In out introduction to fracture, we have reviewed the definition of fracture energy $\Gamma$. Recall the definition of the energy release rate:

\[G = - \frac{d\Pi}{da} = \frac{d(F \Delta - U) }{da}\]

Lets use this to determine the fracture energy of peeling a tape.

We condsider peeling at 90 degrees. In this case if we peel the tape some length $\Delta$, the crack will also grow the same length $\Delta$, thus $\Delta = a$. Note that the strain energy is zero ($U = 0 $) because we assume the tape does not stretch. Our equation for the energy release rate becomes:

\[G = \frac{d(Fa)}{da}\]

Thus at crack growth:

\[G_c = F\]