The qualitative picture of the fracture of a body may be well-understood, across disparate scales of length and time, from the distortion of electron clouds, to the jiggling of atoms, to the motion of dislocations, to the extension of the crack, to the drop of the load-carrying capacity of the body. This statement by itself, however, is of limited value: it offers little help to the engineer trying to prevent fracture of a structure. Hypes of multiscale computation aside, no reliable method exists today to predict fracture by computation alone.

The pragmatic approach is to divide the labor between numerical computation and experimental measurement. Some quantities are easier to compute, and others easier to measure. A combination of computation and measurement solves problems economically.

Of course, what is easy changes when circumstances change. As new tools and applications emerge, it behooves us to renegotiate a more economical division of labor. The history of fracture mechanics offers excellent lessons on such divisions and renegotiations.