Calculation methods to assess threshold and propagation behaviour of mechanically short cracks at high temperatures

Lifetime prediction is an important part in designing components like rotating disks for aircraft engines or gas turbines. The structures are assumed to be inherently flawed, thus requiring a damage tolerant design, which ensures these flaws will not result in a propagating crack, at least not until the next inspection. Therefore, the KitagawaTakahashi diagram, which combines the endurance limit and the long crack threshold of the linear elastic fracture mechanics, is often used within the design process. For an accurate lifetime prediction, the crack propagation in the stage below and above the long crack threshold is crucial. Currently available computational methods struggle in the area of short crack growth above the intrinsic threshold and result either in a nonor extreme conservative prediction. Recently, a new model, which takes increasing crack closure for short cracks into account, was proposed by Maierhofer et. al [MAI14]. This project aims to further develop the above-mentioned model and transfer it to IN718 and its typical maximum service temperature in turbine applications of 650 °C. Furthermore, the distinct environmental influence in this alloy at high temperatures is considered. Therefore, tests are conducted in laboratory air and vacuum. The results will in later application enable to distinguish between cracks on the surface, thus connected to the environment, and cracks inside of components assumed to agree with vacuum conditions. For this purpose, a good database is required and created by threshold tests conducted on Single Edge Nochted (SEN) samples. The tests are done for different Rratios and follow a procedure designed by Tabernig [TAB00]. To account for state of the art manufacturing processes as well as classical approaches, the tests are carried out for wrought, cast and per Laser Powder Bed Fusion process (LPBF) manufactured material. These threshold tests are preceded by a basic material characterization, which contains microstructural examinations, tensile tests, incremental step tests and fatigue crack growth (FCG) tests on corner crack (CC) specimens. This initial characterization is necessary for an adequate analysis of the threshold results, especially of the short crack region, which is strongly influenced by microstructural features and Results for the basic material characterization of every material condition as well as cyclic Rcurves for the wrought and LPBF material are shown in this report. The material characterization shows severe differences in the different conditions, which manifests in the results of the threshold tests as well. As expected, the environmental influence is significant and is found in both, the wrought and LPBF material.