銅の切欠き試験片における疲労破壊の研究

き裂先端における負荷応力と内部応力について

抄録

In order to make clear the mechanism of the fatigue crack propagation, it is important to find out what sort of stress it is at the tip of the crack that will start again when a cracked specimen is subjected to repeated loading. It may be considered that there is unstable equilibrium in the strength of the material at the tip of the crack, and that the applied stress determines whether the crack will propagate itself or not.
There are many factors that affect the strength of the material ahead the tip of the crack. These factors, however, may be divided into two groups, the structural factors and the mechanical factors. The former group consists of the crystal forms, grain size, behaviours of the dislocation, point defects and behaviours of solute atom (Impurities), etc. The latter is defined by the interrelation between the external stress and the distribution of the internal stress (Residual stress).
Therefore, to clarify the mechanism of the crack propagation from the point of view of the mechanical factor, the residual stresses at the tip of the crack must be measured, first of all. But, hitherto, as the stress measurement technique of a localized region such as the crack tip has not been established, it seems that no investigation has so far been made yet on this line.
After considerations on these points, in the present study, the authors measured the microscopic and macroscopic residual stresses in the crystals which exist at the tip of 12 sorts of the fatigue crack in annealed copper specimens, respectively, with aids of flow stress curve during the uniaxial stretching process and the crystal oscillation X-ray microbeam diffraction photographic technique.
The results are as follows.
(1) The microscopic and macroscopic residual stresses measured in the crystals ahead the tip of the crack increased with increase in the crack length, while the minimum stress required to propagate the fatigue crack σ_min.c decreased with the crack length.
(2) The ratios of stress concentration α_f.c and α_r.c which were confined by dividing the strength of the material at the tip of the crack by σ_min.c increased with the crack length.
However, it seems to be necessary to make further inquiry into the case of α_f.c since no arrangement of the dislocations were taken into consideration in the estimation of the strength of the material at the crack tip, though the density alone was duly considered.
(3) If the macroscopic residual stress is regarded as the mean stress, it may be considered that the stress state of the cyclic loading at the crack tip was compressed during the fatigue process. In this situation, however, it is clear experimentally that at the initiation of plastical deformation in the zone new crack appeared at the crack tip.