A novel test facility has been designed and set up for the investigation of the influence of stationary temperature, and thus thermally induced stress gradients with respect to the damage evolution of cooled gas turbine components. Thermally induced stress gradients differ from geometrically induced stress gradients. From the point of view of stress mechanics, they are independent from external loads. From the perspective of material mechanics, their impact on service life is influenced by locally different material properties and strength. However, the impact of thermally induced stress gradients on the cyclic life of high loaded, cooled components is not precisely known. In order to increase knowledge surrounding these mechanisms, a research project was launched. To achieve high temperature gradients and extended mechanical stress gradients, large heat fluxes are required. The authors developed a test bench with a unique radiant heating to achieve very high heat fluxes of q̇ ≥ 1.6MW/m 2 on cylindrical specimen. Special emphasis has been placed on homogenous temperature and loading conditions in order to achieve valid test results comparable to standard low cycle or thermo-mechanical fatigue tests. Different test concepts of the literature were reviewed and the superior performance of the new test rig concept was demonstrated. The austenitic stainless steel 316L was chosen as the model material for commissioning and validation of the test facility. The investigation of thermally induced stress gradients and, based on this analysis, low-cycle fatigue tests with superimposed temperature gradients were conducted. Linear elastic finite element studies were performed to calculate the local stress-strain field and the service life of the test specimens. The test results show a considerable influence of the temperature gradient on the low-cycle fatigue life of the investigated material. Both the temperature variation over the specimen wall and thermally induced stresses are stated to be the main drivers for the change in low-cycle fatigue life. The test results increase the understanding of fatigue damage mechanisms under local unsteady conditions and can serve as a basis for improved lifetime calculation methods.

High velocity oxygen fuel (HVOF) sprayed stellite coating is often adopted on contact surfaces of the shrouds and stubs of titanium turbine blades, in order to increase fretting fatigue strength and reduce fretting wear. To confirm the effectiveness of the sprayed stellite coating, we conducted friction-type fretting tests originally developed to simulate the load condition of shrouds and stubs, vibratory force was carried only by friction force. We also investigated the fretting fatigue mechanism of coated materials by observing non-propagating cracks in the tested specimens and analyzed crack propagation behavior at the interface using fracture mechanics. The fretting fatigue tests confirmed that the sprayed stellite coating could double the fretting fatigue strength of Ti-6A1-4V. The fretting fatigue mechanism of Ti-6A1-4V with a sprayed stellite coating was revealed as follows: Final fracture was led by a newly initiated micro-crack at the interface as cracks initiated on the coating surface did not continuously propagate into the substrate but mainly propagated along the interface. The micro-crack propagation at the substrate depends on the length of interface crack. The longer interface crack delays micro-crack propagation to the substrate because the lines of force are less concentrated at the crack tip. Hence, the interface crack makes the fretting fatigue strength of coated Ti-6A1-4V higher than that of uncoated specimens. This finding was supported by the cross-sectional observations of non-propagating cracks in the tested specimens, and by FE analysis which revealed that Δ K of the substrate crack initiated at the interface decreases with an increase in delaminated crack length. Ti-6A1-4V with a sprayed stellite coating offers better wear resistance than uncoated Ti-6A1-4V because the coating’s friction coefficient is lower than that of Ti-6Al-4V, although the wear rates of coated and uncoated materials against the consumption energy per cycle were almost the same.

Room-temperature fatigue tests were conducted on Ti 834 with prior creep strains accumulated under constant load at 550°C and 600°C, respectively. Microstructural and fractographic examinations on specimens with prior creep strain > 3% revealed the failure process consisting of multiple surface crack nucleation and internal void generation by creep, followed by fatigue crack propagation in coalescence with the internally distributed damage, leading to the final fracture. The amount of prior creep damage increased with creep strain. The fatigue life of Ti 834 was significantly reduced by prior creep straining. The behavior is rationalized with the integrated creep-fatigue theory.

Extrusion additive manufacturing technologies may be utilized to fabricate complex geometry devices to control the flow distortions at the inlets of gas turbine engines. However, the success of these devices depends upon their ability to withstand the dynamic mechanical loads experienced in service. One such device is a StreamVane ® . A current candidate material for StreamVane fabrication is ULTEM 9085 — a thermoplastic polymer material. ULTEM 9085 was selected as the material of choice for StreamVane production because of its high strength-to-weight ratio. In this study, quasi-static and cyclic fatigue tests were performed on ULTEM 9085 samples fabricated by fused deposition modeling. First, ASTM D638 tensile-tensile material tests were conducted on three different build orientations to verify the mechanical strength of ULTEM 9085 listed by Stratasys. The three different orientations that were tested were Orientation XZZ, Orientation XYZ, and Orientation ZXY with measured ultimate tensile strengths of 69.6 MPa, 56.3 MPa, and 37.6 MPa, respectively. Fatigue properties were investigated utilizing the procedure outlined in ASTM D7791. S-N curves were generated using data collected at stress levels of 80%, 60%, 30% and 20% of the ultimate tensile stress with an R-ratio of 0.1 for the build orientation XZY. Next, the modified Goodman approach was used to estimate the fully reversed (R = −1) fatigue life. The initial data suggested that the modified Goodman approach was very conservative. Therefore, four different stress levels of 25%, 20%, 15% and 10% of ultimate tensile stress were used to characterize the fully reversed fatigue properties. Because of the extreme conservatism of the modified Goodman model for this material, a simple phenomenological model was developed to estimate the fatigue life of ULTEM 9085 subjected to fatigue at different R-ratios. The calculated fatigue life using the model and the data collected for the fully reversed test correlated very well. The data from this investigation demonstrated that StreamVane designs using ULTEM 9085 are more than capable of withstanding the static and dynamic loads in currently planned turbomachinery applications.

Described within this paper are the results derived from an investigation into the root cause of cracks which are prematurely forming within the aft end rotor rim attachments of large combustion turbine compressors. The overall objective for this program was to use these results to develop a means for projecting the remaining service life of similarly affected rotors based on non-destructive measurements which identified the degree of fatigue damage sustained in the critical disc rim fillets. Projected rotor retirement was identified by systematically calculating the change in stress intensity for a crack as it propagated from a corner of the attachment fillet through the disc rim. Risk of catastrophic failure was identified when the stress intensity along the front of a given crack size and depth exceeded the fracture toughness of the rotor material, risking failure. Estimates of both crack initiation and propagation relied on properties derived from fatigue tests of specimens obtained from a retired rotor and stress results from a transient thermal analysis of the stages. When compared, projections of crack growth correlated well against field measurements of damaged rims.

An energy based fatigue damage and lifing assessment method is developed for a high temperature material, Inconel 625, and Aluminum 6061-T6. A newly developed experimental method is used for interrogating accumulated fatigue damage and evolution for low and high cycle fatigue (LCF/HCF) at continuum scales. The proposed fatigue lifing assessment method is based on assessing the total strain energy dissipated to cause fatigue failure of a material, known as the fatigue toughness. From the fatigue toughness and experimentally determined fatigue lives at two different stress amplitudes, the cyclic parameters of the Ramberg-Osgood constitutive equation that describes the hysteresis stress-strain loop of a cycle are determined. Stress controlled mechanical fatigue tests are performed to construct room temperature stress-life (S-N) curves and to determine damage progression based on accumulated fatigue damage. The predicted fatigue life obtained from the present energy based approach is found in good agreement with experimental data.

The combined high and low cycle fatigue (CCF) test on full scale turbine blade in the laboratory is an important method to evaluate the life. In fact, the low cycle fatigue which is usually caused by the centrifugal force can be confirmed easily. While, the high cycle fatigue which is usually caused by the vibration and aerodynamic force is often hard to determine. So the previous scholar has proposed the contrast method to determine the high cycle load in the field. This method utilizes the new and used blades to determine the high cycle within certain limits. While it can’t be applied effectively in the whole life range with the low cycle-high cycle-ultra high cycle fatigue theory raised. So this paper put forward the modified contrast method to realize the optimization. Firstly, the CCF tests are carried out on the turbine blade systematically. Then, the CCF damage properties, including the crack propagation, the fracture morphology and the dynamic characteristic are analyzed. Lastly, the new modified contrast method is proposed with the new coordinate axes, new fitting criterions and amend method. Through comparisons we conclude that: the new method is slightly complicated, but the evaluate precision has significantly increased. So it could be used to deal with data for CCF tests on full scale turbine blade in the future.

Slender turbine blades are susceptible to excitation. Resulting vibrations stress the blade’s fixture to the rotor or stator. In this paper, high cycle fatigue at the edge of contact between blade and rotor/stator of such fixtures is investigated both experimentally and numerically. Plasticity in the contact zone and its effects on e.g. contact tractions, fatigue determinative quantities and fatigue itself are shown to be of considerable relevance. The accuracy of the finite element analysis is demonstrated by comparing the predicted utilizations and slip region widths with data gained from tests. For the evaluation of edge of contact fatigue, tests on simple notched specimens provide the limit data. Predictions on the utilization are made for the edge of contact of a dovetail set-up. Tests with this set-up provide the experimental fatigue limit to be compared to. The comparisons carried out show a good agreement between the experimental results and the plasticity-based calculations of the demonstrated approach.

Thermal degradation of precipitation-hardened nickel based superalloys has been demonstrated to be reversible through full solution rejuvenation heat treatment processing. The specific concern with full solution rejuvenation heat treatment of single crystal alloys is the formation of recrystallized grains on surfaces with residual stress. The threshold temperature for recrystallization and the effect of heat treatment temperature and time on recrystallization depth were evaluated on service run industrial gas turbine blades comprised of nickel based single crystal alloys René N5 and RR2000. Recrystallization of rejuvenated blades was observed on the root surfaces of blades which had been shot peened at original manufacture and/or during a prior repair. Blades which did not receive peening at manufacture were free of recrystallization in critical areas following full solution rejuvenation heat treatment. Given that gas turbine blade roots operate at relatively low temperatures compared to the airfoil, creep is not considered a life limiting damage mechanism for this region of the blade. Rather, high cycle fatigue is considered the primary damage mechanism of concern. As such, fatigue testing of shot peened and heat treated (recrystallized) René N5 specimens was carried out at 650°C at various stress levels in comparison with baseline (non-recrystallized) specimens to determine the extent to which recrystallization would limit fatigue endurance at blade root operating conditions. It was found that recrystallization did not reduce the fatigue endurance relative to baseline samples at the tested conditions. The findings indicate that repair including full solution rejuvenation heat treatment of previously peened blades comprised of René N5 alloy is feasible provided that recrystallization be limited to root surfaces.

Working in a harsh environment with high temperature gas and high rotation speed, hollow turbine blades of gas turbine engine commonly endure both creep damage and low cycle fatigue damage at the same time. It is difficult to predict the life of hollow turbine blades accurately because of a strong coupling effect between creep and low cycle fatigue (creep/fatigue) damage. To address this difficulty, one effective way is to carry out creep/fatigue tests on full scale hollow turbine blades in a bench environment. The present creep/fatigue test methods on full scale hollow turbine blades usually do not consider the temperature gradient between the wall of cooling hole and blade surface. It does not matches the actual working circumstance of hollow turbine blades, under which the temperature gradient at the blade body can reached 150°C or even more higher. This investigation proposes a new experimental setup of creep/fatigue tests on full scale hollow turbine blades, in which real hollow turbine blades are heated by the eddy current induction heating furnace and cooled by cooling air which goes through the hollow blade’s cooling hole. During the tests, the temperature gradient between the wall of cooling holes and blade surfaces were controlled by adjusting the power of eddy current induction heating furnace and the flux of cool-air. Several thermocouples are employed to measure and validate the temperature gradient at the key/critical section, among them three are embed inside the wall of cooling hole by cutting the hollow turbine blade along cooling hole into three parts, and two are glued on the blade’s surface. Tests results show that, when the eddy current induction heating furnace is working stably with an output power of 6.4 Kw and meanwhile the flux of cooling air which goes through the hollow turbine blade’s cooling hole reaches 10 liter per minute, the temperature gradient of the hollow turbine blade at the key/critical section can be well simulated in a bench environment. Eight full scale hollow turbine blades have been tested at four different stress levels in this investigation. The test data are processed to obtain the field life based on equivalent damage principle. The field life data of the hollow turbine blades fits well with that of their flight experience.

Fatigue testing for an important turbine rotor material (X12CrMoWVNbN10-1-1 steel) was carried out over a wide range of strain and stress amplitudes at 873K. Particular attention was paid to the effect of control mode on the cyclic deformation behavior and life assessment at elevated temperature. Two main domains were observed depending both on the strain and stress amplitudes, where the effect of control mode was different. In the micro plastic deformation domain, the cyclic softening is slight and there is no clear difference in fatigue behavior between the stress and strain modes. In the plastic damage regime, stress cycling causes more significant softening or damage than strain cycling. The dependence of damage behavior on the evolution of dislocation substructure was focused. On the other hand, it is not possible to use strain based life model to predict fatigue life with the test results under a different control mode. A unified energy-based model is proposed based upon the deformation mechanism and the experimental results, which can assess the low cycle fatigue life with different control modes. The results obtained in this study could have significant implications in the design of structures.

An investigation of cycling rate effects on fatigue life behavior is being conducted on Aluminum (Al) alloys. This effect, along with specimen diameter, highlights the major difference between ultrasonic and servohydraulic fatigue test procedures. Ultrasonic fatigue testing is conducted on a 3.2 mm diameter hourglass specimen operating at 26 kHz, and servohydraulic axial testing is conducted on an ASTM E 466-07 standard dogbone specimen with a 25.4 mm gage length and 4.45 mm diameter operating at 35 Hz. Previous works have shown that cycling rate increases of 100–1000 times can reduce the fatigue crack growth rates in the stress intensity region between threshold and critical by at least an order of magnitude for aluminum and steel. For high cycle fatigue (HCF), however, where the majority of cycles to failure are accumulated before crack initiation, not during propagation, the effects of cycling rate on total loading cycles to failure needs further understanding, which may lead to more accurate and/or less conservative HCF design of critical gas turbine engine components. Fatigue behavior of Al 6061-T6 was assessed between the failure range of 10 4 –10 10 cycles using ultrasonic and servohydraulic testing procedures. Though aluminum is not a widely used alloy in gas turbine engine applications, understanding frequency-based fatigue life discrepancies associated with the choice of empirical methods is of paramount importance to component design; Aluminum 6061-T6 is the most cost-effective way to gaining this understanding. Comparisons were made between the fatigue behavior results using cycling rate (also stated as strain-rate or frequency) comparisons for fatigue crack growth study. The comparisons show promising results correlating the fatigue behavior trends of servhydraulic and ultrasonic fatigue data.

In this investigation, we have applied an integrated experimental-analytical framework for fatigue life assessment and prediction of a thick-wall component of a high-pressure (HP) steam turbine. Emphasis is placed on the development of an effective experimental and analytical procedure for life characterization on the basis of low cycle and high cycle fatigue (LCF/HCF) in order to improve the safety, reliability, and affordability of real world steam turbine operations. Stress-control constant amplitude fully reversed fatigue tests were performed in room temperature and 500°C to serve two purposes: (a) to obtain experimental stress-life (S-N) curves and (b) to assess the values of the parameters of the energy-based framework to predict the fatigue life. The experimental and the predicted S-N curves are compared with each other in case of both the room and the elevated temperature to examine the soundness of the present energy-based model to predict fatigue life. The present lifing model was found to be able to predict both the room and elevated temperature LCF/HCF life of the thick-wall component with excellent accuracy. Furthermore, the elevated temperature fatigue life is found to be lower than the room temperature fatigue life due to the lower fatigue toughness at elevated temperature.

Electrical resistance (ER) is a relatively new approach for real-time monitoring and evaluating damage in SiC/SiC composites for a variety of loading conditions. In this study, ER of woven silicon carbide fiber-reinforced silicon carbide composite systems in their pristine and impacted state were measured under cyclic loading conditions at room and high temperature (1200C). In addition, modal acoustic emission (AE) was also monitored, which can reveal the occasion of matrix cracks and fiber. ER measurement and AE technique are shown in this study to be useful methods to monitor damage and indicate the failure under cyclic loading. Based on the slope of the ER evolution, an initial attempt has been made to develop a method allowing a critical damage phase to be identified. While the physical meaning of the critical point is not yet clear, it has the potential to allow the failure to be indicated at its early stage.

During an exercise to determine the optimum conditions for metal removal and tool wear, three ultra high strength steels investigated were 18% Ni maraging steel, a Ni-Cr-Mo steel (designated REX 685), and a 5% Cr-Mo-V steel (designated H50). These were machined in the fully heat treated condition (120 tsi U.T.S.). Metallurgical etch inspection techniques were used to assess the metallurgical surface conditions created by the machining operations, which were turning, milling, drilling, reaming and grinding. Stress corrosion and fatigue tests were carried out on specimens machined under optimum and sub-optimum conditions, “sub-optimum” being used to describe machining under conditions outside those recommended but which can develop during normal workday practice.

An overview of the disk life prediction and verification program, which includes very special disk fatigue test equipment called the Ferris Wheel, is described. This rig has been designed to simulate the effect of centrifugal force of blades on jet engine disk and is capable of doing LCF test in a very short time; (the period required for a Ferris Wheel test is one tenth that of a cyclic spinning test). This feature enables us to evaluate actual disk LCF life statistically and improve the reliability of disk LCF life predicted. An example problem is also included and the procedure of the evaluation of the disk life described in detail.

During transient operation, gas turbine blades are subjected to cyclic thermal stresses. For the first-stage blade of a Brown Boveri gas turbine, time-dependent temperature and stress distributions are computed by finite difference method to determine maximum total strain and to predict blade life failure. These calculations together with similarity conditions result in allowed temperature range fluctuations as a function of cycle time for a particular damage. The influence of important parameters like Biot and Fourier number was determined. Thermal fatigue tests of these blades in fluidized beds were carried out to investigate their resistance and to ascertain the usefulness of the computation technique. The predicted number of cycles to failure agreed quite well to those found in fluidized bed.

In this paper, a new accelerated fatigue test (AFT) method based on Miner’s Law is advanced, which is used for vibratory fatigue test of aero-engine compressor blading. By using this method, a sample life distribution can be obtained and it saves time about 50–60%. This approach is validated by experiments and the test data are compared by two statistical methods — namely, F-test and t-test.

This paper describes a new balancing concept: S pray A utomated B alancing O f R otors (SABOR). The F uel A ir R epetitive E xplosion (FARE) process of metal film deposition is used to apply discrete amounts of metallic or ceramic particles to a spinning rotor at a controlled angular location on the rotor. In this way, without stopping the rotor, its mass eccentricity is changed in a manner designed to reduce the vibration sensed by a balancing machine. The system design and control for implementing SABOR are described. Preliminary results are presented from a demonstration rig which confirm the ability of the method to control both angle of application and rate of application to the extent necessary for effective automated balancing. The method is used to balance a spinning disk without stopping it. Results are presented which show that the bond strengths of materials deposited by the FARE process for use in small gas turbine engines are acceptable for the level of centrifugal stresses expected. Fatigue tests of sprayed samples show the data to be indistinguishable from that available for specimens hand-ground according to current balancing practice.

The microstructure and fracture surfaces of two commercially available silicon nitride ceramics have been characterized using techniques of scanning and analytical transmission electron microscopy. These results have been correlated to mechanical property data (obtained from static and dynamic fatigue tests using four-point flexural test methods both at room temperature and at elevated temperature). For one of the materials, the results clarified failure mechanisms. For the other, the results showed how processing variables can affect microstructure (and thus mechanical behavior), and emphasized particularly that mechanical tests of billet-derived specimens may be unreliable in predicting the mechanical behavior of production components.