Resistance to Crack Growth in Human Bone is Greater in Shear Than in Tension

It has been proposed that longitudinal shear stresses create bone microdamage, which suggests that bone is weak in shear and may not be adapted to prevent crack growth under shear loading. However, based on the similarities between bone and other fiber-reinforced composites that are tough, i.e. resistant to crack growth, we hypothesized that resistance of human bone to crack growth under shear loading is greater than under tensile loading. Because bone from older individuals and women has demonstrated increased propensity to fracture, we also hypothesized that bone from these individuals has less resistance to crack growth under shear and tension loading. Using compact shear and compact tension specimens, the critical strain energy release rate (Gc) of human bone was measured for longitudinally oriented cracks under tension (mode I) and shear (mode II) loading for male and female cadavers ranging from 55 to 89 y. Average tensile fracture toughness (GIc) of male and female human bone was 339 N m−1 (S.D. = ± 132). Average shear fracture toughness (GIIc) of human bone over the same range was 4200 N m−1 (S.D. = ± 2516 N m−1). Shear toughness was greater than tensile toughness (approximately 13 times), which is consistent with other fibrous composite materials. Fracture toughness decreased with age, but the fits were weak and significant for shear loading only. Tension and shear toughness did not depend on gender. We concluded that the resistance to crack propagation under shear loading is greater than under tensile loading, a finding which suggests that bone adapts to prevent crack growth in shear. We also found that bone toughness is equivalent in men and women and that bone toughness gradually decreases with age between 55 and 89 y