Modelling of ductile crack propagation by numerical methods is generally based on the node release technique. This technique allows a crack extension over a length equal to the mesh size. Crack extension occurs when energetic, damage or geometric conditions occur at the crack tip. The criteria generally used for that are: a dissipative energy with the cohesive zone model (CZM), a critical damage with the Gurson-Tvergaard-Needleman model, critical damage given by SRDD model or a critical crack opening angle (CTOAc). Results are generally sensitive to mesh size.
For the simulation of crack extension in a ductile material it is necessary to know the fracture resistance of the material. Several approaches are used, some of which are global and others are local. The impact fracture energy measured by impact tests or obtained by static tests as the critical J energy parameter is considered as a global parameters that is constant during crack extension. The yield locus after damage or cohesive zone energy is a local criterion of this type. They are also considered as constant with crack extension. Use of the crack growth resistance curve (R-curve) and particularly the δR -Δa curve slope as the CTOA is another approach. In this approach, CTOA can be assumed to be non-constant with crack extension.
Initially and particularly in standard methods for determining crack extension in pipes such as the BTCM, HLP, and HLP-Sumitomo, fracture resistance is described as the specific fracture energy Rf obtained either from the Charpy-V energy or modified Charpy energy, or from DWTT energy obtained by using either standard or embrittled specimens. A new two curves method based on the critical CTOAc values is now available.