Fracture toughness from load variation

Fracture toughness data (room temperature measurements) for various continuous fiber composites under different loading modes arc presented in Table 4. The data from different sources are reasonably consistent for a given material system with the same fiber content. Small differences may result from the variations in the degree of cross-linking for thermosets and the degree of crystallinity for the thermoplastic matrices. 

The above discussion considered cracks from a macroscopic viewpoint but even for a pure mode I loaded crack, cracks are often deflected at a micro-structural level. This crack deflection is a result of variations in the fracture toughness within real materials and the influence of localized stress fields. These observations imply that mixed mode effects can play a role even in a failure that appears to be a result of pure mode I loading, i.e., with the crack propagating normal to the maximum stress. Crack deflection on the microstructural scale are discussed further in the next section. 

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