ABSTRACT
It is probably not necessary to go back to the Viking ships and the American
horse buggy to realize that flexibility and amount of resilience are the essential
elements of structural quality. Without these features the structure is simply
not able to absorb the energy of external forces. Of course, the structure can be
grossly overloaded and then all bets are easily off, especially when a flaw or a
crack is present. The material breaks, usually in tension, as the crack spreads
across the direction of loading and the stored-up strain energy is potentially avail-
able to propagate this crack. Such a statement, of course, suggests that we are
dealing with the self-destructive mechanism of the material because of the pre-
sence of strain energy in a resilient structure. The only question remains is
what portion of the stored energy is directly converted into fracture energy.
And it is not surprising that modern fracture mechanics is more concerned
with the conversion of energy into fracture than with the traditional forces and
stresses acting on the structure. This situation, however, points clearly to the
complexity of the fracture control process, where any oversimplification of this
problem may be dangerous. As the conventional design principles, developed
over many years of practical experience, the history of fracture mechanics tea-
ches that any fracture control options depend heavily on the engineering judg-
ment in combining the three main branches of technical knowledge involving
