Prediction of the uniaxial tensile curve of 4-step 3-dimensional braided preform

Prediction of the uniaxial tensile curve and tensile strength of 3-dimensioanl braided preforms is the first step in predicting the uniaxial tensile strength of 3-dimensional braided composite. This paper presents an analytical model to predict the uniaxial tensile strength and uniaxial tensile curve within the whole tensile strain range. First, the equations of spatial configuration of trace yarn in each yarn group were obtained from the coordinates of center point of the trace yarn cross-section in preform and the microstructure of 3-dimensional braided preform was described in mathematical way. Based on the mathematical description of trace yarn and microstructure of preforms, the uniaxial tensile strain relations between 3-dimensional braided preform and braiding yarn at small and large strain were established. Then, from the energy conservation law that the sum of strain energy of all braiding yarns equals to the work of external force applied to the preform, the uniaxial tensile load at every uniaxial tensile strain points of braiding yarn can be calculated and then the whole uniaxial tensile curve of 3-dimensional braided preform can be obtained (also for uniaxial tensile strength). The maximum strain failure criterion is used in calculations. From experimental verification, the method proposed in this paper predicting the uniaxial tensile load and tensile strain simultaneously from the uniaxial tensile curve of braiding yarn is valid and simple in application.