Brittle fracture of ductile steels

Brittle fracture of normally ductile steels has occurred primarily in large, continuous, box-like structures such as box beams, pressure vessels, tanks, pipes, ships, bridges, and other restrained structures, frequently joined with welded construction.
  1. A stress concentration must be present. This may be a weld defect, a fatigue crack, a stress-corrosion crack, or a designed notch, such as a sharp corner, thread, hole, or the like. The stress concentration must be large enough and sharp enough to be a "critical flaw" in terms of fracture mechanics.
  2. A tensile stress must also be present. This tensile stress must be of a magnitude high enough to provide microscopic plastic deformation at the tip of the stress concentration. One of the major complexities is that the tensile stress need not be an applied stress on the structure, but may be a residual stress that is completely within the structure. In this case, the stress is not obvious or easily measured, as is the applied stress. The part or structure can be completely free of an external or applied load - just lying on a bench or floor, for example - and still experience instantaneous, sudden, catastrophic brittle fracture. This type of occurrence is within the experience of many persons who have worked with metals, particularly welded, torch cut, or heat treated steels.
  3. The temperature must be relatively low for the steel concerned. The problem is that the definition of metal/temperature interrelationships is inexact, very much subject to the type of test used to try to understand whether or not a particular steel is actually subject to brittle fracture under certain conditions. However, regardless of the type of test used to try to establish the ductile/brittle transition temperature, the general results are the same: The lower the temperature for a given steel, the greater the possibility that brittle fracture will occur. For some steels, for example, the ductile/brittle transition temperature under certain conditions may be above room temperature.

As noted, the absence of any one of the four factors - stress concentration, high tensile stress, relatively low temperature, susceptible steel - that contribute to brittle fracture of a ductile steel will prevent this problem from occurring.
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