Stable and Unstable Crack Growth in Viscoelastic Media

The problem of failure of load-bearing structures by fracture is generally important in all phases of our society. It may concern small household items as well as expensive structures of civil or space applications and accordingly may cause varying degrees of economic distress. While the state of failure is usually easily determined as either “not failed” or “completely failed,” the estimation of how close to either state a structure is, poses a much more difficult problem. It is important to recognize, however, that from an engineering point of view, the latter problem is the important one because it would allow, in principle, the prediction of the conditions leading to fracture and thus to a close estimate of the service life of a structure. Inasmuch as failures by fracture involve the growth of cracks it appears that keeping track of the size of a crack in a particular structure provides a means of assessing quantitativelythe strength prior to complete failure. If one agrees that the description of structural strength is rationalized in terms of the size of defects, it follows that one must attempt to understand the laws that govern the growth of such defects in order to predict complete failure. Fracture of materials is a complicated process which encompasses atom&tic aspects, as well as microscopic and large-scale continuum mechanical considerations. Although one of these aspects should not be considered without the other we shall be concerned in the following primarily with the continuum-mechanical formulation of the problem of fracture growth in viscoelastic materials. From this