Fracture Toughness Improvement in a Carbon Steel Due to Normalization

Quantification of the fracture toughness improvement in a carbon steel due to normalization was investigated using A516-70 (normalized) and A36 (as-rolled) specimens tested at −40 and −46°C, respectively. The A516-70 was in its Charpy transition range and the A36 was on its Charpy lower shelf. This precluded fracture toughness comparison based on ductile tearing. Plasticity considerations excluded the plane strain fracture toughness idealization, and variations in specimen thickness disallowed use of the crack-opening-displacement method. A previously developed three-dimensional finite element (3DFE) estimate of the plastic stress singularity strength at maximum load was used to index fracture toughness which was found insensitive to specimen thickness for those thicknesses examined. The fracture toughness of the normalized steel was 2¼ times that of the as-rolled steel. The 3DFE analysis of the plastic stationary crack indicated, for the tougher A516-70, crack profile blunting and curvature reversal. Through-thickness FE mesh refinement revealed a local maximum value of plastic stress singularity strength removed from the plate midthickness, which confirmed observations made by P. S. Theocaris using the method of caustics. The fracture toughness of A516-70 was predicted reasonably well by a Charpy transition toughness correlation. As expected, the transition regime fracture toughness of the A36 steel was not predictable from its Charpy lower shelf energy.