Fracture mechanics testing [KIc, J-integral, and crack-tip opening displacement (CTOD)] of a QT-steel (AISI 4137 H) has been carried out in order to compare the testing methods. The tests were performed at room temperature, and the material toughness was varied by applying annealing temperatures from 300 to 650°C.
The J-initiation values were successfully determined with a specially developed d-c potential-drop (PD) method. The technique is based upon pulsed current and reference electrodes. Only the potential-drop difference caused by a current difference is measured; this eliminates the thermo-electric effect and offset voltage associated with PD equipment.
On the basis of the PD measurements both the J-initiation value and the R-curve (J-Δa) could be determined from a single-specimen test.
The R-curves could be fitted to a third-order polynomials, but even if the initiation values were nearly constant for replicate specimens, the slope of the R-curve would vary significantly. The mechanisms of crack initiation and growth are different, and the observed results indicate that the crack growth properties, as characterized with the R-curve, of the present steel quality vary from one specimen to the next.
The J-initiation values determined with the multispecimen technique recommended in ASTM E 813 showed considerably more scatter in comparison with the present PD technique. The results suggest that the J-initiation value should be determined on each specimen and not calculated with extrapolation techniques.
Author Information
Thaulow, C
The Norwegian Institute of Technology, Trondheim, Norway
Hauge, M
The Foundation for Scientific and Industrial Research at The Norwegian Institute of Technology, Trondheim, Norway
Gunleiksrud, Å
The Foundation for Scientific and Industrial Research at The Norwegian Institute of Technology, Trondheim, Norway
Paauw, AJ
The Foundation for Scientific and Industrial Research at The Norwegian Institute of Technology, Trondheim, Norway
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