54 Brittle Bones

materials have a small and they begin to fail toward or almost immediately after being stressed beyond their limit. Bone, cast iron, ceramic, and concrete are examples of brittle materials. Materials that have relatively large plastic regions under tensile are known as . Examples of ductile materials include aluminum and copper. The following figure shows how brittle and ductile materials change shape under stress. Even the cartilage that makes up tendons and ligaments is relatively brittle because it  behaves less like example (c) and more like examples (a) and (b). Luckily, those tissues have adapted to allow the deformation required for typical movement without the brittle nature of the materiel coming into play. We will learn about that adaptation in the next chapter.

A drawing of rod of unknown material is shown with force arrows indicating the rod is under tension. Three drawings of outcomes of stretching the rod are shown. (a) the rod fractures without noticeable deformation. (b) The rod fractures after stretching and thinning. (c) The rod fractures after significant stretching and thinning.
Profile (a) is an example of the material that fractures with no plastic deformation, i.e., it is a brittle material. Profile (b) is an example of a material that fractures after very little plastic deformation. These two profiles would be classified as having low ductility. Profile (c) in contrast is a material that plastically deforms before fracture. This material has high ductility. Image Credit: Sigmund (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Materials that are very can undergo significant plastic deformation under stress, as apposed to stress. Very malleable materials can be pounded into thin sheets. Gold is the most malleable metal.[1]


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