TOTAL ASSESMENT OF THE CHARACTERISTICS OF SMALL CRACK PROPAGATION IN THE MATERIAL FOR STRUCTURAL USE

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Reassessment of the structural integrity of the machine-structure system has revealed that there are still critical problems to be solved for the total safety of the system. Total assesment of the behavior of small crack initiation and propagation in structural steel under low and high cycle fatigue regime is among these problems. In the series of present research project, special attention was paid to the following three topics. (1) Characteristics of near threshold propagation of small crack. (2) Microdefects and the nature of fatigue process in high hardened steel. (3) Fatigue fracture under extremely low cycle fatigue.Results obtained are summarized in the followings. (1) Near threshold propagation behavior of small crack in dual phase microstructure of unnotched specimen can be under the control of not only the crack growth resistance of the second phase microstructure during the process of conventional growth of single crack but also the condition of coalescence of multiple crack … More s near by the main crack. In addition, this coalescence becomes the causes of high structure sensitivity during crack propagation of relatively large crack. (2) Fatigue fracture of high hardened steel can be characterized by the concept that the matrix material can be regarded as the material having a number of small pits corresponding to small non-metallic inclusion. The level of fatigue strength can be under the control of the maximum sized inclusion in the surface layer of specimen so that the upper limit of the fatigue strength of material can be attained by the inclusion free microstructure. Furthermore, a reduction of fatigue strength of nitrided titanium can be explained to be due to possible defects in the hardened layer of the specimen. (3) Fatigue life of structural steel in extremely low cycle fatigue range can be under the control of two fracture criteria which are related to the difference of crack nucleation site in the material, i.e., the internal cracking and surface cracking, respectively.This is typical phenomenon for dual phase microstructure having relatively large brittle second phase and ductile matrix. Less