Aircraft strength testing. Aircraft testing workshop

The creation of any aircraft is a long and complex process, the result of the joint efforts of a huge team, many divisions and departments. The Ilyushin aviation complex, along with the experimental design bureau, also includes a large number of structural laboratories necessary for conducting full tests, including tests of structural strength that are very important for the future aircraft.

1. (clickable up to 1400)

During static and life tests of full-scale prototypes of experimental products, the calculated conclusions are confirmed experimentally. The tests confirm the correctness of the design of the structure for the given loads, and the issue of the correctness of determining the loads is resolved with the help of flight strength tests, which are carried out by specialists at the LII with the participation of specialists from the department. Today we’ll take a closer look at just such a complex.

We were met and given a tour by the deputy head of the laboratory of the strength testing complex "AK named after S.V. Ilyushin", candidate of technical sciences - Vladimir Ivanovich Tkachenko...
2.

Vladimir Ivanovich spoke about the types of strength tests, and specifically about the research that is carried out in this laboratory.

There are two independent disciplines of strength calculations - static strength calculations and resource calculations. Static tests, during which the load on the airframe elements exceeds the operational load by 1.5 times. The load on the wing during flight exceeds 1000 tons. Create the most approximate conditions of the structure due to the stress-strain state of the structure.

The operational load in the calculations is assumed to be 67% (this is outside the airworthiness standards). If, for example, we multiply this value by the safety factor (for calculations the value taken is 1.5, which takes into account the life of the airframe), then we get exactly the 100% design load, although such a load never occurs during flight...
3.

The wing, as the most damaged part of the aircraft structure, is tested with design loads of up to 120%. The fuselage, although it has various structural cutouts and cavities, and it would seem to have less strength, in flight is not subject to the same loads that the wing receives. Therefore, testing with 100% load is sufficient for it...
4.

This Il-76TD (RA-76751), produced in 1988, first flew for Aeroflot and managed to fly 2,500 hours, and in 1994, after landing on Khodynskoye Field, it was placed at the disposal of the Design Bureau for installation and flight testing of new PS- engines. 90.
However, the engines were never installed on this vehicle, and it was decided to leave this side for life testing. A special program was developed for this purpose. Life tests of the Il-476 are now being carried out under a similar program...
5.

The IL-76 was initially designed for 20,000 flights. But in order to provide it with such characteristics, it was necessary to carry out the entire complex of strength and then life tests. And then, to this day, continue to carry out tests to ensure service life extension...
6.

This is exactly the kind of research that is carried out in this laboratory. The landing gear has been removed from the plane. The aircraft is suspended under powerful beams on special suspensions, which also include hydraulic cylinders that can create a load of tens of tons on a structural element. The automatic tracking system, developed jointly with TsAGI, allows you to stabilize the suspension and ensure the desired flight condition.

The forces of these hydraulic cylinders are proportional to their diameters. The load is evenly distributed among the structural elements using additional beams and brackets. On the left wing half-span there are pylons for standard D-30 engines, while on the right wing there are pylons and reinforced structural elements and mounting points for PS-90 engines, which are heavier and more powerful than the 30s...
7.

On average, to determine the service life, a flight lasting 3-4 hours and a service life of 20-25 years are taken into account. These values ​​are confirmed first. In the future, to increase the resource, they begin to conduct additional tests, which can last for years. Typically, the service life due to the condition of the material (corrosion, fatigue, wear) is less than that due to flights, and it is more difficult to extend such a service life. Now, based on test results, the flying Il-76 with D-30 engines has had its service life extended to 10,000 hours...
8.

A program flight usually lasts 20 minutes and is carried out with a full load on the structural elements of the wing and fuselage (loads on the wing are carried out with a safety factor of 2). The wing is subjected to pressure and various vibrations through the action of hydraulic cylinders. The load for calculation is summed up from all cylinders. In the final stage of flight, the wing is subjected to pressure loads that simulate landing. According to the program that is currently being worked out on the 76th, it is necessary to carry out 20,000 such flights...
9.

If damage occurs, testing is stopped and the unit is repaired. After this, the testing process resumes. Damage to the structure and individual elements is detected different ways both visual (if large) and instrumental (there are several special techniques), capable of finding even minimal cracks...
10.

Typically, the further the service life is extended, the more restrictions are imposed on the operation of the aircraft. For example, flights are limited by weather conditions or transferred from passenger flights to cargo transportation...
11.

We also looked inside the plane. To create a load on the floor, ring weights are placed in various places...
12.

The navigator's place and again the loads on the floor...
13.

Wiring harnesses from sensors of measuring equipment are stretched throughout the cabin...
14.

View of the wing through the porthole, entangled in a network of beams, brackets and wires...
15.

Studying the insides of the plane was not without an attentive local “controller” ;)
16.

If cracks are detected, they can now be fixed or eliminated using adhesive methods, so-called “stoppers”. Such innovations began with the creation and testing of the wing for the Il-86, which during development required different, higher strength characteristics...
17.

Today, there are still a very large number of IL-76s in operation around the world, including with foreign operators, which in turn requires additional research on the resource. Therefore, these types of strength tests on this machine will continue...
18.

Below are two new pylons for PS-90 engines, handed over by the plant for installation and strength testing...
19.

The entire wing is hung with various levers and counterweights, combined into one common complex system...
20.

From this cabin, located several meters above the floor, the operator controls the test programs.
There are two such cabins built in the hangar...
21.

Well, then we got acquainted with another amazing aircraft - a wooden full-size mock-up of the Il-96-300, created mainly to solve the problems of interior layout.
22.

Even part of the wing and engine were recreated on the model...
23.

Not only the interior, but also the external design features are modeled in sufficient detail...
24.

Having climbed on board, the first thing we do is look into the cockpit, because it is also included in this model...
25.

Inside, at first glance, everything looks like a real 96. The differences become noticeable only upon closer examination. For production, in most cases, wood and plywood were used. Although, in some places, real interior trim elements are installed...
26.

There are several salons, just like in real Il. There is plenty of free space inside. They say that this model was also used in the development of the interior equipment of the presidential Il-96 cabin...
27.

In the frame below - the 103rd car (five-seater Il-103), which has already passed the entire scope of tests, and is now also in this department, nestled next to its older sister...
28.

And finally, one more general form to the laboratory...
29.
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The creation of any aircraft is a long and complex process, the result of the joint efforts of a huge team, many divisions and departments. The Ilyushin aviation complex, along with the experimental design bureau, also includes a large number of structural laboratories necessary for conducting full tests, including tests of structural strength that are very important for the future aircraft. During static and life tests of full-scale prototypes of experimental products, the calculated conclusions are confirmed experimentally. The tests confirm the correctness of the design of the structure for the given loads, and the issue of the correctness of determining the loads is resolved with the help of flight strength tests, which are carried out by specialists at the LII with the participation of specialists from the department. Today we’ll take a closer look at just such a complex. We were met and given a tour by the deputy head of the laboratory of the strength testing complex "AK named after S.V. Ilyushin", candidate of technical sciences - Vladimir Ivanovich Tkachenko...
Vladimir Ivanovich spoke about the types of strength tests, and specifically about the research that is carried out in this laboratory. There are two independent disciplines of strength calculations - static strength calculations and resource calculations. Static tests, during which the load on the airframe elements exceeds the operational load by 1.5 times. The load on the wing during flight exceeds 1000 tons. Create the most approximate conditions of the structure due to the stress-strain state of the structure. The operational load in the calculations is assumed to be 67% (this is outside the airworthiness standards). If, for example, we multiply this value by the safety factor (for calculations the value taken is 1.5, which takes into account the life of the airframe), then we get exactly the 100% design load, although such a load never occurs during flight...
The wing, as the most damaged part of the aircraft structure, is tested with design loads of up to 120%. The fuselage, although it has various structural cutouts and cavities, and it would seem to have less strength, in flight is not subject to the same loads that the wing receives. Therefore, testing with 100% load is sufficient for it...
This Il-76TD (RA-76751), produced in 1988, first flew for Aeroflot and managed to fly 2,500 hours, and in 1994, after landing on Khodynskoye Field, it was placed at the disposal of the Design Bureau for installation and flight testing of new PS- engines. 90. However, the engines were never installed on this vehicle, and it was decided to leave this side for life testing. A special program was developed for this purpose. Life tests of the Il-476 are now being carried out under a similar program...
The IL-76 was initially designed for 20,000 flights. But in order to provide it with such characteristics, it was necessary to carry out a full range of strength and then life tests. And then, to this day, continue to carry out tests to ensure service life extension...
This is exactly the kind of research that is carried out in this laboratory. The landing gear has been removed from the plane. The aircraft is suspended under powerful beams on special suspensions, which also include hydraulic cylinders that can create a load of tens of tons on a structural element. The automatic tracking system, developed jointly with TsAGI, allows you to stabilize the suspension and ensure the desired flight condition. The forces of these hydraulic cylinders are proportional to their diameters. The load is evenly distributed among the structural elements using additional beams and brackets. On the left wing half-span there are pylons for standard D-30 engines, while on the right wing there are pylons and reinforced structural elements and mounting points for PS-90 engines, which are heavier and more powerful than the 30s...
On average, to determine the resource, a flight lasting 3-4 hours and a service life of 20-25 years are taken into account. These values ​​are confirmed first. In the future, to increase the resource, they begin to conduct additional tests, which can last for years. Typically, the service life due to the condition of the material (corrosion, fatigue, wear) is less than that due to flights, and it is more difficult to extend such a service life. Now, based on test results, the flying Il-76 with D-30 engines has had its service life extended to 10,000 hours...
A program flight usually lasts 20 minutes and is carried out with a full load on the structural elements of the wing and fuselage (loads on the wing are carried out with a safety factor of 2). The wing is subjected to pressure and various vibrations through the action of hydraulic cylinders. The load for calculation is summed up from all cylinders. In the final stage of flight, the wing is subjected to pressure loads that simulate landing. According to the program that is currently being worked out on the 76th, it is necessary to carry out 20,000 such flights...
If damage occurs, testing is stopped and the unit is repaired. After this, the testing process resumes. Damage to the structure and individual elements is detected in various ways, both visual (if large) and instrumental (there are several special techniques), capable of finding even minimal cracks... Typically, the further the service life is extended, the more restrictions are imposed on the operation of the aircraft. For example, they limit flights due to weather conditions or transfer from passenger flights to cargo transportation...
We also looked inside the plane. To create a load on the floor, ring weights are placed in various places...
The navigator's place and again the loads on the floor...
Wiring harnesses from sensors of measuring equipment are stretched throughout the cabin...
View of the wing through the porthole, entangled in a network of beams, brackets and wires...
Studying the insides of the plane was not without an attentive local “controller” ;)
If cracks are detected, they can now be fixed or eliminated using adhesive methods, so-called “stoppers”. Such innovations began with the creation and testing of the wing for the Il-86, which during development required different, higher strength characteristics...
Today, there are still a very large number of IL-76s in operation around the world, including with foreign operators, which in turn requires additional research on the resource. Therefore, these types of strength tests on this machine will continue... Below are two new pylons for PS-90 engines, handed over by the plant for installation and strength testing...
The entire wing is hung with various levers and counterweights, combined into one common complex system... From this cabin, located several meters above the floor, the operator controls the test programs. There are two such cabins built in the hangar...
Well, then we got acquainted with another amazing aircraft - a wooden full-size mock-up of the Il-96-300, created mainly to solve the problems of interior layout.
Even part of the wing and engine were recreated on the model...
Not only the interior, but also the external design features are modeled in sufficient detail...
Having climbed on board, the first thing we do is look into the cockpit, because it is also included in this model...
Inside, at first glance, everything looks like a real 96. The differences become noticeable only upon closer examination. For production, in most cases, wood and plywood were used. Although, in some places, real interior trim elements are installed...
There are several salons, just like in real Il. There is plenty of free space inside. They say that this model was also used in the development of the interior equipment of the presidential Il-96 cabin...
In the frame below - the 103rd car (five-seater Il-103), which has already passed the entire scope of tests, and is now also in this department, nestled next to its older sister...
And finally, another general view of the laboratory...

There are two types of aircraft testing:
First type.
Factory tests - these include:

• testing of prototype aircraft;
• testing of lead production aircraft;
• testing of production aircraft;
• acceptance tests.

Second type.
State tests - they include:

• testing of prototype aircraft;
• tests of serial aircraft (military tests are carried out for military aircraft).

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Factory tests of prototype aircraft

The purpose of these tests is to determine the tactical and flight properties of the aircraft, as well as the correspondence of these properties to the calculated ones, according to the data design bureau(KB). The test program makes it possible to determine the flight and operational characteristics of the entire aircraft, as well as to check the operation of its main components.
The test is carried out by employees of the factory flight test station. The brigade consists of a leading pilot, a leading engineer, special services specialists and an on-board technician. The team monitors the entire construction process prototype aircraft from the beginning to the end. Before starting the plant, the following documents are submitted to the testing station:

• Calculation of aircraft aerodynamics;
• Aircraft stability calculations;
• Calculation of aircraft strength.

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Preparatory work before the first flight

As soon as the aircraft is delivered to the flight test station, preparations for the first flight begin. The preparation process consists of the following stages:

• assembly and leveling of the aircraft in accordance with the design bureau drawings;
• control measurements of the aircraft for compliance with design bureau drawings;
• weighing and determining the center of gravity of the aircraft;
• determining the most favorable alignment for the first flight;
• checking the operation of aircraft components;
• adjustment of the aircraft brake group;
• checking the operation of controls;
• test runs without takeoff - to check the steering wheel and brakes at speeds up to 30 km/h;
• checking the operation of the tail rudders at speeds up to 50 km/h;
• checking the operation of the chassis;
• If the area of ​​the test airfield allows, then control take-offs are carried out at a height of 1-2 m.

After all the preparatory work, the prototype aircraft is ready for its first flight.

First flight

The first flight of the aircraft is scheduled only after all defects identified during ground tests have been eliminated. It is conducted by the lead pilot. The result of the first flight is the determination of the following defects and properties of the aircraft:

• smooth takeoff of the aircraft;
• presence of vibrations of the housing and units;
• the presence of deformations, bends and twisting in the elements of the fuselage, wings and tail elements;
• operation of units and equipment in various modes;
• operation of controls at different speeds;
• main indicators of units (engine temperature, oil level, water temperature, engine speed)
• smooth operation of the motor;
• fuel consumption at different modes flight;
• position of stabilizers during landing;
• landing run length;
• operation of the braking system.

After the first flight, another 10-15 flights are carried out to fine-tune the aircraft. Usually, in order to have confidence in the safety of further operation, the aircraft is tested at speeds exceeding the design allowable.

Testing of the leading production aircraft

The lead production aircraft are the first two aircraft new series. The test program for these aircraft consists of the following stages:

• determining the ceiling of the aircraft and the time of rise to different heights;
• determination of landing speed and maximum speeds at different altitudes;
• determination of aircraft controllability at various altitudes;
• determination of engine operating modes under various loads;
• determination of fuel consumption and flight range at optimal speed;
• checking the operation of aircraft instruments and components;
• determining the time and length of aircraft acceleration.

The equipment and equipment for testing the lead production aircraft does not differ from that used when testing prototype aircraft.

Testing of production aircraft

Such aircraft tests are carried out by special commissions, which include representatives of customer services. The number of aircraft approved for flight testing is specified in the contract. Typically no more than ten production aircraft

Multi-engine aircraft testing

The difference between this type of test is the inclusion in the flight program of the following two items:

1. control of the aircraft when one or more engines fail;
2. determination of maximum flight altitude for various engine combinations.

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Preparation of test report

When the aircraft has completely passed the entire test program, a test report is prepared. It consists of several sections:

• Flight and weight characteristics of the aircraft and equipment;
• A list of all changes and improvements made during the testing process;
• Analysis of compliance of flight characteristics with the design data required for this type of aircraft;
• Comparison of the aircraft with foreign analogues;
• Conclusions and recommendations from the flight test station.

The report is compiled by the leading engineer of the flight test station and signed by all specialists who took part in the tests. After the report is approved, the aircraft is presented for state tests.
The following documentation is included with the aircraft:

• aerodynamics calculation;
• stability calculation;
• strength calculation;
• aircraft testing materials;
• aircraft equipment diagrams;
• technical description of the aircraft (compiled by representatives of the design bureau).

State tests

Such tests are carried out if the aircraft was designed and built under government order. The purpose of these tests is:

• checking the aircraft's compliance with tactical and technical requirements;
• identifying defects in the design and equipment of the aircraft.

The state testing program consists of the following stages:

• familiarization flights;
• instrument calibration;
• flights to determine the optimal flight mode;
• flights to determine stability under various loads on;
• determination of lifting speed, ceiling and fuel consumption;
• determination of maneuverability;
• night flights and blind flights;
• during takeoff and landing;

The principle of safe damage. Flight safety aircraft directly related to the durability of structures.

A design is said to be safe to operate if it requires minimal inspection and repair while satisfactory performance of essential functions. Satisfactory performance means there is a negligible probability of structural failure for civil aviation aircraft or an acceptably low probability of failure for military aircraft. The safety of passengers and crew of civil aviation aircraft is of paramount importance. Methods for calculating structures that are reliable in operation have been developed mainly for civil aviation aircraft.

Modern aircraft have a semi-monocoque structure, consisting of thin-walled sheets supported by beams (trusses) and stringers to prevent loss of stability. The outer skin or wall forms the aerodynamic contour of the unit - fuselage, wing, stabilizer. Stiffeners are attached to the inner surface of the skin and absorb concentrated loads. This structure has been the main subject of aerodynamic research for many years and significantly distinguishes the apparatus from conventional building structures.

The required service life of a civil aviation aircraft is determined based on comprehensive economic considerations. It is 10-15 years. The designer is primarily trying to ensure longer operation of the aircraft without the formation of cracks. To do this, he uses a developed calculation methodology with which he minimizes stress concentrations and tries to keep the stresses as low as possible, based on the flight performance requirements. For parts that are difficult to repair or replace, the designer may attempt to provide the required durability without cracking, equal to the service life of the aircraft. For many designs this is not feasible. In addition, there is a risk of structural damage from maintenance vehicles, rocks on the runway, and collapsing propeller or engine parts. The designer must minimize loss of strength resulting from fatigue cracks or damage during aircraft operation. He solves this problem as follows:

selects materials and determines the dimensions of parts to ensure adequate strength of structures in the presence of cracks;

applies elements of reliability (paths of variable loads and plugs that prevent the development of cracks);

selects materials that have a low rate of fatigue crack development.

One of the modern means of increasing the reliability of structures while simultaneously increasing the service life, reducing material consumption and improving economic efficiency- design and determination of service life based on the principle of safe damage. This takes into account the presence of initial metallurgical and technological defects in structural elements and the formation of cracks in them as operational damage accumulates.

The development and implementation of the principle of safe damage is possible only by using methods of fracture mechanics. Determining the stress-strain state of structural elements containing defects such as cracks is the most important and complex stage of strength calculations. In accordance with generally accepted concepts, the stress-strain state of a body with a crack is completely characterized by the values ​​of the stress intensity factor. Almost all currently known criteria for brittle and quasi-brittle fracture, as well as dependencies describing the growth of fatigue cracks, are based on their preliminary definition.

The concept of "safe damage" refers to a structure designed to minimize the possibility of aircraft failure due to the propagation of undetected defects, cracks or other similar damage. When producing structures that allow any damage, two main problems have to be solved. These problems consist of ensuring controlled safe growth of defects, i.e., safe operation with cracks, and forced containment of damage, as a result of which either residual durability or residual strength must be ensured. In addition, the calculation of allowable damage does not eliminate the need for careful analysis and fatigue calculations.

The basic assumption on which the concept of safe damage is based is that defects always exist, even in new designs, and that they may remain undetected. Thus, the first condition for the tolerance of a defect is the condition that any structural element, including all additional links for transmitting the load, must allow safe operation if there are cracks.

Control of safe growth of defects. The occurrence of fatigue cracks can be avoided by creating a structure in which the stresses at all points are below a certain level. However, a decrease in stress levels leads to an increase in the weight of structures. In addition, cracks can occur not only from fatigue, but also for other reasons, for example, due to accidental damage received during operation or due to material defects. Therefore, in actual design, it is assumed that there will be a number of small cracks in the structure when it leaves the factory. Larger of these cracks may develop during use.

The most important element of the principle of safe damageability is the period of time during which a crack can be detected. Due to various accidents, the probability of detecting a crack during inspection is unstable. Sometimes barely visible cracks are detected in the most remote areas of the structure, and at the same time very large ones can be missed cracks in other places. Thus, there is a known case when, during the inspection of a Boeing 747, a crack 1800 mm long under the lining in the pressurized cabin of the aircraft was missed.

Therefore, a destruction control program must be drawn up for the structural elements that determine the load-bearing capacity of the airframe. An important element of a destruction control program is the development of inspection methods. For each element, appropriate verification methods must be developed and proposed. For individual parts of elements, the use of non-destructive testing methods of varying sensitivity may be required. The timing of the inspection is established based on an analysis of the available information on crack growth, taking into account the specified initial size of the defect and the size of the detected defect, which depends on the sensitivity of the flaw detection method used. The timing of the inspection should be set on the basis that, provided that the required safety factor is ensured, an undetected defect does not reach a critical size before the next inspection. Typically, the time intervals between successive inspections are scheduled so that two inspections are completed before any crack reaches a critical size.

The principle of safe damageability of aircraft structures has necessitated the wider use of non-destructive testing methods technical condition all functional systems. Capabilities of various non-destructive testing methods for detecting fatigue cracks. Non-destructive testing methods are constantly being improved.

Fatigue, corrosion and crack resistance. In the practice of operating aircraft, there are numerous cases of destruction of parts of elements and assemblies due to material fatigue. Such failure is the result of alternating or repeated loads. Moreover, fatigue failure requires a significantly lower maximum load than static failure. In flight and during ground movement, many parts and components of the aircraft structure are subject to variable loads and, although the rated stresses are often low, stress concentrations that generally do not reduce static strength can lead to fatigue. destruction. This is confirmed by the practice of operating not only aircraft, but also ground vehicles. Indeed, it is almost always possible to observe fatigue failures and very rarely - failures from static loads.

A feature of fatigue failure is the absence of deformation in the fracture zone. Similar phenomena are observed even in materials such as mild steels, which are highly ductile under static fracture. This is a dangerous feature of fatigue failure because there are no signs that precede failure. Incipient fatigue symptoms are usually very small and difficult to detect until they reach a macroscopic size. Then they spread quickly and complete destruction occurs in a short period of time. Thus, timely detection of fatigue cracks is a challenging task. Most often, fatigue cracks originate in the area of ​​changes in shape or defects in the surfaces of parts.

Such defects, as well as a small change in the working section of the parts, do not affect the static strength, since plastic deformation reduces the effect of stress concentration. At the same time, during fatigue failure of parts, plastic deformations are, as a rule, small, as a result of which stress reduction in the concentration zone does not occur and concentration is taken into account stress is significant, so it is important when designing parts operating under variable loads to make them lighter and safer with respect to fatigue failure.

Thus, factors influencing fatigue resistance include: stress concentrators, part sizes, the relative importance of static and cyclic loads, and corrosion, especially friction corrosion, which is the result of small repeated movements of two contacting surfaces.

Fatigue failures are usually caused by many thousands or millions of loading cycles. However, they can occur after hundreds or even tens of cycles.

All elements, parts and components of the aircraft are subject to dynamic loads when moving on the ground and in flight. Variable loads of various types acting on structural elements, parts of units and devices cause corresponding alternating stresses, which ultimately lead to fatigue failure. The rate of processes of mechanical destruction of loaded parts and assemblies, respectively, and the time to destruction depend on the structure and properties of materials, on stresses caused by active loads, temperature and other factors. However, the nature of failure due to fatigue of the material has a unique appearance, different from brittle fracture.

Fatigue failure of a part usually begins near a metallurgical or technological defect, a zone of stress concentration, as well as in the presence of technological defects in products.

As is known, static destruction is determined mainly by the probability of a large load occurring in flight, for example, from an air gust, as a result of which a load will be applied to the aircraft that exceeds the static strength limit of the structure, i.e. the possibility of static failure is essentially a matter of the likelihood of a large load occurring.

Fatigue failure under these assumptions is the result of applying a sufficient number of load cycles or a sufficient number of aircraft flights over a certain distance.

The main difference between fatigue and static loading is the following:

the main factor of fatigue strength for a given load distribution, even taking into account the spread of data, is the number of load changes or service life; for static strength and destruction - effective load;

The nature of the probabilistic approach to fatigue loading differs significantly from the nature of the probabilistic approach to static loading - for specific operating conditions, the probability of the influence of a single large load on an aircraft, for example, from an air gust, exceeding the static destructive load, does not depend on the operating time. This can happen at the beginning and end of the service life. The probability of fatigue failure changes during operation, increasing significantly towards the end of its service life. In this case, designers and scientists believe that the assigned resource or service life limit and the corresponding level of probability should be such that the frequency of failure repeatability has a sufficiently small value that, if possible, would be generally accepted. This probability value is 10 9, which is accepted as a basis by leading foreign and domestic aviation companies.

Aviation experts believe that corrosion, like fatigue damage, determines the service life of an aircraft structure to the same extent. Often sources of corrosion are damage to the structure when loading the aircraft on the ground and scratches to the skin.

It is known that corrosion damage to the structure entirely depends on the operating conditions of the aircraft and the quality of maintenance.

The instructions, first of all, draw attention to corrosion of the main structural elements. It has been established that corrosion is caused to a greater extent by internal rather than external factors. Thus, the cause of corrosion is liquids spilled in the buffet area (especially fruit juices) and toilets.

Areas of the fuselage structure most susceptible to corrosion and fatigue cracks (shaded).

The least dangerous type of corrosion is general (uniform) corrosion. But in real operating conditions, uniform corrosion in its pure form is rare and is usually accompanied by ulcerative lesions. The effect of such corrosion on fatigue resistance.

It can be seen that, depending on the area and depth of corrosion damage, the fatigue life of the D16T alloy is significantly reduced. In this case, the area of ​​corrosion damage reduces fatigue resistance to a lesser extent than the diameter and depth of corrosion pits.

During operation, the processes of accumulation of fatigue and corrosion damage alternate with partial overlap. It is generally believed that corrosion damage develops while parked, and fatigue damage develops during flights. Corrosion damage is a stress concentrator.

Provisions and approaches used in justifying resources within 103 l. h over a period of 20-25 years of operation, necessitate the use of the progressive principle of “safe damage” when ensuring flight safety at the present stage, along with the “safe resource” principle.

This latter principle allows fatigue damage to occur on structural members during the time interval between two successive inspections, provided that this interval is not too long and the damage does not reach its limit state and will not lead to failure of the structure as a whole.

Consequently, the aircraft strength criterion, which states that the formation of cracks is inadmissible, is incorrect for the structure as a whole, since under conditions of long-term operation of aircraft it is almost impossible to avoid fatigue cracks in its individual elements. It is necessary to detect cracks in time and prevent them further development beyond the maximum permissible dimensions.

Thus, the strength life of an aircraft should be determined on the basis of a strength criterion that takes into account the intensity of crack initiation and development for the structure as a whole and in elements that do not lead to a catastrophic outcome.

There is a concept according to which it is believed that within 30 minutes. 101 l. h must ensure safety, and then up to 60 * 103 l. h - operation is ensured due to the survivability of structures.

Let us recall that the survivability of an aircraft or its functional systems is understood as a property that ensures the normal performance of specified functions in flight (or flights) with individual malfunctions or damage to their elements or components. It is ensured by the presence of a reserve, specific design solutions that favor a sufficiently slow development of damage and sufficient strength in the presence of a fault, easy accessibility for damage detection and objective monitoring, if possible.

Experience shows that during long-term operation wear of components, fatigue and corrosion damage are the most common failures.

Fatigue cracks lead to a decrease in the strength of the structure and determine its strength reliability. Therefore, when designing, it is necessary to ensure compliance following conditions: the development and propagation of cracks in structural elements must be so slow that the residual static strength when cracks develop to the size of its visual detection is sufficient for trouble-free operation of the aircraft without restrictions.

Let's look at some test results for aircraft fuselage skin samples with a pressurized cabin. Thus, a diagram of the development of a fatigue crack in the fuselage panels of a DC-10 aircraft is shown. The residual strength of the fuselage of the DC-10 aircraft was studied on panels measuring 4267 x 2642 mm with a radius of curvature of 30 mm. The tests were carried out under combined loading conditions simulating inertial loads and boost pressure in the passenger cabin. To do this, we took a panel from the top of the skin with an existing initial crack of 12 mm. As can be seen, at the first stage of testing at a nominal pressure of 0.65 Pa up to 15,000 cycles, crack growth was practically not observed. After making a cut in the load-bearing element and a slight increase in internal pressure, the crack growth rate began to increase, but did not reach a dangerous value. At 46,000 cycles, the central frame was destroyed, followed by the destruction of both frames, which resulted in a sharp increase in the rate of crack development and the destruction of other load-bearing elements. Complete destruction of the panel occurred with a crack length of 1157 mm and at a pressure exceeding 1.53 times the nominal pressure in the cabin.

Similar tests carried out on other panels with a set of load-bearing elements showed the possibility of creating structures with increased survivability and applying the principle of “safe” damage to the structure while ensuring control of its technical condition during maintenance.

However, the most dangerous are fatigue failures of fuselage structural elements. Thus, cracks in the fuselage skin of the Comet aircraft, which arose near the cutouts for the windows, were the cause of two crashes of aircraft of this type.

The main reason for the appearance of cracks is repeated loads of the fuselage skin with a pressurized cabin of the Comet aircraft and design flaws. As is known, the skin of an aircraft experiences repeated tension-compression loads. They caused the development of cracks in places of stress concentration. After completion of the cladding modifications, no cracks of this type were observed.

The survivability design allows for certain damage sizes that must satisfy more general regulatory requirements. For example, the Douglas company believes that the residual structural strength of a passenger aircraft should be ensured with a crack in the wing up to 400 mm long with a stringer destroyed in the middle, and in the fuselage with a longitudinal crack up to 1000 mm long with a titanium stopper destroyed in the middle or with a transverse a crack up to 400 mm long with a spar destroyed in the middle.

Lockheed defines the following acceptable damage to the fuselage: a 300 mm long crack with a frame or stringer destroyed in the middle is allowed in the skin; longitudinal crack in the casing - up to 500 mm; a crack running from the corner of a cutout, up to 300 mm, with the destruction of one frame or stringer.

The ICAO requirements indicate that the minimum level of residual strength of damaged structures must correspond to the value of the maximum operational load equal to 66.6% of the design load for the most important design loading cases.

GOST 27.002 83 defines durability as the property of an object to remain operational to a certain state with an AT maintenance system installed. The limit state may be caused by: an irreparable violation of flight safety requirements due to a violation of the structural strength; unavoidable deviation of the parameters of units and devices beyond the tolerance limits; irreparable decrease in efficiency; the need to fulfill overhaul in accordance with the current regulatory and technical documentation.

Like reliability, durability is laid down during the design of the aircraft, ensured in production and maintained during operation. For AT, durability is determined from the conditions of flight safety and the feasibility of its further use based on comparative efficiency and the possibility of replacement with more advanced models. When designing AT products, possible loads during operation and operating modes are taken into account; select the appropriate material for parts and processing methods. For elements operating under friction conditions, materials are selected that are most wear-resistant under the expected operating conditions, etc.

All this allows designers not only to create workable structures, but also to carry out appropriate calculations and ensure the required durability standards for the equipment being designed.

Durability as a property of a structure depends on numerous factors that can be divided into strength, operational and organizational.

Strength factors include design, production, technological, load and temperature factors. Among them: stress concentrators in structural elements and residual stresses arising from imperfect technology and due to plastic deformations during assembly of components and repairs; properties of materials and their changes during operation, including initial static strength; fatigue limit; stress intensity factor for failures such as detachment and shear.

Experts believe that, using modern achievements of science, engineering and technology, it is possible to ensure the durability of structural parts of long-haul aircraft up to 40-103 hp. h. Without cracks, the aircraft can fly 30 x x 103 liters. h. If we assume that the economically advantageous resource (or duration of operation) is 60,103 l. h, then we can guarantee approximately half of this period, and the remaining half of the aircraft will be operated with acceptable damage to parts and assemblies and their replacement during repairs.

In the course of creating new types of aircraft, an increasing amount of work falls on ground testing - modern modeling techniques and test benches make it possible to obtain results with good accuracy that previously required test flights. Of course, it is impossible to do without flight tests completely - before the first flight it is necessary to find out some basic set characteristics that fundamentally confirm the airworthiness of the aircraft, after which ground and flight tests continue in parallel.

Testing new aircraft has always been a dangerous profession. Back in the 50s. In the last century, a test pilot died on average once a week all over the world. Now the tests have become at least an order of magnitude safer. This has been greatly facilitated by the development of technology, which makes it possible to carry out an increasing amount of testing on the ground.

Both the aircraft airframe and individual systems are subjected to ground tests on special stands. All strength tests of an aircraft airframe can be divided into two large groups: static, during which the level of static strength of the aircraft structure is determined, and repeated-static (resource) tests, which are aimed at determining the fatigue strength and operational survivability of the aircraft structure.

In other words, static tests determine the ability of a structure to withstand high single loads that may arise in critical situations during aircraft operation: during sudden maneuvers, gusts of wind, turbulence, system failures, etc.

Life tests determine fatigue strength - the ability of a structure to withstand repeated loads without the formation of cracks, as well as operational survivability - the ability of a structure to resist the development of cracks and other defects that can lead to its destruction.

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