Thermoelastic Properties of 3-D Braided Composites: Experiment and Predictions

Thermoelastic constants are obtained both analytically and experimentally for a class of 3-D braided composites. In the experiment, preforms of square and rectangular cross-sections are braided by the 4-step 1x1 method using an E-glass yarn and a high modulus graphite yarn. The preforms are then consolidated by a resin transfer molding (RTM) procedure with an epoxy resin. The consolidated composite bars of various cross-sectional sizes are cut and prepared for testing under a 4-point-bend loading condition for their elastic bending properties, and their effective thermal expansion coefficients in the temperature range of 25°C to 100°C. In the analysis, a brief introduction is given for the geometric description of the yarn structures in 3-D preforms braided by the 4-step 1x1 method. Based on the yam structure, this class of preforms is shown to be composed of an interior core region and a surface layer region; in each region, a distinctive yarn structure exists. The effective thermoelastic constants in each region are extracted using a simple 3-D micromechanics model based on their respective yarn structures; and these constants are then used to forecast the various properties in conjunction with the 4-point-bending and the thermal expansion tests. A comparison between the tested and the forecasted results is made.