This work presents a physically based probabilistic model for LCF life of the conventional cast (CC) nickel-based superalloy MAR-M247. The model explicitly accounts for the effects of casting defects. Over 100 LCF tests were conducted at 850 °C and 950 °C on specimens with and without defects. Specimens were extracted from turbine blades that had manufacturing-related casting defects. Additionally, artificial defects were introduced into initially defect-free specimens. A multi-stage CT- and μCT-based specimen extraction strategy was developed to precisely position casting defects within specimens. Defects were parameterized orthogonally to the loading direction based on their size, shape and orientation using SEM fractography. For all tested strain ranges, a dominant influence of defect size on fatigue life was found. The effects of defect shape and orientation were small in comparison. Significant scatter of material parameters was observed experimentally. Electron backscatter diffraction (EBSD) revealed significant variations in grain size and orientation depending on the specimen extraction position. The model accounts for this by using individual values for stress ratio Rσ and Young’s modulus E. LCF fatigue life was modeled using an approach based on the cyclic crack tip opening displacement ΔCTOD. Defect size was accounted for as an initial crack length based on the Feret diameter dFeret of each defect. Additionally, an El Haddad-type threshold term was used to account for defect size dependence in the high-cycle regime. Validation against experimental data showed robust prediction accuracy across all defect types, defect sizes, specimen geometries, temperatures, and strain ranges. Model inaccuracies arising from the fracture mechanics based approximation solution were estimated analytically. For defects large compared to the specimen geometry, a tendency towards more conservative fatigue life assessment was shown. A probabilistic framework was established using Monte Carlo simulations. Material scatter was modeled using experimentally derived distributions for E and Rσ. Fatigue life distributions were quantified under realistic operating temperatures and loads. The results support risk-based maintenance and optimized gas turbine operation in modern decarbonized energy systems.

A gas turbine (GT) operating in a renewable-integrated grid is subjected to repeated start-stop cycles and quick load ramps. Such conditions highlight a risk of potential increase of cyclic damage in its critical rotating components like the turbine blades. A multiyear government-funded project is in progress to evaluate the low cycle fatigue (LCF) behavior of a cast blade material with defects. The aim is to develop a probabilistic fatigue crack nucleation life model. Strain-controlled LCF tests were conducted on defective and defect-free flat and round specimens extracted from cast MAR-M247 GT blades [1, 2]. As the third in a continuum of publications, this paper investigated the use of the so-called IBESS model [3] as a deterministic base for a proposed probabilistic framework. Key fatigue and fracture mechanics concepts were briefly reviewed for a better understanding, followed by a detailed description of the IBESS modification of the NASGRO [4] crack growth equation. The model was applied on the data generated from the LCF tests. While the model overestimated the predictions of cycles to crack nucleation for most specimens, it showed promise for further improvements. Future work is expected to define an appropriate failure criterion and achieve better corrections for the size and geometry of both the crack and the specimen.

Addressing anthropogenic climate change is a great challenge that requires integrating renewable energy resources into the grid. The intermittent nature of renewables creates a demand for more flexible gas turbine operation conditions. This increases the number of startup and shutdown cycles and leads to gas turbine components experiencing a significant increase in low cycle fatigue (LCF) loading. This study examines the LCF performance of a widely used rotor blade material to support flexible power plant operation. The study is part of a multi-year government-funded project developing a probabilistic fatigue lifetime model for cast material, including defects like cast porosity. The present work is the second contribution in an ongoing series of studies [1]. The focus of this work is the investigation of the influence of defect size and shape on LCF performance. Because defects introduce a variety of uncertainties, understanding their influence is required for developing a probabilistic approach. Additional model input parameters subject to uncertainties, such as grain sizes and orientations that significantly depend on blade location and design, are also investigated.

In this paper, the initiation and propagation behaviour of short fatigue cracks in nickel-base alloy Inconel 718 is studied extensively in the fatigue loading regime by ex-situ scanning electron microscopy on component-near notched specimens in the temperature range between 400°C and 600°C. To investigate the influence of different surface treatments, both specimens with polished and shot-peened surfaces were tested. For polished specimens, most fatigue cracks initiated at oxidised non-metallic inclusions, whereas for shot-peened specimens, the process-induced surface roughness also served as crack starters. It was found, that the observed scatter in lifetime for repetitive tests is mainly caused within the short crack fatigue regime below 50μm, while cracks larger than 100μm showed similar growth characteristics.

Intermittent renewable energy sources are becoming increasingly important. This creates a demand for more flexible gas turbine operation conditions, to fill supply gaps and manage peak loads. As a result, the number of startup and shutdown cycles — the low cycle fatigue (LCF) loading of a gas turbine component — increases substantially. This work investigates the LCF performance of a common rotor blade material, to support the future transition towards more flexible power plant operation. One of the goals of this multi-year government funded project is the development of a probabilistic life model of cast material including defects such as porosity. The focus of this work is the investigation of the influence of casting defects on the LCF performance of cast MAR-M247. A comprehensive test program on defective and defect-free specimens at realistic engine conditions, i.e., temperature and stress was conducted. A first deterministic fatigue life model that accounts for material defects is proposed.

Austenitic stainless steels have been extensively tested in hydrogen environments; however, limited information exists for the effects of hydrogen on the fatigue life of high-strength grades of austenitic stainless steels. Moreover, fatigue life testing of finished product forms (such as tubing and welds) is challenging. A novel test method for evaluating the influence of internal hydrogen on fatigue of orbital tube welds was reported, where a cross hole in a tubing specimen is used to establish a stress concentration analogous to circumferentially notched bar fatigue specimens for constant-load, axial fatigue testing. In that study (Kagay et al, ASME PVP2020-8576), annealed 316L tubing with a cross hole displayed similar fatigue performance as more conventional materials test specimens. A similar cross-hole tubing geometry is adopted here to evaluate the fatigue crack initiation and fatigue life of XM-19 austenitic stainless steel with high concentration of internal hydrogen. XM-19 is a nitrogen-strengthened Fe-Cr-Ni-Mn austenitic stainless steel that offers higher strength than conventional 3XX series stainless steels. A uniform hydrogen concentration in the test specimen is achieved by thermal precharging (exposure to high-pressure hydrogen at elevated temperature for two weeks) prior to testing in air to simulate the equilibrium hydrogen concentration near a stress concentration in gaseous hydrogen service. Specimens are also instrumented for direct current potential difference measurements to identify crack initiation. After accounting for the strengthening associated with thermal precharging, the fatigue crack initiation and fatigue life of XM-19 tubing were virtually unchanged by internal hydrogen.

In this investigation, the fatigue and crack growth behavior of the ductile cast iron material (DCI) GJS-500 was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests, out-of-phase thermomechanical fatigue (OPTMF) tests as well as low cycle fatigue tests combined with the replica testing method (LCF-R) within the temperature range RT-500 degrees C. The studied material exhibits an embrittlement at temperatures nearby 400 degrees C, leading to a significant higher fatigue crack growth rate and a reduced lifetime, when compared with RT and 500 degrees C. A possible explanation for the observed higher fatigue crack growth rate coupled with a significant lifetime reduction, is intergranular embrittlement.

Today's and future parameters of stationary gas turbines and aircraft engines require intensive and highly efficient cooling of hot gas path components. High temperature and thermally induced stress gradients with impact on fatigue life are the consequence. Thermally induced stress gradients dyer from geometrically induced stress gradients with respect to stress mechanics by the independence from external loads and material mechanics by the influence of temperature on material properties and strength. Regarding the contribution and evaluation on damage, the latter characteristic feature in turbomachinery is currently notfully understood Therefore, a test facility has been designed, set up, and reported in GT2018-76519 for the investigation of the influence of stationary temperature, and thus thermally induced stress gradients, on the damage evolution of cooled high-temperature components. To achieve high temperature and thermally induced stress gradients, large heat fluxes are required. A unique radiation heating has been developed allowing very high heat fluxes of q > 1.5 MW/m2 for testing of hollow cylindrical specimens. The conventional cast nickel-base alloy Mar-M247 has been chosen to study the influence of thermally induced stress gradients on fatigue life. The low-cycle fatigue testing of the hollow cylindrical specimens has been conducted both with and without superimposed stationary temperature gradients. In addition, Complex Low-Cycle Fatigue (CLCF) tests with symmetric and nonsymmetric loading conditions have been performed to provide the necessary database for the adaptation of a viscoplastic deformation model. To calculate the local stress strain field and service life of the test specimens, linear elastic and viscoplastic finite element studies have been performed and were assessed by means ofa fracture mechanics-based lifetime model. The test results show the considerable influence of the temperature gradient on the low-cycle fatigue life for the investigated material. Both the radial temperature variation over the specimen wall with a hot outer surface and a cooled inner surface as well as the thermally induced stresses are stated to be the main drivers for the change in low-cycle fatigue life. The test results enhance the understanding of fatigue-damage mechanisms under local unsteady conditions and can be used as a basis for improved service life predictions.

Digital image correlation (DIC) is a highly accurate image-based deformation measurement method achieving a repeatability in the range of σ= 10−5 relative to the field-of-view. The method is well accepted in material testing for non-contact strain measurement. However, the correlation makes it computationally slow on conventional, CPU-based computers. Recently, there have been DIC implementations based on graphics processing units (GPU) for strain-field evaluations with numerous templates per image at rather low image rates, but there are no real-time implementations for fast strain measurements with sampling rates above 1 kHz. In this article, a GPU-based 2D-DIC system is described achieving a strain sampling rate of 1.2 kHz with a latency of less than 2 milliseconds. In addition, the system uses the incidental, characteristic microstructure of the specimen surface for marker-free correlation, without need for any surface preparation—even on polished hourglass specimen. The system generates an elongation signal for standard PID-controllers of testing machines so that it directly replaces mechanical extensometers. Strain-controlled LCF measurements of steel, aluminum, and nickel-based superalloys at temperatures of up to 1000 °C are reported and the performance is compared to other path-dependent and path-independent DIC systems. According to our knowledge, this is one of the first GPU-based image processing systems for real-time closed-loop applications.

In this investigation, the fatigue behaviour of a ductile cast iron with high content of silicon and molybdenum, was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests as well as out-of-phase thermomechanical fatigue (OPTMF) tests within the temperature range RT – 500 °C. The studied material shows an embrittlement at temperatures nearby 400 °C. A possible explanation for the observed lifetime reduction is intergranular embrittlement (IE). A mechanism based lifetime model is proposed for assessing the lifetime. The model is based on the assumption that the crack advance per cycle is correlated with the cyclic crack tip opening displacement (ΔCTOD) attributed to the crack tip blunting caused by accumulation of plastic and creep deformations ahead of the crack tip. Intergranular embrittlement is accounted for by introducing a temperature and strain rate dependent prefactor in the crack growth law, which only acts in a certain temperature range. The model is calibrated for a GJS material and successfully applied to predict the lifetime of this material when undergoing isothermal and non-isothermal mechanical loadings. A probabilistic interpretation of the scatter of the investigated material is presented in conjunction with the random nature of the initial defect size distribution.

In this paper, the influence of different phase angles, load ratios and dwell times on the thermomechanical fatigue (TMF) crack growth of the nickel-based superalloys MAR-M247 CC (HIP) and CM-247 LC is studied. The thermomechanical fatigue crack growth tests are performed under in-phase (IP) and out-of-phase (OP) TMF loading between 300°C and 950°C. It is shown, that the applied load ratio plays a crucial role for the resulting fatigue crack growth rates and whether IP or OP TMF loading is more detrimental. A digital optical microscope is installed, which allows in situ observation of the fatigue crack growth, the active damage mechanisms and the damage evolution even at high temperatures. The in situ observations are accompanied by fractographic investigations, which confirm, that IP TMF loading preferentially leads to interdentritic fracture. Finally, first steps towards a quantification of the active damage mechanisms are taken using digital image processing.

Aluminium cast alloys are used for engine components, such as pistons and cylinder heads. The micromechanical properties of an AlSi12 cast alloy under monotonic and cyclic loadings are investigated. Therefore a microstructure-based two dimensional finite element model is generated. The characteristic shape of primary precipitates is analyzed and translated into an artificial microstructure. The quality of the generated microstructure is evaluated based on the stress distribution along the primary particle boundaries. The effect of the temperature dependent material behavior of the aluminium matrix is studied with respect to the resulting stress distribution along the particle boundaries. The results are discussed in terms of a possible change of fracture mechanisms from a brittle type fracture at low temperatures to an increasingly ductile fracture at high temperatures.