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Figure 4. Toughening by crack bridging in dentin. (a) An optical micrograph showing the development of crack bridging in dentin dehydrated using whisky. Microcracks ("daughter cracks"), initiated primarily at tubules, form ahead of the main crack ("mother crack"); their inability to link perfectly with the main crack leads to regions of uncracked material which spans the crack. Such "uncracked ligaments" carry load that would otherwise be used to drive the crack, and thereby act to toughen the dentin. (b) A three-dimensional x-ray tomography reconstruction of a section of the crack in (a), showing the three-dimensional nature of the uncracked-ligament bridging. The nominal direction of crack growth is indicated in both cases.
