Study On Free Vibration Characteristics For Composite Laminates And Stiffened Plates With Multiple Delaminations

Composite laminates and stiffened composite plates have been used commonly in the engineering structures, because they have the properties of high specific strength and specific stiffness, fatigue resistance, vibration resistance and design versatility, etc. However they are easy to suffer various degrees of multiple delamination damages, caused by debonding between interlaminar interfaces during manufacturing and/or low velocity impact in the service. Delaminations will not only reduce the static stiffness, but also vary their vibration characteristic. It is therefore important to understand the vibration characteristic (including frequency and modes) of multiple delaminated composite plates to the accuracy in design and the safety in use of composite structures under the dynamic loading.Based on the first-order shear deformation theories of composite laminated plate and beam, a free vibration FEA model and method of the laminates with multiple through-width delaminations and that of the composite stiffened composite plates with multiple circle-embedded delaminations have been developed in this thesis and a linearity contact model to simulate the interacting of delaminate area was established during vibration analysis process.For the laminates plate with multiple through-width delaminations, the effects of the location, length, depth and numbers of delaminations, and boundary condition of the laminates upon vibration characteristic were discussed in detail. For the composite stiffened plates with multiple circle-embedded delaminations, the study of the effects of the size, depth, numbers of delaminations and the heights of the stiffeners upon vibration characteristic were also carried out.The research work of this paper is an important sub-part of the projects of the National Natural Science Foundation in China "the study on the nonlinear dynamic characteristics and damage mechanism of composite stiffened structure", Grant No. 10272025, and "The mechanism of advanced grid structure (AGS) with damage", Grant No. 10302004, and National Basic Research Program of China "New theory of optimum design for extra-light porous materials and their structures", Grant No. 2006CB601205.