CC BY
25
UDC 620.179.1
M. V. Karuskevich
National aviation university
AIRCRAFT LIFE PREDICTION BY THE PARAMETERS OF FOIL SENSORS AND SKIN SURFACE
Abstract: Two approaches for aircraft fatigue monitoring are described: a) the application of foil indicators; b) the direct observation and quantitative estimation of surface deformation relief parameters of alclad aluminium alloys. The evolution of surface state has been monitored at various regimes of fatigue loading.
Aircraft, fatigue, foil indicator, single-crystal, deformation relief
Introduction
Due to the advances in technologies and in the development of analytical and tools methods of fatigue crack prediction the failure rate in aircraft structures coursed by fatigue has decreased significantly last years. Nevertheless, metal fatigue is still one of the main causes of unforeseen crashes.
Components that fail by fatigue usually undergo three separate stages of damage:
a ) initiation of a fatigue crack; b) propagation of the fatigue crack; c) final sudden failure.
It is obvious that the quicker you reveal the first stage of fatigue the less probability of disastrous failure.
Fatigue analysis includes a set of theoretical and experimental procedures, but taking into account the complicated character of aircraft loading during operation and the stochastic nature of metal fatigue, one may assume that at present only reliable and adequate instrumental diagnostic of actual accumulated fatigue damage can prevent unexpected failure of structural components.
There are two approaches for estimation of accumulated fatigue damage: a) application of specimen-witness; b) direct diagnostic of material state.
A set of diagnostic methods are based on using specimen-witnesses, mounted on the surface of the object to be inspected. Such devices are usually called fatigue sensors or indicators of fatigue damage. The description of the most effective devices is given in papers [1, 2]. The indicators subjected to the operating spectrum of cyclic loads, change their state or even may be destroyed and in such way indicating the degree of damage accumulation in the tested structural element.
Direct inspection may be performed by applying
© M. V. Karuskevich 2006 j.
non-destructive methods, such as acoustic emission testing, high frequency ultra sonic, penetration, eddy current test methods, etc.
Our investigations show that quantitative estimation of accumulated fatigue damage may effectively be conducted by computer-aided optical analysis of the surface state.
1. Fatigue damage foil indicators
Foil indicators in its simplest form may be made of aluminium polycrystalline foil and in more sophisticated form - of aluminium single-crystal plate.
Both methods are based on the proved possibility of the quantitative estimation of the accumulated fatigue damage using the parameters of the deformation surface relief formed on the surface under mechanical loading.
The single-crystal fatigue damage indicator was created at the National Aviation University [2].
Two possible ways of manufacturing of the single-crystal fatigue damage indicators have been considered. Firstly, such indicator can be made of single-crystal of aluminium with cleanliness of 99.999%, which were grown by Bridgman's method. In this case the cylindrical single-crystals of 20 mm in diameter are cut by the electric spark unit on disks of 1.0 mm wide. Then they are the mechanically polished, and at the final stage by means of the electrolytic polishing their width is up to 0.2 mm. For the electrolytic treating the solution of 50%H3PO4 + 39%H2SO4 + 3%CrO2 + 8%H2O electrolyte is used.
The indicators are fixed on the specimen surface for fatigue test by means of glue cyanoacrylate (C5H5NO2) based.
This test the single-crystal indicators were fixed on the specimen of constructional aluminium alloy
D16AT.
Taking into account a wide range of operational loads, it is necessary to control indicators sensitivity. The carried out experiments have shown, that the sensitivity of single crystal indicators depends on their crystallographic orientation. The crystallo-graphic orientation defines the magnitude of shear stress in slip systems, propensity of a crystal to multiple or individual slip, etc.
Fig. 1 - Specimen for fatigue test with glued single-crystal indicator.
The crystallographic orientation of single-crystal indicators is defined by the crystallographic orientation of the axis of a cylindrical single-crystal rod and the crystallographic orientation of the indicator plate.
A possibility of the indicators production from single-crystal cylinders with axis orientation <100>, <110> and <111> are under consideration.
The alternative approach to growing up the single-crystals is application of critical deformation and annealing method. The corresponding experiments were held on the samples of aluminium alloy AD-1, which is the technical aluminium. Consecutive annealing of the samples at the temperature of 500 °C, deformation up to the certain level of strain, defined by the previous investigations and repeated annealing at the temperature of 550 °C enable us to get the grains of size up to 50 mm. Such a multi-crystal structure can be used for the manufacturing of single-crystal indicators. Besides, such a structure allows testing of the multi-crystal specimen, monitoring the state of separate grains, considering with some assumption, that the properties of separate grains are similar to the properties of single-crystals. The size of grains allows us to define the crystallographic orientation of the grains by using the radiographic method of Laue, the precision of which is not less than 2 degrees.
The testing have been conducted on the hydropulsating machine MUP-20.
The calculation of density of slip lines was performed by visual control of the state of a surface using the
metallographic microscope MMP-4 with magnification of x400. The evolution of deformation relief of the single-crystal indicator surface was investigated under the regular cyclic loading and some regimes of the program loading. In all cases the relation between density of slip lines and number of cycles of loading and level of strain was observed.
As it was shown earlier [1] the single-crystal indicators can be applied for controlling the accumulation of damage both under a cyclic and static loading. In the presented paper we tried to combine these opportunities in the program regime.
Fig. 2 illustrates the measurements results of density of the slip lines ( k ) on the single-crystal, which surface of coincides with the crystallographic plane {110} and the direction [221] along the axis of loading. The cyclic loading was performed in two stages with transition from the lower level of loads (maximum stress of cycle equal 140 MPa) to the higher (maximum stress of cycle equal 180 MPa). The magnitude of the stress under the static loading was 400 MPa.
The slip lines formed under cyclic loading were located at the angle of 82 degrees to the axis of loading. On the surface of the single-crystal as a result of the static loading action the slip lines of another orientation are formed. The angle of their inclination to the axis of loading under the magnitude of the relative deformation not more then 1.7% was -57 degrees. This distinguishes them from the bands of fatigue nature. Thus, the possibility of indication of the static overloads acting simultaneously with cyclic loading, was demonstrated.
The investigation of the single-crystal indicators with different crystallographic orientation has proved that the orientation substantially influences both the intensity of process of the slip bands formation and the external image of the slip lines. In some cases the defected structures that are formed on the surface can not be presented quantitatively applying the described technique, that is, by calculating the density of the slip lines. So, during the investigation of the single-crystals with a plane of surface {100} and the direction along the sample [100] the structures are formed, for the quantitative estimation of which the methods of the fractal geometry [2] is applied.
Fig. 2 - Evolution of PSB's density under fatigue and static overload.
Foil indicator may be manufactured of polycrystalline foil as well. In such case for quantitative
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estimation of accumulated fatigue damage the presented below procedure can be used.
2. Computer-aided optical analysis of fatigue damage for alclad aluminium alloys.
Studies on aluminum single crystals under fatigue [1,2] conducted at the National Aviation University and the published results of some authors [3], showed a close correlation of the accumulated fatigue damage level with the density of PSB's. These results are the basis for the proposed method. So, it was proposed to use the same approach for analysis of surface state of polycrystal structural materials [4].
Aluminium alloys D16AT, 2024T3 and 7075T6 have been chosen for experiment. These materials are widely used for manufacturing of modern aircraft skin in Ukrainian, Russian and Western aircraft industry.
Flat specimens with a hole in the center, in order to induce fracture localization were used in fatigue test procedure. Such stress concentrator indicates the point for surface state checking as well.
The thickness of the specimen is 1.5 mm and the diameter of the hole is 4 mm. These dimensions were chosen taking into account that sheets 1.5 mm thickness are used in many cases for aircraft skin production, where the 4 mm hole imitates a constructive hole for rivets. Riveted aluminum structures are found to vary degrees on virtually all aircraft. In aircraft structures rivets are used to joint sheets of the skin, or to mount skin on frames and stringers. The number of rivets in the structure of a modern 200 seat passenger airplane is more than 1.5 million. Thus, such kind of stress concentrator is typical.
All damage parameter measurements have been performed at the stress concentrator, where stress level is maximum.
Special computer-aided optical equipment has been designed for deformation relief monitoring. The main objective was to use standardized systems of
mass production with stable characteristics and relatively low in cost. The present investigation of deformation relief and the quantitative estimation of the accumulated fatigue damage have been conducted with the system containing metallographic light microscope with the about X400 enlargement, digital camera with the number of pixels 1600x1200 and portable PC.
The three-dimensional character of observed pattern and its correspondence to the known scheme of intrusions and extrusions formation have been confirmed by means of Scanning Electron Microscopy (SEM) investigation by using microscope Zeiss DSM950.
Images of cyclically loaded specimen surfaces have been processed by special software. The developed program saves the surface images in BMP format and gives the possibility to determine quantitatively the damage parameter D. Such parameter is equal to the area of specimen surface with deformation tracks (PSB's) divided by total considered surface.
Cyclic deformation test has been carried out with a hydraulic pulsating machine MUP-20. Tests have been performed under load control at frequency of 11 Hz. The shape of loading cycle is sinusoidal. The researches have been carried out in the wide range of stress conditions. A set of experimental curves that show the dependence of accumulated damage parameter on the number of cycles have been obtained. All curves and that presented below have been obtained by the approximation with exponential function. As an example the result of fatigue test of D-16 specimen and damage monitoring under the maximum stress of 81,7 MPa and load ratio R = 0 is presented. It expresses the relationship of damage parameter D
and current number of cycles N c (Fig.3). Results
presented have been approximated by the function
D = 0,0027 N c 0394 with correlation coefficient
R2 = 0,7865.
The test was stopped after the nucleation of fatigue crack of 1.0 mm length as it has been considered as the critical state condition.
As it is seen from the graph, the minimum scatter is on the initial stage of the fatigue process, whereas the final stage of the damage accumulation process has maximum level of scattering.
The data obtained can be also presented as a relationship of damage parameter D and percentage of residual life. So, it is possible to predict aircraft units life by the parameter D.
It is much more difficult to predict fatigue failure at random action of loads. The further research plan intends to carry out testing under a wide range of loading operation regimes. Both regular and program loading regimes will be materialized in order to simulate service conditions.
Fig.3 - The dependence of damage parameter D on the number of load cycles.
As a result of scheduled researches, the following exemplary procedure for aircraft fatigue analysis might be proposed:
1. Operating range of loading, load distribution along the structure, and material characteristics are determined. According to recommendations of International Civil Aviation Organization (Doc. 9051-
0.5 Q 0.45 M 0.4 § 0.35 § 0.3 0.25 ^ 0.2
0.15
0.1
0.05
30
number of cycles, N X 104
AN/896, ICAO, 1987) the load range must be based on statistic tests data obtained by means of generalized load researches for the particular airplane type.
2. Structure parts to be investigated are determined. The location of a possible damage can be determined by analysis or on the basis of endurance tests for the whole structure or its separate elements. If the estimation is performed by analysis, the following parameters are to be taken into account: a) strain measurement data for defining the places of high stresses concentration and magnitude of the concentration; b) places where residual deforma-tions are arisen during previous tests; c) places of possible fatigue damages defined by fatigue analysis; d) structure places which according to operation experience of similar structural elements are susceptible to fatigue.
3. Laboratory fatigue tests of structure elements with monitoring of surface state of the foil indicator or alclad surface state are carried out to create data base. The test program is scheduled taking into account operating range of loads. For each state both the damage parameters is estimated and factor of service life expiration is calculated as a relation of the number of cycles corresponding to a given state to cycle number to failure under given loading condition. The result of such monitoring presents regression models for life prediction.
4.Monitoring of fatigue process of aviation structures in operation or under full-scale test is performed by means of inspection of foil indicators or skin surface in determined areas in accordance with requirements of Item 2 and by procedure stated in Item 3.
5. The quantitative analysis of accumulated damage of the structure inspected part is conducted by estimation of damage parameters and residual life by use of regression models, composed on laboratory test results.
Conclusion
Accumulated fatigue damage estimation of aircraft units may be performed by the analysis of surface pattern, created by the cyclic loads on the surface of foil indicators or directly on the surface of skin unit.
In case of foil indicator it might be recommended to use two damage parameters: slip line density for single-crystal and relief saturation parameter D for polycrystalline foil indicators.
As the deformation relief on the surface of the surface layer of alclad alloys is observed in the very first cycles of loads, the computer-aided optical inspection of initial stages of fatigue damage is possible.
The new approach may be used for indication of more dangerous points of aircraft structures, for prediction of fatigue crack under full scale test of aircraft structures as well as for residual service life estimation.
References
1. Zasimchuk E.E., RadchenkoA.I., Karus-kevich M.V. Single-crystals as an Indicator of Fatigue Damage//Fatigue Fract. Engng. Mater.Struct. -1992.-Vol.15, N 12. - P. 1281-1283
2. Karuskevich M.V., Gordienko Yu., Zasimchuk E.E. Forecasting the critical state of deformed crystal by analysis of smart defect structure.// Fractal characteristics and percolation critical indexes. Proceedings of the seventh conference on sensors and their applications, held in Dublin, Ireland.-1995, 10-13 September.-112-117.-Dublin.
3. Гурьев А.В., Савкин А.Н. Роль микропластических деформаций в усталости металлов//Меха-ническая усталость металлов.- Киев: Наукова думка, 1983.-C.122-129.
4. Игнатович С.Р., Карускевич М.В., Каруске-вич О.М., Хижняк С.В., Якушенко О.С. Мониторинг втомного пошкодження алюмУевих конструкцмних сплавiв, Вюник НАУ NAU, 1(19) 2004. -C. 88-92.
Поступила в редакцию 25.07.2006 г.
Рецензент: д-р техн. наук, проф. Дмитриев С.А. Национальный авиационный университет, Киев.
Анота^я: Розглянуто два способи мон1торингу втоми ав1аи,1йних конструкцй: а) викори-стання фольгових ¡ндикатор1в; б) безпосередне спостереження \ к1льк1сна оцнка параметре поверхневого деформацйного рельефу плакованих алюм1н1евих сплав1в. Еволюця стану поверхн1 дослджена при р1зноман1тних режимах цикл1чного навантаження.
Аннотация: Рассмотрено два способа мониторинга усталости авиационных конструкций: а) применение фольговых индикаторов; б) непосредственный контроль и количественная оценка параметров поверхностного деформационного рельефа плакированных алюминиевых сплавов. Эволюция состояния поверхности исследована при различных режимах циклического нагружения.
