The report [53] describes the following fatigue experiments, performed by Hodgkinson, on cast and wrought iron: A bar of cast iron 3 inches square, was placed on supports about 14 feet a
Trang 1rules as may enable the Engineer and Mechanic, in their respective spheres, to apply the Metal with confidence, and shall illustrate by theory and experiment the action which takes place under varying circumstances in Iron Railway Bridges which have been constructed;
A report [53] summarizing the results of the investigations was published in 1849 This report was widely publicized and reprinted without the complete appendices [6, 54, 55] The report contains the first systematic investigation of the effect of moving loads on railroad bridge structures and fatigue of rails The pertinent experiments were performed
by E Hodgkinson, Captain H James and Lt D Galton
The commissioners [53] stated their research goals as follows: The questions to be examined may be arranged under two heads, namely—
1 Whether the substance of metal which has been exposed for a long period to per-cussion and vibrations, undergoes any change in the arrangement of its particles, by which it becomes weakened?
2 What are the mechanical effects of percussions, and of the passage of heavy bodies,
in deflecting and fracturing the bars and beams upon which they are made to act?
The report [53] describes the following fatigue experiments, performed by Hodgkinson,
on cast and wrought iron:
A bar of cast iron 3 inches square, was placed on supports about 14 feet asunder A heavy ball was suspended by a wire 18 feet long, from the roof, so as to touch the centre of the side of the bar By drawing this bar out of the vertical position at right angles to the length
of the bar, in the manner of a pendulum, to any required distance, and suddenly releasing it,
it could be made to strike a horizontal blow upon the bar, the magnitude of which could be adjusted at pleasure, either by varying the size of the ball or the distance from which it was released Various bars, (some of smaller size than the above) were subjected by means of this apparatus to successions of blows, numbering in most cases as many as 4,000 The magnitude
of the blow, in each set of experiments being made, greater or smaller, as occasion required The general result obtained, was that when the blow was powerful to bend the bars, through one half of their ultimate deflection (that is to say, the deflection which corresponds to their fracture by dead pressure) no bar was able to stand 4,000 of such blows in succession; but all the bars (when sound) resisted the effects of 4,000 blows, each bending them through one third of their ultimate deflection
The report [53] describes the following fatigue experiments, performed by James and Galton:
Other cast iron bars, of similar dimensions, were subjected to the action of a revolving cam, driven by a steam engine By this they were quietly depressed in the centre, and allowed
to restore themselves, the process being continued to the extent, even in some cases, of
a hundred thousand successive periodic depressions for each bar, and at a rate of about
Trang 2four per minute Another contrivance was tried, by which the whole bar was also, during the depression, thrown into a violent tremor The results of these experiments were, that when the depression was equal to one-third of the ultimate deflection, the bars were not weakened This was ascertained by breaking them, in the usual manner, with stationary loads
in the centre When, however, the depressions produced by the machine were made equal to one-half of the ultimate deflection, the bars were actually broken by less than nine hundred depressions This result corresponds with and confirms the former
The experimental setup used by James and Galton was illustrated in the discussion by Timoshenko of a paper by Peterson [56]
The report [53] describes the following fatigue experiments, performed by Willis, James and Galton: “By other machinery, a weight equal to half of the breaking weight, was slowly and continually dragged backwards and forwards from one end to the other
of a bar of similar dimensions to the above A sound bar was not apparently weakened
by ninety-six thousand transits of the weight.”
The report [53] concluded that
It may, on the whole, therefore be said, that as far as the effects of reiterated flexure are concerned, cast iron beams should be so proportioned, as scarcely to suffer a deflection of one-third of their ultimate deflection And, as it will presently appear, that the deflection produced by a given load, if laid on the beam at rest, is liable to be considerably increased
by the effect of percussion, as well as by motion imparted to the load, it follows, that to allow the greatest load to be one-sixth of the breaking weight, is hardly a sufficient limit for safety, even upon the supposition that the beam is perfectly sound
In wrought iron bars, no very perceptible effect was produced by 10,000 successive deflections by means of a revolving cam, each deflection being due to half the weight, which, when applied statically, produced a large permanent flexure
The report [53] also summarizes the experimental and analytical studies performed by Willis on the effect of moving loads on the deflection of bridge bars and beams Willis [57] studied the effect of a moving load on a beam, in an attempt to understand the “action which takes place under varying circumstances in iron railway bridges.” This study used the experiments performed by James and Galton on full scale bridge type of structures and small scale tests on model systems performed by himself The experimental results were compared with theoretical analyses by Stokes The main conclusions of these studies were that the deflections experienced by the bridge structures due to moving loads are higher than those due to static loads They could be over twice those produced by static loads of the same magnitude
The report [53] recommended that
, as it has been shown that to resist the effects of reiterated flexure, iron should scarcely
be allowed to suffer a deflexion equal to one-third of its ultimate deflexion, and since the deflexion produced by a given load is increased by the effects of percussion, it is advisable
Trang 3that the greatest load in railway bridges should, in no case, exceed one-sixth of the weight which would break the beam when laid on at rest in the centre
The commissioners conclude their report with the statement that they “lament that the limited means which have been placed at our disposal, and the great time required for such investigations, have compelled us to leave in an imperfect state, or even to neglect altogether, many interesting and important branches of experimental inquiry ” This conclusion [58] was further reinforced as follows: “we understand that the labors of this important Commission were prematurely stopped by cutting off the necessary funds for carrying on the experiments Surely, seeing the important uses to which, on land and sea, iron is now employed, it was not a wise economy to put an end to an inquiry which promised to be of such great national importance.” These comments are reminiscent of the laments of present day researchers, who quite often bemoan the lack of adequate funding and time to accomplish their research, and the premature termination of promising research activities
DISCUSSION AND CONCLUSION
As can be seen from the research efforts discussed in the previous section, the practicing engineers correctly described the fatigue crack growth process during the early 1840s While Edwards [25] and Rankine [27] were apparently the first ones to publish a description of the fatigue process, others [26, 29–31] arrived independently at similar descriptions of the fatigue failure process Rankine developed and experimentally verified an engineering solution for preventing failure of railway axles with reentrant corners This solution was adopted and Rankine went on to other research, which established his reputation
While the correct qualitative description of the crack growth process was developed, no attempt was apparently made to quantify it The scientific and engineering communities were misled by the observation that the failure surfaces of axles that failed in service had
a crystalline appearance, while axles that were failed when new had a fibrous appearance Various explanations for the change in the appearance of the failure surfaces were offered based on the prevalent theories These included magnetism, electrical charging, and the caloric theory of materials Even thought convincing proof that materials do not exhibit
a change from fibrous to crystalline fracture was presented by Stopfl [31] and Kirkaldy [59], who showed that the change can be caused by changing the loading rate and by the presence of notches in the specimens, the crystallization theory of fatigue persisted into the early 1900s
Besides railroad axle failures, early railroad bridge failures were common These were due to the increase in railroad engine and car weight, and train speed The numerous bridge failures led to the creation of the royal commission to inquire into the application of iron to railway structures Its report [53] includes the first systematic fatigue experiments
Trang 4in England and the development of the first fatigue design criterion Unfortunately, this significant fatigue research has been overlooked because the fatigue results were only published in the report of the commission and not in the scientific literature Also, it should be noted that funding for this significant research effort was terminated before it could be properly completed [53, 58]
Other significant fatigue research was performed before Wöhler’s systematic fatigue test program and the formulation of the so-called Wöhler’s laws of fatigue
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Geognosie, Bergbau und Hüttenkunde, 1838, 10, 215–234.
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31 Stopfl, P., “Achsenbrüche an Lokomotiven, Tender und Wagen, ihre Erklärung und Beseitigung” [Axle Failures in Locomotives, Tenders and Wagons – Their Causes and Methods
of Avoiding], Organ für die Fortschritte des Eisenbahnwesens in technischer Beziehung, 1848,
3(2), 55–67
32 McConnell, J E., “On the Deterioration of Railway Axles”, Proceedings of the Institution
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33 Anon, “Chemins de Fer – Formation d’une Commission de Statistique”, nnales des Ponts et
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Association, 1925, 15 July, 11–16.
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38 Engerth, W., “Ueber die Veränderung der Textur des Eisens, welches bei stattfindender Torsion
zugleich StöBen ausgesetzt ist”, Zeitschrift und das als besondere Beilage herausgegebene Notizen- und Intelligenzblatt des österreichischen Ingenieur-Vereines für das Jahr 1851 1851,
3, 34–36
39 von Burg, Ritter., “Ueber die von dem Civil-Ingenieur Hrn Kohn, angestellten Versuche,
um den Einfluss oft wiederholter Torsionen auf den Molekularzustand des Schmiedeisen
auszumitteln”, Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften, Wien, 1851,
6, 149–152
40 Schrötter, A., “Ist die krystallinische Textur des Eisens von Einfluss auf sein Vermögen
magnetisch zu Werden?”, Sitzungsberichte der Kaiserlichen Ajademie der Wissenschaften,
Wien, 1857, 23, 472–481.
41 Tredgold, T., Practical Essay on the Strength of Cast Iron and Other Metals, 4th Edition with
Notes by Eaton Hodgkinson, London:John Weale 1842
42 Hood, C., “On Some Peculiar Changes in the Internal Structure of Iron, Independent of, and
Subsequent to, the Several Processes of Its Manufacture”, Philosophical Magazine, 1842,
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43 Vignoles, C., “On Straight Axles for Locomotives”, Reports of the British Association for the
Advancement of Science, 1842, 12, 104–105.
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Reports of the British Association for the Advancement of Science, 1842, 12, 105–106.
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Hollow Axles”, Minutes of Proceedings of the Institution of Civil Engineers, 1843, 2, 89–94.
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47 Thorneycroft, G B., “On the Manufacture of Malleable Iron; with the Results of Experiments
on the Strength of Railway Axles”, Minutes of Proceedings of the Institution of Civil Engineers,
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London: Wm Clowes and Sons, for Her Majesty’s Stationery Office 1849;XX + 453 pages
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1850
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164, 50–56
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of Various Kinds of Wrought-Iron and Steel, 2nd Edition 1863.
Trang 8Appendix B
Final Report for the USAF High Cycle Fatigue
Program
Otha Davenport
INTRODUCTION
In the mid 1980s, high performance gas turbine engines with relatively high thrust to weight ratios were beginning to reach the boundaries of the empirically based design systems for the phenomena that was to become High Cycle Fatigue (HCF) Simply stated, HCF is the formation and propagation of cracks in a structure that results for many millions (or billions) of cycles at stresses well below the yield strength of the material The relatively small stresses can be the result of any or all of a myriad of stimuli that can
be extraordinarily complex
As jet engines provided higher performance at lower weights, difficulties in unexplored areas developed In the 1960s, the challenge was compressor stall when the throttle was abruptly moved, or the aircraft maneuvered Tools and methods for stall predic-tion, avoidance, and recovery were developed and became the standard for the engine manufacturers
In the 1970s, unexpected and unpredicted structural failures were the next major technical challenge for engine designers Extensive testing and analysis, along with char-acterization of a new generation of materials, showed that the large mechanical and thermal strain associated with the full range of throttle movements from starting through maximum power was developing and propagating cracks in the major rotating compo-nents This was characterized as Low Cycle Fatigue (LCF) and the USAF developed and promulgated the Engine Structural Integrity Program as a development and sustainment process to manage the application of the LCF tools and methods throughout the life cycle and preclude unexpected structural failures
As HCF field failures continued to occur through the late 1980s and early 1990s, the effects of these failures became more and more onerous on the war fighting commands
It became imperative for the engineering staffs within the government and the engine manufacturers to better understand the many dimensions of this phenomenon The first study of HCF was conducted by ASC/EN in 1989, followed by a USAF Scientific Advisory Board review in 1991 The issue was recognized as a major issue by the
∗This document was prepared by Otha Davenport, Director of Engineering, Propulsion Product Group,
Wright-Patterson AFB, retired.
493
Trang 9then Assistant Secretary of Defense, Dr John Deutch in 1994 who said “Engines are
the major readiness issue for USAF ” Further, the Secretary of the Air Force, the
Honorable F Whitten Peters stated, “This is not only a readiness issue, it is a retention
issue ” Meanwhile, the Air Force Chief of Staff, Gen Fogleman, stated in 1995: Part of the problem is the workload that our flightline troops and our engine shops are
experiencing due to new inspection requirements to combat high cycle fatigue.
A further emphasis was added as new configurations of higher thrust to weight engines were being developed with higher aerodynamic loading with little or no inherent mechanical damping as integrally bladed rotors were chosen as the preferred config-urations The USAF Air Force Research Laboratory responded to this imperative in
1995 with the National High Cycle Fatigue Initiative which grew into the International High Cycle Fatigue Program with the participation of the United Kingdom Ministry of Defense (MoD)
The program began with and has preserved the objective of the development of a physics-based design system that encompasses the underlying but necessarily complex interactions of the steady and the unsteady aerodynamic forces, the damped mechanical response of the structure, the capability of the material to withstand the stresses that are applied and the experimental tools to measure these characteristics and to verify the physics-based analytical tools as they are developed The HCF failures have generally been experienced in only a very small percentage of a population of engines This implies that these failures occur in the extremes of a probabilistic distribution of the contributing factors and are difficult, if not impossible, to precisely replicate in a laboratory or test facility
The program has accomplished many of the objectives initially outlined and many physics-based design tools have been transitioned to routine use in company design systems, however these tools have yet to be integrated into a comprehensive design process for HCF New instrumentation concepts and products have greatly expanded the understanding of the excitation forces and mechanical responses New and innovative material test methods have greatly shortened the time required to characterize the HCF properties of materials New damping approaches are yielding products to reduce the vibratory responses of lightly damped components New processes for surface treatments
to impart large compressive surface stresses have largely matured as a result of this program and are in production on several engines
The most elusive objective that has yet to be reached is the ability to integrate the inherent probabilistic nature of HCF A characterization protocol has been developed to better understand the nature of these statistical distributions of significance but a method
to demonstrate a desired level of resistance to HCF has yet to emerge
Even at the current stages of maturity of the various HCF tools and methods, the impact
of the HCF program on current and development engines has been enormous The field
Trang 10engine inspection workload for HCF has been reduced by over 90% and the proportion
of engine mishaps resulting from HCF has been reduced from 54 to 7%, far exceeding the HCF program goal of a 50% reduction in mishaps
Further, these same tools are enabling technologies for the next generation of high performance jet engines and for the foundation of the F135 and F136 engines for the F-35 Joint Strike Fighter that is intended for use by the USAF, USMC, USN, and many allied and friendly countries Without these tools and methods, the development programs would likely encounter many unexpected development difficulties with the accompanying delays and cost growth
BACKGROUND
Emergence of HCF as critical issue
In the mid 1980s HCF began to emerge as a critical issue for the USAF The designs
of engines prior to the F100 and F110 used primarily steel compressor blades and rotors while solid nickel blades and rotors were used in the turbines The introduction of titanium blades and rotors in the fans and compressors allowed higher blade loading, greater rotor speeds, all in a material with less inherent material damping than the traditional steel materials Relatively early in the fielding of the F-16 aircraft, HCF issues began to appear with no indication of the existence of these problems in the engine development and qualification programs The F-16, being the most prevalent aircraft in the USAF inventory and being a single engine fighter, accentuated the issue because an engine failure caused loss of the aircraft in about 80–90 % of the cases
In one engine model, the first stage compressor blades and disk posts failed and the fleet was impacted with blade change outs This was followed by the development
of ultrasonic inspection techniques to find the cracking before it led to failures One characteristic of HCF also began to emerge at this time, namely the probabilistic nature
of the failure mode In all, only about 1 in 1000 blades ever indicated cracking The underlying cause of these cracks was not well understood but the traditional methods of reducing or eliminating HCF were all applied These included reduction of the net section stresses and reduction of fretting in the blade/disk interface While these redesigns served
to reduce the incidence of cracking they were not adequate to eliminate the problem and ultrasonic inspections continued for many years
In another engine model, the HCF cracking occurred in the last stage of a three-stage fan Engine failures were caused by the loss of fan blades and disk segments Much attention was focused on this issue as it only began to appear after the fleet had accumulated slightly over 1,000,000 flying hours with no indication of the issues being present This particular issue caused great disruption of the fleet maintenance process
as fan removal was required frequently for an effective inspection Again the traditional