In Chapter 4, we examined the mechanical properties of bone when it is fractured monotonically by a load sufficient to exceed the failure stress of the material. Structures may also fail more gradually. There are two principal ways in which this can happen: creep and fatigue. In both, a stress less than the ultimate stress is applied, and damage from this stress grows and accumulates until failure occurs. In creep, the stress is applied continuously; in fatigue, the stress is applied cyclically. Obviously, these phenomena may occur simultaneously in the components of an engineering structure like a bridge, where the weight of the structure provides creep loading and the periodic passage of vehicles produces fatigue. Similarly, in the skeleton, bones often support more or less constant loads for prolonged periods of time (such as the vertebral bodies in your spine as you sit reading this book), and equally often carry cyclic loads (such as when you walk to class). In this chapter we shall see that creep and fatigue are closely related and explore the ways in which bone limits and repairs the damage that they produce. We also discuss what seems to happen when the limits of this capacity for damage control are exceeded and the bone fails by what is called a stress fracture.
KeywordsFatigue Life Cortical Bone Stress Fracture Fatigue Damage Strain Energy Release Rate
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