Crack deflection

Often in composite materials both elements are quite brittle, so that individually they both have a low fracture toughness. It might therefore be reasonable to expect that the fracture toughness of the composite material would also be low. However, this is not necessarily the case. With careful microstructural design, composites can achieve notably higher levels of toughness than their individual components.

Central to this microstructural design is the ability of the microstructure to cause cracks to deflect away from what would otherwise be rapid crack propagation paths, leading to catastrophic sudden failure. Interfaces between the different components of the composite material are of particular interest in this regard.

For a crack propagating normal to an interface to deflect along the interface, and not continue to propagate through the bulk of the material, the criterion

\[ \frac{{{{\rm{\Gamma }}_\text{i}}}}{{{{\rm{\Gamma }}_\text{bulk}}}} < \frac{1}{{2\pi }} \]

needs to be satisfied, where \( \Gamma_\text{i} \) is the interfacial strain energy release rate ('fracture resistance') and \( \Gamma_\text{bulk} \) is the bulk strain energy release rate / fracture resistance. Click here to see a full derivation for this criterion for crack deflection.

The following animation shows the two ways a crack could grow through a material.

The design of the composite material will therefore enable cracks propagating perpendicular to the interfaces between the different components, as a consequence of a suitable level of applied stress, to be deflected along the interface between the components.

Incorporating such 'weak' interfaces in composite materials is beneficial in increasing their fracture toughness. It also makes their failure 'graceful', rather than catastrophic. A well-designed fibre composite is seen to have a very obvious fibrous fracture surface when a crack has finally propagated across the composite and the whole fracture surface has been exposed

Crack deflection is used in laminated materials to make them tough. An example of this is laminated SiC with weak graphite interlayers between each SiC lamina. The graphite interlayers are oriented so that the individual sheets of graphite within the interlayers lie preferentially parallel to the laminae, rather than perpendicular to them. As the graphs and diagrams show schematically in the animation below, significantly more energy can be absorbed by the laminated SiC in comparison with a solid block of SiC.

Laminated glasses are very popular to use because of their high toughness and their transparency to visible light. Windscreens of modern cars are made from laminated glass. Small stones thrown up from the road impacting windscreens behave like indenters, leaving behind a relatively small volume of damage on the windscreen, rather than the windscreen shattering into thousands of small pieces, as happens with toughened glass when hit sufficiently hard by a small stone.

It is common to see laminated glass being used in tourist attractions. Examples are staircases in various Apple stores worldwide, the Grand Canyon viewing platform in the U.S.A., the glass walkway on Tianmen Mountain in China, the glass floor at the CN Tower in Toronto, Canada and the revolving glass floor at the Space Needle in Seattle in the U.S.A.

Glass walkway on Tianmen Mountain (Wikimedia Commons)