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Research on the Relationship of Low Velocity Impact Response and Damage of Composite Laminate and St

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Tutor: ZhangXiaoJing
School: Shanghai Jiaotong University
Course: Aircraft design
Keywords: composite,dropping-weight impact,response,damage,stiffener skin
CLC: V229.7
Type: Master's thesis
Year:  2012
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Abstract:
Composite structures are unavoidably subjected to low velocity impact during service and maintenance, and are vulnerable to this kind of threatens. Under low velocity impact, material and structures can be seriously damaged by multiple modes, and thus are considerably dangerous for structural integrity and flying safety. Researches about impact response, impact damage, as well as the relationship between them are necessary and constructive for composite material characterization and structure design.In this paper, low-velocity impact of three layups of composite laminate is tested via dropping weight method. The impact response is recorded. The characteristics and diversity of contact force and energy are analyzed. It is shown that the shape of contact force versus time curve apparently changes while the energy level increases, relative to the damage resistant ability of laminate. Impact energy, absorbed energy and damage dissipated energy are obtained, and their relationships with dent depth and delamination areas are discussed. Peak contact force and maximum contact force are different and clearly defined, that the maximum contact force is only a threshold which is determined by the resistant ability of laminate.Additionally, the impact process is simulated via FEM models. Taking use of Hashin initiation criterion and equalized linear evolution law, the gradual damage propagation is achieved. Cohesive elements layers that obey traction-separation laws are inserted to simulate the delamination. Response curves are obtained and damage is predicted, both of which fit the test results.On basis of standard specimen tests, impacts on the stiffener skins of the same materials are carried out using a portable dropping weight machine. Two typical sites are impacted. Response parameters and damage parameters are achieved and analyzed. Statistics show that the contact force curve will change gradually while the impact energy increases; Dent depth and damage area are also in reasonable relationship with impact energy. The dominated damage modes and spread locations are pointed out. Important parameters are compared between two sites¡¯impacts, and between standard specimen and stiffener skin.FEM models are also established to simulate the stiffener skin impacts. Response curves are obtained and damage is predicted, both of which fit the test results. Furthermore, explanations and proofs are given for the uniformities and minor diversities.
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