Through this series of investigation, safety against fracture on HT780 steel composed of martensite structure has been studied from various points of view comparing with ordinary ferrite steel. In this paper, property of ductile fracture initiation is evaluated on the relationship of equivalent plastic strain and stress triaxiality by analyzing damage amount quantitatively under repeated prestraining of tension and compression. Main results can be summarized as follows;
1) In case that ductile crack occurs from the center of the specimen, difference of the critical line of ductile crack initiation which is given on the relationship of equivalent plastic strain and stress triaxiality between HT780 (martensite) and ferrite steel, is small. But in case that ductile crack occurs from the surface of the specimen, this critical line of martensite steel shows worse than that of ferrite due to the difference of work hardenability.
2) Under repeated prestraining of tension and compression including Baushinger effect, property of the ductile crack initiation can be evaluated by effective damage concept in which the strain during showing Baushinger effect is not considered additionally as damage evolution of material.
3) Effective damage amount under repeated prestraining of tension and compression can be calculated by following back stress. In this report, the validity of this calculation was verified by positron annihilation lifetime spectrometry. Furthermore, it was suggested that in reverse loading process brought by dispersing of dislocations, the amount of damage decreased temporary.
