Similitude and the Paris crack growth law

Whilst fracture mechanics based concepts have been found to be a good tool for predicting the growth of large cracks, many researchers have questioned the validity of the similitude hypothesis in Region I. This paper examines some of the data that were first used to relate crack growth (da/dN) to the stress intensity factor (K) and reveals that even in this case the similitude hypothesis appears to be invalid in Region I, where the crack growth was low. This lack of similitude is then further illustrated by considering crack growth in Ti-6AL-4V.