Interlaminar fracture surfaces

Fig. 12. Scanning electron micrographs of interlaminar fracture surface for composite laminates in RT-aii and 77K-LN,.

Figure 10. SEM micrographs of the Interlaminar fracture Surfaces of GD-31 composite. The domains resembling the separation of CTBN particles are clearly shown In the micrograph. The marker Indicates 10 pm.

Figure 11. SEM micrograph of the interlaminar fracture surface of GD-31 composite at lower magnification. The cracks are shown to branch from one ply to adjacent plies.

Summary of Mode I and Mode II interlaminar fracture toughness values for unidirectional carbon fiber 828 mPDA epoxy matrix composites with different fiber surface treatments ... 

Figure 12. EE from the interlaminar fracture of graphite/epoxy composites made with (a) untreated, and (b) surface treated fibers (Epon 828/mpda).

The saw-tooth fracture surface can be interpreted as a sequence of interlaminar and intralaminar delaminations oscillating back and forth at regular intervals [4], The amplitud(j of the saw-tooth pattern determined from photographs is consistent with the assumption that it is equal to the thickness of the centre 90°-ply of the cross-ply specimens. For the symmetric lay-up with two 90°-plies at the centre, the wave-length of the saw-tooth pattern is more than doubled (factor around 2.1) compared with the non-symmetric lay-up. The steady-state delamination in the cross-ply specimens is oscillating between two 0°-plies on either side of the centre 90°-ply with a wavelength that seems to depend on ply thickness. 

Fig. 7 shows that the values from the second laboratory tend to be lower than those from the first for both types of cross-ply lay-up, while those for the unidirectional lay-up agree fairly well. The scatter still seen in the R-curves for the cross-ply laminates with a single fracture surface topography (Fig. 7) can probably, at least in part, be attributed to different amounts of fibre-bridging (compare Fig. 1). Another factor is micro-cracking in front of the delamination that may make accurate determination of the delamination length difficult. This would also offer an explanation for the steep rise seen in the R-curves of those specimens for which the delamination does not deviate into the unidirectional plies. This is discussed in detail in [6]. Small (local and short-term) deviations of the delamination into the unidirectional plies not recognised in the visual inspection of the fracture surfaces might also contribute to the scatter by temporarily reducing Gic. Finally, the oscillating interlaminar - intralaminar type of delamination propagation could also account for some of the observed scatter. The analysi > presented in [4] concludes that the intralaminar G is considerably smaller than the...

The crack plane in tension remained generally perpendicular to the direction of the applied load, without any deviation into the interlaminar plies. Oxidation of the composites did not change this general trend. SEM was performed on the tensile fracture surfaces to characterize the nature of the tensile failure. Micrographs of the fracture surfaces of the 5X and 8X as-prepared composites show a relatively rough fracture surface, but with very limited fiber pull-out. This corresponds well with the relatively low tensile failure strains of 0.2% seen in both of these composites. 

M.A. Pinto, VB. Chalivendra, Y.K. Kim, and A.E Lewis, Effect of surface treatment and Z-axis reinforcement on the interlaminar fracture of jute/epoxy laminated composites. Eng. Fract. Mech. 114,104-114 (2013). 

The actual contribution of this microfailure mechanisms to the interlaminar fracture energy of the composites tested under particular conditions are a function of the number of events taking place, the real area of fracture surface formed, and the size (length and width) of the damage zone (DZS) around the main crack. The larger the latter becomes the more side cracks have to be expected, and the more energy is consumed by plastic deformation of the polymer matrix material. 

The ILSS of a u.d. composite may be calculated by two extreme approaches. The most optimistic estimate for ILSS is obtained by neglecting stress concentrations. Consider a triangular array of fibres that is subject to a longitudinal shear stress equal to the interlaminar shear strength in a horizontal plane. On the assumption that at fracture the horizontal resin bridges between the fibres bear the matrix yield stress X and that an effective fraction a of the fibre surface bears the interface strength x, it can be derived that ... 

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