Fatigue nucleation site of cold expansion hole varying as fatigue load level varies

Fatigue nucleation sites of cold expansion holes are commonly believed to exactly locate at the inlet side hole edge because of the relatively lower compressive residual stress imparted by cold expansion. However, this work shows that the locations of fatigue nucleation sites can vary as fatigue load level varies. In present paper, when the maximum stress of fatigue load (R = 0.1) is larger than 210 MPa, fatigue nucleation sites have precisely located at inlet side hole edge, while, when the maximum stress of fatigue load (R = 0.1) is below 210 MPa, fatigue nucleation sites have moved far away from hole edge. Numerical calculation analysis can well explain the reason why the above described feature generate and propose that where fatigue crack will nucleate for cold expansion holes is strictly governed by the factual stress state under the action of residual stress superimposing applied fatigue load rather than merely by residual stress. As the hole designed nominal stress is generally low, under this circumstance the fatigue origin is preferred to nucleate in tensile residual stress zone which is relatively far away from hole edge. So, it is strongly recommended that in service and in regular inspection, except for hole edge, the tensile residual stress region must be paid enough attention to ensure the structure integrity and safety of cold expansion holes. Furthermore, the experimental results also show that even if the cold expansion holes undergo strain fatigue, SSCX keeps achieving an evident fatigue gain.