Meso- and micro-stress analyses in an 8HS graphite/polyimide woven composite subjected to biaxial in-plane loads at room temperature

The stress distributions in an eight harness satin (8HS) graphite/PMR-15 (polyimide) composite subjected to a variety of inplane simple and biaxial loading conditions at room temperature were determined in this study. The stresses were calculated numerically on the micro- and meso-scales using the finite element method. By considering visco-elastic properties of the polyimide resin the residual thermal stresses from manufacturing were also determined in the stress analysis. It has been shown that there is a critical external in-plane loading condition for which the micro- and meso-stresses in the composite are the highest. Under in-plane biaxial loads consisting of tension along the warp and fill tows in addition to in-plane shear there exists the highest combination of transverse and shear meso-stresses in the tows. The micro-stresses in the matrix between the fibers are also the highest for this biaxial combination of in-plane loads. Therefore, the combination of in-plane biaxial tension and shear is the critical loading condition for the initiation of transverse cracks in the tows. The numerical methodology developed in this study for the evaluation of visco-elastic meso- and micro-stresses in high temperature woven graphite/polymer composites should help aerospace designers optimize in-service loading conditions to prevent fracture in the tows and to increase the strength of aerospace structures utilizing such advanced composite systems.