Volume 5, Issue 9 (October 2008)
Test Results from Round Robin on Precracking and CTOD Testing of Welds
A weld fracture toughness round robin test program was initiated by ASTM to evaluate a proposed annex to E1290-93 on fracture toughness testing of ferritic steel weldments. The focus of the round robin was on specimen preparation procedures and their effect on crack tip opening displacement (CTOD). The local compression procedure was used to reduce the influence of residual stress on precrack curvature. Specimens were tested at −196 °C, which is on the lower shelf for this metal, to minimize variability in fracture toughness and maximize influence of residual stress. Seven labs participated in the round robin and among them conducted 51 tests per E1290-93 to measure CTOD. Statistical analysis of the data showed that crack curvature decreased by 5.2 % ±2.1 % for each 1 % increase in local compression, over the range of compressions tested (0–2.9 % with a nominal target value of 1 %). However, mean values of curvature varied significantly between labs, and when results from all labs are considered, local compression of about 1 % did not reliably reduce crack front curvature. It appears that the procedure used is more important than the amount of local compression, as evidenced by the straight cracks obtained by two labs that exercised tight control on local compression. The statistical analysis revealed that the mean value of measured CTOD for all data with less than 10 % curvature was not statistically different than the mean for less than 20 %, as it is in BS7448 (1997). Perhaps the most surprising observation from this round robin is that there does not appear to be a correlation between CTOD measured at −196 °C and amount of local compression or crack front curvature.