Study Of Peformance And Optimum Design Of Cocured Composite Lanimates With Embedded Viscoelastic Layer

In recent ten years, cocuring damping materials in composites has been shown to be successful in greatly increasing the damping of composite structures. Cocuring refers to the process of inserting materials within composite laminates before the composite is cured, therefore the embedded materials undergo the temperature and pressure cycle that is necessary to cure the composite. It can increase the damping of composite structures effectively and avoids the stripping phenomenon based on keeping the stiffness and strength of the composites. In 1996, this structure was applied successfully in turbo-fan by University of California and National Aeronautics and Space Administration (NASA). Our country also pay much attention to this new cocured composite structure and is doing some beforehand study on application in satellite structures.The damping and mechanical properties of the cocured composites should be sensitive to the temperature and the frequency of the vibration load with an embedded viscoelastic layer as a special layer. Because that, the properties of viscoelastic materials are sensitive to temperature and frequency.In present, the damping of the cocured composite was usually predicted by modal strain energy method. However, the loss factor calculated by this method characterize the damping of the structure free vibrate with natural frequency. However, the response frequency is same to the pumping frequency and not the natural frequency. Therefore, the modal loss factor can not character the damping of the structure in actual work accurately. In this paper, a new method for the determination of damping in cocured composite laminates with embedded viscoelastic layer is developed based on modal strain energy method, mode-superposition and time-temperature superposition principle. The effects of the frequency of the load on the damping of the cocured composite are also taken into account. The method was applied in determination of the damping of the cocured composites with the pumping frequency of 1Hz and different temperature. There is a good agreement between the analytical result and experimental result, the method put forward was validated.Most of the existing research on cocured composite with embedded viscoelastic layer concentrates on the static mechanical properties and not consider the effect of the temperature and frequency. The dynamical mechanical properties approach the practical mechanical properties. The dynamical mechanical properties provide design parameters for advance, proper, safe and economy design for composites. In this paper, the dynamic mechanical properties of the cocured composite with embedded viscoelastic layer was investigated by dynamic mechanical thermal analysis (DMTA) and was compared to that of fiber reinforced composite without embedded viscoelastic layer. It finds that, the dynamical properties of the cocured composites are completely different form that of the fiber reinforced composites, and similar to that of the embedded viscoelastic material. There is a peak damping near the vitrification temperature of the embedded viscoelastic material, and the loss factor is 2.5 to 13 times of that of fiber reinforced comopsits.The embedded materials undergo the temperature and pressure cycle that is necessary to cure the composite inserting within composite laminates before the composite is cured. The properties of the embedded material will be changed due to the temperature and pressure and penetration of the resin during the cure cycle. And it will change the properties of the integral composite structure. However, there is little report about it. In this paper, the effects of the temperature and pressure and penetration of resin during the cocuring with composite were validated by scanning electronic microscope and field emission scanning electron microscope. The effects of cocured with composites on the dynamic mechanical properties of the embedded viscoealstic material was investigated by dynamic mechanical thermal analysis (DMTA). It finds that, the damping of the viscoelatic material decreased while the bending stiffness increased due to cocuring. The main reason is the penetration of the resin, and temperature and pressure have little effect on it for comparing.The existing results indicate that, although the embedded viscoelastic layer has little effect on the tensile and compress properties of the composite structure, it decrease the bending stiffness or strength and interlaminar shear propertiesof the composite structure. And it dose not meet the challenge of integral design of structures. Some related researchers put forward the embedded layers were perforated with a series of small holes to allow the resin to flow through the damping layer and completely couple the structure to improve bending stiffness and interlaminar shearing strength of these cocured composite laminates. However, it decreases the damping of the cocured composites. The contradiction between the damping and bending stiffness make it difficult to determine the size of the perforation area. In this paper, the properties of cocured composite with embedded perforated viscoelastic layer are investigated. The effect of the size of the perforation area on the damping and bending stiffness of the cocured composite structure was studied first of all. And then, an optimum design method for ratios of perforation area was developed using unified object function method. There are two sub-object functions, one is the loss factor, and the other is the bending modulus of the cocured composite laminates. The method provides a reference for coordination of the damping and bending stiffness of the cocured composites with embedded perforated viscoelastic layer.