The accurate prediction of ductile fracture behaviour plays an important role in structural integrity assessments of critical engineering structures under fully plastic regime, including nuclear reactors and piping systems. Many structural steels and aluminium alloys generally exhibit significant increases in fracture toughness, characterized by the J-integral, over the first few mm of stable crack extension (Δa), often accompanied by large increases in background plastic deformation. Conventional testing programs to measure crack growth resistance (J-Δa) curves employ three-point bend, SEN(B), or compact, CT. However, laboratory testing of fracture specimens to measure resistance curves (J-Δa) consistently reveals a marked effect of absolute specimen size, geometry, relative crack size (a/W ratio) and loading mode (tension vs. bending) on R-curves. These effects observed in R-curves have enormous practical implications in defect assessments and repair decisions of in-service structures under low constraint conditions. Structural components falling into this category include pressurized piping systems with surface flaws that form during fabrication or during in-service operation.
This paper presents the on-going work to develop specimens and test procedures to study geometry effects (e.g., triaxiality effects) in the brittle to ductile transition of carbon-manganese steels, the basic idea being to compare the results obtained on these specimens with the results obtained on CT specimens. A clamped SENT specimen was chosen for this study. Finite element computations have been made to optimize the specimen shape and to develop the η-factor, the shape factor F (to compute K) and the normalized compliance μ. Preliminary tests have been conducted, showing that some adjustments of the test procedure should be made. Tests on new specimens are on-going.