Microstructural effects on fracture mechanisms and fatigue life in extremely low cycle fatigue

摘要

This paper is divided into two main parts.In the first part,microstructural effects on fatigue fracture behavior in the Extremely Low Cycle Fatigue (ELCF) regime are investigated using various kinds of materials having different microstructures.In the case of steel with a duplex microstructure such as annealed low carbon steel and ferritic ductile cast iron,two types of fracture mode are observed depending on the level of the plastic strain range DELTA epsilon p.In the conventional low cycle fatigue regime,a crack originating from the surface leads to final fractue of the specimen (surface fracture mode).In the ELCF regime,however,final fracture of the specimen is controlled by the coalescence of microvoids inside the material (internal fracture mode).A method to detect the internally accumulated fatigue damage in ELCF has also been proposed.Secondly,the applica-bility of Manson-Coffin law to ELCF is discussed in relation to the transition between fracture modes.In materials which exhibit the transition,the fatigue life curve is composed of two straight lines (Manson-Coffin type) with different slopes which correspond to the surface and the internal fracture modes,respectively.A new concept of 'Fatigue Life as Determined by Competition of Two Failure Limit Lines' is proposed to explain this characteristic of fatigue life in the ELCF regime.The effect of repeated annealing on fatigue life in the ELCF regime is also discussed to prove this concept.