YOUNGS MODULI AND SHEAR MODULI IN CORTICAL BONE

Young's modulus and shear modulus are determined for cortical bone fro m mammals and birds and for antler bone, using three-point bending at a range of span-to-depth ratios between 25 and 10. Young's modulus is obtained by extrapolating the values for the flexural modulus E(app) t o infinite span-to-depth ratios. The shear modulus is obtained from th e dependance of E(app) on this ratio. The main determinant for the mec hanical properties is the mineral content. For mammalian bone the freq uency of Haversian systems correlates negatively with stiffness and re sistance to shear. However, because Haversian systems have a lower min eral content than laminar bone (the main component), material and stru ctural determinants can not be separated at present. The ratio of Youn g's modulus to shear modulus is of the order of 20:1. This high value is discussed in terms of the Cook-Gordon theory of controlled crack pr opagation as well as in its significance for protecting hollow bones f rom failing upon local impact.