Working Content> MacroModels > Solids  

If an object is pulled too much, the object will break.  We expect that this will occur in a homogeneous material in the same scaling way that the object deforms -- in terms of the scaled variables stress and strain. This works pretty well, though in realistic objects that are not perfectly uniform, defects (such as small cracks,  irregularities, or inclusions of other materials) can strongly modify the perfectly uniform results we describe here.

In a stress – strain curve for uniform solids, the approach to a break first shows up as a non linear region beyond that where Young’s modulus prevails.  The stress can either level off as the object rapidly stretches. This produces a flattened curve -- a rapid increase of the extension (strain) with a very small increase in force (stress).  At some point, the object will simply break, and there, the stress strain curve stops.

Breaking strain, as a result of stress, is usually a bad thing for a biological organism.  Typically you do not want your leg to buckle as your run, nor does a monkey want its arm to break as it hangs in a tree.  Therefore, such biological elements are typically made to withstand a reasonable amount of force.  However, there are instances when an organism does want something to break, as when a dandelion wants to shed its seeds in the wind.

Karen Carelton and Joe Redish 10/20/11