Fatigue damage-fracture mechanics interaction in cortical bone

Fatigue loading causes accumulation of damage that may lead to the initiation of a macrocrack and result in a catastrophic failure of bone. The objective of this study was to examine the influence of fatigue damage on crack growth parameters in bovine cortical bone. Nineteen rectangular beam specimens (4 X 4 X 48 mm) were machined from bovine tibiae. The long axis of the beams was aligned with the long axis of bones. Using a four-point bending fatigue setup, ten specimens were fatigue-damaged to different levels as indicated by stiffness loss. A through-thickness notch was machined at the center of each damaged and undamaged beam. The notched specimens were then monotonically loaded beyond failure using a three-point bending protocol. Critical stress intensity factor, K-I and work to critical load, W-Q, were significantly lower in the damaged group than in the undamaged group (p<0.03). When the undamaged specimens were assigned a percent stiffness loss of zero and pooled with the damaged group, significant negative correlations of percent stiffness loss with K-I (R=0.58, p<0.01), W-Q (R=0.54,p<0.02), maximum load, P-max (R=0.59,p<0.008), deflection at maximum load, Delta(max) (R=0.48, p<0.04), structural stiffness, S-max (R=0.53, p<0.02), W-max (R=0.55, p<0.02), and load at 1.4 mm deflection (a value beyond failure but without complete fracture), P-1,P-4 (R=0.47, p<0.05), were found. Post hoc analysis revealed that the average load-deflection curve from the damaged group was transformable into that from the undamaged group through a special shift on the load-deflection plane. Fatigue damage reduces bone stiffness and resistance to crack initiation, maximum load-carrying capacity, and deflection before and after failure in cortical bone. The data suggest there is a single rule that governs the overall effect of fatigue damage on the fracture behavior of cortical bone. (Bone 30:509-514; 2002) (C) 2002 by Elsevier Science Inc. All rights reserved.