  It is the purpose of this short paper to show that the well known Griffiths
theory of brittle fracture can be re-interpreted as a simple case of a modified
Hamiltonian, and corresponds to a stationary action solution using the
Euler-Lagrange equations. In this derivation it is still necessary to employ
the equivalence with Irwins stress intensity factor approach to obtain the
Griffith-Irwin equation. It is then shown that if physics of fracture is
modelled using a simplified crack tip stress wave unloading model, then the
Griffith-Irwin relationship can be derived in a single process, which is a
major simplification of the procedure. Also the simplified crack tip stress
wave unloading model employed gives a much clearer view of the basic physics of
fracture.
  Keywords: Fracture, Griffiths, Hamiltonians, stress waves.
