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Aviation History
1957
1957 - 0052.PDF
52 FLIGHT, 11 January 1957 Research, Development and Technical Issues . . • of failure during the vital period are raised, I doubt if one can comebelow a strength such that the pilot can safely apply at any time a manoeuvre of reasonable severity, since otherwise the confidenceof the crew and passengers in the ruggedness of the aircraft must be in doubt, anyway, throughout its later life. The definition of"reasonable severity" is the difficulty. If a reduced standard of strength is acceptable later in the lifeof the aircraft, were the original standards too high? The standards specified in the past have, on the whole, resulted in satisfactoryservice experience and it would, I think, need substantial argu- ments to reduce them. These arguments might be provided asgreater attention is paid to anti-fatigue design and fatigue strength, since an element in the specification of static strength is anendeavour to cover fatigue requirements in the broad. At present, design tensile stresses in civil aircraft are perhaps more governedby considerations of fatigue than of static strength; as anti-fatigue design improves, or as materials with lower static strength butbetter fatigue properties are used (as some designers advocate), the whole question of static strength requirements will needfurther review. The use to which the aircraft is to be put must determine whichdesign approach to adopt—this matter is to be seen not as a "fight to the death" between "safe life" and "fail safe," and it is not agood service to aviation to seek to make it so. For example, for smaller aircraft for use in "out back" areas with limited main-tenance and inspection facilities, there is good reason for using the "safe life" approach; the operator is put in the straight-forward position that he changes pans at a stated time and the single spar construction may well give him the more economicaircraft. For aircraft taking a heavy beating in continuous low altitude use, the overall economy throughout the aircraft's work-ing life may best be met by "safe life," the cost of the new com- ponents being offset by the inspection and repair costs into whichthe "fail safe" design may run in its later life. For larger aircraft used for the most part in the high-altitudecruise condition, and worked by operators with excellent inspec- tion and maintenance facilities, both approaches are perfectlyreasonable and a combination of "safe life" and "fail safe" may well be appropriate. For economy in inspection and maintenance,a reasonably long crack-free service must be attained in a "fail safe" design, even if the crack is not catastrophic, and it is thenessential that the cracks can be easily found and the part repaired. In some parts of the structure there may well be difficulties ofinspection and maintenance and these parts must be put entirely beyond suspicion by a "safe life" approach. In the present state of knowledge there must, I think, be a testto confirm that the trouble-free life is not too short; to determine where cracks are to be expected, and their significance; to deter-mine safe lives where necessary; and to develop inspection tech- niques and procedures that can be relied upon to find the cracksbefore it is too late. Recent test work in England has shown what MI: would expect—that it is not where fatigue trouble has beenanticipated and designed against that fatigue failures are found, but in local details and particularly in those that cause a rapiddiffusion of load; there have been somewhat higher stress con- centrations than were estimated. In nearly all our major tests,cracks and failures have been produced at places other than main joints, and they can usually be explained by considering the wayin which the load is diffusing in the area. Concentration nn the joints alone is not sufficient, Th: complete aircraft fatigue test provides knowledge whichit is difficult to get with certainty in any other way. A longer operating life can be allowed with safety if test results are availableto substantiate the design. The method includes comprehensive strain gauging, and it is usually possible to extend the fatigue lifeby minor modifications at points of high stress revealed in the tests. In the application of "fail safe" principles the tests producethe cracks and establish their rate of growth; they prove that the alternative stress routes actually exist and that they operate success-fully after the crack has form ad. Increases in the life of civil aircraft to 20 years at full utilizationis an aim which, if achieved, would make a further impact on the already favourable trend of aviation economics; such achievementwill be largely governed by success in design against fatigue, and is therefore dependent on the acquisition of the detailed knowledgewhich only testing can provide. The more fatigue testing is done, the more it seems to be justified. We hav: not found anything better than the water tank test. Nodifference has been detected between tests done under water and those done in air. There are, of course, uncertainties in thecorrespondence between a tank test and service use, but I doubt if they are serious and further experience will shed more light onthem. It is, it is true, difficult to represent the effects of main- tenance and factors, such as corrosion, that are time-dependent.Some commentators have been concerned as to whether corrosion taking place in the tank may not produce a serious adverse effecton the fatigue life of the specimen; this is not so, for less corrosion takes place in the tank than occurs to the aircraft in service as a result of flying or standing in the rain. The question is whether the tank over-estimates the life, rather than under-estimates it on this score. The broad indication ironr work in England is that from thefatigue standpoint the cabin is an easier design problem than the wing. On the other hand, the cabin is more vulnerable todanger from external sources, and therefore the form of construc- tion adopted and the stress levels used should have this very im-portant consideration in view. As for tailplanes, our experience has been good; the structure is usually relatively simple andstraightforward, and it may also be that designers tend to be more conservative with the tail. But undercarriages are items whichrequire careful attention. The tendency is to design the oleo to have a low value of the reaction coefficient, and this means thatif the undercarriage is designed under considerations of static strength, then a relatively large fraction of the ultimate load isapplied on every landing. The use of large light-alloy forgings gives this particular point; unless the design and heat treatment of theforging is suitable, large "locked-in" stresses can be caused, and lead, in conjunction with the landing load, to early fatigue failure.Our experience has led us to the following principles in handling large forgings; these have helped to achieve success in voidingserious "locked-in" stresses. The forgings should be quenched in boiling water and the alloy used should be such that it suffersonly minor loss of mechanical properties as a result of this treat- ment. The forging should be heat-treated in the fully-machinedcondition whenever possible. Where the dimensional control obtainable in a boiling water quench is not good enough, a verysmall amount of post-heat-treated machining is allowable, pro- vided it is symmetrically disposed. Before leaving the subject of fatigue it is interesting to notethat the problem of the rate of growth of cracks, which is vital to the "fail safe" approach to design, is an example of an almostforgotten research. In 1921, Dr. A. A. Griffith put forward an analysis of the propagation of cracks in brittle materials and ad-vanced the hypothesis, which he was able to support by experi- mental evidence, that rapid propagation of the crack takes placeif the rate of work done by the release of strain energy in the material exceeds the rate of energy demanded to produce crackextension. This research was apparently forgotten, except perhaps by specialists, and was not connected by engineers with theproblem of crack propagation in ductile materials used for struc- tures; indeed, it is probably fair to say that until an aircraftaccident brought the question of crack propagation into strong prominence, most engineers had not studied the question seriouslyat all, though some steel structural designers had encountered and considered the problem of "brittle fracture" particularly in suchstructures as oil-storage tanks. Recent work has shown, as would be expected, that the originalGriffith theory does not apply to ductile materials, probably be- cause the energy absorbed in the plastic phase of deformation ofa ductile material must be taken into account. However, although attempts to evaluate the rate of work required to create newcracked surface, using present theories, give disappointing results, a great deal of empirical data is now accumulating. It appears frompresent work that the relationship between stress, and the length of crack which is critical in the sense that it will run rapidly, isaffected by the width of the plate and to some extent of its thick- ness, but not by its length. The relation is unaffected by cyclicloading, or pre-straining, but is greatly influenced by the presence of reinforcing. In Fig. 16 are shown some typical results on the influence ofcrack-stopping reinforcement, for which I am indebted to the Bristol Aeroplane Company. An artificial crack was made in the 6 7 8 9 10 It TOTAL LENGTH OF CRACK (in) Fig. 16. Test on specimens in 0.028in plate with crack-stoppers: A, integral stopper; B, Reduxed stopper; C, riveted stopper; D, fast- cracking threshold for unreinforced plate; E, inner edge of stopper; F, inner rivet line.
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