Composite materials are good in tension because the fibers are stable until fracture in this configuration. Their performance in compression is less good as the fibers are now loaded in compression and can become unstable to buckling before the onset of plastic deformation. This behavior is illustrated in the diagram below.

Sample (a) has a low density of fibers and the lowest energy for the distortion puts the buckling of the fibers out of phase. The 'high n' mode shown, rather than the single curve of the n = 1 mode for a free column comes about because the elastic energy of the matrix is lower. This case is not of commercial interest because V_F is too low.

For sample (b), the fibers buckle in phase, again with a high n buckling mode. The shear modulus of the matrix, G_M , is important in this case, and the stress that causes buckling is given by: s_C = G_M ( 1 - V_F) .

From: Hull, "An Introduction to Composite Materials,"
Cambridge (1992)