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Summary: The measured fatigue strength of a material can be affected by specimen size:tests using a large stressed volume may show a low fatigue strength due to the increased probability of finding weak regions. A Weibull analysis revealed an important size effect in bone and predicted this effect with an accuracy of 12%. This approach also explained apparent inconsistencies in the published data and made it possible to separate and quantify the effects of frequency, loading mode, and material source. The effect of frequency is the same for human and bovine bone, and the differences between different types of loading (tension, compression, and bending) are small (maximum: 12%). By extrapolating to the volume of whole bones, it is concluded that large bones will have a fatigue strength much lower, by a factor of 2-3, than that measured by conventional tests. Failure within 105 cycles is expected to occur at cyclic stresses of 23-30 MPa in human long bones and of 32-43 MPa in bovine bones. Repair is therefore needed to prevent failure at physiological stress levels.
Fatigue failures (stress fractures) occur in humans (17) and animals (20). Fatigue damage, in the form of microcracks, has been linked to phenomena such as osteonal remodeling (14) and adaptation to altered stress environments (21). This implies that the damage that occurs during normal, physiological loading is severe enough to cause failure if it is not repaired. Damage caused in vivo has been detected (9,13,19), but recent work (15,22,23) has suggested that the fatigue life of bones at in vivo stress levels is so high that the bones will never fail and therefore do not need to be repaired. Fatigue failures occur due to repetitive, cyclic loading. Tests are usually conducted at constant cyclic amplitude, and it is found that the number of cycles to failure, Nf, is a function of the stress range, As, which is the difference between the maximum and minimum stress in the cycle. The relationship can be expressed as a power law with two constants, A and ct: In some tests the strain range, Delta epsilon, is used instead of Delta sigma, and a similar relationship results. The fatigue strength of the material, Delta sigma^sub o^, is defined as the value of...