Anggit Murdani

Effect of Crack Initiation Size on Fatigue Strength in 0.85% Carbon Steel

The fatigue limit of a plain specimen of a tool steel JIS SK85 was investigated. The initial crack length in the fatigue process is related to the size of a single crystal in the case of low carbon steel. However, that is independent of the crystal size in the case of pure aluminum, a lamellar pearlitic steel, etc. In this study, the relationship between the initial crack length and the fatigue limit was investigated. The original microstructure of this material includes a spherical microstructure. However, after heat treatment under some conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of heat-treated SK85 could be evaluated by an equation in which the parameters are the hardness and initial crack length. Finally, this result was discussed based on the observations of crack growth behavior and fracture surface.

EFFECT OF NaCl SOLUTION SPRAYING ON FATIGUE LIVES OF SMOOTH AND SLIT SPECIMENS OF 0.37% CARBON STEEL

The fatigue crack initiation life and growth rate are affected by experimental conditions. A corrosive environment can be created in a laboratory by means of dropping salt water onto the specimen surface, spraying chloride mist into the experimental chamber, etc. In the case of smooth specimens of some metals, fatigue life is shortened and the fatigue limit disappears under such corrosive experimental conditions. In this study, the effects of intermittent spraying of 3% NaCl solution-mist on corrosion fatigue behavior were investigated. The material used was 0.37% carbon steel. This is called JIS S35C in Japan. Spraying of 3% NaCl solution-mist attacked the surface layer of the specimen. It is well known that the pitting, oxidation–reduction reaction, etc. affect the fatigue strength of metals in a corrosive environment. We carried out corrosion fatigue tests with smooth specimens, holed specimens and slit specimens. Then the effects of such specimen geometry on the fatigue strength were investigated when the NaCl solution-mist was sprayed onto the specimen surface. In the case of lower stress amplitude application in slit specimens, the fatigue life in a corrosive atmosphere was longer than that in the open air. It is discussed that the behavior is related to the crack closure which happens when the oxide builds up and clogs the crack or slit.