Towards a Process for the Assessment of Mixed Mode I+II Fracture Toughness

In fracture mechanics, a flaw behavior in pressure vessels is assessed with respect to the material fracture toughness. Fracture toughness which most Fitness-for-Service (FFS) codes relies on, only considers mode-I crack opening. However, in presence of tilted flaws, like quasi-laminar hydrogen flakes, this mode-I toughness may be too severe, and a mixed mode I+II fracture toughness seems to be more appropriate. In order to address the assessment of the fracture toughness curve, mixed mode I+II tests were performed by the authors on ferritic steel samples by adjusting the standard mode I CT specimen geometry to a geometry subjected to mixed mode I+II. Then, XFEM simulations of the mixed mode tests were performed in order to calculate the J-integral along the crack front. Based on tests and calculations results, the paper explains how the authors work towards proposing a method to measure the material fracture toughness in case of flaws subjected to mixed mode (I+II) loading.