Small tensile specimens of these materials stretch plastically by a large amount before breaking. Yet large components fabricated from these same materials sometimes fracture without much plasticity at nominal stresses below the yield stress. Such fractures are caused by cracks or crack-like defects.
Large components can harbor cracks or defects which tend to grow under regular service loads.
Sometimes the initial flaw size and the amount of growth are so small that the crack remains harmless. However, small flaws that grow slowly and progressively can become serious problems. Eventually, the crack reaches a critical size. At this stage, the crack can penetrate through the cross section of a component in a fraction of a second. It should be understood that the conventional tensile properties of yield strength, tensile strength and elongation have virtually no bearing on the vulnerability of a material to crack extension and fracture.