X線による切欠き材の疲労破壊に関する研究細束X線による焼入時効した低炭素鋼材の疲労の観察 (X線材料強度特集号)

概要

論文の詳細を見る
Fatigue fracture occurs in any kinds of engineering materials, cold-rolled ones, heat-treated ones, etc. It is considered to be very significant to find out a monotonously varing parameter which represents fatigue progress of these materials, in understanding the mechanism of fatigue fracture as well as in the prediction of fracture by fatigue. For this purpose, the authors have set about a series of studies on microstructural change during the fatigue process of low-carbon steel specimens, which are treated in different manners before fatigue test, by means of back-reflection X-ray microbeam diffraction technique. In the previous papers, the fatigue process preceding macro crack initiation of annealed or cold-rolled specimens was investigated. In this paper, the specimens were initially quench-aged and fatigue process preceding macro crack initiation of these specimens was studied. It was intended to find out the feature characterizing the fatigue process in three kinds of materials, the annealed, the cold-rolled and the quenchaged steels and their features are briefly described in articles (1)-(4). The causes for the increasement in fatigue strength of quench-aged steel was discussed and the results are summarized in article (5). (1) In the early stage of fatigue process, the micro lattice strain range within a grain changes notably. The total misorientation β increases while the subgrain size t decreases rather slowly. As fatigue proceeds, the micro lattice strain range within a subgrain decreases and t continuous to decrease more slowly, although β takes nearly constant value. The width of subboundaries are narrowed. As macro crack initiation is coming near, the changes of these quantities begin to diminish. Therefore, the rearrangement of dislocations to develop substructure is a chracterizing feature of the later stage of fatigue process. The degree of the development of substructure can be used as one of the crystallographic parameters which express the degree of progress in "fatigue damage" of cyclically stressed specimens with any initial state. (2) Excess dislocation density in a subgrain at the number of stress cycles of macro crack initiation takes the small value of 2-5×108 lines/cmcm2. The amount of increase in excess dislocation density at subboundaries during the fatigue process till crack initiation is of the same order notwithstanding the variety of initial state. (3) During the fatigue process of specimens cyclically stressed above fatigue limit, well-developed substructure is observed in severely deformed grains at the same stage as the appearance of persistent slip bands. A crack takes place, at last, at the place of some persistent slip bands. On the other hand, the diffraction patterns reflected from grains without persistent slip bands do not show a notable difference from the initial patterns. From these points of view, it can be said that well-developed substructure plays an important role in crack initiation. (4) The degree of fatigue progress in cyclically stressed specimens is observed to differ from grain to grain, even if these grains are located in the same irradiated area of X-ray microbeam. This implies that there is no remarkable interaction between the adjacent grains, at least until the macro crack initiation of high cycle fatigue. (5) The structural causes for the improvement in fatigue strength are listed as follows; the initial locking of dislocations, the highness of frictional stress and the rapidness of work-hardening rate. These facts reveal the difficulty of dislocation movement mainly in the early stage of fatigue process.
公益社団法人日本材料学会の論文

X線による切欠き材の疲労破壊に関する研究細束X線による焼入時効した低炭素鋼材の疲労の観察 (X線材料強度特集号)

スポンサーリンク

概要

著者

関連論文

スポンサーリンク

X線による切欠き材の疲労破壊に関する研究 細束X線による焼入時効した低炭素鋼材の疲労の観察 (X線材料強度特集号)

スポンサーリンク

概要

著者

関連論文

スポンサーリンク

X線による切欠き材の疲労破壊に関する研究細束X線による焼入時効した低炭素鋼材の疲労の観察 (X線材料強度特集号)