Determination of conditional plies characteristics for braided CFRP structures by identification method

Structure features of braided materials make unobvious possibilities of using traditional multilayered material models, or so-called layer-by-layer analysis models (LLAM), successfully used for composites manufactured by winding or laying. The objective of the article is to demonstrate applicability of using LLAM to predict the elasticity constants for the composites manufactured by braiding. The conclusion is based on comparing the results of calculations and experiments. Experimental values of elasticity characteristics (elastic moduli and Poisson’s ratios) of five different carbon fiber reinforced plastic (CFRP) structures under study were determined during tension tests of strip samples and ring specimens cut-out from thin-walled CFRP braided shells with an internal diameter of 270 mm. Deformations in the tests were measured using strain gauges, elasticity characteristics were determined from linear approximation of initial parts of stress-strain curves. Structural elements with the same direction of fibers were combined into conditional unidirectional plies (CUP). Experimental values of elasticity characteristics of all structures were used as input data for solving the problem of identifying the CUP characteristics. Relationship between the CUP characteristics and the characteristics of complex structures was set in accordance with the traditional LLAM. Two types of CUP were considered in braided structures: a) the yarns were laid along the rectilinear shell generator, b) the yarns were laid along the helix line at specified angles to the rectilinear generator. It was supposed that mentioned CUPs had different elasticity characteristics despite of the same types of fibers and binder. The solution of identification problems enabled one to determine elasticity characteristics of two types of CUP which give minimal possible differences between predicted and experimental values of the elasticity characteristics for all examined structures. These differences did not exceed the experimental data scatter. Such result is proof that traditional LLAM can be used for calculating and predicting elasticity characteristics of braided structures.