Volume effects are a fundamental determinant of structural failure. A material exhibits a volume effect if its failure properties are dependent on the specimen volume. Many brittle ceramics exhibit volume effects due the presence of “critical” flaws. The number of flaws, their locations, and the nature of the stress field within the stressed volume play a role in determining the structure’s failure properties. Structures composed of large volumes of material have higher probabilities of containing a flaw of critical size than do small volumes. Consequently, large material volumes tend to fail at lower stresses compared to smaller volumes when tested under similar conditions. Effects similar to those documented in brittle ceramic and composite structures have been proposed to affect the mechanical properties of bone. We hypothesized that for cortical bone material, (1) small volumes have greater yield strengths and longer fatigue lives than large volumes, and (2) this volume effect explains variability not accounted for by such microstructural variables as porosity, osteon diameter, and density.

In this investigation, waisted, rectangular, equine third metacarpal diaphyseal cortical bone tissue specimens with nominal cross-sections of 12 mm and gage lengths of either 10.5, 21, or 42 mm, were tested monotonically (n=24) and cyclically (n=18) in uniaxial tension to determine the specimen volume effect on yield strength and fatigue life, respectively. Osteon diameter, osteon density, and bone porosity were quantified and served as the microstructural variables.

Yield strength and fatigue life were negatively correlated with volume for log transformed variables, supporting the hypothesis that small stressed volumes of cortical bone material possess greater yield strengths and longer fatigue lives than similarly tested large stressed volumes. The hypothesis that a volume effect contributes to fatigue life and yield strength variability in addition to microstructural effects was supported by multiple regression analyses of volume and microstructural data with mechanical properties. Variation in specimen volume was associated with comparable amounts of variability in fatigue life and yield strength when compared to variations in microstructural features. The present results demonstrate that specimen volume and microstructural effects independently affect fatigue life and yield strength of equine cortical bone tissue.