Tensile Property And Failure Mode Of Three-Dimensional Braided Composite

Three-dimensional (3D) braided composites are widely used in the fields of aerospace industry, automobile industry and biomedical engineering due to their excellent mechanical properties, low delimitation tendency, high energy absorbing capability and out fatigue resistance. Prediction approach for their performances has become increasingly important.For3D four-directional braided composites, based on the three unit cells model, a new approach for calculating fibre volume fraction is proposed which is through braiding angle, the number of monofilament and its diameter, the number of columns and rows of yarns, and the thickness and width of the composites and a good agreement with the experimental results was obtained.In experimental, the influences of three dimensional braiding structure parameters (such as braiding angle, fibre volume fraction, the number of axial yarns to the number of braiding yarns ratio,3D four-directional/five-directional, thickness) on tensile stiffness and tensile strength of composites are evaluated, and the failure mode of composites is also studied. The results showed that3D braided composites have good tensile performance and the yarn braiding angle, composites size, fibre volume fraction and the number of axial yarns to the number of braiding yarns ratio are important factor to the improvement of properties. In addition, composites have two kinds of failure modes:one is the crack propagation along the fibre bundles, another is bundle fracture, and the latter is the main failure mode.In theoretical, the influences of the number of columns and rows of yarns on longitudinal tensile modulus of3D braided composites are researched. Studies found that longitudinal tensile modulus decreases with the increase of the number of columns and/or rows and the influences of volume proportions of surface region and corner region on mechanical properties of3D braided composites cannot be ignored when the number of columns and rows are very small.An approach for testing and predicting longitudinal tensile modulus of3D braided composites is proposed in this paper. The main contents include: 1. An approach for predicting longitudinal tensile modulus of3D braided composites which the number of rows of yams in composites to be calculated is larger than2:This approach is based on the number of columns and rows of yarns and longitudinal tensile modulus of three kinds of specimens, and based on the number of columns and rows of the composites to be calculated. These composites have same fibre, resin, yarn packing factor, braiding angle, pitch length and fibre volume fraction with the three kinds of tested composites specimens, but have different number of rows and/or columns of yarns in the preforms.For the three kinds of tested specimens, the length, width and thickness can be set as250mm,25mm and2-10mm respectively. As to the number of rows and columns of yarns in the tested composites specimens, the number of rows of yarns is set as larger than2based on the composites to be calculated and the number of columns of yarns can be set based on the width25mm of specimens.2. An approach for predicting longitudinal tensile modulus of3D braided composites which the number of rows of yams in composites to be calculated is equal to2:This approach is based on the number of columns and rows of yarns and longitudinal tensile modulus of two kinds of specimens, and based on the number of columns and rows of the composites to be calculated. These composites have same fibre, resin, yam packing factor, braiding angle, pitch length and fibre volume fraction with the three kinds of tested composites specimens, but have different number of rows and/or columns of yarns in the preforms.For the two kinds of tested specimens, the length, width and thickness can be set as250mm,25mm and2-10mm respectively. As to the number of rows and columns of yarns in the tested composites specimens, the number of rows of yarns is set as2based on the composites to be calculated and the number of columns of yarns is set based on the width25mm of specimens.