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The paper focuses on the fracture of a SENB specimen in a Drop Weight Tear Test. Deep fracture profile occurring during stable propagation is investigated.
Typically in the models that correlate the instrumented test results to CTOA values, the position of the fracture development is assumed straight, i.e. independent to specimen thickness. With this assumption in mind, the residual ligament computation is apparently easy. On the contrary, a remarkable fracture tunneling is present: it looks like different pipeline materials present a different attitude to crack tunneling.
In order to know the real shape of the fracture, silicone casts of DWTT fractured specimens were made when stable propagation is achieved. The casts are analyzed through tomographycal approach. Through the image processing of DWT tests, it is possible to automatically compute the angle of opening at the superficial crack tip during propagation as well as the overall kinematics during loading.
The attention is finally centered on two different steels used in inshore gas piping, X60 and X100, detailing the comparison on the effective stress field acting nearby the crack tip and the energy really involved for the fracture propagation.
The loads are applied making use of inclined constraints that allows to change the stress field ahead the crack tip, during the tests. As illustrated within the paper, the highest slopes allow to realize a persistent state of stress ahead the crack tip. This diffusive state of stress has an influence on X100 fracture behavior much more evident than that observed on X60.