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 are 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 has been performed for the first panel before joining. The second panel includes no additional treatment after drilling pilot holes and final reaming. Bolts are 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 are 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 is based on the experimental study of residual stress fields. The first stage, being a subject of the present study, includes the analysis of residual stress fields, which arise after removing bolts and separation of skin from stringers. Hole drilling and gradual crack growth were used to determine the components of residual stresses. Deformation response is measured by electronic speckle-pattern interferometry. High quality interferograms, which provide a reliable resolution of the interference fringes of ultimate density over the hole edge or directly along the notch borders, have been obtained for both ways of local removing the material. The first point-wise method based on drilling a probe hole, provides 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 provides 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 is 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.