Fracture Orientations
• Transverse (L-C & L-R)– Cuts through osteons
• Longitudinal (R-L & C-L)– Splits osteons along
longitudinal axis
• Radial (C-R and R-C)– Splits osteons radially
Fracture Orientation
Fracture Toughness by Orientation
• Transverse cracking has been found consistently toughest orientation
• Radial cracking is found to be the least tough.
• Bone location is also thought to effect fracture toughness
Mode II Fracture Toughness• Transverse Mode II
fracture resistance found significantly stronger than longitudinal.
Mode III Fracture Toughness
• Transverse Mode III fracture resistance found stronger than longitudinal.
Angle Oriented Fracture Toughness
• A compact tension test study by Behiri shows mode I fracture toughness dependence on orientation angle with respect to bone axis.
• This study also showed stable crack propagation for 30° test and catostrophic failure for 90° specimen
Effect of bone microstructure
• The cement line is found to provide a weak path for fracture.
• Osteons are much stronger than the cement line, which is why transverse cracking has highest K value
• Excessive repair and remodelling of osteons leads to lower fracture toughness
• Increased mineral content lowers toughness• Mechanical properties of collagen effect fracture
resistance• Wet and dry density effects fracture resistance
Fracture Mechanisms
• Intrinsic mechanisms: microstructural damage mechanisms that operate ahead of the crack tip and act to increase resistance to crack initiation
• Extrinsic mechanisms: Shield the crack from the applied driving force
• Rising R-curve behavior is the direct result of extrinsic toughening mechanisms
Extrinsic Toughening Mechanisms
Uncracked Ligament Bridging
• Intact bridges of material span across the crack wake and sustain part of the applied load
• Most recent studies have indicated that crack bridging is the primary mechanism responsible for rising R-curve behavior
Uncracked Ligament Bridging: Transverse (radial) Orientation
Uncracked Ligament Bridging: Longitudinal Orientation
Uncracked Ligament Bridging
• The magnitude of the contribution of uncracked ligament bridging to crack growth resistance is determined by:– Size of the bridging zone – Area fraction of the bridges in the zone– Their load-bearing capacity
• Steady-state toughness may be reached when bridges spanning the crack wake are created and destroyed at the same rate.
Uncracked Ligament Bridging: Effects of Aging
• Test specimens from the aged (right) show much smaller uncracked-ligament crack bridges than a specimen from a young (left) donor.
Uncracked Ligament Bridging: Effects of Aging
• Studies have also shown that there is a significant reduction in density of bridges in aged (bottom) vs. young specimens (top)
Collagen Fiber Bridging
• Typically associated with resisting propogation of microcracks.
• Collagen fiber bridges are on a much smaller scale than uncracked ligament bridges
Fatigue of Cortical Bone
• Blunting and resharpening of the crack tip has been proposed as a mechanism for fatigue failure
Fatigue of Cortical Bone
• On a shorter timescale (a), evidence of uncracked-ligament bridging is shown
• On a longer timescale (b), evidence of time-dependent crack blunting is suggested by the larger crack opening in the lower panel
Fatigue of Cortical Bone
• Add picture from Nalla – aspects of in vitro fatigue
Aging
• Crack initiation toughness and crack growth toughness of bone decreases with age