Fiber pull-out toughness

Fig. 6.2. Variation of fiber pull-out toughness, f po, as a function of discontinuous fibers of length, f. After...

Mai (1985) has also given a review of the fracture mechanisms in cementitious fiber composites. The total fracture toughness, / i, is given by the sum of the work dissipation due to fiber pull-out, fiber and matrix fraetures, fiber-matrix interfacial debonding and stress redistribution, i.e.. 

Fig. 7.5. (a) Transverse impact fracture toughness and (b) fiber pull-out length versus testing temperature for carbon fiber-epoxy matrix composites with and without PVAL coatings on fibers. After Kim and... 

In general, the use of FE signals accompanying the deformation and fracture of composites offer elucidation of failure mechanisms and details of the sequence of events leading upto catastrophic failure. The extent of interfacial failure and fiber pull-out are also potential parameters that can be determined. FE can assist in the interpretation of AE and also provide an independent probe of the micro-events occurring prior to failure. FE has been shown to be sensitive to the locus of fracture and efforts are underway to relate emission intensity to fracture mechanics parameters such as fracture toughness (Gjp). Considerable work still remains to fully utilize FE to study the early stages or fracture and failure modes in composites. 

The second step is to deposit a thin layer of a suitable interface material onto the surface of each filament in the preform. The purpose of this layer is twofold to provide a barrier to protect the fiber from the CVI atmosphere and to prevent strong bonding between the fiber and the matrix so that high toughness is achieved be debonding, bridging, and fiber pull-out (cf. Section 5.4.2.7). 

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