Abstract
Uniaxial tensile fatigue tests were carried out on high strength steel to investigate the relation between fatigue characteristics and fracture origins. The examination compared two surface finishing conditions, one grinding polished by the emery paper and the other electro-chemically polished. The emery paper finished specimens showed a stepwise S-N diagram while the electro-chemically polished specimens had a straight S-N diagram. The sites of fracture origins changed from surface to interior of test pieces as the stress level was reduced on both kinds of test piece. An examination of the initial stress intensity factor range ΔKini of fracture origins was interpreted to show that large surface defects in the grinding condition lowers the fatigue strength of surfaceoriginating fractures, resulting in the stepwise S-N diagram. Interior-originating fractures showed that cracks originate and propagate from inclusions with much smaller ΔKini than the surfaceoriginating cracks.
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