Compressive response and failure of braided textile composites: Experiments and analysis.

Abstract

Textile composites have similar mechanical attributes when compared with other fiber reinforced composites, however, because of cost effective manufacturability, they are being considered as a viable alternative for structural applications in the aerospace and automotive industries. This thesis focuses on the compressive response of a 2D flat triaxial braided composite (2DTBC) under conditions that are similar to those encountered when a tubular structural member undergoes axial compressive crush. During crush, the walls of the member are subjected to predominantly biaxial stress state of compression (lengthwise) and tension (widthwise), while, near the end of the tube where the loading is introduced, a combined bending and compression type of biaxial stress state is predominant. Experiments on flat 2DTBCs were carried out under two types of load states: compression/tension (C/T) and bending/compression (B/C). C/T tests were carried out on a special planar biaxial load frame. External loads and full field planar incremental strain fields (the Deltaepsilon_x, Deltaepsilon_y and Deltagamma_xy) were captured during the loading process via digital speckle photography (DSP). Failure mechanisms were investigated and supplemented by post experiment microscopy. Similarly, load and strain data were obtained from the B/C tests, which was based on a novel eccentric <italic>Elastica</italic> experimental configuration. The experimental results provided fundamental insight into the failure mechanisms of 2DTBCs and motivated the development of robust micromechanics based strength models for the 2DTBCs. In addition, the biaxial experimental data provide grounds for the validation of failure theories that have been conceived on measurements based on uniaxial loading. An analytical model based on constituent properties and textile geometry as input was developed to determine the elastic orthotropic stiffness properties of a 2DTBC. A finite element (FE) based micromechanics model of the 2DTBC was developed to predict the compressive <bold>strength</bold> of 2DTBC under multiaxial stress states, based only on constituent properties and geometry as the input. The predictions of these models captured the details of the experimentally observed failure mechanisms and were found to be in good agreement with the experimental data. These FE models, under biaxial loading conditions, provided failure envelopes that compared well against the experimental data.