Bending Moments in Fuselage Frames Obtained by the Photoelastic Method

Abstract

SummaryThe experimental work described in this paper, which was divided into two sections, was done to determine the usefulness of photoelasticity as a method for analysing fuselage frame problems. In the first part, a series of tests was made by the photoelastic method on models without any supporting shell, which were in equilibrium under the applied i loads, to determine the bending moment distribution in various types of frames and rings.In all cases the theoretical bending moments were calculated from the principles of minimum strain energy. The experimental results agreed with theory to within 5 per cent, except close to the null points and joints.The second part was devoted to shell-supported models, and the shear loads were reacted out through the skin to firm supports at the end of adjacent bays. Again the bending moments were determined for the entire frame from the isochromatic fringe pattern.The shell-supported tests proved to be of considerable interest when compared with the theoretical curves of Ref. 4 for the corresponding relative stiffness parameters. In some cases agreement was good but in others there were marked differences. A possible reason for this was that compression effects in the skin were altering the distribution and value of the bending moments in the frame.It was felt that there was plenty of scope for more interesting work on fuselage frame analysis, from the standpoint of shear flow determination, intermediate frame reaction, and skin-to-frame relative stiffness effects. All of these might feasibly be examined photoelastically with a certain degree of advantage over lengthy and laborious theoretical investigations.