Abstract
ie authors have newly designed and manufactured a large rotating bending fatigue testing machine (cantilever type, max. bending moment 7 t-m, standard size of specimen 125φ mm) intending to investigate the fatigue strength of marine propellar shaft. In this paper the construction of the testing machine is outlined and test results of 125φmm smooth surface specimens made of 0.2 carbon forged steel and size effect referring to 10φ mm specimens are described. The following facts have been revealed by these results. (1). The size effect on fatigue limit of acid open-hearth steel is 8.5%, while that of electric furnace steel 18%. (2). It should be noted that the fracture of electric furnace steel is shown by some pits in the vicinity of the center. These pits may be caused by some faults in material, but seem to have no effect on the fatigue strength. (3). Propagating speed of fatigue crack under constant bending load may be presumed 1.0∼1.5μ/c at the initial stage of crack, and about 10μ/c at the stage, where the rupture is nearing. (4). Remarkable temperature rise of specimen is noted only when the working stress exceeds 90% of fatigue limit. Temperature rises to a maximum steady value after 2 or 3 hours, and keeps almost constant till final fracture.
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