CREEP CONTRIBUTES TO THE FATIGUE BEHAVIOR OF BOVINE TRABECULAR BONE

Repetitive, low-intensity loading from normal daily activities can gen erate fatigue damage in trabecular bone, a potential cause of spontane ous fractures of the hip and spine. Finite element models of trabecula r bone (Guo et al., 1994) suggest that both creep and slow crack growt h contribute to fatigue failure. In an effort to characterize these da mage mechanisms experimentally, we conducted fatigue and creep tests o n 85 waisted specimens of trabecular bone obtained from 76 bovine prox imal tibiae. All applied stresses were normalized by the previously me asured specimen modulus. Fatigue tests were conducted as room temperat ure; creep tests were conducted at 4, 15, 25, 37, 45, and 53 degrees C in a custom-designed apparatus. The fatigue behavior was characterize d by decreasing modulus and increasing hysteresis prior to failure. Th e hysteresis loops progressively displaced along the strain axis, indi cating that creep was also involved in the fatigue process. The creep behavior was characterized by the three classical stages of decreasing , constant, and increasing creep rates. Strong and highly significant power-law relationships were found between cycles-to-failure, time-to- failure, steady-state creep rate, and the applied loads. Creep analyse s of the fatigue hysteresis loops also generated strong and highly sig nificant power law relationships for time-to-failure and steady-state creep rate. Lastly, the products of creep rate and time-to-failure wer e constant for both the fatigue and creep tests and were equal to the measured failure strains, suggesting that creep plays a fundamental ro le in the fatigue behavior of trabecular bone. Additional analysis of the fatigue strain data suggests that creep and slow crack growth are not separate processes that dominate at high and low loads, respective ly, but are present throughout all stages of fatigue.