PR-306-143732-R02 Lode Parameter Effect on Critical Strain in Mechanical Damage Defects

In this report, an advanced failure initiation criterion based on plastic deformation is presented and its use is discussed. The approach is based on the actual stress state and the plastic strain development within the defect region, as these two quantities have been demonstrated to drive the failure of ductile metallic materials. Two steel grades were selected to benchmark the suggested criterion and were fully characterized throughout a comprehensive laboratory testing campaign; where specific specimen geometries were designed so that a given stress state could be achieved in terms of stress triaxiality and Lode parameter. The latter has emerged in more recent studies as having a significant effect, in addition to stress triaxiality. Recent experiments showed that the Lode parameter, which distinguishes between axisymmetric and shear dominated stress states, has an effect on material ductility, especially at low stress triaxiality.

The effect of the Lode parameter on plastic strain to failure was experimentally observed in our case studies. It confirms the importance of accurately evaluating stress and strain fields in the pipe wall deformed by mechanical damage, but also the need of reliable material data base to enable engineers to conduct conservative engineering critical assessments as predictive as possible.

In addition, the introduced criterion has been numerically implemented in a FE code and tuned so that simulations are in good agreement with experiments. The criterion is so called "un-coupled model" since the plastic behavior is not affected by development of ductile damage during the calculation. It is used as a post-processing script which evaluates a damage indicator for each calculated increment, and failure initiation is expected when the criterion reaches unity.

Damage model was investigated on pipe specimens only, and its application has not been extended to actual defects on real pipelines in the scope of this study. It could be in a subsequent project.