The principle of similitude analysed from plastic zones estimates ahead crack tips

The similitude principle is the foundation for the use of Linear Elastic Fracture Mechanic (LEFM) because it permits to relate design nominal stress, σn, with experimental fracture toughness data. This relation is done assuming that if the laboratory specimen is under the same Stress Intensity Factor (KI) of real structure, both will have the same fracture behaviour and both will have plastic zones (pz) equals from crack tip. However, this work shows that similar pieces with the same KI have different pz, leading to the conclusion that the principle of similitude based on KI is not completely accurate. This is also observed even under low load level where the disturbance generated in the stress field by the presence of the crack is located very close at its tip. The numerical pz estimates are done by Finite Element Method and two analytical stress fields are analysed, one is obtained from KI and another obtained from KI plus T-stress. In a different way that is advocated, this work shows that T-stress addition does not solve either pz estimates or the Principle of Similitude based on KI.