Computational Physics Group

Stress Redistribution in Skin/Flange Assemblies

K. Matous and George J. Dvorak

Department of Mechanical Engineering, Aeronautical Engineering and Mechanics
Rensselaer Polytechnic Institute
110 8th Street, Troy, NY 12180

Abstract

A simple prestressing procedure is proposed for reduction of stress concentrations at the leading edges of adhesive bondlines in composite skin/flange assemblies. A detailed finite element analysis of a specific geometry of such an assembly is presented, which accounts for nonlinear viscoelastic deformation of the adhesive. Simple design diagrams based on elastic analysis are constructed for evaluation of prestress forces that reduce or completely eliminate adhesive stress concentrations caused by either tension stresses or bending, applied to the skin in the direction transverse to the longitudinal flange axis.

Conclusion

The result suggest a relatively simple method of adhesive stress 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. Possible viscoelastic deformation of the adhesive tends to reduce over time the stress maxima at the leading edges of the flange bondline. Therefore, design diagrams based on elastic stress analysis should suffice, and led to conservative designs in most applications. This would also obviate a detailed evaluation of the material parameters needed in the nonlinear adhesive analysis. Since the adhesive stress distributions depend both on adhesive and adherend elastic moduli and details of the joint geometry, a finite element evaluation of the stress distributions is needed 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 tapered free edges of the laminated flange. Indeed, failure of the joint often originates in the tapered 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.

Acknowledgment

© 2009 Notre Dame and Dr. Karel Matous