Quasi-static tensile behavior and damage of carbon/epoxy composite reinforced with 3D non-crimp orthogonal woven fabric

Abstract

This paper presents a comprehensive experimental study and detailed mechanistic interpretations of the tensile behavior of one representative 3D non-crimp orthogonal woven (3DNCOW) carbon/epoxy composite. The composite is tested under uniaxial in-plane tensile loading in the warp, fill and +/- 45 degrees bias directions. An "S-shape" nonlinearity observed in the stress-strain curves is explained by the concurrent contributions of inherent carbon fiber stiffening ("non-Hookean behavior"), fiber straightening, and gradual damage accumulation. Several approaches to the determination of a single-value Young's modulus from a significantly nonlinear stress-strain curve are discussed and the best approach recommended. Also, issues related to the experimental determination of effective Poisson's ratios for this class of composites are discussed, and their possible resolution suggested. The observed experimental values of the warp- and fill-directional tensile strengths are much higher than those typically obtained for 3D interlock weave carbon/epoxy composites while the nonlinear material behavior observed for the +/- 45 degrees-directional tensile loading is in a qualitative agreement with the earlier results for other textile composites. Results of the damage initiation and progression, monitoried by means of acoustic emission, full-field strain optical measurements, X-rays and optical microscopy, are illustrated and discussed in detail. The damage modes at different stages of the increasing tensile loading are analyzed, and the principal progressive damage mechanisms identified, including the characteristic crack patterns developed at each damage stage. It is concluded that significant damage initiation of the present material occurs in the same strain range as in traditional cross-ply laminates, while respective strain range for other previously studied carbon/epoxy textile composites is significantly lower. Overall the revealed advantages in stiffness, strength and progressive damage behavior of the studied composite are mainly attributed to the absence of crimp and only minimal fiber waviness in the reinforcing 3DNCOW preform.