ADI材におけるS-N曲線の二段折れ曲がり現象の要因

概要

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We throughly observed fracture surfaces of austempered ductile iron (ADI) cylinders which fractured in the very high cycle region under internal pressure fatigue tests by SEM, and investigated the reason for the unique fatigue behavior, i.e. the fracture origins changed from casting defects to small graphite with decreasing applied stress, and the S-N diagram drops again after levelling off at l0^6〜10^7 cycles (two drop S-N diagram). As a result, it was clarified that (1) ADI has special spheroidal graphites which have the possibility to induce stress concentration on the micron order because of the wedgewise precipitation of carbon into the ferrite layer at the interface of graphite and matrix ; (2) at low stress levels, there exists a highly structure-sensitive crack propagation initiated from this type of spheroidal graphite, and its crack growth rate is estimated to be very low compared with that at high stress levels ; and (3) the "two drop S-N diagram" is caused by both phenomena that cracks from casting defects are arrested and that the propagating mechanism of (2)appears at low stress levels.
社団法人日本機械学会の論文
1995-02-25