We demonstrate a dislocation density-based crystal plasticity (CP) model approach for simulating mesoscale deformation and damage. The existing CP framework is extended to be compatible with the oxygen-free phosphorous copper microstructure that is the focus of this study. The key aim is to introduce relevant plastic deformation mechanisms and to develop a failure model capable of depicting creep damage in the material. The effect of local variations in material is evaluated, and the model response is compared with experiments and characterisation. The basis of this work is CP material modelling, including grain orientation and size, obtained using electron backscatter diffraction and experimental test data of real relaxation test specimens. This will yield a realistic description of texture and grain shape and, ultimately, accurate stress-strain response at the microstructural level for further evaluation of performance with respect to material creep(-fatigue) damage.

Oxide-dispersion strengthened steels are promising materials for extreme service conditions including nuclear reactors core. In service conditions, nuclear fuel claddings are exposed to the fission gas pressure at temperatures about 700 °C. This paper presents novel results on ODS creep properties from a round robin of inner gas pressure creep test. A gas pressure creep test, simulating fission gas loading, was designed and achieved by four different European teams. Lifetime and specific behavior of ODS steel tube are prospected. Based on a mechanical clamping achieving gas tightness, short length tubes samples are tested by different laboratories. In situ laser measurements exhibit the radial expansion of ODS steel tubes before failure. Post-mortem, geometrical characterizations are performed to determine hoop strains at failure. A consistent creep lifetime is observed by all the teams even with slightly different testing apparatus and clamping systems. Under inner gas pressure, ODS steels exhibit a typical failure by leakage associated to a very small radial expansion. This behavior results from a brutal failure (burst) without evidence of tertiary creep stage. This failure mode of ODS cladding in creep conditions is consistently observed on all samples of the study. Inner gas pressure creep tests were compared, for the first time, by four European laboratories on ODS steel tube. This technique, simulating the fission gas pressure loading, is applied on small and mechanically clamped samples. This technique shows a remarkable consistency between the different laboratories results and demonstrates to be efficient for ODS steel cladding tube qualification. The results show a correlation between the creep properties and the microstructure.

Mechanical properties and creep resistance are among the key issues in the selection of fuel cladding materials for future Gen-IV nuclear reactor applications. As for today’s nuclear reactors, the Fukushima accident has increased the interest in accident-tolerant fuel cladding materials. Thin-walled fuel cladding tubes may exhibit anisotropic creep properties due to the manufacturing process. A newly developed Pneumatic Loading Apparatus (PLA) is capable of testing the multiaxial creep properties of thin-walled tubular specimens with internal pressure and an additional axial force. Because of its innovative design, the PLA is moveable and can be implemented in testing in extreme environments. The system functionality and accuracy was demonstrated by two biaxial creep tests performed on austenitic stainless steel DIN 1.4970 and E110 zirconium alloy specimens. Both materials showed anisotropic creep behavior and the PLA testing concept and equipment proved its capability to perform reliably and accurately biaxial creep testing of fuel cladding materials.

Hot steam lines operate at high temperatures and are designed against creep. In service they will consume the creep life at the highest rate in locations with the most adverse combination of material strength (weakness) and high stress (e.g. system loads). Adverse effects in stress state are promoted by features of geometry and discontinuities in the materials properties. In practice it is essential to find the areas of maximum damage, as these will determine the locations and timing to inspect and finally to repair or replace before failures or unplanned outages. The inspection experience from power plant steam systems have provided the established views on the expected locations of the early creep damage, and rules on timing the next inspection. The experience has also shown that optimal timing is materials dependent, and that the steel grade X20CrMoV11-1 (X20, 11% Cr steel) performs particularly well in the inspection statistics. This paper describes a case with observed creep cavitation and cracking in a branch weld of a X20 steam header that was replaced after 135 000 service hours. The characteristics of the observed damage and its evolution are discussed.

Gas turbine blades operate under severe loading conditions, and for satisfactory service must be designed to withstand multiple damage mechanisms. On the other hand, if the assumptions on the future service are sufficiently conservative or actual service conditions more benign than foreseen, there is potential for blade life extension. This generally does not justify fully omitting intermediate inspections, as intervening damage by e.g. overheating or foreign objects is not excluded, and restoring coatings or other repairs could be needed. Nevertheless, a significant extension to the time of final replacement can be possible. In this work, example cases are used to highlight the technical challenges and results of blade life extension. Such aspects include assessment of in-service stress and temperature histories, interpretation of condition assessment, and materials performance. The example cases refer to ex-service blade samples extracted at the time approximately corresponding to the nominal blade life as defined by the engine supplier. Both land based and aircraft turbines are considered, and in spite of the differences in design and service profiles, similarities can be pointed out. The results for the example cases generally suggest significant potential for life extension, for reasons related to the case-specific service histories.