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Catastrophic brittle fracture service failures have occurred in structures made from steels which were considered to be ductile as judged by the customary tension tests. Many of these failures occurred with extreme suddenness, without any noticeable deformation and under conditions where the nominal stress was at very low values—well below the yield strength values determined by tension tests. Another feature of these failures is that they occurred at temperatures within the range of temperatures where notch-bar impact tests show a marked decrease in energy-absorbing capacity and the nature of fracture changes from ductile, with a large proportion of shear failure and considerable deformation to brittle, with fracture largely of the cleavage type accompanied by little or no noticeable deformation. Detailed studies have indicated that low-strength catastrophic failures appear to be the result of a small notch, crack, or other defect, combined with the presence of unfavorable residual stress, a sufficiently low temperature, and a certain minimum level of load stress. To avoid brittle fracture, or at least to minimize the probability of its occurrence in ships and pressure vessels, steps are now being taken to limit the operating temperature relative to the transition temperature of the steel used, the residual stress, the load stress, or the defect size. However, there is little or no indication that such practices have been given any consideration for “blast-resistant” protective structures or any other type of reinforced concrete structures. There is apparently no report on the resistance of such structures at low atmospheric temperatures. This appears to be a serious oversight when it is realized that the transition range of steels used for reinforcing steel bars may be expected to lie in the range of ordinary atmospheric temperatures and that the deformations on the surface of reinforcing bars along with other irregularities may provide numerous locations for the initiation of cracks which could propagate into catastrophic failures.
Tarr, Allan L.
Army Research Office, Washington, D. C.