible to such cracking, were exposed to four different environments: Condition A: 4% by weight of sulfuric acid in distilled water, plus a poison (phosphorus dissolved in carbon disulfide).
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