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Study on Dynamics and Stability of Scarf-repaired Composite Laminates

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Tutor: YuYin
School: Shanghai Jiaotong University
Course: Aircraft design
Keywords: composite laminate,scarf repair,dynamics,stability,finite element analysis
CLC: V229.7
Type: Master's thesis
Year:  2012
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Abstract:
Composite laminates have been widely applied in the aircraft structures. Since damages of laminates occurred during the whole service life is inevitable, repairs must be executed in time when the damage meet the limit of repair tolerance for the sake of structural safety and economical efficiency. The scarf repair has turned to be one of the fundamental repairing methods of laminates with the characteristics of wide-field of application, efficiency, good repairing effect and small impact on aerodynamic shape.The majority of present relevant researches focus on the static strength of scarf-repaired laminates, while less attention is paid to its stability and dynamics characteristics. However, the influences of scarf repair on stability and dynamics of composite laminates must be paid highly attentions as well. Take the large number of application of thin-walled composite structures in airplanes for example, the scarf repair should be designed not to cause premature buckling of the repaired structures. For those laminates whose dynamics should be concerned in working circumstances, coupling resonance with surrounding structures caused by repairs must be avoided. For the same consideration, the major national S&T program on large civil aircraft being implemented at present also demands urgent researches on the impacts of scarf repair on stability and dynamics of composite laminates. Based on engineering background above, the effects of different scarf-repairing parameters on the stability and dynamics of laminates are analyzed by means of finite element method in this paper.Firstly, a digital design method for scarf-repairing plan of laminates is proposed based on engineering reality. The application of this method to the repair of typical delamination of skin demonstrates that the method can fulfill digital design of scarf repair quickly and effectively.Then, three-dimensional finite element analysis models with different repair parameters are built up via ABAQUS to investigate the effects of scarf angle , the aperture D and over-laminating plies on the stability of repaired laminates subjected to four kinds of typical loading. Studies show that the stability of repaired laminates is insensitive to the change of D but is more sensitive to the change of under uniaxial compression. Decreasing D or increasing can both strengthen the stability of repaired structure under biaxial compression while the change of brings more notable effect.When the repaired laminates is subjected to in-plane shear loading, the increase of D results in higher stability with a constant value of . It is also found that there is a particular value of which brings in the weakest repaired pattern when D is constant. This value is5degrees in this paper and should be avoided for those panels who are mainly exposed to shear loading. The over-laminating plies have an important influence on the critical buckling loads and modes and can strengthen stability of repaired laminates dramatically when loaded with shear stress.As far as the compression-shear-combined load is concerned, there is also a small value of which results in the lowest critical buckling load when D is constant. Increasing D may weaken the stability under some particular load cases when is fixed. The stability of all repaired panels under four kinds of loading above can gain a outstanding reinforcement with the presence of over-laminating plies regardless of load cases.More research is made to examine the influences of D, , over-laminating plies and material property of adhesive layer on natural frequencies and steady-state harmonic response of repaired structures. The results indicate that the structural fundamental frequency is raised significantly by repair and the structural resonance frequency of harmonic response is near the first natural frequency. Scarf repair also lower the amplitude of resonance dramatically and the reduced magnitude of amplitude increases with the increase of ? . Structures with larger D and over-laminating plies have lower natural frequencies when scarf angle is fixed. When scarf angle is smaller than 10 degrees, small increase of ? could elevate natural frequencies significantly. Both of the over-laminating plies and material property of adhesive layer have little effect on structural natural frequencies. What¡¯s more, the impact of over-laminating plies on the amplitude of resonance depends on the stiffness effect and quality effect caused by the presence of over-laminating plies. When ? is constant, the quality is the dominant factor for repairing patterns with smaller D and larger resonance amplitude could be gained. Otherwise, the stiffness is the dominant factor and the presence of over-laminating plies lowers the amplitude.
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