抄録
Measurements and observations on the closures of both propagating fatigue cracks and socalled non-propagating fatigue cracks initiated from notch roots in three kinds of steel during rotating bending tests were made successively at various stress levels in one stress cycle by using the plastic replica method.
The results for the propagating cracks having about the same propagation rate (0.05μ/cycle) show that fatigue cracks can be closed at the crack tips for up to three fourths in the annealed 0.13%C steel, two thirds in the annealed 0.54%C steel and one half in the heat treated 0.54%C steel, of the respective full stress ranges. Therefore, the stress up to these levels does not cause further propagation of the cracks.
On the other hand, the result for the non-propagating cracks in the annealed 0.13%C steel shows that the crack tip is kept closed even at the maximum stress level. Therefore, the fatigue damage is hardly accumulated by the repetitions of the stress which has produced the non-propagating cracks.
The plastic deformation remaining in the wake of crack extension is considered to be the factor controlling the crack closure mentioned above.
