Shear fracture - kink

Some modes may dominate for example, for large bending strains in a flexible structure, fibre fracture in tension and fibre kinking in compression wdl dominate near both surfaces. Matrix cracks can cause delamination when they reach a ply interface. If the structure is stiff enough to resist with a significant force, then local indentation damage, and shear-driven delamination in the interior, wdl occur. Figure 9.2 shows schematically the different modes of fadure in three zones of a laminate. The peanut shape deformations (3) have this shape because the compression under the impact force suppresses the delaminations. 

In compression, the mechanism of failure changes from fibre fracture to fibre buckling. Here, the modulus of the matrix is a critical factor in supporting the fibre and by prevention of kinking (of the fibres) in a shear band. Soutis [53] has discussed the modelling of these types of failure using Eqns (12.17) and (12.18) ... 

Deformation occurs by microbuckling with a shear mechanism, forming kink bands on the fracture surface at 45° to the fiber axis. The onset of buckling is temperature dependent and although Tonen MP fails without buckling at 25°C, it does buckle at 140°C. West [112] found that the compressive strength is approximately proportional to the square of the Tg. 

Fi. 10. (a.b) Polye.sler fibres after a single bend, (c-e) Flex fatigue in polyester kink-band fracture, (f) Flex fatigue shear splitting in nylon, (g) Flex fatigue shear splitting in polyester. For further explanation. 

If a composite with unidirectional fibres is loaded in tension or compression perpendicular to the fibre direction or in axial shear in fibre direction, it can fail without failure of the fibres by fracture, buckling, or kinking. These cases are therefore called matrix-dominated failure. 

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