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