Sachin R. Shinde

A Numerical and Experimental Investigation of Fretting Wear and a New Procedure for Fretting Wear Maps

In this investigation, the fretting wear phenomenon was investigated experimentally and analytically. For the analytical investigation, a combined finite discrete element model (FDEM) was developed to study the effects of displacement amplitude and normal force on fretting wear. The FDEM was used to investigate the wear of a fretting Hertzian line contact calculated using the Archard and dissipated energy wear theories. Although the FDEM results from each theory were in agreement, the dissipated energy approach more accurately predicted the wear volumes obtained from experimental measurements. A fretting wear test rig (FWTR) was used to verify and corroborate the results and conclusions of the numerical investigation. The results indicated that the dissipated energy equation better describes wear in fretting contacts than the Archard equation. An energy dissipation rate map was developed from the experimental results, showing the effect of normal force and displacement amplitude on frictional energy loss. The energy dissipation rate map was used to create a steady-state fretting wear map.

Fretting Wear Modeling of Coated and Uncoated Surfaces Using the Combined Finite-Discrete Element Method

A new approach for modeling fretting wear in a Hertzian line contact is presented. The combined finite-discrete element method (FDEM) in which multiple finite element bodies interact as distinct bodies is used to model a two-dimensional fretting contact with and without coatings. The normal force and sliding distance are used during each fretting cycle, and fretting wear is modeled by locally applying Archard’s wear equation to determine wear loss along the surface. The FDEM is validated by comparing the pressure and frictional shear stress results to the continuum mechanics solution for a Hertzian fretting contact. The dependence of the wear algorithm stability on the cycle increment of a fretting simulation is also investigated. The effects of friction coefficient, normal force, displacement amplitude, coating thickness, and coating modulus of elasticity on fretting wear are presented.

Oxy-Fuel Gas Turbine, Gas Generator and Reheat Combustor Technology Development and Demonstration

Future fossil-fueled power generation systems will require carbon capture and sequestration to comply with government green house gas regulations. The three prime candidate technologies that capture carbon dioxide (CO2 ) are pre-combustion, post-combustion and oxy-fuel combustion techniques. Clean Energy Systems, Inc. (CES) has recently demonstrated oxy-fuel technology applicable to gas turbines, gas generators, and reheat combustors at their 50MWth research test facility located near Bakersfield, California. CES, in conjunction with Siemens Energy, Inc. and Florida Turbine Technologies, Inc. (FTT) have been working to develop and demonstrate turbomachinery systems that accommodate the inherent characteristics of oxy-fuel (O-F) working fluids. The team adopted an aggressive, but economical development approach to advance turbine technology towards early product realization; goals include incremental advances in power plant output and efficiency while minimizing capital costs and cost of electricity [1]. Proof-of-concept testing was completed via a 20MWth oxy-fuel combustor at CES’s Kimberlina prototype power plant. Operability and performance limits were explored by burning a variety of fuels, including natural gas and (simulated) synthesis gas, over a wide range of conditions to generate a steam/CO2 working fluid that was used to drive a turbo-generator. Successful demonstration led to the development of first generation zero-emission power plants (ZEPP). Fabrication and preliminary testing of 1st generation ZEPP equipment has been completed at Kimberlina power plant (KPP) including two main system components, a large combustor (170MWth ) and a modified aeroderivative turbine (GE J79 turbine). Also, a reheat combustion system is being designed to improve plant efficiency. This will incorporate the combustion cans from the J79 engine, modified to accept the system’s steam/CO2 working fluid. A single-can reheat combustor has been designed and tested to verify the viability and performance of an O-F reheater can. After several successful tests of the 1st generation equipment, development started on 2nd generation power plant systems. In this program, a Siemens SGT-900 gas turbine engine will be modified and utilized in a 200MWe power plant. Like the 1st generation system, the expander section of the engine will be used as an advanced intermediate pressure turbine and the can-annular combustor will be modified into a O-F reheat combustor. Design studies are being performed to define the modifications necessary to adapt the hardware to the thermal and structural demands of a steam/CO2 drive gas including testing to characterize the materials behavior when exposed to the deleterious working environment. The results and challenges of 1st and 2nd generation oxy-fuel power plant system development are presented.Copyright

The Effects of Dwell on the LCF Behavior of IN617

Much has been studied on the individual effects of creep and fatigue on alloy life. However, not much is known of the combined effects of these two mechanisms. Therefore, a study was put into place to determine the effects of dwell on fatigue life and deformation mechanisms, for IN617, a solid-solution strengthened Ni-base alloy used widely in the power generation and aerospace industries. Low cycle fatigue (LCF) tests were conducted from 649–982°C with either tensile or compressive dwell. Fracture surfaces of the test specimens as well as longitudinal and transverse sections were examined via scanning electron microscopy to determine the damage and failure mechanisms. Test results confirmed tensile dwell lives that were significantly lower than those seen in compressive dwell. The mechanics for the reduction in cyclic life for tensile dwell was attributed to creep damage accumulation at grain boundaries that led to widespread intergranular cracking and failure. Tensile dwell life reductions were largest in tests at moderate (649–760°C) temperatures. The failed specimens for this temperature range showed the most evidence of grain boundary cavitation and intergranular cracking. At higher test temperatures, the tensile dwell sensitivity for IN617 was significantly reduced or almost entirely eliminated at high temperatures (871–982°C). This was attributed to the lower stresses that developed at these temperatures for a given strain range. The LCF testing and subsequent analysis indicated that a substantial tensile stress during dwell time coupled with moderate temperatures, to allow for diffusion creep, lead to grain boundary damage that can reduce cyclic life.

Influence of Extremum Temperatures on TMF of a Ni-Base Superalloy

Significantly reducing the minimum temperature while maintaining maximum temperature of thermomechanical fatigue (TMF) cycles can reduce the life even when mechanical strain ranges are similar. This applies to in-phase (IP) and out-of-phase (OP) TMF cycles. This reduction in life has generally been attributed to a combination of changes in microstructure arising from aging and increases in the cyclic inelastic strain promoted by increases in the elastic modulus as the minimum cycle temperature is reduced. TMF cycles under both IP and OP conditions were conducted with maximum cycle temperatures within the 750-950C range and with minimum cycle temperatures of either 100 or 500C. A reduction in minimum temperature was observed to promote a decrease in TMF life by as much as a factor of ten for all TMF experiments. The reduction in TMF life is primarily controlled by increases in the inelastic strain range associated with increases in the elastic modulus that arise when the minimum temperature is reduced.

An Analytical Stress-Strain Hysteresis Model for a Directionally-Solidified Superalloy Under Thermomechanical Fatigue

Cyclic plasticity and creep are the primary design considerations of 1st and 2nd stage gas turbine blades. Directionally-solidified (DS) Ni-base materials have been developed to provide (1) greater creep ductility and (2) lower minimum creep rate in solidification direction compared to other directions. Tracking the evolution of deformation in DS structures necessitates a constitutive model having the functionality to capture rate-, temperature-, history-, and orientation-dependence. Historically, models rooted in microstructurally-based viscoplasticity simulate the response of long-crystal, dual-phase Ni-base superalloys with extraordinary fidelity; however, a macroscopic approach having reduced order is leveraged to simulate LCF, creep, and creep-fatigue responses with equally high accuracy. This study applies uncoupled creep and plasticity models to predict the TMF of a generic DS Ni-base, and an anisotropic yield theory accounts for transversely-isotropic strength. Due to the fully analytic determination of material constants from mechanical test data, the model can be readily tuned for materials in either peak- or base-loaded units. Application of the model via a parametric study reveals trends in the stabilized hysteresis response of under isothermal fatigue, creep-fatigue, thermomechanical fatigue, and conditions representative of in-service components. Though frequently considered in design and maintenance of turbine materials, non-isothermal fatigue has yet to be accurately predicted for a generalized set of loading conditions. The formulations presented in this study address this knowledge gap using extensions of traditional power law constitutive models.Copyright

Approach for Stabilized Peak/Valley Stress Modeling of Non-Isothermal Fatigue of a DS Ni-base Superalloy

Turbine blades derived from directionally solidified (DS) Ni-base superalloys are increasingly employed in the first and second stages of gas turbine engines, where thermal and mechanical cycling facilitate cyclic plasticity and creep. The elongated grains, which are aligned with the primary stress axis of the component, provide (1) greater creep ductility, and (2) lower minimum creep rate in solidification direction compared to other directions. Tracking the evolution of deformation in these structures necessitates a constitutive model having the functionality to capture rate, temperature, history, and orientation dependence. Historically, models rooted in microstructurally based viscoplasticity simulate the response of long-crystal, dual-phase, Ni-base superalloys with extraordinary fidelity; however, a macroscopic approach having reduced order is leveraged to simulate low-cycle fatigue (LCF), creep, and creep-fatigue responses with equally high accuracy. This study applies uncoupled creep and plasticity models to predict the thermomechanical fatigue (TMF) of a generic DS Ni-base, and an anisotropic yield theory accounts for transversely isotropic strength. The microstructure of the subject material contains γ-matrix (FCC Ni) and γ′-particles (cuboidal Ni3Al). Because of the fully analytic determination of material constants from mechanical test data, the model can be readily tuned for materials in either peak- or base-loaded units. Application of the model via a parametric study reveals trends in the stabilized hysteresis response of under isothermal fatigue, creep fatigue, idealized thermomechanical fatigue, and conditions representative of in-service components. Though frequently considered in design and maintenance of turbine materials, non-isothermal fatigue has yet to be accurately predicted for a generalized set of loading conditions. The formulations presented in this study address this knowledge gap using extensions of traditional Ramberg-Osgood and Masing models.