Fracture properties of bovine tibial bone

Linear elastic fracture mechanics predicts that the fracture stress of precracked materials is dependent on the length of the initial crack and crack tip radius of curvature, as supported by the Griffith and Inglis equations. In order to determine the applicability of these equations and the effects of other variables, tensile specimens of bovine tibia were produced with edge cracks of known dimensions and tested to fracture. Longitudinal sheet tensile specimens were taken from the midposterior diaphysis of bovine tibiae that had been kept frozen in saline soaked towels. Each specimen had a milled gauge length of 25 mm, 16 mm width and 2 mm thickness. All specimen preparation was performed under a saline drip. An edge crack, centered along the gauge length, was milled in the specimen perpendicular to its long axis. The crack lengths used were 4, 6, 8, 10 and 12 mm. The crack tip was formed with drill bits having nominal diameters of 1 32 , 1 16 , and 3 32 in. All the combinations of crack length and crack tip radius were repeated five times for a total of 75 specimens. The testing order was randomly selected. Each specimen was tested in tension to fracture at a constant deformation rate of 7.5 × 10 −3 mm s −1, on an Instron mechanical testing device, and the fracture stress was measured. A linear load-deflection curve to fracture was exhibited by all of the specimens. The weight percent calcium of each specimen was determined by atomic absorption spectrophotometry. Microradiographs were used to determine the fractional void area and to histologically evaluate each bone sample. The weight percent calcium and the fractional void area were found to not significantly influence the fracture stress. A plexiform structure was present throughout the specimens. A two-way analysis of variance found that the fracture stress was dependent on the crack length ( F = 161.79, p < 0.0001) and not on the crack tip radius of curvature ( F = 1.37, p > 0.26). The relationship between the fracture stress and crack length can be expressed as σ frac= 7.12c −1 2 −54.2 MPa which is similar to the Griffith equation. The surface energy, intrinsic flaw size, and an estimate of the opening mode critical stress intensity factor were found to be 3.98 kJm −2, 1.82 mm and 11.2 ± 2.6 MNm −3 2 , respectively.