Although it has been known since the 1960’s that the fracture toughness of metal sheets is higher in the near plane stress regime, sometimes with a peak value at intermediate thickness much larger than the plane strain value, the number of convincing experimental data remains very limited. Furthermore, there is almost no predictive model that has been able to capture the full variation of fracture toughness from thin sheets up to the plane strain regime.

This is surprising as such effects take place in the millimetre range of thicknesses for most steel, aluminium or titanium alloys, which is important for many applications. As a matter of fact, the difficulties are numerous from the testing of microstructurally similar materials for a large range of thicknesses using rigorous fracture mechanics methods, to modelling ductile fracture phenomena that are intrinsically 3D. A major reason for the thickness effect is indeed associated to the dissipation by crack tip necking in the near plane stress regime.

In this talk, recent progress on this subject will be described, putting emphasis on the importance of strain hardening and the need to use advanced non-local micromechanics based models.