Assessing the quality of inspection for tapered aircraft fastener holes using an engineer’s blue contact test

Tapered interference-fit fasteners (e.g. Taper-lok) are commonly used by the aerospace industry due to their high fatigue strength. The inspection relies on a ‘true’ geometry gauge coated with a thin film of engineer’s blue being brought firmly into contact with the tapered hole. The pattern created on the gauge is then used to identify bearing surface area and distribution. This research investigates if the inspection reflects the hole geometry, and under what conditions the inspection may be assumed valid. The results show that the shade of the blue on the gauge as inspected through computer vision correlates with the unloaded geometry evaluated via a roundness tester, in particular when the red channel is taken alone. The inspection method falls short of accurately representing small features, particularly those tangential to the gauge removal direction. Thresholding visually or by computer vision may reasonably be used to identify the contact region pattern, however factors limiting blue migration mean the contact area may appear smaller than it actually is.