Fiber buckling shear mode

Figure 3-55 Extensional Mode and Shear Mode of Fiber Buckling...

Dow and Rosen s results are plotted in another form, composite material strain at buckling versus fiber-volume fraction, in Figure 3-62. These results are Equation (3.137) for two values of the ratio of fiber Young s moduius to matrix shear modulus (Ef/Gm) at a matrix Poisson s ratio of. 25. As in the previous form of Dow and Rosen s results, the shear mode governs the composite material behavior for a wide range of fiber-volume fractions. Moreover, note that a factor of 2 change in the ratio Ef/G causes a factor of 2 change in the maximum composite material compressive strain. Thus, the importance of the matrix shear modulus reduction due to inelastic deformation is quite evident. 

Figure 20.26 Variation of longitudinal compressive strength of carbon fiber epoxy resin laminate with temperature showing transition from shear mode to buckling mode failure. Failure depends on the shear modulus of the matrix and shear strength of the fibers and a similar effect is observed with the uptake of water. Source Reprinted with permission from Ewins PD, Potter RT, Phil Trans R Soc London, A294, 507-517, 1980. Copyright 1980, The Royal Society of Chemistry.

In composites, Drzal and Madhukar (1993) observed that the failure mode depended on the level of fiber/matrix adhesion at low levels, the mechanism was global delamination buckling at intermediate levels, fiber microbuckling at high levels fiber compressive (shear) failure. This is illustrated in Fig. 27. 

For the shear buckling mode in Figure 3-55, the fiber displacements are equal and in phase with one another. The matrix material is alternately sheared in one direction and then the other as the x-direction is traversed. However, changes in deformation in the y-direction are ignored. Thus, the shear strains are presumed to be a function of the fiber-direction coordinate alone. The matrix is sheared according to... 

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