In part I, the characteristics of an emergent crack-tip dislocation loop were studied by considering half-elliptical shape dislocation loops. In existing models, the shape of dislocation loops had been not variant but fixed as 2-dimensional line or half-circular. In this work, the shape variation was included and the energy surface and iso-energy contour with the size and shape of dislocation loops were investigated. The self energy of crack-tip dislocation loop were exactly calculated by considering the energy correction term using 3-D weight function methods.
As the result of energy surface observation, it could be found out that the minimum activation shape of an emergent crack-tip dislocation loop and its energy barrior were the values of saddle point on the energy surface under some critical load, and above the critical load a 2-D dislocation loop could be emitted spontaneously without any activation energy barrior.
In comparison with the existing models, the circular emission path in the half-circular dislocation loop model was not stable on the result of energy surface observation, and the emission distance chosen as the core cutoff radius in the 2-D dislocation line model seemed to be too small. And, due to the facts mentioned above, the critical load for spontaneous dislocation emission could be found out to be overestimated in the existing models.
In comparison with the theoretical and experimental results, the proposed half-elliptical model had the smaller differences than the existing models.
In part II, the fracture behaviors accompanied by crack-tip deformation were sudied by computer simulation technique. In this work, the crack-tip deformation was simplified by two dislocational processes, the succesive emission and glide on a single slip plane, and simulated with increasing mode-III load. The choosed succesive emission condition was the positive net force criterion at some nucleation distance.
As the results of computer simulation, it could be found out that the local stress intensity factor, k, was monotonically increased with increasing the applied load, and the increase of k could be proposed as one possible micromechanism for explaining the fracture behaviors accompainied by crack-tip deformation.
Some fracture phenomena were examined by the k increase mechanism. The fracture toughness, $K_c$ could be found out to be decreasing with increasing fraictional stress and it could e confirmed that the activation energy of the ductile/brittle transition and the dislocation mobility is identical. The results mentioned above had good consistences with experimental results and therefore the proposed micromechanism(the increase of k) seems to be a possible mechanism for explaining the actual fracture behaviors accompaning crack-tip deformation.
