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 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 preliminary program was previously conducted at room temperature using deeply notched specimens (Le Delliou, 2012). Finite element computations were made to optimize the specimen shape and to develop the η-factor, the shape factor F (to compute K) and the normalized compliance μ.
For the present program, new specimens have been machined with shallower notches (a/W = 0.4), to get a0/W = 0.5 after fatigue pre cracking. Fatigue pre cracking was conducted in 4-point bending to avoid damaging the back of the notch. Moreover, the specimens have been cut in the TS (Transverse-Short) direction of the plate to get lower toughness properties, and less plasticity during the tests.
Tests at room temperature have been conducted first to validate the revised test procedure. Then, the SENT specimens have been tested at −100°C, −60°C, and −40°C, together with CT specimens.