Damage in composite materials is challenging to detect because of complexbehavior. Using nonlinear analytical techniques, the behavior of a disbond in analuminum honeycomb material is illuminated. By analyzing the superharmonicfrequencies under sinusoidal excitation, quadratic stiffness was seen in a disbondedpanel and tracked as the damage area increased. A cubic nonlinearity was alsoobserved, but only when the damaged area was larger. It is hypothesized that thequadratic nonlinearity is due to the change in face sheet stiffness as the top sheetvibrates off and into the honeycomb core. The cubic nonlinearity is not as easilyexplained due to the changes in response amplitude not following the analyticalapproximations. The lack of a nonlinear backbone curve indicates that the cubicnonlinearity is not related to stiffness but most likely damping. The elastic behaviorof the epoxy adhesive under high strains and geometric constraints is thought tocontribute to the cubic nonlinearity. More work into understanding the relationshipbetween the epoxy viscoelastic behavior and the panel response is necessary.
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