Finite Element Analysis to Investigate the Effect of Loading Modes on the CTOA of DWTT Specimens

Abstract

To replace the traditional Charpy-test-based fracture arrest methodology, the crack tip opening angle (CTOA) has been proposed as a fracture propagation parameter for prediction of fast ductile fracture propagation and arrest of axial cracks in pipelines. Transferability of CTOA from the DWTT specimens determined according to ASTM E3039 to other specimens and especially pipes is critical to the applications of CTOA. As part of the research effort to investigate the transferability of crack tip opening angle (CTOA) from the drop weight tear test (DWTT) specimens to pipelines, this work focuses on the effect of loading modes (bending vs. tension) of DWTT specimens on CTOA based on finite element analysis. The damage-mechanics model (Modified Mohr-Coulomb Model) and element deletion technique to simulate crack initiation and propagation is found to be accurate and agrees well with test data. This model accounted for variations in stress triaxiality and Lode angle at the crack front. For the DWTT model under bending, the surface CTOA is considerably larger than the interior CTOA consistent with the experimental observations. However, for the DWTT specimen under tension, the CTOA on surface and interior are almost the same. The interior CTOA is found to be very similar in all cases. The CTOA distribution as a function of distance behind the crack tip is found to be very consistent up to one half-thickness behind the crack tip, where CTOA starts to increase gradually. For both the bending and tension DWTT simulations, steady-state crack growth was achieved at a reasonably low crack extension. These results support the transferability of CTOA from standard DWTT tests according to ASTM E3039 to other loading cases.