The assessment of crack initiation and/or propagation has been the
subject of many past discussions on fracture mechanics. Depending on how
the chosen failure criterion is combined with the solution of a
particular theory of continuum mechanics, the outcome could vary over a
wide range. Mod- elling of the material damage process could be elusive
if the scale level of observation is left undefined. The specification
of physical dimension alone is not sufficient because time and
temperature also play an intimate role. It is only when the latter two
variables are fixed that failure predictions can be simplified. The
sudden fracture of material with a pre-existing crack is a case in
point. Barring changes in the local temperature, * the energy released
to create a unit surface area of an existing crack can be obtained by
considering the change in elastic energy of the system before and after
crack extension. Such a quantity has been referred to as the critical
energy release rate, G e, or stress intensity factor, K Ie. Other
parameters, such as the crack opening displacement (COD),
path-independent J-integral, etc., have been proposed; their relation to
the fracture process is also based on the energy release concept. These
one-parameter approaches, however, are unable simultaneously to account
for the failure process of crack initiation, propagation and onset of
rapid fracture. A review on the use of G, K I, COD, J, etc., has been
made by Sih [1,2].