The Effect Of Reducing-yarn Technique On Microstructure And Mechanical Properties Of Three Dimensional Braiding Composites

Nowadays, the three-dimensional integrated-braiding composites are one of the high-performance composites with rapid development. The major advantage of 3-D integrated-braiding composites are that the braided preforms can be any thickness , multiplicate shapes and can not delamination along the thickness direction in the theory . Applying the technique of the 3-D integrated-braiding, the different-shaped preforms of the composites can be braided directly, such as the T-shape cross section composites and the equal - thickness cones with changed cross-section along the braiding direction. But the problem is that the theory is behind practice in practical production. So many tests must be done in order to obtain the exact shape of the preforms, which waste a lot of manpower and the performances of preforms are instability.The paper studied the reason of reducing-yarn> the theory of the 3-D braiding for cross-section changed preforms for the first time. In order to ensure the process of reducing-yarn braiding following the movement rule of four-step braiding ,The following two principles must be obey: the first one is ensuring that the process of the braiding is sequence; the second one is ensuring that the structure of the preforms is changed uniformly. Then a concrete technique of the reducing-yarn presents in the paper. The preforms of a changed cross-section along the braiding direction have been braided by using reducing-yarn technique which obtains a full integrated shape.In the paper, the preforms microstructure of the four-step 3-D integrated-braiding composites was analyzed by the topology- analysis method developed by Wang. This analysis describes yarn structure of braided preforms after reducing-yarn and obtains the movement rule of braiding yarns. In the reducing-yarn procedure, the braiding yarns is carried by the carriers and tends to change the movement along the original path. After reducing-yarn and several braiding circles, the braiding yarns return to original path. On the basis of microstructure, the change of the fiber volume fraction of the 3-D integrated-braiding composites present in this paper. The relations between braiding process and structure parameters of the braiding and reducing-yarn techniques areestablished.At last, the mechanical properties of 3-D braiding composites are studied in this paper. The study dealing with two aspects: the strength and tensile modulus. The result shows that the strength has changed little, and the effect is mainly on the tensile modulus after reducing yarn.The conclusions of this paper can make a theory foundation and can be applied for designing the braiding process of the cross-section changed 3-D braiding performs and promoting the development of the 3-D braiding process and its application.