As the crack gets longer however these conditions are reversed and beyond the ‘critical Griffith length’ l g the crack is producing more energy than it is consuming, so it may start to run away in an explosive manner. proportional to the square of the crack length…On the other side of the account book is the surface energy…needed to form the new surfaces and clearly increases as only the first power of the depth of the crack…When the crack is shallow it is consuming more energy as surface energy than it is releasing as relaxed strain energy and therefore conditions are unfavorable for it to propagate. It is quantified by the work of fracture-the energy needed to produce a new surface by propagation of a crack through the material-a concept introduced by Alan Griffith during his research on fracture mechanics.Ī strained material contains strain energy which would like to be released just as a raised weight contains potential energy and would like to fall…The relief of strain energy …. Toughness is opposite to brittleness, and is related to but not identical to ductility. The worst sin in an engineering material is not lack of strength or lack of stiffness, desirable as these properties are, but lack of toughness, that is to say, lack of resistance to the propagation of cracks. Just two figures, however, are not sufficient to characterize a material, especially when it's used to build a structure. The two properties together describe a solid about as well as you can reasonably expect two figures to do. Gordon summarizes these ideas succinctly.Ī biscuit is stiff but weak, steel is stiff and strong, nylon is flexible and strong, raspberry jelly is flexible and weak. Strength measures how much stress a material can withstand before failing. Stiffness describes how much a material lengthens when pulled (that is, strains when stressed), and is quantified by its Young’s modulus.

In Chapter 1 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss two mechanical properties of a material: stiffness and strength. Recently, I reread Structures and read for the first time its sequel The New Science of Strong Materials: Or Why You Don’t Fall Through the Floor. It showed me to how engineers think about mechanics.

When I was in graduate school, I read a fascinating book by James Gordon titled Structures: Or Why Things Don’t Fall Down. Structures: Or Why Things Don't Fall Down,

Structures or why things don t fall