On the origin of the toughness of mineralized tissue: microcracking or crack bridging?

Two major mechanisms that could potentially be responsible for toughening in mineralized tissues, such as bone and dentin, have been identified-microcracking and crack bridging. While evidence has been reported for both mechanisms, there has been no consensus thus far on which mechanism plays the dominant role in toughening these materials. In the present study, we seek to present definitive experimental evidence supporting crack bridging, rather than microcracking, as the most significant mechanism of toughening in cortical bone and dentin. In vitro fracture toughness experiments were conducted to measure the variation of the fracture resistance with crack extension [resistance-curve (R-curve) behavior] for both materials with special attention paid to changes in the sample compliance. Because these two toughening mechanisms induce opposite effects on the sample compliance, such experiments allow for the definitive determination of the dominant toughening mechanism, which in the present study was found to be crack bridging for microstructurally large crack sizes. The results of this work are of relevance from the perspective of developing a micromechanistic framework for understanding fracture behavior of mineralized tissue and in predicting failure in vivo.