Computational Physics Group

A prestressing procedure for reduction of adhesive peel and shear stresses at the leading edge of a skin/flange assemble is analyzed for tensile and bending loads applied to the skin. Both an analytical solution based on the Green's functions and finite element solution are presented for specific examples, together with design diagrams. Substantial shear stress reduction is obtained with the proposed procedure.

The results suggest a relatively simple method of adhesive stresses reduction in a skin/flange assembly loaded either by skin tension and/or bending, acting transverse to the longitudinal axis of the flange. While certain special fixtures would be required for prestressing, the expected enhancement of load bearing capacity and/or endurance may well be worth the extra cost. Since the adhesive stress distributions depend both on adhesive and adherend elastic moduli, details of the joint geometry, and the GFA cannot account correctly for peel stress in the adhesive and is inaccurate in evaluating the shear stress, therein a finite element method is strongly preferred. The proposed design diagrams based on elastic stress analysis should suffice and lead to conservative designs in most applications. Finite element evaluation of the stress distributions is used for construction of the design diagrams. Scaling of solutions obtained for a single load magnitude is indicated in the elastic case. In an actual composite structure, both flange and skin are made of a laminate consisting of several fibrous layers. Layup details may influence the adhesive stresses at the leading edge of the bondline, and also the interlaminar stresses at the free edges of the laminated flange. Indeed, failure of the joint often originates in the flange end and extends along ply interfaces before reaching the adhesive layer. Since the laminates were homogenized in our analysis, the results do not reflect that level of detail. However, inasmuch as the goal was to minimize the stress concentrations at the bondline leading edge by superposition of the prestress and applied loading stress distributions, the differences between the layered and homogenized solutions should not have a large effect on the loading combinations found to generate the minimized stress distributions.

The authors appreciate financial support of this work by the Ship Structures and Systems S&T Division of the Office of Naval Research. Dr. Yapa D.S. Rajapakse served as program monitor.

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© 2006 UIUC and Dr. Karel Matous