| Current fracture toughness testing is conducted with planar specimens, such as the compact tension C(T) specimen and the single edge bend SE(B) specimen. The crack front in a planar specimen has a constraint condition which varies through the thickness from plane stress on the lateral surfaces to plane strain in the middle of the specimen. This change in constraint makes the geometry of planar specimens three-dimensional; however, two-dimensional analyses based on the assumption of plane stress or plane strain are often performed.; An alternative specimen geometry is the notched round bar (NRB). This axisymmetric geometry has the advantage of constant constraint in the circumferential direction. Additionally, the NRB geometry can be modeled using an axisymmetric formulation--essentially a two-dimensional analysis.; The suitability of the NRB geometry for fracture toughness testing was studied. Notched round bars with finite notch root radii p were tested without fatigue precracking using methodologies similar to those described in ASTM test methods E399, E813, and E1152, and in a draft ASTM test method for transition fracture toughness testing of ferritic steels. Four different materials were tested: 2024T351 aluminum alloy, overaged 2024, a modified A302B steel, and nylon 6/6. Apparent fracture toughness {dollar}Ksb{lcub}Jc{rcub}{dollar} as a function of {dollar}sqrt{lcub}rho{rcub}{dollar} was a linear relation for 2024-T351 and modified A302B. Apparent initiation fracture toughness {dollar}Jsb{lcub}Q{rcub},{dollar} determined from NRB J-R curves, as a function of {dollar}rho{dollar} was a linear relation for overaged 2024 and nylon 6/6. Sharp-crack fracture toughness was determined by extrapolated apparent fracture toughness values to zero {dollar}rho .{dollar} The NRB results compared favorably to the results obtained from precracked C(T) and SE(B) specimens.; In addition to using an innovative approach for determining sharp-crack fracture toughness, two other innovative approaches were used. First, the load separation method was used to determine {dollar}etasb{lcub}pl{rcub}{dollar} for calculating {dollar}Jsb{lcub}pl{rcub}{dollar} from load-displacement test records and inferring crack length without direct measurement. Second, a yield function which included hydrostatic stress effects on yielding in the nonlinear finite element analyses was used. |
