We have proposed a new deformation processing method for enlarging partial diameter of shaft. The method is called a collar-forming method. To examine fatigue damage of the part processed (called a collar) during the forming process, rotary bending fatigue tests and tensile strength verification tests were carried out on smooth test specimen as well as two kinds of collared test specimen, namely the specimen with burrs and without burr. Moreover, stress concentration and strain hardening in the root of collar were quantitatively evaluated by the finite element method. The following were clarified from those results: Though there are burrs, the fatigue damage in the root of a collar during the forming process is not caused at all when an increase rate of shaft diameter is within 1.7. The processing stage in which the increase rate is within 1.7 is still a stage of the work hardening, and is not a stage in which the fatigue life is shortened by increasing of macro-damage. There are not any signs of fatigue deterioration in the root of collar formed by the processing method from a practical viewpoint, either but there is sound to the fatigue damage enough.
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