Development of stress corrosion crack initiation test and monitoring techniques (PhD)

Stress corrosion cracking is a major issue in the plant life management of ageing plants.  For optimal life-time management, a reliable method to predict the time to failure of a component has to be developed and validated against experimental data.  Generally, the time-to-failure due to stress corrosion cracking consists of 3 parts: (1) incubation-precursor stage, (2) crack initiation and (3) crack propagation and finally mechanical fracture due to overload of the remaining ligament. In a nuclear power plant two types of materials show high crack growth rates: Ni-based alloys used for dissimilar welds between the stainless steel piping and the carbon steel reactor vessel and highly irradiated stainless steels use as the structural material for some in-core components. The life time of these components are mainly determined by the time to crack initiation. Therefore models and test methods to predict the time to crack initiation and initiation thresholds are of great industrial importance.

The first objective of the work is to develop a suitable test method to investigate crack initiation. This test method should have a sound scientific and engineering base to advance the tests used at the moment (four-point bending, O-ring, etc.).

The second objective of the work is to develop an on-line detection method for early crack initiation such as electrochemical noise detection or local electrochemical sensing techniques like the SRET (Scanning reference electrode technique). This should be used in conjunction with the test methods to reveal more scientific inside in the crack initiation phenomena.

The third objective is to obtain a valid parameterisation of a statistical model for crack initiation, which is based on a constant nucleation rate (with statistical distribution of initiation sites through a simulation of the microstructure) and contains deterministic rules for crack interaction and growth (taken from existing knowledge at SCK-CEN or open literature).  The model is iteratively run until the end of the experiment is reached and the predicted crack size distribution can be compared to the observed one.