MINISTRY OF SUPPLY AERONAUTICAL RESEARCH COUNCIL REPORTS AND MEMORANDA An Experimental Investigation of Stress Diffusion in Non-buckling Plates

The purpose of this report is to provide experimental resuKs for comparison with theoretical analyses of stress diffusion problems. The structures considered consist of plane reinforced sheet which has been assumed not to buckle. Symmetrical loads are applied to the edge booms connected to the sheet by continuous no-slip joints. Attention is concentrated on the stress distribution near the ends of the parallel strips of plate.-An outline of the existing theoretical work which is applicable to this type of problem is given. The stringer-sheet theory, the only one capable of dealing adequately with unreinforced sheet, is compared with the photoelastic results. It is shown that the stringer-sheet theory overestima[tes the peak shear stresses near the corners of the strips and consequently also the rate of diffusion of load from boom to sheet. It is also shown that the experimental shear stresses. . are in reasonable agreement with those predicted by a more exact plane-stress theory. This theory predicts that the peak shear stress in the plate is 2/= times the direct stress in the boom at the end of the panel. However, with the type of joint considered here, the maximum shear stress is likely to be much higher than the value given by this prediction. Some attention is also given to transverse end stiffeners and it would seem that these normally have very little effect on the shear stresses. • The photoelastic models were made from the Allylstrene plastic called C.R.39. The no-slip joints were obtained by gluing the stiffeners to the plates. 1. Introduction.-The problems of stress analysis known as stress diffusion and shear lag S have been confronting aeronautical engineers for about fifteen years. The papers published during this period on these problems have been mainly theoretical, and indeed very few con-firmatory experiments appear to have been carried out. Until recently it has been the practice to allow the thin plate skin of aircraft to buckle between the stringers at loads below the ultimate ; in some cases buckling occurred at working loads. For this reason and alsobecause the theory is thereby simplified, it was usually assumed that the plates take no direct load and merely transfer load from one stringer to the next by shear. Skin buckling is now no longer considered permissible or desirable, owing firstly to the need for more accurate aerofoil shapes, and secondly, to the investigations which have shown that under current …