Aircraft Engineering TH E MONTHLY ORGAN OF THE AERONAUTICAL ENGINEERING PROFESSION VOL XXVI No 310 DECEMBER 1954 previous conditions under which the wing remains permanently elastic are violated, the stresses and strains at the end of the first HE causes of the crashes of the two Comets are not yet sub cycle are different from those at the beginning. On proceeding to jects for comment since, being at present under investigation investigate the second cycle, however, it is found that the conditions Tby the British Court of Inquiry, they must be treated as sub at the end are the same as they were at the end of the first and all judice. subsequent cycles will be similar. The wing is then said to have It can, however, be said that the conclusions reached at the Royal shaken down to an elastic state and, therefore, the life is determined Aircraft Establishment are so different from what were expected by other considerations to which we are accustomed. when the examination was first put in hand that they have made the aeronautical world realize, as never before, how little has hitherto Dangerous Cases been known of the effects of cyclic changes between the conditions encountered by an aeroplane when flying at ground level, operating So far so good, but the paper goes on to show that two other types at high altitude and reverting to ground level again on a number of of stress-strain cycle may occur which are dangerous even over a few occasions. cycles. When the temperature range increases beyond certain limits Accepting the view formed by the R.A.E. staff that the root cause more serious conditions arise. The web of the wing structure yields was cyclic changes in pressurization leading to failure through in both tension and compression and in subsequent cycles though fatigue, this is, of course, a special case of the problem. no large deformations occur the web continues to yield alternately in tension and compression. As the author writes, little is known The Present Case about the behaviour of materials subjected to strain cycles of this type, but it seems likely that failure will occur after a small number Quite different from this is the matter DR E. W. PARKES raises in of reversals. this issue, though it is somewhat analogous and may lead to failure The fourth type of behaviour envisaged is that of incremental through what is, in effect, fatigue; but of the wing structure and not collapse when large deformations occur. In such cases the wing of the fuselage. becomes progressively deformed with each cycle of loading which A good deal has been written on the effects of extremes of tempera may lead to failure of the material or at any rate jamming of controls ture on various materials used in aircraft structures. Many aspects or elastic or aero-elastic troubles. of this were covered at a special conference held by the Stress A most informative table is given which shows clearly the condi Analysis Group of the Institute of Physics at Bristol last year. We tions inviting the occurrence of the four various forms of stress- published summaries of the papers read at last year's conference in strain systems with a summary of the results. our February 1954 issue and since then a number of the individual An earlier table gives the elastic thermal stress in a cellular wing papers have appeared in full in subsequent issues. Among the papers with the skin and web of various materials. This shows that the most summarized was one by DR PARKES entitled 'The Design of Wings favourable combination for pure elastic thermal stress, without for Minimum Thermal Stress', which may be said to have followed considering the question of reversals which we have been discussing, on two papers by him which had appeared in these columns, in is with the skin of steel and the web of light alloy. February and December 1953 respectively, on 'The Alleviation of Thermal Stresses' and 'Transient Thermal Stresses in Wings'. His present paper studying the behaviour of wings under repeated ther Tests Needed mal stress may be considered to be the logical outcome of these. DR PARKES' study is, of course, in the nature of an assessment of the problem, and to simplify the argument he makes a number of Innocuous Types assumptions which are based on the most drastic temperature There are, it appears, four types of stress-strain systems that may gradient which can be obtained. He has shown in his earlier papers be set up under the repeated thermal loadings to which an aircraft that sufficient accuracy can be achieved by these assumptions, and may be subjected during its life of frequent changes of altitude—or, they do enable him to state clearly the four types of cycle that must in fact occur. It does seem, however, that experiment is needed and of course, of speed, though it is with the former that we are most concerned. indeed is probably being carried out to confirm how accurately the boundary conditions between the different regimes are deter In the case of permanent elasticity it is found that the conditions at the end of the cycle are the same as those at the beginning and all mined by the simplified analysis; and whether corrective factors subsequent cycles will be similar. can be applied to them to bring them into closer conformity with test results if these prove appreciably different. In the second case, of shakedown to elastic state, in which the
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Dec 1, 1954
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