Journal of COMPOSITE MATERIALS, Vol. 43, No. 25/2009 DOI: 10.1177/0021998309345329 Micromechanical Strength Modeling and Investigation of Stitch Density Effects on 3D-Orthogonal Composites Ryan L. Karkkainen * and Jerome T. Tzeng Weapons and Materials Research Directorate U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005 ABSTRACT A three-dimensional orthogonal woven S2-glass composite is investigated using finite element micromechanics to characterize the stiffness and strength. The methods are applied to a targeted parametric investigation of the effects of stitch density on strength properties and potential benefits of through-thickness reinforcement, such as resistance to transverse shear and delamination, with some consequent loss of in-plane properties. Direct modeling of the exact microstructure from SEM visualization allows for precise knowledge of the mechanics and failure modes of the microstructure under various loading conditions. Modeling results are verified by comparison to experimental data. In-plane stiffness and strength are predicted with 90% or better accuracy. Transverse shear stiffness was less well predicted, but strength was still predicted within 86% accuracy. KEY WORDS: Textile Reinforcement, Micromechanics, Finite Element Method INTRODUCTION Three-dimensional (3D) textile composites offer key benefits to structural robustness for applications involving impact, multi-dimensional loading, or thick sections with relatively large through-thickness or delamination stresses. Such is also the case for any two-dimensional (2D) woven or braided composite structure, which inherently provide reinforcement in multiple directions, and may include some out-of-plane reinforcement by nature of the undulation of interwoven fiber tows. However, the three-dimensional orthogonal weave incorporates fiber tows directly in the through-thickness direction which is of critical importance to delamination * Author to whom correspondence should be addressed. E-mail: ryan.karkkainen@arl.army.mil