Observations of non-linear stress effects on crack growth in interference fit fasteners

Interference fit fasteners are a ubiquitous part of modern airframes, incorporated to increase the fatigue life at critical locations. When inserted into a structure, they have a compound effect of inducing residual stresses and changing the stress concentration. Evidence suggests that this stress state may become non-linear under high tensile loading, as the fastener and hole surface may separate. This paper illustrates the importance of including this separation in fatigue analysis of structures containing interference fit fasteners by showing that separation results in an otherwise unexplained acceleration in crack growth. Test specimens with and without fasteners are subjected to a specifically designed test spectrum, and crack growth rates are measured using quantitative fractography. This investigation reveals a statistically significant acceleration in fatigue crack growth only found in the specimens containing interference fit fasteners. Simulations that model the separation effect predict an acceleration, but classic crack growth equations fall short. Ignoring these separation effects is shown to underpredict the growth of cracks under certain conditions by a factor of two or more.