Synergy effects in the fatigue crack growth of hole cold expanded specimens under variable cyclic loading

The influence of variable load spectra in the fatigue crack growth (FCG) was analyzed. A 4.1% interference was pre-set to some specimens by an expansion operation. As-drilled hole and hole expanded specimens were subjected to FCG tests under a combination of baseline cyclic loading and periodic overloads or underloads applied. Fatigue life was of approximately 1 million of cycles for the as-drilled hole, and 3.5 million of cycles for the hole cold expanded. The pre-setting operation and variable load spectra interaction exhibited a synergy, which was associated with multiple periods of retardation and arresting observed in the crack growth rate.