The manufacturing procedure for coiled tubing induces different surface mor
phologies on the inner and outer surfaces. Since fatigue is a surface pheno
menon, this can lead to different fatigue behavior at the two locations. Th
is has been demonstrated in full-scale bending tests and in axial tests con
ducted on small coupon samples in closed-loop servohydraulic strain control
. Furthermore, service environments can cause localized defects on the oute
r and inner surfaces of coiled tubing, such as grinding marks, corrosion pi
ts, tool marks, or injector block damage, which can have a first order effe
ct on fatigue strength. However, it is demonstrated in this article that th
ere is an inherent tendency for cracks to initiate on the inner surface of
coiled tubing. This tendency leads to the hypothesis that a ''threshold sev
erity'' for external surface defects could exist, below which the fatigue s
trength of the tubing is not affected. Limited data confirm this hypothesis
, and suggest that the severity could depend on the grade of the material.