Analytical and Experimental Results for Bonded Single Lap Joints with Preformed Adherends

Abstract

A theoretical and experimental study is conducted to investigate the load transfer of a single lap joint in which the adherends are preformed so that the angle between the line of action of the applied in-plane force and the bond line is reduced. The preforming of the adherend reduces the moment resultant in the ad here nd at the edge of the overlap region, which reduces both the maximum peel and shear stresses in the adhesive and gives a more uniform shear distribution in the adhesive. An increase in static load transfer of up to 120% is shown, and several orders-of-magnitude increase in fatigue life are achieved with modest preform angles. Thus, sizable benefits can be obtained in the fatigue life or additional load capacity of bonded single lap joints by preforming the adherends. Results are presented from an analytical and experimental investigation to determine the effect that preforming the adherend has on the stress distributions and strengths of bonded single lap joints. The analytical investigation used a closed-form analysis similar to that of the classical analysis of Goland and Reissner3 and a two-dimensional quasi-nonlinear finite-element analysis similar to that of Ref. 4 to study in detail the stress distributions in the ASTM D 1002-72 specimen5 with a 1.27-cm (0.50-in.) overlap and preformed adherends. The finite element analysis is used to check the accuracy of the closed-form analysis. A parametric study is conducted using the closed-form analysis to determine the optimum adherend preform angle for typical adherend and adhesive material properties and joint geometries. The ex- perimental investigation includes numerous tension-to-failure tests to determine the effect that preforming the adherends has on the ultimate static strength of typical bonded joints. A photoelastic test is also conducted to show the changes in the stress distribution in the joint due to preforming the adherends. Due to the high stress concentrations near the edge of the overlap region in the adhesive, joints are usually designed for fatigue rather than static strength. In fact, design-allowable stresses for single lap joints are generally taken to be 30% of the static ultimate strength because of fatigue. The reductions in stress concentrations obtained by preforming the adherends of single lap joints should significantly improve the fatigue strength. Thus, fatigue tests are also conducted to determine the effect of preforming the adherends on the fatigue strength of typical bonded single lap joints. Because of program and time constraints, only a limited number of low-cycle fatigue tests are performed. Test variables include both preform angle and overlap length.