Experimental and Theoretical Investigation of Size Effects on the Ductile/Brittle Fracture Toughness of a Pressure Steel

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 fracture toughness on the upper shelf increases with the size of the specimens, which have the similar geometry and then decreases gradually to the plane strain fracture toughness. The ductile fracture toughness 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—22 mm for the increase of the high-stress triaxiality zone and the plastic deformation zone size not changing much which is less than the residual ligament width. While the fracture toughness of the lower shelf 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 to 22 mm for the increase of the high out-of-plane constraint. Theoretical analysis with the primary definition of the fracture toughness J integral, the constraint level and the plastic deformation volume was performed to investigate the different size effects for different temperatures. Finite Element Analysis simulations with continuum damage model to get the distribution and variety of the stress triaxiality as an important factor of fracture strain and fracture toughness.