Stress Concentrations , Fatigue , Fracture

The fundamental topic in this document is the development of cracks in steel. For structures subjected to cyclic loads, such cracks can develop over time and ultimately reach the size at which some dramatic failure occur. While this problem is rare in structures made of “mild steel,” the use of “high tensile” steel changes that situation. It is straightforward to double the strength of mild steel by adding carbon, but this may increase the material’s brittleness by a factor of fifteen (Gordon 1978). If the stress in the structure was also doubled to take advantage of the increased strength, then the capacity of the material to cope with cracks is reduced by a factor of as much as 2215=60, as explained by the equations in this document. This is a dramatic reduction which may make cracks the governing design concern.