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RESEARCH PAPERS

J. Eng. Gas Turbines Power. 1985;107(1):2-9. doi:10.1115/1.3239694.

An experimental program to research, develop, and demonstrate an axially staged, premixed low emissions combustor is described, with particular emphasis on the many mechanical and thermal problems which were encountered and which seem characteristic of the type. Of these, the risks of autoignition and flashback in the premixing duct were perhaps, the most severe. Their elimination imposed severe constraints on premixing duct geometry and indeed, dictated the layout of the whole combustor. Nevertheless, it proved possible to retain conventional technology to a large extent including the basic combustor structure and mounting arrangements and normal fuel injector accessibility. Large emissions reduction capability was demonstrated to be possible though not within conventional combustor dimensions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):10-17. doi:10.1115/1.3239668.

Experimental values of weak and rich limits, for a wide range of fuel-oxygen-inerts mixtures (H2 , CO, CH4 , C3 H8 , O2 , Ar, He, N2 , CO2 ), are presented for a number of tube diameters. The significance of the results is discussed, and with the use of other published data, a weak limit correlation is proposed that demands knowledge only of the fuel composition, its calorific value, and the specific heat of the stoichiometric products at 298 K. The correlation is applicable to simple or complex mixtures, with or without inert addition, and applies over an initial temperature range from 298–1000 K. It is also capable of application to a much wider range of fuels than those investigated experimentally. The accuracy of the parameter is as good as, or better, than those currently in use.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):18-23. doi:10.1115/1.3239682.

Further experimental research has been carried out on the ignition improvement by oxygen addition in a small pilot combustion chamber taken from an existing aeroengine. Both uniform mixture method and local oxygen injection method have been investigated. The results show that oxygen addition improves low-pressure ignition performance significantly. Local oxygen injection makes much better use of oxygen added. Experimental results show low evaporation percentage of fuel spray responsible for the poor ignition performance. Based on thermal ignition theory, a semianalytical equation has been obtained to predict the relative enlargement of maximum ignition velocity by oxygen addition, the equation can correlate experimental data very well. To form a stagnant region with suitable oxygen concentration is the most important factor in the design of ignitor with oxygen addition.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):24-37. doi:10.1115/1.3239693.

An analytical study is made of the substantial body of experimental data acquired during recent Wright-Patterson Aero Propulsion Laboratory sponsored programs on the effects of fuel properties on the performance and reliability of several gas turbine combustors, including J79-17A, J79–17C (Smokeless), F101, TF41, TF39, J85, TF33, and F100. Quantitative relationships are derived between certain key aspects of combustion, notably combustion efficiency, lean blowout limits and lean light-off limits, and the relevant fuel properties, combustor design features, and combustor operating conditions. It is concluded that combustion efficiency, lean blowout limits, and lean lightoff limits are only slightly dependent on fuel chemistry, but are strongly influenced by the physical fuel properties that govern atomization quality and spray evaporation rates.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):38-47. doi:10.1115/1.3239695.

In-flame optical measurements of soot particulates in a turbulent, recirculating (i.e., complex flow) model laboratory combustor are described. A nonintrusive optical probe based on large-angle (60 and 20 deg) intensity ratio scattering was used to yield a point measurement of soot particulate in the size range of 0.08 to 0.38 μm. A shale-derived JP-8 stock, isooctane, and mixtures of isooctane with various ring and aromatic compounds blended to yield the smoke point of the JP-8 stock were separately injected as a liquid spray through a twin-fluid atomizer. One blend was also introduced prevaporized through a hollow-cone nozzle. The addition of ring compounds to the base isooctane as well as operation on JP-8 increased the amount of soot produced, although the total amount of soot produced depended on fuel type for those fuels of equivalent smoke point. The spatial distribution of soot as well as the amount produced was found to be sensitive to nozzle atomization quality and injection momentum. The amount of soot produced was reduced by a reduction in fuel loading. However, injection of fuel in a prevaporized state both increased the amount of soot produced and changed spatially the region over which soot was distributed. Scanning electron micrographs of extracted samples established that the optical technique resolved the large particle wing of the soot size distribution.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):48-53. doi:10.1115/1.3239696.

An existing steady-state, locally homogeneous flow model of turbulent spray combustion was modified to predict NO emission from a spray flame and soot emission from a gas-jet flame. The effect of turbulent fluctuations on the reaction rates was accounted for. The predicted NO emission from an n-pentane spray with a changing injection velocity could be correlated with the convective time scale of the flow. Calculation of soot emission from a burning turbulent gas jet indicated that the centerline soot concentration reaches a peak upstream of the maximum temperature location and then decreases due to soot oxidation and dilution by air entrainment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):54-59. doi:10.1115/1.3239697.

Heat transfer and boundary layer measurements were derived from flows over a cooled flat plate with various free-stream turbulence intensities (Tu = 1.6–11 percent), favorable pressure gradients (k = νe /ue 2 •due /dx = 0÷6•10−6 ) and cooling intensities (Tw /Te = 1.0–0.53). Special interest is directed towards the effects of the dominant parameters, including the influence on laminar to turbulent boundary layer transition. It is shown, that free-stream turbulence and pressure gradients are of primary importance. The increase of heat transfer due to wall cooling can be explained primarily by property variations as transition, and the influence of free-stream parameters are not affected.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):60-67. doi:10.1115/1.3239698.

A two-dimensional, boundary-layer program, STAN5, was modified to incorporate a low-Reynolds number version of the K-ε, two-equation turbulence model for the predictions of flow and heat transfer around turbine airfoils. The K-ε, two-equation model with optimized empirical correlations was used to account for the effects of free-stream turbulence and transitional flow. The model was compared with experimental flat plate data and then applied to turbine airfoil heat transfer prediction. A two-zone model was proposed for handling the turbulent kinetic energy and dissipation rate empirically at the airfoil leading edge region. The result showed that the predicted heat transfer coefficient on the airfoil agreed favorably with experimental data, especially for the pressure side. The discrepancy between predictions and experimental data of the suction surface normally occurred at transitional and fully turbulent flow regions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):68-75. doi:10.1115/1.3239699.

Two-dimensional arrays of circular jets impinging on a surface parallel to the jet orifice plate are considered. The jet flow, after impingement, is constrained to exit in a single direction along the channel formed by the jet orifice plate and the impingement surface. Experimental results for the effects of streamwise nonuniform array geometries on streamwise flow distributions and heat transfer characteristics are presented. A flow distribution model for nonuniform arrays is developed and validated by comparison with the measured flow distributions. The model is then employed to compare nonuniform array streamwise resolved heat transfer coefficient data with previously reported uniform array data and with a previously developed correlation based on the uniform array data. It was found that uniform array results can, in general, serve as a satisfactory basis from which to predict heat transfer coefficients at individual spanwise rows of nonuniform arrays. However, significant differences were observed in some cases over the first one or two rows downstream of the geometric transition line of the nonuniform array.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):76-83. doi:10.1115/1.3239700.

This paper describes the measurement of heat flux distributions obtained for a Garrett TFE 731-2 hp turbine. Measurements were obtained for a full turbine both with and without injection and for the nozzle guide vanes with and without a rotor. A shock tube is used as a short-duration source of heated air and miniature thin-film gages are used to obtain the heat flux measurements. Results are presented for values of the blowing parameter (ρc Vc /ρ∞ V∞ )at SLOT , in the range of 0.8–1.3. The injection gas (air) as a percentage of turbine weight flow, Wc /Wo , was in the range of 2.1–3.5 percent. A comparison is presented between results obtained with the rotor operating at 100 percent of corrected speed and those obtained with the rotor replaced by a row of flow straighteners. The results suggest that: (i) the reduction of heat flux due to injection is a function of the blowing parameter, the temperature ratio, and the physical location relative to the tip or hub endwall and (ii) the presence of the rotor has a significant affect on the vane trailing edge Stanton number, increasing it by 15 to 25 percent. The vane leading edge and midchord regions were generally unaffected.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):84-91. doi:10.1115/1.3239701.

This paper describes the results of an experimental investigation into the film cooling effectiveness of two rows of holes inclined in the stream and spanwise directions. The effects of hole and row spacings and combinations of inclinations are investigated in the presence of free-stream pressure gradients and turbulence for a typical range of blowing rates.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):92-98. doi:10.1115/1.3239702.

A method for predicting cooling air flow rates using tests on cylindrical models of typical turbine blade leading edges has been extended to include blades with inserts and blades with reversed-angled holes. When an insert is used, the pressure loss across the insert can be determined from flow tests and added to other losses in the flow path to determine cooling flow rates. Calculated and experimentally determined flow rates are compared with good agreement. The second experiment was performed to determine internal loss coefficients for reverse-angled holes oriented so the flow makes a reverse turn to enter the holes. The reversed flow case produced significantly greater internal loss coefficients than when the same holes were oriented in the direction of flow. These results were used to predict flow from arrays of reverse- angled holes and from a cylinder containing both reverse-angled holes and nonreversed holes. In all cases, good agreement was found between predicted and measured flow rates.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):99-104. doi:10.1115/1.3239703.

This paper describes an investigation of the sensitivity of the heat transfer coefficient under the film to the state of the approach boundary layer for injection through a row of holes on a flat plate. The investigation is done for a range of blowing parameters using a heat-mass transfer analogy. Injection angles of 35 deg and 90 deg are covered. Additionally, for the same injection geometries, the effect of injection in the presence of mild adverse, mild favorable, and strong favorable mainstream pressure gradients is investigated. The results indicate that the heat transfer coefficient under the film is sensitive neither to the condition of the approach boundary layer nor to the presence of a mild adverse pressure gradient, but it is significantly lowered by a favorable pressure gradient, particularly at low blowing parameters.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):105-110. doi:10.1115/1.3239667.

The influence of injection of cooling films through a row of holes on the heat transfer coefficient on a flat plate is investigated for a range of mass flux ratio using a heat-mass transfer analogy. Injection angles of 35 deg and 90 deg are covered. The experimental technique employed uses a swollen polymer surface and laser holographic interferometry. The results presented show the change in local heat transfer coefficient over the no-injection values at the centerline and off-centerline locations for various streamwise stations. The effect of injection on laterally averaged heat transfer coefficients is also assessed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):111-116. doi:10.1115/1.3239669.

An experimental investigation of film cooling on a rough surface has been performed using a mass transfer technique. Two injection geometries (one and two rows of holes) and six roughness patterns were used. The density of the injected gas was close to that of the free-stream air. The presence of roughness causes a decrease in the spanwise-averaged effectiveness for both injection geometries at low blowing rates, and an increase in the effectiveness for one-row injection at high blowing rates, but not for two-row injection. The results for two-row injection (with or without roughness) can be correlated by a parameter used to correlate slot-injection data. The influence of roughness pattern on the average effectiveness and on the lateral uniformity of the effectiveness is described.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):117-122. doi:10.1115/1.3239670.

The local film cooling effectiveness on a gas turbine blade with a row of discrete cooling jets has been measured using a mass transfer technique. Particular emphasis is placed on phenomena near the end wall of the blade. This region contains a horseshoe vortex system modified by a passage vortex. On the concave (pressure) surface the film cooling performance is not greatly altered by the presence of the end wall. On the convex surface of the blade the film cooling is essentially absent in a triangular region extending from near the region of peak curvature on the blade to its trailing edge. This unprotected region closely corresponds to the location of the passage vortex as indicated by flow visualization. The passage vortex sweeps away the injected coolant flow from the surface. Upstream of the unprotected area the injected flow is skewed toward the middle span of the blade. The influence of the end wall extends about one-half chord length up from the end wall in the present experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):123-126. doi:10.1115/1.3239671.

A methodology is developed for the inverse design and/or analysis of interior coolant flow passage shapes in internally cooled configurations with particular applications to turbine cascade blade design. The user of this technique may specify the temperature (or heat flux) distribution along the blade outer fixed surface shape and the unknown interior coolant/blade interface. The numerical solution of the outer gas flow field determines the remaining unspecified blade outer surface quantity—surface heat flux if temperature was originally specified or vice versa. Along the unknown coolant flow passage shape the designer has the freedom to specify the desired temperature distribution. The hollow blade wall thickness distribution is then found from the solution of Laplace’s equation governing the temperature field within the solid portion of the hollow blade, while satisfying both boundary conditions of temperature and heat flux at the fixed outer blade surface, and the specified temperature boundary condition on the evolving inner surface. A first order panel method, coupled with Newton’s N-dimensional interation scheme, is used for the iterative solution of the unknown coolant/blade interface shape. Results are shown for a simple eccentrical bore pipe cross section and a realistic turbine blade cross section. The inverse design procedure is shown to be efficient and stable for all configurations that have been tested.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):127-134. doi:10.1115/1.3239672.

The state of the boundary layers near the leading edge of a high-speed turbine blade has been investigated, in cascade, using an array of surface-mounted, constant-temperature, hot-film anemometers. The measurements are interpreted with the aid of inviscid and viscous prediction codes. The effects of Reynolds number, compressibility, incidence, and free-stream turbulence are described. In all cases, the initial development of the boundary layers was extremely complex and, even at design conditions, separation and reattachment, transition and relaminarization were found to occur.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):135-141. doi:10.1115/1.3239673.

To study the heat transfer in ceramic coatings applied to the heated side of internally cooled hot section components of the gas turbine engine, a mathematical model is developed for the thermal response of plasma-sprayed ZrO2 -Y2 O3 ceramic materials with a Ni-Cr-AL-Y bond coat on a Rene 41 rod substrate subject to thermal cycling. This multilayered cylinder with temperature dependent thermal properties is heated in a cross-flow by a high veloctiy flame and then cooled by ambient air. Due to high temperature and high velocity of the flame, both gas radiation and forced convection are taken into consideration. Furthermore, the local turbulent heat transfer coefficient is employed which varies with angular position as well as the surface temperature. The transient two-dimensional (heat transfer along axial direction is neglected) temperature distribution of the composite cylinder is determined numerically.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):142-146. doi:10.1115/1.3239674.

Acoustic emission techniques have recently been used in a number of studies to investigate the performance and failure behavior of plasma-sprayed thermal barrier coatings. Failure of the coating is a complex phenomena, especially when the composite nature of the coating is considered in the light of possible failure mechanisms. Thus it can be expected that both the metal and ceramic components (i.e., the bond coat and ceramic overlay) of a composite thermal protection system influence the macroscopic behavior and performance of the coating. The aim of the present work is to summarize the “state-of-the-art” in terms of this initial work and indicate where future progress may be made.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):147-151. doi:10.1115/1.3239675.

This study is concerned with the effect of reduced pressure on heat transfer to particulate matter injected into a thermal plasma. Reduced pressures in conjunction with the high temperature in a thermal plasma lead to a strong increase of the mean free path in the plasma, and therefore, the Knudsen effect plays an important role in this situation. Based on the heat-conduction-potential jump approach, calculated data are presented for small spherical particles, typical for applications in plasma processing (for example, plasma spraying) and in plasma chemistry. The Knudsen effect severely reduces the heat flux to a particle, even for larger particles (up to 100 μm) at reduced pressures. For small particles (<10 μm) this effect is even felt at atmospheric pressure.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):152-159. doi:10.1115/1.3239676.

A review is presented of various rotordynamic problems which have been encountered and eliminated in developing the current flight engines and of continuing subsynchronous problems which are being encountered in developing a 109 percent power level engine. The basic model for the HPOTP, including the structural dynamic model for the rotor and housing and component models for the liquid and gas seals, turbine-clearance excitation forces, and impeller-diffuser forces, are discussed. Results from a linear model are used to examine the synchronous response and stability characteristics of the HPOTP, examining bearing load and stability problems associated with the second critical speed. Various seal modifications are examined and shown to have favorable consequences with respect to bearing reactions and stability. Differences between linear and nonlinear model results are discussed and explained in terms of simple models. The transient nonlinear model is used to demonstrate forced subsynchronous motion similar to that observed in test data for models which are lightly damped but stable. The subsynchronous motion results from bearing clearance nonlinearities. Simulation results indicates that synchronous bearing loads can be reduced but that sub-synchronous motion is not eliminated by seal modifications.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):160-164. doi:10.1115/1.3239677.

A technique is described which provides early detection of rolling element bearing failure through direct observation of the bearing outer race with a high-gain eddy current probe. Data is presented that compares REBAM™—Rolling Element Bearing Activity Monitor—to traditional monitoring approaches that employ case mounted velocity and acceleration transducers. Generally, the REBAM approach simplifies the analysis of bearing condition because of its proximity to the bearing and its insensitivity to extraneous vibration sources due to the relative mounting approach employed. A summary of field results is presented and its suitability to high speed gas turbine monitoring is discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):165-169. doi:10.1115/1.3239678.

This paper discusses the experimental evaluation of performance of squeeze film dampers without centralizing retaining springs. Rotor amplitudes in the damper plane and damping coefficient have been considered to assess the system performance. Tests were conducted on two damper configurations that were to go in the rotor assembly of a certain gas turbine engine. Land width and film thickness were varied, and experiments were conducted for different values of excitation frequency, oil supply pressure, and unbalance. The test program showed that the parameters are to be individually optimized to obtain better overall performance in the damper systems without centralizing springs. The damper configurations subjected to severe dynamic conditions due to unbalance on overhung mass were also experimentally analyzed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):170-180. doi:10.1115/1.3239679.

An experimental investigation was conducted to identify an optimum oil-buffered shaft seal for use on centrifugal compressors, with the primary objective being minimal seal oil exposure to process gases that cause seal oil degradation or are toxic. Types of seals tested included smooth bore cylindrical bushings, spiral groove cylindrical bushings, radial outward-flow face seals, and radial inward-flow face seals. The influence of shaft speed, gas pressure, seal oil differential pressure, oil bypass flow rate, and oil supply temperature on process side seal oil flow rate was determined. The investigation revealed some surprising relationships between seal oil flow rates and the escape of process gas.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):181-186. doi:10.1115/1.3239680.

Aero-engine bearings operate in an extraordinarily high speed range (high DN number) and severe conditions. It is especially necessary to measure and adjust the bearing thrust load in the engine development phase, but it is very difficult to measure the thrust load accurately, because bearings and bearing housings are subjected to elevated temperature and oil environment. Open space permitted for installation of thrust measurement transducers is small and limited around the bearing housing. We tried to measure the thrust load by applying “Unit Cells,” which are installed between bearing and bearing housing. “Unit Cells” which have been specially designed to measure the bearing thrust load are very small and temperature-compensated load cells. We have been successful in measuring the actual thrust load using the above “Unit Cells,” both in the steady-state and transient condition. Repeatability and hysteresis of the data have been satisfactory. We have established the effect of seal clearance on the thrust load by the measurement. We also have obtained the dynamic characteristics of the thrust load versus rotor speed in low bypass fan engines. Procedure and obtained data are presented in detail.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):187-196. doi:10.1115/1.3239681.

The experimental portion of a joint government/industry/university research study on the vibrational characteristics of twisted cantilevered plates is presented. The overall purpose of the research study was to assess the capabilities and limitations of existing analytical methods in predicting the vibratory characteristics of twisted plates. Thirty cantilevered plates were precision machined at the Air Force’s Aero Propulsion Laboratory. These plates, having five different degrees of twist, two thicknesses, and three aspect ratios representative of turbine engine blade geometries, were tested for their vibration mode shapes and frequencies. The resulting nondimensional frequencies and selected mode shapes are presented as a function of plate tip twist. The trends of the plate natural frequencies as a function of the governing geometric parameters are discussed. The effect of support compliance on the plate natural frequency and its impact on numerically modeling twisted plates is also presented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):197-204. doi:10.1115/1.3239683.

A shock spectrum procedure is developed to estimate the peak displacement response of linear flexible rotor-bearing systems subjected to a step change in unbalance (i.e., a blade loss). A progressive and a retrograde response spectrum are established. These blade loss response spectra are expressed in a unique non-dimensional form and are functions of the modal damping ratio and the ratio of rotor spin speed to modal damped whirl speed. Modal decomposition using complex modes is utilized to make use of the unique feature of the spectra for the calculation of the peak blade loss displacement response of the rotor system. The procedure is applied to three example systems using several modal superposition strategies. The results of each are compared to true peak displacements obtained by a separate transient response program.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):205-211. doi:10.1115/1.3239684.

This paper summarizes the results of an investigation to establish the impact of mistuning on the performance and design of blade-to-blade friction dampers of the type used to control the resonant response of turbine blades in gas turbine engines. In addition, it discusses the importance of friction slip force variations on the dynamic response of shrouded fan blades.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):212-219. doi:10.1115/1.3239685.

In this paper, the transient thermal stress problem in a hollow cylinder or a disk containing a radial crack is considered. It is assumed that the cylinder is reinforced on its inner boundary by a membrane which has thermoelastic constants different than those of the base material. The transient temperature, thermal stresses, and the crack tip stress intensity factors are calculated in a cylinder which is subjected to a sudden change of temperature on the inside surface. The results are obtained for various dimensionless parameters and material constants. The special cases of the crack terminating at the cylinder-membrane interface and of the broken membrane are separately considered and some examples are given.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):220-224. doi:10.1115/1.3239686.

A study measuring the effects of a molten sulfate/chloride salt on the creep/fatigue behavior of a nickel base turbine blade superalloy, Udimet 720, at 1300°F (704°C) is described. Cyclic stress–cycles to failure (S-N) curves were generated at high mean stress levels, with mean stress, maximum stress, or the ratio of minimum to maximum stress (R ratio) held constant. In salt, it was found that when maximum stress is above the yield, with the cyclic component 20 percent of the maximum, failure occurs by stress corrosion fatigue in orders of magnitude less time than for corresponding loading conditions in air. It is significant, from a failure analysis point of view, that fatigue fracture is intergranular in these circumstances. Similar fatigue behavior may be expected for other nickel base alloys, however, at substantially lower maximum stresses in as much as Udimet 720 exhibits superior short time rupture strength, i.e., resistance to this form of stress corrosion, over the other blade alloys evaluated in this environment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):225-230. doi:10.1115/1.3239687.

An exact analysis is presented for the creep deformation of a curved bar subjected to the simultaneous actions of bending moment and axial force. Dimensionless interaction curves and charts, which relate the variables, axial force, location of neutral axis, maximum stress and strain rate, bending moment, and change in curvature rate, are also provided for engineering practice convenience. The constitutive relationship of the curved bar is described by the Prandtl-Nadai Creep Law.

Topics: Deformation , Creep , Force , Stress
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):231-237. doi:10.1115/1.3239688.

A simplified inelastic analysis computer program (ANSYMP) was developed for predicting the stress-strain history at the critical location of a thermomechanically cycled structure from an elastic solution. The program uses an iterative and incremental procedure to estimate the plastic strains from the material stress-strain properties and a plasticity hardening model. Creep effects can be calculated on the basis of stress relaxation at constant strain, creep at constant stress or a combination of stress relaxation and creep accumulation. The simplified method was exercised on a number of problems involving uniaxial and multiaxial loading, isothermal and nonisothermal conditions, dwell times at various points in the cycles, different materials, and kinematic hardening. Good agreement was found between these analytical results and nonlinear finite element solutions for these problems. The simplified analysis program used less than 1 percent of the CPU time required for a nonlinear finite element analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 1985;107(1):238-241. doi:10.1115/1.3239689.

Aluminum alloy RR 58 (AA 2618) has been viewed during exposure to cyclic loading, which is applied using a load frame inserted into a scanning electron microscope. Thus observation of surface interactions of the material microstructure with a propagating crack is feasible. Photomicrographs and (for the purposes of analysis and presentation) dynamic, real-time video-recordings used to document the processes will be displayed and the nature of the observations presented in relation to existing physical understanding of fatigue. Some additional ideas will be included based on the results presented herein.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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