Beside the approach discussed above, Rivlin and Thomas introduced a method to determine the fracture energy by measuring an unnotched and a single notched sample. During the measurement, the stress of a notched sample is recorded with progressing stretch ratio until crack propagation occurs marking the critical stretch ratio λ c (Figure ). Next, the work of extension of the unnotched sample is calculated up to λ c so that the fracture energy can be calculated following Γ = W (λ c )h 0 with h 0 , the initial length of the sample (identical to L 0 ). Alternatively, notched and unnotched specimen can be measured and compared in a force-displacement plot (Figure ). Again, a force is applied to displace a notched sample until the onset of crack propagation is observed at the critical displacement length L c . Integrating the area under the extension curve of the unnotched sample up to L c gives rise to the critical elastic energy U(L c ). As a result, equation 6 can be rewritten to