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by Gennady MOROZOV, Cand. Sc. (Tech.), Deputy General Director, State Research Center "All-Russia Institute of Aircraft Materials" (VIAM), Director of Test Center
As different from many other areas of modern engineering, materials used in aircraft and rocket production must meet really the highest standard for strength and dependability - and that for some very obvious reasons. In view of the vital importance of this work, our Institute has been charged with the task of constantly keeping an eye of the development of such "airborne" materials (metals, alloys, composites, etc.) and issuing appropriate recommendations to the manufacturers. And it would be no exaggeration to say that we feel quite equal to this task, having at our disposal some really unique technical and functional testing potential.
But to begin from the beginning - the establishment of a system of quality control and management of aircraft materials- from their development to mass production-began practically right after the end of the Great Patriotic War against Nazi Germany. During the war, control of the "viability", or service life, of combat aircraft was slackened as something of secondary importance. In those years their lifespan was often counted in weeks, and even in days. And it was only in 1952 that it was decided to introduce a federal system of obligatory certification of all "airborne" materials. Any and all arbitrary changes in their composition were off limits and there was a system of stringent regimentation of technological and manufacturing procedures. This "menu of regimented parameters" continued to grow, and if in the beginning aircraft designers attached importance mostly to statistical strength factors, later on they also had to take into account what specialists call low-cycle fatigue and fracturing resistance as factors making it possible to assess the service life and the carrying capacity of an aircraft structure. And for materials operating at high temperatures the list of obligatory parameters also had to include
short-time and long-time durability and plasticity, or ductability.
And even that was not the end. In addition to the aforesaid parameters, designers had to bear in mind a whole range of characteristics which determine the working capacity, both - of separate parts and components and of the "product" as a whole. As proved by experience, ignoring them, or having an insufficient knowledge of the performance of some material in operational conditions, could lead to some unpredictable, and even tragic consequences. To give just one example, the leading producer of aircraft engines - the Rolls Royce - found itself on the verge of bankruptcy after an accident with one of the aircrafts which was caught in a sandstorm over the Sahara. Its engine was equipped with a compressor with blades made of an insufficiently tested composite material. And they turned into a kind of brush, or shreds of carbon fibers.
Apart from strength characteristics, there is also a whole number of factors which have no apparent effect on night safety, but which simply can not be ignored. Here is just one example. Some of our manufacturers here tried to use imported enamels and plastic flooring for aircraft cabins without checking first their compatibility with homemade ground coatings. As a result the covering became highly polarized and one of the airliners turned practically "blind" and in another one passengers were exposed to electric shocks.
The development of modern aircraft, meeting the present-day comfort and safety standards, is really impossible without the advent of some "unheard of materials which ensure the flawless performance of parts and components under the combined impact of temperature and stress loads. And, bearing all this in mind, we simply cannot do without an up-to-date versatile R&D base capable of conducting all kinds of tests and trials.
Bearing all this in mind, we at VIAM have designed and built a set of testing stands with all the necessary instruments and devices capable of measuring the whole range of the "vital" parameters and finding new methods of assessment of the "working capacity" of materials and developing the criteria of their safety and dependability. Having this kind of a "base" to rely on, we can take care of up to 96 percent of homemade aircraft materials with what is known as ultra-fine dendrite* monocrystalline structure, aluminum-lithium alloys, highly effective combination coatings, etc. Deserving of special mention are refractory homogenous nickel systems ZHS-32 and- 47, which are unparalleled anywhere in the world.
With all that, what really makes out center different from its foreign "brethren"? First of all, we boast a set of the latest technological stands and equipment made in the USA, Germany, Japan and also Russia (up to one third of the total). Their number already exceeds 700 units, including some really unique ones.
And now let's take a look at the range of studies which can be conducted at our Center. To begin with, these are what we call static and cyclic tests of materials of aircraft engines and the airframe, and for the assessment of their technical characteristics we use what we call "programmed loading". We are investigating in details the physical properties of metal structures, using electron microscopy, X-ray structural, phase, micro-roentgeno-spectral, photo and thermal-physics analyses. Nor do we overlook non-metallic components. They are all submitted to what we call climatic, fire safety, heat, optical and erosion tests. This is followed by chemical analyses and methods of nondestructive control-ultrasonic, magnetic, vortex-current, X-ray, heat, capillary and electric ones.
In our lab for the assessment of mechanical parameters of structural
* Dendrites-mineral aggregates (or crystals) of arborescent form. -Ed.
materials (named after S. Kishkina) the range of stresses generated by the available equipment makes it possible to test such things as fabrics used for the coverings, or jacketing of delta-planes and also models of power packs of aircraft. The possibility of reproducing "stretching-compression" stresses of up to hundreds of tons (maximum of 250 tons) applied to on aircraft frames, and what we call large "working space" of our units make it possible to conduct fatigue and fracture-resistance tests not only of separate large-size components, but on whole sections of mechanisms, exposing them to vibrations of 15-20 Hz.
Using servo-hydraulic mechanisms*, we can carry out both - standard fatigue tests and apply "rigid" loads when the end of a test is signaled by sample deformation.
Apart from mechanical impacts, we also study temperature, or heat loads, which are necessary for the assessment of aircraft equipment of today and also that developed for future uses. With this aim in mind we have special chambers in which temperature can be varied in a range of -140 to +400 0 C and also chambers where temperature can be raised to +2,000 0 C.
Studies of the mechanisms of destruction of structural materials have proved that of great importance in this process is the quality of the surface of various parts. That involves not only their highest possible cleanness, but also the formation, or sedimentation, of an outer layer, whose mechanical characteristics could increase by an order of magnitude the longevity of steel structures and also of aluminum and titanium alloys. A measure of the Institute's achievements in this field is an admission of the board of directors of the world-famous General Electric that our technology of what we call directional crystallization of blades of gas-turbines is
* Servomechanism - an automatic device for controlling large amounts of power by means of very small amounts of power and automatically correcting performance of a mechanism. - Ed .
30 times as effective as the American one.
And it is also important to know the conditions of these or those parts after several hundreds of hours of their operation. It turns out that there occur in them some irreversible changes leading to the development of microdefects, then cracks, followed by complete disintegration. In the past such parts were simply disregarded, but after our studies of the temperature and stress factors causing the above damage of the surface layer, we were able to develop a method of "rejuvenation" of various components.
As has already been said, an important characteristic of structural materials, especially non-metallic ones, is their resistance to dust, hail, rain, fog, etc. An experimental unit assembled at our Center makes it possible to conduct tests on erosion-resistance of external parts of an aircraft within a broad range of particle masses (from 20 to 1,000 mcm), their velocities (up to 500 m/s) and various angles of collision, or impact.
We pay considerable attention to finding ways and means of ensuring the fire safety of aircrafts' passenger compartments. They are clad or finished today mostly with plastics and composite materials which are highly flammable and produce plenty of smoke during fires. Another important factor is heat release which can be measured by the one and only unit available in Russia and the CIS (at our Center) which meets both Russian (AP) and international (FAR) standards. It can be used for studying the impact of various risk factors and use the data obtained for optimizing the composition and structure of new materials being developed.
As has been proved by our experience - the worsening of the technical conditions of aircraft in the process of their operation occurs not only due to the mechanical wear and deterioration of parts, but also because of the processes of corrosion, aging and what we call bio-erosion (impact of microorganisms). This problem is especially acute in areas with a harsh climate - high temperatures and humidity, increased solar radiation and high levels of chlorides in the environment.
All of these problems are being attended to by what we call the Climate Station of our Center which has two sections. One is located in Moscow - on top of a 7- storey house (area of some 300 m 2 ) and has more than 20 testing stands for materials testing in the atmosphere of an industrial area. The other one is in the Krasnodar Territory - in Gelenjik (shore and off-shore stationary station). Specialists there study the performance of various structures in a marine sub-tropical climate. They study the impacts of temperature, relative humidity of air, intensity of precipitation, the solar radiation flux in three wavebands, including the ultraviolet.
Alongside the traditional materials (steels of various modifications) they have started using in aircraft engineering composites and alloys possessing some hitherto unattainable properties. The technical potential at our disposal now makes it possible to assess these new materials, including their crystallographic structure, formed artificially. For studies of their properties we had to conduct investigations at the atomic level with the help of raster and transmission-type electron microscopes*.
Aircraft engineers, and probably not they alone, often speak of the "safe damage" level. This means in practical terms that if some crack has appear on the surface of an aircraft structures, it can still continue to operate but on one condition'- one has to keep a close check on the conditions of such flaws because they can lead up to the ultimate destruction of parts and components and an accident.
The most vulnerable in this respect is the engine. And to ensure its safe performance and the timely detection of any flaws we can use an ultrasonic robotic unit which detects even the finest (1-2 mc) cracks.
It really takes plenty of time describing all of the testing units of our Center and the methods used by our specialists. As has been said before, many of them are really unprecedented and unparalleled. Some of them have been designed and built by ourselves, such as the "Thermal-physical complex for controlling the system of cooling of gas-turbine blades" or the "family" of X-ray computer units DRON-3 and DRON-4 for studying the structure of metallic materials. Some of these systems have been adapted by our specialists to our specific conditions.
Tens of Russian and foreign patents have been obtained for all of this equipment and methods.
The practical value of our research complex is demonstrated by the fact that not one aircraft in this country - from gliders to high-tech combat aircraft, passenger liners and space probes - has been developed without our participation. And it is important to recall at this point that prolonging the service life of one aircraft by only one year brings economic returns of up to one min dollars. We think that facts and figures of this kind really speak for themselves.
* See: V. Bykov, "Microscope Examines... Atoms", Science in Russia, No. 1, 2001. - Ed.
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