Research On Material Performance Of 3-D Braided Composites Based On Co-braided Optical Fiber Sensors

The 3-dimensional multi-directional braided composites have been considered as the third-generation strengthen composites for their outstanding performance and are now being widely used in many fields, such as aeronautics & astronautics. Optical fiber sensors co-braided into the 3-D braided composite structures can be used to detect internal structural parameters of the structure, and are helpful to promote correlated researches on braided composites' health monitoring, manufacturing process and stiffness or strength prediction. The relationship among micro unit cell, composing properties, meso-braided structures and optical fiber sensing technology used in the testing of the braided composites are studied in this thesis are firstly established for the optical fiber co-braided in the structure using two different methods. The unit cell models for the optical fiber sensors co-braided into the structures by two ways are firstly constructed, the contacting track between optical fiber and yarns and the geometric relationship in the unit cell are discussed subsequently, fiber volume fraction formula of each constituent is given, so does some corresponding conclusions. Based on the above geometric analysis, stiffness properties of the two unit cell models are developed by the means of correct inclined laminate theory, Young's modulus and Poisson ratio are calculated, A Michelson polarized sensing experiment system is established to validate the efficacy of above analysis. The experimental results are corresponding to the prediction value. Loading transferring and assignment analyses in the unit cell model are always important to the structural performance parameters testing. In this paper, theoretical research is conducted on the Boolean operation of the internal cell model and the constructions of 3-D Green's functions of heterogeneous medium; The constituent condition and boundary condition are also presented; Heterogeneous components equilibrium equations based on variation principle are proposed. Geometrical modeling is constructed by using FEM structural analysis software; Effective constituent condition and boundary condition of unit cell model equivalent to experimental condition are also discussed. Fiber Bragg grating sensors have the virtue of good sensitivity and are applied to point detecting and quasi-distributed testing. In this paper fiber Bragg grating sensing system for tension test to determine the internal strain are implemented, comparison are given between the theoretical and stimulation results under the effective boundary conditions. Tensile strength experiments and failure analysis are carried out for 3-D braided composites by co-braided optical Bragg gratings. Mechanical properties are investigated from the macroscopic view, so does the failure mechanism of braided structures from the microscopic view. SEM analyses of the specimens fracture on the basis of tensile experiments are conducted. All these results provide an experimental basis for further studies on the strength prediction and failure criterion's setup by the means of co-braided optical fiber sensors.