Prestress of Adhesively Bonded 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 diagram based on elastic analysis
is constructed for evaluation of prestress forces that reduce or
completely eliminate adhesive stress concentrations caused by
tension stress, applied to the skin.
Conclusion
The result suggest a relatively simple method of adhesive stress
reduction in a skin/flange assembly loaded by skin tension. 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 lead 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 diagram. 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 \cite{Brien1}. 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
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.
© 2006 UIUC and Dr. Karel
Matous