Model of a macrocrack in a composite
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The dependence of the crack resistance K* of a composite on the volume fiber content Vf may both be monotonically decreasing (x=26 in Fig. 5a) and have a maximum at a certain volume fiber fraction (x=10, x=8 in Fig. 5a). Within the framework of our model, it is possible to have an indefinitely increasing K* at sufficiently small x — this corresponds to a material insensitive to a crack of a certain size. Qualitatively, this behavior of the model is consistent with experiment: in  K* for boroaluminum (x small) increases with increase in Vf, in  the K* for a tungsten-copper composite (x large) falls with increase in Vf.
As the crack length increases, K* tends to a constant value which can be taken to be characteristic of a given structure (see Fig. 5b). However, the necessary crack length may be too great for the performing of the corresponding experiment.
The fracture toughness of the matrix has a very important influence on crack resistance (see Fig. 5c). If we regard the composite structure as amplifying the crack resistance of the matrix, then the amplification is nonlinear.
The crack resistance of the composite depends importantly on the fiber characteristics on a length of the order of the critical length (mean distance between fiber defects failing in a given test).
The fact that the crack resistance of the composite structure depends on a large number of factors both opens up broad possibilities of controlling the crack resistance of composites and, at the same time, leads to considerable scatter of the experimental data.
KeywordsExperimental Data Fracture Toughness Crack Length Composite Structure Important Influence
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