Abstract
The authors have newly designed and manufactured a large rotating bending fatigue test machine (Cantilever type, Max. bending moment 7 t-m, Standard diameter of specimen 125 mmφ) and tested specimens of 135 mm φ under sea water corrosion.The specimens have been taken from steel bars forged from 1. 4 ton ingots made by acid open-hearth process (forged ratio = 6) and annealed in the furnace. The chemical and mechanical properties of the specimensare as shown below : Chemical Properties Mechanical PropertiesC 0.22 Tensile strength (kg/mm2) 46.447.0Si 0.23 Yield point (kg/mm2) 24.024.5mMn 0.42 Elongation (50 mm, %) 36.837.2P 0.013 Reduction of aea (%) 58.163.2S 0.04310 mm φ specimens of the same material having simillar shape have been also tested under sea water corrosion for comparison.The following facts have been revealed by the tests : (1) Fatigue strengths of 130 mm φ and 10 mm φ specimens at 108 stress reversal were 11.2kg/mm2 and 3. kg/mm2 respectively.(2) Depth of cracks found on the corroded surface of specimens is calculated by the following formula : l=C×log10(N/Nc)where l : Depth of corrosion cracks (mm) C : Constant, determined by the method of corrosion (=0.86 in this experiment) Nc : Limit of stress reversal at which corrosion cracks will originate (=1.7×106 in this experiment) N : No. of stress reversals.(3) When N does not exceed 1.7×106, cracks will not originate.(4) A reversal stress limit (σc) at which surface cracks have originated on large corroded specimens may be pressumed about 5kg/mm2.(5) There is also a reversal stress limit (σ0) at which fine cracks on the non-corroded specimens begin to develope, σ0 is assumed about 11 kg/mm2 for the steel tested(6) Failure of corroded specimens may occur when reversal stress has increased as the result of reduction of effective area by surface cracks and exceeds σ0.(7) Fatigue limit of large corroded specimens is designated by σ0.(8) There may be no fatigue limit for small corroded specimens.
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