Fields of Residual Stresses near Filled Assemblage Holes of the Aircraft Wing Panel

Abstract

Fatigue resistance tests were performed on two geometrically identical and structurally similar models of the lower wing panel of a commercial aircraft. The panels differed in the method of installation of the mounting bolts joining the skin and the stringer. In the first panel, cold hardening of the holes was performed before installation of the skin and stringer. In the second panel, no additional processing of the holes was performed after drilling and reaming. The bolts were installed with an interference fit within the range of 1.3 to 2.1% for the first panel and 2.9 to 3.2% for the second panel. Variations in the values of the interference fit are due to the presence of tolerance fields for the diameters of both the bolts and the mounting holes. A comparison of these technologies was based on a study of residual stress fields. An analysis was performed of the residual stress component values in the vicinity of the mounting holes in the skin, which were filled with interference-fit bolts. The residual stress components were determined using the hole drilling method and the successive crack length increase (SCLI) method. The deformation response was measured using electron speckle interferometry. The first (discrete) method based on drilling a probing hole makes it possible to quantitatively determine the components of residual stresses starting from a distance of 1.1 mm from the contour of the mounting hole. The second (continuous) method consists of successively increasing the length of an artificial notch. A new version of the SCLI method ensuring determination of the parameters of fracture mechanics for notches propagating in the contact interaction zone has been developed. It consists in making an initial through hole in the middle of the distance between the studied mounting holes, from the contour of which a sequence of artificial notches begins. The end point of this sequence is the outer contour of the bolt. This approach ensures analysis of the residual stress fields arising in two bolt installation technologies by comparing the values of stress intensity factors (SIF). Both experimental approaches reveal the advantages of joining with bolts installed in a hardened hole. The high efficiency and reliability of the methods for determining residual stresses using optical interference measurements of the strain response to local material removal has been substantiated and clearly demonstrated. These methods are based on obtaining high-quality interferograms, which provide resolution of interference fringes of maximum density directly on the contour of the probing hole or on the banks of an artificial notch.