A Study on the Influence of Artificial Defect on Fatigue Limit in Austempered Ductile Iron

摘要

Rotary bending fatigue tests were carried out torninvestigate the influence of artificial defect on fatigue limit inrnannealed and austempered ductile iron. Main resultsrnobtained are as follows.rn(1) Artificial defect(micro hole type, dia.<0.4mm) onrnspecimen surface did not bring about a obviousrnreduction of fatigue limit in austempered ductilerniron(ADI) as compared with annealed ductile iron.rn(2) According to the investigation ofrnarea~(2/1)_c which isrnthe critical defect size to crack initiation at artificialrndefect,rnarea~(2/1)_c of ADI is larger than that of annealedrnductile iron. This shows that the situation of crackrninitiation at artificial defect in ADI is more difficult inrncomparison with annealed ductile iron.rn(3) One of the reason for the low rate of crack initiationrnfrom artificial defect in ADI is that the resistance ofrnmatrix to crack initiation is higher than that of annealedrnductile iron.rn(4) In case that thernarea~(2/1)_c of artificial defect and graphiternnodule is the same size, the rate of crack initiation fromrngraphite nodule is higher than that from artificial defect.rnThis reason is that the serious ruggedness aroundrngraphite nodule is formed by austempering treatment.