Study On Microstructure, Mean Stiffness And Compression Properties Of 3-D Braided Composites

This paper describe research concerning the microstructure ,mean stiffness and compression properties of three -dimentionally braided composite materials, the braided process and yarn path in 4-step 3-D(lxl) rectangular braided composites are analysed, based on 3-D solid yarn and unit cell geometric model, microstructure inside 3-D braided preforms is analysed and some geometric models are set up. The paper also presents an analytic description of interior, surface and corner unit of 3-D braided composites and establishes several mathematic models.Models of braided performs and composites have been established with local-cut method. They can easily describe the complexity of braided structure and show the direction of yarns in braided performs and composites, and can accurately calculate the length of yarns. A problem that yarns between inner cell , surface cell and coner cell cannot be connected effectly has been solved in this paper.The final yarn structures in composites have changed comparing with those in the preforms after which are impregnated with matrix and consolidated into the final shape. This paper proposed that each different combination of row m and column n of main body yarns can scheme two type of section plot of perform and the section of perform can be designed . The expressions of fiber volume fraction are deduced. An optimization design proposal is give to the process of braided composites and structure parameters.When all yarns of preform in jamming condition, The biggest yarn volume fraction of right face model is equal to that of inner cell of 45° model, but not equal to that of surface cell and coner cell. when we employed the biggest braiding angel r=54.7° ,The biggest yarn volume fraction of inner cell ,surface cell and corner cell of 1x1 braided performs in 45°model are follows: Vymax = 67.96% , Vsymax= 58.23% and Vcymax = 37.31%. compactness of Vyaainner , surface and the corner area are decrease grudaully. This paper proposed a concept of changed braiding angle in the process from braided preforms to final composites .meanwhile the changed braiding angle has been used in model established and the process of calculation of mean stiffness. Calculations of dimensions of representative unit cell have been given when performs changed to composite materials.Application of the model established to predict mean stiffness of 3-D braided compoites was conducted in this paper, including four direction and five direction braided composites.The experimental results show that the RVE mechanical model can reasonably predict the effective elastic constants of 3-D braided composite materials and the results show close agreement with theoretical predictions. We see yarn in braided composites as rod of inclined structures and use the structural model of three cell to predict monolithic stiffness of 3-D braided composites with method of volume average and method of weighted average of three cell's stiffness. In addition, the mechanical experiments of braided composite materials were carried out, such as tensile, compression and experiments. The braiding angle and fiber volume fraction have notable effect on module of elasticity.In this paper we proposed a hypothesis of difference fiber volume fraction and presented expressions respectively. The fiber volume fraction of unit cell is not equal to that of each lamina ,which need calculate eachly.The paper also advanced conception of weighted average of three cell's stiffness. The results of calculation agree with my experimental data and other documents.Based upon compression failure experiments of braided composites, some mitigating areas can be seen from the response curve of stress and stain. The phenomenon indicates that it is the failure of fiber bundles(lamina) bring on. three typically macroscopic failure modes of compression of 3-D braided composites have found, they are wholly unfasten failure, 45 degree breaking section failure and drum-like breaking failure. Through the compression experiments of braided...