The effect of nonlinear contact upon natural frequency of delaminated stiffened composite plate

The effect of nonlinear contact upon natural frequency of the stiffened composite plate with pre-damages, such as delamination of skin panel and/or debonding interfaces between skin panel and stiffeners, is studied by the finite element method, based on hump resonance principle. A formula of element stiffness and mass matrices for the composite stiffened plates is deduced by using the first order shear deformation theory combined with the selecting numerical integral scheme, while that of global damping matrix is constructed by Raleigh mode damping model in conjunction with Adam’s’ strain energy method. A virtual interface nonlinear contact element, called Hertz’ discontinuous spring, is also employed for simulating the separate/contact mode state along the interfaces of delamination and/or debonding regions during the vibration process. The dynamic analysis method for the plates is established by a precise time integration method, and the natural frequencies are determined from the frequency/response curve. By some numerical examples, the delaminating and debonding effects upon the natural frequencies of the plates are discussed. The method and conclusions would be useful for composite structural designers.