Effect of Crack-tip Shielding by Dislocations on Fracture Toughness - in Relation to Hydrogen Embrittlement -

Effect of crack-tip shielding by dislocations is the most fundamental mechanism governing the fracture toughness of crystalline materials. Brittle-to-ductile transition (BDT) caused by increasing temperature is a general phenomenon observed not only in metals and alloys but also in various crystalline materials such as ionic crystals or semiconductors. The increase of fracture toughness in BDT is closely related to the shielding effect due to dislocations multiplied around a crack-tip. The present paper reviews the fundamental theory of crack-tip shielding and its experimental evidence, and also shows the reason why the nature of interatomic bonding has a remarkable influence on macroscopic fracture toughness, based on the shielding theory. Hydrogen embrittlement has attracted much attention in the fields of materials science and mechanical engineering although there still remain many arguments on its mechanism. In this paper, the phenomena being characteristic to hydrogen embrittlement are reviewed, and its mechanism is also discussed from the viewpoint of dislocation shielding.