Size effects on the ductile/brittle fracture properties of the pressure vessel steel 20g

Detailed un-standard experiments of fracture toughness in which SENB specimens of five different thicknesses were included were carried out to investigate the size effect on the ductile and brittle fracture. It is found that the ductile fracture toughness parameter-the critical strain energy release rate increases with the size of the specimens which have the similar geometry shape and then decreases gradually to the plane strain fracture toughness. On the upper shelf the critical strain energy release rate increases with the size in the range of 4–16 mm for the increment of the plastic deformation zone size and plastic fracture strain under general yielding conditions, and then drops down from 16 mm to 22 mm for the plastic deformation zone size not changing much which is less than the residual ligament width and the increase of the proportional of the high stress triaxiality zone to the whole specimen. While on the lower shelf the critical strain energy release rate increases with the thickness in the range of 4–8 mm for the plastic deformation zone size increasing under small scale yielding conditions, and then drops down from 8 mm to 22 mm for the increase of the high out-of-plane constraint. Theoretical analysis with the primary definition of the strain energy release rate, the constraint level and the plastic deformation volume was performed to investigate the different size effects for different temperatures. FEA simulations with continuum damage model of GTN models to get the results of the stress triaxiality as an important factor of fracture toughness increasing with the reduction of the thickness.