Ferris Wheel Testing Low Cycle K. Jimboh Fatigue Test of Gas Turbine Manager. Engine Discs

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1 `L`/f r Ya MEC` a^cf i 80-GT-1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345E 47 St. New York N.Y 017 ^] `I 3- t f^^_ l F! tl Pllf f r C''1., I _ ^I:'; Or car,]t i i t 1L.I ^^ n i,i, ^ ail I", `u ^.1^ ^I'^:= Ir t I ^ 7 f -;,^I' Copyright 1980 by ASME Ferris Wheel Testing Low Cycle K. Jimboh Fatigue Test of Gas Turbine Manager Engine Discs H. Aono Section Manager T. Kawashima Engineer, IHI Co., Ltd., Tokyo, Japan 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. ABSTRACT An overview of the disc life prediction and verification programme which includes very special disc 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 disc 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 disc LCF life statistically and improve the reliability of disc LCF life predicted. An example problem is also included and the procedure of the evaluation of the disc life described in detail. INTRODUCTION It is imperative aero-gas turbine engine designers evaluate the low cycle fatigue (LCF) lives of turbine and compressor discs. Various methods of assessing disc life have been proposed and discussed, but no fatigue life evaluation method is reliable enough for practical usage. At the design stage, therefore, conventional methods, which make use of strain (stress) - fatigue life characteristic (S-N curve) and modified Goodman Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the Gas Turbine Conference & Products Show, New Orleans, Louisiana, March -13, Manuscript received at ASME Headquarters November 5, Copies will be avialable until December 1, diagram, are sometimes applied. The accuracy of fatigue life evaluation is not satisfactory unless the S-N curve has been obtained from test specimens or cyclically spun actual discs which have stress distributions similar to those on the design being evaluated. But, it is almost impossible to get such S-N curves. Therefore, generally, material LCF data on uniaxial specimens are corrected by considering such factors as a notch effect, a loading form and a temperature effect, and used to estimate fatigue lives. Low cycle fatigue lives are strongly influenced by not only local stresses but also stress and thermal distribution. Heat treatment, method of manufacture and surface finish are all factors which are known to influence LCF life, too. Therefore, the fatigue life which is estimated from S-N curves of specimens do not necessarily predict the actual disk life accurately. This is the reason why LCF tests of actual components with the objective of achiveing full validation for design purpose are needed. Fatigue is a highly statistical phenomenon exhibiting scatter on life for a given stress. Scatter defines the reliability of the life and the limiting stress. The effect of extraneous scatter leads to unnecessarily conservative design. Therefore, the amount of data required for statistical analysis is needed to ensure a good estimation of probabilities of failure. LCF data of actual discs must be treated statistically, too. There are several LCF test methods for actual discs. The methods that can provide good simulation are an engine cyclic test and a cyclic spin test. They have disadvantages as they are very expensive,

2 time-consuming and so on. Therefore, they are not suitable to provide sufficient data for statistical analysis in a short time. To overcome these disadvangates, IHI installed a Ferris Wheel Multi-Axial Low Cycle Fatigue Test Rig. Ferris Wheel Test Rig is a disc fatigue test equipment designed to simulate the effects of centrifugal force of blades on jet engine disc. This rig makes it possible to present much more data than a spin test rig can offer. In addition, the disc is not rotating, and the initiation and propagation of fatigue cracks can be observed easily. These features permit accurate evaluation of the disc fatigue life. In this paper, the outline of a disc life verification program in which Ferris Wheel Test Rig is included and the general view of the rig are described. The typical example of disc life evaluation is also presented and discussed. DISC DESIGN AND LIFE EVALUATION PROGRAM Figure 1 illustrates a flow diagram outlining the definition of a disk life evaluation program. Disc stress analysis combines the temperature distribution with the centrifugal loading by means of computer programs. For the calculation of nominal stress, an axisymmetric isoparametric FEM computer program, developed at IHI, is employed. In addition to calculation of nominal stress, bolt holes and blade attachment slots are analyzed for stress concentration utilizing a two dimensional isoparametric FEM computer program which was also developed at IHI. Considering not only the results of stress analysis but also the mission cycle, the minimum service life is estimated based on the material fatigue data (mean value - 3e - ). Blade attachment slots generally have stagger angles. These angles cause the slot-wise stress distribution. The basic study for the magnitude of local stress concentrations associated with such details as blade attachment slots is done utilizing the three dimensional photo-elastic method and other experimental techniques. The results of basic study are used to increase the accuracy of disc stress analysis and fatigue life estimation. Manufactured discs are assembled and experimental engine endurance tests are started. But engine tests are preceded by rig tests to confirm the accuracy of disc design. Ferris Wheel test is arranged as follows; (1) Detailed stress analysis and definition of test condition (external load and disc temperature distribution). (2) Collecting sufficient amount of disc life data for statistical analysis. (3) Statistical analysis of fatigue data which includes the information of crack initiation life and crack propagation. In applying statistics to test data, a basic Weibull analysis is used. The scatter of fatigue life is clearly I L ENGINE SPECIFICATION DISC DESIGN I MISSION CYCLE THERMAL STRESS DISC STRESS EVALUATION ANALYSIS ANALYSIS LCF LIFE, DESIGN I MATERIAL Ti PROPERTIES I FULL SIZE DISC TEST" ^ STRESS ANALYSIS CYCLIC SPIN FERRIS WHEEL TEST TEST DISC LIFE VALIDATION STATISTICAL ANALYSIS LI FUNDAMENTAL INVESTIGATION ENGINE TEST Fig. I Disc Life Design & Verification Programme presented by this analysis. Cyclic Spin Tester is used along with Ferris Wheel Test Rig. The time required for a cyclic spin test is ten times more than that of a Ferris Wheel test. For this reason, the amount of data obtained from a cyclic spin test is very little in comparison with that of a Ferris Wheel test which is done in the same period. The results of cyclic spin tests are, therefore, used to confirm the validity of the test conditions and the results of Ferris Wheel tests. The retired discs are also cycled in the full size component test program to determine the excess life. The fatigue life evaluated i,n the full size component test program is compaired with the design life and that comparison enables us to improves the prediction and component test method. FERRIS WHEEL TEST RIG THE MULTI-AXIAL TEST RIG This rig is mainly based on the achievement of Mr. S. J. Leshenski of Avco Lycomming. (1), (2) Figure 2, 3, 4 show the general view of Feffis Wheel Test Rig. The capabilities of the rig are tabulated in Table 1. The rig is based on the closedloop electro-servo hydraulic control principle. Hydraulic cylinders are evenly spaced around the load frame. These cylinders are ganged in four I 2

3 V Fig. 2 Ferris Wheel Test Rig (Load Frame & Controller) Fig. 3 Ferris Wheel Test Rig (Load Frame, rear view) Table - 1 SPECIFICATION U PHYSICAL Performance ; DA 1 mm at 3 Hz Actuator displacement ; 50 mm DA Load Cell rating; 11, 360 kg dynamic LVDT stroke length; mm DA FRAME Disc diameter ; mm to mm Actuator adjustment for radial load ; + 3' Specimen pitch distance ; min mm Fig.4 Ferris Wheel Test Rig (Heat Coil & Specimen) approximately equal group. Each group has a load cell and a displacement feedback tranceducer. The displacement feedback tranceducer detects the deflection of disc position. Each group has a load limiter, too. The load limiter accepts the signal of load cell and detects the maximum tensile load and compressive force to prevent them from being applied to the disc. The signals detected by load cells and displacement feedback tranceducers are supplied to the control system. The control system always keeps the disc in the center of the load frame and controls the uniformity of the load. For more accurate evaluation of disc life, test load program must be designed so as to accumulate LCF damage which simulates that encontered during the flight duty cycle. To meet the demand, a function generator which is controlled by a microcomputer is being installed. LOAD ACCURACIES Drift ; Less than % of load setting Load readout ; + 0.1% of Range Load control; +0.3% of Range CONTROL ACCURACIES Disc centering ; +1 mm from 0 to full load Disc strain sensing ; mm Failsafe control ; +0.5% of full scale The strength of materials decreases in high temperature condition. Ferris Wheel Test Rig employs a high frequency induction-heating system to heat discs during test. This heating system has the capability of controlling temperatures in excess of 1200 C. At the first stage of the fatigue test, it is necessary to modify the heating condition to get the desired temperature distribution. This preliminary test requires extensive thermocoupling including several thermocouples located radially along the disc profile. Using the heat coil and air cooling system which is specially designed for each disk profile, temperature variation around the disc can be within the 3

4 range of +5 C. The multi point recorder can record the temperature up to 24 points. The digital thermometer always displays the temperatures of test specimen. EXAMPLE OF DISC FATIGUE LIFE EVALUATION In this section, a typical example of disc life evaluation is described. It was planed to make the disc with forged titanium alloy. This is a near of alloy of chemical composition 6A1-5Zr-O.5Mo-O.25Si. Two different processes from forging the material to surface finish were proposed for manufacture of the disc. To know which process is superior to the other, it was necessary to statistically evaluate the lives of the discs which are manufactured by each process. At this time, a Ferris Wheel Test Rig became available and life evaluation system was completed. It was decided to evaluate the disc life by full size disc test procedure in the programme. Analysis Definition of life. Before the fatigue test starts, it is essential that the term of "life'' be defined. In this test, blade attatchment slot sections (dovetails) were thought to be most critical. Therefore, the life of this disc was defined as the number of cycles to a 0. 8 mm long crack on the dovetail side-face to a probability of 1: 00. (3) Stress analysis. In the case of the stress distribution analysis of actual disc, the centrifugal loading was analyzed combined with the thermal stress. The thermal stress was calculated with actual disc thermal distribution measured during the engine endurance test. Stress distribution of the disc in Ferris Wheel testing was determined including thermal stress so as to estimate that of the disc in the operating engine. Figure 5 gives the stress distribution analyzed by two dimensional FEM computer programme. In figure 5, the solid line shows the equal stress level of actual disk and the dotted line shows that of the test disc. The stress distribution was gained without recontouring the disc profile in this case. But, if the critical sections are located near the bore or the bolt holes, it is rather difficult to obtain a wellsimulated stress distribution under static condition and need to recontouring the disc geometry. For the evaluation of disc fatigue life, as described before, it is impossible to ignore the information of a slot-wise stress distribution. This disc had a i5 stagger angle. Figure 6 shows the relative -,stress distribution along the slot obtained by three dimensional photelastic method. This result indicates that the maximum stress is about 1. 5 times as high as the nominal value. Therefore, the fatigue life of this disc had to be evaluated by considering this slot-wise stress concentration combined with nominal stress calculated by two dimensional FEM programme ` ^ 0 1 A Fig.6 Example of Slot -wise Stress Distribution P max A 0.a P max \ Z 0 i 1. 5 j ^\\ ' ^ I.0 Engine condition _ I 2%N LGF test condition / I ^ ^ I =3.7 MP max a Z.50 ^1^^\ 1. \1.0 Fig. 5 Example of Calculated Stress Distribution (Slot-wise average) o 3-D PHOTOELASTIC DATA TIME (SECOND) Fig. 7 Load Cycle Pattern 4

5 Dummy blade. Dummy blades, which connect actuators and disc, were made of same material as the actual blade. All specifications required for the manufacture of actual blade (heat-treatment, surfacefinish, shot-peening, etc.) were adapted. Fatigue test Load cycle pattern. Figure 7 shows the load cycle pattern. This load cycle had been used to get the S-N curve of specimens, which was used to estimate the disc life. This made it possible to compare the disc life with test piece life directly. The maximum load was determined so as to obtain the same stress distribution in the dovetail region under static test condition as occured in spin test. Thermal distribution. It was necessary to reproduce the thermal distribution, which had been used to analyze the test condition, on the disc. Adjusting the distance between the disc and the heat coil and using air-cooling system, temperature variation around the disc were within the range of +5'C. In this preliminary test, more than twenty four thermocouples were used to monitor the disc temperature. Four thermocouples were used to monitor and supply feedback signals to the controller during the fatigue test. Inspection of crack initiation and propaeation The fatigue test was interrupted every 500 cycles for visual inspection of the disc and dummy blades. Dye penetrant and the measure-scope check were carried out to detect cracks. The appearance of cracks was illustrated and photographically recorded. Visual observation of disc cracks has been found to be tedious and therefore automatic detection system utilizing Acoustic Emission is currently being investigated. Cyclic spinning test. In parallel with the fatigue test by Ferris Wheel, cyclic spinning test was performed to verify the Ferris Wheel test condition. This disc was manufactured using the same material batch as the discs for Ferris Wheel testing. This disc was cycled between 3, 000 RPM and 18, 000 RPM. The disc temperature was almost constant becouse temperature gradient simulation was almost impossible The result and discussion Data Comparison between Spin Test and Ferris Wheel Test. In cyclic spin test, time per cycle is very long. In addition, considerable time is required to disassemble and assemble before and after inspection. Ten specimens completed the Ferris Wheel Test and only one specimen completed the cyclic spin test. The results of fatigue test are presented in figure 8 in the form of S-N curve. In this figure, the fatigue lives of Ferris Wheel tested discs present the number of cycles to a same crack length as the maximum EO000m o 6ST ANDARD SPECI EN A T1q^ 30 O PROCESS A (FERRIS WHEEL) 5 o PROCESS A (CYCLE SPIN) o PROCESS B (FERRIS WHEEL) 5 20 LIFE TO FAILURE X 3 CYCLE Fig.8 Observed LCF Lives of Discs Fig.9 Dovetail Crack (Cyclic Spin Test) one which was found in the inspection of spin tested disc. Figure 9 presents the fatigue crack of spin test. The fatigue life of spin tested disc is in the scatter range of Ferris Wheel tested discs manufactured by the same process. This means that Ferris Wheel testing condition simulated the effect of rotation. Figure 8 shows that a change in manufacturing process will be effective. But, the life of crack initiation must be understood statistically. Because crack initiation and crack propagation are quite different phenomena. Estimation of fatigue crack initiation. The discs were inspected every 500 cycles, and each crack length was measured and recorded. The number of cycles to a 0.8 mm long crack was estimated by interpolating or exterpolating the crack length.

6 Observation of crack propag ation. Fatigue cracks sometimes close when the applied load is removed. During the crack observation, about a half of cyclic load was applied to the disc and made the crack open. The applied load made it easy to observe the crack. The cracks were drawn on the figures and some of them were photographically recorded. Figure (a) to (c) shows examples of crack growth. Figure 11 presents the actual run, based on these figures and photographs, of some cracks and the development along the presumed direction. Cracks are initiated at almost same heights in the blend area between the side wall and base of the slot. It is likely that most of them will propagate toward the periphery. The results of detailed observation of crack propagation will be used to refine the new design software which includes fracture mechanics and is in the stage of basic research now. Analysis and discussion. The distribution of dovetail fatigue life was regarded as to be approximated by a Welbull distribution. The number of cycles to a 0.8 mm long crack to a probability of 1: 1, 000 was read off the graphs and this number of cycles was assumed to be the disc life in this test. Figure 12 gives a typical example of the probability of dovetail LCF life to a 0.8 mm crack and presents the definition of the disc life. Each disc has 49 dovetails. At 20, 000 cycles the test was terminated and 20 dovetails survived in the case of this disc. Statistical analysis of test data which include suspended data was performed. The disc lives were also plotted on a Weibull paper. In figure 13 the result is presented. The test points form smooth line. This means that the estimated life derived from this graph does not have noticeable error. And it was concluded that the discs should be made by means of process B. SUMMARY This paper has presented an overview of the disc life evaluation program which includes very special disc fatigue test rig called Ferris Wheel Test Rig, and a typical example of disc life verification by the program has also be described. By installing Ferris Wheel Test Rig, it becomes possible to do disc LCF test in very short period and (a), 000 cycles (b) 12, 000 cycles (c) 14, 000 cycles Fig. Observed Crack Propagation (Ferris Wheel Test) E + H HM ^ 1 0 C4 a 0. PEAK RADIAL STRESS POSITION MEASURED CRACK RUN PRESUMED CRACK RUN Fig.11 Example of Crack Run X3 CYCLES TO 0. 8mm CRACK Fig. 12 Dovetail Lives to 0. 8mm Long Crack 6

7 90 50 N o.6, June 1974, pp Leshienski, S. J., "Low Cycle Fatigue Testing of Turbine Engine Discs, " Closed loop, Spring 1973, pp. 3-8, MTS System Corporation 3 Sattar, S.A., Sundt, C.V., "Gas Turbine Engine Disc Cyclic Life Prediction," J.Aircraft, Vol. 12, No.4, April 1975, pp H Hl ON1 a X 3 DISC LIFE (CYCLE) Fig.13 Low Cycle Fatigue Life Distribution increase the amount of. data. This advantage enables us to evaluate LCF life statistically and improves the reliability of LCF life prediction. The accuracy of the Ferris Wheel test was confirmed by comparing the data with those of cyclic spin test. By performing the fatigue test with the disc not rotating, the initiation and propagation of fatigue cracks was easily observed. The detailed information of crack propagation is now used to calibrate the new disc design system in which the fracture mechanics analysis is used for predicting crack growth in disc. ACKNOWLEDGEMENTS The authors mention especially here that this Ferris Wheel Disc Fatigue Rig was mainly based on the achievement of Mr. S. J. Leshenski of Avco Lyc oming. They gratefully acknowledge the personnel of MTS System Corporation and MTS (Japan) who manufactured this test rig and the personnel of JEOL who developed the induction-heating system. They are also grateful to Mr. T. Yokomitsu for his works on the experiments. REFERENCES 1 Leshienski, S. J., "Multi-axial Low-cyclefatigue Test Rig, " Experimental Mechanics, Vol. 14 7