Fields of Residual Stresses near Open Assemblage Holes of Aircraft Wing Panel

Abstract

The results of fatigue tests of two geometrically identical and similar in design models of the lower wing panel of a commercial aircraft were analyzed. The panels differed in the way of installing mounting bolts, which connect the skin and stringers. Cold expansion of holes drilled both in the skin and stringer was performed for the first panel before joining. The second panel included no additional treatment after drilling pilot holes and final reaming. The bolts were mounted with an interference fit varying from 1.3 to 2.1% and from 2.9 to 3.2% for the first and the second panel, respectively. Changes in the interference fit were the consequence of a scatter attributed to the presence of a tolerance zone for the diameters of both bolts and mounting holes. A two-step comparison of both technologies was based on the experimental study of residual stress fields. The first stage, being a subject of the present study, included the analysis of residual stress fields arising after removal of the bolts and separation of the skin from stringers. Hole drilling and gradual crack growth were used to determine the components of residual stresses. The deformation response was measured by electronic speckle pattern interferometry. High quality interferograms, which provided a reliable resolution of the interference fringes of ultimate density over the hole edge or directly along the notch borders, were obtained for both ways of local removal of the material. The first (pointwise) method, based on drilling a probe hole, provided a quantitative determination of the residual stress components, starting from 1.4 mm distance from the assemblage hole edge. The second technique implements the crack compliance method of subsequent lengthening of the notch, starting directly from the mounting hole edge. This approach provided for a quantitative analysis of residual stress fields, related to different bolt mounting technologies, proceeding from the comparison of SIF values. A high level of compressive residual stresses near open holes was characteristic for both types of panels. Both experimental approaches showed the benefits of joints, where bolts are mounted into cold-expanded (reinforced) holes. For this case, the estimation of the relaxation parameters of the principal component of residual stresses in the direction of the external load is presented.