Crack Growth Resistance of Ceramic Matrix Composites and Anisotropic Stiffness Prediction/Measurement

A Durability and Damage Tolerance (D&DT) analysis of an S200 Nicalon/SiNC and Oxide/Oxide Ceramic Matrix Composite (CMC) was conducted to determine the crack growth resistance (GIc) of Wedge Loaded DCB (WDCB) at Room and Elevated temperatures (RT/ET) and compared with experimental tests observations. Wedge Loading gives proper crack path without mixed mode effects and can be used at high temperature in a furnace. Load displacement, GIc, electrical resistivity and acoustic emission was measured by tests and compared to FE based Multi Scale Progressive Failure Analysis (PFA) of the WDCB specimen. The critical damage events studied included damage initiation, damage propagation, fracture initiation, and fracture propagation as the components were being loaded. Effect of defects on Modulus (E11, E22, and E33) was conducted by Electrical Resistance (ER) Measurement at Room temperature (RT). Multi-Scale modeling simulation considered de-homogenized nano-assisted micromechanics analytical formulation, a Mori Tanaka based stiffness correction including void shape, size, distribution and orientation effects. Emitted/received signal amplitude by ER Vs. time was used to evaluate reduction of stiffness in all directions resulting in anisotropic stiffness of As-Built specimens. WDCB specimen was tested to failure at RT/ET to produce reliable GIc values with minimum specimen size. Many parameters that contribute to specimen failure included interface coating thickness, mixed mode failure evolution, interlaminar defects, delamination damage, crack bridging, and fiber fracture which were all studied in detail in this work. All simulations correlated well with test.

Advanced Welding Materials and Technologies for Repair of Turbine Engine Components Manufactured of High Gamma Prime Nickel Based Superalloys

The current study reports on modification of conventional welding materials with titanium, boron and silicon to enhance weldability by reducing the solidus temperature and promoting the formation of low temperature eutectics. Melting behavior, weld microstructure, tensile and stress rupture strength are reported for several alloy compositions. Some examples of repairs of IGT and aero HPT blades using the materials and technologies are provided.

Technology-Based Re-Contouring of Blade Integrated Disks After Weld Repair

The widespread adoption of blade integrated disks (blisks) made of titanium demands tailored re-generation processes to increase sustainability and economic efficiency. High standards regarding geometrical accuracy and functional properties as well as the unique characteristics of each type of damage complicate the repair. Thus, flexible and well-designed processes are necessary. Typically, material deposit is followed by a milling or grinding process to restore the original shape. Here, not only the individual repair processes have to be controlled, but also their interaction. For example, depending on the resulting microstructure of the welded seam, the re-contouring process needs to be adapted to minimize tool wear as well as shape deviations of the complex blade geometries. In this paper, the process chain for a patch repair is examined, consisting of a TIG welding process followed by 5-axis ball nose end milling. Conventional TIG as well as a modified TIG process producing a finer grain structure and enhanced mechanical properties of deposited material were investigated. Grain refinement was achieved by SiC particles added to the weld pool. Based on the characteristics of the fusion material and static stiffness of the component, a methodology is introduced to minimize shape deviation induced by the subsequent milling process. Special attention is given to tool orientation, which has a significant impact on the kinematics and resulting process forces during milling. An electromagnetic guided machine tool is used for compensation of workpiece deflection.

Approach for Zero Defect Manufacturing: Geometric Calibration of Five-Axis Machine Tools for Blisk Manufacturing Process

Five-axis machining is a key technology of blisk manufacturing process. Blisks generally require high accuracy due to their high performance and safety-critical conditions. However, recent research show that the design of the blisks and turbine blades are getting more complex and require even higher accuracy. This leads also to the application of wide and rare area of movement axes of the machine. Thus, the machine accuracy has to be assured within the overall machine volume. The geometric accuracy demonstrates the base accuracy of the machine. This paper presents a geometric calibration method optimized for the axes movement area of blisk machining process. The accurate calibration of the five-axis machine tool is challenging and hardly possible due to limited error measurement of standard measurement devices. Some measurement methods enable complete calibration of the machine but with complex, time-consuming process and expensive measurement devices. Also, due to the rare axes travel, there is no standard calibration method for the blisk machining process. The calibration method in this paper is developed based on so called ‘R-test’ method. The machine and the errors are modelled mathematically for the measurement. An adapter is applied for the measurement of maximum axis positions. Automation units are developed for the full machine integration and automation of calibration procedure. With the developed method, the machine is calibrated from 130 μm to 10 μm in maximum measurement time of 90 minutes. The calibration quality is validated at an independent measurement position with continuous movement of the five axes.

Analysis of Performance of a Twin-Shaft Gas Turbine During Hot-End Damage in the Gas Generator Turbine

In this study, an analysis of the performance of a twin-shaft industrial gas turbine (IGT) during hot-end damage in the gas generator turbine (GGT) at high-power operation has been carried out using a validated Simulink IGT model. The Simulink model is based on fundamental thermodynamics and allows the implementation of correlation coefficients in the GGT module to predict the performance of the IGT system during a hot-end GGT damage incident. Measured field data from a twin-shaft IGT operated as a power generation unit denoting a reduction in performance due to hot-end GGT damage are considered for the analysis. Four hot-end GGT damage incidents across a range of measured field data have been identified and considered for the analysis. The results show that the Simulink model can predict the change of physical parameters (pressure, temperature) across the IGT system for each GGT damage incident. Hot-end damage increases the flow capacity and reduces the efficiency of the GGT. Future work will validate the dynamic change of flow capacity and efficiency during different GGT damage incidents.

Measurement and Evaluation of Shaft Torsional Vibrations Using Shaft Instantaneous Angular Velocity

Online evaluation of possible failures of a turbine is a key factor for a successful long-term turbine operation. Fluctuations of generator air-gap torque, caused for example by non-stationary conditions of electrical power grid, influence shaft torsional vibrations as well as rotating blades vibrations. Symptoms of shaft torsional vibrations are not measurable by normally used relative shaft vibrations sensors, so special measurement must be used. The direct consequence of shaft torsional vibrations is local acceleration or deceleration of shaft circumference when it is measured by a stationary sensor. This article deals with a measurement method using an optical probe measuring the passage of black and white stripes of a zebra tape which is stuck on the rotor. The shaft torsional vibrations manifest themselves as a phase modulation of the optical probe output signal, so the sampling rate influence the achievable resolution of the calculated shaft vibrations. The presented method for the calculation of the shaft torsional vibrations is based on the evaluation of shaft instantaneous angular velocity. The advantage of this method is a direct compensation of possible non-regular geometry of the zebra tape. The analysis of shaft torsional vibrations evaluation using this method is supplemented by two case studies from the authors’ current work.

Development of ODS Coating for High Temperature Turbine Components Using DED Additive Manufacturing

Current and future designs for advanced turbine systems, such as Integrated Gasification Combined Cycle (IGCC), advanced Natural Gas Combined Cycle (NGCC), and the emerging supercritical CO2 (SCO2) systems require increasing turbine inlet temperature (TIT), which is well beyond the substrate melting temperature. The well-known approach is coating the turbine blade with thermal barrier coatings (TBC) combined with internal cooling channel in the substrate. However, due to thermally grown oxide (TGO) and thermal expansion mismatch stresses, TBC spallation failure is a major concern. Furthermore, neither the ceramic coating layer nor the metallic bond coat in current TBC system can provide structural support to house the internal cooling channels. In this research, a method to fabricate high temperature protective structural coating on top of critical gas turbine components by additive manufacturing (AM) technique using oxide dispersion strengthening (ODS) metal powder is presented. A novel combined mechanochemical bonding (MCB) plus ball milling process is utilized to produce near spherical and uniformly alloyed ODS powders. AM-processed ODS coating by direct energy deposition (DED) method on MAR-247 substrate, with laser powers from 100W to 200W were carried out. The ODS coated samples were then subjected to thermal cyclic loadings for over 2200 cycles. For comparison, in our earlier studies, under the same cyclic testing condition, typical tested TBC coupons showed spallation failure after ∼400 cycles. Correlation of the measured ODS coating Young’s modulus using a unique non-destructive micro-indentation testing method with evolution of the ODS microstructures are studied to identify optimum AM processing parameters for best performance of the ODS samples. In particular, stability of secondary γ′ phase in the ODS coating after thermal cycles is analyzed. Test results revealed a thin steady durable alpha alumina oxide layer on the best performance ODS samples. After 2,200 thermal cycles, strong bonding at ODS/substrate interface is also maintained for most of the ODS coated samples. Test results also showed stable substrate microstructure due to the protective ODS coating even after 2,200 thermal cycles. These preliminary test results showed strong potential for applications of AM-assisted ODS coating on advanced gas turbine components.

Constituent Development for Higher-Temperature Capable Ceramic Matrix Composites

This paper highlights research that is addressing the need for improved high-temperature-capable CMCs, with a focus on CMC constituents and an understanding of their processing, microstructure, and behavior in relevant service environments. The most pervasive lifetime and temperature limitations for SiC/SiC CMCs are related to oxidation, creep and stress rupture of the fibers, oxidation-induced instability of the fibermatrix interface, and instability of the matrix at temperatures > 1400°C. Consequently, we are addressing these shortcomings by developing technologies to enable higher-temperature capable SiC fiber, oxidation-resistant fiber-matrix interfaces, and improvements in processing of refractory matrices for both turbine engine and hypersonic applications.

Gas Turbine Bearing Wear Monitoring Method Based on Magnetic Plug Inductance Sensor

In this paper, a gas turbine bearing wear monitoring method based on the magnetic plug inductance sensor is presented. Using the induced magnetic field of the magnetic pulse generated by the inductance coil, this method is applied to measure ferromagnetic wear debris in the oil, and the condition of bearing wear can be monitored and predicted on line. To estimate the mass of different particle size of ferromagnetic debris, the sample databases of debris mass were built, the oil capturing experiment was conducted, and the mapping model for the voltage signal of the sensor and the captured accumulation of ferromagnetic wear debris based on the BP (Error Back Propagation) neural network was established. Moreover, the kernel method was used to calculate the voltage distribution of the debris sensor in the step time of wear prediction, and the mean value and confidence boundary value of the signals in the expected step time were obtained. Moreover, the prediction model of the bearing wear was established by a linear regression method to predict the mass of ferromagnetic wear debris generated by bearing wear. Finally, a lubricating oil debris detection system was designed, and the bearing wear test was conducted on the bearing wear testing rig. The results showed that the monitoring method can continuously monitor and dynamically predict the condition of bearing wear online with the advantages of stability and automatic purifying lubricant oil.

Investigating Discrepancies in Vibration Bending Fatigue Behavior of Additively Manufactured Titanium 6Al-4V

The vibration bending fatigue life uncertainty of additively manufactured Titanium (Ti) 6Al-4V specimens is studied. In this investigation, an analysis of microscopic discrepancies between 10 fatigued specimens paired by stress amplitude is correlated to the bending fatigue life scatter. Through scanning electron microscope (SEM) analysis of fracture surfaces and grain structures, anomalies and distinctions such as voids and grain geometries are identified in each specimen. This data along with previously published results are used to support assessments regarding bending fatigue uncertainty. Corrections on stress and scatter based on microscopic features are implemented to the stress versus fatigue life comparisons. The results of this investigation show that the bending fatigue life uncertainty can be bounded by cold-rolled Ti 6Al-4V data when correcting the tested stress amplitude values with stress concentration effects and variation due to microstructure geometries. The understanding gained from this study is important for future development of a predictive vibration bending fatigue life model that will include the probability of geometry, density, and location of voids as an artifact of LPBF build parameters.