Fatigue Strength of Induction-Hardened Test-Piece with Crack

Abstract

One of the reasons why the fatigue limit remarkably rises in the notched specimen of steel which is induction-hardened, is that, the cracks produced do not propagate any further. In this connection, some experiments on rotary bending were made with the specimens of a plain carbon steel with 0.18%C, some of which are ordinary plain specimens and others have some crack, both were tested for two cases, ie. non-heat treated and induction-hardened. The results are as follows : (1) without any heat treatment fatigue limit a) ordinary plain specimens 20 kg/mm² b) specimens with some crack 10 kg/mm² (2) induction hardened a) ordinary plain specimens 54 kg/mm² b) specimens with some crack 48 kg/mm² The reduction rate of the crack is apparently less in the latter than in the former, which is less hardend. To explain this seemingly contradictory fact, Prof. Ishibashi's opinion has been applied that the tip of crack must be subjected only to tension stress during rotating bending tests.