Dynamic Analysis of Honeycomb Sandwich Beam with Multiple Debonds

Analytical formulation for the evaluation of frequency of CFRP sandwich beam with debond, following the split beam theory, generally underestimates the stiffness, as the contact between the honeycomb core and the skin during vibration is not considered in the region of debond. The validation of the present analytical solution for multiple-debond size is established through 3D finite element analysis, wherein geometry of honeycomb core is modeled as it is, with contact element introduced in the debond region. Nonlinear transient analysis is followed by fast Fourier transform analysis to obtain the frequency response functions. Frequencies are obtained for two types of model having single debond and double debond, at different spacing between them, with debond size up to 40% of beam length. The analytical solution is validated for a debond length of 15% of the beam length, and with the presence of two debonds of same size, the reduction in frequency with respect to that of an intact beam is the same as that of a single-debond case, when the debonds are well separated by three times the size of debond. It is also observed that a single long debond can result in significant reduction in the frequencies of the beam than multiple debond of comparable length. 1. Introduction Sandwich construction is formed by bonding two thin facings to a thick core and has very high strength and stiffness properties achieved by increasing the thickness of the core without any weight penalty. The manufacturing defects like incomplete wetting or entrapped air pockets into resin-dominant layer can result in nonuniform adhesion between the face sheets and the core or skin-to-core debond. In-service circumstances such as low velocity impact by foreign objects or accidental tool drops during maintenance operations can lead to local debond. Overloading and elevated temperature regime may also induce debond at the weakest point of skin-to-core interface. During service, the debond may propagate and trigger new damage modes such as face sheet wrinkling, dimpling, and core shear cracks. As the structures using sandwich materials require extremely high level of reliability, debond must be detected immediately after its occurrence to ensure safety and the durability of the structures. In the author’s recent analytical study on vibration characteristics of sandwich beams with debond, the well-known split beam theory was modified considering the core stiffness of sandwich beam and accordingly the equations of motion were derived [1]. In the case of an aluminium skinned cantilever sandwich