| Abstract Scope |
Most of our engineering metals suffer from a dense population of preexisting cracks as proposed by Griffith. The cracks have been discovered to originate in the casting process, by the folding over of the oxide film on the surface of the liquid metal, top dry side to top dry side, so there is no bonding between the opposed surfaces. These unbonded double film cracks are formed mainly during the chaotic turbulence of pouring. The cracks, called 'bifilms', are often nm thin, but can be of large area. They are clear as ‘quasi-cleavage’ facets on fracture surfaces. They play a major role in fatigue, so that nearly all assumed fatigue failures in service applications are mainly preexisting bifilms, with a minor fatigue contribution which completes the failure. Castings techniques have now been developed to eliminate fatigue in components. |