A study of the mechanical behavior of a 2-D carbon-carbon composite

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Virginia Polytechnic Institute and State University

The objective of this study was to observe and characterize the out-of-plane fracture of a 2-D carbon-carbon composite and to gain an understanding of the factors influencing the stress distribution in such a laminate. The experimental portion of this study consisted of performing an out-of-plane tensile test in a scanning electron microscope and determining the modes of failure. Failure was found to be interlaminar, with cracks propagating along the fiber-matrix interface.

Finite element analyses of a two-ply carbon-carbon composite under in-plane, out-of-plane, and thermal loading were performed. Stress distributions were studied as a function of stacking sequence, undulation aspect ratio, and undulation offset ratio. The results indicated that under out-of-plane loading σx and τxz were strongly dependent on the geometric parameters studied, but σz and σy were relatively independent of geometry. Under in-plane loading all components of stress were strong functions of the geometry, and large interlaminar stresses were predicted in regions of undulation. The thermal analysis predicted the presence of large in-plane normal stresses throughout the laminate and large interlaminar stresses in regions of undulation.

An elasticity solution was utilized to analyze an orthotropic fiber in an isotropic matrix under uniform thermal load. The analysis reveals that the stress distributions in the fiber are singular when the radial stiffness Crr is greater than the hoop stiffness C₀₀. Conversely, if Crr < C₀₀ the maximum stress in the composite is finite and occurs at the fiber-matrix interface. In both cases the stress distributions are radically different than those predicted assuming the fiber to be transversely isotropic (Crr = C₀₀).