Modelling And Detection Of Delamination In Plate Structures

Based on its mechanic property and lightweight advantage, plate structure being made of carbon-fibre reinforced resin composite materials is therefore widely used in aircraft and spacecraft engineering. However, as a result of low-velocity impact, air entrapments and stress concentrations caused by manufacturing processes, transporting or installing, internal defects are prone to arise in composite materials. One of the most serious defects leading to potential failure in composite laminates is delamination. This defects reduces the strength and stiffness of structure and will spread to cause a catastrophic collapse due to huge degradation of buckling resistance, which leads that composite plate structures fail to provide good usability and safety. Therefore, in order to avoid disastrous consequences caused by delamination in composite plate structure, ensure the safety of aircraft and spacecraft and improve their service life, it is of certain importance to measure the location and severity of delamination in composite plate structures of aircraft and spacecraft through the study of the impact of delamination to plate structure, which give rise to make the structure safe and reliable and is responsible for the reliability of structure bearing capacity. A new type of non-destructive measurement based on the simplicity and high accuracy of measurement for natural frequency is therefore proposed to determine the delamination location in composite plate structures by using normalized natural frequencies.Regarding the delamination in wing structure of aircraft, composite laminate with embedded delamination with an infinite small thickness hypothesis is modelled by VICONOPT based on exact stiffness method and Wittrick-Williams algorithm, whilst the dynamic response and vibration mode of the structure under free vibration is analysed by controlling variable method. Through discussing the impact of the geometry size and location in axis of the delamination to the composite laminate, the relationship between the delamination location and natural frequency is obtained, which provides a theoretical argument for the inverse problem of delamination detection.Inputting the actual data of free vibration experiment for a delaminated plate into detection program of using normalised natural frequency variation and comparing the predicted delamination range with the actual damage location are presented in chapter 3, which validates the procedure simplicity and accuracy of this method with high accuracy. Moreover, through inputting the actual data of free vibration experiment for a delaminated cantilever beam, a reliable predicted result is obtained by the detection method using the normalised natural frequency variation.Concerning the actual boundary condition of the wing structure, the effects of delamination detection model under different boundary conditions to delamination in various severity and at varying widthwise position is presented in chapter 4. It concludes that delamination detection method based on frequency measurement can be used in delamination localisation for composite whilst delamination arises in a small severity, which leads to a linear degradation of natural frequency. This method aims at localises the delamination before its developing extent leads to structural failure, which ensures the safety and utility of the structure.