Astm stp 495 1971

METAL FATIGUE DAMAGE MECHANISM, DETECTION, AVOIDANCE, AND REPAIR With Special Reference to Gas Turbine Components S S Manson, editor ASTM SPECIAL TECHNICAL PUBLICATION 495 List price $21.00 04-495000-30 ~l~ AMERICAN SOCIETY FOR TESTING AND MATERIALS 1916 Race Street, Philadelphia, Pa 19103 by American Society for Testing and Materials 1971 Library of Congress Catalog Card Number: 70-158437 NOTE The Society is not responsible, as a body, for the statements and opinions advanced in this publication Printed in Baltimore, Md September 1971 Foreword The work presented in the publication Metal Fatigue Damage-Mechanism, Detection, Avoidance, and Repair was sponsored by the ASTM-ASME Joint Committee on the Effect of Temperature on the Properties of Metals and financed by The Metal Properties Council The Applied Research Panel under the Chairmanship of M Semchyshen and the Gas Turbine Panel Chaired by G J Wile, cooperated in this project Mr S S Manson, who headed the Task Group under the Applied Research Panel, was responsible for the coordination of the papers presented in this publication Related ASTM Publications Fatigue at High Temperature, STP459 (1969), $11.25 Effects of Environment and Complex Load History on Fatigue Life, STP 462 (1970), $22.00 Manual on Low Cycle Fatigue Testing, STP 465 (1969), $12.50 Achievement of High Fatigue Resistance in Metals and Alloys, STP 467 (1970), $28.75 Contents Introduction Fatigue Mechanisms in the Sub-Creep Range-J C Grosskreutz Mechanisms of Fatigue in the Creep Range-C H Wells, C P Sullivan, and M Gell Fatigue Damage Detection-J R Barton and F N Kusenberger 61 123 Field Practices in the Repair of Fatigue Damaged Jet Engine ComponentsH G Popp, L G Wilbers, and K J Erdeman 228 Avoidance, Control, and Repair of Fatigue Damage-S S Manson 254 Copyright by ASTM Int'l (all rights reserved); Mon Dec 13:17:27 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproduction STP495-EB/Sep 1971 Introduction Fatigue is one of the most common of the failure mechanisms of gas turbine engine components Both the cool parts and hot parts are susceptible to this mode of failure, and the origin may be either mechanical or thermal The importance of fatigue as a limiting factor in jet engine reliability was brought out clearly in a study conducted some time ago by the NACA (the forerunner of the NASA) This study was made in the middle 1950's (Ref 1) on military jet engines, using statistics obtained from Air Force overhaul bases as a framework for the analysis All the major components studied-bearings, compressor blades, combustors, turbine nozzle vanes and buckets, and turbine disks were, to a serious extent, life limited, because of fatigue problems Some of the problems encountered were due to the high performance demanded of the engines by the military requirements; others were due to the fact that the jet engine was then in its early formative years and experience was limited However, the major reason for the prevalence of the fatigue problem relates to the very nature of the service to which jet engine components are subjected High fluctuating loads, high temperatures and temperature gradients, frequent starts and stops, stress concentrations resulting from complex geometrical shapes and from surface discontinuities produced by service conditions-all contribute to making the components fatigue-prone Add to this the fact that light weight and high performance provide the strongest of motivations in gas turbine design and service, and it becomes clear that fatigue will continue to be an important limitation in the life of gas turbine engines The NACA study (Ref 1), it is true, related to gas turbine engines of early design in which information now available could not be incorporated Also, in military engines, performance comes first; cost of overhaul and frequent part replacement is accepted as an appropriate price to pay for the high performance sought It might be properly asked whether the picture has not changed in the intervening years, and whether trade-offs between performance and life cannot be suitably made in commercial gas turbines for both stationary and aircraft use To a limited extent the answer to both of these questions is affirmative Much research has been conducted in the intervening years since the NACA study was made, and this information can be well used to increase fatigue life of c o r n National Aeronautics and Space Administration Lewis Center Staff, "Factors that Affect Operational Reliability of Turbojet Engines," NASA TR R-54, 1960 Copyright*1971 by ASTM International www.astm.org Copyright by ASTM Int'l (all rights reserved); Mon Dec 13:17:27 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized METAL FATIGUE DAMAGE ponent parts Furthermore, the trade-off constants are reasonably well known; increase in life for a reduction in temperature can be reasonably determined But performance is still a magic incentive, and engines for high-speed flight count more than ever on high temperature and operating efficiency Furthermore, turbines are becoming more complex; cooled turbine buckets require intricate passages that produce stress concentration while still requiring the material to operate at the limit of its capability in regard to both temperature and temperature gradient There seems little likelihood that fatigue, as an important failure mechanism, will go away on account of reduction in severity of service or the availability of better materials If fatigue is really to be combatted, it must be met forthrightly and dealt with thorough knowledge The mechanisms involved must be understood, and these mechanisms must then provide the guidance for methods of avoidance or mitigation The survey contained in this Special Technical Publication has been prepared with such an objective in mind Following an expression of urgent need expressed by members of the Joint Committee on the Effect of Temperature on the Properties of Metals, it was decided to prepare a "state-of-the-art" survey on fatigue of gas turbine components Such a survey could well serve as a framework for future research on this subject It seemed appropriate to include the following: Basic Mechanisms of Fatigue in the Sub-Creep Range Since many components of gas turbines operate at moderately low temperatures, the mechanism of fatigue in this temperature range must be defined Furthermore, the mechanisms at the lower temperatures would act as a baseline for superimposing further effects needed to understand the fatigue mechanisms at higher temperatures Basic Mechanisms of Fatigue in the Creep-Range Many components, such as nozzle guide vanes, turbine buckets, and combustion liners operate at temperatures wherein creep becomes an important, if not the dominating, mechanism It is conventional to define the temperature at which creep starts to become important as half the absolute melting temperature At this temperature, and above, new mechanisms can affect the fatigue life Thus, it is important to define these mechanisms and show their relation to fatigue life Detection of Fatigue Damage Fatigue manifests itself as damage in many ways, ranging from minor misorientation of atoms to gross cracking An extensive literature already exists on fatigue damage detection wherein the part or specimen involved must be metal- Copyright by ASTM Int'l (all rights reserved); Mon Dec 13:17:27 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions aut INTRODUCTION lurgically sectioned or in some way invalidated for further useful service Nondestructive techniques have recently been greatly expanded and improved for fatigue detection after the damage has proceeded to a considerable extent Nondestructive techniques also have been developed for observing flaws, such as inclusions, that are likely to become nuclei for fatigue failure The most advanced and successful of these techniques should be collated and evaluated Study of Field Practices for Repair of Fatigue Damage Only an actual field survey can reveal the nature of fatigue damage as encountered in field service and to discover the extent of present practices used in the field to repair such fatigue damage The NACA study made use of reports issued by the military overhaul bases, together with actual field visits, in order to obtain pertinent information To bring the information up to date it appeared desirable to arrange visits to overhaul centers of the commercial airlines Such visits, if made by personnel from an engine producer, could serve to coordinate the aspects that relate to initial design with those that relate to service operation Avoidance, Control, and Repair of Fatigue Damage Avoiding fatigue, or repairing it once it has been nucleated, is an extremely broad subject However, even in a preliminary survey, such as undertaken in the present study, much useful information can be crystallized into a single compendium Drawing on an understanding of the mechanism of fatigue provides the most useful approach The most important single guiding principle is recognizing that the avoidance of fatigue is the joint domain of the designer, the fabricator, the service staff, and the maintenance staff If he does not fulfill his rote properly, any one of these participants can destroy the most meticulous efforts of the others Thus, it is important to outline not only approaches that are in current use, but also to draw on possible opportunities that have not yet been fully exploited With the foregoing outline as a framework, each section was assigned to an individual or group of individuals who were equipped by training, interest, and experience to provide a useful document in their specialty It was recognized that exhaustiveness of treatment would not be possible because of time and space limitations, and because the state of the art is still rapidly unfolding However, it was felt that a useful purpose would be served by describing the progress that has been made, and directing attention to gaps in understanding that needed to be filled While the emphasis in these chapters has been application to aircraft gas turbines, much of the information is applicable not only to stationary gas turbines but also to machines other than gas turbines in which fatigue plays an Copyright by ASTM Int'l (all rights reserved); Mon Dec 13:17:27 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authoriz M E T A L FATIGUE DAMAGE important role in governing life It is hoped that this volume will serve all who are interested in the subject of metal fatigue S S M a n s o n Chief, Materials and Structures Division, Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio 44135; symposium chairman Copyright by ASTM Int'l (all rights reserved); Mon Dec 13:17:27 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized MANSON ON AVOIDANCE,CONTROL,AND REPAIR 333 more in life could be obtained by removal of about 0.002 in after every million cycles of loading (50 percent of the nominal life at the loading conditions involved) Other investigators have found similar improvements due to surface removal Thompson and Wadsworth[86] found that removal of approximately 0.001 in after each 25 percent of life prolonged life substantially After 2.25 times the normal life, the specimen was unbroken and looked as good as new A more engineering-oriented approach to the problem of surface removal is described by Christensen[87] Figure 83 shows some of the results The test specimen contained a central hole constituting a source of strain concentration where the crack nucleated The S - N curve for the test specimen without surface removal is shown as A Two specimens were tested by removing 0.015 in of surface at intervals of 50,000 cycles, the results of which are shown by the line P Q R S T U V W (The stress variation as shown by this line is partly due to material removal and partly to changes in external loading conditions) It can be seen from the figure that life was increased appreciably by the surface removal Based on the fatigue curve A the summation of the cycle ratios (applied cycles at each stress level divided by fatigue life at the stress level) is about 3.5, suggesting that surface removal was beneficial Figure 84 illustrates, on the other hand, one of the cautions[87] that must be exercised in seeking to extend life by this method, namely, to ensure that cracks present in the surface must be completely removed The S - N curve for the filleted specimens is shown by A B Five specimens were first loaded at 25 ksi for 100,000 cycles, half the life that would have caused failure at the stress level Two of these specimens were then machined to remove 0.0015 in as well as remachining the fillet to a radius of in When subsequently tested at 42.5 ksi, 45 HOLEDIAMETER[ - - - ~ INCREASED IN I '{-~:', I 40 ~ ~ 35 30 =~ x 25 < INCREMENT ~ ~( I UI I HOLE 20 P ~ s ,l 1~4 I i ~-~ i FAILURE ~ ,R:O.2 15 o.o3o-IN, i~x~ ~ , I=~'-~'~'~C'~t~,~,"'~"~':'}~P.~'~'>I~,'~C:~'~' ~ i ~ ~ , ~ "