1 Lecture #13(14A). Experimental estimation of service life Plan: 1. Methods of fatigue tests 2. Program composition of tests 3. Estimation of the directive stresses 1. Methods of fatigue tests The fullest information on endurance of a structure can be received at its test under operating conditions. However fatigue failure of the plane in operation is inadmissible and the preliminary decision is demanded for this question. Reliably enough fatigue strength can be established only an experimental method. Therefore all structures for which questions fatigue strengths play an essential role, are tested by repeated loads a whole structure of the plane and separate "weak" elements for which questions fatigue strength plays an essential role. Usually the weak elements are considered the most loaded details having complex outlines, with sharp differences rigidity on length and concentrators of stresses as holes or notch. Here it is possible to attribute fasten units of a wing, units of fastening of tail unit, rudders, ailerons and flaps, the landing gear, frames of fastening of engines and other. Tests of the isolated details do not give of full confidence about secure work of these details in a structure, as their actual conditions of loading essentially can differ from conditions of loading during tests. Moore reliable results are got from test of a whole structure. Such tests are carried out to reveal the weakest elements of a structure, to study an initial stage of fatigue failure of a structure and to estimate safe service life. As results of tests changes are introduced in the structure for increase of its survivability, and also develop recommendations for control surveys the planes structure and to its repair. Fig. 1. Test in pool.
The programs of tests are generated so that they most full reflected actual loading the plane under operating conditions. The sequence of the application of overpressure in a cabin and loads on a wing and tail unit usually corresponds to their change for typical flight. Tests of a fuselage had carried out in pool with water. Thus overpressure in a cabin is created due to change of pressure by the water filling internal volumes of a fuselage. Tests of a fuselage in pool exclude explosive character of destruction, rather precisely allow to establish the begin of structure destruction upon pressure drop in fuselage. The water level in pool is adjusted with the top edge of a fuselage in order to exclude influence of hydrostatic pressure of water on a structure. Realization of such tests is connected to the big difficulties of the technical order and significant material inputs. An essential lack of tests in pool is corrosion of elements of a structure which influences on fatigue strength and garbles the data fatigue tests. Therefore now often the structures are tested by air overpressure. Such tests reflect conditions of operation of a structure more full and eliminate the phenomenon of the accelerated corrosion. At test of a structure by air it is easier to watch formation of a crack and its development if, certainly, danger of explosive destruction of a structure is excluded. The last is reached by filling of internal volumes structure by polyfoam or any other material. Thus energy of air decreases according to reduction of its volume, and danger of explosive destruction becomes insignificant. Necessity to take and to keep blocks of polyfoam during survey of an internal part of a structure increases expenditures of labor and time for realization of this responsible operation at tests. Until recently in practice of laboratory researches mainly static methods of loading, providing slow enough application and removal of loads were used. These methods having of many advantages, because small speed of loading practically cannot be used for reproduction of loads with low level as it would result in excessive increase of duration of tests. Therefore now the dynamic test methods of structures are introduced becouse they allow to overlap static and vibrating loading and allowing for same time of tests to reproduce more full spectrum of loads exerting on a structure in operation. The supersonic planes are subjected to long aerodynamic heating becouse they are checked on fatigue strength in conditions simultaneous them loading and heating. At heating temperature stresses appear in a structure which are commensurable with stress from external loading. Therefore they are the important part of a spectrum fatigue loadings. To reproduce temperature stresses, various elements of a structure are heated up according to their actual heating under operating conditions. Heating is carried out by an air stream which moves on channels to various parts of a structure or heating devices. 2. Program composition of tests The basic material for composition of the test program for structures of the plane as a whole or its units on repeated static loads are the data by statistics on character of loading the plane in various conditions of operation. These data are extracted by statement of special 2
researches of the exerting loadings on the plane in flight, taxiing,running, at taking-off and landings. Such experiments are carried out in the wide plan as on the experimental planes of various types allocated for this purpose at scientific research institutes, and on the planes - leaders who are taking place in operation and specially for this purpose supplied necessary recording equipment. In connection with these conditions of their operation the greater amount of flight hours is provided for planes - leaders in comparison with other planes of the same series. The collecting statistical material of such researches canbe the basis for judgement about character and alternation of loads on the plane and its units for one flight, including takeoff and landing. Thus, obviously, all factors, capable to influence on the loadings should be taken into account.there are type of the plane and assignment of flight, climatic and weather conditions, height and speed of flight and many other things. For composition of the full strict test program on repeated static loads it must be discounted that in various flyings various conditions of flight can be and consequently, and loads are essential also. Thus alternations of conditions of flight and conditions of loading can be natural and random. Therefore recurrence them in the program it is possible only to some probability. Anyhow, the analysis of the saved up statistical material and received on the planeleaders datas allow with sufficient degree of conformity to establish sizes of loading, their character, frequency of repeatability and the order of alternation, which the plane are exposed during one typical flight (taxiing, takeoff, climb, flight, gliding, landing) or groups of flights in various conditions. The loading, appropriate to this typical flight or their group and concerning the certain amount of flight hours, is accepted for one cycle of loading. Thus it is supposed, that while in service the plane will test the loadings representing multiple recurrence of this typical cycle. There are three categories of complexity of tests of aviation structures on durability and techniques of recalculation for them: test with unit-stage loading, tests with combined loading under the schematic program, tests with multistage loading under the complex program simulating conditions in operation. Tests with unit-stage loading are the most simple and are applied for a long time. The size of repeated loads with constant amplitude should be chosen such that the accepted level of load has caused the same damages in a structure, as well as all set of the loadings in operation. Often planes are tested by load 0,67Рu or 0,5Рu. They are applied to an estimation of endurance of a wing, a fuselage, tail unit, control system, landing gear of the plane. The strick and full program of plane tests on repeated static loads should reproduce complete load spectrum in time. It is carried out program loading with the help of hydrojacks (the number of them reach several tens pieces), controlled by the computing device. In the test stend monitoring system should be installed for control of loading. Basically it is possible to provide alongside with the programmed natural alternation of loads too random loading. At such tests gauging stresses and deformations of a structure is conducted with the help of recording devices. 3
At test with combined loading under the schematic program loads of several kinds and levels are put to a structure. Transport planes (their hermetic cabins) are exposed for estimation of their service life to such tests. The program of loading is made in such manner that all loadings exerting aboard the plane from the moment of takeoff to a stop enter into one cycle (takeoff, climb, a cruiser mode, descent, landing, taxiing). In this case mean service life T m is estimated as: NT T 0 m (1) where Т 0 is duration of typical flight; η - the safety factor which is taking into account dispersion of kinds and levels of loads for a cycle; N - number of cycles before destruction. Р 4 t t = 100 500 houres Fig. 2. The multistage loading tests. The multistage loading tests give closest fit to conditions of loading in operation. Thus the program of loading is made on the basis of the operational loadings repeatability - a loading spectrum. Loads are grouped in a level and frequency on the given number of flight hours (100... 500 h). As the program corresponds to the certain number of flight hours service life is equal to product of number of loading cycles N on number of flight hours Т 0, which appropriate to one loading cycle : NT T 0 m. (2) Also programs are applied in which loadings of various size alternate in a chaotic casual sequence as it happens at operation of the plane. Such tests are rather difficult, but realization
of them is the extremely expedient. This technique of tests gives reliably enables to reveal dangerous places in the structures by fatigue strength. However realization of tests under such program demands enormous expenses, and realization of them is difficult and toilful. Therefore only the part of planes is exposed to such researches. The service life of the other planes is determined with the help of theoretical reasons on basis of the data from tests on repeated static loads. The simplest program of tests for repeated static loads is the program with one-stage loading. Thus the unit is exposed to some given on size and character of distribution loading with the subsequent unloading. The service life of a structure is estimated by number of loading cycles before destruction. P 5 P l Fig. 3. One-stage loading. That such tests could be taken as a principle for estimation of the unit service life, it is necessary to prove character and a level of loading cycle. The loading during one typical flight (or groups of flights), appropriates to the certain quantity of flight hours k. It is necessary to replace all spectrum of loading some certain quantity of simple one-stage cycles n (loading - unloading) with any given level and the most typical distribution of loads. Thus it is necessary to secure equivalence of such replacement in sense received "damage" by structure. However now reliable theoretical methods of substantiation of equivalence of such replacement are not present, therefore replacement is necessary carrying out so that n simple loading cycles were obviously heavier for a structure, than a difficult spectrum of loads for k flight hours. In practice the size of loading of a simple cycle is accepted within the limits of (0.4-0.67)P u from ultimate load, and the equivalent quantity of cycles should be coordinated to the available statistical data of flight tests. So, for example, in USA in firm North American it was accepted, that 3000 repeated loadings by the load which equal 67 % from a ultimate load, corresponds to a 2000 flight t
hours the maneuverable plane - fighter. From such tests the plane service life before destruction is determined by the formula: N Tm k1 (3) n where N - number of simple cycles of the loading exerted on a structure before destruction; n - number of simple cycles of the loading accepted as equivalent loadings for k 1 of flight hours. For firm North American we have n=3000, k 1 = 2000fh. At test with one-stage loading new structures and structures fulfilled in operation significant term it is possible to establish factor of recalculation more precisely. Quantity of cycles before destruction can be recalculated in hours of service life by means of factor K e Ti Ke (4) N0 N i where No - number of cycles before the destruction, sustained by a new structure; Ni - number of cycles before the destruction, sustained by a structure after Тi of flight hours The size of factor Кe is depended from type of the plane, a structure and used material. Value Кe should be determined for each plane. For maneuverable planes at test by loading 0,67Рu factor Ke is equal Ke = 0,7... 0,8, at tests with 0.5Рu - Кe= 0,20... 0,25. Mean service life is estimated with help Кe under the formula: Т m =Кe*N 0. (5) 6 3. Estimation of the directive stresses The fulfilled fatigue tests has allowed to establish, that for maintenance of the given service life it is necessary to establish in appropriate way a level of service stresses in operation. Calculation on static strength provides safety of the plane from loads of the big size which seldom meeting in operation. At this calculation the service life is not considered in general. Experience shows, that it is possible to achieve of the given service life by reducing a level of service stresses. For increase service life of a structure it is necessary to decrease of working stresses in bottom and partly in top panels of a wing, in a longitudinal set and a fuselage skin. There is the empirical formula for estimation of increase of plane mass Δm from an increment of full service life ΔT ( flight hours): Δm 5.5*10-7 m 0 * ΔT (6) here m 0 - take-off mass of the plane (kg). For example, for the plane with take-off mass m 0 = 17000 (kg) the increase of a service life on ΔT = 10000 flight hours can be achieved by increase of structure mass on 83 kg. Importance an exact method for estimation of fatique strength for "regular" zones of a structure is determined: 1) influence of a stresses level in longitudinal elements on unit mass;
2) damage because of premature discarding all park of the given type planes owing to a fatal mistake in a choice of a material and a level of stresses in elements of a longitudinal set. Experience shows, that fatigue characteristics of wings of the big aspect ratio are rather close to endurance curve for a flat specimen with the non-loaded hole. On fig. 4 curve of durability is constructed by design stresses for "regular" zone of the bottom surface of a wing from conditions of the given service life for some transport planes. The design stress for each plane is conditionally given to load factor n y u = 3,75 and the design service life was determined with safety factor η=3. From this base fatique curve the choice is carried out so-called directive stresses in details of a longitudinal force set of the bottom surface for a big aspect ratio wing on required durability. Thus, at design load factor in the tension panels of a wing directive stresses should operate for maintenance of a design service life and directive stresses become criterion of maintenance of the given durability. On fig. 4 the flight duration is equal 1.5 hour 400 dir, MPa 7 375 350 325 300 275 250 3.8 4 4.2 4.4 4.6 4.8 lgt Fig. 4. Directive stresses. The method of an estimation service life of details in "irregular" zones differs by using of experimental fatique curve for a concrete joint or notch. This method provides for the big volume of tests on powerful tearing machines of natural specimens of panel joints. For example the service life of typical irregularities of a wing was estimated from the tests of the
more than 200 specimens of 14 types of joints of the top and bottom panels and two types of the experimental compartments simulating the basic structure features of a wing in a zone of operational hatches, typical and stiff ribs. In calculation of transport airplane stressman can take as first iteration in quality failing stress t fsp directive stresses dirsp = t fsp for spar in view of service life which is equal:, (7) where σ ut sp - is the ultimate tension stress for material of spar; σ dir p =σ dir sk =σ dir st - are directive stress for panel, stringers and skin because they are manufactured from same material; σ ut p =σ ut sk =σ ut st - are the ultimate tension stress for panel, stringers and skin because they are manufactured from same material. 8