Fracture behavior of C-Mn steel and weld metal in notched and precracked specimens: Part II. micromechanism of fracture
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I.
INTRODUCTION
THE Ritchie-Knott-Rice
(RKR) model tu of cleavage fracture in ferrite steel shows that the unstable cleavage extension .onsets at the moment when the microscopic cleavage fracture stress, tri , related to the critical crack length, is exceeded by the principal tensile stress over a characteristic distance ahead of a crack tip. Though a great number of investigations have made contributions to understanding this criterion, there are still four questions which have attracted attention: (1) Does the value of try vary with testing conditions such as loading rate, temperature, and notch acuity? (2) Is the cleavage fracture propagation only controlled by the principal tensile stress? (3) What is the critical event in a cleavage fracture of C-Mn steel, and can it be changed by testing conditions? (4) Can the difference in o-I measured in notched and precracked specimens be accounted for by the statistical theory? It has been concluded in many works [~,2[ that the value of o-I does not vary much with testing conditions such as temperature, loading rate, etc. But some current studies [3,41 fmd that o)varies for specimens with different notch acuities. In those works, on the basis of statistical theory about brittle fracture, the authors considered that it resuits from the difference of highly stressed volume ahead of the notch root and the crack tip, which causes the change in the probability of the largest "eligible" particles to be sampled as the critical nucleus of cleavage fracture. Some recent studies Is,6] have shown that the value
J.H. CHEN, Professor, G.Z. WANG, Assistant, and H. MA, Lecturer, are with the Welding Department, Gansu University of Technology, Lanzhou, Gansu, People's Republic of China. Manuscript submitted January 4, 1989. METALLURGICAL TRANSACTIONS A
of o-I decreased with the increase of temperatures in some cases. These facts imply that some links in the cleavage fracture process may be connected with the effective shear stress. In the early work, [81 the critical event was thought to be the propagation of a ferrite grain-sized microcrack into the surrounding ferrite matrix. In recent years, it has been accepted that the critical event was the propagation of a second-phase particle-sized microcrack into neighboring ferrite grain.J3,4'7'91 But one of the authors of this paper has recently found that the critical event of C-Mn weld metal is the propagation of a ferrite grain-sized microcrack 30 to 4 0 / z m in size into the matrix in Charpy V specimens tested at - 6 0 ~ [lu Therefore, the critical event still needs to be studied. In this study, in conjunction with the finding in Part I [131 that the value of try is different in notched and precracked specimens of C-Mn base metal and C-Mn and Ti-B weld metals (Table I), the crack-initiating particles, the microcracks remaining in specimens, and the microstructure in the region initiating the crack were carefully investigated. On the basis of the results obtained, the four questions mentioned above are discussed, and the difference
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