Strain rate dependence of work of fracture tests on bone and similar tissues: Reflections on testing methods and mineral content effects.

This paper is concerned with the effect of different strain rate on the Work of Fracture (Wf) of various vertebrate mineralised tissues, controlling for the effect of mineral content and Young's modulus of elasticity. Using specimens of uniform shape and size values for the Work of Fracture of specimens tested at various deformation rates, and also the energy absorbed by notched specimens in impact, are reported. The results indicated that, of those tested, for most bone specimens the Work of Fracture measurements were constant like in the case for a 'material property'. Variations due to loading conditions (deformation rate) were small, with the exemption of antler, which is relatively poorly mineralised and in which the Work of Fracture values increased by a factor of 4 across the range from quasistatic loading to impact. The Tattersall and Tappin (1966) test has shown itself to offer some great advantages: if the quest is for a fracture toughness test for an unknown tissue it offers reliability, it is perhaps more forgiving to handling errors, it also suffers less of the influence of strain rate effects and uses relatively simple instrumentation. It is also able to demonstrate the remarkable toughness of antler bone which other more commonly used fracture toughness methods cannot do.

The effects of ageing and changes in mineral content in degrading the toughness of human femora.

Static toughness and impact strength of human cortical bone decline with age. This decline is strongly associated with an increase in the ash content of the bones but other factors, associated with age, are probably also responsible for the degradation of the bones' mechanical properties.

Effect of formaldehyde fixation on some mechanical properties of bovine bone.

The risk of infection of investigators working on the biomechanics of human bone from a variety of modern pathogens including the human immunodeficiency virus or the hepatitis B virus has increased recently. New safety procedures are needed to reduce that risk. The procedure we follow in our laboratory employs brief (< 3 h) fixation in formaldehyde, and we report here the effects it has on some mechanical properties of bovine bone. Results in quasistatic loading tests were almost unaffected by our fixation protocol, but a significant decrease in impact strength was found. These results indicate that there may be some interaction between fixation and strain rate dependent effects and, therefore, some caution is needed when using common biomechanical measurement methods on fixed bone material.

Dependence of mechanical properties on fibre angle in narwhal tusk, a highly oriented biological composite.

The successful modelling of the mechanical properties of mineralized tissues depends critically on the knowledge of the off-axis behaviour of individual unidirectional lamellae. Information on this is lacking. In this work we attempt to rectify the situation. Young's modulus, measured in bending and tension, and the tensile strength and ultimate strain to failure of the dentine of the narwhal Monodon monoceros, were determined on specimens that had almost unidirectional fibres, whose direction differed considerably from specimen to specimen. Modulus and strength decreased steadily with the degree of off-angle loading, falling to about 45% of maximum for modulus, and 35% of maximum for strength. Ultimate strain showed a less uniform behaviour, and remained remarkably high at large angles. Differences in mechanical behaviour were not related to the very small differences in mineral content measured between specimens. These findings have strong implications for modelling the anisotropic behaviour of bone, because dentine is very much like bone in most important respects. Predictions using classical composite theory are reasonably satisfactory, as long as the mineral crystals are assumed to be platelets, not rods.

The mechanical properties of the dentine and cement of the tusk of the narwhal Monodon monoceros compared with those of other mineralized tissues.

Values for Young's modulus of elasticity, ultimate and yield stresses, ultimate and yield strains, work under the stress-strain curve and work of fracture were obtained from tensile and bending tests on specimens of narwhal tusk dentine and cement, femoral bone from young and mature cattle, and reindeer antler. Compared with the cattle bone the narwhal tissues had low Young's moduli, low yield stresses, rather low ultimate stresses and high ultimate strains. In all these properties they were similar to reindeer antler. The calcium content and hardness of the narwhal tissues were compared with those of human and cattle dental tissues. The narwhal dentine was considerably softer and less mineralized than human and cattle dentine. Human cementum was softer and less mineralized than cattle cementum, and was like narwhal cementum. In general, the mechanical properties of the narwhal tusk tissues were as would be expected from their mineral content, except that the stiffness of the cementum was low. It is likely that narwhal dentine is not very similar to human and cattle dentine in its mechanical properties.