Experimental Investigation On Mechanical Properties Of Honeycomb Sandwich Structure With Defects

With the rapid development of aeronautics, astronautics and automobile manufacturing, honeycomb sandwich structures attract more and more research interest around the world. This kind of structure has many advantages, such as light weight, high strength, good shock absorption performance and so on. However, during the process of preparation and using, stochastic defects of different kinds will be generated. These defects may disadvantage the mechanical properties of the structure. So it is necessary to do experimental study on the mechanical properties of honeycomb sandwich structures with defects. Amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) is a non-contact, high precision, full-field and real time technology, which has become a research focus on the measurement of structure deformation in vibration.In this thesis, the vibration characteristics of honeycomb sandwich structures with or without defects are investigated and the main contents are as follow:(1) The measurement principle and the optical arrangement of AF-ESPI method is introduced. Based on the zero order Bessel function J0, the calculation method for the out-of-plane deformation using the fringe pattern obtained by the AF-ESPI is given.(2) The numerical results of the specimen without defects are calculated and in good agreement with the experimental results, which verifies the reliability and accuracy of the AF-ESPI method. And on this basis, the vibration characteristics of a cantilever thin plate containing an edge crack are investigated using the AF-ESPI method. The impact of the crack parameters including the length and location on the vibration characteristics are studied. From the experimental results, it can be concluded that the resonant frequencies are evidently influenced by the crack parameters but the mode shapes are hardly sensitive to these factors. In addition, the stress intensity factor induced by the resonant vibration is evaluated.(3) The vibration characteristics of honeycomb sandwich structures with or without defects are investigated using the same optical arrangement. These defects are generated during impact failure, three-point bending failure and both of the two. From the experimental results, it can be concluded that the resonant frequencies are sensitive to these defects, while the mode shapes are insensitive. The displacement response will distort or mutate near the crack. And if the physical parameters like mass matrix and stiffness matrix change, the phenomenon of mode jumping will happen. Moreover, the mechanism and influence of the mode jumping are discussed based on the traditional matrix perturbation method.