This study investigated using small precracked round bars (CRBs) for measuring fracture toughness values representative of material fracture resistance in large, thick components (so-called plane strain fracture toughness). The three alloys tested with CRBs (titanium-10V-2Fe-3Al, HY-130 steel, and A508 class 2A steel) cover a broad range of fracture behaviors. K or J values at crack initiation were calculated using the experimental load and displacement records. The fracture tests were complemented with scanning electron microscope observations and finite element analyses of round bars with different ratios of crack depth to bar diameter.
The CRB data were compared with independently measured toughness data obtained with larger bend specimens. In two cases, toughness values measured with precracked round rs for a given material and mode of fracture were in good agreement with those measured with larger standard fracture mechanics specimens; the exception was A508 steel at 204°C, and the lower values in this case may have occurred because the microstructural features influencing fracture were large in relation to the uncracked ligament size of the CRB. Finite element simulations of the CRB indicate that in the fully plastic regime the constraint factor Q is low (-0.75 to -0.85) for shallow crack specimens but increases with deformation (from -0.6 to -0.1) for deeply cracked bars. Experimentally, this difference in onstraint was evidenced by a transition from ductile to cleavage fracture mode in (embrittled) HY-130 steel specimens with shallow and deep cracks, respectively.
This limited feasibility study showed the potential of small CRB specimens for measuring representative plane strain fracture toughness. More work is needed to optimize specimen size and precracking methods and to improve and validate test evaluation methods.