Fields of residual stresses near filled assemblage holes of the aircraft wing panel

Fatigue tests of two geometrically identical and similar in design models of the lower wing panel of a commercial aircraft were performed. The models differ in the way of mounting bolts which join the skin and stringers. Cold expansion of holes drilled both in the skin and stringer has been performed for the first panel before joining. No additional treatment of holes was performed in the second panel after drilling pilot holes and final reaming. Bolts are mounted with the interference fit ranging from 1.3 to 2.1% and from 2.9 to 3.2% for the first and the second panel, respectively. The range of the interference fit values is attributed to the tolerance fields for the diameters of bolts and assemblage holes. A comparison of both technologies proceeded from the experimental study of residual stress fields. The second stage, which is the subject of present paper, includes the analysis of the values of residual stress components in the vicinity of skin holes filled by bolts mounted with the interference fit. The components of residual stressed were determined using the method of hole drilling and successive cracking (crack compliance) method. The deformation response was measured by electronic speckle-pattern interferometry. The first point-wise method, based on drilling a probe hole, provides the quantitative determination of residual stress components, starting from a distance of 1.1 mm from the assemblage hole edge. The second technique consists in successive extension of the notch length. A new version of the crack compliance technique providing the determination of fracture mechanics parameters for notches propagating in the contact zone has been developed. The essence of this approach consists in drilling the initial hole equidistant from two assemblage holes of interest. The edge of this through hole is a starting point of the sequence of artificial notches, the end point of this sequence is the outer contour of the bolt. This approach provides quantitative analysis of residual stress fields proceeding from the comparison of the values of stress intensity factors (SIF) related to different technologies of bolt mounting. Two experimental approaches reveal advantages of joining with bolts mounted into cold-expanded holes. The high efficiency and accuracy of the methods for residual stress determination which employ optical interferometric measurements of the deformation response to local removing of the material is substantiated and clearly demonstrated. The methods are based on recording high-quality interferograms, which provide the resolution of interference fringes of the ultimate density directly on the hole edge and along borders of the artificial notch.