Published works on studying effects of specimen size on fatigue endurance limit and fatigue life were reviewed. Specimen size effect is apparent in many bending fatigue tests. However, axial tension and compression loading fatigue tests showed that the fatigue limit decreased slightly with increasing specimen size. Design margin for the size effect on fatigue limit under axial tension and compression loading condition that is the condition of the data used to construct Design Fatigue Curve (DFC) is not considered to be necessary. If it is considered conservatively, the value less than 1.1 is enough. Axial tension and compression loading fatigue tests using cylindrical specimens having different diameters showed that the fatigue life increased with increasing specimen diameter. This phenomenon is considered to occur due to the crack growth life which is not negligible in relatively large diameter specimens. Based on this, design margin factor on number of cycles is not necessary to be considered in DFC. These design margins based on the axial tension and compression loading fatigue test results are less than the values in current design codes. This indicates that the current design fatigue curves in the codes have large margins. These considerations will be verified after the large scale fatigue tests that are planned in Design Fatigue Curve (DFC) subcommittee in the Atomic Energy Research Committee in the Japan Welding Engineering Society.

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