Evolution of Phase Strains During Tensile Loading of Bovine Cortical Bone

Synchrotron X-ray scattering is used to measure average strains in the two main nanoscale phases of cortical bone - hydroxyapatite (HAP) platelets and collagen fibrils - under tensile loading at body temperature (37 degrees C) and under completely hydrated conditions. Dog-bone shaped specimens from bovine femoral cortical bone were prepared from three anatomical quadrants: anterio-medial, anterio-lateral, and posterio-lateral. The apparent HAP and fibrillar elastic moduli - ratios of tensile stress as applied externally and phase strains as measured by diffraction - exhibit significant correlations with the (i) femur quadrant from which the samples are obtained, (ii) properties obtained at the micro-scale using micro-computed tomography, i. e., microstructure, porosity and attenuation coefficient, and (iii) properties at the macro-scale using thermo-gravimetry and tensile testing, i. e., volume fraction and Young's modulus. Comparison of these tensile apparent moduli with compressive apparent moduli (previously published for samples from the same animal and tested under the same temperature and irradiation conditions) indicates that collagen deforms plastically to a greater extent in tension. Greater strains in the collagen fibril and concomitant greater load transfer to the HAP result in apparent moduli that are significantly lower in tension than in compression for both phases. However, tensile and compressive Young's moduli measured macroscopically are not significantly different during uniaxial testing.