## 3 The MibaLeb Project at a Glance
The showcase being considered is a pressurized hot water pipe made of AISI 347 (X6 CrNiNb18-10) as being widely used in German nuclear power plants. In pipes of the primary cooling system of nuclear power plants, the damage mechanisms result primarily from loading due to mechanical and thermal stresses, which can be both static and dynamic. In the operation of nuclear power plants, thermomechanical fatigue damage is of great concern, especially in austenitic materials, which have a low thermal conductivity, that can result in locally different thermal expansions during temperature stratification in the pipelines. If these thermal expansions lead to a local stress buildup across the yield point of the material, local deformation of the component occurs. Thermal stresses on the piping of light water reactors (LWR) range from 250 \({}^{\circ}\)C to 330 \({}^{\circ}\)C [6]. Moreover, the influence of the LWR environment on the fatigue life of components needs to be considered. Effects of sulphur content, dissolved oxygen, and hydrogen, in combination with the effects of strain amplitude, frequency and temperature have been the objective of many studies in recent years [7]. Although a lot of in-service pipes are available much of the information is initially missing. This starts with materials data in general, which has been provided over the past, partially available in huge data bases, but just provided in accordance with the requirements set at the time. As an example, fatigue data have been made available but just to the point of an S-N curve. However, what to do, when the material condition is slightly different from what has been made available in databases, publications or guidelines? What to do when the material considered has degraded due to aging? MibaLeb has shown that a huge number of obstacles turn up, that need to be clarified before a considerable residual life assessment of an aging infrastructure can be made. Ordering a material such as AISI 347 (X6 CrNiNb18-10) off the shelf does not mean, that a proper homogeneous material is to be delivered if its quality is not approved in accordance with nuclear engineering and specification rules. Homogeneity in a material can be far from being guaranteed in a material that has been aged when compared to the material's pristine condition. This inhomogeneity is the result of a variety of degradation mechanisms being related to aging, that need to be recognized and quantified, if possible. If this is to be made possible, then the following steps need to be achieved:  
* Appropriate S-N data need to be obtained through procedures, where not 20 to 30 single fatigue experiments are required;
* Degradation mechanisms characterizing a material's aging, cannot be limited to cracks only, which can be inspected by visual means, but need to be extended to further mechanisms such as dislocations, slip bands, martensitic transformation and possibly others, even occurring under environmental conditions such as humidity, temperature and more, provided such mechanisms can be monitored by means of NDT;
* Reliable numerical modeling needs to be made available, that allows all of an infrastructure's and the materials' information to be assembled, organized and interrelated, such that it can be made available as a digital twin of the infrastructure in the longer term, and where the diagnostics data are the major input while the prognostic data are the major outputs. Getting those steps established for any specific case is a continuous and possibly never-ending process. Within MibaLeb it is shown to some extent, how all of those three steps can be tackled for a specific case. The case is a pressurized hot water pipe made of AISI 347 (X6 CrNiNb18-10), exposed to a series of pressurized hot water flow conditions, which change the material's condition, without having a crack to be seen. To get an idea of an aged material's residual fatigue life requires fatigue tests to be performed and the respective S-N curves to be obtained.